AQUEOUS FLUORORESIN COATING COMPOSITION TECHNICAL FIELD

[0001] The present invention relates to an aqueous fluororesin coating composition which strongly adheres to various metal substrates, and can form a coating film with sufficient thickness having excellent water vapor resistance and corrosion resistance in a single coating, a coating film formed by applying the composition, and an article having the coating film.

FIELD

[0002] Fluororesins have excellent heat resistance, chemical resistance, electrical properties, and mechanical properties, in addition to having a very low coefficient of friction, non-tackiness, and water and oil repellency, leading to widespread use in all types of industrial fields such as chemistry, machinery, electrical devices, and the like.

[0003] In particular, utilizing the non-tackiness and water and oil repellency of fluororesins, fluororesin coatings are used in the coating of cookware such as frying pans and rice cookers, fixing rolls/belts for fixing toners of office equipment, and various other fields, with the fields of applicability having further spread in recent years to inkjet nozzles, chemical plant equipment, and the like.

[0004] However, when coating various substrates with fluororesins, coating the substrate directly with a fluororesin causes inferior adhesion and is very difficult due to non-tackiness, which is a property of fluororesins. Therefore, when performing fluororesin coating, primer coating compositions having adhesiveness to substrates and having adhesiveness to the fluororesin coating (topcoat coating) film that is applied thereto have been commonly used.

[0005] A heat-resistant resin (so-called engineering plastic) having adhesiveness to substrates and capable of withstanding high temperatures greater than or equal to the melting point of the fluororesin is used as an ingredient of such a primer coating composition. For example, Patent Document 1 discloses precursors such as polyimide, polyamide-imide, polyether sulfone, and the like and microparticles such as polyphenylene sulfide and the like. Such a heat-resistant resin is called a binder.

[0006] Meanwhile, an organic solvent (solvent-based coating) or water (aqueous coating) is used as the medium of the fluororesin coating composition containing the primer coating composition, with an aqueous (water-based) coating having been particularly preferably used in recent years from the perspective of environmental load and harm to the human body. In aqueous coating compositions, since heat-resistant resins (binders) which impart adhesiveness to substrates are ordinarily water-insoluble, the particles thereof are dispersed in a liquid of the coating composition. However, a water- soluble polyamide-imide can also be used at this time (Patent Document 2).

[0007] When a water-soluble polyamide-imide (water-soluble PAI) is used as a heat-resistant resin (binder), a high adhesive force can be achieved with a small amount because the substance dissolves uniformly in the aqueous fluororesin coating composition.

[0008] In addition, since water-soluble polyamide-imides have high viscosity, thickeners can be reduced or not used at all, thereby enhancing the purity of the coating film and making it possible to achieve more favorable performance. Further, using a water-soluble polyamide-imide is also advantageous in that a dispersion step or management of the degree of dispersion, which is necessary when a powder of various engineering plastics commonly used as a heat-resistant resin (binder) is used, becomes unnecessary, thereby yielding excellent productivity and also facilitating quality control.

[0009] Accordingly, it is desirable to use a water-soluble polyamide-imide as a heat-resistant resin (binder) that imparts adhesiveness with a substrate in an aqueous coating composition.

However, with a coating film obtained from a fluororesin composition using a conventional water-soluble polyamide-imide, the water vapor resistance and corrosion resistance are insufficient.

[0010] To date, a fluororesin coating composition using a water-soluble polyamide-imide together with a polyether sulfone resin has been proposed as an aqueous coating composition having excellent water vapor resistance and corrosion resistance (Patent Document 3).

[0011] Furthermore, an aqueous fluororesin coating composition has been proposed which contains a water-soluble polyamide-imide resin, a polyetherimide, and a fluororesin, wherein the amount of polyetherimide is 50 to 75 mass% of the total mass% of the water-soluble polyamide-imide resin and the polyetherimide (Patent Document 4).

[0012] Furthermore, an aqueous fluororesin coating composition has been proposed which comprises a water-soluble polyamide-imide resin, a polyether ether ketone, and a fluororesin, wherein the fluororesin is a perfluororesin (Patent Document 5).

[0013] On the other hand, the water-soluble polyamide-imide is uniformly dissolved in the aqueous fluororesin coating composition, and a high adhesive force can be obtained even with a small amount of composition. Therefore, the aqueous fluororesin coating composition using the water-soluble polyamide- imide can be expected to be used as a one-coat coating composition which is used by applying only a single coating. A one-coat coating has excellent cost and productivity because a primer coating and a topcoat coating are not used, and the number of times that coating is performed can be reduced.

[0014] Furthermore, the fluororesin coating film obtained from the one-coat coating can also improve the hardness and abrasion resistance of the coating film due to the heat-resistant resin used as the binder resin, as compared with a coating film containing a standard primer coating and topcoat coating.

These advantages make aqueous fluororesin coating compositions suitable for one-coat coatings usable in a variety of applications.

[0015] An aqueous fluororesin coating composition suitable for a one-coat coating is required to have, in addition to performance as a fluororesin coating material, thick coatability that enables forming a thick coating film having sufficient durability by a single coating, and applicability to various substrates. However, although the fluororesin coating compositions proposed thus far exhibit satisfactory performance as primer coatings, these compositions cannot exhibit sufficient performance as a one-coat coating in terms of thick coatability and versatility applicable to various substrates.

PRIOR ART DOCUMENTS

Patent Documents

[0016] Patent Document 1 : Japanese Examined Patent Publication H4- 71951

[0017] Patent Document 2: Japanese Patent No. 3491624

[0018] Patent Document 3: Japanese Patent No. 4534916

[0019] Patent Document 4: Japanese Patent No. 6722721

Patent Document 5: Japanese Unexamined Patent Application 2021-91754

SUMMARY OF THE INVENTION

[0020] An object of the present invention is to provide: an aqueous fluororesin coating composition which strongly adheres to various metal substrates, and can form a coating film with sufficient thickness having excellent water vapor resistance and corrosion resistance in a single coating; a coating film formed by applying the composition; and an article having the coating film.

[0021] In order to achieve the aforementioned object, the aqueous fluororesin coating composition of the present invention contains a water- soluble polyamide-imide resin, an aromatic polyether ketone, a fluororesin, and a filler, wherein the fluororesin is a perfluororesin, and the proportion of the binder resin is 35 to 55 mass% based on the total of the amount (resin solid fraction) of binder resin containing the water-soluble polyamide-imide resin, the aromatic polyether ketone, and other binder resins, and the amount of the fluororesin.

[0022] That is, the present invention is as follows.

(1 ) An aqueous fluororesin coating composition, comprising a water-soluble polyamide-imide resin, an aromatic polyether ketone, a fluororesin, and a filler, wherein the fluororesin is a perfluororesin, and the proportion of the binder resin is 35 to 55 mass% based on the total of the amount (resin solid fraction) of binder resin containing the water-soluble polyamide-imide resin, the aromatic polyether ketone, and other binder resins, and the amount of the fluororesin.

(2) The aqueous fluororesin coating composition of (1 ), wherein the filler is a squamous filler.

(3) The aqueous fluororesin coating composition according to (1 ) or (2), wherein the filler is mica.

(4) The aqueous fluororesin coating composition according to (1) to (3), wherein the proportion of water-soluble polyamide-imide resin is 15 to 50 mass% based on the total of the amount (resin solid fraction) of binder resin containing the water-soluble polyamide-imide resin, the aromatic polyether ketone, and other binder resins, and the amount of the fluororesin.

(5) The aqueous fluororesin coating composition according to any one of (1 ) to (4), wherein the fluororesin includes a non-meltprocessable polytetrafluoroethylene.

(6) The aqueous fluororesin coating composition according to (5), wherein the proportion of non-meltprocessable polytetrafluoroethylene is 35 mass% or higher based on the total of the amount (resin solid fraction) of binder resin containing the water-soluble polyamide-imide resin, the aromatic polyether ketone, and other binder resins, and the amount of the fluororesin.

(7) A coating film of the aqueous fluororesin coating composition of (1 ) to (6).

(8) The coating film according to (7), which is a one-coat coating film having a thickness of 40 pm or more.

(9) An article having a coating film of the aqueous fluororesin coating composition of (1) to (6).

DETAILED DESCRIPTION

[0023] The present invention can provide: an aqueous fluororesin coating composition which strongly adheres to various metal substrates, and can form a coating film with sufficient thickness having excellent water vapor resistance and corrosion resistance in a single coating; a coating film formed by applying the composition; and an article having the coating film.

[0024] The present invention will be described in detail hereinafter. 1 . Aqueous fluororesin coating composition

[0025] The aqueous fluororesin coating composition of the present invention contains a water-soluble polyamide-imide resin, an aromatic polyether ketone, a fluororesin, and a filler.

Aqueous fluororesin coating composition

[0026] The “aqueous fluororesin coating composition” of the present invention is an aqueous (water-based) dispersion containing a water-soluble polyamide-imide resin, an aromatic polyether ketone, a fluororesin, and a filler, wherein the fluororesin is a perfluororesin, and the proportion of the binder resin is 35 to 55 mass% based on the total of the amount (resin solid fraction) of binder resin including the water-soluble polyamide-imide resin, the aromatic polyether ketone, and other binder resins, and the amount of the fluororesin. The aqueous fluororesin coating composition of the present invention can form a sufficiently thick fluororesin coating film with excellent water vapor resistance and corrosion resistance on various substrates in a single coating, and therefore demonstrates particularly excellent functionality as a one-coat coating.

Water-soluble polyamide-imide resin (water-soluble PAI)

[0027] 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, preferably having repeating units expressed by the following general formula: Formula 1

(wherein, R ¹ is a trivalent organic group, while R ² is a divalent organic group). [0028] The water-soluble PAI used in the present invention is obtained by copolymerizing a diisocyanate compound or a diamine compound as an amine component and a tribasic acid anhydride or a tribasic acid halide as an acid component in a polar solvent. While the synthesis conditions of the water- soluble PAI are varied and not particularly limited, synthesis is ordinarily performed at a temperature of 80 to 180°C, and in order to reduce the effects of moisture in the air, synthesis is preferably performed in an atmosphere of nitrogen or the like.

[0029] While the diisocyanate compound is not particularly limited, an example thereof is a diisocyanate compound expressed by Formula (1 ) below. In Formula (1 ), X is a divalent organic group.

Formula 2

OCN-X-NCO (1)

[0030] Examples of the divalent organic group represented by X include: alkylene groups having 1 to 20 carbon atoms; arylene groups such as phenylene groups or naphthylene groups which are unsubstituted or substituted with lower alkyl groups having 1 to 5 carbon atoms such as methyl groups, or lower alkoxy groups having 1 to 5 carbon atoms such as methoxy groups; divalent organic groups formed by bonding two of the arylene groups described above 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-); divalent organic groups formed by bonding two lower alkylene groups having 1 to 5 carbon atoms via the arylene groups described above; and the like. 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.

[0031] From the perspective of enhancing reactivity, the adhesive strength of the coating film, and the like, the divalent organic group represented by X is preferably a divalent organic group formed by bonding two of the arylene groups described above 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 bonding two of the arylene groups described above 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 bonding two phenylene groups via a single bond or a lower alkylene group having 1 to 5 carbon atoms. Even when two or more types of diisocyanate compounds are used in combination, it is preferable to use two or more types selected from among these preferable modes. In addition, from the perspective of reactivity, the arylene group is preferably unsubstituted, while from the perspective of enhancing the adhesive strength of the coating film, the arylene group is preferably substituted with a lower alkyl group having 1 to 5 carbon atoms such as a methyl group or the like or a lower alkoxy group having 1 to 5 carbon atoms such as a methoxy group or the like.

[0032] 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, isophorone diisocyanate, and the like.

[0033] While not particularly limited thereto, exemplary diamine compounds include a compound in which an isocyanate group is substituted with an amino group in Formula (1) above. Specific examples of diamine compounds include xylene diamine, phenylene diamine, 4,4’- diaminodiphenylmethane, 4, 4’ -diaminodiphenyl ether, 4,4’-diaminodiphenyl sulfone, 3, 3’ -diaminodiphenyl sulfone, 3,3’-dimethylbiphenyl-4,4’-diamine, isophorone diamine, and the like.

[0034] The use of 3,3'-dimethylbiphenyl-4,4'-diisocyanate and/or 3,3'- dimethylbiphenyl-4,4'-diamine as the amine component (diisocyanate compound, diamine compound) is preferable because the substrate adhesive strength and water vapor resistance of the coating film can be improved. Furthermore, 3,3’-dimethylbiphenyl-4,4’-diisocyanate is preferably used from the perspective of enhancing the working environment (International Patent Publication WO2016/175099). [0035] In the reaction, 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 perspective of facilitating the reaction, a diisocyanate compound is preferably used.

[0036] An example of a tribasic acid anhydride is a tricarboxylic acid anhydride. Although not particularly limited, the compound is preferably an aromatic tribasic acid anhydride, more preferably an aromatic tricarboxylic acid anhydride, and even more preferably a compound expressed by Formula (2) or Formula (3) below. From the perspective of heat resistance, cost, and the like, a trimellitic acid anhydride is particularly preferable.

Formula 3

(R is a hydrogen atom, an alkyl group having 1 to 10 carbons, or a phenyl group, while Y is -CH2-, -CO-, -SO2-, or -O-.)

[0037] A tribasic acid anhydride halide is preferably used as a tribasic acid halide, with an example thereof being a tricarboxylic acid anhydride halide. The tribasic acid anhydride halide is preferably a tribasic acid anhydride chloride. Although not particularly limited, the compound is preferably an aromatic tribasic acid anhydride chloride, more preferably an aromatic tricarboxylic acid anhydride chloride, and even more preferably a compound in which the - COOR group in Formula (2) or Formula (3) above is replaced by a -COCI group. From the perspective of heat resistance, cost, and the like, a trimellitic acid anhydride chloride (anhydrous trimellitic acid chloride) is particularly preferable.

[0038] From the perspective of reducing the environmental load, a tricarboxylic acid anhydride is preferably used, with a trimellitic acid anhydride particularly preferable.

[0039] In addition to the tribasic acid anhydride and the tribasic acid halide, a polybasic acid or polybasic acid anhydride such as a dicarboxylic acid and tetracarboxylic acid dianhydride can be used as an acid component to improve hydrophilicity so long as properties such as the heat resistance of the water- soluble PAI and the like are not impaired.

[0040] The dicarboxylic acid is not particularly limited, but examples thereof may include terephthalic acid, isophthalic acid, adipic acid, sebacic acid, and the like. The tetracarboxylic acid dianhydride is not particularly limited, but examples thereof may include pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, biphenyl tetracarboxylic dianhydride, and the like. One type of each of a polybasic acid and a polybasic acid anhydride may be used alone, or two or more types may be used in combination.

[0041] From the perspective of maintaining the properties such as the heat resistance of the water-soluble PAI and the like, the amount of the polybasic acid and polybasic acid anhydride (for example, dicarboxylic acid and tetracarboxylic dianhydride) other than tribasic acid anhydride and tribasic acid halide that are used is preferably 0 to 50 mol%, more preferably 0 to 30 mol%, and even more preferably 0 to 15 mol% of all acid components.

[0042] From the perspective of the molecular weight and degree of crosslinking of the water-soluble PAI that is produced, the usage ratio of the diisocyanate and/or diamine compound to the acid component (tribasic acid anhydride and/or tribasic acid halide and dicarboxylic acid and/or tetracarboxylic dianhydride or the like used as necessary) is preferably that the total amount of the diisocyanate compound and/or diamine compound is 0.8 to 1.1 mols, more preferably 0.95 to 1.08 mols, and even more preferably 1.0 to 1 .08 mols, per the total amount of 1 .0 mol of the acid components.

[0043] A PAI obtained by reacting a diisocyanate compound and/or a diamine compound with an acid component can be used directly as the water- soluble PAI. It can also be used after being protected with a blocking agent.

When a diisocyanate compound is used as a raw material compound, a terminal isocyanate group blocking agent (terminal blocking agent) may be optionally used for the purpose of stabilizing the PAI. By protecting the PAI with a blocking agent, the water-soluble PAI becomes a compound that has no isocyanate groups (-NCO groups) or has a reduced amount of isocyanate groups (-NCO groups) in comparison to a PAI obtained by reacting an isocyanate compound with an acid component.

[0044] Alcohol is an example of a blocking agent, with examples of alcohols including lower alcohols having 1 to 6 carbon atoms such as methanol, ethanol, propanol, and the like. Examples of blocking agents include 2-butanone oxime, 5-valerolactam, and £-caprolactam, and the like. The blocking agent is not limited to these exemplary compounds. One type of blocking agent may be used alone, or two or more types may be used in combination.

[0045] As a polar solvent used in polymerization, N-methyl-2-pyrrolidone (NMP), N-ethylmoropholine, N-formylmorpholine, N-acetylmorpholine, N,N’- dimethylethylene urea, N,N-dimethylacetamide or N,N-dimethylformamide, y- butyrolactone, and the like can be used. Although NMP has preferably been used until now due to the availability and high boiling point thereof, it is preferable to use N-ethylmorpholine or N-formylmorpholine from the perspective of the effects on the human body, REACH regulations, legal regulations of the US FDA, or the like.

[0046] While the amount of solvent used is not particularly limited, it is preferably 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 perspective of the solubility of the resin obtained. [0047] From the perspective of ensuring the strength of the coating film, the number average molecular weight of the water-soluble 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. In addition, from the perspective 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.

[0048] The number average molecular weight of the water-soluble PAI can be managed by sampling PAI at the time of synthesis, measuring the number average molecular weight, and continuing synthesis until the target number average molecular weight is obtained. The number average molecular weight can be measured by gel permeation chromatography (GPC) using a standard polystyrene calibration curve.

[0049] The acid value of the water-soluble PAI combining the carboxyl groups in the resin and carboxyl groups with ring-opened acid anhydride groups is preferably 10 mg KOH/g or more. The acid value is more preferably 25 mg KOH/g or more, and even more preferably 35 mg KOH/g or more. These ranges are preferable ranges from the perspective of facilitating dissolution or dispersion of the water-soluble PAI. In addition, when the basic compound described later is included, the amount of carboxyl groups reacting with the basic compound is sufficient, in addition to also being a preferable range in that water solubilization becomes easy.

[0050] Furthermore, from the perspective of preventing gelation over time, the acid value of the fluororesin coating composition that is ultimately obtained is preferably 80 mg KOH/g or less. The acid value is more preferably 60 mg KOH/g or less, and even more preferably 50 mg KOH/g or less.

[0051] The acid value can be obtained using the following method. First, 0.5 g of water-soluble PAI is collected and 0.15 g of 1 ,4- diazobicyclo[2,2,2]octane is added thereto. In addition, 60 g of N-methyl-2- pyrrolidone and 1 mL of ion-exchanged water are added and stirred until the PAI is fully dissolved to prepare a solution for evaluation. The solution for evaluation is titrated with a 0.05 mol/L potassium hydroxide ethanol solution by potentiometric titration to obtain an acid value. The acid value is an acid value combining the carboxyl groups in the resin and carboxyl groups with ring- opened acid anhydride groups.

[0052] Further, a basic compound may also be reacted to increase the solubility of the PAI in water. The basic compound reacts with the carboxyl groups contained in the PAI to form a salt of the basic compound and the PAI. The action of the basic compound can increase the solubility of the PAI in water.

[0053] In the present invention, examples of basic compounds include: alkylamines such as triethylamine, tributylamine, N,N- dimethylcyclohexylamine, N,N-dimethylbenzylamine, triethylene diamine, N- methylmorpholine, N,N,N’N’-tetramethylethylene diamine, N,N,N’N”,N”- pentamethyldiethylene triamine, N,N’,N’-trimethylaminoethylpiperadine, 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; ammonia; and the like. From the perspective of increasing the solubility of the PAI in water, alkylamines and/or alkanolamines are suitable.

[0054] From the perspective of facilitating the water solubilization of the PAI and enhancing the strength of the coating film, the basic compound is preferably used in an amount of 2.5 equivalents or more, more preferably 3.5 equivalents or more, and even more preferably 4 equivalents or more, with respect to the carboxyl groups and ring-opened acid anhydride groups contained in the resin. In addition, from the perspective of maintaining strength, the amount of the basic compound used is preferably 10 equivalents or less, more preferably 8 equivalents or less, and even more preferably 6 equivalents or less.

[0055] Specific water-soluble PAIs and manufacturing methods thereof are described in Patent Document 3, International Patent Publication WO2016/175099, Japanese Unexamined Patent Application 2016-89016, Japanese Unexamined Patent Application 2016-17084, Japanese Unexamined Patent Application 2018-2802, and the like.

[0056] The water-soluble PAI used in the present invention is ordinarily used as a solution in the preparation of 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.

[0057] The organic solvent is not particularly limited as long as the solvent has high polarity and a high boiling point, with various polar solvents capable of being used for the polymerization of PAI available. As in the case of the solvent used in polymerization, although NMP has been preferably used until now due to the availability and high boiling point thereof, it is preferable to use N-ethylmorpholine, N-formylmorpholine, 3-methoxy-N,N-dimethyl propanamide, or N-ethyl-2-pyrrolidone from the perspective of the effects on the human body, REACH regulations, legal regulations of the US FDA, or the like.

[0058] The organic solvent described above may be the same as a solvent that may be contained in the aqueous medium described later in the fluororesin coating composition of the present invention.

[0059] The water-soluble PAI preferably has a concentration of 1 to 50 mass% and more preferably 5 to 40 mass% of the water-soluble PAI solution in terms of viscosity.

[0060] Examples of commercially available products of such a water- soluble PAI solution include HPC-1000-28 and HPC-2100D-28 manufactured by Hitachi Chemical Co., Ltd., and HPC-2100D-28 is preferable.

Aromatic polyether ketone

[0061] The aromatic polyether ketone used in the present invention is a polymer that is a crystalline thermoplastic resin having a linear polymer structure in which benzene rings are bonded by ether and ketone groups.

[0062] Examples of the aromatic polyether ketone include polyether ketone (PEK), polyether ether ketone (PEEK), polyether ketone ketone (PEKK), polyether ether ketone ketone (PEEKK), polyether ketone ester, and the like. The aromatic polyether ketone may be used alone or two or more may be used in combination.

[0063] The aromatic polyether ketone is preferably at least one selected from the group consisting of PEK, PEEK, PEKK, PEEKK, and polyether ketone esters, but PEEK is more preferable.

[0064] The polyether ether ketone (PEEK) used in the present invention is a polymeric compound having at least the following repeating units. Either a homopolymer or a copolymer thereof may be used.

Formula 4

[0065] Polyether ether ketone (PEEK) is typically manufactured by reacting 4,4'-difluorobenzophenone and hydroquinone in diphenylsulfone in the presence of alkali metal carbonate (for example, potassium carbonate and/or sodium carbonate).

Examples of commercially available products of polyether ether ketone (PEEK) used in the present invention include VICOTE (registered trademark) manufactured by VICTREX and the like.

Fluororesin

[0066] In the present invention, a perfluororesin is used as the fluororesin. The perfluororesin refers to a fluororesin in which all of the hydrogen atoms in the molecular chain are substituted with fluorine atoms, and specific examples thereof include polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene/hexafluoropropylene/perfluoro(alkyl vinyl ether) copolymer, and the like. [0067] With the present invention, when PFA is used, the perfluoro(alkyl vinyl ether) alkyl groups in the PFA preferably have 1 to 5 carbon atoms, and more preferably 1 to 3 carbon atoms. Herein, the amount of perfluoro(alkyl vinyl ether) in the PFA is preferably in a range of 1 to 50 mass%.

[0068] With the present invention, a non-thermomeltable polytetrafluoroethylene (PTFE) is preferably used as the perfluororesin. The non-thermomeltable polytetrafluoroethylene is a high-molecular weight polytetrafluoroethylene (PTFE) that does not exhibit melt fluidity at the melting point or higher, and may be either a homopolymer of tetrafluoroethylene (TFE) (homopolymer of TFE) or a TFE copolymer in which a monomer that is copolymerizable with TFE is contained in a range of 1 mass% or less (modified PTFE), or both may be used in combination.

[0069] In the present invention, thermally meltable perfluororesins may be used together with the non-thermomeltable polytetrafluoroethylene (PTFE). Examples of the thermomeltable perfluororesins include, for example, low molecular weight thermomeltable polytetrafluoroethylene (thermomeltable PTFE), tetrafluoroethylene perfluoro (alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene hexafluoropropylene copolymer (FEP), tetrafluoroethylene/hexafluoropropylene/perfluoro (alkyl vinyl ether) copolymer, and the like, which can be manufactured by solution polymerization, emulsion polymerization, suspension polymerization, and other conventionally known methods.

Furthermore, other fluororesins may be added as necessary.

[0070] The fluororesin of the present invention can be used by dispersing a powder obtained by separating and drying a resin obtained by a known polymerization method, a powder obtained by further pulverizing the aforementioned powder, or a powder that has been refined and granulated by the method described in Japanese Examined Patent Publication No. S52- 44576 or the like in the coating composition. Further, a fluororesin dispersion (dispersion) polymerized by emulsion polymerization can be used directly, or a fluororesin dispersion stabilized by adding a surfactant or adjusted to a high fluororesin concentration by concentrating with a known technique such as the method described in US Patent No. 3,037,953 and the like can also be used. A stabilized fluororesin dispersion is preferable in that the dispersed state can be maintained over a long period of time without the aggregation or precipitation of the fluororesin.

[0071] In the fluororesin coating composition of the present invention, the fluororesin is dispersed as particles in an aqueous medium. The fluororesin described above preferably consists of particles having an average particle diameter of 0.01 to 50 pm. When the average particle size is less than 0.01 pm, the dispersibility of the particles is inferior and there is a risk that the resulting coating composition may have inferior mechanical stability and storage stability. When the average particle size is greater than 50 pm, the particles lack uniform dispersibility and when applied using the obtained coating composition, a coating film with a smooth surface may not be obtained and the coating film physical properties may be inferior. A more preferable upper limit is 5 pm, with an even more preferable upper limit of 0.5 pm, and a more preferable lower limit is 0.05 pm. The mechanical stability described above is a property such that a non-redispersible aggregate is difficult to produce even when subjected to strong stirring or a shearing force with a homogenizer or the like at the time of feeding and redispersion.

[0072] The concentration of the fluororesin dispersion used in the coating composition of the present invention is preferably 20 to 70 mass%, with the use of a composition adjusted to a 40 to 70 mass% by concentration preferable in that it becomes easy to adjust the fluororesin concentration in the coating composition. Examples of commercially available products of the fluororesin dispersion used in the present invention include Teflon (registered trademark) PTFE 31 -JR, PTFE 34-JR, PFA 334-JR, PFA 335-JR, and FEP 120-JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.

[0073] In the present invention, "resin solid fraction" refers to the weight of the fluororesin and the binder resin in the coating film obtained from the fluororesin coating composition. Specifically, it refers to the total mass of binder resins (water-soluble PAI, aromatic polyether ketone, and other binder resins) and fluororesins in the residue after the fluororesin coating composition of the present invention is applied on the object to be coated, dried at a temperature of 80 to 100°C or lower, and baked at approximately 380°C for 45 minutes.

[0074] The aqueous fluororesin coating composition of the present invention can be suitably used as a one-coat coating composition. A one-coat coating is required to have strong adhesion to a substrate, durability and abrasion resistance of a coating film to achieve long-term use and a sufficient film thickness, and the film thickness is generally at least 20 pm or more, preferably 30 pm or more, but when considering variation in film thickness, a film thickness of 40 pm or more, which is thicker than the above range, is particularly desired because it can be formed without defects such as cracks and the like.

[0075] In the aqueous fluororesin coating composition of the present invention, by setting the ratio of the binder resin in the resin solid fraction to 35 to 55 mass%, an excellent one-coat coating composition can be provided, capable of forming a film thickness of 40 pm or more without defects such as cracks and the like.

[0076] In the aqueous fluororesin coating composition of the present invention, the proportion of the water-soluble polyamide-imide of the resin solid fraction (fluororesin and binder resin) is preferably in a range of 15 to 50 mass%, more preferably in the range of 15 to 40 mass%, and particularly preferably in the range of 15 to 30 mass%.

[0077] In the aqueous fluororesin coating composition of the present invention, the aromatic polyether ketone is considered to contribute to the adhesive strength of the coating film to the substrate, film formation, and hydrolysis resistance performance, and thus a ratio of 5 mass% or more of aromatic polyether ketone is preferably maintained in the resin solid fraction (fluororesin and binder resin).

[0078] As described above, the use of non-thermomeltable polytetrafluoroethylene (PTFE) as the perfluororesin in the aqueous fluororesin coating composition of the present invention is preferred, particularly from the perspective of water vapor resistance and corrosion resistance of the coating film, and the ratio of non-thermomeltable polytetrafluoroethylene (PTFE) to resin solid fraction (fluororesin and binder resin) is preferably 35 mass% or more, more preferably 40 mass% or more, even more preferably 45 mass% or more, and particularly preferably 50 mass% or more.

Filler

[0079] The aqueous fluororesin coating composition of the present invention contains a filler. Various organic and inorganic materials can be selected as fillers in the present invention, depending on the desired properties. Examples of organic substances include engineering plastics such as polyphenylene sulfides, polyether ether ketones, polyether sulfones, polyphenyl sulfones, polyamides, polyimides, phenolic resins, urea resins, epoxy resins, urethane resins, melamine resins, polyester resins, polyether resins, acrylic resins, acrylic silicone resins, silicone resins, silicone polyester resins, and the like. Examples of inorganic substances include metal powders, metal oxides (aluminum oxide, zinc oxide, tin oxide, titanium oxide, and the like), glass, ceramics, silicon carbide, silicon oxide, calcium fluorides, carbon black, graphite, mica, barium sulfate, and the like. Various shapes of materials can be used in the aqueous fluororesin coating composition of the present invention, including particle shape, fiber shape, and squamous (flake) shape, and the like, but of these, squamous fillers can be preferably used because the occurrence of cracking can be efficiently suppressed even by a relatively small amount. Examples of the squamous filler include mica, graphite, metal flakes, and the like.

[0080] Insulative squamous fillers such as mica are preferred as the squamous filler from the perspective of further improving the corrosion resistance of the coated metal substrate, but on the other hand, graphite, metal flakes, and the like are preferred for being electrically conductive and capable of producing electrically conductive coating films, and selection based on application is possible.

[0081] In the aqueous fluororesin coating composition of the present invention, the average particle diameter of the filler can be appropriately selected depending on the substance, shape, and required properties. For example, the average particle diameter of the squamous filler is preferably 1 to 100 m, more preferably 5 to 50 pm. The average particle size of the filler is determined as the particle diameter (d50) at 50% integration (volume basis) by laser diffraction.

Other components

[0082] The aqueous fluororesin coating composition of the present invention may also contain various additives used in ordinary coatings in accordance with the required properties such as dispersibility, conductivity, foaming prevention, enhanced wear resistance, and the like, examples of which include: surfactants (for example, polyoxyethylene alkyl ether or polyoxyethylene alkyl phenyl ether-based nonionic surfactants such as Liocol manufactured by Lion, Inc., the TRITON and TERGITOL series manufactured by the Dow Chemical Company, and Emalgen manufactured by KAO, Inc.; sulfocuccinate-based, alkyl ether sulfonic acid sodium salt-based, or sulfate mono-long-chain alkyl-based anionic surfactants such as Repal manufactured by Lion, Inc. and Emal and Pelex manufactured by KAO, Inc.; polycarboxylate or acrylate-based polymer surfactants such as Leoal manufactured by Lion, Inc. or OROTAN manufactured by the Dow Chemical Company; and L-77 manufactured by Momentive, and the Surfynol Series manufactured by AirProduct (Surfynol 420, Surfynol 440, Surfynol 465, Surfynol 485, and the like)); film forming agents (for example, polymeric film forming agents such as polyamides, polyamide-imides, acrylics, and acetates; higher alcohols or ethers; and polymeric surfactants having a film-forming effect); and thickeners (for example, water-soluble celluloses, solvent dispersion thickeners, sodium alginates, caseins, sodium caseinates, xanthan gums, polyacrylic acids, and acrylic esters), and the like.

Aqueous medium

[0083] The aqueous fluororesin coating composition of the present invention contains water as the main medium. However, although not preferable from the perspective of the environment or costs, it is also possible to add a polar solvent that is compatible with water or to disperse an organic solvent that is incompatible with water in order to appropriately adjust the rheology properties such as the liquid viscosity of the aqueous fluororesin coating composition on to enhance the dispersibility of the aromatic polyether ketone, the fillers, and the like. In addition, by adding a polar solvent, the heat- resistant resin (binder) is dissolved and becomes more uniform in the drying process after coating. As a result of the increased density of the coating film or the fact that the heat-resistant resin (binder) becomes more likely to penetrate the indented portions of the recesses and protrusions of the substrate, an effect of enhancing the adhesive force with the substrate can be expected.

Stainless steel (SUS)

[0084] Stainless steel (SUS) is an alloy manufactured by adding chromium, nickel, or the like to iron, and is broadly categorized into austenitic stainless steel, martensitic stainless steel, ferritic stainless steel, and austenitic/ferritic stainless steel. There is a wide variety of stainless steel depending on the components of the alloy, with representative examples of stainless steels prescribed by the JIS standards including SUS304, SUS303, SUS316, SUS410, SUS430, SUS630, and the like.

Manufacturing process for aqueous fluororesin coating composition

[0085] The aqueous fluororesin coating composition of the present invention can be prepared by conventionally known methods or the like. For example, the composition can be obtained by appropriately mixing the aromatic polyether ketone, fluororesin, filler, and other additives that are blended as necessary with the water-soluble PAI solution described above dissolved in water containing an organic solvent. In the aqueous fluororesin coating composition of the present invention, the aromatic polyether ketone, fluororesin, filler, and the like may be prepared beforehand by preparing a dispersion (dispersion solution) thereof, and mixing the obtained dispersion.

[0086] The aqueous fluororesin coating composition of the present invention preferably has a viscosity of 0.1 to 50,000 mPa s at 25°C. When the viscosity is less than 0.1 mPa s, dripping or the like may easily occur when applied to an object to be coated, potentially making it difficult to obtain the target film thickness. When the viscosity exceeds 50,000 mPa s, the coating workability may be diminished and the film thickness of the resulting coating film may not be uniform, potentially diminishing the surface smoothness or the like. A more preferable lower limit is 1 mPa s, and an even more preferable upper limit is 30,000 mPa-s. The viscosity described above is a value obtained by taking a measurement using a BM-type single-cylinder rotary viscometer (manufactured by Tokyo Keiki Co., Ltd.).

2. Coating film

[0087] The “coating film” of the present invention is a coating film obtained by applying the aqueous fluororesin coating composition of the present invention. The aqueous fluororesin coating composition of the present invention demonstrates excellent performance as a one-coat coating, and the "coating film" of the present invention naturally includes coating films achieved by one-coat coating, and also includes laminated coating films made by applying the aqueous fluororesin coating composition of the present invention as a primer layer that adheres to the substrate, and then coating a plurality of layers thereon.

[0088] The “coating film” of the present invention can be formed by various typically used coating methods such as spray coating, dip coating, spin coating, and the like, for example, and is preferably heated to at least the melting point of the fluororesin in order to achieve melt-fluidity and obtain a uniform coating film.

[0089] As described above, one-coat coating films generally have a film thickness of at least 20 pm, preferably 30 pm or more, but a film thickness of 40 pm or more is particularly desirable to obtain sufficient performance over a long period of time.

3. Coated article

[0090] The “coated article” of the present invention is an article having a coating film obtained by applying the aqueous fluororesin coating composition of the present invention.

[0091] Examples of the “coated article” of the present invention can include articles that require non-tackiness and water and oil repellency, for example, cookware such as frying pans, rice cookers, and the like; heat-resistant release trays in factory lines or the like (such as a bread-baking processes and the like); office equipment-related articles such as fixing rollers/belts/inkjet nozzles and the like; industrial equipment-related products at chemical plants such as various sliding members, piping, and the like; and various other articles.

EXAMPLES

[0092] The following examples describe the preparation and performance evaluation of the aqueous fluororesin coating composition of the present invention, but the invention is not limited in any way to these examples.

[0093] The following reagents were used in the examples and comparative examples.

Water-soluble polyamide-imide (PAI) resin

[0094] HPC-2100D-28 manufactured by Hitachi Chemical Co., Ltd.

(Solution with a PAI concentration of approximately 28 mass%, a water concentration of 22 to 32 mass%, and an N-formylmorpholine concentration of 30 to 40 mass%)

Aromatic polyether ketone

[0095] PEEK powder : VICOTE (registered trade name) Coatings 704 manufactured by VICTREX

Other binder resins

Polyether sulfone (PES) resin

PES Powder: SUMIKAEXCEL PES 4100MP manufactured by Sumitomo Chemical Co., Ltd.

Fluororesin

[0096] PTFE aqueous dispersion: Teflon (registered trademark) PTFE 34- JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (PTFE concentration: 58 mass%)

FEP Aqueous dispersion: Teflon (registered trademark) FEP 120-JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (FEP concentration: 54 mass%)

Filler

[0097] Mica : Iriodin 123 manufactured by Merck Performance Materials LLC

Carbon black: Carbon black aqueous dispersion (concentration 25 mass%) Example 1

[0098] 117 g of pure water was placed in a 1 L stainless steel container, and 20 g of a nonionic surfactant aqueous solution (concentration 81 mass%) was added while stirring at 140 rpm using a stirrer (made by Yamato Scientific Co. LTD.). Powdered 72 g of PEEK powder was added to the surfactant dispersion and dispersed by stirring for 10 minutes. Furthermore, 50 g of a carbon black aqueous dispersion (25 mass%) was added and stirred for 10 minutes, and then 342 g of PTFE aqueous dispersion and 10 g of mica were added sequentially and stirred for 10 minutes. Next, 210 g of water-soluble PAI was added and further stirred for 10 minutes to obtain an aqueous fluororesin coating composition.

Examples 2 to 7

[0099] The amounts of each component were adjusted to achieve the coating compositions (mass%) listed in Table 1 below, and with the same procedures as in Example 1 (for example, in Example 2, 24 g of PEEK powder and 48 g of PES powder were used instead of 72 g of PEEK powder, and 128 g of FEP aqueous dispersion solution and 224 g of PTFE aqueous dispersion solution were used instead of 342 g of PTFE aqueous dispersion) an aqueous fluororesin coating composition was obtained.

Comparative Examples 1 to 4

[0100] Fluororesin coating compositions were obtained using the same procedure as in Example 1 while adjusting the amount of each component so as to obtain the coating compositions (composition ratio in the resin solid content (mass%)) shown in Comparative Examples 1 to 3 of Table 1. Furthermore, as Comparative Example 4, a commercially available solventbased PTFE one-coat coating material (Polyflon (registered trademark) PTFE tough coat enamel TC-7809BK) was used.

[0101] A coating film for use in performance evaluation was produced using the following procedure.

Preparation of test piece for substrate adhesive strength

[0102] First, a 95 mm x 50 mm piece of aluminum (JIS A1050 compliant product, thickness: 1 mm) was used as a substrate, wiped with isopropyl alcohol, and then subjected to shot blasting with #60 alumina to obtain a surface roughness (Ra) of from 1 to 5 m. Subsequently, the fluororesin coating compositions of each of the examples and the comparative examples were spray-coated (coating composition 0.5 to 0.7 g) using a spray gun (W- 101 -101 G, manufactured by Anest Iwata Inc.) and baked for 20 minutes at 120°C and then for 20 minutes at 390°C to form a coating film.

[0103] A test piece for evaluation was similarly produced using stainless steel (JIS SUS304, thickness: 1 mm) and steel (JIS SS400, thickness 2 mm) instead of aluminum.

Performance evaluation method

[0104] With respect to the obtained test pieces (aluminum substrate, stainless steel substrate and iron substrate), the substrate adhesive strength of the coating film was evaluated in accordance with JIS K5600-5-6 cross-cut method (initial value). However, the number of cuts was set to 11 , and the number of pieces which did not release from the 100 squares was evaluated. The test pieces were left in 0.4 MPa of water vapor at 150°C for 100 hours and then allowed to sit and cool until reaching ambient temperature. The condition of the coating film was observed, and the substrate adhesive strength of the coating film was measured using the same method (cross-cut method) as described above. Test pieces were subjected to a steam pressure treatment 2 times by performing steam exposure for 100 hours each to make 200 hours, and then the same observations and evaluation of substrate adhesive strength of the coating film by the cross-cut method were performed.

The number of squares which did not release after repeating the steam pressure treatment 2 times is shown in Table 1. (A level where measurement was not performed was described as "Not measured".)

Thick coatability evaluation

[0105] Each of the coating compositions of the Examples and Comparative Examples was applied to a 95 mm x 250 mm aluminum substrate (product conforming to JIS A1050, 1 mm thick) in a gradation manner in the long-side direction by the same method as in the case of the test piece for evaluation of substrate adhesive strength to prepare a test piece having a film thickness of approximately 10 to 50 pm after baking. These were observed at a magnification of 35 times using a digital microscope (KH-1300 manufactured by HIROX Co., Ltd.) to confirm a boundary between portions with favorable coating film and portions where a defect such as cracking or the like occurred. The film thickness on the favorable coating film side of the boundary was measured using a Kett Overcurrent Film Thickness Meter LZ370, and this was determined to be the maximum film thickness that could be applied without failure.

[0106] The results are shown in Table 1. Herein, for example in Example 1 , "> 49" indicates that the maximum film thickness in the prepared test piece was 49 pm and defects such as cracks and the like did not occur in any portion, including this portion. In Comparative Example 2, "<20" indicates that the minimum film thickness in the prepared test piece was 20 pm, and cracks occurred even in the thinnest film portion.

Table 1 Table 1 (continued)

[0107] It was confirmed that the aqueous fluororesin coating composition of the present invention exhibited extremely excellent adhesion to all of aluminum, stainless steel and steel, but the commercially available solventbased PTFE one-coat coating composition could hardly adhere to stainless steel.

[0108] The aqueous fluororesin coating compositions of the present invention of Examples 1 to 7 could be applied to a thickness of 40 pm or more without generating cracks. However, Comparative Example 1 in which the binder ratio in the resin component was higher, Comparative Example 2 in which the binder ratio was lower, and Comparative Example 3 in which no aromatic polyether ketone was contained, all failed to provide a coating film having a thickness of 40 pm or more without generating cracks.

[0109] The aqueous fluororesin coating composition of the present invention can form a sufficiently thick coating film with a single coating which strongly adheres to various metal substrates, and has excellent water vapor resistance and corrosion resistance, and thus can be used as an excellent one-coat coating.