Powder coating composition

Abstract

An object of the present invention is to provide a hot-melt fluororesin powder coating composition containing fillers that can produce a coating film with excellent corrosion resistance and surface properties. The present invention is a powder coating composition, which is a powder mixture, containing: first hot-melt fluororesin particles in which a filler is dispersed in the particles; and second hot-melt fluororesin particles having an average particle diameter of 10 to 200 µm; and charge adjusting agent particles, wherein the amount of filler is 7 wt.% to 15 wt.%, based on the total amount of the powder mixture.

Description

TITLE OF THE INVENTION POWDER COATING COMPOSITIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of Japanese Patent Application No. 2024-009910 filed January 26, 2024, the disclosures of which are incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present invention relates to a hot-melt fluororesin powder coating composition containing a filler, which can be thickly coated by electrostatic powder coating.CONVENTIONAL TECHNOLOGY

[0003] Fluororesins have excellent heat resistance, chemical resistance, electrical properties, and mechanical properties in addition to having a very low coefficient of friction and tack-free properties, leading to widespread use in all types of industrial fields such as chemistry, machinery, electrical devices, and the like. In particular, hot-melt fluororesins demonstrate liquidity at temperatures above a melting point, and therefore, the generation of pin holes can be suppressed when formed in a coating, thereby allowing the fluororesins to be used in coating compositions for fluororesin coatings.

[0004] Patent Document 1 discloses an aqueous liquid coating of a fluororesin. These liquid coatings are used as so-called slurry coatings with high concentration and high viscosity, allowing for a thick coating. However, slurry paints require a drying process, and there are concerns about environmental impact due to evaporation of solvents.

[0005] On the other hand, hot-melt fluororesin powder coatings have advantages where thick coating is possible without a volatile liquid medium, the coating can be reused, and no VOC (volatile organic compound) is generated. Electrostatic coating is commonly used as a method of powder coating using a hot-melt fluororesin powder coating, in which a material to becoated and a powder coating are charged and coated. Furthermore, when fillers are used to add various properties such as conductivity, abrasion resistance, and friction resistance to hot-melt fluororesin powder coatings, or to adjust appearance such as color and gloss, the hot-melt fluororesin powder coating and filler particles can be mixed together and used. However, dispersing filler particles in hot-melt fluororesin powder particles is preferable in terms of achieving thicker coating, coating film durability, prevention of filler from escaping from the coating film, and prevention of variation in the coating film (for example, see Patent Document 2).

[0006] In addition, coatings must be able to be applied in thick coats to improve coating durability and corrosion resistance, improve productivity, and reduce process costs. The present inventors have proposed a hot-melt fluororesin powder coating composition with a high film thickness that can be applied in one thick coat and a high limit film thickness for overlapping coats (Patent Document 3). Patent Document 3 discloses a powder paint composition that ultimately contains about 1 % conductive filler based on the total composition, by using hot-melt fluororesin particles that contain a relatively small amount of conductive filler. However, demand for corrosion resistance and conductivity of coating films has been increasing year by year, and there is a need for coatings that further improve on these properties.PRIOR ART DOCUMENTSPATENT DOCUMENTS

[0007] Patent Document 1 : Japanese Unexamined Patent Application Publication 2003-041126

[0008] Patent Document 2: Japanese Examined Patent Publication H5- 73147

[0009] Patent Document 3: Japanese Unexamined Patent Application Publication 2022-127831SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide a hot-melt fluororesin powder coating composition containing fillers that can produce a coating film with excellent corrosion resistance and surface properties, in addition to being able to provide a thick coating.MEANS FOR SOLVING THE PROBLEM

[0011] The present invention is a powder coating composition, which is a powder mixture, containing: first hot-melt fluororesin particles in which a filler is dispersed in the particles; and second hot-melt fluororesin particles having an average particle diameter of 10 to 200 pm; and charge adjusting agent particles, wherein the amount of filler is 7 wt.% to 15 wt.%, based on the total amount of the powder mixture.

[0012] In the powder coating composition of the present invention, the ratio of the first hot-melt fluororesin particles to the second hot-melt fluororesin particles is preferably 40 to 70: 30 to 60 wt.%, and the amount of charge adjusting agent particles is preferably 0.01 wt.% or more and 5 wt.% or less, with regard to the total amount of the powder coating composition. The average particle diameter of the second hot-melt fluororesin particles is preferably larger than the average particle diameter of the first hot-melt fluororesin particles. Furthermore, the filler is preferably a conductive filler, and the conductive filler is more preferably a carbon material having a graphene structure. Furthermore, the charge adjusting agent particles are preferably graphite. Furthermore, the hot-melt fluororesin is preferably a perfluoro resin.

[0013] Another aspect of the present invention is a coating film manufactured from the powder coating composition, and the film thickness is preferably 100 pm or more.EFFECT OF THE INVENTION

[0014] The present invention provides a hot-melt fluororesin powder coating composition containing fillers that can produce a coating film withexcellent corrosion resistance and surface properties, in addition to being able to provide a thick coating.MODE FOR CARRYING OUT THE INVENTION

[0015] The powder coating composition of the present invention is a powder coating composition, which is a powder mixture containing (1) first hot- melt fluororesin particles, (2) second hot-melt fluororesin particles, and (3) charge adjusting agent particles.(1 ) First hot-melt fluororesin particles

[0016] The first hot-melt fluororesin particles are described below. The first hot-melt fluororesin particles of the present invention are particles in which a filler is dispersed in a hot-melt fluororesin, and are manufactured from the hot- melt fluororesin and the filler.

[0017] The hot-melt fluororesin used in the present invention may be appropriately selected from resins known as hot-melt fluororesins. Examples include polymers or copolymers of a monomer selected from tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluoro(alkyl vinyl ether), vinylidene fluoride and vinyl fluoride, copolymers of the monomers and ethylene, propylene, butylene, pentene, hexene, or another monomer having a double bond, or acetylene, propyne, or another monomer having a triple bond, and the like. Specific examples of the hot-melt fluororesin include low molecular weight hot-melt polytetrafluoroethylenes (hot-melt PTFE), tetrafluoroethylene perfluoro(alkyl vinyl ether) copolymers (PFA), tetrafluoroethylene hexafluoropropylene copolymers (FEP), tetrafluoroethylene hexafluoropropylene perfluoro (alkyl vinyl ether) copolymers, tetrafluoroethylene ethylene copolymers, polyvinylidenefluorides, polychlorotrifluoroethylenes, chlorotrifluoroethylene ethylene copolymers, and the like.

[0018] Of these hot-melt fluororesins, perfluoro resins such as hot-melt PTFE, PFA and FEP, tetrafluoroethylene, hexafluoropropylene and perfluoro(alkyl vinyl ether) copolymers are particularly preferably used from theperspective of tack-free properties and heat resistance of a coating film. Of these, PFA is preferable from the perspective of heat resistance. When PFA is used, an alkyl group of the perfluoro(alkyl vinyl ether) in the PFA preferably has 1 to 5 carbon atoms, and more preferably 1 to 3 carbon atoms. Furthermore, the amount of the perfluoro(alkyl vinyl ether) in the PFA is preferably within a range of 1 to 50 wt.%.

[0019] Furthermore, the hot-melt fluororesin used in the present invention is preferably a hot-melt fluororesin having fluidity at a temperature above the melting point, from the perspective of favorable moldability during high temperature melting. Specifically, the melt flow rate (MFR) of the hot-melt fluororesin is preferably 0.1 g / 10 min or more, and more preferably 0.5 g / 10 min or more. Examples of the resin include PFA, FEP, and tetrafluoroethylene, hexafluoropropylene, and perfluoro(alkyl vinyl ether) copolymers. PFA, which has a high melting point and excellent thermal fluidity, is particularly preferable. On the other hand, if the MFR is too high (melt viscosity is too low), appearance defects due to sagging or pulling may easily occur during repeated coating and baking, and formation of a thick film becomes difficult, which is not preferable. Specifically, the MFR of the hot-melt fluororesin is preferably 30 g / 10 min or less, more preferably 25 g / 10 min or less, and particularly preferably 20 g / 10 min or less.

[0020] The hot-melt fluororesin used in the present invention may be mixed with two or more hot-melt fluororesins depending on the properties desired. Furthermore, a non-hot-melt polytetrafluoroethylene may also be included.

[0021] In the first hot-melt fluororesin particles of the present invention, the filler is dispersed inside the particles. Herein, the filler material is preferably dispersed uniformly inside the particles. Whether or not the filler is uniformly dispersed within the particles can be confirmed by determining whether the filler is uniformly dispersed when observing the surface of the particles with an electron microscope or the like. In order to uniformly disperse the filler in thehot-melt fluororesin to manufacture resin particles, a method described in Patent Document 2 can be used, for example.

[0022] Various types of fillers can be used as the filler dispersed in the particles. Examples include metal powders, metal oxides (aluminum oxide, zinc oxide, tin oxide, titanium oxide, and the like), glass, ceramics, silicon carbides (SiC), silicon oxides, boron nitrides, calcium fluorides, carbon black, graphites, micas, barium sulfates, various resin particles, and the like. Fillers having a variety of shapes, such as particle shaped, fiber shaped, flaked shaped fillers, and the like, can be used as the shape of the filler.

[0023] In particular, the present invention is effective when using a conductive filler, and examples of the conductive filler include metals, metal oxides (zinc oxides, tin oxides, titanium oxides, indium oxides, and the like), titanium carbides, titanium nitrides, carbon fibers, carbon black, graphite, carbon nanotubes (CNT), and other carbon materials having a graphene structure, particles coated therewith, and composite particles. In order to achieve high conductivity, a combination of carbon black and carbon fiber is preferably used. Furthermore, in the present invention, a material having relatively low insulating properties such as silicon carbide (SiC), and particularly, having a volume resistivity of 108Q cm or less, can also be used as the conductive filler. Silicon carbide (SiC) is preferably used in order to improve the wear resistance of the coating film.

[0024] Furthermore, thick coating is difficult when polar particles with hydrophilic surfaces such as mica, aluminum oxide, boron nitride, and silicon oxide are used (the electrical properties of the fluororesin and the filler are different, and therefore, variations in charging during electrostatic coating are thought to occur), but such a filler can be used. Mica can provide a brilliant look to the coating film, and is therefore preferably used.

[0025] Fillers having a variety of shapes, such as particle shaped, fiber shaped, flaked shaped fillers, and the like, can be used as the shape of the particles. A preferred mixing amount depends on the properties required andthe type of filler and size of the particles, but is preferably 5 to 30 wt.%, more preferably 10 to 25 wt.%, and particularly preferably 15 to 20 wt.%.

[0026] The average particle diameter of the first hot-melt fluororesin particles is within a range of 2 to 100 pm, preferably 3 to 75 pm, more preferably 5 to 50 pm, and particularly preferably 8 to 35 pm. If the average particle diameter is small, not only is electrostatic powder coating difficult due to the effects of wind, but manufacturing is difficult in the first place and aggregation tends to occur during storage, causing defects. Furthermore, if the average particle diameter is too large, a charge is difficult to apply and thus desorption tends to occur. Therefore, electrostatic coating becomes difficult, and the surface of the obtained coating film becomes rough.

[0027] Note that in the present specification, “average particle diameter” refers to the particle diameter at an integrated value of 50% of the particle diameter distribution (based on volume) obtained by laser diffraction / scattering (d50).(2) Second hot-melt fluororesin particles

[0028] The second hot-melt fluororesin particles of the present invention are particles containing a hot-melt fluororesin having an average particle diameter of 10 to 200 pm.

[0029] The second hot-melt fluororesin particles can be manufactured from the resin used in the first hot-melt fluororesin particles described above. The second hot-melt fluororesin particles differ from the first hot-melt fluororesin particles described above in that a filler is not included. Of hot-melt fluororesins, perfluoro resins such as PFA and FEP, tetrafluoroethylene, hexafluoropropylene and perfluoro(alkyl vinyl ether) copolymers are particularly preferably used from the perspective of tack-free properties and heat resistance of a coating film. Of these, PFA is preferable from the perspective of heat resistance.

[0030] The average particle diameter of the second hot-melt fluororesin particles is preferably within a range of 15 to 150 pm, more preferably 20 to100 pm, and particularly preferably 25 to 70 pm. The average particle diameter of the second hot-melt fluororesin particles should be larger than that of the first hot-melt fluororesin particles. Commercially available powder coatings of hot-melt fluororesins can be used as the second hot-melt fluororesin particles. Note that the particles do not contain a filler, but may contain a small amount of an additive such as an antifoaming agent or the like outside the particles (in a powder mixed condition).(3) Charge adjusting agent particles

[0031] Various types of conductive particles can be used as the charge adjusting agent particles of the present invention. Examples include metal powders, carbon fibers, carbon blacks, graphite, carbon nanotubes (CNT) and other carbon materials with a graphene structure, metal oxides (zinc oxides, tin oxides, titanium oxides, indium oxides, and the like), titanium carbides, titanium nitrides, and the like. Of these, carbon materials having a graphene structure are preferably used, and graphite is particularly preferably used. A function of the charge adjusting agent particles is believed to be the charge adjusting agent particles adhering to and coating the particles where the electrostatic properties differ between the first hot-melt fluororesin particles containing the filler and the second hot-melt fluororesin particles not containing a filler, so as to homogenize the electrostatic properties of the surfaces and provide uniform mixing without aggregating and separating when the particles are mixed together. Herein, the reason that graphite is preferable is because dry mixing is performed at high speed, and brittle graphite is pulverized to form fine sheets, which can adhere to and be coated on the particles.(4) Optional components

[0032] The powder coating composition of the present invention may also contain an additive of an organic / inorganic material as an optional component within a range that does not affect the physical properties of the powder coating composition. Examples include polyarylene sulfides, polyether ether ketones, polyamides, polyimides, and other engineering plastics, metal powders, metal oxides (aluminum oxide, zinc oxide, tin oxide, titanium oxide, and the like),glass, ceramics, silicon carbides, silicon oxides, calcium fluorides, carbon black, graphites, micas, barium sulfates, and the like. Additives having a variety of shapes, such as particle shaped, fiber shaped, flaked shaped fillers, and the like, can be used as the shape of the additive. The amount is preferably 10 wt.% or less, and more preferably 5 wt.% or less, based on the total amount of the powder coating composition.(5) Powder Paint Composition of the Present Invention

[0033] The powder coating compositions of the present invention are characterized in that they contain 7 to 15% by weight of filler material, based on the total amount of composition. The amount of filler in the powder coating composition of the present invention is preferably 9 to 14 wt.%, and more preferably 11 to 13 wt.%, relative to the total amount of composition. An amount of filler in the above ranges enables combining high electrical conductivity with surface smoothness. With the conventional technology, a large amount of filler material is difficult to mix evenly, and a smooth and uniform coating film cannot be obtained. In contrast, the powder paint composition of the present invention uses first hot-melt fluororesin particles in which the filler is dispersed in the particles and mixed with two types of second hot-melt fluororesin particles to obtain a paint film that has excellent uniformity, despite the fact that the total composition contains a large amount of filler. Furthermore, by using the first hot-melt fluororesin particles with filler dispersed in the particles, the shear force can be reduced when mixing the first hot-melt fluororesin particles, the second hot-melt fluororesin particles, and the charge adjusting agent particles, allowing for simple and efficient production of the composition.

[0034] The first hot-melt fluororesin particles and the second hot-melt fluororesin particles are blended so that the ratio of the amount of filler is in the above range. Preferably, the ratio of the first hot-melt fluororesin particles to the second hot-melt fluororesin particles is 40 to 70: 30 to 60 wt.%, more preferably 45 to 60: 40 to 55 wt.%, and yet more preferably 60: 40 wt.%. Furthermore, the amount of the charge adjusting agent particles is preferably0.01 to 5 wt.%, more preferably 0.1 to 3.0 wt.%, and even more preferably 0.2 to 2.0 wt.%, with regard to the entire powder coating composition.(6) Manufacturing method

[0035] A method of manufacturing the powder coating composition of the present invention is described below. The powder coating composition of the present invention is obtained by mixing the first hot-melt fluororesin particles, the second hot-melt fluororesin particles, and the charge adjusting agent particles. Examples of mixing methods that can be used include a method of mixing the particles in a dry condition (dry blending I dry mixing) and a fluid mixing method using a Turbula mixer or the like that stirs by rolling a container for mixing itself. Examples of devices used for dry blending include, but are not limited to, cutter mixers, Henschel mixers, V-type blenders with a chopper, double-cone mixers with a chopper, rocking mixers, and the like. The powder coating composition of the present invention contains a larger amount of filler material than conventional technology, so the effect of the filler material can be fully demonstrated without applying a strong shear force.(7) Coating film prepared by the powder coating composition of the present invention

[0036] The “coating film” of the present invention is a coating film obtained by coating the powder coating composition of the present invention. A primer layer that adheres to a substrate and contains a fluororesin is preferably provided in order to adhere to the substrate. The method of coating the powder coating composition of the present invention can be any conventionally known powder coating method, but electrostatic powder coating is preferable. After coating, a coating film free of pinholes and other defects is obtained by heating to a temperature higher than the melting point of the hot-melt fluororesin. The powder coating composition of the present invention can be preferably used in: cookware such as frying pans, rice cookers, and the like; heat-resistant release trays in factory lines or the like (such as a bread-baking process and the like); office equipment-related products such as fixing rollers / belts / inkjet nozzles and the like; industrial equipment-related products at chemical plantssuch as piping and the like; and other products requiring tack-free properties and water and oil repellency.EXAMPLES

[0037] The present invention will be described below in further detail based on examples and comparative examples. However, the present invention is not limited to these examples.< Preparing aluminum test piece >(A) Substrate surface treatment (shot blasting)

[0038] A surface of an aluminum substrate (JIS A1050 compliant product, 95 mm x 150 mm, 1 mm thick) was degreased using isopropyl alcohol, and then a sandblaster (Pneuma-blaster SGF-4(A)S-E566, manufactured by Fuji Manufacturing Co., Ltd.) was used to roughen the surface by shot blasting using #60 alumina (Showa Blaster, manufactured by Resonac Co., Ltd.).(B) Undercoating (primer application)

[0039] An air spray coating gun (W-88-10E2 cp1 mm nozzle (manual gun), manufactured by Anest Iwata Corporation) was used to spray and coat a liquid primer coating (fluororesin Teflon (registered trademark) coating, Aqueous primer PJ-BN910, manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.) onto the substrate treated in (A) described above at an air pressure of 3 to 4 kgf / cmA2. Coating was performed such that a coated liquid weight was approximately 0.9 to 1.4 g per sheet of the substrate, and then drying was performed in a forced draft circulation furnace at 120°C for 15 minutes to form a coating film with a film thickness of 8 to 12 pm. The coating environment was a temperature of 25°C with humidity of 60% RH.< Evaluation method >(1 ) Thick coatability

[0040] Using an electrostatic powder coating machine (Hand Gun System GX7500CS, manufactured by Nihon Parkerizing Co., Ltd.), the aluminum substrate treated in (A) and (B) above was placed in a vertical condition and a grounded condition. Then, a powder was electrostatically coated at a coatingvoltage of 20 to 40 kV (negative) and a discharge rate of about 50 g / min from a distance of approximately 25 cm until the powder did not adhere. The coating environment was a temperature 25°C with humidity of 60% RH. The coated aluminum substrate was baked in a forced draft circulation furnace at 380 °C for 30 minutes to form a coating film. The coating amount, presence or absence of falling powder, presence or absence of electrostatic repulsion, and the appearance of the obtained coating film were checked. Coating films with a coating amount of 2.8 g (corresponding to a film thickness of 100 pm) or more, no falling powder, no electrostatic repulsion, foaming and other defects were deemed as passed (o).(2) Surface smoothness

[0041] Using an electrostatic powder coating machine (Hand Gun System GX7500CS, manufactured by Nihon Parkerizing Co., Ltd.), the aluminum substrate treated in (A) and (B) above was placed horizontally in a grounded condition, a powder was electrostatically coated at a coating voltage of 20 to 40 kV (negative) and a discharge rate of about 50 g / min from a distance of approximately 25 cm to a coating amount of approximately 2.8 g (corresponding to a film thickness of 100 pm), and then baked at a prescribed temperature for 30 minutes. This was repeated five times (380°C for the first time and 360°C for the second and subsequent times) to obtain a coated material with a film thickness of 500 pm or more.The surface roughness (Ra) was measured using the resulting coated material. Surface roughness was measured using a HANDYSURF E-45A manufactured by Tokyo Seimitsu in accordance with JISB0601 with an evaluation length of 4 mm and a cutoff value of 0.8 mm by the touch needle method. A surface roughness of less than 2.0 pm was considered acceptable (O).(3) Conductivity (300 pm)

[0042] Using an electrostatic powder coating machine (Hand Gun System GX7500CS, manufactured by Nihon Parkerizing Co., Ltd.), electrostatic coating was performed on a horizontally installed glass substrate (float glass,95 mm x 150 mm, 2 mm thick) such that the film thickness was 100 to 120 pm each time. Baking was performed for 30 minutes at a prescribed temperature, and was repeated three times (380°C for the first time, 360°C for the second and subsequent times). After baking, the coated film was peeled off in boiling water to obtain the film. A surface resistance value was measured by Hiresta UX manufactured by Nittoseiko Analytech Co., Ltd. using an UA probe at an applied voltage of 100 V. If the surface resistance was less than 107Q, the product passed the test (o).(4) Conductivity (500 pm)

[0043] For powder coating compositions that were evaluated as acceptable (o) in (3), the surface resistivity was also measured in the same manner for a 500 pm thick coating film. If the surface resistance was less than 109O, the product passed the test (o).(5) Corrosion resistance

[0044] Corrosion resistance refers to the property of a product to withstand certain conditions. For the coating compositions of the present invention, the resistance of the coating compositions when treated with hydrochloric acid solution after electrostatic painting was used as the evaluation index. Corrosion resistance was considered to be improved if the product is well bonded to the coated substrate, and can form a uniform and constant thickness. Specifically, using an electrostatic powder coating machine (Hand Gun System GX7500CS, manufactured by Nihon Parkerizing Co., Ltd.), the aluminum substrate treated with (A) and (B) above (except that (B) used a 65:35 mixture of EJ-CL107 and SG-CL600 (both products of Chemours-Mitsui Fluoroproducts) in place of the PJ-BN910) was placed horizontally in a grounded condition, electrostatically coated with a powder at a coating voltage of 20 to 40 kV (negative) and a discharge rate of about 50 g / min from a distance of approximately 25 cm to a coating amount of approximately 2.8 g (corresponding to a film thickness of 100 pm), and then baked at a prescribed temperature for 30 minutes. This was repeated three times (380°C for the firsttime and 360°C for the second and subsequent times) to obtain a coated material with a film thickness of 300 pm or more.

[0045] A 5% hydrochloric acid aqueous solution (95°C) was applied to the coated surface of the resultant coating using a Yamazaki type lining tester (LA- 15, manufactured by Yamazaki Seiki Laboratory). The surface of the coating was visually observed every 24 hours and the time until blistering occurred was compared.< Raw materials >• Graphite A: SGP-5 manufactured by SEC Carbon Co.• Graphite B: UF-G5 manufactured by Resonac CorporationPFA aqueous dispersion

[0046] A dispersion of a tetrafluoroethylene / perfluoropropyl vinyl ether (TFE / PPVE) copolymer was prepared by a method in accordance with Examples 1 to 3 described in Japanese Patent publication 5588679. (MFR of solid resin = 16.6 [g / 10 min], Average particle diameter: 0.186 pm, Comonomer (PPVE) ratio: 3.3 wt.%, PFA content in dispersion: 30.6 wt.%)• PFA powder coating: fluororesin Teflon (registered trademark) coating powder topcoat MJ-508 manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd., Average particle diameter d50: approximately 50 pm• Perfluoroheptene (Chemours-Mitsui Fluoroproducts Co., Ltd., Opteon (registered trademark) SF10)• 60% nitric acid aqueous solution (manufactured by FUJIFILM Wako Pure Chemical Corporation)(Preparation Example)(Preparation of first hot-melt fluororesin particles 1 )

[0047] 246.5 g of pure water was added to a 2 L stainless steel beaker,41.2 g of graphite A was added, and then an ultrasonic dispersion treatment was performed for five minutes using an ultrasonic generator (UE-1002288-8A Ultrasonic generator, manufactured by Ultrasonic Engineering Co., Ltd.). Theobtained dispersion solution was further added into a stainless steel container containing 522 g of an PFA aqueous dispersion, and stirred at 600 rpm for 5 minutes using a downflow type propeller type 4-bladed stirrer. Then, 11 .6 g of a 60% nitric acid aqueous solution was added thereto, and after confirming a rapid increase in viscosity, 132.1 g of perfluoroheptene was added to generate coarse particles of aggregates in the liquid. The coarse particles of agglomerates removed by filtration were washed three times with pure water. After washing, the sample was placed on a tray and dried at 120°C for 1 hour and at 290°C for 3 hours to obtain the first hot-melt fluororesin particles 1 . The average particle diameter of the obtained particles was d50: 31.0 pm. The amount of graphite in the obtained hot-melt fluororesin particles 1 was calculated to be 20 wt.% based on the weight change during heating at 600°C for 30 minutes in a nitrogen environment using a tubular electric furnace TMF- 300N (manufactured by As One Corp.).(Example 1 )

[0048] In a small Henschel mixer (FM10B made by Nippon Coke & Engineering Co., Ltd.), 1200 g of the first hot-melt fluororesin particles 1 produced in the above preparation example, 782 g of PFA powder coating as the second hot-melt fluororesin particle (MJ-508 manufactured by Chemours- Mitsui Fluoroproducts Co., Ltd., average particle diameter d50: 51.2 pm), and 18 g of graphite B as a charge adjusting agent were added, and the powder coating composition was mixed and stirred at 1000 rpm for 10 minutes. The ratio of filler to total powder coating composition was adjusted to be 12.0 wt.%.(Example 2)

[0049] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 1100 g of the first hot-melt fluororesin particles 1 and 882 g of the PFA powder coating were used. The ratio of filler to total powder coating composition was adjusted to be 11 .0 wt.%.(Example 3)

[0050] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 1000 g of the first hot-meltfluororesin particles 1 and 982 g of the PFA powder coating were used. The ratio of filler to total powder coating composition was adjusted to be 10.0 wt.%.(Example 4)

[0051] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 900 g of the first hot-melt fluororesin particles 1 and 1082 g of the PFA powder coating were used. The ratio of filler to total powder coating composition was adjusted to be 9.0 wt.%.(Example 5)

[0052] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 800 g of the first hot-melt fluororesin particles 1 and 1182 g of the PFA powder coating were used. The ratio of filler to total powder coating composition was adjusted to be 8.0 wt.%.(Preparation of first hot-melt fluororesin particles 2)

[0053] In the above preparation example, 171 .0 g of pure water, 22.5 g of graphite A, 597.4 g of PFA aqueous dispersion, 12.8 g of 60% nitric acid solution, and 146.2 g of perfluoroheptene were used to obtain the first hot-melt fluororesin particles 2. The amount of graphite in the hot-melt fluororesin particles 2 was adjusted to be 11 % by weight.(Comparative Example 1 )

[0054] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 950 g of the first hot-melt fluororesin particles 2 instead of the first hot-melt fluororesin particles 1 , 1000 g of the PFA powder coating, and 50 g of graphite B were used. The ratio of filler to total powder coating composition was adjusted to be 5.2 wt.%.(Comparative Example 2)

[0055] A powder coating composition was obtained by a similar method as in Example 1 described above, except that 400 g of the first hot-melt fluororesin particles 1 and 1582 g of the PFA powder coating were used. The ratio of filler to total powder coating composition was adjusted to be 4.0 wt.%.(Comparative Example 3)

[0056] A powder coating containing only the first hot-melt fluororesin particles 1 was used as Comparative Example 3. The ratio of filler to total powder coating was adjusted to be 20.0 wt.%.(Comparative Example 4)

[0057] In a small Henschel mixer (FM10B manufactured by Nippon Coke & Engineering Co., Ltd.), 1880 g of PFA powder coating and 120 g of graphite A were added, and the powder coating composition was mixed and stirred at 1000 rpm for 10 minutes to obtain the powder coating composition. The ratio of filler to total powder coating composition was adjusted to be 6.0 wt.%.

[0058] The compositions of the examples and comparative examples are summarized in Table 1. Furthermore, the results of evaluation methods (1) through (5) are summarized in Table 2.Table 1Table 2

[0059] In Examples 1 to 5, when 8 to 12 wt.% of filler was included in the final composition, not only did the composition exhibit good thick coatability, surface smoothness, and conductivity (300 pm and 500 pm), but also corrosion resistance was markedly improved compared to Comparative Example 1 , which contained 5.2 wt.% of filler in the final composition. This improvement in corrosion resistance can be attributed to the fact that the composition of the example has a relatively large number of fillers uniformly dispersed in the composition, and thus a favorable coating film was formed by electrostatic coating.

[0060] In Comparative Examples 1 , 2, and 4 with low filler content, the surface resistance was larger than 107Q during evaluation of conductivity (300 pm) and thus did not show sufficient conductivity, so the evaluation of conductivity (500 pm) was not performed. On the other hand, Comparative Example 3, which contained 20% filler by weight, had good conductivity, but had insufficient thick coatability and surface smoothness.

[0061] The present invention is not limited to the disclosed content of the examples described in this specification or to the embodiments of the invention disclosed in this specification and encompasses the content of inventions appropriately modified based on the particulars disclosed in this specification as long as the content does not conflict with the spirit of the present invention.INDUSTRIAL APPLICABILITY

[0062] The hot-melt fluororesin powder coating compositions of the present invention can be applied thickly by an electrostatic powder coating process to obtain a coating film with excellent surface smoothness, high corrosion resistance, and excellent electrical conductivity.

Claims

Claims1. A powder coating composition, which is a powder mixture, comprising: first hot-melt fluororesin particles with a filler dispersed in the particles; second hot-melt fluororesin particles with an average particle diameter of 10 to 200 pm; and charge adjusting agent particles, wherein the amount of filler is 7 to 15 wt.%, based on the total amount of the powder mixture.

2. The powder coating composition according to claim 1 , wherein the ratio of the first hot-melt fluororesin particles to the second hot-melt fluororesin particles is 40 to 70: 30 to 60 wt.%, and the amount of charge adjusting agent particles is 0.01 to 5 wt.% based on the total amount of the powder coating composition.

3. The powder coating composition according to claim 1 or claim 2, wherein the filler is a carbon material having a graphene structure.4 The powder coating composition according to claim 1 or claim 2, wherein the charge adjusting agent particles are made of graphite.

5. The powder coating composition according to claim 1 or claim 2, wherein the hot-melt fluororesin is a perfluoro resin.

6. A coating film having a thickness of 100 pm or more, comprising: a powder coating composition according to claim 1 or claim 2.

7. An article, comprising the coating film according to claim 6.

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