JPH08311396A - Aqueous base coating material composition and formation of coating film - Google Patents

Abstract

PURPOSE: To obtain a thermosetting aqueous base coating material composition providing a coating material showing sagging and pulverization not being influenced by humidity of coating environment due to possession of pseudo-plastic viscosity of shear rate dependence. CONSTITUTION: This thermosetting aqueous base coating material composition is obtained by dispersing a polyester resin composed of 1-30wt.% of an end hydroxyl group-containing polyalkanedienediol, its hydrogenated substance or its mixture, its crosslinking agent and a coloring pigment to an aqueous medium containing a base to neutralize at least 50% of the total of the carboxyl group of the resin based on the total amount of the whole reactive components. A bivalent and a polyvalent basic metal hydroxides amount to 5-20% of the neutralization equivalent of the base.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermosetting aqueous base coating composition, and more particularly to an aqueous base coating composition containing a coloring pigment used for finish coating of automobile bodies and the like.

In recent years, there has been an increasing interest in the use of water-based paints from the viewpoints of environmental protection, resource saving and energy saving. For example, in the coating of automobile bodies, an intermediate coating layer such as a chipping sealer layer and a plummer layer is applied on the electrodeposition coating film, and further a solid or metallic color base coat and optionally a transparent top coat are applied. It is desirable to use an aqueous paint as much as possible in each of these layers to reduce the amount of organic solvent used.

In an aqueous base coating composition using a polyester resin (including alkyd) as a vehicle resin, a carboxyl group of the resin is neutralized with a base to be water-soluble, and then dissolved or dispersed in an aqueous medium. In a conventional water-based coating composition that complies with Newton's law of viscosity, a low viscosity that is desirable for providing fine skin by spraying and the like and a desirable high viscosity for preventing sagging after application are not compatible. Therefore, the increase in viscosity due to the evaporation of water content after coating and the increase in non-volatile content is used as a principle for preventing sagging. However, this method is susceptible to changes in temperature and humidity. That is, on the high-temperature and low-humidity side, the water volatilization rate is fast, and therefore, good skin cannot be obtained due to armpits and insufficient leveling due to an excessive increase in coating viscosity. On the other hand, on the low-temperature and high-humidity side, the rate of water vaporization is slow, and the coating viscosity hardly increases, so that it easily sags.

On the other hand, in the case of a non-Newtonian fluid whose apparent viscosity decreases as the shear rate increases, the apparent viscosity becomes sufficiently low for atomization in the high shear rate region such as during spraying, and therefore, during coating. In the low shear rate region such as, it becomes high enough to prevent sagging.

DISCLOSURE OF THE INVENTION The present invention relates to a polyester-based water-based coating composition, wherein a part of a base that neutralizes the resin for dissolving or dispersing the resin in an aqueous medium is used as a part of a base such as calcium hydroxide. The use of a basic metal oxide having a valency or higher makes the coating material exhibit pseudoplastic viscosity, and is based on the finding that it has a high yield value.

According to the present invention, (a) a hydroxyl group-terminated polyalkadiene diol having an average degree of polymerization of 5 to 50, a hydrogenated polyalkadiene diol alone, or a mixture thereof (1 to 30) based on the total amount of all reaction components is used. A polyester resin having an acid value of 5 to 150 and a hydroxyl value of 20 to 250, which is included by weight, (b) a cross-linking agent of the polyester resin, and (c) a color pigment, and the total amount of carboxyl groups of the polyester resin. There is provided a thermosetting aqueous base coating composition which is dissolved or dispersed in an aqueous medium containing an amount of a base which neutralizes at least 50% thereof. The improvement is that the neutral metal equivalent of divalent or higher occupies 5 to 20% of the neutralization equivalent of the base.

Thus, by using a divalent or higher valent basic metal hydroxide, such as calcium hydroxide, magnesium hydroxide, or barium hydroxide, as a part of the neutralizing base, the paint can be spray-coated. It has a low viscosity in a high shear rate region, but exhibits a shear rate-dependent pseudoplastic viscosity such that it has a high viscosity in a low shear rate region such as coating. This makes it possible to simultaneously satisfy the coating workability of the coating material, the dispersion stability, and the coating film performance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As is well known, polyester resins are produced by condensing polyhydric carboxylic acids and polyhydric alcohols as essential ingredients. Other reaction components can include minor proportions of monocarboxylic acids, hydroxycarboxylic acids, lactones, etc., and alkyd resins include drying oils or semi-drying oils and their fatty acids.

The polyester resin of the present invention has a content of 10 to 60%, preferably 2%, based on the total weight of all reaction components.
It must contain 0 to 50%, especially 30 to 40% of aromatic polycarboxylic acids. Examples thereof are phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, pyromellitic dianhydride, but isophthalic acid, terephthalic acid, trimellitic anhydride are preferred. . If too much is used, the viscosity of the resin will increase during water solubilization or dispersion.

As the remaining acid component, hexahydrophthalic anhydride, tetrahydrophthalic anhydride, 1,4- and 1,3-cyclohexanedicarboxylic acid, maleic anhydride, succinic anhydride, adipic acid, sebacic acid, azelaic acid, etc. There is. As mentioned previously, small proportions of hydroxycarboxylic acids such as 4-hydroxybenzoic acid, hydroxypivalic acid, 12-hydroxystearic acid and the like, and monocarboxylic acids such as benzoic acid, t-butylbenzoic acid and the like may also be included.

The polyester resin of the present invention contains, as a polyhydric alcohol component, 1 to 30%, preferably 2 to 20%, and especially 3 to 15% of a hydroxyl group-terminated polyalkadiene diol alone, based on the total amount of all reaction components. It must contain hydrogenates alone or in mixtures. These diols are derived from conjugated alkadiene oligomers or polymers with a degree of polymerization of 5 to 50. The molecular weight is 1,
It is preferably in the range of 000 to 4,000, particularly 1,500 to 3,000. 1,4-polyisoprene diol, 1,4- and 1,2-polybutadiene diol, hydrogenated products thereof are preferred. As a commercial product, Epol (hydrogenated polyisoprene diol, molecular weight 186
0, average degree of polymerization 26, Idemitsu Petrochemical Co., Ltd., PIP
(Polyisoprene diol, molecular weight 2,200, average degree of polymerization 34, manufactured by Idemitsu Petrochemical Co., Ltd.), Polytail HA
(Hydrogenated polybutadiene diol, molecular weight 2,200,
Average degree of polymerization 39, Mitsubishi Kasei Co., Ltd., Polytail H
(Hydrogenated polybutadiene diol, molecular weight 2,350,
Average degree of polymerization 42, manufactured by Mitsubishi Kasei Co., Ltd., R-45H
T (polybutadiene diol, molecular weight 2,270, average degree of polymerization 42, manufactured by Idemitsu Petrochemical Co., Ltd.) and the like. The use of these diols as part of the polyhydric alcohol component results in high molecular weight polyester resins that give relatively low viscosities at high degrees of neutralization. However, if it is used too much, a satisfactory coating hardness cannot be obtained.

As the remaining polyhydric alcohol component, ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol and 2,2- Diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanedimethanol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3
-Propanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol, hydrogenated bisphenol A and other diols, and trimethylolpropane, trimethylolethane, glycerin, pentaerythritol and other three There are polyols with a valency or higher.

In the case of alkyd resins, up to 30%, preferably up to 25%, especially 10 to 10% of the total weight of all reaction components.
It must contain 20% of fatty acids having 6 or more carbon atoms or fats and oils containing them. Examples of these fats and oils components are castor oil, linseed oil, dehydrated castor oil, tung oil, safflower oil, soybean oil, tall oil, coconut oil, palm kernel oil and their fatty acids. Palm oil and palm kernel oil are preferred.

As a reaction component other than the above, Cardura E
There are monoepoxide compounds such as (trade name: manufactured by Ciel Kagaku) and lactones. Lactones form ring-opening additions to the polyester chains of polyhydric carboxylic acids and polyhydric alcohols to themselves form polyester chains. Examples thereof include β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, γ-caprolactone, γ-caprylolactone, crotlactone, δ-valerolactone, and δ-caprolactone. However, ε-caprolactone is preferred. Its amount is 3 to 30%, preferably 5 to 20%, in particular 7 to 15% of the total weight of all reaction components. Addition of lactone improves chipping resistance.

For the synthesis of polyester, the above reaction components are used in a conventional manner in a nitrogen stream at, for example, 150 to 250 ° C. and 4-1.
It can be performed by heating for 0 hours and condensation. In that case, as the catalyst, known catalysts such as dibutyltin oxide, antimony trioxide, zinc acetate, manganese acetate, cobalt acetate, calcium acetate, lead acetate, tetrabutyl titanate, and tetraisopropyl titanate can be used. The reaction components may be added all at once, or may be added in several batches.

The molar ratio of the reaction components and the reaction conditions are such that the polyester resin obtained has an acid value of 10 to 100, preferably 20 to 50, and a hydroxyl value of 30 to 200, preferably 5.
0-150, weight average molecular weight 10,000-200,
It should be adjusted to reach 000, preferably 25,000 to 160,000.

The aqueous varnish contains 0.5 to 1.2 equivalents of a base in an amount that neutralizes at least 50% of the total amount of carboxyl groups contained in the resin, that is, the total amount of carboxyl groups.
It is preferably prepared by dissolving or dispersing in an aqueous medium containing 0.7 to 1.0 equivalent of a base.

According to the invention, the neutralization equivalent of the base is 5 to 2
0% is occupied by a basic metal hydroxide having a valence of 3 or more. Examples are hydroxides of calcium, magnesium, barium, strontium, tin, zinc, aluminum and the like. Calcium hydroxide, magnesium hydroxide and barium hydroxide are preferred. If the amount is too small, pseudoplastic viscosity does not develop, and if too large, it does not dissolve in the aqueous medium.

Other bases which make up the remaining 80 to 95% of the neutralization equivalent include inorganic bases such as alkali metal hydroxides and aqueous ammonia, and methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine. , Isopropylamine, diisopropylamine, diethylenetriamine, triethylenetetramine, monoethanolamine, diethanolamine, 2-amino-2-methylpropanol, morpholine, N-methylmorpholine, N-ethylmorpholine, piperazine, dimethylethanolamine, diethylethanolamine, There are amines such as dimethyldodecylamine. Triethylamine, dimethylethanolamine and diethylethanolamine are preferred. Excessive base is not preferable because it accelerates the hydrolysis of the resin during storage.

The aqueous medium is usually ion-exchanged water, but if necessary, it may contain a small amount of a water-miscible organic solvent. Examples are diethylene glycol monoethyl- or monobutyl ether, propylene glycol monomethyl- or monoethyl ether, ethylene glycol monoethyl ether, methanol, ethanol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, dimethyl. Formamide,
N-methylpyrrolidone and the like.

The coating is carried out by dissolving or dispersing a resin in an aqueous medium containing a base to produce an aqueous varnish, and adding a pigment paste dissolved in a part of the aqueous varnish and a crosslinking agent thereto.

The coating composition of the present invention must contain a crosslinking agent that reacts with the polyester resin. Examples of such crosslinking agents are epoxy resins, melamine resins, triglycidyl isocyanurate, dicyandiamide and the like.
In the case of the alkyd resin, a drier of cobalt naphthenate, lead naphthenate, zinc naphthenate or the like may be used.
Melamine resins are preferred. The amount depends on the type of the cross-linking agent, but in the case of a melamine resin, it is generally in the range of polyester resin: melamine resin = 6: 4 to 8: 2 as a solid content. The melamine resin is typically a methylolated melamine which is a condensate of melamine and formaldehyde, and a resin obtained by etherifying a part or all of the methylol group with an alcohol such as methanol, ethanol, n-butanol or isobutanol. is there. Sulfonic acids such as paratoluenesulfonic acid, dodecylbenzenesulfonic acid, and dinonylnaphthalenesulfonic acid, and salts of these sulfonic acids with organic amines that dissociate at the baking temperature of the coating film can be used as a catalyst.

The coating composition of the present invention contains a conventional pigment. Examples are metallic flakes, luster pigments such as aluminum flakes, titanium mica, titanium dioxide-coated mica-like iron oxide, titanium dioxide, lead white, zinc white, zinc sulfide, graphite, carbon black, yellow iron oxide, red oxide. Iron, Phthalocyanine Blue, Phthalocyanine Green, Quinacridone, Indanthrone, Isoindolinone, Perylene, Anthrapyrimidine, Benzimidazolone, Cadmium Yellow, Cadmium Red,
Color pigments such as chrome yellow and diketopyrrolopyrrole, and calcium carbonate, barium sulfate, silicic acid-based pigments,
Extender pigments such as silicate type and aluminum hydrate type. The pigment is prepared by using a part of the polyester resin aqueous varnish dissolved or dispersed in an aqueous medium containing a base to form a pigment paste, and the pigment paste is added to the rest of the aqueous varnish or the resin powder-dispersed aqueous varnish. The pigment content of the coating composition is expressed as a weight ratio of pigment / resin solid content in the coating composition, and is usually 0.01 / 1 to 1/1, preferably 0.03 / 1 to 0.9 /. It is within the range of 1.

The aqueous base coating composition of the present invention is suitably used for forming a composite coating film comprising a base coat and a transparent top coat used for finish coating of automobile bodies, for example. In this case, it is preferable to use the two-coat one-bake coating method in which the water-based base paint of the present invention is applied as the base coat, the transparent top coat paint is applied thereon by wet-on-wet, and both are simultaneously cured. In that case, in order to obtain a coating film having a good finish, it is preferable to preheat the base coat at 60 to 100 ° C. for 2 to 10 minutes before applying the transparent top coat paint. A transparent topcoat must always be applied for a metallic finish, but the topcoat can be omitted for a solid color finish. Such coating methods and the topcoat paints used are well known in the paint industry and do not themselves constitute a part of the invention and need not be described in further detail.

The invention is illustrated by the following examples. "%" In the examples is based on weight.

Production Example 1 155 parts by weight of coconut oil and 2 parts of trimethylolpropane were placed in a reaction vessel having a heating device, a stirrer, a nitrogen introducing pipe and a fractionating tower.
After charging 48 parts by weight and 1.7 parts by weight of dibutyltin oxide, heating was started under dry nitrogen to melt the raw material, and then 2
The temperature was gradually raised to 10 ° C., transesterification was performed, and the mixture was cooled.
Next, 267 parts by weight of isophthalic acid, 59 parts by weight of adipic acid, 33 parts by weight of neopentyl glycol, and 41 parts by weight of epol (hydrogenated polyisoprene diol, molecular weight = 1860, average repetition number n = 26, manufactured by Idemitsu Petrochemical Co., Ltd.) Was added and the temperature was gradually raised to 220 ° C. to carry out a dehydration esterification reaction. After dehydration esterification to a resin acid value of 10, the mixture was cooled to 150 ° C., 40 parts by weight of trimellitic anhydride was added and reacted to a resin AV40, cooled to 140 ° C., and 77 parts of ε-caprolactone was further added to obtain 1 The reaction was carried out for a time and the reaction was completed. 75 parts by weight of Solvesso # 150 (aromatic hydrocarbon solvent, manufactured by Esso Chemical Co., Ltd.) and 75 parts by weight of butyl cellosolve were added to the obtained polyester resin to obtain a polyester resin varnish I having a resin solid content of 83%. The obtained polyester resin I has a hydroxyl value of 137,
The acid value was 37, the number average molecular weight was 2850 (in terms of polystyrene), and the water tolerance was 0.8.

Production Examples 2 to 4, 6, 7 Polyester resins II to IV and VI, VII were obtained in the same manner as in Production Example 1 based on the formulations shown in Table 1. The properties of the obtained polyester resin are shown in Table 1.

[0028]

[Table 1]

1) Epol: hydrogenated polyisoprene diol, molecular weight = 1860, average number of repetitions n = 26, manufactured by Idemitsu Petrochemical Co., Ltd. 2) PIP: polyisoprene diol, molecular weight = 240
0, average number of repetitions n = 34, manufactured by Idemitsu Petrochemical Co., Ltd. 3) Polytail HA: hydrogenated polybutadiene diol,
Molecular weight = 2200, average number of repetitions n = 39, manufactured by Mitsubishi Kasei Co., Ltd. 4) R-45HT: polybutadiene diol, molecular weight =
2270, average number of repetitions n = 42, manufactured by Idemitsu Petrochemical Co., Ltd. 5) Solvesso # 150: aromatic hydrocarbon solvent, manufactured by Esso Chemical Co., Ltd. 6) Average molecular weight: polystyrene conversion by GPC 7) Nonvolatile matter: 150 ° C. Residue after heating for 1 hour at 8) Water tolerance: 0.5 g of polyester varnish was taken in a 100 ml beaker and dissolved in 10 ml of tetrahydrofuran (commercial reagent), and ion-exchanged water was dropped using a burette. The water tolerance is defined as the amount (ml) of ion-exchanged water added dropwise when the mixed solution becomes cloudy.

Production Example 5 100 parts by weight of the polyester resin I obtained in Production Example 1, 5.5 parts by weight of Cymel 253 (a melamine resin manufactured by Mitsui Cytec Co., Ltd.), and 70 parts by weight of 3.8 parts by weight of n-butanol were used.
Condensation was performed at 0 ° C. for 1 hour to obtain a polyester resin V having a resin solid content of 80%. The obtained polyester resin V had a hydroxyl value of 129, an acid value of 35, a number average molecular weight of 3310 (in terms of polystyrene), and a water tolerance of 0.85.

Production Example 8 To 1100 parts by weight of the polyester resin obtained in Production Example 1, 0.41 parts by weight of calcium hydroxide and 3.90 parts by weight of dimethylethanolamine were added and mixed, and 192.1 parts by weight of deionized water was added. Parts were added to obtain an aqueous polyester resin varnish A having a nonvolatile content of 28%. The obtained aqueous polyester resin varnish A was treated with No. The 4-Ford cup was diluted with ion-exchanged water until the viscosity reached 60 seconds, and the viscosity was measured with an E-type viscometer. As a result, the yield value was 18 dyne / cm ² .

Production Examples 9 to 20 The polyester resins I to VII obtained in Production Examples 1 to 7 were treated in the same manner as in Production Example 1 based on Table 2 to obtain aqueous polyester resin varnishes BM. The properties of the resulting aqueous polyester resin varnish are shown in Table 2.

[0033]

[Table 2]

[0034]

[Table 3]

1) Ratio of basic compound to total amount of carboxyl group of polyester resin 2) Mol% of metal hydroxide in basic compound 3) Yield value: 0.5 rpm to 1 using an E-type viscometer
The result of viscosity measurement up to 00 rpm was Casson plotted to obtain a yield value.

Examples 1 to 8 and Comparative Examples 1 and 2 To the aqueous polyester resin varnishes A to I and M shown in Table 2, an aluminum flake paste dissolved in a part of the same aqueous polyester resin varnish was added, and a melamine resin and p-Toluenesulfonic acid was added and mixed well with stirring to obtain an aqueous base paint shown in Table 3. This paint was washed with Ford Cup No. Dilute to 60 seconds / 20 ° C. with 4 and spray paint on tin plate in a coating booth kept at 25 ° C. and 85% RH, bake at 150 ° C. for 30 minutes,
High-humidity workability and coating film performance were evaluated. The results are shown in Table 3.

As for the sagging property, a cationic electrodeposition coating (Power Top PU-50 manufactured by Nippon Paint Co., Ltd.) was electrodeposited to a dry film thickness of about 25 μm on a 20 cm × 30 cm pressed steel plate, and an intermediate coating ( Olga P made by Nippon Paint Co., Ltd.
-2 sealer) by air spray coating to a dry film thickness of about 40 μm and baking at 140 ° C. for 30 minutes. 3 in 4
Adjust the viscosity to 0 sec / 20 ° C, apply two-stage coating with an air spray so that the dry film thickness is 15 μm or 20 μm, set the coated plate in the coating booth (25 ° C., 85% RH), and then set it at 80 ° C. × 5 minutes. Preheat, then apply clear paint with the following composition to Ford Cup # 4 for 20 seconds to adjust the viscosity, and use wet spray to apply a wet-on-wet coating to give a dry film thickness of 40 μm. After baking and curing for 30 minutes, accumulation and sagging of the obtained coated plate were visually evaluated. The results are shown in Table 3.

Compounding of clear paint DIANAL HR-554 (manufactured by Mitsubishi Rayon Co., Ltd., 58.3 parts, thermosetting acrylic resin varnish, solid content 60%) ACR-461 (manufactured by Nippon Paint Co., Ltd., 63.6 parts, heat) Curable acrylic resin varnish, solid content 55%) U-VAN 20N-60 50.0 parts Tinuvin 900 (UV absorber made by Ciba-Geigy) 2.0 parts SANOL LS-292 (manufactured by Sansha Machine Gosei Co., Ltd. 1.0 part) Antioxidant) n-Butanol 1.3 parts Solvesso 100 5.0 parts

[0039]

[Table 4]

[0040]

[Table 5]

1) Aluminum flake paste manufactured by Toyo Aluminum Co., Ltd., containing aluminum flakes 65% 2) Cymel 212: melamine resin manufactured by Mitsui Cytec Co., Ltd. 3) Rubbing the cured coating film 10 times with gauze containing xylene The state of the coating film was observed. 4) Atomization property: Visual evaluation of atomization level of atomized particles when coated with the same discharge amount and air pressure. ○: Very good, △: Slightly bad, ×: Poor 5) Appearance: Coating skin level Visual evaluation ○: Very good, △: Somewhat bad, ×: Poor 6) Sagging property: Electrodeposition, 20 cm × 30c coated up to the middle coat
spray coating to a base film thickness of 15 or 20μ on a press plate of m, and a coating booth (25 ° C, 85% R
After setting the coated plate vertically in H), preheating and clear coating are performed as usual. Visual evaluation of accumulation and sagging in the pressed part of the obtained coated plate ◎: Very good, ○: Good, △: Slightly good, ×: Poor

Front page continuation (72) Inventor Yoshitaka Okude 19-17 Ikedanaka-cho, Neyagawa City, Osaka Japan Paint Co., Ltd.

Claims (9)

[Claims] 1. (a) A hydroxyl group-terminated polyalkadiene diol alone having an average degree of polymerization of 5 to 50, a hydrogenated polyalkadiene diol alone, based on the total amount of all reaction components.
Or an acid value of 5 containing 1 to 30% by weight of the mixture.
To 50, a polyester resin having a hydroxyl value of 20 to 250, (b) a cross-linking agent for the polyester resin, and (c) a color pigment, and the total amount of at least 50% of the total amount of carboxyl groups contained in the polyester resin. A thermosetting base coating composition obtained by dissolving or dispersing 5 to 20% of the neutralization equivalent in an aqueous medium containing a base occupied by a divalent or higher valent basic metal hydroxide. Stuff. 2. The thermosetting water-based base coating composition according to claim 1, wherein the polyester resin contains a fatty acid having 6 or more carbon atoms up to 30% by weight or an oil or fat containing the same, based on the total amount of all reaction components. Composition. 3. The thermosetting aqueous base coating composition according to claim 1, wherein the polyester resin contains 10 to 60% by weight of an aromatic polycarboxylic acid based on the total amount of all reaction components. . 4. The thermosetting aqueous base coating composition according to claim 1, wherein the polyester resin contains 3 to 30% by weight of a cyclic lactone based on the total amount of the reaction components synthesized. 5. The thermosetting aqueous base coating composition according to claim 1, wherein the polyester resin has a weight average molecular weight of 10,000 to 200,000. 6. The number average molecular weight of the polyalkadiene diol and hydrogenated polyalkadiene diol is 1,000.
The thermosetting waterborne base coating composition according to any one of claims 1 to 5, which is ˜4,000 polyisoprene diol, polybutadiene diol and hydrogenated products thereof. 7. The cross-linking agent is a melamine resin, and the ratio thereof is a solid content of polyester resin: melamine resin ratio =
The thermosetting water-based base coating composition according to any one of claims 1 to 6, which is 6: 4 to 8: 2. 8. The thermosetting aqueous base coating composition according to claim 1, wherein the divalent or higher valent basic metal hydroxide is calcium hydroxide, magnesium hydroxide or barium hydroxide. 9. A base coat composition is applied onto an object to be coated,
9. A thermosetting water-based base according to claim 1, which is used as a base coat composition in a method for forming a composite coating film, in which a transparent top coat composition is applied wet-on-wet and both are simultaneously cured. A method comprising using a coating composition.