JP7670250B2 - Resin composition for powder coating, powder coating, and article having coating film of said powder coating - Google Patents

Description

The present invention relates to a resin composition for powder coating, a powder coating, and an article having a coating film of the powder coating.

In recent years, regulations on organic solvents have become stricter due to problems such as air pollution, and environmentally friendly paints have been attracting attention. Among these, powder paints have been in the spotlight as solvent-free paints from the perspective of environmental protection, and acrylic powder paints in particular have excellent coating performance such as weather resistance and stain resistance, so they have attracted attention for use in automobile parts such as aluminum wheels, metal exteriors, and home appliances. However, powder paints have the disadvantage of inferior coating film smoothness compared to solvent-based paints.

In response to this, powder coatings have been proposed that contain epoxy-group-containing acrylic resins obtained by copolymerizing (meth)acrylic acid alkyl esters, epoxy-group-containing acrylic monomers, and other copolymerizable vinyl monomers, and curing agents that have functional groups that can react with epoxy groups (see, for example, Patent Documents 1 and 2). However, although the cured coating films obtained from these powder coatings have improved smoothness, they have a problem of insufficient thread rust resistance.

JP 2002-69368 A JP 2000-103866 A

The problem that the present invention aims to solve is to provide a resin composition for powder paint, a powder paint, and an article having a coating film of said paint, which can give a cured coating film having excellent smoothness, curability, weather resistance, and thread rust resistance.

As a result of intensive research conducted by the inventors to solve the above problems, they discovered that a cured coating film obtained from a resin composition for powder paint containing a specific acrylic resin whose essential raw materials are an acrylic monomer having a specific epoxy group, a vinyl monomer having an aromatic ring, and other monomers, has excellent smoothness, curability, weather resistance, and thread rust resistance, and thus completed the invention.

That is, the present invention relates to a resin composition for powder coating containing an acrylic resin (A) whose essential raw materials are an acrylic monomer (a1) having an epoxy group including 3,4-epoxycyclohexylmethyl(meth)acrylate, a vinyl monomer (a2) having an aromatic ring, and an unsaturated monomer (a3) other than the monomers (a1) and (a2), wherein the monomer components that are the raw materials for the acrylic resin (A) contain 5 to 40 mass% 3,4-epoxycyclohexylmethyl(meth)acrylate, 10 to 40 mass% of the monomer (a2), and 20 to 70 mass% of the monomer (a3), and the epoxy equivalent of the acrylic resin (A) is 450 to 1000 g/eq, the powder coating, and an article coated with the coating.

The resin composition for powder coating of the present invention can form a cured coating film that has excellent smoothness, curability, weather resistance and thread rust resistance, and can therefore be suitably used in coatings for coating various articles.

The resin composition for powder coating of the present invention is a resin composition for powder coating containing an acrylic resin (A) whose essential raw materials are an acrylic monomer (a1) having an epoxy group including 3,4-epoxycyclohexylmethyl (meth)acrylate, a vinyl monomer (a2) having an aromatic ring, and an unsaturated monomer (a3) other than the monomers (a1) and (a2), in which the monomer components that are the raw materials for the acrylic resin (A) contain 5 to 40 mass% 3,4-epoxycyclohexylmethyl (meth)acrylate, 10 to 40 mass% of the monomer (a2), and 20 to 70 mass% of the monomer (a3), and the epoxy equivalent of the acrylic resin (A) is 450 to 1000 g/eq.

First, the acrylic resin (A) will be described. The acrylic resin (A) has an epoxy group and is obtained by copolymerizing the monomer (a1), the monomer (a2), and the monomer (a3).

The monomer (a1) is a monomer having an epoxy group, and since a coating film having excellent filamentary rust resistance can be obtained, it is important that it contains 3,4-epoxycyclohexylmethyl (meth)acrylate, but other monomers such as, for example, glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, (meth)allyl glycidyl ether, and (meth)allyl methyl glycidyl ether can also be used in combination.

In the present invention, "(meth)acrylic acid" refers to either or both of methacrylic acid and acrylic acid, "(meth)acrylate" refers to either or both of methacrylate and acrylate, "(meth)acrylamide" refers to either or both of methacrylamide and acrylamide, and "(meth)acryloyl group" refers to either or both of methacryloyl group and acryloyl group.

The monomer (a2) is a vinyl monomer having an aromatic ring, such as styrene, α-methylstyrene, p-methylstyrene, p-methoxystyrene, divinylbenzene, etc., of which styrene is preferred. These monomers (a2) can be used alone or in combination of two or more.

The monomer (a3) is an unsaturated monomer other than the monomers (a1) and (a2), and examples thereof include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, and n-heptyl (meth)acrylate. n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, (meth)acrylamide, N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-isopropyl (meth)acrylamide , N-t-butyl(meth)acrylamide, N-t-octyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, (meth)acrylonitrile, N,N-dimethylaminoethyl(meth)acrylate, 3-(meth)acryloyloxypropyltrimethoxysilane, 3-(meth)acryloyloxypropyltriethoxysilane, 3-(meth)acryloyloxypropylmethyldimethoxysilane, 2-methoxyethyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 3-hydroxypropyl(meth)acrylate, 4-hydroxy-n-butyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxy-n-butyl(meth)acrylate, 3-hydroxy-n-butyl(meth)acrylate, 1,4-cyclohexanedimethanol mono(meth)acrylate, glycerin mono(meth)acrylate monofunctional unsaturated monomers such as acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate, and lactone-modified (meth)acrylate having a hydroxyl group at the end; ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, and 1,4-butanediol di(meth)acrylate Examples of such unsaturated monomers include bifunctional unsaturated monomers such as 1,6-hexanediol di(meth)acrylate, hydroxypivalic acid ester neopentyl glycol di(meth)acrylate, bisphenol A di(meth)acrylate, bisphenol A EO-modified di(meth)acrylate, and isocyanuric acid EO-modified diacrylate; and trifunctional or higher unsaturated monomers such as isocyanuric acid EO-modified triacrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane EO-modified tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetraacrylate, dipentaerythritol hexa(meth)acrylate, and dipentaerythritol penta(meth)acrylate. Of these, alkyl (meth)acrylates having an alkyl group with 1 to 4 carbon atoms are preferred for the purpose of increasing the glass transition temperature of the cured coating film and improving the filament rust resistance. These acrylic monomers (a3) can be used alone or in combination of two or more kinds.

The amount of 3,4-epoxycyclohexylmethyl (meth)acrylate used is 5 to 40 mass% of the monomer component that is the raw material of the acrylic resin (A), but 10 to 30 mass% is preferred because this further improves the smoothness and curing properties of the coating film.

The amount of monomer (a1) used is preferably 20 to 40 mass% of the monomer components that are the raw materials for the acrylic resin (A), as this further improves smoothness and curing properties.

The amount of the monomer (a2) used is 10 to 40 mass% of the monomer component that is the raw material of the acrylic resin (A), but 10 to 30 mass% is preferred because this further improves the smoothness, curing properties, weather resistance, and thread rust resistance of the coating film.

The amount of the monomer (a3) used is 20 to 70 mass% of the monomer component that is the raw material of the acrylic resin (A), but 30 to 70 mass% is preferred because this further improves the smoothness, curing properties, weather resistance, and thread rust resistance of the coating film.

In addition, the epoxy equivalent of the acrylic resin (A) is 450 to 1000 g/eq, but 480 to 700 g/eq is more preferable, as this provides excellent smoothness, curing properties, weather resistance, and thread rust resistance of the coating film.

In addition, the glass transition temperature of the acrylic resin (A) is preferably 60 to 90°C, and more preferably 60 to 80°C, since this further improves the smoothness, curing property, weather resistance, and thread rust resistance of the coating film.

In the present invention, the glass transition temperature is
FOX formula: 1/Tg = W1/Tg1 + W2/Tg2 + ...
(Tg: glass transition temperature to be determined, W1: weight fraction of component 1, Tg1: glass transition temperature of homopolymer of component 1)
The glass transition temperature of the homopolymer of each component is calculated based on the values given in the "Adhesive Technology Handbook" published by Nikkan Kogyo Shimbun or the "Polymer Handbook" published by Wiley-Interscience. The glass transition temperature of the homopolymer of 3,4-epoxycyclohexylmethyl methacrylate is 35°C.

Furthermore, the number average molecular weight of the acrylic resin (A) is preferably 1,000 to 10,000, more preferably 2,000 to 8,000, since it has excellent fluidity when melted and resistance to filamentous rust. Here, the number average molecular weight is a value converted into polystyrene based on gel permeation chromatography (hereinafter abbreviated as "GPC") measurement.

The acrylic resin (A) can be obtained by a known polymerization method using the monomers (a1) to (a3) as raw materials, but the solution radical polymerization method is preferred because it is the simplest.

The solution radical polymerization method is a method in which each monomer, which is the raw material, is dissolved in a solvent and a polymerization reaction is carried out in the presence of a polymerization initiator. Examples of solvents that can be used in this case include hydrocarbon solvents such as toluene, xylene, cyclohexane, n-hexane, and octane; alcohol solvents such as methanol, ethanol, isopropanol, n-butanol, isobutanol, and sec-butanol; ether solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and diethylene glycol dimethyl ether; ester solvents such as methyl acetate, ethyl acetate, n-butyl acetate, isobutyl acetate, and amyl acetate; and ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. These solvents can be used alone or in combination of two or more.

Examples of the polymerization initiator include ketone peroxide compounds such as cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, and methylcyclohexanone peroxide; 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, n-butyl-4,4-bis(tert-butylperoxy)valerate, and 2,2-bis(4,4-ditert-butylperoxy)cyclohexane. peroxyketal compounds such as 2,2-bis(4,4-ditert-amylperoxycyclohexyl)propane, 2,2-bis(4,4-ditert-hexylperoxycyclohexyl)propane, 2,2-bis(4,4-ditert-octylperoxycyclohexyl)propane, and 2,2-bis(4,4-dicumylperoxycyclohexyl)propane; cumene hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, peroxides; dialkyl peroxide compounds such as 1,3-bis(tert-butylperoxy-m-isopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, diisopropylbenzene peroxide, and tert-butylcumyl peroxide; diacyl peroxide compounds such as decanoyl peroxide, lauroyl peroxide, benzoyl peroxide, and 2,4-dichlorobenzoyl peroxide; peroxycarbonate compounds such as bis(tert-butylcyclohexyl)peroxydicarbonate; organic peroxides such as peroxyester compounds such as tert-butylperoxy-2-ethylhexanoate, tert-butylperoxybenzoate, and 2,5-dimethyl-2,5-di(benzoylperoxy)hexane; and azo compounds such as 2,2'-azobisisobutyronitrile and 1,1'-azobis(cyclohexane-1-carbonitrile).

The powder coating resin composition of the present invention contains the acrylic resin (A) described above, but it is preferable that it contains a curing agent (B) having a functional group capable of reacting with an epoxy group, as this further improves the coating film properties.

The curing agent (B) is a curing agent having a functional group capable of reacting with an epoxy group, and examples thereof include polycarboxylic acid compounds such as suberic acid, azelaic acid, 2,4-diethylglutaric acid, sebacic acid, undecanedicarboxylic acid, dodecanedicarboxylic acid, brassylic acid, tetradecanedicarboxylic acid, pentadecanedicarboxylic acid, hexadecanedicarboxylic acid, heptadecanedicarboxylic acid, octadecanedicarboxylic acid, eicosanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and butanetricarboxylic acid, anhydrides of these polycarboxylic acids, and polyhydric phenol compounds. Among these, aliphatic polycarboxylic acid compounds and their anhydrides are preferred, and dodecanedicarboxylic acid is more preferred, since they provide a coating film with high strength. These curing agents (B) can be used alone or in combination of two or more.

In the powder coating resin composition of the present invention, the blending amounts of the acrylic resin (A) and the curing agent (B) are such that a coating film with high strength can be obtained, and therefore the equivalent ratio (A/B) between the number of equivalents of epoxy groups in the acrylic resin (A) and the number of equivalents of functional groups capable of reacting with epoxy groups in the curing agent (B) is preferably 0.5 to 1.5, and more preferably 0.8 to 1.2.

Various known and commonly used additives such as organic or inorganic pigments, leveling agents, flow control agents, antifoaming agents, light stabilizers, ultraviolet absorbers, and antioxidants can be added to the powder coating resin composition of the present invention within the scope of the invention, without impairing the effects of the present invention. A catalyst can also be added to promote the curing reaction during baking.

The powder coating resin composition of the present invention may further contain silica and hydrolyzable silane compounds such as alkoxysilanes and silane coupling agents, within the range that does not impair the effects of the present invention, in order to improve the filament rust resistance of the coating film. These compounds may be used alone or in combination of two or more kinds.

Examples of suitable silane coupling agents include glycidylalkoxysilane and aminoalkoxysilane. Among these, glycidyltrialkoxysilane is preferred because it provides a coating film with excellent filamentary rust resistance, and glycidyltrimethoxysilane is more preferred.

The amount of the silane coupling agent in the powder coating resin composition is preferably 0.01 to 3 mass%, and more preferably 0.01 to 1 mass%, since this results in a coating film with excellent resistance to filamentary rust.

The powder coating of the present invention can be prepared by a variety of known and commonly used methods, including, for example, a method of mechanically grinding in which the acrylic resin (A), the curing agent (B), and, if necessary, various additives such as pigments and surface conditioners are mixed together, the mixture is then melt-kneaded, and then finely ground and classified.

The powder coating material of the present invention can be applied to exteriors, home appliances, automotive products, motorcycle products, safety fences, etc., but since it produces a coating film with a high appearance that is excellent in smoothness, curing properties, weather resistance, and thread rust resistance, it is also suitable for application to metal components such as aluminum wheel alloy components.

Methods for applying the powder coating of the present invention include various known and commonly used methods such as electrostatic powder coating. In addition, the method for forming a cured coating film after applying the powder coating of the present invention can be appropriately selected depending on the type of substrate and the purpose, but since a coating film with excellent weather resistance and filiform rust resistance can be obtained, it is preferable to bake at a temperature range of 120 to 250°C for a period of 5 to 30 minutes. In addition, the coating film thickness is preferably in the range of 50 to 200 μm.

The present invention will be described in more detail below with reference to specific examples. The epoxy equivalent and number average molecular weight of the acrylic resin were measured by the following method.

[Method for measuring epoxy equivalent]
Measured by hydrochloric acid-pyridine method. 25 ml of hydrochloric acid-pyridine solution was added to the resin, and it was dissolved by heating at 130°C for 1 hour. Then, it was titrated with 0.1N potassium hydroxide alcohol solution using phenolphthalein as an indicator. The epoxy equivalent was calculated from the amount of 0.1N potassium hydroxide alcohol solution consumed.

[Method for measuring number average molecular weight]
Measured by GPC.
Measurement device: High-speed GPC device ("HLC-8220GPC" manufactured by Tosoh Corporation)
Column: The following columns manufactured by Tosoh Corporation were used, connected in series.
"TSKgel G5000" (7.8mm I.D. x 30cm) x 1 "TSKgel G4000" (7.8mm I.D. x 30cm) x 1 "TSKgel G3000" (7.8mm I.D. x 30cm) x 1 "TSKgel G2000" (7.8mm I.D. x 30cm) x 1 Detector: RI (differential refractometer)
Column temperature: 40°C
Eluent: tetrahydrofuran (THF)
Flow rate: 1.0 mL/min Injection volume: 100 μL (sample concentration 4 mg/mL in tetrahydrofuran solution)
Standard sample: A calibration curve was prepared using the following monodisperse polystyrene.

(Monodisperse polystyrene)
"TSKgel Standard Polystyrene A-500" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene A-1000" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene A-2500" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene A-5000" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-1" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-2" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-4" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-20" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-40" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-80" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-128" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-288" manufactured by Tosoh Corporation
"TSKgel Standard Polystyrene F-550" manufactured by Tosoh Corporation

(Synthesis Example 1: Synthesis of acrylic resin (A-1))
A reaction vessel equipped with a stirrer, a thermometer, a condenser, and a nitrogen gas inlet was charged with 67 parts by mass of xylene, and the temperature was raised to 135° C. under a nitrogen atmosphere. A mixture consisting of 20 parts by mass of styrene (hereinafter abbreviated as “St”), 40 parts by mass of methyl methacrylate (hereinafter abbreviated as “MMA”), 40 parts by mass of 3,4-epoxycyclohexylmethyl methacrylate (hereinafter abbreviated as “ECHMMA”), and 6.0 parts by mass of t-butylperoxy 2-ethylhexanoate was added dropwise thereto over 6 hours, and the temperature was maintained for 6 hours after the end of the dropwise addition to carry out a polymerization reaction. Thereafter, the solvent was removed under a reduced pressure of 20 mmHg at 160° C. to obtain a solid acrylic resin (A-1) having a number average molecular weight of 3,000, a glass transition temperature of 73° C., and an epoxy equivalent of 490 g/eq.

(Synthesis Example 2: Synthesis of acrylic resin (A-2))
A solid acrylic resin (A-2) having a number average molecular weight of 4,000, a glass transition temperature of 62° C., and an epoxy equivalent of 650 g/eq was obtained by the same operation as in Synthesis Example 1, except that the composition of the monomers and polymerization initiator was changed to 15 parts by mass of St, 35 parts by mass of MMA, 20 parts by mass of butyl methacrylate (hereinafter, abbreviated as "BMA"), 30 parts by mass of ECHMMA, and 4.0 parts by mass of t-butylperoxy 2-ethylhexanoate.

(Synthesis Example 3: Synthesis of acrylic resin (A-3))
A solid acrylic resin (A-3) having a number average molecular weight of 2,500, a glass transition temperature of 67° C., and an epoxy equivalent of 580 g/eq was obtained by the same operation as in Synthesis Example 1, except that the composition of the monomers and polymerization initiator was changed to 30 parts by mass of St, 25 parts by mass of MMA, 15 parts by mass of BMA, 20 parts by mass of ECHMMA, 10 parts by mass of glycidyl methacrylate (hereinafter, abbreviated as “GMA”), and 8.0 parts by mass of t-butylperoxy 2-ethylhexanoate.

(Synthesis Example 4: Synthesis of acrylic resin (A-4))
A solid acrylic resin (A-4) having a number average molecular weight of 3,000, a glass transition temperature of 65° C., and an epoxy equivalent of 520 g/eq was obtained by the same operation as in Synthesis Example 1, except that the monomer composition was changed to 10 parts by mass of St, 40 parts by mass of MMA, 20 parts by mass of BMA, 10 parts by mass of ECHMMA, and 20 parts by mass of GMA.

(Synthesis Example 5: Synthesis of acrylic resin (RA-1))
A solid acrylic resin (RA-1) having a number average molecular weight of 3,000, a glass transition temperature of 66° C., and an epoxy equivalent of 470 g/eq was obtained by the same procedure as in Synthesis Example 1, except that the monomer composition was changed to 15 parts by mass of St, 35 parts by mass of MMA, 20 parts by mass of BMA, and 30 parts by mass of GMA.

(Synthesis Example 5: Synthesis of acrylic resin (RA-2))
A solid acrylic resin (RA-2) having a number average molecular weight of 3,000, a glass transition temperature of 62° C., and an epoxy equivalent of 390 g/eq was obtained by the same operation as in Synthesis Example 1, except that the monomer composition was changed to 15 parts by mass of St, 30 parts by mass of MMA, 10 parts by mass of BMA, 30 parts by mass of ECHMMA, and 15 parts by mass of GMA.

The monomer compositions of the acrylic resins (A-1) to (A-4) and (RA-1) to (RA-2) synthesized in Synthesis Examples 1 to 4 above are shown in Table 1.

(Example 1: Production and evaluation of powder coating material (1))
A resin composition containing 85.0 parts by mass of the acrylic resin (A-1) obtained in Synthesis Example 1, 15.0 parts by mass of dodecanedicarboxylic acid (hereinafter abbreviated as "DDDA"), 0.5 parts by mass of benzoin, and 0.3 parts by mass of a leveling agent ("Resiflow LF" manufactured by ESTRON) was melt-kneaded using a twin-screw kneader ("APV Kneader MP-2015" manufactured by TSUBACO Yokohama Sales Co., Ltd.), and then finely pulverized and further classified using a 200-mesh wire screen to obtain a powder coating material (1).

[Preparation of cured coating film for evaluation]
The powder coating material obtained above was electrostatically powder coated onto an untreated aluminum plate (A-1050P) (7 cm x 15 cm) so that the film thickness after baking would be 80 to 120 μm, and then baked at 170° C. for 20 minutes to produce a cured coating film for evaluation.

[Evaluation of smoothness]
The cured coating film for evaluation obtained above was judged using standard plates for visually judging the smoothness of powder coating films by PCI (Powder Coating Institute). There are 10 standard plates, numbered 1 to 10, with the larger the number, the better the smoothness. The smoothness of the prepared cured coating film for evaluation was judged by visual inspection to see which standard plate the film corresponded to, and the smoothness was evaluated.

[Evaluation of Curability]
0.5 g of the powder coating material obtained above was dropped onto a hot plate heated to 160° C., and the time until the tack disappeared (gel time) was measured. Based on the gel time, the curability was evaluated as follows.
◯: Gel time is less than 160 seconds △: Gel time is 160 seconds or more and less than 200 seconds ×: Gel time is 200 seconds or more

[Weather resistance evaluation]
The cured coating film for evaluation obtained above was exposed for 1,000 hours using a QUV ultraviolet fluorescent lamp type accelerated weathering tester (manufactured by Q-Lab Corporation, controlled wavelength 310 nm, light irradiation: 0.71 W/m2, 60°C; wet: humidity 90% or more, 50°C, light irradiation/wetting cycle = 4 hours/4 hours), and the specular reflectance (gloss value) (%) of the coating film after exposure was evaluated as the retention (gloss retention: %) of the specular reflectance (gloss value) of the cured coating film before exposure [(100 x specular reflectance of coating film after exposure)/(specular reflectance of cured coating film before exposure)]. A higher retention value indicates better weather resistance. The specular reflectance was measured using a Micro Trigloss manufactured by BYK Corporation.
◯: Gloss retention rate of 80% or more △: Gloss retention rate of 70% or more but less than 80% ×: Gloss retention rate of less than 70%

[Evaluation of thread rust resistance]
The cured coating film obtained above was cut with a cutter knife into two straight scratches of 13 cm each so as to reach the base material of the substrate, and the following test was performed using a CASS tester. Test 1 involved spraying saltwater (prepared by dissolving 2.6 g of copper (II) chloride hydrate, 10 cc of glacial acetic acid, and 500 g of ordinary salt in 10 L of ion-exchanged water) for 6 hours under conditions of a temperature of 50°C, a spray amount of 1.2 to 1.8 cc/h, and a spray pressure of 0.1 MPa, and Test 2 involved leaving the coating for 96 hours under conditions of a temperature of 60°C and a humidity of 85%, for a total of five cycles. After the CASS test was completed, the filamentous rust that had developed from the scratches on the coated plate was visually confirmed, and the filamentous rust resistance of the coating film was judged based on the length of the longest filamentous rust that had grown, as follows:
◯: The longest length of the rust thread is 2.0 mm or less. △: The longest length of the rust thread is more than 2.0 mm and less than 3.0 mm. ×: The longest length of the rust thread is more than 3.0 mm.

(Examples 2 to 4: Production and evaluation of powder coating materials (2) to (4))
Powder coating materials (2) to (4) were prepared in the same manner as in Example 1, except that the acrylic resin (A-1) blended in Example 1 was changed to acrylic resins (A-2) to (A-4), and various physical properties were evaluated.

(Comparative Examples 1-2: Production and Evaluation of Powder Coatings (R1)-(R2))
Powder coating materials (R1) to (R2) were prepared in the same manner as in Example 1, except that the acrylic resin (A-1) compounded in Example 1 was changed to acrylic resins (RA-1) to (RA-2), and various physical properties were evaluated.

The formulations and evaluation results of the powder coatings (1) to (4) obtained in Examples 1 to 4 above, and the powder coatings (R1) to (R2) obtained in Comparative Examples 1 and 2 are shown in Table 2.

It was confirmed that the cured coating films obtained from the powder coating resin compositions of the present invention in Examples 1 to 4 have excellent smoothness, curability, weather resistance, and filament rust resistance.

On the other hand, in Comparative Example 1, 3,4-epoxycyclohexylmethyl (meth)acrylate was not used as the monomer raw material for the acrylic resin, and it was confirmed that the filament rust resistance of the coating film was insufficient.

In addition, Comparative Example 2 is an example in which the epoxy equivalent of the acrylic resin is below 450 g/eq, and it was confirmed that the coating film had insufficient resistance to filamentous rust.

Claims (4)

A resin composition for powder coating containing an acrylic resin (A) whose essential raw materials are an acrylic monomer (a1) having an epoxy group including 3,4-epoxycyclohexylmethyl (meth)acrylate, a vinyl monomer (a2) having an aromatic ring, and an unsaturated monomer (a3) other than the monomers (a1) and (a2), wherein the monomer components that are the raw materials for the acrylic resin (A) contain 5 to 40 mass% 3,4-epoxycyclohexylmethyl (meth)acrylate, 10 to 40 mass% of the monomer (a2), and 20 to 70 mass% of the monomer (a3), and the epoxy equivalent of the acrylic resin (A) is 450 to 1000 g/eq. The resin composition for powder coating according to claim 1, further comprising a curing agent (B) having a functional group capable of reacting with an epoxy group. A powder coating obtained from the resin composition for powder coating according to claim 1 or 2. An article having a coating of the powder coating material according to claim 3.