AQUEOUS FLUORORESIN COATING COMPOSITION CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No. JP 2019-221,441 filed December 6, 2019, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to an aqueous fluororesin coating composition which strongly adheres to metal substrates, in particular stainless steel (SUS) substrates, and can form a coating film having excellent water vapor resistance and corrosion resistance, a coating film formed by applying the coating composition on a substrate, and an article having the coating film.

BACKGROUND TECHNOLOGY

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 chemical processin, mechanical devices, and electrical devices.

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 expanded in recent years to inkjet nozzles, chemical plant equipment, and the like.

However, when coating many substrates with fluororesins, coating the substrate directly with a fluororesin is often undesirable, as the fluororesin coating suffers from poor adhesion to the substrate and is very difficult due to the non-tackiness of fluororesins. Therefore, when performing fluororesin coating, primer coating compositions having adhesiveness to substrates and having adhesiveness with fluororesin coatings applied thereto have been commonly used.

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 such a primer coating composition. For example, Patent Document 1 discloses precursors such as polyimide, polyamide-imide, and polyethersulfone and microparticles such as polyphenylene sulfide. Such a heat-resistant resin is also called a binder. Meanwhile, an organic solvent (solvent-based coating) or water

(aqueous coating) is used as the medium of the fluororesin coating composition including 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 toxicity. 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 before use; however, a water-soluble polyamide-imide can also be used at this time (Patent Document 2). 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. Therefore, the content of the fluororesin can be increased, not only making it possible to use the substance as a primer coating, but also enabling the use thereof as a one- coat coating capable of expressing effects with only one layer without a primer.

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 better 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. 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.

Therefore, it is undesirable to apply the coating film in cookware applications such as frying pans or rice cookers requiring these properties.

In addition, although aluminum has primarily been used as a material for cookware such as frying pans and rice cookers, there is a demand to use stainless steel (SUS) as a material because it can be used in IH cookers/IH cooking heaters, is advantageous in terms of cost, and has a high-class feel.

However, stainless steel (SUS) is problematic in that the adhesiveness of the coating composition is inferior compared to aluminum. Therefore, there is a demand for a coating composition which has high adhesiveness to stainless steel (SUS).

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).

Further, Patent Document 4 proposes a fluororesin coating composition which improves the water vapor resistance or the like of a coating film that is formed using a water-soluble polyamide-imide containing 3,3’-dimethylbiphenyl-4,4’-diisocyanate and/or 3,3’- dimethylbiphenyl-4,4’-diamine as structural units. However, even this fluororesin coating composition does not achieve water vapor resistance and corrosion resistance sufficient for application to cookware. Although N-methyl-2-pyrrolidone (NMP) has been conventionally used as a dissolved/diluted/synthetic solvent of a water-soluble polyamide- imide resin, the toxicity (in particular, reproductive toxicity) of NMP has been regarded as problematic in recent years, so a fluororesin coating composition containing a water-soluble polyamide-imide resin using N- formylmorpholine with low toxicity as a solvent instead of NMP has also been proposed (Patent Document 5). However, this fluororesin coating composition also does not solve the problem of water vapor resistance or corrosion resistance for application to cookware and the adhesiveness to stainless steel (SUS) substrates, in particular, was insufficient.

PATENT DOCUMENTS

Patent Documents

Patent Document 1: JP H04-71951 B Patent Document 2: JP 3491624 B Patent Document 3: JP 4534916 B

Patent Document 4: WO2016/175099 Patent Document 5: JP 2016-89016 A

SUMMARY OF INVENTION

The object of the present invention is to provide an aqueous fluororesin coating composition which strongly adheres to metal substrates, in particular stainless steel (SUS) substrates, while combining excellent water vapor resistance and corrosion resistance so as to be suitably used in cookware such as fry pans or rice cookers, in addition to also being excellent from an environmental and safety hygienic standpoint. To achieve the object described above, the aqueous fluororesin coating composition of the present invention contains a water-soluble polyamide-imide resin, a polyether ether ketone, and a fluororesin, wherein the fluororesin is perfluororesin.

That is, the present invention is as follows. (1) An aqueous fluororesin coating composition, including: a water- soluble polyamide-imide resin, a polyether ether ketone, and a fluororesin, wherein the fluororesin is a perfluororesin.

(2) The aqueous fluororesin coating composition according to (1), wherein the fluororesin is a melt processable perfluororesin.

(3) The aqueous fluororesin coating composition according to (1), wherein the fluororesin is a non-melt processable polytetrafluoroethylene.

(4) The aqueous fluororesin coating composition according to (1), wherein the fluororesin is a melt processable perfluororesin and a non- melt processable polytetrafluoroethylene.

(5) The aqueous fluororesin coating composition according to any one of (1) to (4), further including a polyether sulfone.

(6) The aqueous fluororesin coating composition according to any one of (1 ) to (5), wherein the amount of the fluororesin is from 35 to 90 mass% relative to the total amount of a binder resin and the fluororesin.

(7) A coating film formed by coating a metal substrate with the aqueous fluororesin coating composition according to any one of (1) to (6).

(8) The coating film according to (7), wherein the metal is stainless steel. (9) A coated article including the coating film according to (7) or (8).

(10) The coated article according to (9), which is cookware.

ADVANTAGEOUS EFFECTS OF THE INVENTION

With the present invention, it is possible to provide an aqueous fluororesin coating composition which has sufficient adhesiveness to metal substrates, in particular stainless steel (SUS) substrates, while combining excellent water vapor resistance and corrosion resistance so as to be suitably used in cookware. In addition, with the present invention, it is possible to provide an aqueous fluororesin coating composition which uses water as a medium and is also excellent from an environmental and safety hygienic standpoint. Further, with the present invention, a coating film containing a large amount of a fluororesin can be provided, thereby enhancing the performance of a fluororesin coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail hereinafter.

1. Aqueous Fluororesin Coating Composition

The “aqueous fluororesin coating composition” of the present invention contains a water-soluble polyamide-imide resin, a polyether ether ketone, and a perfluororesin. Aqueous Fluororesin Coating Composition

The “aqueous fluororesin coating composition” of the present invention is an aqueous (water-based) dispersion containing a water- soluble polyamide-imide resin, a polyether ether ketone, and a perfluororesin. The aqueous fluororesin coating composition of the present invention is ordinarily suitably used as a primer coating (undercoat) for adhering a fluororesin layer to a substrate; however, the composition can also be used as a one-coat coating that does not use a primer coating.

Water-soluble Polyamide-imide Resin (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, preferably having repeating units represented by the following general formula:

[Formula 1] (wherein, R ¹ is a trivalent organic group, while R ² is a divalent organic group). 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 from 80 to 180°C, and in order to reduce the effects of moisture in the air, synthesis is performed in an atmosphere of nitrogen or the like.

While the diisocyanate compound is not particularly limited, an example thereof is a diisocyanate compound represented by Formula (2) below. In Formula (2), X is a divalent organic group.

[Formula 2] OCN-X-NCO (2)

Examples of the divalent organic group represented by X include: alkylene groups having a carbon number of from 1 to 20; arylene groups such as phenylene groups or naphthylene groups which are unsubstituted or substituted with lower alkyl groups having a carbon number of from 1 to 5 such as methyl groups or lower alkoxy groups having a carbon number of from 1 to 5 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 a carbon number of from 1 to 5, an oxy group (-0-), a carbonyl group (-CO-), or a sulfonyl group (-SO2-); and divalent organic groups formed by bonds of two lower alkylene groups having a carbon number of from 1 to 5 via the arylene groups described above. The carbon number of the alkylene group is preferably from 1 to 18, more preferably from 1 to 12, even more preferably from 1 to 6, and particularly preferably from 1 to 4. From the perspective of enhancing the adhesive strength of the coating film, 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 a carbon number of from 1 to 5, an oxy group (-0-), 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 a carbon number of from 1 to 5, and even more preferably a divalent organic group formed by bonding two phenylene groups via a single bond or a lower alkylene group having a carbon number of from 1 to 5 is used. 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 a carbon number of from 1 to 5 such as a methyl group or a lower alkoxy group having a carbon number of from 1 to 5 such as a methoxy group.

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.

While not particularly limited thereto, exemplary diamine compounds include a compound in which an isocyanate compound is substituted with an amino group in Formula (2) 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.

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. Further, from the perspective of enhancing the working environment, 3, 3’-dimethylbiphenyl-4, 4’ -diisocyanate is preferably used (Patent Document 4).

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.

An example of a tribasic acid anhydride is a tricarboxylic 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 represented by Formula (3) or Formula (4) below. From the perspective of heat resistance, cost, and the like, a trimellitic acid anhydride is particularly preferable. (R is a hydrogen atom, an alkyl group having a carbon number of from 1 to 10, or a phenyl group, while Y is -CH2-, -CO-, -SO2-, or -0-.)

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 (3) or Formula (4) 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.

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

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 long as it does not impair the properties such as the heat resistance of the PAI. 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.

From the perspective of maintaining the properties such as the heat resistance of the PAI, 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 from 0 to 50 mol%, more preferably from 0 to 30 mol%, and even more preferably from 0 to 15 mol% of all acid components.

From the perspective of the molecular weight and degree of crosslinking of the PAI that is produced, the usage 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 dianhydride or the like used as necessary) is preferably from 0.8 to 1.1 mols, more preferably from 0.95 to 1.08 mols, and even more preferably from 1.0 to 1.08 mols in terms of the total amount of the diisocyanate compound and/or diamine compound with respect to a total of 1.0 mol of the acid components.

As a PAI, a PAI obtained by reacting a diisocyanate compound and/or a diamine compound with an acid component can be used directly.

It can also be used after being protected with a blocking agent.

When a diisocyanate compound is used as the starting 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 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.

Alcohol is an example of a blocking agent, with examples of alcohols including lower alcohols having a carbon number of from 1 to 6 such as methanol, ethanol, and propanol. Examples of blocking agents include 2-butanone oxime, d-valerolactam, and e-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.

As a polar agent 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, g-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.

While the amount of solvent used is not particularly limited, it is preferably from 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.

From the perspective of ensuring the strength of the coating film, the number average molecular weight of the PAI is preferably no lower than 5,000, more preferably no less than 10,000, even more preferably no lower than 13,000, and particularly preferably no lower than 15,000. In addition, from the perspective of ensuring solubility in water, the number average molecular weight is preferably no greater than 50,000, more preferably no greater than 30,000, even more preferably no greater than 25,000, and particularly preferably no greater than 20,000.

The number average molecular weight of the 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.

The acid value of the PAI combining the carboxyl groups in the resin and carboxyl groups with ring-opened acid anhydride groups is preferably no less than 10 mgKOH/g. The acid value is more preferably no less than 25 mgKOH/g and even more preferably no less than 35 mgKOH/g. These ranges are preferable ranges from the perspective of facilitating dissolution or dispersion of the PAI. In addition, when the basic compound described above 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.

Further, from the perspective of preventing gelation overtime, the acid value of the fluororesin coating composition that is ultimately obtained is preferably no greater than 80 mgKOH/g. The acid value is more preferably no greater than 60 mgKOH/g and even more preferably no greater than 50 mgKOH/g.

The acid value can be obtained using the following method. First,

0.5 g of 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.

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.

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; or ammonia, and the like. From the perspective of increasing the solubility of the PAI in water, alkylamines and/or alkanolamines are suitable.

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 no less than 2.5 equivalents, more preferably no less than 3.5 equivalents, and even more preferably no less than 4 equivalents 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 content of the basic compound is preferably no greater than 10 equivalents, more preferably no greater than 8 equivalents, and even more preferably no greater than 6 equivalents.

Specific water-soluble PAIs and production methods thereof are disclosed in Patent Documents 3, 4, and 5, JP 2016-17084 A, JP 2018- 2802 A, and the like.

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. The organic solvent is not particularly limited as long as the solvent has high polarity and a high boiling point, with various polar agents 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 or N-formylmorpholine from the perspective of the effects on the human body, REACH regulations, legal regulations of the US FDA, or the like.

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

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

Examples of commercially available products of such a water- soluble PAI solution include HPC-1000-28 and HPC-2100D-28 available from Hitachi Chemical Co., Ltd., with HPC-2100D-28 being preferable.

Polyether Ether Ketone (PEEK)

The “polyether ether ketone (PEEK)” used in the present invention is a polymeric compound having at least the following repeating units as represented in Formula 5. Either a homopolymer or a copolymer thereof may be used. Polyether ether ketone (PEEK) is typically manufactured by reacting 4,4-difluorobenzophenone with hydroquinone in diphenylsulfone in the presence of alkali metal carbonate (for example, potassium carbonate and/or sodium carbonate).

Commercially available products of polyether ether ketone (PEEK) used in the present invention include VICOTE™ available from Victrex PLC.

In the binder resin used in the present invention (water-soluble PAI, PEEK, or other heat-resistant resins), the composition ratio of the water- soluble PAI is no less than 30 wt.% and preferably no less than 40 wt.%. PAI is a thermosetting resin that is thought to have an effect of forming a film without flowing during the firing of the coating film, and in small amounts, a uniform film cannot be formed, with defects likely to occur.

PEEK is preferably contained in an amount of no less than 10 wt.% in the binder resin. It is thought that PEEK has an effect of enhancing the adhesive force to substrates, hydrolysis resistance performance, and film formability, in addition to having an effect of enhancing hydrolysis resistance performance and yielding a coating film having high adhesive force to a substrate, film formability for forming a uniform film, and excellent hydrolysis resistance performance. Fluororesin

In the present invention, the “perfluoro resin” refers to a fluororesin in which all of the hydrogen atoms in the molecular chain are substituted with fluorine, with specific examples thereof including polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene/hexafluoropropylene/perfluoro(alkyl vinyl ether) copolymer, etc. In the present invention, a melt processable fluororesin is preferably used as the perfluororesin and exhibits melt fluidity at the melting point or higher because, when used as a coating film, the occurrence of pin holes can be suppressed and a uniform, smooth coating film can be obtained. Even among melt processable perfluororesins, PFA is particularly preferable due to the excellent heat resistance thereof.

When PFA is used, the perfluoro(alkyl vinyl ether) alkyl groups in the PFA preferably have a carbon number of from 1 to 5 and more preferably have a carbon number of from 1 to 3. Flere, the amount of perfluoro(alkyl vinyl ether) in the PFA is preferably in a range of from 1 to 50 mass%.

Moreover, in the present invention, a non-melt processable polytetrafluoroethylene (PTFE) is preferably used as the perfluoro resin.

As a result, the stress remaining in the coating film after heating can be reduced and the cost can also be reduced. Furthermore, the simultaneous use of the melt processable perfluoro resin and the non-melt processable polytetrafluoroethylene (PTFE) yields the respective advantages described above and is more preferable. Furthermore, other fluorine resins may be added as necessary.

Non-melt processable Polytetrafluoroethylene The non-melt processable polytetrafluoroethylene preferably used in the present invention 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 no greater than 1 mass% (modified PTFE), or both may be used in combination. By using such a PTFE, the cost can also be reduced.

Melt processable Fluororesin

Examples of the “melt processable fluororesin” used in the present invention include a low-molecular-weight melt processable polytetrafluoroethylene (melt processable PTFE), tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymers (PFA), tetrafluoroethylene/hexafluoropropylene copolymers (FEP), tetrafluoroethylene/hexafluoropropylene/perfluoro(alkyl vinyl ether) copolymers, tetrafluoroethylene/ethylene copolymers, polyvinylidene fluorides, polychlorotrifluoroethylenes, and chlorotrifluoroethylene/ethylene copolymers. These can be manufactured by conventionally known methods such as solution polymerization, emulsion polymerization, or suspension polymerization.

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 Application Publication No. S52-44576 or the like in the coating composition. Further, a fluororesin resin dispersion (dispersion) polymerized by emulsion polymerization can be used directly, or a fluororesin resin dispersion stabilized by adding a surfactant or adjusted to a high fluororesin resin concentration by concentrating with a known technique such as the method described in US Patent No. 3,037,953 can also be used. A stabilized fluororesin resin dispersion is preferable in that the dispersed state can be maintained over a long period of time without the coagulation or precipitation of the fluororesin.

The concentration of the fluororesin dispersion used in the coating composition of the present invention is preferably from 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™ PTFE 31 -JR, PTFE 34-JR, PFA 334-JR, PFA 335-JR, and FEP 120-JR available from Chemours-Mitsui Fluoroproducts Co., Ltd.

When the aqueous fluororesin coating composition of the present invention is used as a primer coating, the amount of the fluororesin is preferably from 35 to 90 mass% and particularly preferably from 45 to 80 mass% relative to the total amount of the binder resin and the fluororesin.

If the amount of the fluororesin is less than 35 mass%, the water vapor resistance and corrosion resistance of the coating film, as well as the adhesion of the top coat, may decrease. In contrast, if the fluororesin exceeds 90 mass%, the corrosion resistance of the coating film may decrease, and furthermore, adhesion force to the substrate and strength of the coating film may decrease.

In addition, when the aqueous fluororesin coating composition of the present invention is used as a one-coat coating, the amount of the fluororesin is preferably from 5 to 90 mass% and particularly preferably from 10 to 70 mass% relative to the total amount of the resin solid content. If the amount of the fluororesin is less than 5 mass%, the water vapor resistance and corrosion resistance of the coating film may decrease. Furthermore, the properties of the fluororesin coating such as releasability may not be sufficiently obtained. In contrast, if the fluororesin exceeds 90 mass%, corrosion resistance of the coating film may decrease along with the adhesion force to the substrate and the strength of the coating film, as in the case of the primer coating.

In the present invention, the “resin solid content” described above refers to the total mass of the binder resin (water-soluble PAI, PEEK, or other heat-resistant resin) and the fluororesin in a residue after the fluororesin coating composition of the present invention is applied to coated object, dried at a temperature of from 80 to 100°C, and then sintered for 45 minutes at approximately 380°C. 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 size of from 0.01 to 50 pm. When the average particle size is less than 0.01 pm, the dispersibility of the particles is poor and there is a risk that the resulting coating composition may have poor mechanical strength 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 poor. A more preferable upper limit is 5 pm, with an even more preferable upper limit of 0.5 pm, while a more preferable lower limit is 0.05 pm. The mechanical stability described above is a property such that a non-redispersible agglomerate 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.

Other Components

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, and enhanced wear resistance, examples of which include: surfactants (for example, polyoxyethylene alkyl ether or polyoxyethylene alkyl phenyl ether-based non-ionic surfactants such as Liocol available from Lion, Inc., the TRITON and TERGITOL series available from the Dow Chemical Company, and Emalgen available from KAO, Inc.; sulfocuccinate-based, alkyl ether sulfonic acid sodium salt-based, or sulfate mono-long-chain alkyl-based anionic surfactants such as Repal available from Lion, Inc. and Emal and Pelex available from KAO, Inc.; polycarboxylate or acrylate-based polymer surfactants such as Leoal available from Lion, Inc. or OROTAN available from the Dow Chemical Company; and L-77 available from Momentive, and the Surfynol Series available from EVONIK (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.

In addition, a variety of organic and inorganic substances can be added to the aqueous fluororesin coating composition of the present invention as binders or fillers in accordance with the required properties. Examples of organic substances include engineering plastics such as polyphenylene sulfides, 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, and silicone polyester resins. 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, and barium sulfate. Substances having a variety of shapes such as particle-shaped, fiber-shaped, and flake-shaped substances can be used as fillers.

Aqueous Medium

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 or to enhance the dispersibility of the PEEK, the fillers, or 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)

Stainless steel (SUS) is an alloy produced 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.

Production Process for Aqueous Fluororesin Coating Composition)

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 PEEK, a fluororesin, and other additives or fillers 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, PEEK, a fluororesin, a pigment, or the like may be prepared by preparing a dispersion (dispersion solution) thereof in advance and mixing the obtained dispersion.

The aqueous fluororesin coating composition of the present invention preferably has a viscosity of from 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 a coated object, 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 the value obtained by taking a measurement using a BM-type single-cylinder rotary viscometer (available from Tokyo Keiki Co., Ltd.). 2. Coating Film

The “coating film” of the present invention is a coating film obtained by applying the aqueous fluororesin coating composition of the present invention to a substrate. A coating film is also included in which the coating composition of the present invention is used as a primer layer to be adhered to a substrate, with a plurality of layers coated and laminated thereon. The “coating film” of the present invention can be formed by a typically used method such as spray coating, dip coating, or spin coating, 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.

3. Coated Article

The “coated article” of the present invention is an article obtained by applying the aqueous fluororesin coating composition of the present invention. Examples of the “coated article” of the present invention include: cookware such as frying pans or rice cookers; heat-resistant release trays in factory lines or the like (such as a bread-baking process); office equipment-related articles such as fixing rollers/belts/inkjet nozzles; industrial equipment-related articles at chemical plants such as piping; and other articles requiring non-tackiness and water and oil repellency.

Cookware requiring high water vapor resistance and corrosion resistance is preferable.

EXAMPLES

(Preparation of Aqueous Fluororesin Coating Composition) The following reagents were used in the examples and comparative examples.

Water soluble polvamide-imide (PAD resin

HPC-2100D-28 available from Hitachi Chemical Co., Ltd. (Solution with a PAI concentration of approximately 28 mass%, a water concentration of from 22 to 32 mass%, and an N-formylmorpholine concentration of from 30 to 40 mass%)

Polvether ether ketone (PEEK) resin

PEEK powder: VICOTE™ Coatings 704 available from Victrex PLC Other binder resins Polyethersulfone (PES) resin PES Powder: SUMIKAEXCEL PES 4100MP available from Sumitomo Chemical Co., Ltd. polyetherimide (PEI) resin

PEI Powder: Ultem 1000F3SP-1000 available from SABIC Fluororesin

PFA Aqueous dispersion (1): Teflon™ PFA 334-JR available from Chemours-Mitsui Fluoroproducts Co., Ltd. (PFA concentration: 60 mass%)

PFA Aqueous dispersion (2): Teflon™ PFA 335-JR available from Chemours-Mitsui Fluoroproducts Co., Ltd. (PFA concentration: 60 mass%) PTFE Aqueous dispersion (1): Teflon™ PTFE 34-JR available from

Chemours-Mitsui Fluoroproducts Co., Ltd. (PTFE concentration: 58 mass%)

Example 1

300 g of purified water was placed in a 1 L stainless steel vessel and using a stirrer (available from Yamato Scientific Co., Ltd.), 5 g of a non-ionic surfactant aqueous solution (concentration: 81 mass%) was added while stirring at 140 rotations/min. 54 g of a PEEK powder was added to the surfactant dispersion and dispersed by stirring for 10 minutes. Further, 43 g of a carbon black aqueous dispersion (water dispersion with a solid content of 25 mass%) was added and stirred for 10 minutes. Next, 148 g of the PFA aqueous dispersion (1) and 260 g of the PTFE aqueous dispersion (1) were added and stirred for 10 minutes. Next, 157 g of water-soluble PAI was added and further stirred for 10 minutes to obtain an aqueous fluororesin coating composition. Examples 2 to 8

Aqueous 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 Table 1 below. Comparative Examples 1 to 4 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 Table 1 below. The composition ratios (mass%) of the resin solid content in the coating compositions of the examples are shown in Table 1 below, with the composition ratios (mass%) of the resin solid content in the coating compositions of the comparative examples shown in Table 2 below. Table 1 - Examples

Table 2 - Comparative Examples

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

Production of Test Piece for Evaluation

First, a 170 mm square piece of aluminum (JIS A1050 compliant product, thickness: 2 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 pm. Subsequently, the fluororesin coating compositions of each of the examples and the comparative examples were spray-coated using a spray gun (W-101-101G, available from Anest Iwata Inc.) and dried for 20 minutes at 120°C and then for 20 minutes at 250°C to form a primer layer (fluororesin coating composition layer). The substrate on which the primer layer was formed was subjected to electrostatic powder coating (coating weight: 3.0 to 3.5 g) with a PFA powder coating (Teflon™ coating MJ-102 available from Chemours-Mitsui Fluoroproducts Co., Ltd.) over the entire surface using a powder spray gun (GX355HW available from Parker Ionics) and this was sintered for 30 minutes at 400°C (substrate temperature) to form a top coat layer (PFA layer).

A test piece for adhesiveness evaluation was similarly produced using stainless steel (JIS SUS304, thickness: 1 mm) instead of aluminum and this was used in evaluation testing.

Performance Evaluation Method

The test piece for evaluation (aluminum substrate) described above was left to stand for 100 hours in 0.8 megapascal steam at 170°C, then allowed to sit and cool to room temperature, after which the state of the coating film was then observed. Next, the substrate was heated to 200°C with a gas stove, then rapidly cooled with water, and the state of the coating film was confirmed. If any swelling or blistering occurred, testing was ended. If the state of the coating film was good, then this was repeated every 100 to 300 hours to perform three cycles of water vapor pressure processing. The adhesive strength of the coating film was then measured by the method described below.

The test piece of the stainless-steel substrate was evaluated in the same manner as in the above method with the exception that the temperature of the water vapor was set to 150°C. Adhesive Strength Measurement Method

The coating film was cut with a cutter so as have a width of 1 cm at the center of the abovementioned test piece, after which the end of the coating film was peeled so as to serve as the gripping part for measuring the adhesive strength.

Using a Tensilon universal testing machine (available from A&D Inc.), the peeled coating film described above was sandwiched between the chucks of the tester in accordance with the measurement method for the peel strength of an adhesive (90-degree peel test method) prescribed by JIS K 6854 and pulled at a rate of 50 mm/minute to measure the adhesive strength (peel strength). The units were kgf (kilogram-force).

The results are shown in the table below. Cases in which no swelling or blistering occurred and the adhesive strength was larger than 0.3 kgf were given an evaluation result of o, while other cases were given an evaluation result of *.

Table 3 Table 4