Method for forming multilayer coating film

Abstract

Provided is a method for forming a multilayer coating film wherein: a base coating composition (Y) includes a hydroxyl group-containing acrylic resin (y1) that has a weight-average molecular weight of 13,000-35,000 and a glass transition temperature of not lower than 20°C but lower than 80°C, and a polyol compound (y2) that has a hydroxyl value of more than 200 mgKOH / g but not more than 1,000 mgKOH / g and a molecular weight of 100-1,000; the solid content of the hydroxyl group-containing acrylic resin (y1) is 50-95 parts by mass per 100 parts by mass of resin solids of the base coating composition (Y); a clear coating composition (Z) includes a hydroxyl group-containing acrylic resin (z1) that has a hydroxyl value of 90-190 mgKOH / g, a weight-average molecular weight of 4,000-6,000, and a glass transition temperature of 15-100°C, a hydroxyl group-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4) that includes at least one metal selected from the group consisting of zinc, bismuth, and tin, and an organic amine catalyst (z5) that has an amidine group; and the solid content of the hydroxyl group-containing acrylic resin (z1) is 50-95 parts by mass per 100 parts by mass of resin solids of the clear coating composition (Z).

Description

Method for forming a multi-layer coating

[0001] The present invention relates to a method for forming a multi-layer coating film.

[0002] Automobile components typically have a multi-layer coating consisting of a base coating and a clear coating on top of it, for the purpose of providing aesthetic appeal and durability. From an energy-saving perspective, the base coating and clear coating are cured simultaneously (two-coat, one-bake method) to reduce the heating process.

[0003] As a coating composition used in the above method, Patent Document 1 discloses a clear coating composition containing an organometallic catalyst consisting of a metal compound and an amidine compound. According to Patent Document 1, this clear coating composition hardens at low temperatures and in a short time.

[0004] International Publication No. 2013 / 047209

[0005] In the two-coat, one-bake method, a clear coating is typically formed using a two-component clear coating composition containing a polyisocyanate compound as a curing agent. In this case, the polyisocyanate compound is expected to penetrate from the uncured clear coating into the uncured base coating, thereby improving the crosslinking density of the base coating.

[0006] As described in Patent Document 1, when an organometallic catalyst consisting of a metal compound and an amidine compound is incorporated into a clear coating composition, the curing time of the uncured clear coating is shortened, making it difficult for the polyisocyanate compound to sufficiently penetrate from the uncured clear coating to the uncured base coating. As a result, the crosslinking density of the base coating is insufficient, and the physical properties of the coating deteriorate.

[0007] The present invention aims to solve the above-mentioned conventional problems and to provide a method for forming a multi-layer coating that yields a coating with good appearance and high hardness.

[0008] To solve the above problems, the present invention provides the following embodiments. [1] A method for forming a multilayer coating film, comprising: applying at least one layer of base coating composition (Y) to an object to be coated to form at least one uncured base coating film; applying a clear coating composition (Z) to the uncured base coating film to form a clear coating film; and heating and curing the uncured base coating film and the uncured clear coating film, wherein the base coating composition (Y) comprises: a hydroxyl group-containing acrylic resin (y1) having a weight-average molecular weight of 13,000 or more and 35,000 or less, and a glass transition temperature of 20°C or more and less than 80°C; and a polyol compound (y2) having a hydroxyl value exceeding 200 mgKOH / g and 1,000 mgKOH / g or less, and a molecular weight of 100 or more and 1,000 or less; and the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). A method for forming a multilayer coating film, wherein the clear coating composition (Z) comprises a hydroxyl group-containing acrylic resin (z1) having a hydroxyl value of 90 mg KOH / g or more and 190 mg KOH / g or less, a weight-average molecular weight of 4000 or more and 6000 or less, and a glass transition temperature of 15°C or more and 100°C or less, a hydroxyl group-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth and tin, and an organic amine catalyst (z5) having an amidine group, wherein the solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the clear coating composition (Z). [2] The method for forming a multilayer coating film according to [1], wherein the polyol compound (y2) has two hydroxyl groups and an alicyclic hydrocarbon group. [3] The method for forming a multilayer coating film according to [1] or [2] above, wherein the hydroxyl group-containing acrylic resin (y1) has a hydroxyl value of 30 mg KOH / g or more and 100 mg KOH / g or less. [4] The method for forming a multilayer coating film according to any of [1] to [3] above, wherein the solid content of the polyol compound (y2) is 2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y).[5] The method for forming a multilayer coating film according to any of [1] to [4] above, wherein the hydroxyl group-containing polyester resin (z2) has a hydroxyl value of 250 mg KOH / g or more and less than 500 mg KOH / g, and a weight-average molecular weight of 1500 or more and 2500 or less. [6] The method for forming a multilayer coating film according to any of [1] to [5] above, wherein the base coating composition (Y) has a solid content concentration of 30% by mass or more and 60% by mass or less. [7] The method for forming a multilayer coating film according to any of [1] to [6] above, wherein the clear coating composition (Z) has a solid content concentration of 45% by mass or more and 60% by mass or less. [8] The method for forming a multilayer coating film according to any of [1] to [7] above, wherein the object to be coated includes a resin portion containing at least one selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin. [9] The method for forming a multilayer coating film according to any of [1] to [8] above, wherein the organic amine catalyst (z5) has a heterocycle containing a nitrogen atom.

[10] The method for forming a multilayer coating film according to any of [1] to [9] above, wherein the metal catalyst (z4) includes at least one selected from the group consisting of zinc, bismuth, or tin acylate compounds, alkoxide compounds, chelate compounds, and oxide compounds.

[0009] The present invention provides a method for forming a multilayer coating that yields a coating with a good appearance and high hardness.

[0010] In this embodiment, a polyol compound (y2) with low molecular weight and high hydroxyl value is used as the base coating component. Because the polyol compound (y2) has a low molecular weight, the base coating composition (Y) (hereinafter also referred to as the uncured base coating) has low viscosity. Therefore, the polyisocyanate compound (z3) can easily penetrate from the uncured clear coating (hereinafter also referred to as the clear coating composition) into the uncured base coating. As a result, the crosslinking density of the base coating increases. In addition, because the polyol compound (y2) has a high hydroxyl value, the crosslinking density becomes even higher, resulting in a multilayer coating with excellent physical properties. Physical properties of the coating refer to, for example, water resistance, hardness, tackiness, and adhesion.

[0011] The clear coating composition contains an organic amine catalyst (z5) having an amidine group, resulting in a fast curing rate. However, because the viscosity of the base coating composition (Y) is sufficiently low, it is believed that the polyisocyanate compound (z3) penetrates into the uncured base coating film before the clear coating composition hardens. The effect of the polyol compound (y2) is more pronounced when the curing rate of the clear coating composition is fast.

[0012] First, the base coating composition (Y) and the clear coating composition (Z) will be described.

[0013] (Y) Base coating composition The base coating composition (Y) according to this embodiment comprises a hydroxyl group-containing acrylic resin (y1) and a polyol compound other than the hydroxyl group-containing acrylic resin (y2).

[0014] The solid content concentration of the base coating composition (Y) is, for example, 30% by mass or more and 60% by mass or less. When the solid content concentration of the base coating composition (Y) is within the above range, curing can be performed in a shorter time and at a lower temperature, contributing to energy saving. The solid content concentration of the base coating composition (Y) may be 35% by mass or more. The solid content concentration of the base coating composition (Y) may be 50% by mass or less. The solid content concentration of the base coating composition (Y) may be 35% by mass or more and 50% by mass or less.

[0015] Solids are also called non-volatile components. Specifically, the solids of a substance are the total components remaining after removing the solvent. The solids concentration is calculated by dividing the total mass of the solids (after removing the solvent) by the total mass of the substance. Alternatively, the solids concentration can be calculated from the residue after heating the substance at 140°C, in accordance with JIS K 5601-1-2, the method for measuring residuals after heating.

[0016] Even if the base coating composition (Y) has a high solid content within the above range, its viscosity is low. The viscosity of the base coating composition (Y) with a solid content of 35% by mass, measured by the flow cup method at 23°C, is, for example, 20 seconds or less. This makes it easier for the polyisocyanate compound (z3) to penetrate from the clear coating composition into the uncured base coating film.

[0017] The above viscosity is that of the base paint composition (Y) within 30 minutes after preparation, assuming the viscosity at the time of actual painting. Hereinafter, the above viscosity is referred to as the painting viscosity η ０ .

[0018] The viscosity of the base paint composition (Y) is measured at 23°C using a No. 4 Ford cup in accordance with the "3. Flow cup method" of JIS K5600-2-2:1999. The viscosity is the average value of the viscosities of five different paint compositions with the same composition. The painting viscosity η ０ of the clear paint composition (Z) is also measured in the same manner.

[0019] (y1) Hydroxyl group-containing acrylic resin The hydroxyl group-containing acrylic resin (y1) is a resin (film-forming component) that serves as the base of the base paint film. The hydroxyl group-containing acrylic resin (y1) reacts with a curing agent such as a polyisocyanate compound and / or a melamine resin to form a crosslinked structure.

[0020] The hydroxyl group-containing acrylic resin has a plurality of acryloyl groups and one or more (typically, two or more) hydroxyl groups in one molecule. The "acrylic resin" is obtained by polymerizing at least one monomer of acrylic acid and its esters, methacrylic acid and its esters.

[0021] The weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) is 13,000 or more and 35,000 or less. When the weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) is 13,000 or more, the hardness and weather resistance of the resulting paint film are improved. When the weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) is 35,000 or less, an excessive increase in the viscosity of the base paint composition (Y) is suppressed, and the penetration of the polyisocyanate compound (z3) into the uncured base paint film is promoted. The weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) may be 15,000 or more, and may be 20,000 or more. The weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) may be 30,000 or less, and may be 25,000 or less. The weight average molecular weight of the hydroxyl group-containing acrylic resin (y1) may be 15,000 or more and 30,000 or less, and may be 20,000 or more and 25,000 or less.

[0022] The weight-average molecular weight can be calculated from the chromatogram measured by gel permeation chromatography, using the molecular weight of standard polystyrene as a reference. For example, the HLC-8200 (manufactured by Tosoh Corporation) is used as the gel permeation chromatograph. The measurement conditions using this are as follows: Column: TSgel Super Multipore HZ-M 3-column eluent: Tetrahydrofuran column inlet Oven: 40°C Flow rate: 0.35 ml Detector: Differential refractive index detector (RI) Standard polystyrene: PS oligomer kit manufactured by Tosoh Corporation

[0023] The glass transition temperature (Tg) of the hydroxyl group-containing acrylic resin (y1) is 20°C or higher and less than 80°C. When the Tg of the hydroxyl group-containing acrylic resin (y1) is 20°C or higher, the stain resistance, scratch resistance, and hardness of the resulting coating film are improved. When the Tg of the hydroxyl group-containing acrylic resin (y1) is less than 80°C, the quick-drying properties of the base coating composition (Y) are improved. The Tg of the hydroxyl group-containing acrylic resin (y1) may be 40°C or higher. The Tg of the hydroxyl group-containing acrylic resin (y1) may be 70°C or lower, or 50°C or lower. The Tg of the hydroxyl group-containing acrylic resin (y1) may be 20°C or higher and 70°C or lower, or 40°C or higher and 50°C or lower.

[0024] The glass transition temperature (Tg) is determined using a differential scanning calorimeter (DSC) by the following method: A hydroxyl group-containing acrylic resin (y1) is heated from 20°C to 150°C at a heating rate of 10°C / min (Step 1), followed by a cooling from 150°C to -50°C at a cooling rate of 10°C / min (Step 2), and then heated from -50°C to 150°C at a heating rate of 10°C / min (Step 3). The value obtained from the chart during the heating in Step 3 is the Tg of the hydroxyl group-containing acrylic resin (y1). For example, a thermal analyzer SSC5200 (manufactured by Seiko Electronics) is used as the DSC.

[0025] The hydroxyl value (OHV) of the hydroxyl group-containing acrylic resin (y1) is, for example, 30 mg KOH / g or more and 100 mg KOH / g or less. When the above hydroxyl value is 30 mg KOH / g or more, the crosslinking density tends to be high. When the hydroxyl value of the hydroxyl group-containing acrylic resin (y1) is 100 mg KOH / g or less, the hydrophilization of the coating film is suppressed, and the water resistance of the multi-layer coating film tends to improve. The above hydroxyl value may be 40 mg KOH / g or more. The above hydroxyl value may be 70 mg KOH / g or less, or 60 mg KOH / g or less. The above hydroxyl value may be 40 mg KOH / g or more and 70 mg KOH / g or less, or 40 mg KOH / g or more and 60 mg KOH / g or less.

[0026] The hydroxyl value can be determined by the neutralization titration method using an aqueous potassium hydroxide solution as described in JIS K 0070.

[0027] The hydroxyl group-containing acrylic resin (y1) can be used alone or in combination of two or more types.

[0028] The solid content of the hydroxyl group-containing acrylic resin (y1) relative to 100 parts by mass of the resin solid content of the base coating composition (Y) is 50 parts by mass or more and 95 parts by mass or less. When the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more, the hardness of the base coating film is improved. When the solid content of the hydroxyl group-containing acrylic resin (y1) is 95 parts by mass or less, a sufficient amount of polyol compound (y2) can be blended, and the polyisocyanate compound can easily penetrate from the uncured clear coating film. The solid content of the hydroxyl group-containing acrylic resin (y1) may be 60 parts by mass or more, and may be 70 parts by mass or more. The solid content of the hydroxyl group-containing acrylic resin (y1) may be 90 parts by mass or less, and may be 88 parts by mass or less. The solid content of the hydroxyl group-containing acrylic resin (y1) may be 60 parts by mass or more and 90 parts by mass or less, and 70 parts by mass or more and 88 parts by mass or less.

[0029] Examples of raw material monomers for the hydroxyl group-containing acrylic resin (y1) include hydroxy acrylate esters such as 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate; hydroxy methacrylate esters such as 2-hydroxyethyl methacrylate and 4-hydroxybutyl methacrylate. Furthermore, if necessary, acrylic acid; acrylic acid esters such as methyl acrylate, butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and isobolonyl acrylate; methacrylic acid; methacrylic acid esters such as methyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and isobolonyl methacrylate; and ethylenically unsaturated monomers having an aromatic ring such as styrene may also be used. These can be used individually or in combination of two or more. Commercially available hydroxyl group-containing acrylic resin (y1) may also be used.

[0030] (y2) Polyol compound The polyol compound (y2) is also a coating film-forming component. The polyol compound (y2) reacts with the polyisocyanate compound (z3) to form a crosslinked structure.

[0031] The polyol compound (y2) is other than the hydroxyl group-containing acrylic resin (y1) and has two or more hydroxyl groups in one molecule. The molecular weight (weight-average molecular weight in the case of a polymer) of the polyol compound (y2) is between 100 and 1,000, making it a low molecular weight compound. Therefore, the polyisocyanate compound (z3) easily penetrates from the clear coating composition (Z) into the uncured base coating film. In addition, the hydroxyl value of the polyol compound (y2) is high, ranging from over 200 mg KOH / g to 1,000 mg KOH / g. Therefore, even with the addition of a small amount, the crosslinking density of the base coating composition (Y) can be improved.

[0032] The molecular weight of the polyol compound (y2) may be 110 or more, or 120 or more. The molecular weight of the polyol compound (y2) may be 800 or less, or 500 or less, or 300 or less. The molecular weight of the polyol compound (y2) may be 110 or more and 800 or less, or 120 or more and 800 or less, or 120 or more and 300 or less.

[0033] The hydroxyl value of the polyol compound (y2) may be 300 mg KOH / g or more, or 400 mg KOH / g or more. The hydroxyl value of the polyol compound (y2) may be 950 mg KOH / g or less, or 900 mg KOH / g or less. The hydroxyl value of the polyol compound (y2) may be 300 mg KOH / g or more and 950 mg KOH / g or less, or 400 mg KOH / g or more and 900 mg KOH / g or less.

[0034] The solid content of the polyol compound (y2) relative to 100 parts by mass of resin solids in the base coating composition (Y) is, for example, 2 parts by mass or more and 10 parts by mass or less. When the solid content of the polyol compound (y2) is 2 parts by mass or more, the polyisocyanate compound penetrates more easily from the uncured clear coating film. When the solid content of the polyol compound (y2) is 10 parts by mass or less, the drying properties of the base coating composition are easily improved. The solid content of the polyol compound (y2) may be 3 parts by mass or more, and may be 5 parts by mass or more. The solid content of the polyol compound (y2) may be 8 parts by mass or less, and may be 7 parts by mass or less. The solid content of the polyol compound (y2) may be 3 parts by mass or more and 8 parts by mass or less, and may be 5 parts by mass or more and 7 parts by mass or less.

[0035] The polyol compound (y2) is not particularly limited as long as it is other than the hydroxyl group-containing acrylic resin (y1) and the hydroxyl group-containing polyester resin (z2) and has two or more hydroxyl groups in one molecule. The polyol compound (y2) may have alicyclic hydrocarbon groups, as this easily improves the physical properties of the resulting coating film (especially hardness and weather resistance). From a similar viewpoint, the polyol compound (y2) may have two hydroxyl groups.

[0036] At least one hydroxyl group of the polyol compound (y2) may be directly bonded to an alicyclic hydrocarbon group or bonded via a C1 or C2 hydrocarbon group. The alicyclic hydrocarbon group may be a five-membered ring or a six-membered ring.

[0037] The polyol compound (y2) may have two hydroxyl groups and an alicyclic hydrocarbon group. Examples of such polyol compounds (y2) include diol compounds such as cyclohexanediol, cyclohexanedimethanol, cyclohexanediethanol, and cyclopentanedimethanol. The arrangement of the two hydroxyl groups in the diol compound is not particularly limited and may be 1,1-, 1,2-, 1,3-, or 1,4-. From the viewpoint of hardness, it may be 1,4-cyclohexanedimethanol, in which the two hydroxyl groups are bonded to cyclohexane via a methylene group.

[0038] (Other hydroxyl group-containing components) The base coating composition (Y) according to this embodiment may include, for example, hydroxyl group-containing acrylic resins other than the hydroxyl group-containing acrylic resin (y1), hydroxyl group-containing polyester resins, and polyol compounds other than the polyol compound (y2) as other hydroxyl group-containing components. These may be used individually or in combination of two or more.

[0039] Other hydroxyl group-containing acrylic resins include, for example, hydroxyl group-containing acrylic resins that satisfy at least one of the following conditions: (a) weight-average molecular weight is less than 13,000, (b) weight-average molecular weight is greater than 35,000, (c) Tg is less than 20°C, and (d) Tg is 80°C or higher. Other polyol compounds include, for example, polycarbonate polyol resins, polyether polyol resins, and polycaprolactone polyol resins.

[0040] It is desirable that the amount of other hydroxyl group-containing components be small. The amount of other hydroxyl group-containing components relative to 100 parts by mass of the total solid content of hydroxyl group-containing components is, for example, 20 parts by mass or less, may be 15 parts by mass or less, may be 10 parts by mass or less, may be 5 parts by mass or less, may be 3 parts by mass or less, or may be 0 parts by mass.

[0041] (Curing agent) The base coating composition (Y) may contain a curing agent. The curing agent further enhances the adhesion between the substrate and the base coating, or the adhesion between the intermediate coating or primer coating applied on the substrate and the base coating. It also improves the coating properties of the base coating.

[0042] The curing agent may include other curing agents such as blocked isocyanate compounds, melamine resins, guanamine resins, urea resins, and other amino resins. These may be used individually or in combination of two or more. The curing agent content is, for example, 5 to 30 parts by mass per 100 parts by mass of the resin solids content of the base coating composition.

[0043] Blocked isocyanate compounds can be prepared by blocking polyisocyanate compounds with a blocking agent. Examples of polyisocyanate compounds include those identical to the polyisocyanate compound (z3) described later.

[0044] Examples of blocking agents include monohydric alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenolcarbinol, and methylphenylcarbinol; cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type terminal diols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycolphenol; polyester-type terminal polyols obtained from diols such as ethylene glycol, propylene glycol, and 1,4-butanediol, and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, and sebacic acid; phenols such as para-t-butylphenol and cresol; oximes such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam. As a blocking agent, active hydrogen compounds such as methyl diketones, methyl ketoesters, and methyl diester compounds, such as alkyl esters including acetylacetone, ethyl acetoate, and diethyl malonate, may be used. Alternatively, blocked isocyanates using pyrazole compounds or imidazole compounds as blocking agents may be used.

[0045] It is preferable that the blocking rate of the blocked isocyanate compound is 100%. This enhances the storage stability of the base coating composition.

[0046] (Pigments) The base coating composition (Y) may contain pigments. The pigments impart design properties to the coating film.

[0047] Examples of pigments include coloring pigments, luminosity pigments, and extender pigments. Examples of coloring pigments include organic coloring pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolon pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments; and inorganic coloring pigments such as lead yellow, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These can be used individually or in combination of two or more.

[0048] Examples of luminous pigments include metal flakes (aluminum, chromium, gold, silver, copper, brass, titanium, nickel, nickel-chromium, stainless steel, etc.), metal oxide flakes, pearl pigments, glass flakes coated with metal or metal oxide, silica flakes coated with metal oxide, graphite, holographic pigments, and cholesteric liquid crystal polymers. These can be used individually or in combination of two or more.

[0049] Examples of extender pigments include calcium carbonate, barium sulfate, clay, and talc. These can be used individually or in combination of two or more.

[0050] The total pigment concentration, i.e., the mass ratio (PWC) of the total pigment to 100% by mass of the resin solids content of the base coating composition (Y), is, for example, 0.1% by mass or more and 60% by mass or less. This makes it less likely that the smoothness of the resulting coating film will be impaired. The PWC of individual pigments is not particularly limited. The PWC of a lustrous pigment may be 1% by mass or more and may be 5% by mass or more. The PWC of a lustrous pigment may be 40% by mass or less and may be 30% by mass or less. The PWC of a lustrous pigment may be, for example, 1% by mass or more and 40% by mass or less and may be 5% by mass or more and 30% by mass or less.

[0051] (Diluting Components) The base coating composition (Y) may contain diluting components. The base coating composition (Y) is diluted with diluting components as appropriate, taking into consideration the coating method, the coating environment such as temperature and humidity, etc. Examples of diluting components include water and non-aqueous solvents. The base coating composition (Y) according to this embodiment may also contain non-aqueous solvents used in the manufacture of each component.

[0052] Non-aqueous solvents include, for example, aliphatic or alicyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, cycloheptane, methylcycloheptane, and mineral split; ketone organic solvents such as acetone, acetylacetone, methyl ethyl ketone, methyl-i-butyl ketone, methyl amyl ketone, and cyclohexanone; aromatic hydrocarbon organic solvents such as benzene, toluene, ethylbenzene, propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, and decalin; ester organic solvents such as methyl acetate, ethyl acetate, n-butyl acetate, and aluminum acetate; cellosolve organic solvents such as methyl cellosolve, ethyl cellosolve, n-propyl cellosolve, i-propyl cellosolve, n-butyl cellosolve, i-butyl cellosolve, i-amyl cellosolve, phenyl cellosolve, and benzyl cellosolve; methyl carbitol, etc. Examples of carbitol-based organic solvents include carbitol carbitol, n-propyl carbitol, i-propyl carbitol, n-butyl carbitol, i-butyl carbitol, i-amyl carbitol, carbitol acetate, phenyl carbitol, and benzyl carbitol; and ether-based organic solvents such as ethylene glycol monoisopropyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-n-butyl ether, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and dioxane. These can be used individually or in combination of two or more.

[0053] (Additives) The base coating composition (Y) according to this embodiment may also contain other additives commonly used in the coating field. Examples of additives include ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, viscosity modifiers, surface modifiers, film-forming aids, and rust inhibitors.

[0054] (Preparation of base coating composition) The method for producing the base coating composition (Y) is not particularly limited, and methods known in the art can be used, such as stirring, kneading or dispersing the above-mentioned components using a disper, homogenizer, roll, sand grind mill or kneader.

[0055] (Z) Clear coating composition The clear coating composition (Z) according to this embodiment comprises a hydroxyl group-containing acrylic resin (z1), a hydroxyl group-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4), and an organic amine catalyst (z5).

[0056] The solid content concentration of the clear coating composition (Z) is, for example, 45% by mass or more and 60% by mass or less. The above solid content concentration is for the clear coating composition (Z) used for painting. The clear coating composition (Z) used for painting is, for example, diluted to a viscosity suitable for painting using a diluent.

[0057] The solid content concentration of the clear coating composition (Z) may be 50% by mass or more, and may be 55% by mass or more. The solid content concentration of the clear coating composition (Z) may be 59% by mass or less, and may be 58% by mass or less. The solid content concentration of the clear coating composition (Z) may be 50% by mass, 59% by mass or less or more, and may be 55% by mass or more and 58% by mass or less.

[0058] The clear coating composition (Z) has low viscosity even if its solid content concentration is high within the above range. The coating viscosity η of the clear coating composition (Z) with a solid content concentration of 58% by mass is measured by the flow cup method at 23°C. ０ For example, it is 25 seconds or less. Coating viscosity η of the clear coating composition (Z) ０This refers to the product immediately after preparation (specifically, within 30 minutes after the hardener and other components have been mixed).

[0059] (z1) Hydroxyl group-containing acrylic resin The hydroxyl group-containing acrylic resin (z1) is a resin (film-forming component) that forms the base of the clear coating film. The hydroxyl group-containing acrylic resin (z1) reacts with a curing agent such as a polyisocyanate compound to form a crosslinked structure.

[0060] The weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) is 4,000 or more and 6,000 or less. When the weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) is 4,000 or more, the hardness and weather resistance of the resulting coating film are improved. When the weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) is 6,000 or less, excessive viscosity increase of the clear coating composition (Z) is suppressed, and the penetration of the polyisocyanate compound (z3) into the uncured clear coating film is promoted. The weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) may be 4,200 or more, or 4,300 or more. The weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) may be 5,800 or less, or 5,500 or less. The weight-average molecular weight of the hydroxyl group-containing acrylic resin (z1) may be 4,200 or more and 5,800 or less, and may be 4,300 or more and 5,500 or less.

[0061] The glass transition temperature (Tg) of the hydroxyl group-containing acrylic resin (z1) is 15°C or higher and 100°C or lower. When the Tg of the hydroxyl group-containing acrylic resin (z1) is 15°C or higher, the stain resistance, scratch resistance, and hardness of the resulting coating film are improved. When the Tg of the hydroxyl group-containing acrylic resin (z1) is 100°C or lower, the quick-drying properties of the clear coating composition (Z) are improved. The Tg of the hydroxyl group-containing acrylic resin (z1) may be 18°C ​​or higher and may be 20°C or higher. The Tg of the hydroxyl group-containing acrylic resin (z1) may be 95°C or lower and may be 90°C or lower. The Tg of the hydroxyl group-containing acrylic resin (z1) may be 18°C ​​or higher and 95°C or lower, and may be 20°C or higher and 90°C or lower.

[0062] The hydroxyl value (OHV) of the hydroxyl group-containing acrylic resin (z1) is 90 mg KOH / g or more and 190 mg KOH / g or less. When the above hydroxyl value is 90 mg KOH / g or more, the crosslinking density is high. When the hydroxyl value of the hydroxyl group-containing acrylic resin (z1) is 190 mg KOH / g or less, the hydrophilization of the coating film is suppressed and the water resistance of the multi-layer coating film is improved. The above hydroxyl value may be 100 mg KOH / g or more, or 110 mg KOH / g or more. The above hydroxyl value may be 180 mg KOH / g or less, or 170 mg KOH / g or less. The above hydroxyl value may be 100 mg KOH / g or more and 180 mg KOH / g or less, or 110 mg KOH / g or more and 170 mg KOH / g or less.

[0063] The acid value of the hydroxyl group-containing acrylic resin (z1) may be, for example, 2 mg KOH / g or more and 30 mg KOH / g or less. This can further improve the smoothness of the resulting coating film. Furthermore, when the coating composition is applied on another uncured coating film, the occurrence of mixed layers can be suppressed. The acid value of the hydroxyl group-containing acrylic resin (z1) may be 3 mg KOH / g or more. The acid value of the hydroxyl group-containing acrylic resin (z1) may be 20 mg KOH / g or less, and may be 15 mg KOH / g or less. The acid value of the hydroxyl group-containing acrylic resin (z1) may be 3 mg KOH / g or more and 20 mg KOH / g or less, and may be 3 mg KOH / g or more and 15 mg KOH / g or less.

[0064] The hydroxyl group-containing acrylic resin (z1) can be used alone or in combination of two or more types.

[0065] The solid content of the hydroxyl group-containing acrylic resin (z1) relative to 100 parts by mass of the resin solid content of the clear coating composition (Z) is, for example, 50 parts by mass or more and 95 parts by mass or less. When the solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more, the hardness of the base coating film is improved. When the solid content of the hydroxyl group-containing acrylic resin (z1) is 95 parts by mass or less, a sufficient amount of polyisocyanate compound (z3) can be blended, and the polyisocyanate compound (z3) easily penetrates into the uncured base coating film. The solid content of the hydroxyl group-containing acrylic resin (z1) may be 55 parts by mass or more, and may be 60 parts by mass or more. The solid content of the hydroxyl group-containing acrylic resin (z1) may be 90 parts by mass or less, and may be 88 parts by mass or less. The solid content of the hydroxyl group-containing acrylic resin (z1) may be 55 parts by mass or more and 90 parts by mass or less, and 60 parts by mass or more and 88 parts by mass or less.

[0066] Examples of raw material monomers for the hydroxyl group-containing acrylic resin (z1) include those similar to those for the hydroxyl group-containing acrylic resin (y1).

[0067] (z2) Hydroxyl group-containing polyester resin The hydroxyl group-containing polyester resin (z2) is also a coating film-forming component. The hydroxyl group-containing polyester resin (z2) reacts with the polyisocyanate compound (z3) to form a crosslinked structure. The hydroxyl group-containing polyester resin (z2) has multiple ester bonds and one or more hydroxyl groups.

[0068] The hydroxyl group-containing acrylic resin (z1) tends to increase the viscosity of the clear coating composition (Z). On the other hand, the hydroxyl group-containing polyester resin (z2) generally has low viscosity and tends to increase the hydroxyl value. By using the hydroxyl group-containing polyester resin (z2) in combination, it is possible to improve the crosslinking density while suppressing the increase in viscosity.

[0069] The hydroxyl value of the hydroxyl group-containing polyester resin (z2) is, for example, 250 mg KOH / g or more and less than 500 mg KOH / g. If the above hydroxyl value is 250 mg KOH / g or more, the crosslinking density may be further increased. If the above hydroxyl value is less than 500 mg KOH / g, the hydrophilization of the coating film is suppressed, and the water resistance of the clear coating film may be further improved. The hydroxyl value of the hydroxyl group-containing polyester resin (z2) may be 260 mg KOH / g or more, and may be 270 mg KOH / g or more. The hydroxyl value of the hydroxyl group-containing polyester resin (z2) may be 480 mg KOH / g or less, and may be 450 mg KOH / g or less. The hydroxyl value of the hydroxyl group-containing polyester resin (z2) may be 260 mg KOH / g or more and 480 mg KOH / g or less, and may be 270 mg KOH / g or more and 450 mg KOH / g or less.

[0070] The weight-average molecular weight of the hydroxyl group-containing polyester resin (z2) is, for example, 1,500 or more and 2,500 or less. When the weight-average molecular weight is 1,500 or more, the hardness and weather resistance of the resulting coating film can be further improved. When the weight-average molecular weight is 2,500 or less, excessive viscosity increase of the coating composition is further suppressed. The weight-average molecular weight of the hydroxyl group-containing polyester resin (z2) may be 1,600 or more, or 1,700 or more. The weight-average molecular weight of the hydroxyl group-containing polyester resin (z2) may be 2,400 or less, or 2,300 or less. The weight-average molecular weight of the hydroxyl group-containing polyester resin (z2) may be 1,600 or more and 2,400 or less, or 1,700 or more and 2,300 or less.

[0071] From the viewpoint of viscosity, the hydroxyl group-containing polyester resin (z2) may have a hydroxyl value of 250 mg KOH / g or more and less than 500 mg KOH / g, and a weight-average molecular weight of 1,500 or more and 2,500 or less.

[0072] The amount of hydroxyl group-containing polyester resin (z2) relative to 100 parts by mass of the total solid content of hydroxyl group-containing components is, for example, 4 parts by mass or more. This makes it easier to improve the smoothness of the resulting coating film. The amount of hydroxyl group-containing polyester resin (z2) relative to 10 parts by mass or more may be 7 parts by mass or more. The amount of hydroxyl group-containing polyester resin (z2) relative to 30 parts by mass or less may be, for example. This makes it easier to improve the drying properties of the clear coating composition (Z). The amount of hydroxyl group-containing polyester resin (z2) relative to 25 parts by mass or less may be 20 parts by mass or less. In one embodiment, the amount of hydroxyl group-containing polyester resin (z2) relative to 100 parts by mass of the total solid content of hydroxyl group-containing components is 4 parts by mass or more and 30 parts by mass or less, 7 parts by mass or more and 25 parts by mass or less, and 10 parts by mass or more and 20 parts by mass or less.

[0073] The hydroxyl group-containing polyester resin (z2) can be obtained, for example, by polycondensation (esterification reaction) of a polyhydric alcohol with a polybasic acid or its anhydride. A commercially available hydroxyl group-containing polyester resin (z2) may also be used.

[0074] The polyhydric alcohols are not particularly limited and include, for example, ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, hydrogenated bisphenol A, hydroxyalkylated bisphenol A, 1,4-cyclohexanedimethanol, 2,2-dimethyl- Examples include 3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate, 2,2,4-trimethyl-1,3-pentanediol, N,N-bis-(2-hydroxyethyl)dimethylhydantoin, polytetramethylene ether glycol, polycaprolactone polyol, glycerin, sorbitol, trimethylolethane, trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol, dipentaerythritol, and tris-(hydroxyethyl) isocyanate. These can be used individually or in combination of two or more.

[0075] Polybasic acids or their anhydrides are not particularly limited and include, for example, phthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, methyltetrahydrophthalic acid, methyltetrahydrophthalic anhydride, hymic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, isophthalic acid, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, lactic acid, dodecenyl succinic acid, dodecenyl succinic anhydride, cyclohexane-1,4-dicarboxylic acid, and endo anhydride. These can be used individually or in combination of two or more.

[0076] The hydroxyl group-containing polyester resin (z2) may be modified using lactones, oils or fatty acids, melamine resin, urethane resin, etc. The oils or fatty acids are not particularly limited and include, for example, oils such as castor oil, dehydrated castor oil, coconut oil, corn oil, cottonseed oil, linseed oil, perilla oil, poppy oil, safflower oil, soybean oil, and tung oil, or fatty acids extracted from these oils.

[0077] (Other hydroxyl group-containing components) The clear coating composition (Z) according to this embodiment may include, for example, hydroxyl group-containing acrylic resins other than the hydroxyl group-containing acrylic resin (z1), and various other polyol compounds as other hydroxyl group-containing components. These may be used individually or in combination of two or more.

[0078] Other hydroxyl group-containing acrylic resins include, for example, hydroxyl group-containing acrylic resins that satisfy at least one of the following conditions: (a) hydroxyl value less than 90 mgKOH / g, (b) hydroxyl value greater than 190 mgKOH / g, (c) weight-average molecular weight less than 4,000, (d) weight-average molecular weight greater than 6,000, (e) Tg less than 15°C, and (f) Tg greater than 100°C. Other polyol compounds include polyol compounds (y2), polycarbonate polyol resins, polyether polyol resins, and polycaprolactone polyol resins.

[0079] It is desirable that the amount of other hydroxyl group-containing components be small. The amount of other hydroxyl group-containing components relative to 100 parts by mass of the total solid content of hydroxyl group-containing components is, for example, 20 parts by mass or less, may be 15 parts by mass or less, may be 10 parts by mass or less, may be 5 parts by mass or less, may be 3 parts by mass or less, or may be 0 parts by mass.

[0080] (z3) Polyisocyanate compound The polyisocyanate compound (z3) is a curing agent that reacts with hydroxyl group-containing components to form a crosslinked structure and cure the paint composition. A portion of the polyisocyanate compound (z3) penetrates into the uncured base coating film (base paint composition) and acts as a curing agent for the base paint composition.

[0081] Polyisocyanate compounds (z3) have at least two isocyanate groups in one molecule. Examples of polyisocyanate compounds (z3) include aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic polyisocyanates having aromatic rings not bonded to isocyanate groups in the molecule (aroliphatic polyisocyanates), aromatic polyisocyanates, and derivatives of these polyisocyanates. Specifically, examples include aromatic polyisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, and metaxylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate; and polymers of these, such as biuret type, nurate type, and adduct type. These can be used individually or in combination of two or more.

[0082] The equivalent ratio (NCO / OH) of isocyanate groups in the polyisocyanate compound (z3) to hydroxyl groups in the hydroxyl group-containing component may be 0.7 or higher, or 0.8 or higher. The equivalent ratio (NCO / OH) may be 2.0 or lower, 1.8 or lower, or 1.5 or lower. In one embodiment, the equivalent ratio (NCO / OH) is 0.7 or higher and 2.0 or lower. When the equivalent ratio (NCO / OH) is within this range, a sufficient amount of polyisocyanate compound (z3) can penetrate the uncured base coating film, while the curing reaction of the clear coating film is also easily carried out. The equivalent ratio (NCO / OH) may be 0.7 or higher and 1.8 or lower, or 0.8 or higher and 1.5 or lower.

[0083] (Other curing agents) The clear coating composition (Z) according to this embodiment may further contain, as other curing agents, at least one selected from the group consisting of, for example, amino resins, epoxy compounds, aziridine compounds, carbodiimide compounds, and oxazoline compounds. The content of the other curing agents is appropriately set depending on the coating film forming components.

[0084] (z4) Metal compound metal catalyst (z4) is a curing catalyst. The metal catalyst (z4) can form a complex with the organic amine catalyst (z5) in the clear paint composition (Z). By heating, the metal catalyst (z4) and the organic amine catalyst (z5) dissociate and act as curing catalysts respectively. When the metal catalyst (z4) and the organic amine catalyst (z5) are used in combination, the low-temperature curability is improved.

[0085] The metal catalyst (z4) contains at least one metal M selected from the group consisting of zinc, bismuth, and tin. The metal catalyst (z4) may be an organometallic catalyst containing the metal M. The metal catalyst (z4) may be, for example, an acrylate compound, an alkoxide compound, a chelate compound, or an oxide compound of the above metal M. These are used alone or in combination of two or more.

[0086] For example, the acrylate compound of zinc has the following general formula (C11): Zn-(O-C(=O)-R ｃ１１ ) ２ (C11) (In the formula, R ｃ１１ represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms.) It is represented by.

[0087] The carbon number of R ｃ１１ may be 3 or more, may be 4 or more, and may be 5 or more. The carbon number of R ｃ１１ may be 9 or less, may be 8 or less, and may be 7 or less. R ｃ１１ may be a linear, branched or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0088] For example, the acrylate compound of tin has the following general formula (C13): Sn-(O-C(=O)-R ｃ１３ ) ２ (C, 13) (In the formula, R ｃ１３ represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms.) It is represented by.

[0089] The carbon number of R ｃ１３ may be 3 or more, may be 4 or more, and may be 5 or more. The carbon number of R ｃ１３ may be 11 or less, may be 10 or less. Rｃ１３ This may be a linear, branched, or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0090] For example, bismuth acylate compounds have the following general formula (C12): Bi-(O-C(=O)-R ｃ１２ ) ３ (C12) (wherein, R ｃ１２ ) represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms.

[0091] R ｃ１２ The number of carbon atoms may be 3 or more, 4 or more, or 5 or more. ｃ１２ The number of carbon atoms may be 9 or less, 8 or less, or 7 or less. ｃ１２ This may be a linear, branched, or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0092] Examples of alkoxide compounds of metal M are given by the following general formula (C2): M(OR ｃ２ ) ｓ (C2) (wherein, R ｃ２ Each of the terms independently represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms, and s represents the valence of the metal M.

[0093] R ｃ２ The number of carbon atoms may be 2 or more, 3 or more, or 4 or more. ｃ２ The number of carbon atoms may be 11 or less, 10 or less, or 9 or less. ｃ２ This may be a linear, branched, or cyclic aliphatic hydrocarbon group, and may be a linear or branched aliphatic hydrocarbon group.

[0094] A chelate compound of metal M is, for example, one of the following general formulas (C3): M(R ｃ３ ) ｔ (C3) (wherein, R ｃ３(where represents an acetylacetonate group, ethylacetonate group, propylacetonate group, isopropylacetonate group, butylacetonate group, propionylacetonate group, or proponylacetonate group, and t represents the coordination number of the metal M.)

[0095] When metal M is tin, the organoxide compound of tin is, for example, the following general formula (C4): Sn(=O)-(-R ｃ１４ ) ２ (C4) (wherein, R ｃ１３ (This represents an aliphatic hydrocarbon group having 1 to 12 carbon atoms.)

[0096] Examples of metal catalysts (z4) include organozinc compounds such as zinc naphthenate; organobismuth compounds such as bismuth oxide, bismuth hydroxide, and bismuth carboxylate; and organotin compounds such as diacetyltin diacetate, dibutyltin dilaurate, dibutyltin diacetate, dioctyltin dilaurate, diacetyltin dioctoate, tin octoate, dibutyltin diacetate, and dibutyltin dioctoate.

[0097] The content of the metal catalyst (z4) is, for example, 0.005% by mass or more and 0.05% by mass or less relative to the resin solids. This can further improve the smoothness and pot life of the resulting coating film. The content of the metal catalyst (z4) may be 0.008% by mass or more, and may be 0.01% by mass or more. The content of the metal catalyst (z4) may be 0.05% by mass or less, and may be 0.03% by mass or less. The content of the metal catalyst (z4) may be 0.008% by mass or more and 0.05% by mass or less, and may be 0.01% by mass or more and 0.03% by mass or less.

[0098] (z5) Organic amine catalyst The organic amine catalyst (z5) has an amidine group. The organic amine catalyst (z5) does not contain metal atoms. The organic amine catalyst (z5) exhibits excellent catalytic activity under heating. In particular, the organic amine catalyst (z5) shows high activity even at low temperatures (e.g., below 100°C), improving the physical properties of the resulting coating film. In addition, with the organic amine catalyst (z5), the curing reaction can be completed in a short time (e.g., below 20 minutes), even at low temperatures. Therefore, post-treatment such as polishing can be performed immediately after the curing treatment.

[0099] By combining a clear coating composition (Z) containing an organic amine catalyst (z5) with a base coating composition containing a polyol compound (y2), low-temperature curing is possible in a two-coat, one-bake method, and a coating film with excellent physical properties can be obtained.

[0100] The amidine group has a structure in which a carbon (C) atom is bonded to one nitrogen (N) atom by a double bond and to one nitrogen atom by a single bond. The general formula for the amidine group is: -C(=NR １ )-N(R ２ ) - is represented by this.

[0101] In the formula, R １ and R ２ These are, independently of each other, a hydrogen atom and a monovalent or divalent hydrocarbon group having 1 to 3 carbon atoms. １ and R ２ This means that a ring structure (a heterocycle containing an N atom) may be formed via one or more C atoms.

[0102] Examples of organic amine catalysts (z5) that do not have the above heterocycle include N'-cyclohexyl-N,N-dimethylformamidine, N'-methyl-N,N-di-n-butylacetamidine, N'-octadecyl-N,N-dimethylformamidine, N'-cyclohexyl-N,N-dimethylvaleroamidine, 1-methyl-2-cyclohexyliminopyrrolidine, 3-butyl-3,4,5,6-tetrahydropyrimidine, N-(hexyliminomethyl)morpholine, N-(α-(decyliminoethyl)ethyl)pyrrolidine, N'-decyl-N,N-dimethylformamidine, N'-dodecyl-N,N-dimethylformamidine, and N'-cyclohexyl-N,N-acetamidine.

[0103] Examples of the above heterocycles include an imidazoline ring, an imidazole ring, a tetrahydropyrimidine ring, a dihydropyrimidine ring, and a pyrimidine ring. The organic amine catalyst (z5) may have an imidazole ring.

[0104] The organic amine catalyst (z5) may have the above-mentioned heterocycle, or it may be a polycyclic compound having the above-mentioned heterocycle.

[0105] Examples of organic amine catalysts (z5) having an imidazole ring include N-(2-hydroxyethyl)imidazole, N-(3-aminopropyl)imidazole, 4-(hydroxymethyl)imidazole, 1-(tert-butoxycarbonyl)imidazole, imidazole-4-propionic acid, 4-carboxyimidazole, 1-butylimidazole, 1-methylimidazole, 2-methyl-4-imidazolecarboxylic acid, 4-formylimidazole, 1-(ethoxycarbonyl)imidazole, reaction products of propylene oxide with imidazole and 2-methylimidazole, 1-trimethylsilylimidazole, 4-(hydroxymethyl)imidazole hydrochloride, 1H-imidazolium, 3-ethyl-1-methyl-benzo Examples include ethyl acetate, copolymers of 1-chloro-2,3-epoxypropane and imidazole, 1-(p-toluenesulfonyl)imidazole, 1,1-carbonylbisimidazole, 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole, 2-phenyl-2-imidazoline pyromelitate, 4-(hydroxymethyl)imidazole picrate, disodium salts of 2-propenoic acid, 4,5-dihydro-2-nonyl-1H-imidazole-1-ethanol, and 2-heptyl-4,5-dihydro-1H-imidazole-1-ethanol, 1-(cyanoethyl)-2-undecylimidazole trimellitate, formate esters of 1-(2-hydroxypropyl)imidazole, sodium imidazole salts, and silver imidazole salts.

[0106] Examples of polycyclic organic amine catalysts (z5) include 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), 1,4-diazabicyclo[3.3.0]octe-4-ene, 2-methyl-1,5-diazabicyclo[4.3.0]one-5-ene, 2,7,8-trimethyl-1,5-diazabicyclo[4.3.0]one-5-ene, 2-butyl-1,5-diazabicyclo[4.3.0]one-5-ene, and 1,9-diazabicyclo[6.5.0]tridece-8-ene.

[0107] The organic amine catalyst (z5) can be used individually or in combination of two or more types.

[0108] The content of the organic amine catalyst (z5) is, for example, 0.1% by mass or more and 1.5% by mass or less relative to the resin solids. This can further improve the smoothness and pot life of the resulting coating film. The content of the organic amine catalyst (z5) may be 0.2% by mass or more, and may be 0.3% by mass or more. The content of the organic amine catalyst (z5) may be 1.0% by mass or less, and may be 0.8% by mass or less. The content of the organic amine catalyst (z5) may be 0.2% by mass or more and 1.0% by mass or less, and may be 0.3% by mass or more and 0.8% by mass or less.

[0109] (Other Catalysts) The clear coating composition (Z) according to this embodiment may contain catalysts other than the organic amine catalyst (z5). Examples of other catalysts include organometallic compounds other than zinc, bismuth, and tin, amine compounds that do not have an amidine group, and boric acid compounds. These may be used individually or in combination of two or more.

[0110] Other organometallic compounds include, for example, organoaluminum compounds such as aluminum trimethoxide, aluminum tris(acetylacetonate), aluminum tri-n-butoxide, aluminum tris(acetacetate ethyl), aluminum diisopropoxy(acetacetate ethyl), and aluminum acetylacetonate; organotitanium compounds such as titanium tetra(monoethyl ethoxide), titanium tetra(monoethyl ethoxide), titanium tetra(monobutyl ethoxide), titanium tetrakis(acetylacetonate), and tetran-butyl titanate; and organozirconium compounds such as zirconium tetra(monomethyl ethoxide), zirconium tetra(monoethyl ethoxide), zirconium tetra(monobutyl ethoxide), zirconium-n-propylate, zirconium-n-butyrate, and zirconium tetrakis(acetylacetonate).

[0111] Examples of amine compounds that do not have an amidine group include trimethylamine, triethylamine, 2-(dimethylamino)ethyl methacrylate, 1-methylpiperidine, 1-methylpyrrolidine, pyridine, 4-dimethylaminopyridine, 4-(1-piperidyl)pyridine, N-methylimidazole, and N,N-dimethylaniline.

[0112] Examples of boric acid compounds include boric acid esters such as trimethyl borate, triethyl borate, tripropyl borate, tributyl borate, triphenyl borate, tri(4-chlorophenyl) borate, and trihexafluoroisopropyl borate.

[0113] The content of other catalysts is, for example, 0.05% by mass or less relative to the resin solids. The content of other catalysts may be 0.03% by mass or less, or 0.02% by mass or less. The content of other catalysts may be 0.01% by mass or more. The content of other catalysts may be 0% by mass or more.

[0114] (Diluting Components) The clear coating composition (Z) according to this embodiment may contain diluting components. The clear coating composition (Z) is diluted with diluting components as appropriate, taking into consideration the coating method, the coating environment such as temperature and humidity, etc. Examples of diluting components include water and the above-mentioned non-aqueous solvents. The clear coating composition (Z) according to this embodiment may also contain non-aqueous solvents used in the manufacture of each component, etc.

[0115] (Additives) The clear coating composition (Z) according to this embodiment may contain other additives commonly used in the coating field. For example, it may contain coloring pigments and / or gloss pigments, to the extent that they do not impair transparency. Furthermore, it may contain ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, viscosity modifiers, surface modifiers, film-forming aids, rust inhibitors, etc.

[0116] [Method for forming a multi-layer coating] A multi-layer coating is formed by a method comprising: applying at least one layer of base coating composition (Y) to a workpiece to form at least one uncured base coating; applying a clear coating composition (Z) to the uncured base coating to form a clear coating; and heating and curing the uncured base coating and the uncured clear coating.

[0117] A primer film may be formed between the object to be coated and the base coating. The primer film can be formed in the same way as the base coating.

[0118] (I) Formation of an uncured base coating film An uncured base coating film is formed by applying the above-mentioned base coating composition (Y) to the object to be coated. The base coating composition (Y) is applied, for example, so that the thickness of the base coating film after curing is 10 μm or more and 50 μm or less.

[0119] The painting method is not particularly limited. Examples of painting methods include air spray painting, airless spray painting, and rotary atomization painting. These methods may be combined with electrostatic painting. Among these, rotary atomization electrostatic painting is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic painting, rotary atomization electrostatic painting machines commonly known as "micro-microbell (μμbell)", "microbell (μbell)", or "metallicbell (metabell)" are used.

[0120] After applying the base coating composition (Y), pre-drying (preheating) may be performed. This suppresses the boiling of the diluent components in the base coating composition (Y) during the curing process, making it easier to suppress the occurrence of bubbling. Furthermore, pre-drying suppresses the mixing of the uncured base coating film and the clear coating composition (Z) applied on top of it, making it difficult for a mixed layer to form. As a result, the smoothness of the resulting coated article is further improved.

[0121] The conditions for pre-drying are not particularly limited. Examples of pre-drying include leaving the product at room temperature for 5 to 15 minutes, or heating it at a temperature of 50°C to 80°C for 30 seconds to 5 minutes.

[0122] (II) Formation of an uncured clear coating The uncured clear coating is formed by applying the above-mentioned clear coating composition (Z) onto the uncured base coating. The clear coating composition (Z) is applied, for example, so that the thickness of the clear coating after curing is 15 μm or more and 60 μm or less.

[0123] The painting method is not particularly limited. For example, the painting method may be the same as that used for the base coating composition. Among these, rotary atomizing electrostatic coating is preferred from the viewpoint of coating efficiency. After painting the clear coating composition (Z), pre-drying may be performed. The conditions for pre-drying are not particularly limited and may be the same as those for pre-drying the base coating film.

[0124] (III) Curing each of the cured and uncured coating films. Each coating film can be cured by heating. In this step, the base coating film and the clear coating film are cured at the same time.

[0125] The heating conditions are set appropriately according to the composition of each coating composition and the material of the object to be coated. The heating temperature is, for example, 60°C to 100°C. According to the clear coating composition (Z) of this embodiment, a clear coating film with high hardness is formed even at such low temperatures. The heating temperature may be, for example, 75°C or lower, or 70°C or lower.

[0126] The heating time can be set appropriately according to the heating temperature. When the heating temperature is between 60°C and 100°C, the heating time may be, for example, between 5 minutes and 20 minutes, or between 5 minutes and 15 minutes. The heating time refers to the time during which the heating device is maintained at the target temperature, and does not take into account the time it takes to reach the target temperature. Examples of heating devices include drying ovens that utilize heat sources such as hot air, electricity, gas, and infrared radiation.

[0127] (Subject to be coated) The material of the subject to be coated is not particularly limited. Examples of subject materials include metal, resin, and glass.

[0128] The shape of the object to be painted is not particularly limited. Specifically, the objects to be painted include automobile bodies and parts for automobile bodies such as passenger cars, trucks, motorcycles, and buses, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and door handles.

[0129] Examples of metals include iron, copper, aluminum, tin, zinc, or alloys thereof (e.g., steel). Typical examples of metals to be coated include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electro-galvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy plated steel sheets, zinc-iron alloy plated steel sheets, zinc-magnesium alloy plated steel sheets, zinc-aluminum-magnesium alloy plated steel sheets, aluminum plated steel sheets, aluminum-silicon alloy plated steel sheets, and tin plated steel sheets.

[0130] Metallic workpieces may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After surface treatment, metallic workpieces may be further coated with electrodeposition paint. The electrodeposition paint may be cationic or anionic.

[0131] Examples of resins include polyethylene resin, EVA resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), acrylic resin, acrylonitrile butadiene styrene (ABS) resin, polycarbonate-polybutylene terephthalate (PC-PBT) resin, polycarbonate-acrylonitrile butadiene styrene (PC-ABS resin), polycarbonate resin, vinyl chloride resin, polyamide resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), acrylonitrile styrene (AS) resin, acetal resin, phenolic resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). The resin-coated object may be degreased.

[0132] The clear coating composition (Z) according to this embodiment is suitable for coating resin substrates because it can be cured at low temperatures. The substrate may include both a resin part (a part formed from resin) and a metal part (a part formed from metal). The substrate may be made of resin. The substrate may include a resin part comprising at least one selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin.

[0133] [Coated Article] This embodiment provides a coated article comprising a base coating film formed on an object to be coated, and a clear coating film formed on the base coating film.

[0134] The thickness of the base coating is not particularly limited and can be set appropriately according to the purpose. The thickness of one layer of the base coating is, for example, 10 μm or more, may be 15 μm or more, or may be 20 μm or more. The thickness of one layer of the base coating is, for example, 50 μm or less, may be 45 μm or less, or may be 40 μm or less. The thickness of one layer of the base coating is, for example, 10 μm or more and 50 μm or less, may be 15 μm or more and 45 μm or less, or may be 20 μm or more and 40 μm or less.

[0135] The thickness of the coating can be measured using an electromagnetic film thickness gauge (for example, SANKO's SDM-miniR). The coating thickness is the average value of the coating thickness at five arbitrary points.

[0136] The thickness of the clear coating is not particularly limited. From the viewpoint of scratch resistance and smoothness, the thickness of the clear coating after drying may be, for example, 15 μm or more, and may be 20 μm or more. The thickness of the clear coating may be 60 μm or less, and may be 40 μm or less. The thickness of the clear coating after drying may be 15 μm or more and 60 μm or less, and may be 20 μm or more and 40 μm or less.

[0137] A primer film may be interposed between the workpiece and the base coating. The primer film improves the adhesion between the base coating and the workpiece (especially resin workpieces). Furthermore, if the surface of the workpiece is uneven, the primer coating makes the painted surface uniform, which helps to suppress unevenness in the base coating.

[0138] The primer coating is formed by a primer paint composition comprising, for example, a film-forming component, a material adhesion component, a viscosity modifier, a diluent, a pigment, and optionally a curing agent. The primer paint composition may optionally contain various of the above additives. The primer paint composition may be solvent-based or water-based. Examples of the film-forming component, curing agent, viscosity modifier, diluent, and pigment include the components exemplified as being incorporated into the base paint composition.

[0139] The thickness of the primer film is not particularly limited. In terms of the smoothness and chipping resistance of the painted article, the thickness of the primer film may be 5 μm or more and 40 μm or less. The thickness of the primer film may be 7 μm or more. The thickness of the primer film may be 25 μm or less. The thickness of the primer film may be 7 μm or more and 25 μm or less.

[0140] The present invention will be described below with reference to examples. However, the present invention is not limited to the examples described below.

[0141] [Examples 1-7, Comparative Examples 1-7] (1) Preparation of base coating composition (Y) and clear coating composition (Z) Each component was mixed according to the components and amounts shown in Tables 3 and 4 to obtain base coating composition (Y) and clear coating composition (Z). The solid content concentration of base coating composition (Y) was 35% by mass, and the solid content concentration of clear coating composition (Z) was 58% by mass.

[0142] (2) Formation of multilayer coating film A substrate made of ABS resin was prepared as the substrate to be coated and wiped with isopropyl alcohol. Next, the above base coating composition (Y) was applied to this substrate using a spray gun (manufactured by Anest Iwata Corporation; W-101-134G) to a dry film thickness of 15 μm. After that, it was dried for 3 minutes in an environment with a temperature of 20 ± 5°C and a relative humidity of 78% or less to form an uncured base coating film on the ABS substrate.

[0143] Next, the freshly prepared clear coating composition (Z) was applied to the uncured base coating film using a spray gun (Anest Iwata Corporation; W-101-134G) to a dry film thickness of 30 μm. After that, the substrate was left for 10 minutes in an environment with a temperature of 20 ± 5°C and a relative humidity of 78% or less to form an uncured clear coating film.

[0144] Next, a coated article having a multi-layer coating with a cured base coating and a clear coating was obtained by heating in a dryer at 70°C for 10 minutes.

[0145] The components used in the examples and comparative examples are as follows. The hydroxyl group-containing acrylic resins (y1) and (z1) were manufactured as follows.

[0146] [Production of hydroxyl group-containing acrylic resin (y1-1)] 57 parts of butyl acetate were charged into a reactor equipped with a stirring blade, thermometer, dropping device, temperature control device, nitrogen gas inlet, and cooling tube, and the temperature was raised to 120°C while stirring and introducing nitrogen gas. Next, a mixture consisting of 0.5 parts methacrylic acid, 56.6 parts 2-ethylhexyl methacrylate, 16.7 parts methyl methacrylate, 15.0 parts styrene, 2.3 parts 2-hydroxyethyl methacrylate, and 8.9 parts lactone-modified 2-hydroxyethyl methacrylate, along with a solution of 2.0 parts t-butyl peroxy-2-ethylhexanate dissolved in 5 parts butyl acetate, was added dropwise to the reactor over 3 hours. After the dropwise addition was complete, the mixture was allowed to mature for 1 hour, and then a solution of 0.2 parts t-butyl peroxy-2-ethylhexanate dissolved in 5 parts butyl acetate was added dropwise to the reactor over 1 hour, and the mixture was allowed to mature for 2 hours while maintaining the temperature at 120°C to complete the reaction. The obtained hydroxyl group-containing resin had a non-volatile content of 60% and a weight-average molecular weight of 13,000. The glass transition temperature was 20°C, and the hydroxyl value was 30 mgKOH / g.

[0147]

[0148] [Production of hydroxyl group-containing acrylic resins (y1-2), (a1), and (a2)] Hydroxyl group-containing acrylic resins (y1-2), (a1), and (a2) were produced in the same manner as hydroxyl group-containing acrylic resin (y1-1), except that the types and amounts of monomers were as shown in Table 1.

[0149] [Production of hydroxyl group-containing acrylic resin (z1-1)] In a reactor equipped with a stirring blade, thermometer, dropping device, temperature control device, nitrogen gas inlet, and cooling tube, 57 parts of butyl acetate were charged, and the temperature was raised to 120°C under stirring while introducing nitrogen gas. To this reactor, a mixture consisting of 0.8 parts methacrylic acid, 26.8 parts 2-ethylhexyl acrylate, 33.0 parts methyl methacrylate, and 39.4 parts 2-hydroxyethyl methacrylate, and a solution of 10 parts t-butyl peroxy-2-ethylhexanate dissolved in 5 parts butyl acetate were added dropwise over 3 hours. After the dropwise addition was complete, the reactor was allowed to mature for 1 hour. Then, a solution of 0.2 parts t-butyl peroxy-2-ethylhexanate dissolved in 5 parts butyl acetate was added dropwise over 1 hour. The reactor was allowed to mature for 2 hours while maintaining the temperature at 120°C to complete the reaction and obtain hydroxyl group-containing acrylic resin (z1-1). The obtained hydroxyl group-containing acrylic resin (z1-1) had a non-volatile content of 60%, a weight-average molecular weight of 4,500, and a glass transition temperature of 20°C. The hydroxyl value (OHV) was calculated to be 170 from the monomer composition.

[0150] [Production of hydroxyl group-containing acrylic resins (z1-2) and (a3)] Hydroxyl group-containing acrylic resins (z1-2) and (a3) ​​were produced in the same manner as hydroxyl group-containing acrylic resin (z1-1), except that the types and amounts of monomers were as shown in Table 2.

[0151]

[0152] Polyol compound y2-1: 1,4-cyclohexanedimethanol, hydroxyl value 780 mgKOH / g, molecular weight 144

[0153] y2-2: 1,1-Cyclohexanediethanol, hydroxyl value 652 mg KOH / g, molecular weight 172

[0154] p1: Polypropylene glycol diol type 2000, manufactured by Wako Pure Chemical Industries, Ltd., hydroxyl value 56 mg KOH / g, molecular weight 2,000 p2: Polycarbonate diol, Duranol T-5650J, manufactured by Asahi Kasei Corporation, hydroxyl value 140 mg KOH / g, molecular weight 800

[0155] Blocked isocyanate compound (y3), trade name "Duranate MF-K60B", melamine resin manufactured by Asahi Kasei Corporation, trade name "RESIMENE 747", manufactured by Preferred Resins.

[0156] Hydroxyl group-containing polyester resin (z2) Product name: Basonol HPE1170B, manufactured by BASF, hydroxyl value 280 mg KOH / g, weight-average molecular weight 1,800

[0157] Polyisocyanate compound (z3), trade name "N3300", manufactured by Covestro, isocyanurate of hexamethylene diisocyanate.

[0158] Metal compound z4-1: Dibutyltin dilaurate z4-2: Bismuth carboxylate

[0159] Organic amine catalyst (z5) DBU: 1,8-diazabicyclo[5.4.0]unde-7-ene

[0160] KAT-1: 1H-Imidazolium,3-ethyl-1-methyl-,benzoate, CAS No. 150999-33-0

[0161] (h) Diluting component: N-butyl acetate

[0162] [Evaluation] The evaluation was conducted as follows. The evaluation results are shown in Tables 3 and 4.

[0163] (Paint viscosity η) ０ The viscosity of the base coating composition (Y) or clear coating composition (Z) immediately after preparation was measured at 23°C using a No. 4 Ford cup in accordance with JIS K5600-2-2:1999 "3. Flow cup method". The average viscosity of five different clear coating compositions (Z) with the same composition was used to determine the coating viscosity η. ０ The viscosity of the paint was determined to be η. ０ The following criteria were used to evaluate the product.

[0164] Base paint composition (Y), solids content concentration 35% Good: Paint viscosity η ０ Failure if less than 20 seconds: Paint viscosity η ０ Over 20 seconds

[0165] Clear coating composition (Z), solids content concentration 58%, good: coating viscosity η ０ Failure if less than 25 seconds: Paint viscosity η ０ Over 25 seconds

[0166] (Smoothness) The smoothness of the multilayer coating film was measured using a surface measuring instrument (BYK Corporation, Wave Scan-dual) to determine the amount of reflected light in the long wavelength region (1200 μm to 12000 μm) (LW ０ The following criteria were used to measure and evaluate LW. ０ The smaller the value, the smoother the surface. In Wave Scan-dual, laser light is shone onto the test specimen at a 60° angle while the light source is moved, and the reflected light is measured. A rating of B or higher indicates high smoothness and an excellent appearance.

[0167] A: LW ０ 20 or less B:LW ０ Over 20 and under 25 C:LW ０ is over 25

[0168] (Initial Adhesion) A single-blade cutting tool specified in JIS K-5600-5-6 was applied perpendicularly to the multi-layer coating to make cuts (parallel lines 1) that reached the workpiece. Furthermore, 10 cuts parallel to these parallel lines 1 were made at equal intervals. Eleven cuts (parallel lines 2) were made perpendicular to these 11 parallel lines 1 and that reached the substrate, also at equal intervals. The spacing between parallel lines 1 and between parallel lines 2 was 2 mm. In this way, a grid pattern was formed with 100 squares enclosed by four straight lines.

[0169] A transparent pressure-sensitive tape, as specified in JIS K-5600-5-6, was applied to the grid-like pattern above, ensuring that no air bubbles were trapped between it and the painted surface. The tape was then quickly peeled off within 0.5 to 1.0 seconds, and the degree of peeling in the grid-like pattern was visually evaluated. The evaluation criteria were as follows: A: No peeling of the paint film was observed. B: Some peeling of the paint film was observed.

[0170] (Water Resistance) The painted articles were immersed in a water tank maintained at 40°C for 240 hours. After that, the painted articles were removed from the water and dried at room temperature for 1 hour. The multi-layer coating was then evaluated visually and by the same tape peel test as above. A rating of B or higher indicates water resistance. Visual evaluation observed gloss reduction and blister formation. A: No tape peeling, no gloss reduction or blister formation observed. B: No tape peeling, but slight gloss reduction and / or blister formation observed. C: Tape peeling, gloss reduction and blister formation observed.

[0171] (Tackiness) Under a 23°C atmosphere, the surface of the multi-layer coating immediately after curing was pressed with a finger to evaluate the presence or absence of tackiness (adhesion). The evaluation criteria are as follows: Tackiness is more likely to occur in coatings cured at low temperatures. Tackiness is one indicator of the degree of curing. If tackiness is felt, it can be said that curing is insufficient.

[0172] Good: No stickiness felt. Poor: Stickiness felt.

[0173] (Amount of polyisocyanate compound penetration) The base coating composition (Y) and the clear coating composition (Z) were sequentially applied to a fluororesin sheet in the same manner as described above, and cured by heating at 70°C for 10 minutes to obtain a multi-layer coating film for evaluation.

[0174] The multilayer coating was peeled off from the fluororesin sheet, and ATR-IR measurement was performed on the surface of the multilayer coating that was in contact with the sheet under the following conditions, and the ester bond origin was 1730 cm². －１ Infrared absorption peak intensity at and 1670 cm⁻¹ derived from urethane bonding －１ The infrared absorption peak intensity was measured. The peak area ratio (urethane bond area / ester bond area) was determined and applied to a calibration curve prepared in advance using the following procedure to calculate the isocyanate amount α (parts of polyisocyanate compound solids per 100 parts of base paint composition (Y) solids) at the interface between the base coating and the substrate. The isocyanate amount α was used as an indicator of penetration depth.

[0175] The higher the isocyanate content α, the better the polyisocyanate compound penetrates from the clear coating composition into the base coating composition. A rating of "acceptable" or higher indicates that the polyisocyanate compound has penetrated sufficiently.

[0176] Good: The amount of isocyanate α is 5 parts or more. Acceptable: The amount of isocyanate α is 1 part or more but less than 5 parts. Poor: The amount of isocyanate α is less than 1 part.

[0177] (ATR-IR measurement conditions) Instrument: Fourier transform infrared spectrophotometer "FT / IR610", manufactured by JASCO Corporation Measurement mode: ATR method (prism: zinc selenide, incident angle: 45°) Resolution: 4 cm －１ Number of cumulative measurements: 16; Wavelength range: 400 cm －１ ~4000 －１

[0178] (Calibration Curve Preparation Procedure) Three standard coatings were prepared by adding 2, 5, and 10% by mass of the polyisocyanate compound (product name "Desmodule N3300, manufactured by Covestro Japan Co., Ltd.") used in the clear coating composition (Z) to the base coating composition (Y), respectively. Each standard coating was applied to a fluororesin sheet and cured by heating at 70°C for 10 minutes to obtain three standard coating films. A portion of the standard coating film was peeled off from the sheet, and the peak area ratio (urethane bond area / ester bond area) was determined in the same manner as above to obtain a calibration curve.

[0179]

[0180]

[0181] The multilayer coatings in the examples had good appearance and high hardness. The multilayer coatings in Comparative Examples 1, 2, and 4-5 had good appearance but poor physical properties. The multilayer coatings in Comparative Examples 3 and 6 had high hardness but poor appearance. The multilayer coating in Comparative Example 7 was inferior in both hardness and appearance.

[0182] The multilayer coating formed by the present invention is suitable for, for example, automobile vehicles and automobile parts.

[0183] This application claims priority under Japanese Patent Application No. 2024-197938, filed in Japan on November 13, 2024, the entirety of which is incorporated herein by reference.

Claims

1. A method for forming a multilayer coating film, comprising: applying at least one layer of base coating composition (Y) to an object to be coated to form at least one uncured base coating film; applying a clear coating composition (Z) to the uncured base coating film to form a clear coating film; and heating and curing the uncured base coating film and the uncured clear coating film, wherein the base coating composition (Y) comprises: a hydroxyl group-containing acrylic resin (y1) having a weight-average molecular weight of 13,000 or more and 35,000 or less, and a glass transition temperature of 20°C or more and less than 80°C; and a polyol compound (y2) having a hydroxyl value exceeding 200 mgKOH / g and 1,000 mgKOH / g or less, and a molecular weight of 100 or more and 1,000 or less; and the solid content of the hydroxyl group-containing acrylic resin (y1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y). A method for forming a multilayer coating film, wherein the clear coating composition (Z) comprises a hydroxyl group-containing acrylic resin (z1) having a hydroxyl value of 90 mg KOH / g or more and 190 mg KOH / g or less, a weight-average molecular weight of 4000 or more and 6000 or less, and a glass transition temperature of 15°C or more and 100°C or less, a hydroxyl group-containing polyester resin (z2), a polyisocyanate compound (z3), a metal catalyst (z4) containing at least one metal selected from the group consisting of zinc, bismuth and tin, and an organic amine catalyst (z5) having an amidine group, wherein the solid content of the hydroxyl group-containing acrylic resin (z1) is 50 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the resin solid content of the clear coating composition (Z).

2. The method for forming a multilayer coating film according to claim 1, wherein the polyol compound (y2) has two hydroxyl groups and an alicyclic hydrocarbon group.

3. The method for forming a multilayer coating film according to claim 1 or 2, wherein the hydroxyl group-containing acrylic resin (y1) has a hydroxyl value of 30 mg KOH / g or more and 100 mg KOH / g or less.

4. The method for forming a multilayer coating film according to any one of claims 1 to 3, wherein the solid content of the polyol compound (y2) is 2 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the resin solid content of the base coating composition (Y).

5. The method for forming a multilayer coating film according to any one of claims 1 to 4, wherein the hydroxyl group-containing polyester resin (z2) has a hydroxyl value of 250 mg KOH / g or more and less than 500 mg KOH / g, and a weight-average molecular weight of 1500 or more and 2500 or less.

6. The method for forming a multilayer coating film according to any one of claims 1 to 5, wherein the base coating composition (Y) has a solid content concentration of 30% by mass or more and 60% by mass or less.

7. The method for forming a multilayer coating film according to any one of claims 1 to 6, wherein the clear coating composition (Z) has a solid content concentration of 45% by mass or more and 60% by mass or less.

8. A method for forming a multilayer coating film according to any one of claims 1 to 7, wherein the object to be coated includes a resin portion comprising at least one selected from the group consisting of ABS resin, PC-PBT resin, PC-ABS resin, polycarbonate resin, vinyl chloride resin, polyamide resin, and polypropylene resin.

9. The method for forming a multilayer coating film according to any one of claims 1 to 8, wherein the organic amine catalyst (z5) has a heterocycle containing a nitrogen atom.

10. The method for forming a multilayer coating film according to any one of claims 1 to 9, wherein the metal catalyst (z4) comprises at least one selected from the group consisting of zinc, bismuth, or tin acylate compounds, alkoxide compounds, chelate compounds, and oxide compounds.

Images