Resin composition for powder coatings

Abstract

The present invention aims to provide a resin composition for powder coatings that yields powder coatings with excellent coating film performance and excellent storage stability. [Solution] A resin composition for powder coatings is used that contains a (meth)acrylic copolymer (A) which has an epoxy group (meth)acrylate (a1) and at least one monomer selected from the group consisting of styrene (a2) and alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​as essential constituent monomers, the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 63% by weight, the glass transition temperature is 45 to 65°C, the epoxy group amount (moles / kg) is 2.6 to 3.1, and the number average molecular weight is 1500 to 2500.

Description

Technical Field

[0001] The present invention relates to a resin composition for powder coatings.

Background Art

[0002] Powder coatings are powder coatings obtained by pulverizing a resin composition obtained by kneading and mixing raw materials such as pigments, curing agents, and resins. Compared with solvent-based coatings, they are pollution-free and have many excellent features such as easy paint recovery and recyclability. Therefore, they are used in a wide range of fields such as the automotive field, the construction field, and household appliances.

[0003] As a powder coating composition that has excellent coating film performance such as coating film appearance and is less likely to cause blocking when made into a powder coating, a (meth)acrylate having an epoxy group and an alkyl (meth)acrylate having 14 to 33 carbon atoms in the alkyl group are used as essential constituent monomers, and a (meth)acrylic acid ester copolymer having an epoxy group concentration of 2.0 to 5.0 mol / kg and a polyester resin having a carboxyl group are included. A powder coating composition is known (Patent Document 1). However, the powder coating composition described in Patent Document 1 does not have sufficient storage stability of the powder coating itself, and a smooth coating film may not be obtained when stored for a long period exceeding one month.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a resin composition for powder coatings from which a powder coating having excellent coating film performance and excellent storage stability can be obtained.

Means for Solving the Problems

[0006] The inventors of this invention arrived at this present invention as a result of their research in order to achieve the above objectives. In other words, the present invention relates to a resin composition for powder coatings containing a (meth)acrylic acid ester copolymer (A) having an epoxy group (meth)acrylate (a1) and styrene (a2) and / or alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​as essential constituent monomers, wherein the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 63% by weight, the glass transition temperature is 45 to 65°C, the epoxy group amount (moles / kg) is 2.6 to 3.1, and the number average molecular weight is 1500 to 2500, and a powder coating containing the resin composition for powder coatings. [Effects of the Invention]

[0007] The resin composition for powder coatings of the present invention provides the effect of yielding powder coatings with excellent coating film performance and excellent storage stability. [Modes for carrying out the invention]

[0008] The resin composition for powder coatings of the present invention is an epoxy group-containing (meth)acrylate (a 1) contains a (meth)acrylic acid ester copolymer (A) [hereinafter referred to as copolymer (A)], in which styrene (a2) and / or alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​are essential constituent monomers. In copolymer (A), the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 80% by weight. Copolymer (A) has a glass transition temperature of 45 to 65°C, an epoxy group content (moles / kg) of 2.6 to 3.1, and a number-average molecular weight of 1500 to 2500.

[0009] Preferred epoxy group-containing (meth)acrylate (a1), which is an essential constituent monomer of copolymer (A) contained in the resin composition for powder coatings of the present invention, includes those having 6 to 30 carbon atoms and having one epoxy group and one (meth)acryloyl group, such as glycidyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate glycidyl ether. Among the epoxy group-containing (meth)acrylates (a1), glycidyl (meth)acrylate is preferred from the viewpoint of blocking resistance, and glycidyl methacrylate is more preferred.

[0010] The copolymer (A) contained in the resin composition for powder coatings of the present invention comprises either styrene (a2) or alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3), or both styrene (a2) and alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​as essential constituent monomers. In particular, it is preferable that both styrene (a2) and alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​are essential constituent monomers.

[0011] Examples of alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​include methyl (meth)acrylate, ethyl (meth)acrylate, and (meth)acrylates in which the alkyl group has 3 to 11 carbon atoms and is either a linear or branched alkyl group. A linear alkyl group refers to an alkyl group in which the alkyl group bonded to the carbonyloxy group in (meth)acrylate (a3) ​​has one methyl group, while a branched alkyl group refers to an alkyl group in which the alkyl group bonded to the carbonyloxy group has two or more methyl groups. Examples of (meth)acrylates having 3 to 11 carbon atoms in the alkyl group and being linear alkyl groups include n-propyl (meth)acrylate, n-butyl (meth)acrylate, and n-dodecyl (meth)acrylate. Examples of (meth)acrylates having 3 to 11 carbon atoms in the alkyl group and being branched alkyl groups include isopropyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. As (meth)acrylate (a3), methyl (meth)acrylate, ethyl (meth)acrylate, and (meth)acrylates having 3 to 4 carbon atoms in the alkyl group and being either a linear or branched alkyl group are preferred, and from the viewpoint of blocking resistance, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and n-butyl (meth)acrylate are even more preferred.

[0012] In the copolymer (A) contained in the resin composition for powder coatings of the present invention, the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 63% by weight. If the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers falls below 55% by weight, the gloss of the coating film deteriorates, and if it exceeds 63% by weight, the strength of the coating film deteriorates. The ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is preferably 55 to 60% by weight, and more preferably 55 to 58% by weight.

[0013] The glass transition temperature of copolymer (A) contained in the resin composition for powder coatings of the present invention is 45 to 65°C. If the glass transition temperature of (meth)acrylic copolymer (A) is below 45°C, the necessary storage stability cannot be obtained, and if it exceeds 65°C, the smoothness, gloss, and strength of the coating film deteriorate. A glass transition temperature of copolymer (A) of 46 to 50°C is more preferable. The glass transition temperature of copolymer (A) can be lowered by using a large proportion of monomers with low homopolymer glass transition temperatures as constituent monomers, and raised by using a large proportion of monomers with high homopolymer glass transition temperatures. Furthermore, the glass transition temperature of copolymer (A) can be lowered by decreasing its molecular weight, and raised by increasing its molecular weight. The polymer (A) contained in the resin composition for powder coatings of the present invention can have its glass transition temperature set to a predetermined range by methods such as setting the total weight ratio of styrene (a2) and the (meth)acrylate (a3) ​​to the preferred range described above, and setting the number average molecular weight of the copolymer (A) to the preferred range described later.

[0014] The glass transition temperature (Tg) of copolymer (A) is the intermediate glass transition temperature measured using the DSC method described in JIS K7121, Method for Measuring Transition Temperatures of Plastics, using DSC20 and SSC / 580 manufactured by Seiko Electronics Industries, Ltd. If copolymer (A) has two or more glass transition temperatures, the glass transition temperature with the largest heat absorption is considered the glass transition temperature of (meth)acrylic copolymer (A).

[0015] The epoxy group content of copolymer (A) is 2.6 to 3.1 mol / kg. If the epoxy group content (mol / kg) of (meth)acrylic acid ester copolymer (A) is less than 2.6, the flexibility of the coating film will be insufficient, and if it exceeds 3.1, the smoothness of the coating film will be insufficient. The epoxy group content of copolymer (A) is the number of moles of epoxy groups in 1 kg of copolymer (A), and can be calculated from the total weight (kg) of all constituent monomers used in polymerization of copolymer (A) and the number of moles of (meth)acrylate (a1) containing epoxy groups used. Alternatively, it can be determined by calculating the epoxy index as described in JIS K7236 Method for Determining the Epoxy Equivalent of Epoxy Resins. The amount of epoxy groups (moles / kg) in copolymer (A) is preferably 2.7 to 3.0, and more preferably 2.8 to 3.0.

[0016] In addition to (a1) to (a3) ​​above, copolymer (A) may also contain at least one monomer selected from the group consisting of alkyl (alkyl group with 12 or more carbon atoms) (meth)acrylate (a4), aromatic vinyl monomers other than styrene (a5), acrylamide monomer (a6), amino group-containing (meth)acrylate (a7), hydroxyl group-containing (meth)acrylate (a8), and aliphatic vinyl ester monomer (a9).

[0017] As the alkyl (alkyl group with 12 or more carbon atoms) (meth)acrylate (a4), alkyl (alkyl group with 12 to 33 carbon atoms) (meth)acrylate is preferred. Examples of alkyl (alkyl group with 12-33 carbon atoms) (meth)acrylates include linear alkyl (meth)acrylates (a41) where the alkyl group is linear, and branched alkyl (meth)acrylates (a42) where the alkyl group has a branched structure. Examples of linear alkyl (meth)acrylates (a41) include n-dodecyl (meth)acrylate, n-hexadecyl (meth)acrylate, n-octadecyl (meth)acrylate, n-tetracosyl (meth)acrylate, and n-dotriacontyl (meth)acrylate. Examples of branched alkyl (meth)acrylates (a42) include 2-methylpentadecyl (meth)acrylate, 2-hexyldecyl (meth)acrylate, 2-octyldecyl (meth)acrylate, 2-hexyldodecyl (meth)acrylate, 2-methylheptadecyl (meth)acrylate, 2-octyldodecyl (meth)acrylate, 2-decyltetradecyl (meth)acrylate, 2-dodecylhexadecyl (meth)acrylate, and 2-tetradecyloctadecyl (meth)acrylate.

[0018] Examples of aromatic vinyl monomers (a5) excluding styrene include hydrocarbyl (alkyl, cycloalkyl, aralkyl or alkenyl) substituted styrenes having a substituent with 1 to 7 carbon atoms (α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, etc.), hydroxyl group-containing styrene compounds having 8 to 15 carbon atoms (hydroxystyrene, etc.), and the like.

[0019] Examples of acrylamide monomers (a6) include (meth)acrylamide, N-alkyl-substituted (meth)acrylamide having an alkyl group with 1 to 10 carbon atoms (preferably 1 to 6 carbon atoms) bonded to the nitrogen atom {N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, etc.}, and the like.

[0020] Examples of amino group-containing (meth)acrylates (a7) include aminoalkyl (meth)acrylates (a71) having an alkylene group with 1 to 6 carbon atoms bonded to the amino group and the acryloyloxy group (aminoethyl (meth)acrylate, aminopropyl (meth)acrylate, etc.), N-alkyl (1 to 6 carbon atoms) substituted aminoalkyl (1 to 6 carbon atoms) (meth)acrylates (a72) (t-butylaminoethyl methacrylate, etc.), N,N-dialkyl (alkyl having 1 to 4 carbon atoms) substituted aminoalkyl (1 to 4 carbon atoms) (meth)acrylates (a73) (dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, etc.), and the like.

[0021] Examples of hydroxyl group-containing (meth)acrylates (a8) include (meth)acrylates (a81) in which a hydrogen atom of the alkyl group contained in alkyl (1 to 6 carbon atoms) (meth)acrylate is substituted with a hydroxyl group (sometimes also referred to as hydroxyalkyl (meth)acrylate (a81)), polyoxyalkylene (meth)acrylates (a82), and the like. Examples of the hydroxyalkyl (meth)acrylate (a81) include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, etc. Examples of the polyoxyalkylene (meth)acrylate (a82) include polyoxyalkylene (meth)acrylates in which the oxyalkylene group has 2 to 4 carbon atoms {polyoxyethylene mono(meth)acrylate, etc.}.

[0022] Examples of the aliphatic vinyl ester monomer (a9) include esters of (meth)allyl alcohol and aliphatic carboxylic acids having 2 ~6 carbon atoms, such as vinyl acetate, vinyl propionate, and vinyl butyrate, etc.

[0023] In the copolymer (A), the total weight of the (meth)acrylate (a1), styrene (a2) and alkyl (meth)acrylate (a3) is preferably 95 to 100% by weight, more preferably 98 to 100% by weight, based on the total weight of the constituent monomers of the copolymer (A).

[0024] The weight of the (meth)acrylate (a1) contained in the constituent monomers of the copolymer (A) is preferably 37 to 45% by weight, more preferably 40 to 45% by weight, particularly preferably 42 to 45% by weight, from the viewpoint of achieving both coating film strength and storage stability, based on the total weight of the (meth)acrylate (a1), styrene (a2) and alkyl (meth)acrylate (a3).

[0025] From the viewpoint of the glossiness of the coating film, the copolymer (A) preferably contains both styrene (a2) and alkyl (meth)acrylate (a3) as essential constituent monomers.

[0026] When the constituent monomers of the copolymer (A) contain both styrene (a2) and alkyl (meth)acrylate (a3), the weight of the styrene (a2) is preferably 40 to 55% by weight, more preferably

[0027] Copolymer (A) can be obtained by polymerizing a constituent monomer composition containing (a1) and (a2) and / or (a3) ​​by a known method. Polymerization methods include solution polymerization, bulk polymerization, suspension polymerization, emulsion polymerization, etc., with solution polymerization being preferred. Solution polymerization can be carried out by dropping monomer (a1), (a2) and / or (a3), (a4) to (a9) as needed, and a polymerization initiator into a heated solvent. The heating temperature of the solvent, i.e., the polymerization temperature, is preferably 70 to 230°C or lower, and more preferably 80 to 180°C or lower, from the viewpoint of the molecular weight and yield of copolymer (A).

[0028] The number-average molecular weight (Mn) of copolymer (A) is 1,500 to 2,500, preferably 1,700 to 2,000. If the Mn content of copolymer (A) is less than 1,500, the blocking resistance decreases, and if it exceeds 2,500, the smoothness and gloss decrease. The Mn content of copolymer (A) can be adjusted by various polymerization conditions, such as polymerization temperature. By increasing the polymerization temperature and decreasing the polymerization concentration (slowing down the dropping rate when performing dropwise polymerization), the Mn content can be reduced. Furthermore, Mn is measured by dissolving copolymer (A) in tetrahydrofuran (THF), using this as the sample solution, and measuring it using gel permeation chromatography (GPC) under the following conditions. Equipment: HLC-8120 manufactured by Tosoh Corporation Column: TSK GEL GMH6 (2 pieces) [Manufactured by Tosoh Corporation] Measurement temperature: 40℃ Sample solution: 0.25% by weight THF solution Solution injection volume: 100μL Detection device: Refractive index detector Calibration curves were prepared using 12 samples of standard polystyrene manufactured by Tosoh Corporation (TSKstandard POLYSTYRENE) with molecular weights of 500, 1050, 2800, 5970, 9100, 18100, 37900, 96400, 190000, 355000, 1090000, and 2890000.

[0029] The copolymer obtained by the polymerization method described above can be used as is as the resin composition for powder coatings of the present invention, and may also contain other components. In particular, the resin composition for powder coatings of the present invention preferably contains a copolymer (A) and further contains a polyester resin (B) having a carboxyl group.

[0030] As the polyester resin (B), a polyester resin (B) that does not have epoxy groups is preferred.

[0031] It can be obtained by condensation polymerization using an acid component (b1) mainly composed of a polycarboxylic acid (preferably a dicarboxylic acid) and an alcohol component (b2) mainly composed of a polyhydric alcohol (preferably a dihydric alcohol) as raw materials, in an amount where the ratio {COOH / OH} of the number of moles of carboxyl groups in (b1) to the number of moles of hydroxyl groups in (b2) is greater than 1.

[0032] The acidic components (b1) include monocarboxylic acids (b11), dicarboxylic acids (b12), and polycarboxylic acids with three or more valent carboxylic acids (b13). (b1) may be one type or two or more types may be used in combination. Monocarboxylic acids (b11) include those with 2 to 30 carbon atoms, such as saturated fatty acids (e.g., acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, etc.), unsaturated fatty acids (e.g., oleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic acid, etc.), and aromatic monocarboxylic acids (e.g., benzoic acid, etc.). Examples of divalent carboxylic acids (b12) include aliphatic dicarboxylic acids with 2 to 50 carbon atoms (e.g., oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, etc.) and aromatic dicarboxylic acids with 8 to 36 carbon atoms (e.g., phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, etc.). Examples of polycarboxylic acids with a valency of 3 or more (b13) include aliphatic polycarboxylic acids with 6 to 36 carbon atoms (e.g., hexanetricarboxylic acid) and aromatic polycarboxylic acids with 9 to 20 carbon atoms (e.g., trimellitic acid, pyromellitic acid). As the acid component (b1), anhydrides of these carboxylic acids, lower alkyl (1-4 carbon atoms) esters (e.g., methyl esters), carboxylic acid halides, etc. may be used.

[0033] As for the acid component (b1), it is preferable that it contains terephthalic acid and / or isophthalic acid from the viewpoint of strength and durability. In other words, it is preferable that the polyester resin (B) has terephthalic acid and / or isophthalic acid as constituent monomers. The content of terephthalic acid and isophthalic acid contained in the acid component (b1) (the content of terephthalic acid and isophthalic acid in the acid component (b1) as monomers constituting the polyester resin (B)) is preferably 70 mol% or more, and more preferably 75 mol% or more, based on the number of moles of (b1), from the viewpoint of strength and durability.

[0034] Examples of alcohol components (b2) include monohydric alcohols (b21), dihydric alcohols (b22), and trihydric or higher polyols (b23). (b2) may be used alone or in combination of two or more types. Examples of monohydric alcohols (b21) include linear or branched alkyl alcohols having 1 to 30 carbon atoms (methanol, ethanol, propanol, isopropanol, 1-decanol, dodecyl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, arachidyl alcohol, behenyl alcohol, and lignoceryl alcohol, etc.). Examples of dihydric alcohols (b22) include alkylene glycols with 2 to 36 carbon atoms (ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol, and 1,12-dodecanediol, etc.) and alkylene ether glycols with 4 to 36 carbon atoms (dietile Examples include polymethyl glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol, etc., alicyclic diols having 6 to 36 carbon atoms (such as 1,4-cyclohexanedimethanol and hydrogenated bisphenol A), (poly)alkylene (2 to 4 carbon atoms) oxide adducts of alicyclic diols (preferably with an average number of added moles of 1 to 30), and (poly)alkylene (2 to 4 carbon atoms) oxide adducts of bisphenols (preferably with an average number of added moles of 1 to 30). Examples of polyols with a valency of 3 or higher (b23) include trimethylolpropane, trimethylolethane, glycerin, pentaerythritol, sorbitol, sorbitan, polyglycerin, and dipentaerythritol.

[0035] From the viewpoint of strength, the alcohol component (b2) preferably contains a dihydric alcohol (b22) and a trihydric or higher polyol (b23), and more preferably contains ethylene glycol and / or neopentyl glycol and 1,6-hexanediol and / or trimethylolpropane. From the viewpoint of smoothness and strength, the content of dihydric alcohol (b22) contained in the alcohol component (b2) is preferably 75 mol% or more and less than 100 mol%, and more preferably 85 mol% or more and less than 100 mol%, based on the number of moles of (b2). From the viewpoint of smoothness, the content of trivalent or higher polyol (b23) contained in the alcohol component (b2) is preferably 25 mol% or less, and more preferably 15 mol% or less, based on the number of moles of (b2).

[0036] The polyester resin (B) can be produced by esterifying a carboxylic acid component (b1) and an alcohol component (b2) using a known method. The esterification reaction temperature is preferably 150 to 280°C, more preferably 160 to 250°C, and even more preferably 170 to 235°C. The esterification reaction time is preferably 30 minutes or more, and more preferably 2 to 40 hours.

[0037] For the esterification reaction, known esterification catalysts can be used as needed.

[0038] The amount of carboxyl groups (moles / kg) in polyester resin (B) is preferably 0.2 to 0.4 mol / kg, and more preferably 0.3 to 0.4 mol / kg, from the viewpoint of smoothness and strength of the coating film. The amount of carboxyl groups in polyester resin (B) is the number of moles of carboxyl groups contained in 1 kg of polyester resin (B). This can be determined by measuring the acid value of polyester resin (B) in accordance with JIS K0070 Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products, and converting it to the number of moles of carboxyl groups per 1 kg of polyester resin (B).

[0039] The hydroxyl value of polyester resin (B) is preferably 0 (mgKOH / g) from the viewpoint of the reactivity of the powder coating. The hydroxyl value is measured in accordance with JIS K0070 Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products.

[0040] When producing polyester resin (B), the ratio of carboxylic acid component (b1) to alcohol component (b2) is preferably 30 / 70 to 49 / 51, and more preferably 40 / 60 to 49 / 51, in terms of the molar ratio of hydroxyl groups to carboxyl groups ([OH] / [COOH]) from the viewpoint of blocking resistance, smoothness, and strength. By setting this range, the Mn of the polyester resin (B) can be set to a preferred range, and the carboxyl group concentration can be set to a preferred range.

[0041] The Mn of the polyester resin (B) is preferably 1,000 to 10,000, and more preferably 2,000 to 6,000, from the viewpoint of blocking resistance, smoothness, gloss, and strength. The Mn content of polyester resin (B) can be measured under the same measurement conditions as for copolymer (A).

[0042] The Tg of polyester resin (B) is preferably 40 to 70°C, and more preferably 45 to 65°C, from the viewpoint of blocking resistance and smoothness. The Tg of polyester resin (B) can be measured in the same way as the Tg of copolymer (A).

[0043] When the resin composition for powder coatings of the present invention contains a copolymer (A) and a polyester resin (B) having carboxyl groups, the ratio of moles of epoxy groups in copolymer (A) to moles of carboxyl groups in polyester resin (B) (epoxy groups / carboxyl groups) is preferably 0.5 to 1.5, and more preferably 0.8 to 1.2, from the viewpoint of coating film strength.

[0044] The total weight of the copolymer (A) and polyester resin (B) in the resin composition for powder coatings of the present invention is preferably 20 to 100% by weight, and more preferably 30 to 100% by weight, based on the total weight of the resin composition for powder coatings, from the viewpoint of smoothness, gloss, and strength.

[0045] The resin composition for powder coatings of the present invention may contain a curing catalyst. By including a curing catalyst, the reaction between carboxyl groups and epoxy groups can be controlled, and the curing time (gel time) of the powder coating obtained from the resin composition for powder coatings can be adjusted.

[0046] Examples of curing catalysts include at least one curing catalyst selected from the group consisting of imidazole skeleton-containing compounds, metal salt complexes of imidazole skeleton-containing compounds, imidazoline skeleton-containing compounds, metal salt complexes of imidazoline skeleton-containing compounds, tertiary phosphines, quaternary phosphonium salts, and quaternary ammonium salts.

[0047] Examples of imidazole skeleton-containing compounds include alkylimidazoles such as 2-ethyl-4-methylimidazole, 1-methylimidazole, 1,2-dimethylimidazole, 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, and 2-isopropylimidazole; carbamylalkyl-substituted imidazoles such as 1-(2-carbamylethyl)imidazole; cyanoalkyl-substituted imidazoles such as 1-cyanoethyl-2-methylimidazole; aromatic-substituted imidazoles such as 2-phenylimidazole, 2-phenyl-4-methylimidazole, and 1-benzyl-2-methylimidazole; and alkenyl-substituted imidazoles such as 1-vinyl-2-methylimidazole. Examples include allyl-substituted imidazoles such as 1-allyl-2-ethyl-4-methylimidazole, and polyimidazoles. Among these, alkylimidazoles and aromatically substituted imidazoles are preferred. Commercially available products from the Curazole series (manufactured by Shikoku Chemicals, Inc.), 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), may be used as curing catalysts.

[0048] Examples of imidazoline skeleton-containing compounds used as curing catalysts include 2-phenylimidazole, 2-methylimidazole, 2-undecylimidazole, and 2-heptadecylimidazole. Commercially available products such as Cureazole 2PZL-T (manufactured by Shikoku Chemicals, Inc.; 2-phenylimidazole) can also be used.

[0049] Examples of metal salt complexes include those obtained by compounding (forming a complex compound of) the imidazole skeleton-containing compound or the imidazoline skeleton-containing compound with a metal salt. Examples of metal salts that make up metal salt complexes include metals such as copper, nickel, cobalt, calcium, zinc, zirconium, silver, chromium, manganese, tin, iron, titanium, antimony, and aluminum, as well as salts such as chloride, bromide, fluoride, sulfate, nitrate, acetate, malate, stearate, benzoate, and methacrylate.

[0050] Examples of tertiary phosphines include triphenylphosphine and tritlylphosphine.

[0051] Examples of quaternary phosphonium salts include benzyltriphenylphosphonium chloride, butyltriphenylphosphonium bromide, ethyltriphenylphosphonium iodide, and ethyltriphenylphosphonium bromide.

[0052] Examples of quaternary ammonium salts include tetraethylammonium chloride, tetraethylammonium bromide, and benzyltrimethylammonium bromide.

[0053] From the viewpoint of curing properties at low temperatures, the curing catalyst is preferably at least one selected from the group consisting of imidazole skeleton-containing compounds and imidazoline skeleton-containing compounds.

[0054] The amount of curing catalyst in the resin composition for powder coatings is preferably 20% by weight or less, and more preferably 0 to 15% by weight, based on the weight of the resin composition for powder coatings, from the viewpoint of smoothness. By setting the range to this extent, the gel time of the powder coating obtained from the resin composition for powder coatings can be set to a desirable range (for example, a gel time of 50 seconds or more and 100 seconds or less at 160°C, as measured in accordance with JIS K 5600-9-1 (Method for measuring the gel time of thermosetting powder coatings at a predetermined temperature)).

[0055] The resin composition for powder coatings of the present invention may also contain a curing agent component. The curing agent component is a polyvalent compound having a functional group that reacts with the carboxyl group of the polyester resin (B), and examples include amine curing agent components, acid curing agent components, blocked isocyanate curing agent components, and phenol resin curing agent components.

[0056] Examples of amine curing agents include aliphatic polyamines, polyaminoamides, ketimines, alicyclic diamines, aromatic diamines, imidazoles, dicyandiamides, polyamides, and β-hydroxyalkylamides.

[0057] Examples of acid curing agent components include aliphatic polycarboxylic acids and acid anhydrides of polycarboxylic acids. Among these, aliphatic polycarboxylic acids having 10 to 30 carbon atoms (decanedicarboxylic acid and dodecanedicarboxylic acid, etc.) are preferred.

[0058] Examples of blocked isocyanate curing agents include those 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.

[0059] The content of the curing agent component in the resin composition for powder coatings is preferably 10% by weight or less, and more preferably 0.5 to 5% by weight, based on the weight of the polyester resin (B), from the viewpoint of strength.

[0060] The resin composition for powder coatings of the present invention may contain, to the extent that it does not impair the effects of the present invention, a polyester resin (X) that has no acid value (acid value = 0) and has hydroxyl groups, and a curing agent component (Y) that has a functional group that reacts with hydroxyl groups. The aforementioned polyester resin (B) has carboxyl groups and therefore has an acid value, and is distinguished from the polyester resin (X).

[0061] The polyester resin (X) is preferably a polyester polyol. Polyester resin (X) can be obtained by condensation polymerization using an acid component (b1) and an alcohol component (b2) as raw materials, similar to polyester resin (B). The hydroxyl group concentration of polyester resin (X) is preferably 0.3 to 0.7 mol / kg, and more preferably 0.35 to 0.65 mol / kg, from the viewpoint of smoothness and strength. The hydroxyl group concentration of polyester resin (X) can be calculated by measuring the hydroxyl value in accordance with JIS K0070.

[0062] The weight ratio ((B) / (X)) of polyester resin (B) to polyester resin (X) is preferably 100 / 0 to 30 / 70, and more preferably 100 / 0 to 40 / 60.

[0063] Examples of curing agent components (Y) having a functional group that reacts with a hydroxyl group include blocked isocyanate curing agents obtained by blocking nurate compounds of isocyanate compounds such as hexamethylene diisocyanate, isophorone diisocyanate, and xylylene diisocyanate with a blocking agent such as ε-caprolactam or methyl ethyl ketone oxime, and self-blocking type isocyanate curing agents having a uretdione bond in the molecule.

[0064] The molar ratio {OH / NCO} of the number of moles of hydroxyl groups in the polyester resin (X) in the resin composition for powder coatings to the number of moles of blocked isocyanate groups in the curing agent component (Y) is preferably 0.5 to 1.5, and more preferably 0.6 to 1.4, from the viewpoint of blocking resistance and strength.

[0065] The resin material for powder coatings of the present invention may contain known additives that can be used in powder coating compositions, such as coloring pigments, extender pigments, inorganic fillers, leveling agents, flow improvers, and defoaming agents.

[0066] Known inorganic and organic pigments can be used as coloring pigments. Examples of inorganic pigments include red iron oxide, chromium titanium yellow, yellow iron oxide, titanium dioxide, and carbon black. Organic pigments include azo, perylene, condensed azo, nitro, nitroso, and futanari pigments. Examples of pigments include cyanine-based, anthraquinone-based, quinacridone-based, and dioxane-based pigments. 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-based pigments include phthalocyanine blue and phthalocyanine green. Examples of anthraquinone pigments include induthlene blue and diantraquinonyl red. Examples of quinacridone-based pigments include quinacridone red and quinacridone violet. Examples of dioxane-based pigments include carbazole dioxazine violet.

[0067] From the viewpoint of smoothness and strength, the weight of the colorant contained in the resin composition for powder coatings is preferably 0.05 to 60% by weight for inorganic pigments and 0.05 to 20% by weight for organic pigments, relative to the total weight of the copolymer (A) and polyester resin (B) contained in the resin composition for powder coatings.

[0068] Examples of extender pigments include talc, silica, calcium carbonate, and barium sulfate.

[0069] Inorganic fillers can be used to improve chemical resistance and rust prevention, and examples include alumina, silica, precipitated barium sulfate, calcium carbonate, clay, talc, mica, zinc phosphate, and aluminum phosphate.

[0070] From the viewpoint of smoothness and strength, the weight of the inorganic filler contained in the resin composition for powder coating is preferably 5 to 60% by weight, more preferably 5 to 50% by weight, and particularly preferably 5 to 35% by weight, relative to the total weight of the (meth)acrylic copolymer (A) and polyester resin (B) contained in the resin composition for powder coating.

[0071] Examples of leveling agents include polyolefin resins with a manganese content of 100 to 1,000 [polyethylene, polypropylene, etc.], olefin-(meth)acrylic acid copolymers [ethylene-(meth)acrylic acid copolymers, etc.], and polyvinylpyrrolidone. The weight of the leveling agent contained in the resin composition for powder coating is preferably 0.1 to 6% by weight, and more preferably 0.5 to 3% by weight, relative to the total weight of the copolymer (A) and polyester resin (B) contained in the resin composition for powder coating, from the viewpoint of blocking resistance, smoothness, gloss, and strength.

[0072] Examples of defoaming agents include mineral oil, silicone oil, and benzoin. The weight of the defoaming agent is preferably 0.1 to 6% by weight, and more preferably 0.5 to 3% by weight, relative to the total weight of the copolymer (A) and polyester resin (B) contained in the resin composition for powder coatings, from the viewpoint of blocking resistance, smoothness, gloss, and strength.

[0073] Examples of fluidity improvers include fatty acid amides, polyethylene wax, oxidized polyethylene wax, and hydrogenated castor oil wax. From the viewpoint of blocking resistance and smoothness, waxes having a melting point of 40 to 160°C are preferred, and waxes having a melting point of 50 to 150°C are more preferred. In this invention, the melting point of the fluidity improver is defined as the dissolution peak temperature (Tpm), measured in accordance with differential scanning calorimetry (DSC) as specified in JIS K7121 (2012). From the viewpoint of smoothness and strength, the weight of the fluidity improver is preferably 0 to 10% by weight, and more preferably 0 to 5% by weight, relative to the total weight of the copolymer (A) and polyester resin (B) contained in the resin composition for powder coatings.

[0074] The resin composition for powder coatings of the present invention can be obtained by pre-mixing a copolymer (A), a polyester resin (B), and other components used as needed using a known mixer such as a super mixer and a Henschel mixer, and then melt-kneading them using a known kneader such as a cone kneader and an extruder. The melt-mixing should be carried out at a temperature at which at least a portion of the raw materials melt and the whole can be mixed. From the viewpoint of kneadability and preventing reactions during mixing, the temperature during melt-mixing is preferably 80°C to 140°C, and more preferably 90°C to 130°C.

[0075] The resin composition for powder coatings obtained by melt-kneading or the like may be pulverized to a predetermined particle size using known pulverizing devices such as hammer mills and jet impact mills, and the pulverized resin composition for powder coatings can be used as a powder coating. The pulverized resin composition for powder coatings of the present invention can be further classified as needed to remove large and fine particles and adjust the particle size distribution. For classification, an air classifier, vibrating sieve, ultrasonic sieve, etc., can be used.

[0076] The resin composition for powder coatings of the present invention is preferably in the form of particles obtained by pulverization or the like. When it is in the form of particles, the volume-average particle size of the particles is preferably 5 to 60 μm, and more preferably 15 to 40 μm, from the viewpoint of smoothness, gloss, and workability of the coating film. The volume-average particle size of the resin composition for powder coatings in this invention was measured using a particle size distribution analyzer that utilizes laser diffraction scattering [product name "Microtrac MT3000II Particle Size Analyzer", manufactured by Nikkiso Co., Ltd.].

[0077] The powder coating of the present invention comprises the above-mentioned resin composition for powder coatings, and can be obtained by pulverizing the above-mentioned resin composition for powder coatings. The powder coating of the present invention can form a coating film (cured coating film) by curing it by heating or the like after it has been applied to the object to be coated.

[0078] The powder coating of the present invention may be the pulverized resin composition for powder coating used as is, or it may be a mixture with other known additives (such as a flow improver and an anti-blocking agent).

[0079] Examples of objects to be coated using the powder coating of the present invention include iron plates, steel plates, aluminum plates, ceramic plates, and those with surface treatments. The aforementioned powder coating film formation may be performed on top of an existing primer film on the object to be coated. Known primers such as electrodeposition paints and primers can be used as the primer film for forming the primer. Application methods include spray painting, electrostatic powder coating, and fluid immersion coating, with electrostatic powder coating being preferred from the viewpoint of coating efficiency.

[0080] The thickness of the coating film made by the aforementioned powder coating is preferably 10 μm to 150 μm, and more preferably 20 μm to 100 μm, from the viewpoint of preventing mottling and transparency of the coating film, and preventing the generation of bubbles on or inside the coating film.

[0081] The curing temperature of the powder coating applied to the object to be coated can be adjusted according to the type and amount of curing agent used. From the viewpoint of coating film curability and industrial considerations, the curing temperature is preferably 100°C to 230°C, and more preferably 140°C to 200°C. The curing time of the aforementioned powder coating can be adjusted according to the type and amount of curing agent used and the curing temperature, and from the viewpoint of coating film curability and industrial use, it is preferably 6 to 60 minutes, more preferably 8 to 30 minutes, and particularly preferably 10 to 20 minutes.

[0082] By using a powder coating containing the resin composition for powder coatings of the present invention to form a cured coating film on a substrate, it is possible to manufacture building materials, electrical products, office equipment, automobile bodies, exterior panels, and parts, etc., that have a coating film with a good balance of high smoothness, excellent gloss, and strength.

[0083] This specification discloses the following:

[0084] The present disclosure (1) is a resin composition for powder coatings containing a (meth)acrylic acid ester copolymer (A) in which an epoxy group (meth)acrylate (a1) and at least one monomer selected from the group consisting of styrene (a2) and alkyl (alkyl group with 1 to 11 carbon atoms) (meth)acrylate (a3) ​​are essential constituent monomers, the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 63% by weight, the glass transition temperature is 45 to 65°C, the epoxy group amount (moles / kg) is 2.6 to 3.1, and the number average molecular weight is 1500 to 2500.

[0085] Disclosure (2) is the resin composition for powder coatings according to Disclosure (1), further comprising a polyester resin (B) having a carboxyl group.

[0086] Disclosure (3) is a resin composition for powder coatings according to Disclosure (2), wherein the ratio (epoxy groups / carboxyl groups) of the number of moles of epoxy groups in the (meth)acrylic acid ester copolymer (A) to the number of moles of carboxyl groups in the polyester resin (B) is 0.5 to 1.5.

[0087] Disclosure (4) is a powder coating comprising a powder coating composition described in any of Disclosures (1) to (3). In this invention, the SP value is an abbreviation for solubility parameter, and serves as a measure of solubility. A higher SP value indicates higher polarity, while a lower SP value indicates lower polarity. The SP value is calculated at 25°C using the method described on pages 151-154 of Volume 14 of Polymer Engineering and Science by Robert F Fedors et al. [Examples]

[0088] The present invention will be further described below with reference to examples, but the present invention is not limited thereto. In the following description, "parts" refers to parts by weight.

[0089] <Manufacturing Example 1> Production of polyester resin (B) having carboxyl groups In a reaction vessel equipped with a reflux condenser, stirrer, thermometer, condenser, and nitrogen gas inlet, the raw materials listed in the "Raw Materials and Amounts Used" column for polyester resin (B) having carboxyl groups in Table 1 were added, and 0.1 parts of di-n-butyltin oxide (manufactured by Nitto Chemical Co., Ltd.) were mixed in. The mixture was then gradually heated to 240°C in a nitrogen stream, and the esterification reaction was carried out while distilling off the generated water to obtain polyester resin (B-1). The carboxyl group concentration, number-average molecular weight, Tg, and SP value of the obtained polyester resin are shown in Table 1.

[0090] [Table 1]

[0091] The compounds used in Table 1 were as follows: Isophthalic acid: Manufactured by Mitsubishi Gas Chemical Company, Inc. Terephthalic acid: Manufactured by Toray Industries, Inc. Neopentyl glycol: Manufactured by Mitsubishi Gas Chemical Company, Inc. Trimethylolpropane: Manufactured by Lanxess

[0092] <Examples 1-3, Comparative Examples 1-2> In a pressure-resistant reaction vessel equipped with a thermometer, stirrer, reflux condenser, and nitrogen inlet, 66.7 parts of xylene (reagent grade 1, manufactured by Nippon Refine Co., Ltd.) were charged. After stirring, the air in the reaction vessel was replaced with nitrogen gas, and then the mixture was heated under reflux. Next, after heating under reflux, the vessel was sealed and the temperature was raised to 150°C. Then, the monomer composition obtained by mixing the raw materials listed in the "Raw Materials and Amount Used" column of Table 1 for the epoxy group-containing (meth)acrylic copolymer (A) was added dropwise over 3 hours. After the entire amount had been added dropwise, the reaction was continued at the same temperature for another 2 hours. Subsequently, the reaction vessel was gradually depressurized to remove the xylene, and resin compositions for powder coatings consisting of epoxy group-containing (meth)acrylic copolymer copolymers (A-1) to (A-3) and (ratio A-1) and (ratio A-2), respectively, were obtained. The number-average molecular weight, epoxy group amount, glass transition temperature, and SP value of the obtained copolymers are shown in Table 2.

[0093] [Table 2]

[0094] The compounds used in Table 2 were as follows: GMA: Glycidyl methacrylate (manufactured by NOF Corporation) MMA: Methyl methacrylate (manufactured by Mitsubishi Chemical Corporation) IBA: Isobutyl acrylate (manufactured by Wako Pure Chemical Industries, Ltd.) Styrene: Manufactured by Idemitsu Kosan Co., Ltd. 2-Hydroxyethyl methacrylate: Manufactured by Mitsubishi Chemical Corporation Di-t-butyl peroxide: Manufactured by NOF Corporation

[0095] <Examples 4-7, Comparative Examples 3-4> According to the formulations shown in Table 3, copolymer (A) and polyester resin (B) were placed in a Buss AG single-screw continuous kneader (product name: Co-Kneader) and melt-kneaded at 120°C. Next, the resulting mixture was pulverized using a pulverizer (product name: Atomizer) manufactured by Fuji Powder Co., Ltd. The resulting pulverized material was classified using a 325-mesh Tyler standard sieve [mesh opening 44 μm], and then fine particles were removed using an airflow classifier DS-2 type [manufactured by Nippon Pneumatic Mfg. Co., Ltd.] to obtain the resin compositions for powder coatings according to Examples 4-7 and Comparative Examples 3-4. The obtained powder coating compositions were used as powder coatings, and their storage stability, as well as the smoothness, gloss, and strength of the coating film, were measured using the following method and are shown in Table 3.

[0096] [Table 3]

[0097] <Storage stability of powder coatings> Each of the powder coatings according to Examples 4-7 and Comparative Examples 3-4 was stored in a 40°C incubator. Coating films were prepared every week from the start of storage using the method described below, and the arithmetic mean roughness (Ra) of the coating films obtained using the surface roughness method described below was measured. The process of creating a coating and measuring the surface roughness was repeated every week until the arithmetic mean roughness (Ra) exceeded 1.2. The storage period from the start of storage up to the week before the week in which the arithmetic mean roughness exceeded 1.2 is recorded in the storage stability column of Table 3. A higher storage stability number indicates a longer storage period during which a smooth coating can be obtained, and thus good storage stability. A storage stability of 5 weeks or more indicates that the powder coating has good storage stability.

[0098] <Physical properties of the coating film> A coated sheet was created by treating a cold-rolled copper sheet (75 x 150 x 0.8 mm) conforming to JIS G 3141 (SPCC~SD) with zinc phosphate. Using an electrostatic coating machine for powder coatings (applied voltage -80kV), powder coating was applied to the substrate to a dry film thickness of 80μm, and the coating film was created by baking at 160°C for 15 minutes.

[0099] (Smoothness of the coating film) The average surface roughness (Ra) of the coating film prepared using the method described above was measured using a surface roughness meter (manufactured by Mitutoyo Corporation), and the values ​​are shown in Table 3. A smaller average surface roughness value indicates higher surface smoothness, and a value less than 0.7 indicates good smoothness.

[0100] (Glossiness of the coating) For the coating film prepared using the method described above, the gloss (60° specular gloss) was measured at an incident angle of 60 degrees in accordance with the method described in JIS K5600-4-7 for the visual characteristics of coating films, and the values ​​are listed in Table 3. A higher value indicates better gloss, and a value exceeding 90% indicates that the coating film has good gloss.

[0101] (Coating film strength based on drop impact test) The strength of the coating film prepared using the method described above was evaluated by a drop impact test conducted in accordance with the DuPont method for the mechanical properties of coating films, as specified in JIS K 5600-5-3. For the drop impact test, a DuPont-type impact tester manufactured by Ueshima Seisakusho Co., Ltd. (impact die size: 1 / 2 inch, weight mass: 500 g) was used. The test was conducted by changing the height of the weight in 5cm increments until cracking or peeling of the coating due to impact deformation was observed. The maximum height at which no cracking or peeling of the coating was observed is shown in Table 3. Note that a higher number indicates higher strength, and a value greater than 30cm indicates that the coating has good strength.

[0102] The results in Table 3 show that the powder coating composition of the present invention has superior storage stability compared to the comparative composition. [Industrial applicability]

[0103] The coating film obtained from the resin composition for powder coatings of the present invention exhibits excellent gloss, smoothness, and strength, as well as superior storage stability. The resin composition for powder coatings of the present invention is particularly suitable for use in coating applications for civil engineering, buildings, and home appliances.

Claims

1. A resin composition for powder coatings containing a (meth)acrylic acid ester copolymer (A) comprising an epoxy group-containing (meth)acrylate (a1) and styrene (a2) and / or alkyl (alkyl group having 1 to 11 carbon atoms) (meth)acrylate (a3) ​​as essential constituent monomers, wherein the ratio of the total weight of styrene (a2) and the (meth)acrylate (a3) ​​to the total weight of the constituent monomers is 55 to 63% by weight, the glass transition temperature is 45 to 65°C, the epoxy group amount (moles / kg) is 2.6 to 3.1, and the number average molecular weight is 1500 to 2500.

2. The resin composition for powder coatings according to claim 1, further comprising a polyester resin (B) having a carboxyl group.

3. The resin composition for powder coatings according to claim 2, wherein the ratio (epoxy groups / carboxyl groups) of the number of moles of epoxy groups in the (meth)acrylic acid ester copolymer (A) to the number of moles of carboxyl groups in the polyester resin (B) is 0.5 to 1.

5.

4. A powder coating comprising the resin composition for powder coatings according to any one of claims 1 to 3.

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