Powder coating composition and method for forming a coating film

Abstract

The present invention provides a powder coating composition that exhibits good coating appearance and properties, as well as good storage stability, even when cured at low temperatures. [Solution] A powder coating composition comprising an acrylic resin (A) and a polyester resin (B) as a film-forming resin, wherein the acrylic resin (A) has epoxy groups, the weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less, the content of units derived from monomers having epoxy groups in the acrylic resin (A) is 20% by mass or more and 60% by mass or less, the polyester resin (B) has carboxyl groups, the acid value of the polyester resin (B) is 10 mg KOH / g or more and 35 mg KOH / g or less, and the absolute value |SP(A)-SP(B)| of the difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B) is 0.22 or less.

Description

【Technical Field】 【0001】 The present disclosure relates to a powder coating composition. Further, the present disclosure relates to a method for forming a coating film. 【Background Art】 【0002】 In recent years, with the growing awareness of reducing environmental impact and the demand for environmentally friendly products, powder coating compositions that do not contain organic solvents have attracted attention in coating compositions. 【0003】 Furthermore, powder coating compositions are relatively inexpensive, have excellent workability, for example, can recover and reuse excess paint, can thicken the coating film (cured product) by multiple layers of overcoating, and can be used immediately after coating. Therefore, there is a high demand as a coating composition for forming a coating film on electronic components, OA equipment, household appliances, building materials, automotive parts, etc. 【0004】 On the other hand, in powder coating compositions, in the process of coating and heating to form a cured coating film, the coating film is formed by heating and melting the powder coating composition. However, since the fluidity of the coating composition obtained by heating and melting is low, there is a problem that the smoothness of the coating film cannot be sufficiently obtained compared to solvent-based coating compositions. Therefore, it is necessary to heat at a high temperature above a certain temperature, and the environmental superiority of the powder coating composition that does not contain organic solvents has not been fully demonstrated. 【0005】 In response to such problems, Patent Document 1 describes a thermosetting powder coating obtained by dispersing a thermosetting resin solution containing a thermosetting resin and an organic solvent in an aqueous solution containing a water-soluble polymer to prepare a suspension, distilling off the organic solvent in the dispersed phase of the suspension to solidify the dispersed phase, and separating the solidified dispersed phase particles from the suspension. The thermosetting resin contains Resin A and Resin B, (SP value of Resin A) - (SP value of Resin B) is 0.1 to 1.0, the glass transition temperature of Resin A is 50 to 100 °C, the number average molecular weight is 2000 to 4000, and the value of (number average molecular weight / 100 + glass transition temperature) is 90 or more. The glass transition temperature of Resin B is 20 to 70 °C, the number average molecular weight is 1000 to 4000, and the value of (number average molecular weight / 100 + glass transition temperature) is 89 or less. The solid content weight ratio of Resin A / Resin B is 5 / 95 to 50 / 50, and the viscosity in the temperature increase test of the thermosetting powder coating at 140 °C is 40 mPa·s or less, etc. are described. 【Prior Art Documents】 【Patent Documents】 【0006】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 2003-105264 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0007】 However, in the powder coating composition described above, it has not been possible to achieve both storage stability such as the coating film appearance after storage (also referred to as resistance to solid-phase reaction) and blocking resistance, and the smoothness of the obtained coating film. 【0008】 The present disclosure aims to provide a powder coating composition that has good coating film appearance (smoothness) and coating film physical properties (resistance to weight drop) of the obtained coating film even when cured at a low temperature, and also has good storage stability (coating film appearance after storage (resistance to solid-phase reaction) and blocking resistance). Furthermore, the present disclosure provides a method for forming a coating film using the above powder coating composition. 【Means for Solving the Problems】 【0009】 To solve the aforementioned problems, this disclosure provides the following embodiments. [1] A powder coating composition comprising an acrylic resin (A) and a polyester resin (B) as film-forming resins, The acrylic resin (A) has epoxy groups, The weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less. In the acrylic resin (A) mentioned above, the content of units derived from monomers having epoxy groups is 20% by mass or more and 60% by mass or less. The polyester resin (B) has a carboxyl group, The acid value of the polyester resin (B) is 10 mg KOH / g or more and 35 mg KOH / g or less. A powder coating composition in which the absolute value of the difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B), |SP(A)-SP(B)|, is 0.22 or less. [2] The acid value of the aforementioned polyester resin (B) is expressed as W (mgKOH / g), In the acrylic resin (A), the content of units derived from monomers having epoxy groups is X (%). Let Y be the weight-average molecular weight of acrylic resin (A). The absolute difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B) is |SP(A)-SP(B)|, which is expressed as Z(cal / cm²). 3 ) 1 / 2 In that case, The following formula: Δf=(Y-4,500) / 1,000-1.552(|Z|+|W+X-60| / 200) The resin composition according to [1], wherein the Δf calculated by is between -1.29 and 1.90. [3] The solubility parameter SP(A) of the aforementioned acrylic resin (A) is 9.0 (cal / cm³). 3) 1 / 2 11.0 (cal / cm or less as described above 3 ) 1 / 2 The powder coating composition according to [1] or [2], which is as follows [4] The epoxy equivalent of the acrylic resin (A) is 240 g / eq or more and 710 g / eq or less. The powder coating composition according to any one of [1] to [3]. [5] The glass transition temperature (Tg A ) of the acrylic resin (A) is 0°C or more and 65°C or less. The powder coating composition according to any one of [1] to [4]. [6] The weight average molecular weight of the polyester resin (B) is 5,000 or more and 100,000 or less. The powder coating composition according to any one of [1] to [5]. [7] The solubility parameter SP(B) of the polyester resin (B) is 10.0 (cal / cm 3 ) 1 / 2 or more and 11.5 (cal / cm 3 ) 1 / 2 or less. The powder coating composition according to any one of [1] to [6]. [8] The glass transition temperature (Tg B ) of the polyester resin (B) is 40°C or more and 70°C or less. The powder coating composition according to any one of [1] to [7]. [9] The minimum melt viscosity is 1 Pa·s or more and 400 Pa·s or less. The powder coating composition according to any one of [1] to [8].

[10] A method for forming a coating film, wherein the powder coating composition according to any one of [1] to [9] is applied onto an object to be coated and heated to form a cured coating film. The method for forming a coating film, wherein the heating is performed at a temperature of 140°C or more and 200°C or less for the object to be coated. [[ID=U+3000]]【Effect of the Invention】 【0010】 According to this disclosure, it is possible to provide a powder coating composition that, even when cured at low temperatures, exhibits good coating film appearance (smoothness) and coating film properties (resistance to weight drop), as well as good storage stability (coating film appearance after storage (resistance to solid phase reactivity) and resistance to blocking). Furthermore, this disclosure provides a method for forming a coating film using the powder coating composition. [Modes for carrying out the invention] 【0011】 The powder coating composition disclosed herein comprises an acrylic resin (A) and a polyester resin (B) as a film-forming resin. The acrylic resin (A) has epoxy groups, The weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less. In the acrylic resin (A) mentioned above, the content of units derived from monomers having epoxy groups is 20% by mass or more and 60% by mass or less. The polyester resin (B) has a carboxyl group, The acid value of the polyester resin (B) is 10 mg KOH / g or more and 35 mg KOH / g or less. The absolute difference |SP(A)-SP(B)| between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B) is 0.22 or less. 【0012】 The powder coating compositions of this disclosure exhibit good coating film appearance (smoothness) and coating film properties (resistance to weight drop) even when cured at low temperatures, and also exhibit good storage stability (coating film appearance after storage (resistance to solid-phase reactivity) and resistance to blocking). While this disclosure should not be interpreted as being limited to any particular theory, the reasons why the powder coating compositions of this disclosure can achieve such effects are thought to be as follows. 【0013】 In other words, the powder coating composition of this disclosure contains an acrylic resin and a polyester resin of a predetermined molecular weight that can react with each other and harden, as film-forming resins, and enhances the compatibility of the acrylic resin and the polyester resin, while keeping the amount of epoxy groups in the acrylic resin and the amount of carboxyl groups in the polyester resin within a predetermined range. Therefore, it is believed that even when cured at low temperatures, the coating film will have good appearance and properties, and will also have good storage stability. Furthermore, since the compatibility of acrylic resins and polyester resins, as well as the amounts of epoxy and carboxyl groups, are interdependent, it is practically difficult to change them as independent parameters. In this disclosure, by setting the molecular weight and epoxy group amount of the acrylic resin, the acid value of the polyester resin, and the compatibility of the acrylic and polyester resins within predetermined ranges, these physical properties can be balanced, resulting in a powder coating composition with good storage stability, good coating appearance and physical properties, and good storage stability. 【0014】 [Acrylic resin (A)] Acrylic resin refers to a polymer having units derived from monomers having (meth)acryloyl groups, and can be prepared by polymerizing a monomer mixture containing vinyl monomers, but is not limited to this. 【0015】 The acrylic resin (A) of this disclosure has epoxy groups. The acrylic resin (A), for example, is obtained by copolymerizing a monomer containing at least one epoxy group with another copolymerizable vinyl monomer. 【0016】 The monomer having an epoxy group is preferably a monomer having a glycidyl group, specifically, glycidyl (meth)acrylate and β-methylglycidyl (meth)acrylate. Glycidyl (meth)acrylate is preferred as the monomer having an epoxy group. These may be used individually or in combination of two or more. 【0017】 In the acrylic resin (A), the content of units derived from the monomer having epoxy groups is 20% by mass or more and 60% by mass or less, preferably 30% by mass or more and 50% by mass or less, and more preferably 35% by mass or more and 45% by mass or less, based on 100% by mass of the total amount of acrylic resin (A). The higher the amount of monomer having epoxy groups, the better the low-temperature curing properties, and the lower the amount of monomer having epoxy groups, the better the storage stability. Furthermore, the content of units derived from monomers having epoxy groups in acrylic resin (A) is equivalent to the content of monomers having epoxy groups contained in the monomer mixture. 【0018】 Other vinyl monomers copolymerizable with the polymerizable monomer having the epoxy group may have at least one unsaturated bond, such as a vinyl group, in their molecule, and include derivatives of acrylic acid and methacrylic acid. 【0019】 The vinyl monomer is not particularly limited and includes, for example, ethylene-based unsaturated carboxylate alkyl monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; cycloalkyl group-containing polymerizable monomers such as cyclopentyl (meth)acrylate and cyclohexyl (meth)acrylate; and 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxyethyl (meth)acrylate with ε-caprolactyl Examples include hydroxyl group-containing ethylene-based unsaturated carboxylic acid alkyl ester monomers such as reaction products with tons; other amide group-containing ethylene-based unsaturated carboxylic acid monomers such as acrylamide, methacrylamide, N-methylolacrylamide, methoxybutylacrylamide, and diacetoneacrylamide; vinyl cyanide monomers such as (meth)acrylonitrile and α-chloroacrylonitrile; saturated aliphatic carboxylic acid vinyl ester monomers such as vinyl acetate and vinyl propionate; and styrene monomers such as styrene, α-methylstyrene, and vinyltoluene. These can be used individually or in combination of two or more types. In this specification, (meth)acrylic acid means acrylic acid or methacrylic acid. 【0020】 The epoxy equivalent of the acrylic resin (A) is preferably 240 g / eq to 710 g / eq, and more preferably 370 g / eq to 420 g / eq. By having the epoxy equivalent of the acrylic resin (A) within the above range, a coating film with sufficient coating hardness can be formed. Furthermore, it is believed that the larger the epoxy equivalent, the more sufficiently the curing reaction between the acrylic resin (A) and the polyester resin (B) proceeds, and the smaller the epoxy equivalent, the better the appearance of the resulting coating film. The epoxy equivalent in this disclosure can be measured by a method in accordance with JIS K 7236. 【0021】 In one embodiment, the weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less, preferably 3,600 or more and 6,400 or less, and more preferably 3,600 or more and 5,500 or less. Having the weight-average molecular weight of the acrylic resin (A) within this range has the advantage of resulting in a paint composition with good resistance to solid-phase reactivity. In another embodiment, the weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less, preferably more than 4,000 and 8,000 or less, and more preferably 4,100 or more and 6,400 or less. Having the weight-average molecular weight of the acrylic resin (A) within this range has the advantage of resulting in a paint composition with good resistance to solid-phase reactivity. In this disclosure, weight-average molecular weight refers to the weight-average molecular weight calculated on a styrene homopolymer basis using gel permeation chromatography (GPC). 【0022】 The solubility parameter SP(A) of the acrylic resin (A) is preferably 9.00 (cal / cm³). 3 ) 1 / 2 More than 11.00(cal / cm 3 ) 1 / 2 More preferably, 10.00 (cal / cm³) 3 ) 1 / 2More than 11.00(cal / cm 3 ) 1 / 2 More preferably, 10.55 (cal / cm³) 3 ) 1 / 2 More than 10.85(cal / cm 3 ) 1 / 2 The following is true: Since the acrylic resin (A) has SP(A) within this range, it can exhibit good compatibility with the polyester resin (B) according to this disclosure. Furthermore, reaction control can be easily performed compared to conventional powder coating compositions. 【0023】 The absolute difference |SP(A)-SP(B)| between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B) is 0.22 (cal / cm³). 3 ) 1 / 2 The following is preferable: 0 (cal / cm²) 3 ) 1 / 2 More than 0.15(cal / cm 3 ) 1 / 2 More preferably 0 (cal / cm 3 ) 1 / 2 More than 0.10(cal / cm 3 ) 1 / 2 More preferably, the following is 0 (cal / cm²). 3 ) 1 / 2 0.05(cal / cm or more) 3 ) 1 / 2 The following applies: 【0024】 Because the absolute value falls within the range described above, the compatibility between the acrylic resin (A) and the polyester resin (B) is good, and even when the powder coating composition is cured at low temperatures, the appearance (smoothness) and physical properties (resistance to dropping weights) of the resulting coating film are good, and the storage stability of the powder coating composition (appearance of the coating film after storage (resistance to solid-phase reactivity) and resistance to blocking) is also good. 【0025】 SP value is an abbreviation for solubility parameter, and it is a measure of solubility. A higher SP value indicates higher polarity, while a lower SP value indicates lower polarity. 【0026】 In this disclosure, the SP value of the resin can be measured by the following method [Reference: SUH, CLARKE, JPSA-1, 5, 1671~1681 (1967)]. 【0027】 As a sample, 0.5 g of resin is weighed into a 100 ml beaker, 10 ml of acetone is added using a volumetric pipette, and the resin is dissolved using a magnetic stirrer. A poor solvent is added dropwise to this sample using a 50 ml burette at a measurement temperature of 20°C, and the point at which turbidity occurs is recorded as the volume added. Deionized water is used as the high-SP poor solvent, and n-hexane is used as the low-SP poor solvent; turbidity is measured for each. The SP value of the organic solvent is δ (cal / cm²). 3 ) 1 / 2 It is given by the following formula. δ=(V ml 1 / 2 δ ml +V mh 1 / 2 δ mh ) / (V ml 1 / 2 +V mh 1 / 2 ) V m =V1V2 / (φ1V2+φ2V1) δ m =φ1δ1+φ2δ2 V i : Molecular volume of solvent (ml / mol) φ i : Volume fraction of each solvent at the turbidity point δ i : SP value of the solvent ml: Low SP poor solvent mixed system mh: High SP poor solvent mixed system 【0028】 Furthermore, if the acrylic resin (A) contains multiple types of acrylic resin (A), the SP value of the acrylic resin (A) can be determined by calculating a weighted average value based on the solid content mass ratio in the acrylic resin (A) components, using the SP values ​​of each monomer. 【0029】 Glass transition temperature (Tg) of acrylic resin (A) A The glass transition temperature is not limited to, but is preferably 0°C to 65°C, and more preferably 0°C to 55°C. By having the glass transition temperature within the above range, a coating film with excellent coating properties can be formed. The glass transition temperature can be measured using a differential scanning calorimeter (DSC) in accordance with JIS K 7121. 【0030】 The amount of acrylic resin (A) in this disclosure is determined by the mixing ratio with polyester resin (B). Specifically, the epoxy equivalent of acrylic resin (A) is preferably 0.7 equivalents or more and 1.5 equivalents or less, more preferably 1 equivalent or more and 1.5 equivalents or less, and even more preferably 1 equivalent or more and 1.3 equivalents or less, relative to 1 equivalent of carboxyl groups of polyester resin (B). By having the amounts of acrylic resin (A) and polyester resin (B) within the above range, the coating film formed from the powder coating composition can be provided with excellent mechanical strength, insulation, flexibility, heat resistance, corrosion resistance, chemical resistance, etc. 【0031】 The method for preparing the acrylic resin (A) having epoxy groups is not particularly limited, as long as it involves copolymerizing the monomer having at least one epoxy group with other vinyl monomers copolymerizable with them. For example, various known methods can be used, and a method in which the various monomers are subjected to a radical polymerization reaction in solution and then desolvented to obtain the desired polymer is preferred in that it allows for easy adjustment of the molecular weight. 【0032】 [Polyester resin (B)] Polyester resin refers to a polymer having ester bonds within its molecule. Typically, polyester resin may be a condensate of a polyol and a polybasic acid, and / or a modified version of said condensate. 【0033】 The polyester resin (B) of this disclosure has carboxyl groups. The polyester resin (B) can be prepared, for example, by condensing a polyol having carboxyl groups with a polybasic acid, but is not limited thereto. 【0034】 Examples of the polyols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6-hexanediol, 2,3-pentanediol, 1,4-hexanediol, 1,5-hexanediol, 2,5-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, 1,2-dodecanediol, 1,2- Examples include linear or branched aliphatic glycols such as octadecanediol, diethylene glycol, triethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, bisphenol A alkylene oxide adduct, and bisphenol S alkylene oxide adduct; trihydric or higher polyhydric alcohols such as trimethylolpropane, glycerin, and pentaerythritol; and 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolbutyric acid, and 2,2-dimethylolvaleric acid. One polyol may be used alone, or two or more may be used in combination. 【0035】 Examples of the polybasic acid include, for example, aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid and their anhydrides, trivalent or higher aromatic polycarboxylic acids such as trimellitic acid and their anhydrides, saturated aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid and their anhydrides, lactones such as γ-butyrolactone and ε-caprolactone, and aromatic oxymonocarboxylic acids such as p-oxyethoxybenzoic acid. Preferably, terephthalic acid and isophthalic acid are used. One or more of the acid components can be used. 【0036】 In one embodiment, the total proportion of terephthalic acid and isophthalic acid in the polybasic acid is preferably 70 mol% to 100 mol%, more preferably 75 mol% to 100 mol%, and even more preferably 80 mol% to 100 mol%, of the total amount of polybasic acid (100 mol%). This range of total proportion is preferable in terms of durability, physical properties, and cost. Here, when the total proportion of terephthalic acid and isophthalic acid used as the acid component is 70 mol% or more, it specifically means that these are used as the main raw materials. 【0037】 In a preferred embodiment, the proportion of isophthalic acid is preferably 70 mol% to 100 mol%, more preferably 80 mol% to 100 mol%, and even more preferably 90 mol% to 100 mol%, of the total amount of polybasic acid (100 mol%). Having the proportion of isophthalic acid within this range results in good weather resistance of the resulting coating film. 【0038】 Examples of the modified material include modified polyester resins such as urethane-modified polyester resin, epoxy-modified polyester resin, acrylic-modified polyester resin, and silicone-modified polyester resin. 【0039】 The acid value of the polyester resin (B) is 10 mg KOH / g or more and 35 mg KOH / g or less, preferably 15 mg KOH / g or more and 30 mg KOH / g or less, and more preferably 17 mg KOH / g or more and 23 mg KOH / g or less. The higher the acid value of the polyester resin (B), the better the low-temperature curability, and the lower the acid value of the polyester resin (B), the better the storage stability, especially the resistance to solid-phase reactivity. In this disclosure, the acid value and hydroxyl value represent the solids acid value and solids hydroxyl value, respectively, and can be measured in accordance with JIS K 0070:1999. 【0040】 The solubility parameter SP(B) of the polyester resin (B) is preferably 10.0 (cal / cm³). 3 ) 1 / 2 More than 11.5(cal / cm 3 ) 1 / 2 More preferably 10.4 (cal / cm³) 3 ) 1 / 2 More than 11.0(cal / cm 3 ) 1 / 2 More preferably, 10.5 (cal / cm²) 3 ) 1 / 2 More than 10.9(cal / cm 3 ) 1 / 2 The following is true: Because SP(B) is within the applicable range, it has good compatibility with acrylic resin (A), and furthermore, reaction control can be performed more easily compared to conventional powder coating compositions. 【0041】 The carboxyl group equivalent of the polyester resin (B) is preferably 1,000 g / eq to 6,000 g / eq, more preferably 1,000 g / eq to 3,800 g / eq, even more preferably 1,600 g / eq to 3,000 g / eq, even more preferably 1,800 g / eq to 3,000 g / eq, and even more preferably 1,800 g / eq to 2,800 g / eq. By having the carboxyl group equivalent within the above range, a coating film with excellent smoothness and adhesion between the coating film and the substrate, such as water resistance and adhesion after moisture resistance treatment, can be formed. Furthermore, a coating film with excellent physical properties, such as impact resistance, cupping resistance, and scratch resistance, can be formed. In this disclosure, the carboxyl group equivalent represents the carboxyl group equivalent of the solid content and can be measured and calculated in accordance with the method for measuring acid value described in JIS K 0070. 【0042】 The weight-average molecular weight of the polyester resin (B) is preferably 5,000 to 100,000, more preferably 5,000 to 50,000, even more preferably 5,000 to 30,000, and even more preferably 5,000 to 20,000. Having the weight-average molecular weight of the polyester resin (B) within the above range can improve the properties of the coating film. 【0043】 Glass transition temperature (Tg) of polyester resin (B) B The glass transition temperature (Tg) is preferably 40°C to 70°C, more preferably 45°C to 65°C, and even more preferably 50°C to 65°C. B As long as the value is within the above range, a coating film with good smoothness can be obtained. In this disclosure, the glass transition temperature can be measured using a differential scanning calorimeter (DSC) in accordance with JIS K 7121. 【0044】 The melt viscosity of polyester resin (B) at 200°C is preferably 1 Pa·s to 15 Pa·s, more preferably 1 Pa·s to 12 Pa·s, and even more preferably 1 Pa·s to 10 Pa·s. By having the melt viscosity of polyester resin (B) at 200°C within the above range, a coating film can be formed that has excellent coating film appearance, such as smoothness, and excellent coating film properties such as impact resistance. The aforementioned melt viscosity refers to the viscosity exhibited by a chain polymer in a molten state, and is measured using a dynamic viscoelasticity measuring device, such as the Rheosol-G3000 (manufactured by UBM), under the measurement conditions of a frequency of 2 Hz and a torsion angle of 0.5°. 【0045】 In the powder coating composition of this disclosure, the total solid content of the acrylic resin (A) and polyester resin (B) is preferably 80% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass, out of 100% by mass of the total solid content of the powder coating composition. 【0046】 In this disclosure, the solid content of a component means the residue after heating the component at 150°C for 1 hour. 【0047】 The powder coating compositions of this disclosure may contain other resin components in addition to the acrylic resin (A) and polyester resin (B), to the extent that the effects of this disclosure are not impaired. Examples of such resin components include acrylic resins other than acrylic resin (A), polyester resins other than polyester resin (B), epoxy resins, and the like. 【0048】 In one embodiment, the powder coating composition of the present disclosure may include, as other acrylic resins, an acrylic resin obtained by copolymerizing at least one monomer having a carboxyl group and / or a monomer having a hydroxyl group with another vinyl monomer copolymerizable with the monomer having an epoxy group. Note that the other acrylic resin differs from acrylic resin (A) in that it does not have an epoxy group. 【0049】 Examples of monomers having a carboxyl group include (meth)acrylic acid. Examples of monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and hydroxyl group-containing ethylene-based unsaturated carboxylate alkyl monomers such as reaction products of 2-hydroxyethyl (meth)acrylate and ε-caprolactone. Monomers having the functional group can be used individually or in combination of two or more. 【0050】 In one embodiment, the powder coating composition of the present disclosure may include a polyester resin having a hydroxyl group as the other polyester resin. The other polyester resin may consist of only one type, or two or more types. The other polyester resin differs from polyester resin (B) in that it does not have a carboxyl group. 【0051】 In one embodiment, the powder coating composition of the present disclosure may contain an epoxy resin. Examples of the epoxy resin include epoxy resins having two or more epoxy groups in one molecule. Specifically, these include glycidyl ester resins; glycidyl ether type resins such as condensation reaction products of bisphenol A and epichlorohydrin, and condensation reaction products of bisphenol F and epichlorohydrin; and alicyclic epoxy resins, linear aliphatic epoxy resins, brom-containing epoxy resins, phenol novolac type epoxy resins, cresol novolac type epoxy resins, and the like. 【0052】 The solid content of the other resin components is preferably 5 parts by mass or more and 30 parts by mass or less, and more preferably 5 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the total solid content of the acrylic resin (A) and the polyester resin (B). In this disclosure, the sum of the solid content of the acrylic resin (A) and the polyester resin (B) is also simply referred to as the resin solid content. 【0053】 [Hardening agent] The powder coating compositions of this disclosure may contain a curing agent component, to the extent that the effects of this disclosure are not impaired. Including a curing agent can further improve low-temperature curing properties and coating film quality. 【0054】 Examples of the curing agents include acid curing agents such as aliphatic polycarboxylic acids such as decanedicarboxylic acid, dodecanedicarboxylic acid, and sebacic acid, acid anhydrides of polycarboxylic acids, and acid group-containing acrylic resins; blocked isocyanate curing agents obtained by blocking nurate compounds of isocyanate compounds such as hexamethylene diisocyanate, isophorone diisocyanate, and xylylene diisocyanate with blocking agents such as ε-caprolactam and methyl ethyl ketone oxime; self-blocking type isocyanate curing agents having uretdione bonds in the molecule; amine curing agents such as aliphatic polyamines, polyaminoamides, ketimines, alicyclic diamines, aromatic diamines, imidazoles, dicyandiamides, polyamides, and β-hydroxyalkylamides (HAA); phenolic resin curing agents; and the like. 【0055】 The curing agent content is preferably 0.5 parts by mass or more and 10 parts by mass or less, more preferably 0.5 parts by mass or more and 5 parts by mass or less, even more preferably 0.6 parts by mass or more and 3.5 parts by mass or less, and even more preferably 0.7 parts by mass or more and 3 parts by mass or less, per 100 parts by mass of the solid content of the polyester resin (B). When the curing agent content is within the above range, a coating film with good gloss, adhesion, etc., can be formed, and furthermore, a coating film with good flexibility, etc., can be formed. 【0056】 [Curing catalyst] The powder coating composition of this disclosure may further contain a curing catalyst. By including a curing catalyst, the gel time of the powder coating composition can be adjusted. For example, the gel time at 160°C can be set to 50 seconds or more and 100 seconds or less. The gel time of the powder coating compositions described herein is the value at 160°C, measured in accordance with JIS K 5600-9-1 (Method for measuring the gel time of thermosetting powder coatings at a specified temperature). 【0057】 The content of the curing catalyst is preferably 0 to 20 parts by mass, more preferably 0 to 15 parts by mass, and even more preferably 0 to 10 parts by mass, per 100 parts by mass of the resin solids. By having the curing catalyst content within this range, the curing speed of the powder coating composition can be controlled, making it applicable to various substrates and painting / drying equipment. 【0058】 The curing catalyst can be appropriately selected depending on the purpose. For example, the curing catalyst is at least one selected from imidazole compounds, imidazoline compounds and metal salt complexes thereof, tertiary phosphine compounds, quaternary phosphonium salt compounds, and quaternary ammonium salt compounds. 【0059】 Imidazole compounds are not particularly limited, but examples include alkylimidazoles such as 2-ethyl-4-methylimidazole, 1-methylimidazole, 1,2-dimethylimidazole, 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, and 2-isopropylimidazole; carbamyl alkyl-substituted imidazoles such as 1-(2-carbamylethyl)imidazole; and 1-cyanoethyl-2-methylimidazole. Examples of imidazoles include cyanoalkyl-substituted imidazoles such as 2-phenylimidazole, 2-phenyl-4-methylimidazole, and 1-benzyl-2-methylimidazole, as well as aromatic-substituted imidazoles such as 1-vinyl-2-methylimidazole, allyl-substituted imidazoles such as 1-allyl-2-ethyl-4-methylimidazole, and polyimidazoles. However, alkylimidazoles and aromatic-substituted imidazoles are preferred. Commercially available products may also be used. Examples of commercially available products include the Curazol series (manufactured by Shikoku Chemicals Co., Ltd.), such as 2MZ-H (2-methylimidazole), C11Z (2-undecylimidazole), C17Z (2-heptadecylimidazole), 1,2DMZ (1,2-dimethylimidazole), 2E4MZ (2-ethyl-4-methylimidazole), 2P4MZ (2-phenyl-4-methylimidazole), 1B2MZ (1-benzyl-2-methylimidazole), and 1B2PZ (1-benzyl-2-phenylimidazole). 【0060】 The imidazoline compounds are not particularly limited, but examples include 2-phenylimidazole, 2-methylimidazole, 2-undecylimidazole, and 2-heptadecylimidazole. Commercial products may also be used. Examples of commercial products include Curesol 2PZL-T (manufactured by Shikoku Chemicals, Inc.; 2-phenylimidazole). 【0061】 Examples of metal salt complexes include those obtained by compounding the imidazole compounds or imidazoline compounds with a metal salt. The metal salts are not particularly limited, but examples include those consisting of metals such as copper, nickel, cobalt, calcium, zinc, zirconium, silver, chromium, manganese, tin, iron, titanium, antimony, and aluminum, and salts such as chloride, bromide, fluoride, sulfate, nitrate, acetate, malate, stearate, benzoate, and methacrylate. Tertiary phosphine compounds are not particularly limited, but examples include triphenylphosphine and tritlylphosphine. 【0062】 The quaternary phosphonium salt compounds are not particularly limited, but examples include benzyltriphenylphosphonium chloride, butyltriphenylphosphonium bromide, ethyltriphenylphosphonium iodide, and ethyltriphenylphosphonium bromide. 【0063】 The quaternary ammonium salt compounds are not particularly limited, but examples include tetraethylammonium chloride, tetraethylammonium bromide, and benzyltrimethylammonium bromide. 【0064】 In one embodiment, the curing catalyst is at least one of imidazole compounds and imidazoline compounds. The powder coating compositions of this disclosure can be cured at even lower temperatures by including these curing catalysts. 【0065】 [Other ingredients] The powder coating composition may include an acrylic resin (A), a polyester resin (B), and other resin components, curing agents, and curing catalysts as needed, as well as coloring pigments and extender pigments that have substantially no coloring power. Extender pigments are effective in adjusting the specific gravity of the powder coating composition and include talc, silica, calcium carbonate, and barium sulfate. The powder coating composition may also contain, as appropriate, known additives that can be used in ordinary powder coating compositions, such as surface modifiers, UV absorbers, antioxidants, and anti-wrinkle agents. 【0066】 (Coloring agent) In one embodiment, the powder coating composition of this disclosure may contain a colorant. Furthermore, known inorganic pigments and organic pigments commonly used in powder coating compositions can be used as the colorant included in the powder coating composition. 【0067】 Examples of chromatic inorganic pigments include red iron oxide, chromium titanium yellow, and yellow iron oxide, while examples of achromatic inorganic pigments include titanium dioxide and carbon black. Examples of chromatic organic pigments include azo, perylene, condensed azo, nitro, nitroso, phthalocyanine, anthraquinone, quinacridone, and dioxane pigments. Specifically, examples of azo pigments include lake red, fast yellow, disazo yellow, and permanent red; examples of nitro pigments include naphthol yellow; examples of nitroso pigments include pigment green B and naphthol green; examples of phthalocyanine pigments include phthalocyanine blue and phthalocyanine green; examples of anthraquinone pigments include induthlene blue and diantraquinonyl red; examples of quinacridone pigments include quinacridone red and quinacridone violet; and examples of dioxane pigments include carbazole dioxazine violet. 【0068】 The colorant content in the powder coating composition is preferably 0.05 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the resin solids. In particular, the inorganic pigment content is preferably 0.05 parts by mass or more and 60 parts by mass or less per 100 parts by mass of the resin solids. The organic pigment content is preferably 0.05 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the resin solids. 【0069】 In one embodiment, the powder coating composition of the present disclosure preferably further comprises an inorganic filler. The inorganic filler can improve chemical resistance by contributing to the blocking of corrosive factors. Examples of inorganic fillers include extender pigments such as alumina, silica, precipitated barium sulfate, calcium carbonate, clay, talc, and mica; and rust-preventive pigments such as zinc phosphate and aluminum phosphate. 【0070】 The amount of inorganic filler is preferably 5 to 60 parts by mass, more preferably 5 to 50 parts by mass, and even more preferably 5 to 35 parts by mass, per 100 parts by mass of the resin solids. 【0071】 [Powder coating composition] The powder coating composition of this disclosure exhibits good coating film appearance (smoothness) and coating film properties (resistance to dropping weights) even when cured at low temperatures, and also exhibits good storage stability (coating film appearance after storage (resistance to solid phase reactivity) and resistance to blocking). 【0072】 The powder coating compositions disclosed herein are The acid value of the aforementioned polyester resin (B) is expressed as W (mgKOH / g), In the acrylic resin (A), the content of units derived from monomers having epoxy groups is X (%). Let Y be the weight-average molecular weight of acrylic resin (A). When Z is the absolute value |SP(A)-SP(B)| of the difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B), The following formula: Δf=(Y-4,500) / 1000-1.552(|Z|+|W+X-60| / 200) The Δf calculated is preferably between -1.29 and 1.90, more preferably between -1.29 and 1.00, and even more preferably between -0.73 and 1.00. When Δf is within the above range, even when cured at low temperatures, the appearance (smoothness) and physical properties (resistance to dropping weights) of the resulting coating film are better, and the storage stability (appearance of the coating film after storage (resistance to solid-phase reactivity) and resistance to blocking) is also better. Note that Δf is a dimensionless quantity. 【0073】 The average particle size of the powder coating composition of this disclosure is not particularly limited, but is preferably 15 μm or more and 200 μm or less, more preferably 25 μm or more and 170 μm or less, and even more preferably 25 μm or more and 150 μm or less. 【0074】 The average particle size can be selected to a desirable range depending on the coating method, for example. For example, when electrostatic coating is performed, the average particle size of the powder coating composition is preferably 15 μm or more and 50 μm or less, more preferably 25 μm or more and 40 μm or less, and even more preferably 25 μm or more and 35 μm or less. When electrostatic coating is performed, having such an average particle size allows the formed coating film to have excellent smoothness. 【0075】 When performing fluid dipping coating, the average particle size of the powder coating composition is preferably 50 μm to 200 μm, more preferably 80 μm to 170 μm, and even more preferably 100 μm to 150 μm. Having such an average particle size allows for excellent smoothness when performing fluid dipping coating. 【0076】 As described above, the average particle size of the powder coating composition of this disclosure can be selected depending on the coating method. In any embodiment, if the average particle size of the powder coating composition is within this range, the appearance (smoothness) and physical properties (resistance to dropping weights) of the resulting coating film are good. 【0077】 In this disclosure, average particle diameter refers to volume-average particle diameter (D50). Volume-average particle diameter (D50) can be measured using a particle size analyzer, such as a laser diffraction / scattering particle size distribution analyzer (Microtrac, manufactured by Nikkiso Co., Ltd.). Specifically, it refers to the value measured using the "Microtrac MT3000II" (manufactured by Nikkiso Co., Ltd.) as the measuring device. In this disclosure, the average particle size of the powder coating composition refers to the average particle size of the powder coating composition containing an acrylic resin (A) and a polyester resin (B). 【0078】 The minimum melt viscosity of the powder coating composition is preferably 1 Pa·s to 400 Pa·s, more preferably 1 Pa·s to 200 Pa·s. By having a specific minimum melt viscosity at a specific temperature, the melting rate of the coating composition can be controlled. Therefore, when the powder coating composition of this disclosure is applied to a part with a complex shape, for example, by a fluid immersion method or an electrostatic powder coating method, a coating film with a uniform thickness can be formed without the occurrence of uneven thickness or stringing. In this disclosure, the minimum melt viscosity is the minimum viscosity obtained when the temperature is raised from 110°C to 160°C at a heating rate of 5°C / min, and can be measured, for example, with a dynamic viscoelasticity measuring device (Rheosol-G3000; manufactured by UBM). 【0079】 In the powder coating composition of this disclosure, the glass transition temperature (Tg) of the film-forming resin is preferably 40°C to 70°C, more preferably 40°C to 65°C. By having the glass transition temperature of the film-forming resin within this range, a coating film with excellent smoothness and adhesion to the substrate, such as water resistance and adhesion after moisture treatment, can be formed. Furthermore, a coating film with excellent physical properties, such as impact resistance and cupping resistance, can be formed. In addition, even when cured at a lower temperature, the powder coating composition of this disclosure can form a coating film with excellent smoothness and excellent physical properties, such as impact resistance, cupping resistance, and scratch resistance, thus reducing the environmental impact. 【0080】 [Method for manufacturing powder coating composition] The powder coating compositions of this disclosure can be manufactured by known methods. For example, the powder coating compositions of this disclosure are manufactured by preparing raw materials consisting of the above-mentioned components, pre-mixing the raw materials using a supermixer, Henschel mixer, etc., and then melt-kneading the raw materials using a kneader, extruder, etc. 【0081】 Melt mixing is carried out at a temperature at which at least a portion of the raw materials are melted and the whole mixture is kneaded. The temperature during melt mixing is generally between 80°C and 130°C, and in some embodiments, between 80°C and 120°C. 【0082】 The resulting molten material is cooled and solidified using cooling rolls, a cooling conveyor, etc., and then ground to the desired particle size through coarse grinding and fine grinding processes. Grinding can be carried out by physical grinding (coarse grinding, fine grinding), for example, using grinding equipment such as a hammer mill or a jet impact mill. Next, classification is performed as desired. For example, it is possible to remove large and small particles and adjust the particle size distribution. Air classifiers, vibrating sieves, and ultrasonic sieves are used for classification. 【0083】 [Coating film formation method] A method for forming a coating film using the powder coating composition of this disclosure is also included in the technical scope of this disclosure. Such a method for forming a coating film includes applying the powder coating composition of this disclosure onto an object to be coated and heating to form a cured coating film, wherein the heating is performed when the temperature of the object to be coated is 140°C or higher and 200°C or lower. 【0084】 (subject to be coated) The substrates to which the powder coating composition of this disclosure is applied are not particularly limited. Examples of substrates include iron plates, steel plates, aluminum plates, ceramic plates, and those with surface treatments. While a single layer of the powder coating composition of this disclosure provides good protective function, the powder coating composition may also be applied as a topcoat over an undercoat. Known undercoats such as electrodeposition paints and primers can be used to form the undercoat. Furthermore, components processed into complex shapes can also be used. 【0085】 For coating a workpiece, the powder coating composition of this disclosure may be applied directly to a steel plate or the like, or, for example, the powder coating composition of this disclosure may be applied as a topcoat over an undercoat. Known undercoats such as electrodeposition paints and primers can be used to form the undercoat. 【0086】 The method for applying the powder coating composition is not particularly limited, and methods well known to those skilled in the art, such as spray coating, electrostatic powder coating, and fluidized bed coating, can be used. However, electrostatic powder coating is preferred in terms of coating efficiency. 【0087】 One embodiment of the coating film formation method of this disclosure is an electrostatic powder coating method. 【0088】 In the coating film formation method of this disclosure, preheating of the workpiece may be performed before applying the powder coating composition. Preheating of the workpiece may be performed using a heating furnace such as an electric furnace or a gas furnace, or by induction heating using an induction heater. In this case, preheating must be performed within a range that allows the workpiece temperature to be maintained between 150°C and 250°C, taking into account the amount of heat stored due to the shape and thickness of the workpiece and the interval between preheating and painting. Generally, the preheating temperature is set about 10 to 30°C higher than the painting temperature of the powder coating composition. 【0089】 The coating thickness when applying the powder coating composition of this disclosure is at least 20 μm and 150 μm or less, from the viewpoint of preventing mottling and transparency of the coating film and preventing the generation of bubbles on or inside the coating film. In one embodiment, it is 40 μm or more and 90 μm or less, for example, 60 μm or more and 80 μm or less. The coating film formed from the powder coating composition of this disclosure can form a uniform coating film and has high smoothness. 【0090】 For example, with the powder coating composition according to this disclosure, the film thickness can be adjusted according to the required conditions such as the appearance of the coating film, such as smoothness, and the physical properties of the coating film. This allows for more efficient coating film formation than conventional methods, and furthermore, excess powder coating composition can be reused. 【0091】 The heating temperature, for example, the baking temperature and time, varies depending on the type and amount of curing agent used. From the viewpoint of preventing the generation of bubbles on or inside the coating film, the temperature is, for example, 140°C to 200°C, preferably 140°C to 190°C, more preferably 150°C to 190°C, even more preferably 160°C to 190°C, and even more preferably 160°C to 180°C. In another embodiment, the baking temperature is preferably 150°C to 170°C. The baking time can be set appropriately according to the baking temperature. For example, at the aforementioned baking temperature, the baking time may be 10 to 40 minutes. 【0092】 According to this disclosure, even when cured at low temperatures, the resulting coating film has good appearance (smoothness) and physical properties (resistance to dropping weights), and also good storage stability (appearance of the coating film after storage (resistance to solid phase reactivity) and resistance to blocking). The powder coating composition of this disclosure is preferably used in building materials, electrical products, office equipment, automobile bodies, exterior panels and parts, etc. [Examples] 【0093】 The present disclosure will be further illustrated by the following embodiments, but will not be limited thereto. 【0094】 (Manufacturing example A-1) Manufacturing of acrylic resin (A-1) In a reaction vessel equipped with a reflux condenser, dropping funnel, stirrer, thermometer, condenser, nitrogen gas inlet, and vacuum device, 15 parts by mass of butyl acetate and 30 parts by mass of T-SOL 100 were charged, and the temperature was raised to 130°C under a nitrogen atmosphere. To this, a monomer solution consisting of 18 parts by mass of methyl methacrylate, 42 parts by mass of ethyl methacrylate, 40 parts by mass of glycidyl methacrylate, and 1 part by mass of Kaya ester O (manufactured by Kayaku Akzo; t-butyl peroxy-2-ethylhexaate) as an initiator was added dropwise at a constant rate through a dropping funnel over 3 hours, and stirring was continued for a further 2 hours to obtain an acrylic resin containing the solvent. This was heated to 130°C under reduced pressure, and the solvent was completely removed by distillation to obtain the acrylic resin (A-1) (SP(A): 10.70, weight-average molecular weight: 4,500, Tg A At 50°C, epoxy equivalents of 390 g / eq were obtained. 【0095】 (Manufacturing examples A-2~A-23) Manufacturing of acrylic resins (A-2) to (A-23) Acrylic resins (A-2) to (A-23) were manufactured in the same manner as acrylic resin (A-1). Table 1 shows the monomer composition and various characteristic values ​​such as epoxy equivalent for each resin. Furthermore, even when the amount of each monomer used was the same, the weight-average molecular weight and other values ​​were changed by appropriately adjusting conditions such as the reaction temperature. 【0096】 [Table 1] 【0097】 (Manufacturing example B-1) Manufacturing of polyester resin (B-1) In a reaction vessel equipped with a reflux condenser, agitator, thermometer, condenser, and nitrogen gas inlet, 55 parts by mass of isophthalic acid, 45 parts by mass of neopentyl glycol, and 0.1 parts of di-n-butyltin oxide as a catalyst are mixed, and the temperature is gradually raised to 240°C in a nitrogen stream. The esterification reaction is carried out while distilling off the water produced, resulting in polyester resin (B-1) (SP(B): 10.70, weight-average molecular weight: 11,200, Tg B A solution with a carboxyl group equivalent of 2,800 g / eq and a melt viscosity of 4.3 Pa·s was obtained at 63°C. The melt viscosity of polyester resin (B) was measured using a Rheosol-G3000 (UBM Corporation, measurement temperature: 200°C, frequency: 2Hz, torsion angle: 0.5°). 【0098】 (Manufacturing examples B-2~B-14) Manufacturing of polyester resins (B-2) to (B-14) Polyester resins (B-2) to (B-14) were manufactured in the same manner as polyester resin (B-1). Table 2 shows the monomer composition and various characteristic values ​​such as carboxyl group equivalents for each resin. 【0099】 [Table 2] 【0100】 <Example 1> (Preparation of powder coating composition 1) 14 parts by mass of acrylic resin (A-1), 100 parts by mass of polyester resin (B-1), 30 parts by mass of Typure R-960 and 15 parts by mass of Sunlight SL-2200 as pigments were blended and mixed for 3 minutes using a Super Mixer (manufactured by Nippon Spindle Co., Ltd.). Next, the mixture was melt-kneaded at 110°C using a Conneeder (manufactured by Buss Co., Ltd.), the resulting mixture was extruded, cooled, coarsely ground, and then further ground using an ACM Pulverizer (manufactured by Hosokawa Micron Corporation). The resulting pulverized material was classified using a Turbo Classifier (manufactured by Nisshin Engineering Co., Ltd.) to obtain powder coating composition 1 with an average particle size of 30 μm. The minimum melt viscosity of powder coating composition 1 was measured using a dynamic viscoelasticity analyzer (Rheosol-G3000; manufactured by UBM) when the temperature was increased from 110°C to 160°C at a heating rate of 5°C / min. 【0101】 (Preparation of evaluation coating 1) A JIS G 3141 (SPCC~SD) cold-rolled steel sheet (75 × 150 × 0.8 mm) treated with zinc phosphate was coated with the powder coating composition 1 obtained as described above using an electrostatic coating machine for powder coatings (applied voltage -80 kV) to a dry film thickness of 80 μm, and then baked at 160°C for 15 minutes to obtain the coating film 1. 【0102】 (Examples 2-38, Comparative Examples 1-9) Powder coating compositions were prepared in the same manner as in Example 1, except that the types and amounts of each component were changed as shown in Tables 3A, 3B, 3C, 3D, and 4 below. Details of the raw materials used are described below. Furthermore, various coating films were formed using the obtained powder coating composition in the same manner as in Example 1. The explanations for each component listed in Tables 1 to 4 are as follows: • Kayaester O: Initiator, t-butylperoxy-2-ethylhexaate; manufactured by Kayakusoku Akzo. Butyl acetate: Organic solvent; aliphatic ester organic solvent; manufactured by Shoei Chemical Co., Ltd. • T-SOL 100: Aromatic organic solvent; manufactured by ENEOS Corporation. • Di-n-butyltin oxide: Manufactured by Sakai Chemical Industry Co., Ltd.; tin-based catalyst • Typure R-960: Pigment, Titanium Dioxide: Manufactured by DuPont • Sunlight SL-2200: Pigment, calcium carbonate: Manufactured by Takehara Chemical Industry Co., Ltd. 【0103】 The powder coating compositions and coating films obtained in Examples 1 to 38 and Comparative Examples 1 to 9 were evaluated as follows. The evaluation results are shown in Tables 3A to 4 below. 【0104】 Ten g of the paint compositions obtained in the examples and comparative examples were placed in 30 mL glass bottles (with screw caps) and left to stand at 40°C for one week. After one week, the condition of the paint compositions was visually observed, and the blocking resistance was evaluated according to the following criteria. A score of △ or higher was considered acceptable. ○: No lumps at all △: There are clumps, but they break apart easily when you pinch them with your fingers. ×: There is a lump, and even when you lift the lump with your fingers, it retains its shape. 【0105】 (Resistance to solid-phase reactivity) The paint compositions obtained in the examples and comparative examples were left to stand at 40°C for 5 weeks. Afterward, a coating film was prepared in the same manner as described above, and the appearance of the resulting coating film was visually observed. The resistance to solid-phase reactivity was evaluated according to the following criteria. A circle (〇) was considered a pass. ○: The whole thing is smooth. △: Slightly peely skin is visible. ×: Orange peel texture is visible throughout. 【0106】 (Appearance of the coating) The appearance of the coating films obtained in the examples and comparative examples was visually observed and evaluated according to the following criteria. A circle (〇) was considered acceptable. ○: The whole thing is smooth. △: Slightly peely skin is visible. ×: Orange peel texture is visible throughout. 【0107】 (Adhesive) In the coatings of the test pieces obtained in the examples and comparative examples, 11 vertical and 11 horizontal cuts were made at 1 mm intervals using a cutter. Cellophane tape (registered trademark) (manufactured by Nichiban Co., Ltd.) was then applied and peeled off, and the number of remaining squares out of 100 squares was counted (grid test). Note that 100 / 100 indicates that the peeling area of ​​the coating is 0%, for example, 90 / 100 indicates that the peeling area of ​​the coating is 10%, and 50 / 100 indicates that the peeling area of ​​the coating is 50%. 100 / 100 was considered a pass. 【0108】 (Pencil hardness) The hardness of the coating films obtained in the examples and comparative examples was evaluated according to JIS K 5600-5-4 (scratch hardness (pencil method)). A hardness of H or higher was considered acceptable. 【0109】 (Flexibility: Resistance to falling weights) The test specimens (coated boards) obtained in the examples and comparative examples were evaluated for their resistance to dropping weights in accordance with JIS K 5600-5-3 (weight drop resistance test). Using a DuPont impact tester (1 / 2-inch impact type; manufactured by Ueshima Seisakusho Co., Ltd.), a 500g weight was dropped from a certain height, and the height at which cracks occurred was measured to evaluate flexibility (resistance to weight drops). In the table, "50<" indicates that no crack occurred even when the weight was dropped from a height of 50cm. A height of 25cm or more was considered acceptable. 【0110】 [Table 3A] 【0111】 [Table 3B] 【0112】 [Table 3C] 【0113】 [Table 4] 【0114】 Thus, the powder coating composition according to this disclosure can form a coating film that has a good balance of high smoothness and excellent physical properties such as impact resistance and scratch resistance. Furthermore, compared to commonly used powder coating compositions, it can form a coating film at a lower temperature, and moreover, as described above, it can form a coating film that has a good balance of high smoothness and excellent physical properties such as impact resistance and scratch resistance. 【0115】 Comparative Example 1 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the coating-forming resins, is less than 3,000. As a result, the blocking resistance and solid-phase reactivity of the paint composition, as well as the appearance, adhesion, and flexibility of the resulting coating film, were not sufficiently satisfactory. Comparative Example 2 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the film-forming resins, is less than 3,000, and the blocking resistance and solid-phase reactivity resistance of the paint composition were not sufficiently satisfactory. Comparative Example 3 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the film-forming resins, is less than 3,000, and the blocking resistance and solid-phase reactivity resistance of the paint composition were not sufficiently satisfactory. Comparative Example 4 is an example in which the weight-average molecular weight of the acrylic resin (A), one of the film-forming resins, is less than 3,000, and |SP(A)-SP(B)| exceeds 0.22. As a result, the blocking resistance and solid-phase reactivity of the paint composition, as well as the appearance of the obtained coating film, were not sufficiently satisfactory. Comparative Example 5 is an example in which the content of units derived from monomers having epoxy groups in acrylic resin (A), which is one of the coating film-forming resins, exceeds 60% by mass, and the solid-phase reactivity resistance of the paint composition and the appearance of the obtained coating film were not sufficiently satisfactory. Comparative Example 6 is an example in which the weight-average molecular weight of the acrylic resin (A), one of the film-forming resins, is less than 3,000, and |SP(A)-SP(B)| exceeds 0.22. As a result, the blocking resistance and solid-phase reactivity of the paint composition, as well as the appearance and pencil hardness of the resulting coating film, were not sufficiently satisfactory. Comparative Example 7 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the film-forming resins, is less than 3,000, and the blocking resistance and solid-phase reactivity resistance of the paint composition were not sufficiently satisfactory. Comparative Example 8 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the film-forming resins, is less than 3,000, and the blocking resistance and solid-phase reactivity resistance of the paint composition were not sufficiently satisfactory. Comparative Example 9 is an example in which the weight-average molecular weight of the acrylic resin (A), which is one of the film-forming resins, is less than 3,000, and the acid value of the polyester resin (B) exceeds 35 mgKOH / g. As a result, the blocking resistance and solid-phase reactivity resistance of the paint composition were not sufficiently satisfactory. [Industrial applicability] 【0116】 According to this disclosure, it is possible to provide a powder coating composition that, even when cured at low temperatures, exhibits good coating film appearance (smoothness) and coating film properties (resistance to dropping weights), as well as good storage stability (coating film appearance after storage (resistance to solid phase reactivity)). The powder coating composition of this disclosure is preferably used in building materials, electrical products, office equipment, automobile bodies, exterior panels and parts, etc.

Claims

[Claim 1] A powder coating composition comprising an acrylic resin (A) and a polyester resin (B) as a film-forming resin, The acrylic resin (A) has epoxy groups, The weight-average molecular weight of the acrylic resin (A) is 3,000 or more and 9,000 or less. In the acrylic resin (A) mentioned above, the content of units derived from monomers having epoxy groups is 20% by mass or more and 60% by mass or less. The polyester resin (B) has a carboxyl group, The acid value of the polyester resin (B) is 10 mg KOH / g or more and 35 mg KOH / g or less. A powder coating composition in which the absolute value of the difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B), |SP(A) - SP(B)|, is 0.22 or less. [Claim 2] The acid value of the polyester resin (B) is W (mgKOH / g), In the acrylic resin (A), the content of units derived from monomers having epoxy groups is X (%). Let Y be the weight-average molecular weight of acrylic resin (A). The absolute difference between the solubility parameter SP(A) of the acrylic resin (A) and the solubility parameter SP(B) of the polyester resin (B) is |SP(A) - SP(B)|, which is expressed as Z (cal / cm²). ３ ) １／２ In that case, The following formula: Δf=(Y-4,500) / 1,000-1.552(|Z|+|W+X-60| / 200) The resin composition according to claim 1, wherein the Δf calculated is -1.29 or more and 1.90 or less. [Claim 3] The solubility parameter SP(A) of the acrylic resin (A) is 9.0 (cal / cm³). ３ ) １／２ More than 11.0 (cal / cm ３ ) １／２ The powder coating composition according to claim 1, which is as follows: [Claim 4] The powder coating composition according to claim 1, wherein the epoxy equivalent of the acrylic resin (A) is 240 g / eq or more and 710 g / eq or less. [Claim 5] The glass transition temperature (Tg) of the acrylic resin (A) Ａ The powder coating composition according to claim 1, wherein the temperature is 0°C or higher and 65°C or lower. [Claim 6] The powder coating composition according to claim 1, wherein the weight-average molecular weight of the polyester resin (B) is 5,000 or more and 100,000 or less. [Claim 7] The solubility parameter SP(B) of the polyester resin (B) is 10.0 (cal / cm ３ ). １／２ or more and 11.5 (cal / cm ３ ). １／２ The powder coating composition according to claim 1, wherein the solubility parameter SP(B) is as described above. [Claim 8] The glass transition temperature (Tg) of the polyester resin (B) Ｂ The powder coating composition according to claim 1, wherein the temperature is 40°C or higher and 70°C or lower. [Claim 9] The powder coating composition according to claim 1, wherein the minimum melt viscosity is 1 Pa·s or more and 400 Pa·s or less. [Claim 10] A method for forming a coating film, comprising applying a powder coating composition according to any one of claims 1 to 9 onto an object to be coated, and heating to form a cured coating film, A method for forming a coating film, wherein the heating is performed at a temperature of 140°C or higher and 200°C or lower for the object to be coated.

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