Water-soluble fluororesin coating composition

Abstract

To provide a water-soluble fluororesin coating composition that strongly adheres to a substrate, can form a coating film that exhibits excellent heat resistance in addition to steam resistance, is excellent also in environment / safety hygiene.SOLUTION: There is provided a water-soluble fluororesin coating composition comprising water-soluble polyamide-imide resin, non-thermofusible polytetrafluoroethylene, and thermofusible fluororesin. In the solids content of the water-soluble fluororesin coating composition, the content of the water-soluble polyamide-imide resin is 10 mass% or more and 25 mass% or less, the content of the non-thermofusible polytetrafluoroethylene is 30 mass% or more and 72 mass% or less, and the content of the thermofusible fluororesin is 5 mass% or more and 40 mass% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous fluororesin coating composition capable of forming a coating film that firmly adheres to a substrate and has excellent heat resistance as well as water vapor resistance, a coating film obtained by applying the same, and an article having the coating film. [Background technology]

[0002] Fluorine resins have excellent heat resistance, chemical resistance, electrical properties, and mechanical properties, as well as an extremely low coefficient of friction, non-stick properties, and water and oil repellency, making them widely used in a variety of industrial fields, including chemical, mechanical, and electrical.

[0003] In particular, taking advantage of the non-stick, water-repellent and oil-repellent properties of fluororesin, fluororesin coatings are used in a variety of fields, including painting cookware such as frying pans and rice cookers, and fixing rolls and belts that fix toner in office equipment.In recent years, the range of uses has expanded even further, to include inkjet nozzles and chemical plant equipment.

[0004] However, when coating various substrates with fluororesin, due to the non-stickiness of fluororesin, it is extremely difficult to paint the fluororesin directly onto the substrate, as this results in poor adhesion. Therefore, when applying a fluororesin coating, a primer paint composition that has adhesion to the substrate and also to the fluororesin coating to be applied on top of it has usually been used.

[0005] Such primer coating compositions use heat-resistant resins (so-called engineering plastics) that have adhesive properties to the substrate and can withstand temperatures above the melting point of the fluororesin, and for example, Patent Document 1 discloses precursors of polyimide, polyamideimide, polyethersulfone, etc., and fine particles of polyphenylene sulfide, etc. Such heat-resistant resins are called binders.

[0006] On the other hand, organic solvents (solvent-based paints) or water (water-based paints) are used as the medium for fluororesin paint compositions, including primer paint compositions, and in recent years, water-based (water-based) paint compositions have been preferred in terms of environmental impact and harmfulness to the human body. In water-based paint compositions, the heat-resistant resin (binder) that imparts adhesion to the substrate is usually water-insoluble, so its particles are dispersed in the paint composition liquid, and in this case, water-soluble polyamideimide can also be used (Patent Document 2).

[0007] When water-soluble polyamideimide is used as a binder, it dissolves uniformly in the aqueous fluororesin coating composition, so even a small amount can provide high adhesive strength. In addition, because water-soluble polyamideimide has a high viscosity, it is possible to reduce or eliminate the use of thickeners, which is known to have the advantage of improving the purity of the coating film and achieving better performance.

[0008] However, the coating films obtained from conventional fluororesin coating compositions using water-soluble polyamideimides have insufficient water vapor resistance and corrosion resistance, making it necessary to add a separate water-insoluble binder. For example, Patent Document 3 proposes the use of a polyethersulfone resin together with a water-soluble polyamideimide. Patent Document 4 proposes a fluororesin coating composition using a polyetherimide resin together with a water-soluble polyamideimide. Other examples disclosed include the use of a polyetherimide resin and a polyethersulfone resin in addition to a water-soluble polyamideimide (Patent Document 5), the use of a polyetheretherketone (PEEK) resin (Patent Document 6), and the use of a polyetherketoneketone resin (Patent Document 7). do.

[0009] Although the coating films obtained from these paints have shown some improvement in water vapor resistance and corrosion resistance, they do not have sufficient heat resistance, which is required for use in environments where they are exposed to high temperatures for long periods of time, such as the fixing rolls of office equipment. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Special Publication No. 4-71951 [Patent Document 2] Patent No. 3491624 [Patent Document 3] Patent No. 4534916 [Patent Document 4] Japanese Patent Application Publication No. 2019-218484 [Patent Document 5] Patent Publication No. 2021-21013 [Patent Document 6] Patent Publication No. 2021-91754 [Patent Document 7] Japanese Patent Publication No. 2023-87722 Summary of the Invention [Problem to be solved by the invention]

[0011] The object of the present invention is to provide an aqueous fluororesin coating composition that can form a coating film that firmly adheres to a substrate, has excellent water vapor resistance and heat resistance, and is also excellent in terms of the environment, safety, and hygiene. [Means for solving the problem]

[0012] To achieve the above object, the aqueous fluororesin coating composition of the present invention comprises a water-soluble polyamideimide resin, a non-thermofusible polytetrafluoroethylene, and a melt processable fluororesin, and is characterized in that, in the solid content of the aqueous fluororesin coating composition, the content of the water-soluble polyamideimide resin is 10% by mass or more and 25% by mass or less, the content of the non-thermofusible polytetrafluoroethylene is 30% by mass or more and 72% by mass or less, and the content of the melt processable fluororesin is 5% by mass or more and 40% by mass or less. The aqueous fluororesin coating composition of the present invention is excellent in heat resistance as well as in water vapor resistance.

[0013] That is, the present invention is as follows. (1) An aqueous fluororesin coating composition comprising a water-soluble polyamideimide resin, a non-thermofusible polytetrafluoroethylene, and a thermofusible fluororesin, wherein, in the solid content of the aqueous fluororesin coating composition, the content of the water-soluble polyamideimide resin is 10% by mass or more and 25% by mass or less, the content of the non-thermofusible polytetrafluoroethylene is 30% by mass or more and 72% by mass or less, and the content of the thermofusible fluororesin is 5% by mass or more and 40% by mass or less. (2) The aqueous fluororesin coating composition according to (1), wherein the content of non-thermofusible polytetrafluoroethylene is 40% by mass or more and 67% by mass or less, and the content of thermofusible fluororesin is 10% by mass or more and 30% by mass or less. (3) The aqueous fluororesin coating composition according to (1) or (2), which does not contain any water-insoluble heat-resistant resin other than non-thermofusible polytetrafluoroethylene and the thermofusible fluororesin. (4) A coating film made of the aqueous fluororesin coating composition described in (1). (5) A coated article having the coating film described in (4). [Effects of the Invention]

[0014] According to the present invention, it is possible to provide an aqueous fluororesin coating composition that has sufficient adhesion to a substrate and is excellent in heat resistance as well as water vapor resistance. Furthermore, since the aqueous fluororesin coating composition of the present invention uses water as a medium, it is also excellent in terms of the environment, safety and hygiene. Cut. DETAILED DESCRIPTION OF THE INVENTION

[0015] The aqueous fluororesin coating composition of the present invention is characterized by containing a water-soluble polyamideimide resin, a non-thermofusible polytetrafluoroethylene, and a thermofusible fluororesin. The present invention will be described in detail below.

[0016] <Water-soluble polyamide-imide resin (water-soluble PAI)> The "water-soluble polyamide-imide resin (water-soluble PAI)" used in the present invention is a water-soluble resin having an amide bond and an imide bond in the main chain, and is preferably a resin represented by the following general formula: [ka] (In the formula, R 1 represents a trivalent organic group, and R 2 represents a divalent organic group. Because water-soluble PAI dissolves uniformly in the water-based fluororesin coating composition, it is distributed uniformly even during the drying process after application, making the coating film denser and allowing it to easily penetrate into the recesses in the substrate's irregularities, thereby improving adhesion to the substrate.

[0017] The water-soluble PAI used in the present invention can be obtained by copolymerizing a diisocyanate compound or a diamine compound as an amine component with a tribasic acid anhydride or a tribasic acid halide as an acid component in a polar solvent. The PAI synthesis conditions vary widely and are not particularly limited, but are typically carried out at a temperature of 80 to 180°C, preferably in a nitrogen or other atmosphere to reduce the influence of moisture in the air.

[0018] The diisocyanate compound is not particularly limited, but examples thereof include diisocyanate compounds represented by the following formula (1): In formula (1), X represents a divalent organic group.

[0019] [ka]

[0020] Examples of the divalent organic group represented by X include alkylene groups having 1 to 20 carbon atoms; arylene groups such as phenylene and naphthylene groups that are unsubstituted or substituted with a lower alkyl group having 1 to 5 carbon atoms such as a methyl group, or a lower alkoxy group having 1 to 5 carbon atoms such as a methoxy group; divalent organic groups formed by two of the above arylene groups bonded via a single bond, a lower alkylene group having 1 to 5 carbon atoms, an oxy group (-O-), a carbonyl group (-CO-), or a sulfonyl group (-SO2-); and divalent organic groups formed by two lower alkylene groups having 1 to 5 carbon atoms bonded via the above arylene group. The number of carbon atoms in the alkylene group is preferably 1 to 18, more preferably 1 to 12, even more preferably 1 to 6, and particularly preferably 1 to 4. .

[0021] From the viewpoints of reactivity and improving the adhesive strength of the coating film, the divalent organic group represented by X is preferably a divalent organic group formed by two of the above-mentioned arylene groups bonded via a single bond, a lower alkylene group having 1 to 5 carbon atoms, an oxy group (-O-), a carbonyl group (-CO-), or a sulfonyl group (-SO2-), more preferably a divalent organic group formed by two of the above-mentioned arylene groups bonded via a single bond or a lower alkylene group having 1 to 5 carbon atoms, and even more preferably a divalent organic group formed by two phenylene groups bonded via a single bond or a lower alkylene group having 1 to 5 carbon atoms. When two or more diisocyanate compounds are used in combination, it is preferable to select and use two or more of these preferred embodiments. Furthermore, from the viewpoint of reactivity, the arylene group is preferably unsubstituted, and from the viewpoint of improving the adhesive strength of the coating film, it is preferably substituted with a lower alkyl group having 1 to 5 carbon atoms, such as a methyl group, or a lower alkoxy group having 1 to 5 carbon atoms, such as a methoxy group.

[0022] Specific examples of diisocyanate compounds include xylylene diisocyanate, paraphenylene diisocyanate, tolylene diisocyanate, naphthalene diisocyanate, 3,3'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 3,3'-dimethoxybiphenyl-4,4'-diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate.

[0023] The diamine compound is not particularly limited, but examples thereof include compounds in which the isocyanate group in the above formula (1) is replaced with an amino group.Specific examples of the diamine compound include xylylenediamine, phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,3'-dimethylbiphenyl-4,4'-diamine, and isophoronediamine.

[0024] As the amine component (diisocyanate compound, diamine compound), it is preferable to use 3,3'-dimethylbiphenyl-4,4'-diisocyanate and / or 3,3'-dimethylbiphenyl-4,4'-diamine, because this can improve the substrate adhesive strength and water vapor resistance of the coating film.Furthermore, from the viewpoint of improving the working environment, it is preferable to use 3,3'-dimethylbiphenyl-4,4'-diisocyanate.

[0025] In the synthesis reaction of PAI, a diisocyanate compound may be used alone, a diamine compound may be used alone, or a diisocyanate compound and a diamine compound may be used in combination. From the viewpoint of facilitating the synthesis reaction, a diisocyanate compound is preferably used.

[0026] Examples of tribasic acid anhydrides include tricarboxylic acid anhydrides. Although not particularly limited, aromatic tribasic acid anhydrides are preferred, aromatic tricarboxylic acid anhydrides are more preferred, and compounds represented by the following formula (2) or formula (3) are even more preferred. Trimellitic acid anhydride is particularly preferred from the viewpoints of heat resistance, cost, etc.

[0027] [ka] (R represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or a phenyl group, and Y represents -CH2-, -CO-, -SO2-, or -O-.)

[0028] As the tribasic acid halide, a tribasic acid anhydride halide is preferably used, for example, a tricarboxylic acid anhydride halide. The tribasic acid anhydride halide is preferably a tribasic acid anhydride chloride. Although not particularly limited, an aromatic tribasic acid anhydride chloride is preferred, an aromatic tricarboxylic acid anhydride chloride is more preferred, and a compound in which the -COOR group in the above formula (2) or (3) is replaced with a -COCl group is even more preferred. From the viewpoints of heat resistance, cost, etc., trimellitic acid anhydride chloride (trimellitic acid chloride anhydride) is particularly preferred.

[0029] As the acidic component, tricarboxylic acid anhydrides are preferably used, and trimellitic acid anhydride is particularly preferred, from the viewpoint of reducing the burden on the environment.

[0030] In addition to tribasic acid anhydrides and tribasic acid halides, polybasic acids or polybasic acid anhydrides such as dicarboxylic acids and tetracarboxylic dianhydrides can be used as the acid component to improve hydrophilicity, as long as the properties of the PAI, such as heat resistance, are not impaired.

[0031] The dicarboxylic acid is not particularly limited, but examples thereof include terephthalic acid, isophthalic acid, adipic acid, sebacic acid, etc. The tetracarboxylic dianhydride is not particularly limited, but examples thereof include pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, biphenyltetracarboxylic dianhydride, etc. The polybasic acid and polybasic acid anhydride may each be used alone or in combination of two or more.

[0032] The amount of polybasic acid and polybasic acid anhydride (e.g., dicarboxylic acid, tetracarboxylic dianhydride) other than tribasic acid anhydride and tribasic acid halide used is preferably 0 to 50 mol %, more preferably 0 to 30 mol %, and even more preferably 0 to 15 mol %, of the total acid components, from the viewpoint of maintaining the properties of PAI such as heat resistance.

[0033] The ratio of the diisocyanate compound and / or diamine compound to the acid component (tribasic acid anhydride and / or tribasic acid halide, and dicarboxylic acid and / or tetracarboxylic acid dianhydride used as needed, etc.) used is preferably such that the total amount of the diisocyanate compound and / or diamine compound is 0.8 to 1.1 mol, more preferably 0.95 to 1.08 mol, and even more preferably 1.0 to 1.08 mol per 1.0 mol of the total amount of the acid component, from the viewpoint of the molecular weight and degree of crosslinking of the PAI to be produced.

[0034] As the PAI, a PAI obtained by reacting a diisocyanate compound and / or a diamine compound with an acid component can be used as it is, or it can be used after being protected with a blocking agent.

[0035] When a diisocyanate compound is used as a raw material compound, a blocking agent for terminal isocyanate groups (terminal blocking agent) may be optionally used to stabilize the PAI. By protecting the PAI with a blocking agent, the PAI does not have an isocyanate group (-NCO group) or has a reduced amount of isocyanate groups (-NCO group) compared to PAI obtained by reacting an isocyanate compound with an acid component.

[0036] Examples of the blocking agent include alcohols, such as lower alcohols having 1 to 6 carbon atoms, such as methanol, ethanol, and propanol. Examples of the blocking agent include 2-butanone oxime, δ-valerolactam, and ε-caprolactam. The blocking agent is not limited to these exemplary compounds. The blocking agent may be used alone or in combination of two or more.

[0037] Examples of polar solvents that can be used in the polymerization include N-methyl-2-pyrrolidone (NMP), N-ethylmorpholine, N-formylmorpholine, N-acetylmorpholine, N,N'-dimethylethyleneurea, N,N-dimethylacetamide or N,N-dimethylformamide, and γ-butyrolactone. NMP has been preferably used so far because of its easy availability and high boiling point, but from the perspective of its effects on the human body and legal regulations such as the REACH regulation and the US FDA, it is preferable to use N-ethylmorpholine or N-formylmorpholine.

[0038] There are no particular restrictions on the amount of solvent used, but it is preferred to use 50 to 500 parts by mass per 100 parts by mass of the total amount of the amine component and the acid component from the viewpoint of the solubility of the resulting resin.

[0039] From the viewpoint of ensuring the strength of the coating film, the number average molecular weight of the PAI is preferably 5,000 or more, more preferably 10,000 or more, even more preferably 13,000 or more, and particularly preferably 15,000 or more. From the viewpoint of ensuring solubility in water, the number average molecular weight is preferably 50,000 or less, more preferably 30,000 or less, even more preferably 25,000 or less, and particularly preferably 20,000 or less.

[0040] The number-average molecular weight of PAI can be controlled by sampling the PAI during synthesis, measuring the number-average molecular weight, and continuing the synthesis until the desired number-average molecular weight is achieved. The number-average molecular weight can be measured by gel permeation chromatography (GPC) using a calibration curve of standard polystyrene.

[0041] The PAI preferably has an acid value of 10 mgKOH / g or more, calculated by combining the carboxyl groups in the resin and the carboxyl groups resulting from ring-opening of the acid anhydride groups. It is more preferably 25 mgKOH / g or more, and even more preferably 35 mgKOH / g or more. These ranges are preferred from the viewpoint of facilitating dissolution or dispersion of the PAI. Furthermore, when a basic compound (described below) is contained, the amount of carboxyl groups that react with the basic compound is sufficient, facilitating water-solubilization. These ranges are also preferred.

[0042] Furthermore, from the viewpoint of preventing gelation over time in the final fluororesin coating composition, the acid value is preferably 80 mgKOH / g or less, more preferably 60 mgKOH / g or less, and even more preferably 50 mgKOH / g or less.

[0043] The acid value can be obtained using the following method. First, 0.5 g of PAI is taken, to which 0.15 g of 1,4-diazabicyclo[2.2.2]octane is added, followed by 60 g of N-methyl-2-pyrrolidone and 1 mL of ion-exchanged water. The mixture is stirred until the PAI is completely dissolved to prepare a solution for evaluation. The evaluation solution is then titrated potentiometrically with a 0.05 mol / L potassium hydroxide ethanol solution to obtain the acid value. The acid value is the combined acid value of the carboxyl groups in the resin and the carboxyl groups resulting from ring-opening of the acid anhydride groups.

[0044] Furthermore, a basic compound may be added to increase the solubility of PAI in water. The basic compound reacts with the carboxyl group contained in the PAI to form a salt between the basic compound and the PAI. The action of the basic compound can increase the solubility of PAI in water.

[0045] In the present invention, examples of the basic compound include alkylamines such as triethylamine, tributylamine, N,N-dimethylcyclohexylamine, N,N-dimethylbenzylamine, triethylenediamine, N-methylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N",N"-pentamethyldiethylenetriamine, N,N',N'-trimethylaminoethylpiperazine, diethylamine, diisopropylamine, dibutylamine, ethylamine, isopropylamine, and butylamine; alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, dipropanolamine, tripropanolamine, N-ethylethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, cyclohexanolamine, N-methylcyclohexanolamine, and N-benzylethanolamine; caustic alkalis such as sodium hydroxide and potassium hydroxide; and ammonia. From the viewpoint of increasing the solubility of PAI in water, alkylamines and / or alkanolamines are suitable.

[0046] From the viewpoints of facilitating water-solubilization of the PAI and improving the strength of the coating film, the basic compound is used in an amount that is 2.5 equivalents or more, more preferably 3.5 equivalents or more, and even more preferably 4 equivalents or more, relative to the carboxyl groups and ring-opened acid anhydride groups contained in the resin. Furthermore, from the viewpoint of maintaining strength, the content of the basic compound is preferably used in an amount that is 10 equivalents or less, more preferably 8 equivalents or less, and even more preferably 6 equivalents or less.

[0047] Specific water-soluble PAIs and methods for producing them are described in Japanese Patent No. 4534916, International Publication No. 2016 / 175099, Japanese Patent Application Laid-Open No. 2016-89016, Japanese Patent Application Laid-Open No. 2016-17084, Japanese Patent Application Laid-Open No. 2018-2802, etc.

[0048] The water-soluble PAI used in the present invention is usually used in the form of a solution for preparing a fluororesin coating composition. The water-soluble PAI solution can be easily obtained by dissolving the water-soluble PAI in water containing an organic solvent.

[0049] The organic solvent is not particularly limited as long as it has high polarity and a high boiling point, and various polar solvents that can be used in PAI polymerization can be used. Like the solvent used in polymerization, NMP has been preferably used so far because it is easily available and has a high boiling point. However, from the viewpoint of its effects on the human body and legal regulations such as the REACH regulation and the US FDA, it is preferable to use N-ethylmorpholine or N-formylmorpholine. The organic solvent may be the same as the solvent that can be contained in the aqueous medium of the fluororesin coating composition of the present invention, which will be described later.

[0050] The water-soluble PAI should have a concentration of 1 to 50 mass % of the water-soluble PAI solution in terms of viscosity. It is preferably 5 to 40 mass %.

[0051] Examples of commercially available water-soluble PAI solutions include HPC-1000-28 and HPC-2100D-28 manufactured by Resonac Corporation, with HPC-2100D-28 being preferred.

[0052] The water-soluble PAI can be used alone or in combination with other binders. Examples of binders that can be used with the water-soluble PAI include polyethersulfone resins, polyetherimide resins, and polyetheretherketone resins.

[0053] <Non-thermoplastic polytetrafluoroethylene> The "non-thermofusible polytetrafluoroethylene" used in the present invention is a high-molecular-weight polytetrafluoroethylene (PTFE) that does not exhibit melt fluidity above its melting point, and may be either a homopolymer of tetrafluoroethylene (TFE) (TFE homopolymer), a TFE copolymer (modified PTFE) containing 1% by mass or less of a monomer copolymerizable with TFE, or a combination of both. This reduces the stress remaining in the coating film after heating and also enables cost reduction.

[0054] <Heat melt processable fluororesin> The "melt processible fluororesin" used in the present invention is a fluororesin that exhibits melt fluidity at or above its melting point. By using a melt processible fluororesin, the occurrence of pinholes can be suppressed when a coating film is formed, and a uniform and smooth coating film can be obtained. Among these, from the viewpoint of the non-adhesiveness and heat resistance of the coating film, a melt processible perfluororesin in which all hydrogen atoms in the molecular chain are replaced with fluorine can be preferably used.

[0055] Examples of melt processable fluororesins include low molecular weight melt processable polytetrafluoroethylene (melt processable PTFE), tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-hexafluoropropylene-perfluoro(alkyl vinyl ether) copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, polychlorotrifluoroethylene, and chlorotrifluoroethylene-ethylene copolymer. Among these, PFA or FEP is preferred, and PFA is particularly preferred.

[0056] FEP may be a copolymer consisting of only tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), or may be a copolymer consisting of polymerized units based on TFE, HFP, and a monomer copolymerizable with TFE and HFP. Examples of polymerized units based on a monomer copolymerizable with TFE and HFP include perfluoro(alkyl vinyl ether). The amount of tetrafluoroethylene (TFE) in FEP is preferably in the range of 75 to 97 mass %.

[0057] When PFA is used as the melt processible fluororesin, the alkyl group of the perfluoro(alkyl vinyl ether) in the PFA preferably has 1 to 5 carbon atoms, and among these, perfluoro(propyl vinyl ether) (PPVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether) (PMVE) are particularly suitable. The amount of perfluoro(alkyl vinyl ether) in the PFA is preferably in the range of 1 to 50 mass%.

[0058] The melt flow rate (MFR) of the melt processible fluororesin is preferably 1 to 100 g / 10 min, more preferably 1 to 80 g / 10 min, and even more preferably 1 to 50 g / 10 min. The MFR can be measured in accordance with ASTM D1238 using a melt indexer at a temperature of 372°C and a load of 5 kg.

[0059] The non-melt processible polytetrafluoroethylene and melt processible fluororesin of the present invention can be used by dispersing in a coating composition a powder obtained by separating and drying a resin obtained by a known polymerization method, a powder obtained by further pulverizing the powder, or a powder obtained by finely granulating the powder by a method such as that described in Japanese Patent Publication No. 52-44576. Furthermore, a fluororesin dispersion obtained by emulsion polymerization can be used as is, or a fluororesin dispersion stabilized by adding a surfactant, or a fluororesin dispersion adjusted to a high concentration by concentrating the dispersion by a known technique such as that described in U.S. Patent No. 3,037,953, can also be used.

[0060] For the non-melt processible polytetrafluoroethylene and melt processible fluororesin of the present invention, excellent non-stickiness can be obtained by finely dispersing the fluororesin particles in the coating film, so it is preferable to use a dispersion obtained by emulsion polymerization as a raw material for the coating. The average particle size of the melt processible fluororesin particles is preferably 0.5 μm or less, particularly 0.3 μm or less. In the present invention, the average particle size means the particle size at 50% of the cumulative value in the particle size distribution determined by laser diffraction / scattering method. The concentration of the dispersion is preferably 20 to 70% by mass, and it is preferable to use a dispersion concentrated to 40 to 70% by mass, as this makes it easier to adjust the fluororesin concentration in the coating composition. Commercially available non-thermofusible polytetrafluoroethylene and thermofusible fluororesin for use in the present invention include Teflon (registered trademark) PTFE 31-JR, PTFE 34-JR, PFA 334-JR, PFA 335-JR, and FEP 120-JR, all manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd., and Teflon (registered trademark) FEP D121, all manufactured by Chemours Co., Ltd.

[0061] <Water-based fluororesin coating composition> The "aqueous fluororesin coating composition" of the present invention is an aqueous (water-based) dispersion containing a water-soluble PAI, a non-thermofusible polytetrafluoroethylene, and a thermofusible fluororesin. The aqueous fluororesin coating composition of the present invention is usually suitably used as a primer coating (undercoat) for bonding a fluororesin layer to a substrate, but can also be used as a one-coat coating without using a primer coating.

[0062] The aqueous fluororesin coating composition of the present invention contains the water-soluble PAI in an amount of 10% by mass to 25% by mass, preferably 15% by mass to 25% by mass, based on the solid content of the composition. By containing the water-soluble PAI in this range, the above-mentioned advantages can be more effectively exhibited.

[0063] Furthermore, the aqueous fluororesin coating composition of the present invention contains the non-thermofusible polytetrafluoroethylene in an amount of 30% by mass to 72% by mass, preferably 40% by mass to 72% by mass, more preferably 40% by mass to 67% by mass, and even more preferably 48% by mass to 67% by mass, based on the solid content of the composition. By containing the non-thermofusible polytetrafluoroethylene in this range, the above-mentioned advantages can be more effectively exhibited.

[0064] Furthermore, the aqueous fluororesin coating composition of the present invention contains the above-mentioned melt processible fluororesin in an amount of 5% by mass to 40% by mass, preferably 5% by mass to 30% by mass, more preferably 10% by mass to 30% by mass, based on the solid content of the composition. By containing the component within the range, the above-mentioned advantages can be more effectively exhibited. In the present invention, the term "solid content" means the total mass of the aqueous fluororesin coating composition of the present invention after removing volatile components such as water.

[0065] In a specific embodiment of the aqueous fluororesin coating composition of the present invention, the composition does not contain any water-insoluble heat-resistant resin (binder) other than the water-soluble PAI, non-thermofusible polytetrafluoroethylene, and thermofusible fluororesin. While water-insoluble heat-resistant resins (e.g., polyetherimide resins, polyethersulfone resins, polyetheretherketone resins, and polyetherketoneketone resins) have been widely used in this technical field (see Patent Documents 3 to 7), this specific embodiment allows for the production of coating films that have water vapor resistance equivalent to or superior to those containing water-insoluble heat-resistant resins, and also have excellent heat resistance. Furthermore, the absence of a water-insoluble heat-resistant resin (binder) allows for the production of aqueous fluororesin coating compositions that are superior in terms of cost and productivity.

[0066] <Other ingredients> Depending on the desired properties such as dispersibility, conductivity, foam prevention, improved abrasion resistance, and color, various additives used in ordinary paints, such as surfactants, film-forming agents, thickeners, and pigments, can also be added as optional components to the aqueous fluororesin paint composition of the present invention. In addition, various organic and inorganic fillers can be added to the aqueous fluororesin coating composition of the present invention depending on the desired properties. Examples of organic fillers include engineering plastics such as polyphenylene sulfide, polyethersulfone, polyphenylsulfone, polyamide, polyimide, phenolic resin, urea resin, epoxy resin, urethane resin, melamine resin, polyester resin, polyether resin, acrylic resin, acrylic silicone resin, silicone resin, and silicone polyester resin. Examples of inorganic fillers include metal powder, metal oxides (aluminum oxide, zinc oxide, tin oxide, titanium oxide, etc.), glass, ceramics, silicon carbide, silicon oxide, calcium fluoride, carbon black, graphite, mica, and barium sulfate. Fillers can be in various shapes, such as particles, fibers, and flakes.

[0067] <Aqueous medium> The aqueous fluororesin coating composition of the present invention uses water as the main medium, but although it is not preferable from the standpoint of the environment or cost, it is possible to add a polar solvent that is compatible with water or to disperse a water-incompatible organic solvent in order to appropriately adjust the rheological properties such as the liquid viscosity of the aqueous fluororesin coating composition or to improve the dispersibility of fillers, etc. In this technical field, N-methylpyrrolidone has traditionally been used as a solvent, but its use has been restricted in recent years due to environmental regulations, etc. As solvents to replace N-methylpyrrolidone, N-formylmorpholine, 3-methoxy-N,N-dimethylpropanamide, etc. have been investigated, and these solvents can also be used in the present invention.

[0068] <Base material> The aqueous fluororesin coating composition of the present invention can be suitably used on both metal substrates and plastic substrates. Examples of metal substrates include aluminum, stainless steel (SUS), copper, iron, and various other alloys. As the plastic substrate, engineering plastics are preferably used, which can withstand the heat treatment required to form the fluororesin coating and have long-term durability. These include so-called super engineering plastics, such as polyaryl ketone resins (PEK, PEEK, PEKK, PEEKK), polyarylene sulfone resins (PPS, etc.), polyimide (PI), polyetherimide (PEI), and polyamideimide (PAI).

[0069] <Method of manufacturing an aqueous fluororesin coating composition> The aqueous fluororesin coating composition of the present invention can be prepared by a conventionally known method, for example, by appropriately mixing the above-mentioned water-soluble PAI solution dissolved in water containing an organic solvent with non-thermofusible polytetrafluoroethylene, a melt-melt processable fluororesin, and other additives and fillers to be blended as necessary. In the aqueous fluororesin coating composition of the present invention, dispersions (dispersions) of the non-thermofusible polytetrafluoroethylene, the melt-melt processable fluororesin, the pigment, etc. may be prepared in advance, and the aqueous fluororesin coating composition may be prepared by mixing these dispersions.

[0070] The aqueous fluororesin coating composition of the present invention preferably has a viscosity at 25°C of 0.1 to 50,000 mPa·s. If the viscosity is less than 0.1 mPa·s, sagging may occur during application to the substrate, making it difficult to obtain the desired film thickness. If the viscosity exceeds 50,000 mPa·s, coating workability may be impaired, the resulting coating film may not have a uniform thickness, and surface smoothness may be poor. A more preferred lower limit is 1 mPa·s, and a more preferred upper limit is 30,000 mPa·s. The above viscosity is a value obtained by measurement using a BM-type single-cylinder rotational viscometer (manufactured by Tokyo Keiki Co., Ltd.).

[0071] <Coating film> The "coating film" of the present invention is a coating film formed by applying the aqueous fluororesin coating composition of the present invention. It also includes a coating film formed by applying the coating composition of the present invention as a primer layer that adheres to a substrate and then applying and laminating multiple layers thereon. The "coating film" of the present invention can be formed by various existing coating methods, for example, spray coating, dip coating, spin coating, or other commonly used methods, and it is preferable to heat the coating film to a temperature above the melting point of the fluororesin in order to melt and flow it and obtain a uniform coating film.

[0072] <Painted items> The "coated article" of the present invention is an article having a coating film formed by coating the aqueous fluororesin coating composition of the present invention. Examples of the "coated articles" of the present invention include cooking utensils such as frying pans and rice cookers, heat-resistant release trays used in factory lines (for example, in the baking process), office automation equipment-related items such as fixing rolls, belts and inkjet nozzles, and industrial equipment-related items such as piping in chemical plants, all of which require non-stick, water- and oil-repellent properties. Preferably, these are cooking utensils which also require high water vapor resistance and corrosion resistance. [Example]

[0073] (Preparation of Water-Based Fluoropolymer Coating Composition) The following reagents were used in the present examples and comparative examples. binder resin Water-soluble PAI: HPC-2100D-28 manufactured by Resonac Corporation (a solution containing approximately 28% PAI by weight, 22-32% water by weight, and 30-40% N-formylmorpholine by weight) Polyether ether ketone (PEEK) resin powder: VICOTE® Coatings 704 manufactured by Victrex Non-thermoplastic polytetrafluoroethylene PTFE aqueous dispersion: Teflon (registered trademark) PTFE 31-JR (PTFE concentration 60% by mass) manufactured by Mitsui Chemours Fluoro Products Co., Ltd. Melt-type fluororesin PFA aqueous dispersion (1): Teflon (registered trademark) PFA 335-JR (PFA concentration 60% by mass) manufactured by Mitsui Chemours Fluoro Products Co., Ltd. PFA aqueous dispersion (2): Teflon (registered trademark) manufactured by Mitsui Chemours Fluoro Products Co., Ltd. Standard) PFA 334-JR (PFA concentration 60% by mass) FEP aqueous dispersion: Teflon (registered trademark) FEP D121 (FEP concentration 55% by mass) manufactured by Chemours Corporation pigment Brown pigment: AM-Brown 8800-S manufactured by Dainichi Seika Color & Chemicals Mfg. Co., Ltd. Black pigment: CHANNEL BLACK AQUOUS DISPERSION manufactured by Chemours White pigment: Chemours White Pigment Millbase

[0074] Specific examples are shown below, but the present invention is not limited to these. Example 1 189 g of pure water was placed in a 1 L stainless steel container, and 10 g of a nonionic surfactant aqueous solution (concentration: 90% by mass) was added while stirring at 200 rpm using a stirrer (YAMATO SCIENTIFIC CO., LTD.). 76 g of brown pigment was added and stirred for 10 minutes. Next, a fluororesin aqueous dispersion (270 g of PTFE aqueous dispersion, 68 g of PFA aqueous dispersion (1)) premixed in a separate container was added and stirred for 10 minutes. 188 g of water-soluble PAI was then added and stirred for 10 minutes to obtain an aqueous fluororesin coating composition.

[0075] Examples 2 to 15 An aqueous fluororesin coating composition was obtained in the same manner as in Example 1, adjusting the amount of each component so as to obtain the coating composition (composition ratio (mass %) in the resin solid content) shown in Table 1 below.

[0076] Comparative Examples 1 to 7 An aqueous fluororesin coating composition was obtained in the same manner as in Example 1, adjusting the amount of each component so as to obtain the coating composition (composition ratio (mass %) in the resin solid content) shown in Table 2 below.

[0077] Table 1 shows the composition ratios (mass %) in the resin solid content of the coating compositions of the Examples, and Table 2 shows the composition ratios (mass %) in the resin solid content of the coating compositions of the Comparative Examples.

[0078] [Table 1]

[0079] [Table 2]

[0080] The coating films used for performance evaluation were prepared by the following procedure. <Creating test specimens for evaluation> An aluminum plate (JIS A1050 compliant, 1 mm thick) measuring 50 mm in width and 100 mm in length was used as the substrate. After degreasing the surface with isopropyl alcohol, masking tape was applied parallel to the long side of the substrate from the edge to 30 mm from the edge, and a #60 alumina short circuit was formed. The surface was then subjected to hot blasting to a surface roughness (Ra) of 1 to 5 μm. Next, the fluororesin coating composition of each Example and Comparative Example was spray-coated (0.2 to 0.25 g of coating composition) using a spray gun (W-101-101G, manufactured by Anest Iwata Corp.) After peeling off the masking tape, the surface was dried at 120°C for 15 minutes to form a primer layer (fluororesin coating composition layer). The primer-coated substrate was electrostatically coated with PFA powder paint (Mitsui-Chemours Fluoroproducts Teflon® Paint MJ-102) (0.5-0.6 g) using a powder spray gun (Parker Ionics GX355HW) and baked at 380°C (substrate temperature) for 20 minutes. The same PFA powder paint was then electrostatically coated on the entire surface and baked at 330°C (substrate temperature) for 10 minutes to form a topcoat layer (PFA layer). The resulting topcoat layer (PFA layer) was 100-120 μm thick, and the edges of the specimens were free of primer.

[0081] The obtained test pieces were used to carry out the following performance evaluations. <Water vapor resistance evaluation> The above evaluation test pieces were left in steam at 170°C and 0.7 MPa for 300 hours, then allowed to cool to room temperature, and the adhesive strength of the coating film was measured by the following method. <Heat resistance evaluation> The above evaluation test pieces were left in a thermostatic chamber (Perfect Oven STPH-202M, manufactured by Espec Corporation) set to 240°C for 600 hours, then left to cool to room temperature, and the adhesive strength of the coating film was measured using the method described below.

[0082] <Adhesive strength measurement method> A 1 cm wide cut was made in the coating film with a cutter parallel to the short side of the test piece, and about 1 cm of the coating film at the end where the primer was not applied was peeled off to serve as a gripping area for measuring adhesive strength. Using a Tensilon universal testing machine (manufactured by A&D Corporation), the peel strength of adhesives (90-degree peel test method) was measured in accordance with the measurement method for adhesive peel strength specified in JIS K 6854. The peeled coating film was clamped in the chuck of the testing machine and pulled at a speed of 50 mm / min to measure the adhesive strength (peel strength). The unit is kgf (kilogram-force).

[0083] The results are shown in Tables 3 and 4.

[0084] [Table 3]

[0085] [Table 4]

[0086] All of the coating compositions of Examples 1 to 15 had excellent water vapor resistance as well as excellent heat resistance. In contrast, the coating compositions of Comparative Examples 1 to 7 did not have both excellent water vapor resistance and excellent heat resistance. For example, the coating composition of Comparative Example 2 exhibited heat resistance comparable to that of the coating compositions of the Examples, and the coating composition of Comparative Example 5 exhibited water vapor resistance comparable to that of the coating compositions of the Examples, but the coating composition of Comparative Example 2 had poor water vapor resistance, and the coating composition of Comparative Example 5 had poor heat resistance. All of the other Comparative Examples were poor in both water vapor resistance and heat resistance.

Claims

1. An aqueous fluororesin coating composition comprising a water-soluble polyamide-imide resin, a non-thermofusible polytetrafluoroethylene, and a thermofusible fluororesin, In the solid content of the aqueous fluororesin coating composition, The content of the water-soluble polyamide-imide resin is 10% by mass or more and 25% by mass or less, The content of the non-thermofusible polytetrafluoroethylene is 30% by mass or more and 72% by mass or less, The content of the melt processible fluororesin is 5% by mass or more and 40% by mass or less. A water-based fluororesin coating composition.

2. The content of non-thermofusible polytetrafluoroethylene is 40% by mass or more and 67% by mass or less, The content of the melt processible fluororesin is 10% by mass or more and 30% by mass or less. The aqueous fluororesin coating composition according to claim 1.

3. 3. The aqueous fluororesin coating composition according to claim 1, which does not contain any water-insoluble heat-resistant resin other than the non-thermofusible polytetrafluoroethylene and the thermofusible fluororesin.

4. A coating film comprising the aqueous fluororesin coating composition according to claim 1.

5. A coated article having the coating film according to claim 4.

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