Coating composition

Abstract

The present invention provides a coating composition containing an acrylic resin (A), a polyisocyanate curing agent (B), and a solvent (C), wherein: the glass transition temperature Tg of the acrylic resin (A) is 70°C or higher; the solvent (C) contains an alcohol solvent (C1) containing at least one of a secondary alcohol and a tertiary alcohol; and the storage elastic modulus (unit Pa) (E'1) of a flat region of a coating film formed by baking the coating composition at 80°C for 30 minutes and the storage elastic modulus (unit Pa) (E'2) of a flat region of a coating film formed by baking the coating composition at 80°C for 30 minutes and subsequently performing curing at 60°C for 72 hours satisfy formula (1) below.　Formula (1): (E'2) - (E'1) ≤ 8.00 × 106

Description

paint composition

[0001] The present invention relates to a coating composition.

[0002] Plastic molded products are used in mobile phones, home appliances, office automation equipment, etc., and the surfaces of these plastic molded products are sometimes coated to provide decoration or functionality. Therefore, coating compositions for coating plastics are sometimes required to provide the coating film with functions such as abrasion resistance, discoloration resistance, sebum resistance, high gloss, high weather resistance, and electrical insulation, depending on the application, in addition to providing decoration. Furthermore, plastic molded products that come into contact with human skin and hands for long periods of time, such as exterior components of mobile phones and interior components of automobiles, are also required to have excellent chemical resistance, such as sweat resistance, lactic acid resistance, hand cream resistance, and sunscreen cream resistance.

[0003] As such a coating composition, for example, Patent Document 1 discloses a coating composition containing a polyol (a) having a hydroxyl value of 180 or more, an acrylic polyol (b) not corresponding to (a), and a polyisocyanate (c). Patent Document 2 also discloses a coating composition containing a hydroxyl-containing acrylic resin (A) having a weight average molecular weight of 3,000 to 20,000 and a hydroxyl value in the range of 100 to 200 mgKOH / g, a polyisocyanate compound (B), a curing catalyst (C), and a surface conditioner (D) essentially comprising a silicone-based surface conditioner (D-1) and an acrylic-based surface conditioner (D-2). Furthermore, Patent Document 3 discloses a coating composition consisting of a base agent containing a hydroxyl-containing acrylic resin (A), a hydroxyl-containing polyester resin (B), and resin beads (C), and a polyisocyanate curing agent. Furthermore, Patent Document 4 discloses a resin composition containing, as a resin component, a mixture of a cellulose derivative (B) other than an acrylic-modified cellulose derivative obtained by graft polymerization or simply bonding an acrylic component to a cellulose derivative and an acrylic resin (C), wherein the composition of the mixture, the weight-average molecular weight of the cellulose derivative (B), and the glass transition temperature of the acrylic resin (C) are within specific ranges, and the resin component has an acid value and a hydroxyl value of 7 to 100 mgKOH / g.

[0004] JP 2022-095131 A JP 2014-019714 A JP 2015-105297 A JP 2014-181343 A

[0005] However, when attempting to further improve adhesion, chemical resistance, and hydrolysis resistance compared to the compositions of the above patent documents, the solubility of the resin decreases, and the paint and coating film may become cloudy. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a paint composition that has good resin solubility and is excellent in adhesion, chemical resistance, and hydrolysis resistance.

[0006] As a result of extensive research by the present inventors, it was found that in order to achieve sufficient adhesion, chemical resistance, and hydrolysis resistance, it is essential that the blended resin be one in which a large amount of low-molecular-weight hydrophilic monomers are polymerized and one that has a high glass transition temperature (Tg). However, when a resin that satisfies these requirements is used, the resin does not dissolve in current solvents and becomes cloudy. Therefore, the present inventors have developed an acrylic resin with a glass transition temperature (Tg) of 70°C or higher, a low-molecular-weight monomer with high water solubility as a component, and a solvent containing a tertiary alcohol added during dissolution, which makes it possible to achieve both adhesion, chemical resistance, and hydrolysis resistance, as well as solubility in the coating. This has led to the present invention.

[0007] That is, the present invention relates to a coating composition containing an acrylic resin (A), a polyisocyanate curing agent (B), and a solvent (C), wherein the glass transition temperature Tg of the acrylic resin (A) is 70°C or higher, the solvent (C) contains an alcohol solvent (C1) containing at least one of a secondary alcohol and a tertiary alcohol, and the storage modulus (unit: Pa) (E'min1) in the flat region of a coating film formed by baking the coating composition at 80°C for 30 minutes and then aging the coating composition at 60°C for 72 hours satisfies the following formula (1): (E'min2) - (E'min1) ≦ 8.00 × 10 ６ (1)

[0008] The crosslink density (unit: mol / cc) (n1) of a coating film formed by baking the coating composition at 80°C for 30 minutes and the crosslink density (unit: mol / cc) (n2) of a coating film formed by baking the coating composition at 80°C for 30 minutes and then curing it at 60°C for 72 hours preferably satisfy the following formula (2): (n2) - (n1) ≦ 8.00 × 10 －４ (2)

[0009] The glass transition temperature Tg of the coating film formed by baking the coating composition at 80°C for 30 minutes and then curing at 60°C for 72 hours is preferably less than 100°C.

[0010] The solvent (C) preferably further contains an ester solvent (C2).

[0011] The contents of the alcohol solvent (C1) and the ester solvent (C2) contained in the coating composition preferably satisfy the following formula (3): 0.01≦(C1) / (C2)≦0.20 (3)

[0012] The acrylic resin (A) preferably contains 75% or more of a monomer having methacrylic acid as a constituent monomer.

[0013] The hydroxyl value of the acrylic resin (A) is preferably 140 mgKOH / g or less.

[0014] According to the present invention, a coating composition can be obtained which has good resin solubility and is excellent in adhesion, chemical resistance, and hydrolysis resistance.

[0015] An embodiment of the present invention will be described below.

[0016] [Paint Composition] The paint composition of the present invention is a paint composition containing an acrylic resin (A), a polyisocyanate curing agent (B), and a solvent (C), wherein the glass transition temperature Tg of the acrylic resin (A) is 70°C or higher, the solvent (C) contains an alcohol solvent (C1) containing at least one of a secondary alcohol and a tertiary alcohol, and the storage modulus (unit: Pa) (E'min1) in the flat region of a coating film formed by baking the paint composition at 80°C for 30 minutes and then aging the paint composition at 60°C for 72 hours satisfies the following formula (1): (E'min2) - (E'min1) ≤ 8.00 x 10 ６ (1) Details of the paint will be explained below.

[0017] <Acrylic Resin (A)> Examples of polymerizable monomers constituting the acrylic resin (A) include polymers having methacrylic acid and other polymerizable unsaturated monomers.

[0018] (Monomers Having Methacrylic Acid) Examples of monomers having methacrylic acid include monoesters of methacrylic acid with dihydric alcohols, such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, and 4-hydroxybutyl methacrylate; methacrylates having a polyoxyethylene chain with a hydroxyl group at the molecular end; methyl methacrylate, 2-isocyanatoethyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate; Examples of the esters of methacrylic acid with alcohols having 1 to 24 carbon atoms include esters of methacrylic acid with alcohols having 1 to 24 carbon atoms, such as n-octyl methacrylate, n-octyl methacrylate, lauryl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, tridecyl methacrylate, and stearyl methacrylate; epoxy group-containing polymerizable unsaturated monomers, such as glycidyl methacrylate and 3,4-epoxycyclohexylmethyl methacrylate; and aminoalkyl methacrylates, such as N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, and N,N-dimethylaminopropyl methacrylate.

[0019] (Other Polymerizable Unsaturated Monomers) Examples of other polymerizable unsaturated monomers include monoesters of methacrylic acid with dihydric alcohols, such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, and 4-hydroxybutyl acrylate; acrylates having a polyoxyethylene chain with a hydroxyl group at the molecular end; methyl acrylate, 2-isocyanatoethyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, lauryl acrylate, and cyclohexyl acrylate. esters of acrylic acid with alcohols having 1 to 24 carbon atoms, such as acrylate, isobornyl acrylate, tridecyl acrylate, and stearyl acrylate; carboxyl group-containing monomers, such as acrylic acid, methacrylic acid, maleic acid, and maleic anhydride; epoxy group-containing polymerizable unsaturated monomers, such as glycidyl acrylate and 3,4-epoxycyclohexylmethyl acrylate; aminoalkyl acrylates, such as N,N-dimethylaminoethyl acrylate, N,N-diethylaminoethyl acrylate, and N,N-dimethylaminopropyl acrylate; vinyl group-containing aromatic compounds, such as styrene, α-methylstyrene, and vinyltoluene; and vinyl group-containing compounds, such as vinyl acetate.

[0020] The acrylic resin (A) preferably contains 75% or more of a monomer having methacrylic acid as a constituent monomer, which results in excellent adhesion, chemical resistance, and hydrolysis resistance.

[0021] [Characteristics of Acrylic Resin] The characteristics of the acrylic resin (A) will be described below.

[0022] (Glass transition temperature Tg of acrylic resin) The glass transition temperature Tg of the acrylic resin (A) is 70°C or higher. When the glass transition temperature Tg of the acrylic resin (A) is 70°C or higher, a coating film having excellent scratch resistance and chemical resistance can be obtained. The glass transition temperature Tg of the acrylic resin (B) is preferably 75°C or higher and 100°C or lower, and more preferably 80°C or higher and 95°C or lower.

[0023] The glass transition temperature Tg of the acrylic resin (A) was calculated using the Fox formula. In this specification, the glass transition temperature (Tg) of an acrylic resin refers to the temperature calculated using the following FOX formula: 1 / Tg = W1 / Tg1 + W2 / Tg2 + ... + Wi / Tgi + ... + Wn / Tgn In the FOX formula, Tg is the glass transition temperature (K) of a polymer composed of n types of monomers, Tg(1, 2, i, n) is the glass transition temperature (K) of a homopolymer of each monomer, W(1, 2, i, n) is the mass fraction of each monomer, and W1 + W2 + ... + Wi + ... + Wn = 1.

[0024] (Hydroxyl value of acrylic resin) The hydroxyl value of the acrylic resin (A) is preferably 140 mgKOH / g or less, more preferably 130 mgKOH / g or less, and even more preferably 120 mgKOH / g or less. The lower limit of the hydroxyl value of the acrylic resin (A) is preferably 30 mgKOH / g or more, more preferably 60 mgKOH / g or more. When the hydroxyl value of the acrylic resin (A) is 140 mgKOH / g or less, sufficient curability to exhibit good coating film performance can be ensured, and scratch resistance and coating film hardness can be ensured. Furthermore, when the hydroxyl value of the acrylic resin (A) is 30 mgKOH / g or more, unreacted hydroxyl groups can be reduced, resulting in excellent chemical resistance.

[0025] Here, the hydroxyl value of the acrylic resin (A) is the number of mg of potassium hydroxide required to completely acetylate the hydroxyl groups in 1 g of the resin with acetic anhydride and then neutralize the free acetic acid generated by acetylation.

[0026] (Weight average molecular weight Mw) The weight average molecular weight Mw of the acrylic resin (A) is preferably 3,000 or more and 18,000 or less. By having it be 3,000 or more, a coating film with good scratch resistance and coating film hardness can be obtained upon crosslinking. By having it be 18,000 or less, solubility in paint is improved. The weight average molecular weight Mw of the acrylic resin (A) is more preferably 5,000 or more and 14,000 or less.

[0027] The weight average molecular weight Mw can be measured by a GPC (trade name "HLC-8220GPC", manufactured by Tosoh Corporation) equipped with an RI, using a TSKgel column (manufactured by Tosoh Corporation). The GPC conditions are as follows: tetrahydrofuran is used as the developing solvent, the flow rate is 0.35 ml / min, and the temperature is 40°C; and TSK standard polystyrene (manufactured by Tosoh Corporation) is used as the standard substance.

[0028] <Polyisocyanate Curing Agent (B)> The polyisocyanate curing agent (B) is a compound having two or more isocyanate groups in one molecule, and is not particularly limited. Examples of the polyisocyanate curing agent (B) include, but are not limited to, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, ... aliphatic diisocyanates such as 1,6-diisocyanatomethyl caproate; 1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexa alicyclic diisocyanates such as cyclohexane and 1,3-bis(isocyanatomethyl)cyclohexane; aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4- or 2,6-tolylene diisocyanate, 4,4'-toluidine diisocyanate, dianisidine diisocyanate, and 4,4'-diphenyl ether diisocyanate; or araliphatic diisocyanates such as 1,4-xylylene diisocyanate, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(α,α-dimethylisocyanatomethyl)benzene; triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene; tetraisocyanates such as 4,4'-diphenyldimethylmethane-2,2',5,5'-tetraisocyanate;Examples include polymerized polyisocyanates such as dimers and trimers of tolylene diisocyanate, and polyphenylpolymethylene polyisocyanates. Among these, aliphatic diisocyanates and alicyclic diisocyanates are preferred in terms of the properties of the resulting coating film, particularly in terms of the coating film being less likely to yellow.

[0029] The polyisocyanate curing agent (B) may be a commercially available product, such as the Burnock (registered trademark) series from DIC Corporation, the Duranate (registered trademark) series from Asahi Kasei Chemicals Corporation, or the Sumidur (registered trademark) series from Sumika Covestro Urethane Co., Ltd.

[0030] The polyisocyanate curing agent (B) may be used alone or in combination of two or more kinds.

[0031] In the coating composition of the present invention, the content of polyisocyanate curing agent (B) is not particularly limited, but usually the molar ratio of the amount of isocyanate groups in the polyisocyanate curing agent (B) to the amount of hydroxyl groups in the acrylic polyol (A) (NCO mol % / OH mol %) is preferably 0.8 to 1.5, more preferably 0.9 to 1.3. When the content of polyisocyanate curing agent (B) is within the above range, the amount of unreacted hydroxyl groups and isocyanate groups is reduced, and therefore a coating film with better properties can be obtained.

[0032] <Solvent (C)> The solvent (C) contains an alcohol solvent (C1) containing at least one of a secondary alcohol and a tertiary alcohol. Of the secondary alcohol and the tertiary alcohol, the solvent (C) preferably contains a tertiary alcohol. Examples of secondary alcohols include hydrocarbon alcohols such as 2-propanol, 2-butanol, 2- and 3-pentanol, 2- and 3-hexanol, 2-, 3- and 4-heptanol, 2-, 3- and 4-octanol, and 3,3-dimethyl-2-butanol; aromatic alcohols, such as cyclic alcohols including diphenylmethanol, 1-phenylethanol, 1-phenylpropanol, 1-phenyl-2-propanol, 1-phenylbutanol, 1-phenyl-2-butanol, 4-phenyl-2-butanol, and cyclohexanol; and steroid alcohols. Examples of tertiary alcohols include hydrocarbon alcohols such as 2-methyl-2-butanol, 2,3-dimethyl-2-butanol, 2-methyl-2-pentanol, and 3-methyl-3-pentanol; cyclic alcohols such as 1-methylcyclopentanol, 1-ethylcyclopentanol, 1-propylcyclopentanol, 1-butylcyclopentanol, and 1-methylcyclooctanol; monoterpene alcohols such as α-terpineol; and alcohols having a ketone functional group such as diacetone alcohol. Furthermore, it is preferable that the solvent (C) further contains an ester solvent (C2). Examples of the ester solvent (C2) include methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, propyl acetate, and 3-methoxy-1-butyl acetate.

[0033] The contents of the alcohol solvent (C1) and the ester solvent (C2) contained in the coating composition preferably satisfy the following formula (3): 0.01≦(C1) / (C2)≦0.20 (3) When the contents of the alcohol solvent (C1) and the ester solvent (C2) satisfy the above ranges, a coating material that is soluble in the substrate can be prepared, and as a result, a coating film with excellent adhesion can be obtained.

[0034] The concentration of the solvent (C) in the coating composition is preferably 0.1% by mass or more and 30.0% by mass or less from the viewpoint of coatability and environmental considerations.

[0035] <Other Components> The coating composition of the present invention can contain a pigment. There are no particular limitations on the pigment, and pigments commonly used in the coating industry, such as color pigments, extender pigments, and scale-like pigments, can be used. The pigments can be used alone or in combination of two or more.

[0036] Examples of color pigments include titanium oxide, iron oxide, carbon black, yellow lead, molybdate orange, ultramarine, Prussian blue, phthalocyanine blue, phthalocyanine green, quinacridone red, naphthol red, benzimidazolone yellow, Hansa yellow, benzimidazolone orange, and dioxazine violet.

[0037] Examples of extender pigments include silica, talc, mica, calcium carbonate, and barium sulfate.

[0038] Flake pigments are pigments with a thin, flat, foil-like shape, and specific examples include metal pigments such as zinc, nickel, chromium, tin, copper, silver, platinum, gold, and aluminum, as well as glass flakes, talc, mica, kaolin clay, and micaceous iron oxide. Metal pigments also include alloy pigments such as stainless steel. Furthermore, flake pigments such as talc and mica may be surface-treated with a metal oxide such as titanium oxide.

[0039] In the coating composition of the present invention, the amount of pigment is preferably 0 to 40% by mass, and more preferably 5 to 25% by mass. The amount of pigment in the coating composition affects the properties of the coating film. If the amount of pigment is too low, the amount of solid components remaining in the coating film will be small, resulting in reduced coating film hardness and scratch resistance. If the amount of pigment is too high, the crosslink density of the coating film will be low, which may result in reduced chemical resistance.

[0040] In addition to the pigments described above, the coating composition of the present invention can contain additives necessary for coating compositions, such as curing catalysts, pigment dispersants, antifoaming agents, anti-sagging agents, ultraviolet absorbers, light stabilizers, anti-fungal agents, etc., in conventional amounts, and are generally known and commonly used in the art.

[0041] Furthermore, the coating composition of the present invention can also be used in combination with other resins, such as acrylic-modified alkyd resins, alkyd resins, silicone resins, fluororesins, or epoxy resins, as appropriate, for the purpose of improving performance within a range that does not affect the effects of the present invention.

[0042] [Properties of Coating Film] The properties of the coating film formed using the coating composition of the present invention will be described below.

[0043] <Storage Modulus> It is preferable that the storage modulus (unit: Pa) (E'min1) in the flat region of a coating film formed by baking the coating composition at 80°C for 30 minutes and the storage modulus (unit: Pa) (E'min2) in the flat region of a coating film formed by baking the coating composition at 80°C for 30 minutes and then aging at 60°C for 72 hours satisfy the following formula (1): (E'min2) - (E'min1) ≦ 8.00 × 10 ６ (1) Here, the storage modulus (unit: Pa) of the flat region of the coating film is a value determined by the method in the Examples section described later.

[0044] <Crosslink Density of Coating Film> It is preferable that the crosslink density (unit: mol / cc) (n1) of a coating film formed by baking a coating composition at 80°C for 30 minutes and the crosslink density (unit: mol / cc) (n2) of a coating film formed by baking a coating composition at 80°C for 30 minutes and then curing at 60°C for 72 hours satisfy the following formula (2): (n2) - (n1) ≦ 8.00 × 10 －４ (2) By adjusting the crosslink density of the coating film to fall within the above range, it is possible to obtain a coating film having good coating film hardness, scratch resistance, and chemical resistance. Here, the crosslink density of the coating film is a value determined by the method described in the Examples below.

[0045] <Glass transition temperature Tg of coating film> The glass transition temperature Tg of a coating film formed by baking a coating composition at 80°C for 30 minutes and then aging at 60°C for 72 hours is preferably less than 100°C, more preferably less than 95°C. When the glass transition temperature Tg of the coating film is within the above range, a coating film with excellent scratch resistance and chemical resistance can be obtained. The glass transition temperature Tg is a value determined by the method in the examples described below.

[0046] The resin composition of the present invention can form a coating film that has excellent chemical resistance, particularly resistance to sunscreen agents, and is therefore useful for coating plastic molded products that will come into contact with human skin and hands for long periods of time, specifically, interior and exterior components of automobiles and motorcycles, components for home appliances such as audio, video and television sets, and components for office equipment such as mobile phones, printers and personal computers.

[0047] [Coating Method] The coating method of the resin composition includes, for example, a step of applying the resin composition to the surface of a substrate and then forming a film by drying or the like. In the coating method of the resin composition of the present invention, in addition to directly applying the resin composition to the surface of the substrate, a primer may be applied to the substrate in advance before applying the resin composition. By applying a primer in advance, the adhesion between the coating film and the substrate can be improved.

[0048] The method for applying the resin composition is not particularly limited, and examples thereof include known application methods such as dipping, spin coating, flow coating, roll coating, spray coating, blade coating, and air knife coating. Among these, spray coating and roll coating are preferred from the viewpoint of easily controlling the film thickness.

[0049] In the method for applying a resin composition, a coating film can be formed by drying the resin composition applied to the surface of a substrate, and for example, when the resin composition is a volatile drying type resin composition, a coating film can be obtained by leaving the resin composition after application at a temperature of 5° C. or higher and lower than 70° C. Similarly, when the resin composition is a thermosetting type resin composition, a coating film can be obtained by leaving the resin composition after application at a temperature of 5° C. or higher and lower than 70° C.

[0050] (Substrate) In the method for coating the resin composition of the present invention, the substrate is not particularly limited, and substrates of various shapes can be selected depending on the application of the substrate. Examples of the substrate include plastic substrates such as PPE (polyphenylene ether) resins, polystyrene resins, olefin polymers such as polypropylene and polyethylene, polycarbonate resins, acrylic resins such as polymethyl methacrylate, and ABS resins.

[0051] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the examples shown below.

[0052] [Preparation of Acrylic Resin Solution] First, a resin solution containing a hydroxyl-containing acrylic resin (A) (hereinafter referred to as hydroxyl-containing acrylic resin) used in the examples and comparative examples was prepared. [Resin Solution 1] Resin solution 1 containing acrylic resin (A) was prepared according to the following procedure. Table 1 shows the solvents, monomers, and polymerization initiators required for preparing acrylic resin (A), as well as the properties of acrylic resin (A). In Table 1, the blending amount of each component is expressed in parts by mass. 38.0 parts by mass of butyl acetate, 12.0 parts by mass of 2-hydroxyethyl methacrylate, 6.5 parts by mass of methyl methacrylate, 4.0 parts by mass of butyl methacrylate, 0.5 parts by mass or more of acrylic acid, 3.5 parts by mass of 2-ethylhexyl acrylate, 24.5 parts by mass of styrene monomer, and 4.0 parts by mass of t-butylperoxy-2-hexanoate were reacted under nitrogen gas at 110°C for 8 hours. Thereafter, the solution was diluted with 7.0 parts by mass of butyl acetate to obtain an acrylic resin solution 1 having a solid content of 55% by mass.

[0053] [Resin Solutions 2 to 9] Resin solutions 2 to 9 were prepared in the same manner as resin solution 1, except that the compounds and formulations were changed as shown in Table 1.

[0054] [Characteristics of Acrylic Resin (A)] The characteristics of the acrylic resin (A) in Resin Solutions 1 to 9 were measured as follows. The measurement results are shown in Table 1.

[0055] (Solid Content) The solid content was calculated from the change in weight of a resin solution weighed in a container and heated in a drying oven at 150° C. for 30 minutes.

[0056] (Hydroxyl Value of Acrylic Resin (A)) After the hydroxyl groups in 1 g of acrylic resin (A) were completely acetylated with acetic anhydride, the number of mg of potassium hydroxide required to neutralize the free acetic acid generated by the acetylation was determined, and this value was taken as the hydroxyl value of the acrylic resin (A).

[0057] (Glass Transition Temperature Tg of Acrylic Resin (A)) The glass transition temperature Tg of the acrylic resin (A) was calculated by Fox's formula.

[0058] (Weight-average molecular weight Mw) The weight-average molecular weight Mw of the acrylic resin (A) was measured using a TSKgel column SuperMultipore HZ-M (manufactured by Tosoh Corporation) by SEC (size exclusion chromatography, product name "HLC-8420GPC", manufactured by Tosoh Corporation) equipped with a differential refractive index (RI) detector. The SEC conditions were tetrahydrofuran as the developing solvent, a flow rate of 0.35 ml / min, and a temperature of 40°C. TSK standard polystyrene (manufactured by Tosoh Corporation) was used as the standard substance.

[0059]

[0060] ​Next, using the above resin solution, test specimens equipped with coating films formed by applying and curing the coating compositions of Examples and Comparative Examples were prepared according to the following procedure. The components used in the coating compositions and their formulations are shown in Tables 2 to 5. In Tables 2 to 5, the amount of each component is expressed in parts by mass, and NV indicates the non-volatile content (mass%) of the resin. Details of the materials in Tables 2 to 5 are as follows: Curing catalyst: TOS-K (Osaka Shinyaku Co., Ltd.) Color pigment: NSP-CZ808(D)BLACK (Nikko Bix Co., Ltd.) Additives: BYK-065 (BYK-Chemie) BYK-307 (BYK-Chemie) BYK-325N (BYK-Chemie) Curing agent: Sumidur N3300 (Covestro) Duranate 24A-100 (Asahi Kasei Corporation) Burnock DN-980S (DIC Corporation) Dilution solvent: ISOPAR E (ExxonMobil)

[0061] Example 1 (Preparation of coating composition) 67.5 parts by mass of resin solution 1 as the hydroxyl group-containing acrylic resin (A), 2.0 parts by mass of TOS-K as a curing catalyst, 2.0 parts by mass of butyl acetate (C2), 2.0 parts by mass of 3-methoxy-1-butyl acetate (C2), 4.0 parts by mass of methyl isobutyl ketone, 16.0 parts by mass of NSP-CZ808 (D) BLACK as a coloring pigment, 0.5 parts by mass of BYK-065, 0.1 parts by mass of BYK-307, 0.4 parts by mass of BYK-325N as additives, and 15.5 parts by mass of a polyisocyanate compound were mixed and then stirred with a disperser to prepare the coating composition of Example 1.

[0062] (Preparation of coated body (substrate: polypropylene resin)) The coating composition was air spray coated onto a polypropylene resin substrate having a thickness of 3.0 mm and a size of 100 mm x 100 mm to a dry film thickness of 50 μm, and then the coating composition was dried at 80° C. for 30 minutes to form a coating film, thereby preparing a coated body. This coated body was used to measure the glass transition temperature Tg and crosslink density of the coating film.

[0063] [Examples 2 to 16, Comparative Examples 1 to 11] Coating compositions and test specimens were prepared in the same manner as in Example 1, except that the components and formulations of the coating compositions were changed as shown in Tables 2 to 5.

[0064] [Evaluation of properties of coating composition] The coating composition was evaluated for the following properties. The evaluation results are shown in Tables 2 to 5.

[0065] (Solid Content) The solid content of the coating compositions of the above Examples and Comparative Examples was measured in accordance with the test conditions of JIS K 5601_1_2. The measurement results are shown in mass %.

[0066] (Solubility when mixed with diluting solvent) The solubility of the coating compositions of the above Examples and Comparative Examples when mixed with diluting solvent was evaluated according to the following criteria: ◯: Easily soluble Δ: Not uniformly soluble ×: Gelled

[0067] (Evaluation of coating film appearance after time has elapsed) For the coating compositions of the above Examples and Comparative Examples, the base agent, curing agent, and dilution solvent were mixed, and the appearance after 6 hours at room temperature was evaluated according to the following criteria: ◯: Same as the coating film appearance immediately after mixing ×: Occurrence of bumps, loss of gloss

[0068]

[0069]

[0070]

[0071]

[0072] [Evaluation of Coating Film] The coating films of the above Examples and Comparative Examples were evaluated as follows. The evaluation results are shown in Tables 6 to 12.

[0073] ​​​​(Storage Modulus) (1) No curing: The coating composition was spray-coated onto an ABS resin substrate (thickness 3.0 mm, size 100 mm x 100 mm) to a dry film thickness of 30 μm by air spraying, and then the coating composition was baked at 80°C for 30 minutes to form a coating film, thereby preparing a test specimen. The storage modulus (unit: Pa) (E'min1) of the flat region of the coating film was determined by the following measurement method. The loss modulus E'' was also determined in the same manner. <Measurement Conditions> Measuring Instrument: RSA-GII (TA Instruments) Measurement Mode: Temperature Dependence Chuck Distance: 20 mm Measurement Width: 5.0 mm Sample Thickness: Dry Film Thickness 30 μm Measurement Temperature Range: 10°C to 200°C Heating Rate: 5°C / min Frequency: 1 Hz Displacement Amplitude: 25 μm Strain: 0.05% (2) With Aging The coating composition was baked at 80°C for 30 minutes and then aged at 60°C for 72 hours to form a coating, and the storage modulus (unit: Pa) (E'min2) of the flat region of the coating was measured. The storage modulus (unit: Pa) (E'min2) with aging was determined using the measurement method described above. (3) Storage Modulus Difference: (E'min2) - (E'min1) The storage modulus difference was determined from the storage moduli (E'min2) and (E'min1) described above.

[0074] (Crosslink Density) (1) No curing In the same manner as in the measurement of storage modulus, the coating composition was baked at 80°C for 30 minutes to form a coating film, and the crosslink density (unit: mol / cc) (n1) of the coating film was measured. The crosslink density n [mol / cc] was calculated from the following logical formula (the molecular weight between crosslinks was calculated from 1 / n): n=E' ｍｉｎ / (3×ρRT) E' ｍｉｎ : storage modulus of the flat region [Pa] ρ: film specific gravity R: gas constant (8.31 × 10 ６ [Pa·cc·mol －１ ・K －１]) T: Temperature of plateau region of storage modulus [K] (2) With aging The coating composition was baked at 80°C for 30 minutes and then aged at 60°C for 72 hours to form a coating film, and the crosslink density (unit: mol / cc) (n2) of the coating film was measured. The crosslink density n2 in the case of aging was determined by the method described above. (3) Crosslink density difference: (n2) - (n1) The crosslink density difference was determined from the above crosslink densities (n2) and (n1).

[0075] (Glass transition temperature Tg of coating film) (1) Without aging Tanδ was determined from the storage modulus and loss modulus determined above, and the peak value was taken as Tg. Tanδ: loss tangent (tanδ=E" / E') E': storage modulus E": loss modulus (2) With aging The coating composition was baked at 80°C for 30 minutes and then aged at 60°C for 72 hours to form a coating film. Thereafter, the glass transition temperature Tg was determined in the same manner as above.

[0076] (Coating Film Appearance) The coating composition was air spray coated onto an ABS resin substrate (thickness 3.0 mm, size 100 mm x 100 mm) to a dry film thickness of 30 μm, and then the coating composition was dried at 80°C for 30 minutes to form a coating film, to prepare a test specimen. Gloss was measured at 60° using a BYK Gardner Micro-Tri-gloss product. The coating film appearance was evaluated according to the following evaluation criteria: ◯: More than 85 Δ: 80 to 85 ×: Less than 80

[0077] (Adhesion) A straight cut was made with a cutter knife at a cutting angle of 60° on the coating surface of the above test specimen, and then another cut was made at an angle of 60° to the first cut, creating 100 grids (2 mm x 2 mm). Cellophane tape was then firmly applied to cover the cuts, and the cellophane tape was peeled off at an angle of 45° to the coating surface. The number of remaining grids out of 100 was evaluated according to the following criteria: ◯: 100 Δ: 90 or more but less than 100 ×: Less than 90

[0078] (Chemical resistance 1: Sun oil) A 70 mm x 150 mm test piece was cut out from the above test body, a piece of gauze was placed on it, 1 g of sun oil was evenly applied on top of it, and it was kept at 80°C for 24 hours. The gauze marks after washing were evaluated according to the following evaluation criteria. ◎: No gauze marks ○: Slight gauze marks can be seen △: Gauze marks can be seen on part of the test piece ×: Gauze marks can be seen over the entire surface of the test piece

[0079] (Chemical resistance 2: Hand cream) A 70 mm x 150 mm test piece was cut out from the above test body, a piece of gauze was placed on it, 1 g of sun oil was evenly applied on top of it, and it was kept at 80°C for 24 hours. The gauze marks after washing were evaluated according to the following evaluation criteria: ◎: No gauze marks ○: Slight gauze marks can be seen △: Gauze marks can be seen on part of the test piece ×: Gauze marks can be seen over the entire surface of the test piece

[0080] (Abrasion resistance 1: dry wiping) Using a Gakushin abrasion tester (Dyeing abrasion fastness tester RT-300 manufactured by Daiei Scientific Instruments Co., Ltd.), a canvas (No. 6) was used, and a load of 500 g was applied to the coating surface of the test specimen, which was then rubbed back and forth 1000 times at 60 rpm. After 1000 times of rubs, the coating surface was washed with running water and allowed to dry naturally, after which the gloss of the coating was visually observed and rated according to the following criteria: ○: Very slight scratches present △: Clear scratches present ×: Base material exposed

[0081] (Heat Resistance) After holding the test specimen at 90°C for 24 hours, adhesion was evaluated. A straight cut was made on the coating surface of the test specimen with a cutter knife at a cutting angle of 60°, and then another cut was made at an angle of 60° to the first cut, creating 100 grids (2 mm x 2 mm). Cellophane tape was then firmly applied to cover the cuts, and the cellophane tape was peeled off at an angle of 45° to the coating surface. The number of remaining grids out of 100 was evaluated according to the following criteria: ○: 100 △: 90 or more but less than 100 ×: Less than 90

[0082] (Moisture Resistance) After the test specimen was kept for 96 hours under conditions of an ambient temperature of 50°C and humidity of 98%, adhesion was evaluated. A straight cut was made on the coating surface of the test specimen with a cutter knife at a cutting angle of 60°, and then another cut was made at an angle of 60° to the first cut, creating 100 grids (2 mm x 2 mm). Cellophane tape was then firmly applied to cover the cuts, and the cellophane tape was peeled off at an angle of 45° to the coating surface. The number of remaining grids out of 100 was evaluated according to the following evaluation criteria: ○: 100 △: 90 or more but less than 100 ×: Less than 90

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090] As shown in Tables 6 to 12, the coating films of the examples using the coating compositions of the present invention obtained good results in all evaluation items.​​​​​​

Claims

1. A coating composition containing an acrylic resin (A), a polyisocyanate curing agent (B), and a solvent (C), wherein the glass transition temperature Tg of the acrylic resin (A) is 70°C or higher, the solvent (C) contains an alcohol solvent (C1) containing at least one of a secondary alcohol and a tertiary alcohol, and the storage modulus (unit: Pa) (E'min1) of a flat region of a coating film formed by baking the coating composition for 30 minutes at 80°C and the storage modulus (unit: Pa) (E'min2) of a flat region of a coating film formed by baking the coating composition for 30 minutes at 80°C and then curing the coating composition for 72 hours at 60°C satisfy the following formula (1): (E'min2) - (E'min1) ≦ 8.00 x 10 ６ (1) 2. The coating composition according to claim 1, wherein the crosslink density (unit: mol / cc) (n1) of a coating film formed by baking the coating composition at 80°C for 30 minutes and the crosslink density (unit: mol / cc) (n2) of a coating film formed by baking the coating composition at 80°C for 30 minutes and then curing the composition at 60°C for 72 hours satisfy the following formula (2): (n2) - (n1) ≦ 8.00 x 10 －４ (2) 3. A coating composition according to claim 1, wherein the glass transition temperature Tg of a coating film formed by baking the coating composition at 80°C for 30 minutes and then curing the same at 60°C for 72 hours is less than 100°C.

4. A coating composition according to claim 1 or 2, wherein the solvent (C) further contains an ester solvent (C2).

5. The coating composition according to claim 4, wherein the content of the alcohol solvent (C1) and the content of the ester solvent (C2) contained in the coating composition satisfies the following formula (3): 0.01≦(C1) / (C2)≦0.20 (3) 6. A coating composition according to claim 1 or 2, wherein the acrylic resin (A) contains 75% or more of a monomer having methacrylic acid as a constituent monomer.

7. A coating composition according to claim 1 or 2, wherein the hydroxyl value of the acrylic resin (A) is 140 mg KOH / g or less.

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