WO2021021768A1

WO2021021768A1 - Aqueous fluororesin coating composition

Info

Publication number: WO2021021768A1
Application number: PCT/US2020/043800
Authority: WO
Inventors: Ryo Nakazawa; Yuqing Liu; Kenji Suzuki
Original assignee: Chemours-Mitsui Fluoroproducts Co., Ltd; The Chemours Company Fc, Llc
Priority date: 2019-07-29
Filing date: 2020-07-28
Publication date: 2021-02-04
Also published as: JP7431524B2; CN114174444B; CN114174444A; JP2021021013A; KR20220042409A

Abstract

Provided is an aqueous fluororesin coating composition which strongly adheres to metal substrates such as stainless steel (SUS), while exhibiting excellent water vapor resistance and corrosion resistance. The fluororesin coating composition has utility as coating for cookware such as fry pans or rice cookers, and is also superior from an environmental and safety hygienic standpoint. The fluororesin coating composition contains water-soluble polyamide-imide resin, polyetherimide, polyethersulfone, and fluororesin.

Description

TITLE OF THE INVENTION

AQUEOUS FLUORORESIN COATING COMPOSITION

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese patent application no. JP2019-138424, filed July 29, 2019, the disclosure of which is

incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an aqueous fluororesin coating composition which strongly adheres to a metal 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

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 processing, 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, and the fields of applicability have 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 the substrate and having adhesiveness to fluororesin coatings applied thereto have been commonly used.

A heat-resistant resin (so-called engineering plastic) having adhesiveness to a substrate 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, and an aqueous (water-based) coating has been particularly preferably used in recent years from the perspective of environmental load and toxicity. In an aqueous coating composition, since a heat-resistant resin (binder) which imparts adhesiveness to a substrate is 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, which not only makes it possible to use the substance as a primer coating, but also enables use as a one-coat coating capable of expressing effects with only one layer without a primer.

In addition, since a water-soluble polyamide-imide has high viscosity, a thickener can be reduced or not used at all, which enhances the purity of the coating film and makes it possible to achieve better performance. Further, using a water-soluble polyamide-imide also has advantages such 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, which yields excellent productivity and also facilitates 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 a frying pan or a rice cooker 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 such can be used in an IH cooker/IH cooking heater, is advantageous in terms of cost, and has a high-class feel.

However, stainless steel (SUS) has a problem 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 by 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 a problem in recent years, and 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, is insufficient.

PATENT DOCUMENTS Patent Document 1 : Japanese Examined Patent Application

Publication No. H4-71951 (Published Japanese patent application

JP59149969A)

Patent Document 2: Japanese Patent No. 3491624

Patent Document 3: Japanese Patent No. 4534916 Patent Document 4: WO2016/175099

Patent Document 5: Japanese Unexamined Patent Application Publication No. 2016-89016

SUMMARY OF THE INVENTION

An object of the present invention is to provide an aqueous fluororesin coating composition which strongly adheres to a metal substrate - in particular, a stainless steel (SUS) substrate - while

combining excellent water vapor resistance and corrosion resistance so as to be suitably used in cookware such as fry pans or rice cookers, and is also 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 polyetherimide, a polyethersulfone, and a fluororesin.

That is, the present invention is as follows.

(1 ) An aqueous fluororesin coating composition containing a water- soluble polyamide-imide resin, a polyetherimide, a polyethersulfone, and a fluororesin, wherein the fluororesin contains a non melt-processible polytetrafluoroethylene and a melt-processible fluororesin.

(2) The aqueous fluororesin coating composition according to (1 ), wherein an amount of the fluororesin is from 35 to 90 mass% relative to a total of the water-soluble polyamide-imide resin, the polyetherimide, the polyethersulfone, and the fluororesin.

(3) The aqueous fluororesin coating composition according to (1 ) or (2), wherein the melt-processible fluororesin is a perfluororesin.

(4) A coating film formed by coating a metal substrate with the aqueous fluororesin coating composition according to any one of (1 ) to (3). (5) The coating film according to (4), wherein the metal is stainless steel.

(6) A coated article including the coating film according to (4) or (5).

(7) The coated article according to (6), which is cookware.

With the present invention, it is possible to provide an aqueous fluororesin coating composition which has sufficient adhesiveness to a metal substrate - in particular, a stainless steel (SUS) substrate - 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, so the performance of a fluororesin coating can be enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail hereinafter. Aqueous Fluororesin Coating Composition

The“aqueous fluororesin coating composition” of the present invention contains a water-soluble polyamide-imide resin, a

polyetherimide, a polyethersulfone, and a fluororesin, wherein the fluororesin is comprised of a non melt-processible polytetrafluoroethylene and a melt-processible fluororesin.

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 polyetherimide, a polyethersulfone, and fluororesing comprising non melt-processible polytetrafluoroethylene and a melt-processible fluororesin. 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, and preferably has repeating units represented by the following general formula:

Formula 1

(wherein R¹ is a trivalent organic group, and R² 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. The synthesis conditions of the water-soluble PAI are varied and are not particularly limited, but 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. 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, and 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 the diamine compound is not particularly limited, an example thereof is a compound in which the isocyanate in Formula (2) above is replaced with an amino group. 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 an amine component (diisocyanate compound, diamine compound) is preferable, because it is possible to enhance the substrate adhesive strength and the water vapor resistance of the coating film. 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. Formula 3

(R is a hydrogen atom, an alkyl group having a carbon number of from 1 to 10, or a phenyl group; and Y is -CH2-, -CO-, -SO2-, or -O-.) A tribasic acid anhydride halide is preferably used as a tribasic acid halide, and an example thereof is 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, and a trimellitic acid anhydride is particularly preferable. As an acid component, a polybasic acid or polybasic acid anhydride such as dicarboxylic acid or tetracarboxylic acid dianhydride may be used in addition to a tribasic acid anhydride and a tribasic acid halide in order to enhance hydrophilicity within a range that does not diminish the properties such as heat resistance of the PAI.

The dicarboxylic acid is not particularly limited, but examples thereof include terephthalic acid, isophthalic acid, adipic acid, sebacic acid, and the like. The tetracarboxylic acid dianhydride is not particularly limited, however, examples thereof 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 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 mol, more preferably from 0.95 to 1.08 mol, and even more preferably from 1.0 to 1.08 mol 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 a 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, and examples of alcohols include 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 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 amount of the solvent that is used is not particularly limited, however, 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 that is obtained.

From the perspective of ensuring the strength of the coating film, the number average molecular weight of the PAI is preferably not lower than 5,000, more preferably not less than 10,000, even more preferably not lower than 13,000, and particularly preferably not lower than 15,000. In addition, from the perspective of ensuring solubility in water, the number average molecular weight is preferably not greater than 50,000, more preferably not greater than 30,000, even more preferably not greater than 25,000, and particularly preferably not 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 not less than 10 mg KOH/g. The acid value is more preferably not less than 25 mg KOH/g and even more preferably not less than 35 mg KOH/g. These ranges are preferable ranges from the perspective of facilitating the 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, and this is also a preferable range in that water solubilization becomes easy.

Further, from the perspective of preventing gelation over time, the acid value of the fluororesin coating composition that is ultimately obtained is preferably not greater than 80 mg KOH/g. The acid value is more preferably not greater than 60 mg KOH/g and even more preferably not greater than 50 mg KOH/g.

The acid value can be obtained by 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 to this. 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 not less than 2.5 equivalents, more preferably not less than 3.5 equivalents, and even more preferably not 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 not greater than 10 equivalents, more preferably not greater than 8 equivalents, and even more preferably not 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, and various polar agents that can be used for the polymerization of PAI are 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

manufactured by Hitachi Chemical Co., Ltd., and HPC-2100D-28 is preferable.

Polyetherimide (PEI)

The“polyetherimide (PEI)” used in the present invention is not particularly limited as long as it is an amorphous polymer having an imide bond and an ether bond in the main chain, however, the polymer obtained by condensation polymerization of 2,2-bis[4-(3,4- dicarboxyphenoxy)phenyl]propane and m-phenylenediamine is preferable.

An example of a commercially available product of the PEI used in the present invention is Ultem 1000F3SP-1000 manufactured by SABIC. In the aqueous fluororesin coating composition of the present invention, PEI is dispersed as particles in an aqueous medium described below or dissolved in an aqueous medium. When dispersed as particles in an aqueous medium, the particles are preferably particles having an average particle size of from 0.1 to 20 pm. When the average particle size of the PEI is within the range described above, the corrosion resistance of the coating film obtained from the aqueous fluororesin coating composition of the present invention is favorable. The average particle size is a value (D50) obtained by measurement using centrifugal precipitation. Further, the maximum particle size (Dmax) obtained by measurement using centrifugal precipitation is preferably smaller than 75 pm.

Polyethersulfone (PES)

The“polyethersulfone (PES)” used in the present invention is a polymer having at least the following repeating units (formula 5). Either a homopolymer or a copolymer thereof may be used, and a polymer that can be dissolved in a polar solvent such as NMP, N-ethylmorpholine, or N- formylmorpholine is preferably used.

Formula 5

Examples of such a PES include homopolymers consisting of only the repeating units described above, and polymers consisting of the repeating units described below (Formula 6). Formula 6

Examples of commercially available products of the

polyethylsulfone (PES) used in the present invention include

SUMIKAEXCEL^® PES manufactured by Sumitomo Chemical Co., Ltd., Ultrazone^® manufactured by BASF, and Veradel^® manufactured by SOLVAY.

In the binder resin used in the present invention (water-soluble PAI, PEI, PES, or other heat-resistant resin), the composition ratio of the water- soluble PAI is not less than 30 wt.% and preferably not less than 40 wt.%. PAI is a thermosetting resin that is thought to have an effect of forming a film without flowing during the thermal curing of the coating film, and in a small amount, a uniform film cannot be formed, and defects are likely to occur. PEI and PES are each preferably contained in an amount of not less than 10 wt.% in the binder resin. It is thought that PEI has an effect of enhancing the adhesive force to a substrate and hydrolysis resistance performance, that PES has an effect of improving film formability and enhancing hydrolysis resistance performance, and that using the compounds in combination yields 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 fluororesin contains a non melt- processible polytetrafluoroethylene and a melt processible fluororesin. Non melt-processible polytetrafluoroethylene

The non melt-processible polytetrafluoroethylene 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 not greater than 1 wt.% (modified PTFE), or both may be used in combination. By using such a PTFE, the stress remaining in the coating film after heating can be reduced, and the cost can also be reduced.

Melt-processible Fluororesin

Examples of the“melt-processible fluororesin” used together with the non melt-processible polytetrafluoroethylene in the present invention include 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.

From the perspective of the non-tackiness and heat resistance of the coating film, the melt-processible fluororesin used in the aqueous fluororesin coating composition of the present invention is preferably a perfluororesin in which the hydrogen atoms in the molecular chain are replaced with fluorine, such as PFA, FEP, or a

tetrafluoroethylene/hexafluoropropylene/perfluoro(alkyl vinyl ether) copolymer. By using a melt-processible fluororesin exhibiting melt fluidity at the melting point or higher, it is possible to suppress the occurrence of pinholes when formed into a coating film, which yields a uniform and smooth coating film. Even among the melt-processible fluororesins, 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. The amount of perfluoro(alkyl vinyl ether) in the PFA is preferably in a range of from 1 to 50 mass%.

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 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 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 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%, and using a composition adjusted to a 40 to 70 mass% by concentration is 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 manufactured by 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. When the amount of the fluororesin is less than 35 mass%, the water vapor resistance and corrosion resistance of the coating film are

diminished, and there is a risk that the adhesiveness of the top coat may be diminished. Meanwhile, when the amount of the fluororesin is greater than 90 mass%, the corrosion resistance of the coating film is diminished, and there is a risk that the adhesive force to a substrate and the strength of the coating film may be diminished.

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. When the amount of the fluororesin is less than 35 mass%, the water vapor resistance and corrosion resistance of the coating film are

diminished, and there is a risk that properties of the fluororesin coating such as mold releasability may not be sufficiently achieved. Meanwhile, when the amount of the fluororesin is greater than 90 mass%, as in the case of a primer coating, the corrosion resistance of the coating film is diminished, and there is a risk that the adhesive force to a substrate and the strength of the coating film may be diminished.

In the present invention, the“resin solid content” described above refers to the total mass of the binder resin (water-soluble PAI, PEI, PES, 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 baked 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, and an even more preferable upper limit is 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 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. 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

characteristics. 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, 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 a main medium. However, although it is not preferable from the perspective of the environment or cost, 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 PEI, the fillers, or the like. In addition, by adding a polar solvent, the heat- resistant resin (binder) is dissolved, and the heat-resistant resin (binder) 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, and representative examples of stainless steels prescribed by the JIS standards include 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 a conventionally known method or the like. For example, the composition can be obtained by appropriately mixing PEI, PES, 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, PEI, PES, 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, which may make 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, which may diminish the surface smoothness or the like. A more preferable lower limit is 1 mPa-s, and a 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.).

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, and a plurality of layers are 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 to obtain a uniform coating film. 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 a frying pan or a rice cooker; heat-resistant release trays in a factory line or the like (such as a bread-baking process); office equipment-related articles such as a fixing roller/belt/inkjet nozzle;

industrial equipment-related articles at a chemical plant 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 manufactured by 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%)

Polvetherimide (PEP resin

PEI Powder: Ultem 1000F3SP-1000 manufactured by SABIC Polyethersulfone (PES) resin

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

Other binder resins

Polyimide (PI): Aqueous emulsion of thermosetting imide; Resem IM-3 manufactured by Chukyo Yushi Co., Ltd. (Imide concentration: 40 mass%)

Fluororesin

PTFE Aqueous dispersion (1 ): Teflon^® PTFE 34-JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (PTFE concentration: 58 mass%)

PTFE Aqueous dispersion (2): Teflon^® PTFE 31 -JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (PTFE concentration: 60 mass%)

PFA Aqueous dispersion (1 ): Teflon^® PFA 334-JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (PFA concentration: 60 mass%)

PFA Aqueous dispersion (2): Teflon^® PFA 335-JR manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. (PFA concentration: 60 mass%)

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

Example 1 First, 300 g of purified water was placed in a 1 L stainless steel vessel, and 15 g of a non-ionic surfactant aqueous solution (concentration: 81 mass%) was added while stirring at 140 rpm using a stirrer

(manufactured by Yamato Scientific Co., Ltd.). Next, 40 g of a PEI powder and 17 g of a PES powder were added to the surfactant dispersion and dispersed by stirring for 10 minutes. Further, 41 g of a carbon black aqueous dispersion (water dispersion with a solid content of 25 mass%) was added and stirred for 10 minutes. Next, 138 g of the PFA aqueous dispersion (1 ) and 240 g of the PTFE aqueous dispersion (1 ) were added and stirred for 10 minutes. Next, 169 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 with 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 6

Fluororesin coating compositions were obtained with 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, and the composition ratios (mass%) of the resin solid content in the coating compositions of the comparative examples are 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 -101 G, manufactured by 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 manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.) over the entire surface using a powder spray gun (GX355HW manufactured by Parker Ionics), and this was baked 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 tests.

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, and 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, the test was ended. If the state of the coating film was good, then this was repeated every 100 hours up 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

A notch was cut into the coating film with a cutter so that the width was 1 cm in the center portion of the test piece, and about 1 cm of the coating film at the end was peeled to form a gripping part for the adhesive strength measurement.

Using a Tensilon universal testing machine (manufactured by 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 (kilograms by weight).

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, and other cases were given an evaluation result of ^c.

Table 3 - Examples

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Table 4 - Comparative Examples

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Claims

1. An aqueous fluororesin coating composition comprising a water-soluble polyamide-imide resin, a polyetherimide, a polyethersulfone, and a fluororesin, wherein the fluororesin comprises a non-melt

processible polytetrafluoroethylene and a melt processible fluororesin.

2. The aqueous fluororesin coating composition according to claim 1 , wherein the amount of the fluororesin is from 35 to 90 mass% based on the total mass of the water-soluble polyamide-imide resin, the polyetherimide, the polyethersulfone, and the fluororesin.

3. The aqueous fluororesin coating composition according to claims 1 or 2, wherein the melt processible fluororesin is a perfluororesin.

4. A coating film formed by coating a metal substrate with the aqueous fluororesin coating composition according to any one of claims 1 to 3.

5. The coating film according to claim 4, wherein the metal is stainless steel.

6. A coated article including the coating film according to claims 4 or 5.

7. The coated article according to claim 6, which is

cookware.