KR100665882B1 - Aqueous paint compositions for automotive surfacer coating and method of coating automobiles by using the same - Google Patents

Abstract

The present invention relates to a water-soluble coating composition for automotive intermediates, more specifically, 5 to 50% by weight polyurethane dispersion resin, 2 to 20% by weight methylated melamine curing agent, polyester dispersion resin, based on the weight of the total coating composition 2 to 5 wt%, butyl glycol and paraffin oil mixed solvent 2 to 10 wt%, ethylene glycol monobutyl ether 1 to 5 wt%, ethylene glycol monohexyl ether 1 to 5 wt%, pigment 1 to 30 wt%, silicone leveling 0.05 to 2% by weight, ultraviolet absorber 0.2 to 2% by weight, fine titanium dioxide 0.1 to 3% by weight, water 25 to 40% by weight and alkali dissolving thickener 1 to 10% by weight and synthetic bentonic thickener 0.1 to 3% by weight It relates to a water-soluble coating composition for automotive intermediate containing in the ratio of. The coating composition of the present invention is applicable to the new coating method of the water-soluble multilayer coating method, and is environmentally friendly, not only shortens the process, but also has a remarkable effect on improving the physical properties of the coating film, such as coating film hardness, popping and pinhole removal, storage stability.

Description

Aqueous paint compositions for automotive surfacer coating and method of coating automobiles by using the same}

The present invention relates to a water-soluble coating composition for automotive intermediates, more specifically, 5 to 50% by weight polyurethane dispersion resin, 2 to 20% by weight methylated melamine curing agent, polyester dispersion resin, based on the weight of the total coating composition 2 to 5 wt%, butyl glycol and paraffin oil mixed solvent 2 to 10 wt%, ethylene glycol monobutyl ether 1 to 5 wt%, ethylene glycol monohexyl ether 1 to 5 wt%, pigment 1 to 30 wt%, silicone leveling 0.05 to 2% by weight, ultraviolet absorber 0.2 to 2% by weight, fine titanium dioxide 0.1 to 3% by weight, water 25 to 40% by weight and alkali dissolving thickener 1 to 10% by weight and synthetic bentonic thickener 0.1 to 3% by weight It relates to a water-soluble coating composition for automotive intermediate containing in the ratio of. The coating composition of the present invention is applicable to new coating methods such as a water-soluble multilayer coating method, and is environmentally friendly and not only shortens the process but also has a remarkable effect on improving the physical properties of the coating film such as coating hardness, popping and pinhole removal, and storage stability. .

High industrialization has provided humans with the benefits of real life. The automobile, which is considered one of the best inventions in the modern society, has been technically approached to meet the growing importance of appearance since the 1980s. However, the catchphrase of global environmental preservation, which is advocating rapidly in recent years, is strongly insisting on each industry to establish countermeasures. Recently, in Europe and developed countries, the use of volatile organic compounds is actively restricted, and sanctions are being imposed on products using the same. Generally known volatile organic compounds are easily volatilized at room temperature, and when present in a gaseous state, are naturally emitted in the air environment and are an important cause of air pollution. Volatile organic compounds have inherent biotoxicity and chemical reactivity, and are known to cause various types of environmental problems. They are causing serious environmental destruction such as tropospheric ozone layer generation, stratospheric destruction, greenhouse effect, and odor generation. .

Recently, legislation regulating these volatile organic compounds has been established, and international regulatory legislation will soon be enacted around OECD countries. In particular, the amount of volatile organic compounds emitted through the automotive coating process is estimated to be more than 240,000 tons per year until 1999. The movement to regulate volatile organic compounds emitted to the atmosphere during the automotive coating process has become an international issue. Situation.

In addition to the important propositions not only for the appearance of the exterior but also for the protection of the environment, automotive paints have a great impact on the national economy and the market. By reducing the use of solvents and harmful heavy metals, and greatly shortening the process of curing the paint, it is possible to reduce the foreign currency and energy by supplying raw materials and to reduce the unit cost through the efficiency of the process. Cost reduction through reduction and optimization of air circulation can be achieved.

In general, 2Coat 1 Bake (2C1B) car coating system, which is used as a new car coating system, is applied to the body of the electrodeposition paint and applied to the vehicle through the middle coating process, and then completely cured under the condition of 150 ℃ X25 minutes. It is the most common automotive coating system as a process of coating and drying a top base coat and a top clear coat continuously thereon. Recently, however, automakers have preferred to develop eco-friendly and economical coating systems, and in response, various coating systems have been developed.

The water-soluble multilayer coating system for automobiles is also commonly referred to as a "water-soluble 3C1B (3Coat1Bake) coating system". The water-soluble medium and top coat bases are water-soluble to minimize the volatile organic compounds and enable eco-friendly coating techniques, and the 2C1B coating system. This is a new technology coating system that can lead to highly economical automobile production by eliminating the intermediate baking process (complete curing under the condition of 150 ℃ X 25 minutes) which is essential for

The existing 2C1B coating system requires a lot of operating cost and energy because it requires a middle coating process and a separate top coat and top clear coat coating process. However, the water-soluble multilayer coating system for automobiles is called an integrated coating system and can provide a lot of economic benefits because the coating of the medium, top coat and top clear coat can be integrated in one coating line. In particular, in the case of the intermediate, since the intermediate baking process (complete curing under the condition of 150 ° C. X 25 minutes) is omitted, it is impossible to make a complete baking type, and only after drying the medium at only 80 ° C. X 2-3 minutes, the top coat and the top coat Take the form of painting a clear coat.

As mentioned above, in order to obtain eco-friendliness and economy at the same time that cannot be realized in the existing 2C1B coating system, in the present invention, polyurethane dispersion resin, methylated melamine curing agent, polyester dispersion resin, butyl as main components of the intermediate coating material Medium amount of glycol and paraffin oil mixed solvent, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, pigment, silicone leveling agent, UV absorber, fine titanium dioxide, water and alkali dissolving thickener and synthetic bentonic thickener The present invention has been completed by improving the overall physical properties of the coating film, such as quick drying, excellent UV blocking effect, workability, adhesion, water resistance, impact resistance, cold chipping resistance, interlayer mixing prevention, and the like.

An object of the present invention, based on the weight of the total coating composition, 5 to 50% by weight of polyurethane dispersion resin, 2 to 20% by weight of methylated melamine curing agent, 2 to 5% by weight of polyester dispersion resin, mixed solvent of butyl glycol and paraffin oil 2 to 10% by weight, 1 to 5% by weight of ethylene glycol monobutyl ether, 1 to 5% by weight of ethylene glycol monohexyl ether, 1 to 30% by weight of pigment, 0.05 to 2% by weight of silicone leveling agent, 0.2 to 2% by weight of UV absorber %, 0.1 to 3% by weight of fine titanium dioxide, 25 to 40% by weight of water, and 1 to 10% by weight of alkali-soluble thickener and 0.1 to 3% by weight of synthetic bentonic thickener. It is.

It is another object of the present invention to provide a coating method for a motor vehicle which coats the coating composition in one integral coating line and then coats the top coat and the top coat.

Hereinafter, the present invention will be described in more detail.

The coating composition of the present invention is a coating composition for automotive intermediate coatings applied to the 3 coat 1 bake coating method for excellent fast drying property, efficiency of the painting process, and improvement of overall physical properties of the final coating film, polyurethane dispersion resin, methylated melamine curing agent, poly Contains ester dispersion resin, butyl glycol and paraffin oil mixed solvent, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, pigment, silicone leveling agent, UV absorber, fine titanium dioxide, water and alkali soluble thickener and synthetic bentonic thickener It is characterized by. Each of these components is explained in detail as follows.

Polyurethane dispersion resin used as a main component in the intermediate coating composition of the present invention contains a solid content of 30 to 70%, a resin synthesized dimethylol propionic acid and diisocyanate, polyester polyol, amine, etc. under acetone process Can be preferably used. Polyurethane dispersion resins with a number average molecular weight of 8,000 to 15,000, an acid value of 4 to 7 mgKOH / g, and a particle size of 100 to 200 nm can be used to obtain excellent fast drying, interpenetration prevention, leveling, etc. Proper wettability, adhesion, water resistance, impact resistance, cold resistance chipping resistance and the like can be enhanced. The polyurethane dispersion resin is used at 5 to 50% by weight based on the total coating composition, thereby affecting the overall physical properties of the coating, such as adhesion enhancement, drying property, and interlayer penetration prevention, and when the amount is less than the above range, the coating film Formability, drying properties, mechanical properties, etc. are poor, and if it exceeds the above range has a problem that the cold resistance chipping resistance and impact resistance of the coating film is somewhat poor.

Polyester dispersion resin is used in order to disperse a pigment, It is preferable that it is number average molecular weights 20,000-30,000, glass transition temperature -30--40 degreeC, and solid content 60-80%. The polyester dispersion resin is used in 2 to 5% by weight based on the total coating composition, if the application amount is less than the above range, the dispersion particle size of the applied pigment may be increased to affect the appearance and the water resistance of the coating film over the above range And poor dryness.

In the present invention, methylated melamine is applied as a curing agent in order to increase adhesion and hardness, mechanical properties of the coating film, and the like.

The methylated melamine resin reacts with the hydroxyl groups in the base paint coated on the middle to increase the density, hardness and the like of the coating film. Methylated melamine resin is a common melamine resin having a methyl group, which may be used in an amount of about 2 to 20% by weight based on the total coating composition, thereby increasing the density and plasticity of the coating film, and when the application amount is less than the above range, it is mentioned. It is difficult to obtain effects such as physical properties, and when the above range is exceeded, problems with heat storage properties are caused.

In addition to the said melamine resin, a block isocyanate hardening | curing agent can be mixed and used as needed. When the block isocyanate is used, it is possible to increase the mechanical properties of the coating by using about 2 to 20% by weight based on the total coating composition. Relatively, when the application amount is less than the above range, it is difficult to improve the physical properties of the coating film, and when the above range is exceeded, the excess isocyanate may react with the water, which causes problems in heat storage and promotes gelation of the paint. . As an example of blocked isocyanate, the thing which blocked isophorone diisocyanate, hexamethylene diisocyanate etc. with dimethylpyrazole, an oxime, etc. is mentioned.

In the present invention, a specific solvent mixture is applied in order to impart excellent drying properties and intermixing resistance. In particular, drying properties and intermixing prevention are the most important parts of water soluble multilayer coating systems. The solvent mixture applied to the present invention is a solvent system capable of forming an azeotrope mixture with water in the form of a mixture. The solvent mixture can maintain a relatively high vapor pressure so that volatilization of water can be easily promoted to speed up drying of the coating film. It has the characteristics to be implemented. As for the azeotropic solvent mixture applied to the present invention, it is preferable to use butyl glycol and paraffin oil mixed solvent, ethylene glycol monobutyl ether, and ethylene glycol monohexyl ether, using 2 to 10 wt%, 1 to 5 wt%, and 1 to 5 wt%, respectively. Preferably, if the application amount is less than the above range can not promote the volatilization of the water can not implement the quick-drying of the coating film, if it exceeds the above range, the volatilization of water and solvent contained in the coating film is moderate Since the coating film is not fully implemented at, the coating film becomes sticky, which causes problems in compatibility with the top coat, and may cause intermixing. In addition, it causes severe popping on the surface of the intermediate coating film, and may cause popping even after the clear coat is finally coated.

As the pigment component in the present invention, conventional inorganic pigments, organic pigments, and the like may be applied, and may affect hiding and glossiness, mechanical properties, and the like of the coating film. These pigments vary depending on the composition of the inorganic and organic pigments, the amount of the application may also be used in about 1 to 30% by weight depending on the pigment. Less than an appropriate amount may cause problems with hiding power, gloss, mechanical properties, etc. with respect to the overall paint composition, and more than an appropriate amount may cause deterioration of dispersibility, storage stability, and basic physical properties of the paint.

In the present invention, a silicone leveling agent having a structure of polyether-modified polydimethyl siloxane is applied to provide leveling and wettability of the coating film. The silicone leveling agent applied in the present invention has a solid content of 50 to 90% by weight, and may be used in an amount of 0.05 to 2% by weight based on the total coating composition. If the total paint composition is less than the above range, the effect is reduced, the effect of a suitable leveling agent can not be exerted, and when applied in excess of the above range may cause a crater shape problem on the surface. Tego Wet 510 or the like may be used as the silicon leveling agent.

In the present invention, in order to prevent the ultraviolet rays that can reach the electrodeposition surface, a UV absorber and fine titanium dioxide are applied to the intermediate portion. In the case of the 2C1B system, some of the ultraviolet light transmitted through the top clear coat and the top coat is absorbed in the middle part and is prevented from reaching the electrodeposition surface. In particular, the intermediate degree applied to the 2C1B system is completely cured for 150 ° C. for 25 minutes, and the film thickness also requires a level of 40 μm, thereby preventing UV rays. However, in the case of 3C1B system, it can not be completely cured because the median degree gives only 80 ℃ X3 minutes of drying conditions. The choice of UV absorbers in 3C1B systems can be important because it can. The UV absorber applied to the present invention is a method of obtaining a UV stabilizing effect by interfering with the starting step of forming free radicals in the photooxidation mechanism. It is a form using the principle. In its form, when applied as a benzotriazole system, it can give an effect of properly blocking ultraviolet rays. Examples of these benzotriazole-based ultraviolet absorbers include Tinuvin 384, manufactured by Ciba. In addition, the fine titanium dioxide applied to the present invention has an effect of effectively dispersing ultraviolet rays in an average size of 20 nm, and when used in combination with an ultraviolet absorber, it can provide an effective ultraviolet blocking effect. The ultraviolet absorber used in the present invention is composed of 90% by weight solids, and may be applied in an amount of 0.2 to 2% by weight based on the total coating composition. When less than the above range for the overall paint composition, the ability to absorb ultraviolet rays may be degraded, and when applied in excess of the above range, the top coat and the top coat may be floated to provide an overall cloudy appearance. In addition, the fine titanium dioxide applied in the present invention is composed of 100% by weight solids, 0.1 to 3% by weight based on the total coating composition can be used. If less than the above range for the overall paint composition, the ability to absorb ultraviolet rays may also be degraded, and when applied in excess of the above range may cause a problem in concealment.

Water-soluble paints are relatively slow to dry and thus have poor flowability in the vertical portion and can easily cause sagging. In order to prevent this, thickeners are used, and as the thickeners, it is preferable to simultaneously apply an alkali-soluble thickener and a synthetic bentonic thickener. Alkali-soluble thickeners are acrylic copolymer emulsion types that use Solthix 100 from Avecia. Examples of the synthetic bentonic thickener include BYK 420 of BYK Corporation. Their amount is 1 to 10% by weight and 0.1 to 3% by weight, respectively. Outside this range, the flowability is bad and sagging may occur.

In the present invention, in addition to the above components can be used by combining the components conventionally used in the intermediate coating composition. That is, components commonly used to prepare paint compositions such as surfactants, dispersants, antifoams, antisettling agents, pH adjusting agents, wetting agents, etc. may be appropriately selected and used.

The paint compositions according to the invention are suitable as intermediate paints in water-soluble multilayer coating systems. That is, the coating composition can be applied to the water-soluble multilayer coating system by painting the base coat and the top coat clear coat in one integral coating line using the coating composition as a medium. After the intermediate coating, it is dried at 80 ° C. X 2 to 3 minutes without being processed by the existing intermediate curing process, and then the top coat and the top coat are coated.

When the present invention is described in more detail as an embodiment as follows, this embodiment does not limit the present invention.

The components used in the Examples and Comparative Examples illustrated below are each carried out using the following components, except as specifically illustrated.

Polyurethane dispersion resin 1: number average molecular weight 12,000, acid value 4 ~ 7 mgKOH / g, particle size 100 ~ 200nm (FLF 2222, Bayer)

Polyurethane dispersion resin 2: Number average molecular weight 20,000, acid value 4-8 mgKOH / g, particle size 150-200 nm

Polyester Dispersion Resin: (Bayhydrol D270, Bayer)

Melamine Curing Agent: Common Melamine Resin Containing Methyl Group (Cymel 325, Cytec)

Solvent 1: Ethylene Glycol Monobutyl Ether (Butyl Cellosolve, Shell)

Solvent 2: Ethylene glycol monohexyl ether (hexyl cellosolve, Shell)

Solvent 3: Solvent mixture using butyl glycol, paraffin oil, etc. as a mixture of low molecular weight polymers having surface activity (Byketol WS, BYK)

Pigments: Organic-inorganic pigments commonly applied (such as titanium dioxide and carbon black)

Silicone leveling agent: Leveling agent having a structure of polyether modified polydimethyl siloxane (Wet 510, Tego)

UV absorbers: benzotriazole-based structures (Tinuvin 384, Ciba)

Fine titanium dioxide: titanium dioxide with an average particle size of 20 nm or less (UV-TITAN L181, Kemira)

Thickener: Alkali-soluble thickener applied (Soltix A-100, Avecia) Synthetic Benton thickener applied (BYK-420, BYK)

Examples and Comparative Examples

Water-soluble coating composition for 3C1B system

Example (Unit g) Comparative example (unit g) One 2 3 One 2 3 4 Polyurethane dispersion resin 1 31.0 31.0 31.0 31.0 31.0 31.0 Polyurethane Dispersion Resin 2 31.0 Polyester Dispersion Resin 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Melamine curing agent 2.3 2.3 2.3 2.3 2.3 2.3 2.3 Silicone leveling agent 0.2 0.2 0.2 0.2 0.2 0.2 0.2 pH regulator 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Solvent 3 3.0 4.0 3.0 2.0 - 3.0 3.0 Solvent 1 2.0 3.0 4.0 - - 2.0 2.0 Solvent 2 4.0 4.0 2.0 2.0 - 4.0 4.0 Normal Butyl Alcohol 5.0 Isopropyl Alcohol 3.0 Synthetic benton thickener 0.3 0.3 0.3 0.3 0.3 0.3 Alkali dissolving thickener 10.0 8.0 10.0 10.0 10.0 10.0 15.0 Surface modified surfactant 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Polymeric Dispersants 1.4 1.4 1.4 1.4 1.4 1.4 1.4 Antifoam 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Titanium dioxide 8.1 8.1 8.1 8.1 8.1 8.1 8.1 Fine titanium dioxide 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Extender pigment (calcium carbonate) 8.1 8.1 8.1 8.1 8.1 8.1 8.1 Sedimentation inhibitors 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Black Pigment (Carbon Black) 0.06 0.06 0.06 0.06 0.06 0.06 0.06 UV absorbers 1.0 1.0 1.0 1.0 1.0 1.0 1.0 water 35.5 32.5 35.5 35.5 35.5 35.5 35.5

The intermediate coating composition was coated on the specimen coated with electrodeposition paint, dried at 80 ° C. for 3 minutes, and subsequently coated with a top coat and a clear coat to form a final coating. The intermediate coating and the final coating were tested under the following test methods and conditions, and the results are shown in the following table.

Test Items Test method and condition Example Comparative example One 2 3 One 2 3 4 Viscosity Early 50 54 51 48 50 54 49 7 days storage at room temperature 53 56 54 53 56 58 52 7 days heat storage (43 ℃) 51 52 50 56 53 55 51 Dispersion stability Hegmann Gauge particle size check ○ ○ ○ ○ △ ○ ○ Room temperature storage 7 days storage at room temperature ○ ○ ○ ○ △ ○ ○ Heat storage 7 days heat storage (43 ℃) ○ ○ ○ ○ △ ○ ○ Midway exterior Visual check ◎ ◎ ◎ △ △ △ △ Final appearance (Clio coat painting) Wave Scan DOI Evaluation (Silver Metallic color CF value) 53.4 54.2 52.9 46.7 46.7 48.3 50.3 Middle dry castle Confirmation ◎ ◎ ◎ △ △ △ ◎ Adhesion 1mm cross cut ○ ○ ○ ○ ○ ○ ○ Impact resistance 500 gX height (10, 20, 30, 40, 50 cm) ○ ○ ○ ○ ○ ○ ○ Water resistance 40 ℃ X7 days deposition ○ ○ ○ ○ △ ○ ○ Water Resistance 1mm cross cut after 40 ℃ X7 day deposition ○ ○ ○ ○ △ ○ ○ Chipping resistance 50g chipping stone price after -40X3 hours ○ ○ ○ ○ △ ○ ○ Sprayer workability Robot sprayer ○ ○ ○ ○ ○ ○ ○ Medium flow Visual observation of the vertical side ○ ○ ○ △ △ △ △ Moderate smoothness Visual observation of the vertical side ○ ○ ○ ○ △ △ △                                                 Evaluation: ◎ Excellent, ○ Good, △ Normal, X Bad

Viscosity: Determination of viscosity with time using Ford cup # .4 (in water-based paints, it is mainly used in the range of 40 to 60 sec, in which the vertical flow is good and there is no sagging)

Dispersion Stability: Dispersion particle size measurement using Hegmann gaude (Dispersion stability of water-soluble paint is judged to be max. 5㎛)

Final appearance: Final CF value measurement using Wave Scan DOI (BYK Gardner), an automotive appearance measuring instrument (currently, the CF level of the solvent-based 2C1B Silver Metallic Coating System is 50 levels, which is more advantageous)

Adhesion: Determination using 100 pieces of 1mm crosscut after removing using Scotch tape.

Impact resistance: judged good when impact resistance of 40cm or more was realized by using 500g circular weight

Water-resistant adhesion: After the water resistance deposition, it is evaluated after adhesion for 1 day at room temperature (determined to be equal to adhesion).

Cold resistance chipping resistance: 50g chipping stone is pushed out at 4 bar pressure using 50g chipping stone after leaving it at -40 ℃ for 3 hours, and the surface of coating film is used. Good judgment when there are 10 or less damages of 1 to 2 mm or less size, normal judgment when 10 or less damages of 2 to 3 mm or less size)

Medium flowability: The degree of painting surface is evaluated after painting on the vertical side.

Moderate dryness: 80 ° C X 3 minutes after drying, the degree of stickiness is evaluated when measuring the contact.

Since the most important part of the water soluble multilayer coating system is the middle part, it is very important to accelerate the drying of the middle part. Although the compatibility of the solvent is very important in order to accelerate the drying even at the coating film thickness of 20 μm, it was confirmed that the volatilization of water can be promoted more quickly in the mixing of the solvent system applied in Examples 1 to 3. The applied solvent system is believed to have a higher vapor pressure characteristic than water to form an azeotropic mixture and to volatilize faster than the inherent properties of water.

In Comparative Examples 1 and 2, the azeotropic effect by the influence of the solvent 1 was halved, and the intermediate dryness was decreased, so that the overall appearance was decreased. In addition, as the moderate drying slowed down, the fluidity was halved, causing sagging in the neck of the car.

In the case of Comparative Example 3, the volatilization of the solvent in the coating film was relatively fast, so that water did not volatilize properly and remained inside the coating film. This result did not result in a stable formation of the coating film, and the coating film was sticky, resulting in easy penetration of the top coat.

As described above, the present invention is an intermediate coating composition included in a water-soluble multilayer coating system (water-soluble 3C1B coating system) for automobiles, and includes polyurethane dispersion resins, polyester dispersion resins, melamine curing agents, specific azeotropic solvent mixtures, and other additives. It is a coating composition that can implement good new car exterior and physical properties even at 80 ℃ X 3 minutes dry conditions by applying. Specifically, the effects are as follows.

First, it is a system that can exclude the intermediate coating process of 150 ℃ X 25 minutes curing conditions in a new car coating system for automobiles. Medium and top coat and top clear coat coatings can be applied in one integral coating line, and the exclusion of the mid coat process can provide significant economic benefits.                     

Secondly, as a new vehicle midway system for automobiles, it is a paint system that can provide sufficient car exterior and physical properties even at 80 ° C. X 3 minutes drying conditions.

Thirdly, it is a coating system capable of providing sufficient final film hardness for automobiles without deteriorating the appearance even under the condition of drying at 80 ° C. X 3 minutes.

Fourth, it is a paint system that maximizes the volatilization of water by mixing and applying a specific solvent to remove the bad effects such as popping and pinhole from the final appearance.

Claims (4)

5 to 50% by weight of polyurethane dispersion resin, 2 to 20% by weight of methylated melamine curing agent, 2 to 5% by weight of polyester dispersion resin, 2 to 10% by weight of mixed solvent of butyl glycol and paraffin oil, based on the total weight of the coating composition , 1 to 5% by weight of ethylene glycol monobutyl ether, 1 to 5% by weight of ethylene glycol monohexyl ether, 1 to 30% by weight of pigment, 0.05 to 2% by weight of silicone leveling agent, 0.2 to 2% by weight of ultraviolet absorber, fine titanium dioxide 0.1 to 3% by weight, 25 to 40% by weight of water, and 1 to 10% by weight of an alkali-soluble thickener and 0.1 to 3% by weight of a synthetic bentonic thickener. According to claim 1, wherein the paint composition isocyanate curing agent 2 to 20% by weight based on the weight of the total coating composition of the water-soluble coating composition for automobiles characterized in that it further comprises. The coating method of claim 1, wherein the coating composition of claim 1 is applied in one integral coating line, and then dried at 80 ° C. for 2 to 3 minutes, and then baked at once after coating the top coat and the top coat. delete