JP2016079343A - Coating material composition thermally cured at low temperature, method for forming multilayer coating film and multilayer coating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermosetting coating material composition being a baking type coating material which is thermally cured at a low temperature and to provide a method for forming a multilayer coating film.SOLUTION: The thermosetting coating material composition includes at least components (I), (II) and (III) as given below: (I) a thermosetting acrylic resin having a number average molecular weight of 2,000 to 10,000 and having a hydroxyl value of 70 to 120 mgKOH/g; (II) an n-butyl etherified melamine resin having a number average molecular weight of 1,000 to 1,500 and having an imino group and a methylol group; and (III) an organophosphate represented by the following formula (Ris a 1-10C alkyl group; and Ris H or a 1-10C alkyl group)SELECTED DRAWING: None

Description

  The present invention relates to a low-temperature baking reactive composition for coating a substrate, a method for forming two or more layers using the same, and a multilayer coating. More specifically, the present invention relates to a coating composition for obtaining a coating film excellent in solvent resistance, curability and appearance, a coating method using the coating composition, and a cured coating film obtained by the coating method. .

  Baking-type paints are mainly used for industrial paints such as automobile paints. In recent years, various efforts have been made to reduce the emission of carbon dioxide, which is a cause of global warming. Lowering the baking temperature in the painting process is one of the effective means.

  As low-temperature curing paints, those using isocyanate compounds or blocked isocyanate compounds have been studied. For example, Patent Document 1 proposes a composition in which an acrylic polyol and a blocked isocyanate are essential components, and a block dissociation catalyst and a melamine resin are added thereto, and the curing temperature thereof is 110 ° C. or higher. Further, Patent Document 2 proposes a composition containing an acrylic polyol, an isocyanate blocked with active methylene, a melamine resin, and a strong acid catalyst. Stability is inferior. In addition, although the isocyanate compound is an essential component in both of the above two examples, the use of the isocyanate compound is not preferable from the viewpoint of safety and hygiene.

  Therefore, a low-temperature curing paint containing only a melamine resin as a curing agent without using an isocyanate compound as a curing agent has been studied. For example, according to Patent Document 3, a thermosetting acrylic resin and a melamine resin having a glass transition temperature of 0 ° C. or less. A paint was combined. This paint gives satisfactory coating film properties by baking at 120 ° C., but further reduction in baking temperature is desired.

  On the other hand, in automobile painting or the like, a multilayer coating film composed of, for example, an undercoat, an intermediate coat, and an overcoat is formed in order to prevent the corrosion of metal and have durability and the aesthetic appearance of the coat. At that time, reducing the number of times of baking as much as possible is effective not only for shortening the process but also for reducing carbon dioxide emissions, so that a plurality of layers are applied to each layer without baking, and the multiple layers are applied after being applied repeatedly. A method of baking at the same time has been put into practical use. Although this method is expected to spread to other industrial coating fields in the future, in order to simultaneously cure films coated in multiple layers at low temperatures and to obtain satisfactory multilayer coating film properties, the formation method is further changed. Need to be considered.

Japanese Patent Laid-Open No. 10-101995 WO2013 / 008879 brochure JP 61-143472 A

  The present invention provides a coating composition having excellent low-temperature curability, excellent coating film properties, and low toxicity by selecting a specific acrylic resin, a specific melamine resin, and a specific catalyst. It aims at providing the coating-film formation method using a thing, and a multilayer coating film.

  As a result of intensive studies to achieve the above object, the present inventor has (I) a thermosetting acrylic resin having a number average molecular weight of 2,000 to 10,000 and a hydroxyl value of 70 to 120 mgKOH / g, (II ) N-butyl etherified melamine resin having a number average molecular weight of 1,000 to 1,500 and having an imino group and a methylol group, and (III) (1)

(In the formula, R ₁ is an alkyl group having 1 to 10 carbon atoms, and R ₂ is hydrogen or an alkyl group having 1 to 10 carbon atoms.)
The present inventors have found that a coating composition containing an organic phosphate ester represented by the above provides excellent low-temperature curability, and has led to the present invention.

  The present invention also provides a method of laminating two or more layers of paint, in which a lower layer portion is coated with a coating composition containing the organophosphate compound shown in the component (III), and the lower layer portion is uncured. In this state, a coating composition containing the above (I) and (II) or a coating composition containing the above (I), (II) and (III) is applied over and over. Provided is a method for forming a multilayer coating film, wherein the layers are simultaneously heated and cured. The present invention also provides a multilayer coating film obtained by the above method.

  The present invention is described in detail below.

  Examples of the monomer component constituting the thermosetting acrylic resin (I) used in the present invention include acrylics such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate. Acid esters, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, methacrylate esters, acrylic acid 2- Actives such as hydroxyethyl, hydroxypropyl acrylate, 4-hydroxybutyl acrylate and other active hydrogen esters, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, etc. Methacrylic acid ester having hydrogen, unsaturated carboxylic acid such as acrylic acid, methacrylic acid, maleic acid and itaconic acid, unsaturated amide such as acrylamide, N-methylol acrylamide and diacetone acrylamide, and styrene, vinyl acetate, acrylonitrile, vinyl Other polymerizable monomers such as toluene glycidyl methacrylate can be mentioned, and an acrylic polyol obtained by polymerizing them in combination is used.

  The thermosetting acrylic resin preferably has a number average molecular weight of 2,000 to 10,000 and a hydroxyl value of 70 to 120 mgKOH / g (in terms of solid content). The hydroxyl group is considered to be a reaction point with the melamine resin of component (II). When the number average molecular weight is less than 2,000 and / or the hydroxyl value is less than 70 mgKOH / g, when cured at low temperature, crosslinking is insufficient and sufficient film properties cannot be obtained, and the number average molecular weight is 10,000. If it exceeds 1, the viscosity will increase and the non-volatile content of the paint will decrease, or the smoothness will tend to deteriorate and an excellent appearance will not be obtained. On the other hand, when the hydroxyl value exceeds 120 mgKOH / g, the stability of the paint is poor.

  In order to heat cure the thermosetting acrylic resin (I), the melamine resin (II) is required. The n-butyl etherified melamine resin (II) used in the present invention is usually obtained by subjecting melamine to an addition reaction or addition condensation reaction of formaldehyde such as formaldehyde or paraformaldehyde with a monohydric alcohol. The functional group has an imino group and a methylol group and is n-butyl etherified. The number average molecular weight is preferably 1,000 to 1,500. More preferably, the number average molecular weight is 1,300 to 1,500. When the number average molecular weight is less than 1,000, the low-temperature cured coating film has poor solvent resistance and pencil hardness. On the other hand, if the number average molecular weight exceeds 1,500, the non-volatile content and the stability of the coating are lowered, which is not preferable.

The organophosphate (III) used in the present invention is one in which R ₁ is an alkyl group having 1 to 10 carbon atoms in the formula (1). Preferably it is a C1-C8 alkyl group, More preferably, it is a C1-C4 alkyl group. When R number of carbon atoms in the alkyl group of ₁ exceeds 10, solvent resistance and pencil hardness is inferior.

In general, in a paint system using a melamine resin as a curing agent, a sulfonic acid compound is used as a curing catalyst. In the present invention, a sulfonic acid compound that is a strong acid (pKa is 1 or less) is low-temperature curable. It is not suitable because it is inferior to the above. Rather, a phosphoric acid ester compound having a weak acid (pKa of 1 or more) and the alkyl group of R _{1 in} the formula (1) having 10 or less carbon atoms is suitable.

  Specific examples of organic phosphates include monoesters such as methyl acid phosphate, ethyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate, 2-ethylhexyl acid phosphate, isodecyl acid phosphate, and the like. A diester or a mixture of a monoester and a diester may be mentioned.

  These organophosphates can also be used by reacting with an amine compound. Examples of amine compounds include primary amines such as ammonia and methylamine, secondary amines such as di-n-propylamine and dimethylamine, tertiary amines such as triethylamine, dimethylaminoethanol and diethylhydroxyamine, n-butylamine and diethylamine. Aliphatic amines, piperidine, morpholine, N-ethylpiperidine, N-ethylmorpholine, heterocyclic amines such as pyridine, aromatic amines such as benzylamine, N-methylaniline, N, N-dimethylamine, etc. These amine compounds can be used alone or in combination of two or more.

  The ratio of the thermosetting acrylic resin to the n-butyl etherified melamine resin is such that the thermosetting acrylic resin (I) is 50 to 70% by weight in solid content and the melamine resin (II) is 30 to 50% by weight in solid content. preferable. When the melamine resin is less than 30% by weight, the number of reaction points is small, and physical properties of the cured film such as solvent resistance and pencil hardness are deteriorated. On the other hand, if the melamine resin exceeds 50% by weight, the self-condensation reaction of the melamine resin proceeds and the cured coating becomes hard and brittle.

  The ratio of the organic phosphate ester is 0.5 to 10% by weight (or phosphorus element equivalent 0.1 to 2.2%) based on the total resin solid content of the thermosetting acrylic resin (I) and the melamine resin (II). 5% by weight) is preferred. If it is less than 0.5% by weight, sufficient physical properties of the coating film cannot be obtained, for example, the gel fraction is lowered. On the other hand, if it exceeds 10% by weight, the stability of the coating is lowered.

In addition to this, the thermosetting coating composition of the present invention can be blended with raw materials commonly used as coatings. Aluminum, mica, nickel, zinc, bismuth, graphite, etc. are used as bright pigments, and titanium oxide, zinc white, carbon black, molybdenum red, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments are used as colored pigments. Inorganic or organic pigments such as isoindoline pigments, selenium pigments, perylene pigments, dioxazine pigments and diketopyrrolopyrrole pigments can be blended. Furthermore, extender pigments such as calcium carbonate, barium sulfate, calcium sulfate, and silicic acid can be blended. In addition, UV stabilizers such as benzophenone, benzotriazole, and triazine, and light stabilizers and antioxidants such as hindered amines are blended in paints to be applied to objects to be used outdoors. Further, a surface conditioner such as silicone, a thickener, and the like can be used.

  The composition of the present invention can be used for solvent-based paints and water-based paints. In the case of a solvent-type paint, an organic solvent is used as a diluent. If it is a water-based paint, a neutralizing agent, an emulsifier, etc. can be mix | blended with water other than a diluent as needed.

  In the multilayer coating film forming method using the coating composition of the present invention, for example, when two layers are laminated, the coating composition of the present invention is used for the upper layer, and the film-forming resin and the organic phosphoric acid are used for the lower layer. Two layers are simultaneously heat-cured using a coating composition containing ester (III). In general, the storage stability of a coating composition having a catalyst tends to depend on the amount of catalyst contained. Also in the coating composition of the present invention, it is expected that the storage stability of the coating is lowered when the amount of the catalyst is too large. On the other hand, curing at low temperatures does not proceed unless the required amount of catalyst is present.

  Further, when two layers are continuously applied without being cured, the components of the upper and lower layers may be mixed around the interface at the time of curing. Therefore, in order to ensure the physical properties of the upper layer coating film, the upper layer curing catalyst is hidden in the lower layer coating, and the catalyst moves from the lower layer to the upper layer during heat curing, thereby promoting low temperature curing of the upper layer coating composition. This method is effective. When a coating composition containing (I) and (II) of the present invention is used for the upper layer or a coating composition containing (I), (II) and (III) of the present invention is used, A coating composition containing a film-forming resin and a catalyst (III) is used. Although the lower layer composition may contain a curing agent as necessary, the catalyst (III) may not have a catalytic effect on the curing reaction of the lower layer composition. The film-forming resin is not particularly limited, but two or more kinds of acrylic resin, polyester resin, urethane resin, epoxy resin, alkyd resin, or these resins can be used.

  The invention is illustrated in more detail by the following specific examples.

Examples and comparative examples

(Production Example 1) Synthesis of thermosetting acrylic resin (I) -1 A four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen blowing pipe and a dropping funnel was placed in a nitrogen atmosphere, and Solvesso 150 ( 80 parts by weight of a mixed solution of Kyoei Chemical Co., Ltd./n-butanol=8/2 is added and heated to 110 ° C. When it reaches 110 ° C., 20 parts by weight of styrene, 32 parts by weight of 2-ethylhexyl methacrylate, 10 parts by weight of 2-ethylhexyl acrylate, 18 parts by weight of 2-hydroxyethyl methacrylate, 8 parts by weight of 4-hydroxybutyl acrylate, methacrylic acid A mixture of 3 parts by weight and 27 parts by weight of t-amylperoxy-2-ethylhexanoate is added over 2 hours. After completion of the addition, the mixture was aged at 110 ° C. for 60 minutes, added with 2 parts by weight of t-amylperoxy-2-ethylhexanoate, and further aged at 110 ° C. for 90 minutes. The number average molecular weight of the obtained resin was 2,800, the hydroxyl value was 120 mgKOH / g (solid content conversion), the acid value was 20 mgKOH / g (solid content conversion), the glass transition point was 10 ° C., and the SP value was 9.8. The non-volatile content was 60% by weight.

(Production Example 2) Synthesis of thermosetting acrylic resin (I) -2 A four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen blowing pipe and a dropping funnel was placed in a nitrogen atmosphere, and Solvesso 150 ( 80 parts by weight of a mixed solution of Kyoei Chemical Co., Ltd./n-butanol=8/2 is added and heated to 110 ° C. When it reaches 110 ° C., 20 parts by weight of styrene, 33 parts by weight of 2-ethylhexyl methacrylate, 10 parts by weight of 2-ethylhexyl acrylate, 18 parts by weight of 2-hydroxyethyl methacrylate, 8 parts by weight of 4-hydroxybutyl acrylate, methacrylic acid A mixture of 3 parts by weight and 26 parts by weight of t-amylperoxy-2-ethylhexanoate is added over 2 hours. After completion of the addition, the mixture was aged at 110 ° C. for 60 minutes, added with 2 parts by weight of t-amylperoxy-2-ethylhexanoate, and further aged at 110 ° C. for 90 minutes. The number average molecular weight of the obtained resin is 3,200, the hydroxyl value is 120 mgKOH / g (in terms of solid content), the acid value is 20 mgKOH / g (in terms of solid content), the glass transition point is 10 ° C., and the SP value is 9.8. The non-volatile content was 60% by weight.

(Production Example 3) Synthesis of Thermosetting Acrylic Resin (I) -3 A 4-necked flask equipped with a stirrer, thermometer, reflux condenser, nitrogen blowing tube, and dropping funnel was placed in a nitrogen atmosphere, and Solvesso 150 ( 80 parts by weight of a mixed solution of Kyoei Chemical Co., Ltd./n-butanol=8/2 is added and heated to 110 ° C. When it reaches 110 ° C., 21 parts by weight of styrene, 40 parts by weight of 2-ethylhexyl methacrylate, 11 parts by weight of 2-ethylhexyl acrylate, 17 parts by weight of 2-hydroxyethyl methacrylate, 5 parts by weight of 4-hydroxybutyl acrylate, methacrylic acid A mixture of 3 parts by weight and 21 parts by weight of t-amylperoxy-2-ethylhexanoate is added over 2 hours. After completion of the addition, the mixture was aged at 110 ° C. for 60 minutes, added with 2 parts by weight of t-amylperoxy-2-ethylhexanoate, and further aged at 110 ° C. for 90 minutes. The number average molecular weight of the obtained resin was 4,000, the hydroxyl value was 95 mgKOH / g (in terms of solid content), the acid value was 20 mgKOH / g (in terms of solid content), the glass transition point was 10 ° C., and the SP value was 9.5. The non-volatile content was 60% by weight.

(Production Example 4) Synthesis of Thermosetting Acrylic Resin (I) -4 A 4-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen blowing tube and a dropping funnel was placed in a nitrogen atmosphere, and Solvesso 150 ( 80 parts by weight of a mixed solution of Kyoei Chemical Co., Ltd./n-butanol=8/2 is added and heated to 110 ° C. When the temperature reaches 110 ° C., 31 parts by weight of styrene, 39 parts by weight of 2-ethylhexyl methacrylate, 13 parts by weight of 2-ethylhexyl acrylate, 10 parts by weight of 2-hydroxyethyl methacrylate, 7 parts by weight of 4-hydroxybutyl acrylate, methacrylic acid A mixture of 3 parts by weight and 15 parts by weight of t-amylperoxy-2-ethylhexanoate is added over 2 hours. After completion of the addition, the mixture was aged at 110 ° C. for 60 minutes, added with 2 parts by weight of t-amylperoxy-2-ethylhexanoate, and further aged at 110 ° C. for 90 minutes. The number average molecular weight of the obtained resin was 5,600, the hydroxyl value was 70 mgKOH / g (in terms of solid content), the acid value was 20 mgKOH / g (in terms of solid content), the glass transition point was 10 ° C., and the SP value was 9.3. The non-volatile content was 60% by weight.

(Production Example 5) Synthesis of thermosetting acrylic resin (I) -5 A 4-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen blowing pipe and a dropping funnel was placed in a nitrogen atmosphere, and Solvesso 150 ( 80 parts by weight of a mixed solution of Kyoei Chemical Co., Ltd./n-butanol=8/2 is added and heated to 105 ° C. When it reaches 110 ° C., 30 parts by weight of styrene, 38 parts by weight of 2-ethylhexyl methacrylate, 12 parts by weight of 2-ethylhexyl acrylate, 10 parts by weight of 2-hydroxyethyl methacrylate, 8 parts by weight of 4-hydroxybutyl acrylate, methacrylic acid A mixture of 4 parts by weight and 10 parts by weight of t-amylperoxy-2-ethylhexanoate is added over one and a half hours. After completion of the addition, the mixture was aged at 105 ° C. for 60 minutes, added with 8 parts by weight of t-amylperoxy-2-ethylhexanoate, and further aged at 105 ° C. for 90 minutes. The number average molecular weight of the obtained resin is 7700, the hydroxyl value is 70 mg KOH / g (solid content conversion), the acid value is 20 mg KOH / g (solid content conversion), the glass transition point is 11 ° C., the SP value is 9.3, and non-volatile The minute was 60% by weight.

(Examples 1 to 23, Comparative Examples 1 to 10)
The thermosetting acrylic resin, melamine resin, and organic phosphate ester used in Examples and Comparative Examples are shown in Table 1, Table 2, and Table 3, respectively.

(Number average molecular weight) The number average molecular weight based on polystyrene as measured by gel permeation chromatography (GPC) using the following apparatus.
Apparatus: Tosoh Corporation HLC-8120GPC
Column: Tosoh Corporation TSKgel GMHXL × 2
TSKgel G2500HXL × 1
TSKgel G2000HXL × 1
Carrier: Tetrahydrofuran Flow rate: 1.0 mL / min
Temperature: 40 ° C
Detection method: Differential refractometer

(Hydroxyl value) The hydroxyl value was calculated by the following formula.

(Acid value) The acid value was calculated by the following formula.

(Glass transition point) The glass transition point (Tg) is calculated using the following FOX equation.
1 / Tg = W1 / Tg1 + W2 / Tg2 + ... + Wi / Tgi + ... + Wn / Tgn
In the FOX formula, Tg is a glass transition temperature (K) of a polymer composed of n types of monomers, and Tg (1, 2, i, n) is a glass transition temperature (K) of a homopolymer of each monomer. W (1, 2, i, n) is a mass fraction of each monomer, and W1 + W2 +... + Wi +.

(SP value) A poor solvent (water, hexane) was dropped into a polymer solution (1 g of thermosetting acrylic resin was diluted with 20 ml of a good solvent (acetone)), and the amount of the poor solvent required to cause turbidity was determined. The SP value was calculated from the following formula.

φ ₁ : SP value of poor solvent (water) (= 23.4)
δ ₁ : Volume fraction of water required to cause turbidity
δ _g : SP value of good solvent (acetone) (= 9.7)
φ ₂ : SP value of poor solvent (hexane) (= 7.3)
δ ₂ : Volume fraction of hexane required to cause turbidity, SP values of water, acetone and hexane were quoted from database values of HSPiP.

(Non-volatile content) About 1 g of a sample is collected in a pre-weighed metal evaporating dish and precisely weighed with a precision chemical balance. These samples are placed in a hot air circulating drier maintained at 120 ± 2 ° C. and heated for 1 hour. Thereafter, the sample is taken out, allowed to cool to room temperature, weighed again with a precision chemical balance, the remaining amount is obtained, and the non-volatile content (= (remaining amount after heating (g) / sample mass (g)) × 100) Is calculated.

The following were used for each raw material shown in Table 2.
U-VAN20SE60, U-VAN125 ... Resimene CE-7103 made by Mitsui Chemicals, Inc.Simel 303 made by INEOS Melamines ... made by Nihon Cytec Industries, Ltd.

Each raw material shown in Table 3 was manufactured by SC Organic Chemical Co., Ltd.
Since Phoslex A-12 and 18 were solids, they were used after being diluted to a solid content of 25% with a mixed solution of Solvesso 150 (manufactured by Kyoei Chemical Co., Ltd.) / N-butanol = 8/2.

In the container, the composition materials described in the clear coating composition shown in Tables 4, 5 and 6 were weighed and stirred with a homodisper for 2 minutes to obtain a clear coating.

  A clear coating was applied to a 0.8 mm thick steel sheet treated with zinc phosphate so that the thickness of the dry coating film was about 25 μm, and baked at 80 to 100 ° C. for 30 minutes. The numbers shown in Tables 4, 5 and 6 represent parts by weight of solids. Moreover, the numerical value in () of organophosphate ester shows phosphorus element conversion amount.

(Coating performance test and evaluation method)
Coating film hardness: The coating film hardness was measured by JIS K5400 8.4.1 tester method, and evaluation by destruction of the coating film was performed.
Solvent resistance: 2 ml of xylene was placed on a test plate in a spot shape, allowed to stand at 25 ° C. for 30 minutes, and the appearance of the coating film was visually determined. Evaluation was performed according to the following criteria.
◎ …… Good ○ …… Slight traces × …… Swelling xx ・ ・ ・ Gel fraction dissolved gel fraction: Gel fraction is acetone / ethanol The value with respect to the weight before extraction of the weight of the undissolved portion when reflux extraction was performed with a 1/1 mixed solution using a Soxhlet extractor for 2 hours was evaluated according to the following criteria.
◎ ... 91% or more ○ ... 86-90%
× ・ ・ ・ ・ ・ 85% or less

(Examples 24-36, Comparative Examples 11-15)
In the container, the composition materials described in the upper layer coating composition shown in Tables 7 and 8 were weighed and stirred with a homodisper for 2 minutes to obtain an upper layer coating material. Similarly, the composition materials described in the lower layer coating composition shown in Tables 7 and 8 are weighed in a container, and stirred with a homodisper for 2 minutes, so that the three lower layer coating materials, lower layer-1, lower layer-2, and lower layer-3 are mixed. Got.

  The lower layer paint is applied to a 0.8 mm thick steel sheet treated with zinc phosphate so that the thickness of the dried coating film is about 25 μm, pre-dried at 60 ° C. for 10 minutes, and then the upper layer paint is applied. The coating was applied so that the thickness of the dried coating film was about 25 μm, and baked at 80 ° C. for 30 minutes.

  The numbers shown in Tables 7 and 8 represent parts by weight of solids. Moreover, the numerical value in () of organophosphate ester shows phosphorus element conversion amount.

Claims (7)

At least the following components (I), (II) and (III):
(I) Number average molecular weight is 2,000 to 10,000, and hydroxyl value is 70 to 120 mgKOH.
N-butyl etherified melamine resin (III) having a number average molecular weight of 1,000 to 1,500 and having an imino group and a methylol group

(In the formula, R ₁ is an alkyl group having 1 to 10 carbon atoms, and R ₂ is hydrogen or 1 to 1 carbon atoms.
10 alkyl groups. )
A thermosetting coating composition comprising an organophosphate represented by the formula:   The thermosetting acrylic resin (I) according to claim 1 is 50 to 70% by weight in solid content, the melamine resin (II) is 30 to 50% by weight in solid content, and the organophosphate (III) is (I) A thermosetting coating composition containing 0.1 to 2.5% by weight in terms of phosphorus element with respect to the total resin content of (II) and (II). (1) The thermosetting coating composition according to claim 1, comprising an organophosphate (III) wherein R _{1 in the} formula is an alkyl group having 1 to 8 carbon atoms. (1) The thermosetting coating composition according to claim 1, comprising an organophosphate (III) wherein R _{1 in the} formula is an alkyl group having 1 to 4 carbon atoms.   In a laminating method in which two or more layers are coated with a paint, a coating composition containing 0.1 to 2.5% by weight of an organic phosphate (III) in terms of phosphorus element with respect to the resin solid content of the paint in the lower layer part A coating composition comprising the thermosetting acrylic resin (I) and n-butyl etherified melamine resin (II) as set forth in claim 1 without coating and curing, or the heat as set forth in claim 1. A coating composition containing a curable acrylic resin (I), an n-butyl etherified melamine resin (II), and an organic phosphate ester (III) is applied repeatedly, and the applied multiple layers are simultaneously heated and cured. A method for forming a multilayer coating film.   The method for forming a multilayer coating film according to claim 5, wherein a coating composition containing a luster pigment or a coloring pigment in addition to the organic phosphate ester (III) is used for the lower layer, and a transparent coating composition is used for the upper layer.   The multilayer coating film obtained by the method in any one of Claim 5 or 6.