WO2024054023A1

WO2024054023A1 - Water-soluble base coat composition

Info

Publication number: WO2024054023A1
Application number: PCT/KR2023/013307
Authority: WO
Inventors: 정다은; 조수영; 김맹기
Original assignee: 주식회사 케이씨씨
Priority date: 2022-09-08
Filing date: 2023-09-06
Publication date: 2024-03-14
Also published as: KR20240035257A

Abstract

The present invention relates to a water-soluble base coat composition comprising a first acrylic resin, a second acrylic resin, a polyurethane dispersion resin, a polyester resin, a urethane resin, and a melamine-based curing agent, wherein the first acrylic resin has a glass transition temperature of 15 to 25°C and a weight average molecular weight 11,000 to 15,000 g/mol and the second acrylic resin has a glass transition temperature 1 to 10°C and a weight average molecular weight 5,000 to 10,000 g/mol.

Description

Water-soluble base coat composition

The present invention relates to water-soluble base coat compositions.

In general, the exterior panel of a car body must be free from deterioration and rust, and must be durable enough to maintain the gloss or color of the paint film. Accordingly, the painting process of a vehicle is typically electrodeposition painting on a car body that has gone through a pretreatment process, followed by intermediate painting to improve adhesion and smoothness, and base painting on the intermediately painted body to enhance the aesthetics of the vehicle body. Afterwards, it is common to apply a clear coat to protect the color of the base coat, improve its appearance, and protect the base coat from the outside. In addition, among the important catchphrases recently applied to each industry, eco-friendliness is being embodied in the form of actively limiting the use of volatile organic compounds.

The volatile organic compounds have the characteristic of easily volatilizing at room temperature, so when they exist in a gaseous state, they are naturally emitted in the atmospheric environment and become an important cause of air pollution. These volatile organic compounds are known to have unique biological toxicity and chemical reactivity, which can cause various types of environmental problems. They cause serious environmental damage, such as the creation of ozone layer in the troposphere, destruction of the stratosphere, greenhouse effect, and odor generation. there is.

Accordingly, in the automotive painting field, there is a transition from solvent-based paint compositions to water-based paint compositions to increase eco-friendliness, and recently, there is a transition from solvent-based metallic coloring base paint compositions to water-based metallic coloring paint compositions.

In this regard, Korean Patent No. 1,221,276 (Patent Document 1) discloses an aqueous base containing polyurethane dispersion resin, acrylic emulsion resin, pigment, leveling agent, silicone-based antifoaming agent, ultraviolet absorber, ultraviolet stabilizer, dispersant, ionized water, and thickener. A coat coating composition is disclosed. However, as shown in Patent Document 1, the conventional water-based coloring base paint composition has a slow drying speed, and the brightness is lowered due to a decrease in volumetric shrinkage due to drying, or the solvent content in the composition is reduced, resulting in lower brightness, and the pigment or solid content in the composition is reduced. There was a problem with sedimentation occurring.

Therefore, the fast drying speed prevents a decrease in brightness due to a decrease in volumetric shrinkage due to drying, improves the decrease in brightness caused by a decrease in the organic solvent content in the composition, and reduces VOC and makes it eco-friendly by lowering the content of the organic solvent. , there is a need for research and development on water-based paint compositions that do not cause problems with precipitation of pigments or solids in the composition.

[Prior art literature]

[Patent Document]

(Patent Document 1) Korean Patent No. 10-1221276 (January 11, 2013)

The present invention seeks to provide a water-soluble base coat composition that prevents deformation of the produced coating film due to reduced volumetric shrinkage due to drying, enables high saturation and high brightness, and produces a coating film with excellent mechanical properties.

In one aspect, the present invention can provide a water-soluble base coat composition including a first acrylic resin, a second acrylic resin, a polyurethane dispersion resin, a polyester resin, a urethane resin, and a melamine-based curing agent.

The water-soluble base coat composition according to the present invention contains resins with different drying speeds, so that deformation of the manufactured coating film due to reduced volumetric shrinkage due to drying is prevented, and high saturation and high brightness can be achieved. In addition, the coating film prepared from the above composition has excellent mechanical properties, especially gloss, impact resistance, water resistance, and cold chipping resistance, and can be usefully used for car body painting.

Hereinafter, the present invention will be described in detail.

A water-soluble base coat composition according to one aspect of the present invention includes a first acrylic resin, a second acrylic resin, a polyurethane dispersion resin, a polyester resin, a urethane resin, and a melamine-based curing agent, wherein the first acrylic resin is glass. The water-soluble base has a transition temperature of 15 to 25 ℃ and a weight average molecular weight of 11,000 to 15,000 g/mol, and the second acrylic resin has a glass transition temperature of 1 to 10 ℃ and a weight average molecular weight of 5,000 to 10,000 g/mol. A coat composition is provided.

As described above, when two types of acrylic resins with different glass transition temperatures and weight average molecular weights are used, the drying speed and crosslinking density of the coating film are adjusted to improve the appearance characteristics and mechanical properties of the coating film, and the organic solvent content in the composition is adjusted. It is environmentally friendly and has excellent workability.

For example, the different glass transition temperatures of the two types of acrylic resin enable high saturation and high brightness to be achieved by controlling the drying speed and surface tension of the coating film of the water-soluble base coat composition of the present invention, resulting in excellent appearance characteristics of the coating film, and also 2 Different weight average molecular weights of different types of acrylic resin have the effect of improving adhesion and mechanical properties by controlling the crosslinking density of the coating film.

1st acrylic resin

The first acrylic resin serves to improve the rheology and hardness of a coating film prepared from a water-soluble base coat composition containing it.

The first acrylic resin can be directly synthesized according to a known method, or a commercially available product can be used. For example, the first acrylic resin is selected from the group consisting of allyl methacrylate (AMA), methyl methacrylate (MMA), ethyl acrylate (EA), hydro ethyl acrylate (HEA) and methacrylic acid (MAA). It may contain units derived from one or more selected monomers. For example, the first acrylic resin includes allyl methacrylate (AMA), methyl methacrylate (MMA), ethyl acrylate (EA), hydro ethyl acrylate (HEA) and methacrylic acid (MAA) as monomers. It may be manufactured from a mixture of:

Additionally, the mixture may further include a divalent acrylic monomer. Divalent acrylic monomers include, for example, 1,4-Butanediol diacrylate, 1,4-Butanediol dimethacrylate, 1,5-pentadiol Dimethacrylate (1,5-Pentanediol dimethacrylate), 1,6-Hexanediol diacrylate, 1,6-Hexanediol dimethacrylate, 1,9-Nonanediol dimethacrylate, 1,10-Decanediol dimethacrylate, Ethylene glycol diacrylate, ethylene Glycol dimethacrylate (Ethylene glycol dimethacrylate), etc. may be mentioned.

The first acrylic resin may have a solid content (NV) of 32 to 38% by weight, or 34 to 36% by weight, based on the total weight of the resin. When the solid content of the first acrylic resin is within the above range, the storage stability of the resin may be improved and workability may be excellent. If the solid content of the first acrylic resin is less than the above range, the viscosity is too low, which causes a problem of insufficient workability of the water-soluble base coat composition containing it, and if it is more than the above range, the viscosity of the first acrylic resin is too high, causing stability during reaction. decreases, dispersion stability deteriorates, and agglomeration may occur over time.

The first acrylic resin may have a viscosity of 8 to 320 cps, or 10 to 300 cps at 25°C. When the viscosity of the first acrylic resin at 25°C is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the viscosity of the first acrylic resin at 25°C is less than the above range, the viscosity of the composition is too low and there is a problem of deterioration of paint flow, adhesion, and scratch resistance due to non-formation of the manufactured coating film, and if it exceeds the above range, A problem may occur in which the appearance characteristics of the manufactured coating film are deteriorated due to insufficient workability of the composition.

The first acrylic resin may have an acid value (Av) of 1 to 10 mgKOH/g, or 3 to 10 mgKOH/g. When the acid value of the first acrylic resin is within the above range, the reactivity of the composition containing it can be adjusted to improve the appearance characteristics of the coating film produced therefrom. If the acid value of the first acrylic resin is less than the above range, the stability of the resin decreases, causing a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and the coating film. Problems with reduced water resistance may occur.

The first acrylic resin may have a hydroxyl value (OHv) of 11 to 20 mgKOH/g, or 11 to 17 mgKOH/g. When the hydroxyl value of the first acrylic resin is within the above range, the reactivity of the composition containing it can be adjusted to improve the appearance characteristics of the coating film produced therefrom. If the hydroxyl value of the first acrylic resin is less than the above range, the stability of the resin decreases, which causes a problem of lowering the storability of the manufactured coating composition. If it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and coating film. A problem may arise where the water resistance of the product is reduced.

The first acrylic resin may have a pH of 6 to 10, or 7 to 9. When the pH of the first acrylic resin is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the pH of the first acrylic resin is less than the above range, the viscosity of the composition is too low and the paint flow, adhesion, and scratch resistance are reduced due to non-formation of the produced coating film, and if the pH is above the above range, the workability of the composition is poor. A lack of this may cause a problem in which the appearance characteristics of the manufactured coating film are deteriorated.

The first acrylic resin may have a glass transition temperature (Tg) of 15 to 25°C, or 16 to 20°C. When the glass transition temperature of the first acrylic resin is within the above range, the hardness and appearance characteristics of a coating film manufactured from a composition containing the first acrylic resin are improved. If the glass transition temperature of the first acrylic resin is less than the above range, the drying speed and crosslinking density of the coating film decrease, causing a problem in that the hardness and appearance characteristics of the manufactured coating film are insufficient, and if it exceeds the above range, the coating film becomes brittle. The appearance characteristics and chipping resistance of the manufactured coating film may be insufficient.

The first acrylic resin may have a weight average molecular weight (Mw) of 11,000 to 15,000 g/mol, or 12,500 to 14,500 g/mol. When the weight average molecular weight of the first acrylic resin is within the above range, the manufactured coating film may have excellent long-term physical properties such as durability, adhesion, hardness, and weather resistance. If the weight average molecular weight of the first acrylic resin is less than the above range, the molecular weight is small, and the water resistance, weather resistance, and scratch resistance of the produced coating film are insufficient, and if it exceeds the above range, the flowability decreases due to the increase in molecular weight, so the water-soluble base containing it The workability of the coat composition becomes poor and the surface smoothness is poor, making it difficult to manufacture a coating film with an excellent appearance.

The first acrylic resin may be in the form of an emulsion dispersed in a solvent such as deionized water. For example, the first acrylic resin is in the form of an emulsion dispersed in a solvent and may have an average diameter of 70 to 170 nm, or 80 to 160 nm. When the first acrylic resin is in the form of an emulsion dispersed in deionized water, it has the effect of lowering the volatile organic compounds (VOC) content of the paint, and when the average diameter is within the above range, the appearance and particle stability are good. do.

The first acrylic resin may be included in the composition in an amount of 3 to 17% by weight, 5 to 15% by weight, or 7 to 13% by weight based on the total weight of the water-soluble base coat composition. When the first acrylic resin is included within the above content range, there is an effect of improving the adhesion, durability, hardness, and scratch resistance of the coating film. If the content of the first acrylic resin is less than the above range, the drying property is lowered, which causes a problem in that the adhesion, gloss, hardness, and durability of the coating film are lowered. If the content of the first acrylic resin is more than the above range, drying progresses quickly and the painting workability and paint flow of the composition are reduced. Due to lack of durability, problems may arise such as deterioration in the appearance of the coating film and its scratch resistance.

2nd acrylic resin

The second acrylic resin serves to improve the rheology and hardness of a coating film prepared from a water-soluble base coat composition containing it.

The second acrylic resin may be the same as that described for the first acrylic resin, except as described below.

The second acrylic resin may have a solid content (NV) of 42 to 50% by weight, or 45 to 47% by weight, based on the total weight of the resin. When the solid content of the second acrylic resin is within the above range, the storage stability of the resin may be improved and workability may be excellent. If the solid content of the second acrylic resin is less than the above range, the viscosity is too low, which causes a problem of insufficient workability of the water-soluble base coat composition containing it, and if it is more than the above range, the viscosity of the second acrylic resin is too high, causing stability during reaction. decreases, dispersion stability deteriorates, and agglomeration may occur over time.

The second acrylic resin may have a viscosity of 180 to 550 cps, or 200 to 500 cps at 25°C. When the viscosity of the second acrylic resin at 25°C is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the viscosity of the second acrylic resin at 25°C is less than the above range, the viscosity of the composition is too low and there is a problem of deterioration of paint flow, adhesion, and scratch resistance due to non-formation of the manufactured coating film, and if it exceeds the above range, A problem may occur in which the appearance characteristics of the manufactured coating film are deteriorated due to insufficient workability of the composition.

The second acrylic resin may have an acid value (Av) of 11 to 20 mgKOH/g, or 12 to 18 mgKOH/g. When the acid value of the second acrylic resin is within the above range, the reactivity of the composition containing it can be adjusted to improve the appearance characteristics of the coating film manufactured therefrom. If the acid value of the second acrylic resin is less than the above range, the stability of the resin decreases, causing a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and the coating film. Problems with reduced water resistance may occur.

The second acrylic resin may have a hydroxyl value (OHv) of 1 to 10 mgKOH/g, or 2 to 9 mgKOH/g. When the hydroxyl value of the second acrylic resin is within the above range, the reactivity of the composition containing it can be adjusted to improve the appearance characteristics of the coating film produced therefrom. If the hydroxyl value of the second acrylic resin is less than the above range, the stability of the resin decreases, which causes a problem of lowering the storability of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and coating film. A problem may arise where the water resistance of the product is reduced.

The second acrylic resin may have a pH of 7 to 9, or 7.5 to 8.5. When the pH of the second acrylic resin is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the pH of the second acrylic resin is less than the above range, the viscosity of the composition is too low and the paint flow, adhesion, and scratch resistance are reduced due to non-formation of the produced coating film, and if the pH is above the above range, the workability of the composition is poor. A lack of this may cause a problem in which the appearance characteristics of the manufactured coating film are deteriorated.

The second acrylic resin may have a glass transition temperature (Tg) of 1 to 10°C, or 2 to 9°C. When the glass transition temperature of the second acrylic resin is within the above range, the hardness and appearance characteristics of a coating film manufactured from a composition containing it are improved. If the glass transition temperature of the second acrylic resin is less than the above range, the drying speed and crosslinking density of the coating film decrease, causing a problem in that the hardness and appearance characteristics of the manufactured coating film are insufficient, and if it exceeds the above range, the coating film becomes brittle. The appearance characteristics and chipping resistance of the manufactured coating film may be insufficient.

The second acrylic resin may have a weight average molecular weight (Mw) of 5,000 to 10,000 g/mol, or 6,000 to 9,000 g/mol. When the weight average molecular weight of the second acrylic resin is within the above range, the manufactured coating film may have excellent long-term physical properties such as durability, adhesion, hardness, and weather resistance. If the weight average molecular weight of the second acrylic resin is less than the above range, the molecular weight is small, and the water resistance, weather resistance, and scratch resistance of the produced coating film are insufficient, and if it exceeds the above range, the flowability is reduced due to the increase in molecular weight, so the water-soluble base containing it The workability of the coat composition becomes poor and the surface smoothness is poor, making it difficult to manufacture a coating film with an excellent appearance.

The second acrylic resin may be in the form of an emulsion dispersed in a solvent such as deionized water. When the second acrylic resin is in the form of an emulsion dispersed in deionized water, it has the effect of lowering the volatile organic compounds (VOC) content of the paint.

The second acrylic resin may be included in the composition in an amount of 3 to 17% by weight, 5 to 15% by weight, or 7 to 13% by weight based on the total weight of the water-soluble base coat composition. When the second acrylic resin is included within the above content range, there is an effect of improving the adhesion, durability, hardness, and scratch resistance of the coating film. If the content of the second acrylic resin is less than the above range, the drying property is lowered, which causes a problem in that the adhesion, gloss, hardness and durability of the coating film are reduced. If the content is more than the above range, the drying progresses quickly and the paint workability and paint flow of the composition are affected. Due to lack of durability, problems may arise such as deterioration in the appearance of the coating film and its scratch resistance.

Polyurethane dispersion resin

Polyurethane dispersion resin improves the drying properties of a water-soluble base coat composition containing it and improves the flexibility of a coating film prepared from the composition.

The polyurethane dispersion resin may be a polyurethane dispersion resin derived from polyester-polycarbonate. For example, the polyurethane dispersion resin may be a polyurethane dispersion resin derived from polyester-polycarbonate diol.

The polyurethane dispersion resin may have a solid content (NV) of 30 to 40% by weight, or 34 to 36% by weight, based on the total weight of the resin. When the solid content of the polyurethane dispersion resin is within the above range, the storage stability of the resin and the water-soluble base coat composition may be improved and workability may be excellent. If the solid content of the polyurethane dispersion resin is less than the above range, the viscosity is too low, which causes a problem of insufficient workability of the water-soluble base coat composition containing it, and if it is more than the above range, the viscosity of the polyurethane dispersion resin is too high, causing a reaction. It has poor stability and dispersion stability may deteriorate, causing aggregation over time.

Polyurethane dispersion resin may be a polyurethane resin dispersed in a solvent. The aqueous medium in which the polyurethane resin is dispersed may be a water solvent or a mixed medium of water and a hydrophilic organic solvent. For example, the water solvent may be constant water, ion-exchanged water, distilled water, ultrapure water, etc. In particular, ion-exchanged water can be used for ease of availability or to prevent particles from becoming unstable due to the influence of salt. In addition, the hydrophilic organic solvent may be a lower monohydric alcohol such as methanol, ethanol, or propanol, a polyhydric alcohol such as ethylene glycol or glycerin, or an aprotic hydrophilic organic solvent such as N-methylmorpholine, dimethyl sulfoxide, or dimethylformamide. . The polyurethane dispersion resin is in the form of a dispersion in deionized water and may have an average diameter of 10 to 120 nm, or 20 to 100 nm.

The polyurethane dispersion resin may have a viscosity of 5 to 550 cps, or 10 to 500 cps at 25°C. When the viscosity of the polyurethane dispersion resin at 25°C is within the above range, the viscosity of the water-soluble base coat composition is appropriate, which has the effect of improving workability. If the viscosity of the polyurethane dispersion resin at 25°C is less than the above range, the viscosity of the composition is too low and there is a problem that the paint flowability, adhesion, and scratch resistance are reduced due to non-formation of the manufactured coating film, and if the viscosity is exceeding the above range, the coating film is not formed. In this case, a problem may occur in which the appearance characteristics of the manufactured coating film are deteriorated due to insufficient workability of the composition.

The polyurethane dispersion resin may have an acid value (Av) of 15 to 30 mgKOH/g, or 18 to 25 mgKOH/g. When the acid value of the polyurethane dispersion resin is within the above range, the reactivity of the composition containing it can be adjusted to improve the appearance characteristics of the coating film produced therefrom. If the acid value of the polyurethane dispersion resin is less than the above range, the stability of the resin decreases, causing a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and coating film. A problem may arise where the water resistance of the product is reduced.

The polyurethane dispersion resin may have a pH of 6 to 10, or 7 to 9. When the pH of the polyurethane dispersion resin is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the pH of the polyurethane dispersion resin is below the above range, the viscosity of the composition is too low and there is a problem of poor paint flow, adhesion, and scratch resistance due to non-formation of the manufactured coating film, and if it exceeds the above range, the workability of the composition is poor. A problem may occur in which the appearance characteristics of the manufactured coating film are deteriorated due to insufficient properties.

The polyurethane dispersion resin may have a glass transition temperature (Tg) of -45 to -20 °C, or -40 to -25 °C. When the glass transition temperature of the polyurethane dispersion resin is within the above range, the flexibility and hardness of the coating film are improved. If the glass transition temperature of the polyurethane dispersion resin is below the above range, the drying speed of the water-soluble base coat composition is delayed, causing a problem in that the solvent resistance and chipping resistance of the produced coating film are insufficient, and if it exceeds the above range, the coating film becomes brittle ( It may become brittle, resulting in insufficient appearance characteristics and hardness of the coating film.

The polyurethane dispersion resin may have a weight average molecular weight of 35,000 to 60,000 g/mol, or 40,000 to 50,000 g/mol. When the weight average molecular weight of the polyurethane dispersion resin is within the above range, the water-soluble base coat composition has excellent drying properties, cold chipping resistance, and impact resistance. If the weight average molecular weight of the polyurethane dispersion resin is less than the above range, there is a problem that the mechanical properties of the produced coating film are deteriorated due to the small molecular weight, and if it exceeds the above range, the flowability is reduced due to the increase in molecular weight, causing the coating film to brittle. This may cause problems such as reduced smoothness and reduced scratch resistance.

The polyurethane dispersion resin may be included in the composition in an amount of 0.05 to 7% by weight, 0.1 to 5% by weight, or 0.2 to 3% by weight based on the total weight of the water-soluble base coat composition. When the polyurethane dispersion resin is contained within the above content range, there is an effect of improving tensile strength, flexibility, elongation, adhesion, wear resistance, scratch resistance, and coating film smoothness. If the content of polyurethane dispersion resin in the composition is less than the above range, there is a problem that flexibility decreases and tensile strength and scratch resistance decrease, and if it exceeds the above range, the viscosity of the composition increases excessively, resulting in reduced workability and drying properties. This may cause problems with deterioration of appearance and mechanical properties.

polyester resin

Polyester resin serves to improve the flexibility and appearance properties of a coating film prepared from a water-soluble base coat composition containing it.

Polyester resin can be produced by reacting a carboxylic acid compound and a diol compound. For example, polyester resins include adipic acid (AA), isophthalic acid (IPA), trimaltic anhydride (TMA), cycloaliphatic acid, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, and maleic acid. At least one carboxylic acid compound selected from the group consisting of anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and derivatives thereof; and 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), trimethylol propane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol 1,4-hexanediol and 3-methyl. It can be prepared by reacting one or more diol compounds selected from the group consisting of pentanediol.

The polyester resin may have a solid content (NV) of 70 to 90% by weight, or 75 to 85% by weight, based on the total weight of the resin. When the solid content of the polyester resin is within the above range, the storage stability of the resin may be improved and workability may be excellent. If the solid content of the polyester resin is less than the above range, the viscosity is too low, which causes a problem of insufficient workability of the water-soluble base coat composition containing it, and if it is more than the above range, the viscosity of the polyester resin is too high, which reduces stability during the reaction. You can.

The polyester resin may have a viscosity of 2,000 to 5,000 cps, or 2,200 to 4,700 cps at 25°C. When the viscosity of the polyester resin at 25°C is within the above range, the viscosity of the water-soluble base coat composition is appropriate, which has the effect of improving workability. If the viscosity of the polyester resin at 25°C is less than the above range, the viscosity of the composition is too low and there is a problem of deterioration of paint flow, adhesion, and scratch resistance due to non-formation of the manufactured coating film, and if it exceeds the above range, the composition Due to lack of workability, a problem may occur in which the appearance characteristics of the manufactured coating film are deteriorated.

The polyester resin may have an acid value (Av) of 1 to 10 mgKOH/g, or 3 to 8 mgKOH/g. When the acid value of the polyester resin is within the above range, the storage stability and crosslinking density of the composition containing it are improved, resulting in good cold chipping resistance. If the acid value of the polyester resin is less than the above range, the stability of the resin decreases, which causes a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and water resistance of the coating film. This deterioration problem may occur.

The polyester resin may have a hydroxyl value (OHv) of 80 to 110 mgKOH/g, or 85 to 105 mgKOH/g. When the hydroxyl value of the polyester resin is within the above range, the storage stability and crosslinking density of the composition containing it are improved, resulting in good cold chipping resistance. If the hydroxyl value of the polyester resin is less than the above range, the stability of the resin decreases, causing a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and the coating film. Problems with reduced water resistance may occur.

The polyester resin may have a glass transition temperature (Tg) of -50 to -25°C, or -40 to -30°C. When the glass transition temperature of the polyester resin is within the above range, the hardness and solvent resistance of a coating film manufactured from a composition containing it is improved. If the glass transition temperature of the polyester resin is less than the above range, the drying speed and crosslink density of the coating film decrease, causing a problem of insufficient hardness and solvent resistance of the manufactured coating film, and if it exceeds the above range, the coating film becomes brittle. The appearance characteristics and chipping resistance of the applied coating film may become insufficient.

The polyester resin may have a weight average molecular weight of 6,000 to 12,000 g/mol, or 7,000 to 11,000 g/mol. When the weight average molecular weight of the polyester resin is within the above range, the manufactured coating film may have excellent long-term physical properties such as durability, adhesion, hardness, and weather resistance. If the weight average molecular weight of the polyester resin is less than the above range, the molecular weight is small, and the water resistance, weather resistance, and scratch resistance of the produced coating film are insufficient, and if it exceeds the above range, the flowability decreases due to the increase in molecular weight, so the water-soluble base coat containing it The composition may have poor workability and poor surface smoothness, making it difficult to manufacture a coating film with an excellent appearance.

The polyester resin may be included in the composition in an amount of 1 to 5% by weight, or 2 to 4% by weight, based on the total weight of the water-soluble base coat composition. When the polyester resin is contained within the above content range, excellent appearance, impact resistance, and cold chipping resistance can be secured. If the polyester resin content is less than the above range, the drying property is lowered, which causes problems in the adhesion, gloss, hardness, and durability of the coating film. If the content exceeds the above range, drying progresses quickly, reducing the painting workability and paint flowability of the composition. A lack of this may cause problems with the appearance of the coating film and deterioration of scratch resistance.

urethane resin

The urethane resin serves to improve the workability of the water-soluble base coat composition containing it and to improve the appearance characteristics of the coating film produced from the composition.

The urethane resin may be a urethane diol-based compound. For example, urethane resins include primary amino alcohols such as amino ethanol or amino isopropanol; or primary diamines such as ethylene diamine, propylene diamine, 2-methyl-pentane diamine-(1,5) or hexane diamine-(1,6); urethane diols obtained by reaction of alkylene carbonates such as ethylene or propylene carbonate. It can be.

The urethane resin may have a solid content (NV) of 30 to 40% by weight, or 34 to 36% by weight, based on the total weight of the resin. When the solid content of the urethane resin is within the above range, the storage stability of the resin may be improved and workability may be excellent. If the solid content of the urethane resin is less than the above range, the viscosity is too low, which causes a problem of insufficient workability of the water-soluble base coat composition containing it. If it is more than the above range, the viscosity of the urethane resin is too high, which may reduce stability during the reaction. .

The urethane resin may have a viscosity of 5 to 600 cps, or 10 to 500 cps at 25°C. When the viscosity of the urethane resin at 25°C is within the above range, the viscosity of the water-soluble base coat composition is appropriate, which has the effect of improving workability. If the viscosity of the urethane resin at 25°C is less than the above range, the viscosity of the composition is too low and there is a problem of deterioration of paint flow, adhesion, and scratch resistance due to non-formation of the manufactured coating film, and if it exceeds the above range, the composition A problem may occur where the appearance characteristics of the manufactured coating film are deteriorated due to insufficient workability.

The urethane resin may have an acid value (Av) of 15 to 30 mgKOH/g, or 18 to 25 mgKOH/g. When the acid value of the urethane resin is within the above range, the gloss and appearance characteristics of the manufactured coating film are good. If the acid value of the urethane resin is less than the above range, the stability of the resin decreases, causing a problem of reduced storage of the manufactured coating composition, and if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which reduces workability and water resistance of the coating film. Deterioration problems may occur.

The urethane resin may have a pH of 6 to 10, or 7 to 9. When the pH of the urethane resin is within the above range, the viscosity of the water-soluble base coat composition is appropriate, thereby improving workability. If the pH of the urethane resin is less than the above range, the viscosity of the composition is too low and the paint flowability, adhesion, and scratch resistance are reduced due to the non-formation of the manufactured coating film, and if it exceeds the above range, the workability of the composition is poor. As a result, a problem may arise in which the appearance characteristics of the manufactured coating film are deteriorated.

The urethane resin may have a glass transition temperature (Tg) of -40 to -10 °C, or -35 to -15 °C. When the glass transition temperature of the urethane resin is within the above range, the hardness and solvent resistance of a coating film manufactured from a composition containing it is improved. If the glass transition temperature of the urethane resin is less than the above range, the drying speed and crosslink density of the coating film decreases, causing a problem of insufficient hardness and solvent resistance of the manufactured coating film, and if it exceeds the above range, the coating film brittles. The appearance characteristics and chipping resistance of the coating film may become insufficient.

The urethane resin may be dispersed in a solvent and may have an average diameter of 30 to 110 nm, or 40 to 100 nm. When the average diameter of the urethane resin is within the above range, the appearance and particle stability are good.

The urethane resin may have a weight average molecular weight (Mw) of 200 to 900 g/mol, or 400 to 700 g/mol. When the weight average molecular weight of the urethane resin is within the above range, the manufactured coating film may have excellent long-term physical properties such as durability, adhesion, hardness, and weather resistance. If the weight average molecular weight of the urethane resin is less than the above range, the molecular weight is small, and the water resistance, weather resistance, and scratch resistance of the produced coating film are insufficient, and if it exceeds the above range, the flowability decreases due to the increase in molecular weight, and the water-soluble base coat composition containing the same Workability may be poor and surface smoothness may be poor, making it difficult to manufacture a coating film with an excellent appearance.

The urethane resin may be included in the composition in an amount of 1 to 5% by weight, or 2 to 4% by weight, based on the total weight of the water-soluble base coat composition. When the content of urethane resin is within the above range, the flexibility and appearance of the coating film are improved. If the content of the urethane resin is less than the above range, the drying property is lowered, which causes a problem in that the adhesion, gloss, hardness, and durability of the coating film are reduced. If the content is more than the above range, the drying progresses quickly and the painting workability and paint flow of the composition are reduced. If there is a lack of it, problems may arise such as deterioration in the appearance of the coating film and its scratch resistance.

Melamine-based hardener

The melamine-based curing agent serves to cure the water-soluble base coat composition by cross-linking with each component and improves the appearance and gloss of the paint film produced from the water-soluble base coat composition.

The melamine-based curing agent may include a hydrophilic melamine resin containing an imino group. For example, the melamine-based curing agent may be in a liquid form containing a hydrophilic melamine resin containing an imino group. As another example, the melamine-based curing agent contains a hydrophilic melamine resin containing an imino group, and may be in a liquid form with a solid content (NV) of 90 to 98% by weight, or 93 to 96% by weight, based on the total weight of the melamine-based curing agent. there is.

Commercially available melamine-based hardeners include Cytec's Cymel-325, Cymel-327 and Cymel-385, INEOS' Resimene HM-2608, Resimene 718 and Resimene 717, and BASF's Luwipal 052 and 072.

The melamine-based curing agent may be included in the composition in an amount of 3 to 8% by weight, or 4 to 7% by weight, based on the total weight of the water-soluble base coat composition. When the content of the melamine-based hardener is within the above range, appropriate coating properties can be secured.

solvent

The water-soluble base coat composition according to the present invention may further include a solvent. The solvent controls the viscosity of the composition and plays a role in reducing the generation of volatile organic compounds (VOC). For example, the solvent may include one or more types of water selected from the group consisting of deionized water, pure water, ultrapure water, and distilled water. The solvent may further include an organic solvent in addition to water.

The solvent may be included in the composition in an amount of 20 to 50% by weight, or 30 to 40% by weight, based on the total weight of the water-soluble base coat composition. If the solvent content in the composition is within the above range, the water dispersibility of the resins in the composition is reduced, the environmental friendliness, which is an advantage of water-based paints, is reduced, and the water is not evaporated sufficiently, resulting in coating film defects such as pinholes in the final coating film. This can prevent problems such as stains.

pigment

The water-soluble base coat composition according to the present invention may further include pigments. Pigments play a role in giving color to the manufactured paint film. For example, the water-soluble base coat composition may use an effect pigment for providing a metallic effect to the coating film, a coloring pigment for providing color and a hiding effect in combination with a coating film-forming material, or a combination thereof.

Examples of effect pigments include water-based aluminum flakes, mica pigments, or mixtures thereof. Examples of coloring pigments include oxide-based inorganic pigments, polycyclic-based organic pigments containing Azo and Vat pigments, anthraquinone-based organic pigments, or mixtures thereof.

The pigment may be included in the composition in an amount of 15 to 35% by weight, or 17 to 32% by weight, based on the total weight of the water-soluble base coat composition. When the pigment content in the water-soluble base coat composition is within the above range, problems such as insufficient hiding power of the manufactured film, reduced stability of the composition, and reduced dispersibility of the pigment can be prevented.

additive

The water-soluble base coat composition according to the present invention may further include one or more additives selected from the group consisting of solvents, neutralizers, catalysts, ultraviolet absorbers, leveling agents, anti-foaming agents, wetting agents and waxes.

Additives may be included in the composition in an amount of 2 to 10% by weight, or 3 to 9% by weight, based on the total weight of the water-soluble base coat composition.

The solvent affects the smoothness of the manufactured coating film, provides storage stability to the water-soluble base coat composition, lowers the minimum coating film formation temperature, and controls the volatility of the solvent during painting operations.

Co-solvents include, for example, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, propylene glycol, N-methyl-2-pyrrolidone, n-propyl alcohol, i-propyl alcohol, n-butanol, propylene glycol monomethyl. Examples include ether, butyl glycol, hexyl glycol, 2-ethylhexyl alcohol, butyl carbitol, etc., but are not limited thereto.

The neutralizer serves to improve the storage stability of the water-soluble base coat composition and adjust the pH (to pH 7.5-9.0).

Neutralizing agents include, for example, trialkylamines such as trimethylamine, triethylamine and tributylamine, N,N-dimethylethanolamine, N,N-dimethylpropanolamine, N,N-dipropylethanolamine and 1- Tertiary amine compounds such as N,N-dialkylalkanolamines such as dimethylamino-2-methyl-2-propanol, N-alkyl-N,N-dialkanolamines, and trialkanolamines such as triethanolamine; ammonia; trimethylammonium hydroxide; sodium hydroxide; potassium hydroxide; and lithium hydroxide.

The catalyst prevents incomplete curing of the water-soluble base coat composition and improves the mechanical properties of the coating film produced therefrom. Examples of catalysts include amine group-containing phosphate-based compounds.

The UV absorber serves to improve the weather resistance of the final coating film by blocking ultraviolet rays from reaching the manufactured coating film.

The ultraviolet absorber may be a benzotriazole-based ultraviolet absorber. For example, the benzotriazole-based ultraviolet absorber is α-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxyphenyl]-1 -Oxapropyl]-ω-hydroxypoly(oxy-1,2-ethanediyl)(α-[3-[3-(2H-Benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4 -hydroxyphenyl]-1-oxopropyl]-ω-hydroxypoly(oxy-1,2-ethanediyl)), α-[3-[3-(2H-benzotrirazol-2-yl)-5-(1,1- dimethylethyl)-4-hydroxyphenyl]-1-oxapropyl]-ω-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4- Hydroxyphenyl]-1-oxapropoxy]poly(oxy-1,2-ethanediyl)(α-[3-[3-(2H-Benzotriazol-2-yl)-5-(1,1-dimethylethyl) -4-hydroxyphenyl]-1-oxopropyl]-ω-[3-[3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxopropoxy]poly( oxy-1,2-ethanediyl)), 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-methyl ester (3-(2H-benzotriazol- 2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-methyl ester), etc.

The leveling agent serves to provide leveling and wettability to the manufactured coating film. The leveling agent may be a cationic, anionic, or nonionic surfactant, and may include, for example, a nonionic surfactant.

The antifoaming agent suppresses the generation of bubbles generated during the manufacture of paint and suppresses or eliminates phenomena such as pinholes or popping that occur during the formation of a paint film. The antifoaming agent may be used without particular limitation as long as it is a typical antifoaming agent that can be used in a coating composition. For example, commercially available antifoam agents include BYK's BYK-011, BYK-015, BYK-072, Air Product's DF-21, Munzing's agitan 281, and Sannovco's Foamster-324.

A wetting agent serves to improve the leveling and wettability of a coating film manufactured from a composition containing it, and is not particularly limited as long as it is commonly used in coating compositions. For example, the wetting agent may be a polyether-modified polysiloxane-based or acetylene alcohol-type wetting agent.

Wax plays a role in preventing problems such as sagging due to the slow drying speed of a coating film manufactured from a composition containing it, and is not particularly limited as long as it is commonly used in coating compositions. For example, the wax may include an ethylene-vinyl-acetate wax.

The water-soluble base coat composition according to the present invention has a Ford Cup viscosity of 30 to 100 seconds, or 50 to 70 seconds based on Ford Cup No. 4 at 25°C, and is contained in an amount of 15 to 50% by weight, or 20 to 20%, based on the total weight of the composition. It may contain 45% by weight of solids.

As described above, the water-soluble base coat composition according to the present invention contains resins with different drying speeds, so that deformation of the manufactured coating film due to reduced volumetric shrinkage due to drying is prevented, and high saturation and high brightness can be achieved. In addition, the coating film prepared from the above composition has excellent mechanical properties, especially gloss, impact resistance, water resistance, and cold chipping resistance, and can be usefully used for car body painting.

Hereinafter, the present invention will be described in more detail through examples. However, these examples are only intended to aid understanding of the present invention, and the scope of the present invention is not limited to these examples in any way.

In the present invention, the functional value such as 'acid value' and 'hydroxyl value' of the resin can be measured by methods well known in the art, and can represent values measured by, for example, titration.

In addition, the 'weight average molecular weight' of the resin can be measured by a method well known in the art, for example, it can represent a value measured by a GPC (gel permeation chromatograph) method.

In addition, the 'glass transition temperature' of the resin can be measured by a method well known in the art, for example, it can be expressed as a value measured by differential scanning calorimetry (DSC).

[실시예][Example]

실시예 및 비교예. 수용성 베이스 코트 조성물의 제조Examples and comparative examples. Preparation of water-soluble base coat compositions

A water-soluble base coat composition was prepared by stirring and mixing each component in the composition as shown in Tables 1 to 3 below so that the viscosity was 55 seconds based on Ford Cup No. 4 at 25°C.

Hereinafter, the manufacturer and product name of each ingredient used in comparative examples and examples are shown in Table 4 below.

시험예: 제조된 도막의 특성 평가Test example: Evaluation of properties of manufactured coating film

After applying and drying the intermediate paint (manufacturer: KCC, product name: FU2290) to the specimen to form an intermediate coating film, the water-soluble base coat compositions of Examples 1 to 21 and Comparative Examples 1 to 11 were applied to a thickness of 15 ㎛ on a flat surface. Bell was painted. Afterwards, air was blown at 80°C for 3 minutes to evaporate the water remaining in the paint (dry). Clear paint (manufacturer: KCC) was applied to a thickness of 40㎛ on the base coat and cured in an oven at 140°C for 25 minutes to form the final coating film. The external characteristics and physical properties of the final coating film were measured in the following manner, and the results are shown in Tables 5 to 7.

(1) Paintability

When painting a bell with a water-soluble base coat composition, the painting workability was evaluated by the spraying state of the composition, and the spraying state was judged and compared and evaluated according to the atomization state.

(2) Appearance

The CF value of the final paint film was measured using Wave Scan DOI (BYK Gardner), an automobile appearance measuring instrument, and it was judged that the higher the CF value, the better the paint film's appearance characteristics. Specifically, if the CF value was 65 or more, it was evaluated as excellent (◎), if it was 60 or more but less than 65, it was evaluated as good (○), if it was 55 or more but less than 60, it was evaluated as average (△), and if it was less than 55, it was evaluated as poor (×).

(3) Glossy

The 20° gloss of the final coating film was measured. If the measured gloss value is 90 or more, it is excellent (◎), if the measured gloss value is 88 or more but less than 90, it is good (○), if it is 86 or more but less than 88, it is average (△), and if it is less than 86, it is poor (×). It was evaluated as .

(4) Adhesion

After painting and curing the base coat composition and clear coat composition of Examples 1 to 21 and Comparative Examples 1 to 11, adhesion was evaluated using the checkerboard method.

Specifically, in the Badukmok method, adhesion was measured by drawing 100 squares (width 2 mm × height 2 mm) on the repainted film with a blade and removing them using tape. At this time, if 100 squares are 100% completely attached, it is excellent (◎), if the remaining squares are more than 70% but less than 100%, it is good (○), if it is more than 50% but less than 70%, it is average (△), and if it is less than 50%, it is bad ( ×).

(5) Impact resistance

When a 500 g weight was dropped from a height of 30 cm onto the final coating film, the surface of the final coating film was observed to evaluate impact resistance. Specifically, if there were no cracks or traces of impact on the surface, it was evaluated as average (△); if there were traces, it was evaluated as average (△); and if the final coating film was broken or cracked, it was evaluated as poor (×).

(6) Water resistance

After immersing the final coating film in a 40℃ constant temperature bath for 10 days, adhesion was measured using the same method as the Badukmok method in item (4), and the water resistance of the coating film was evaluated by applying the same evaluation criteria.

(7) Seven flow characteristics

The specimen on which the intermediate coating film was formed was hung vertically and the water-soluble base coat compositions of Examples 1 to 21 and Comparative Examples 1 to 11 were applied in the same manner as described above, dried and cured, and the surface of the prepared coating film was observed to determine the application of the composition. Flowability was evaluated.

After applying intermediate and clear coating as described above to the previously painted steel plate with a hole with a diameter of 5 mm, it was observed that paint was formed at the bottom of the hole. The final film thickness ㎛ at the starting point where flow generation was observed was recorded as the flow limit thickness. At this time, it was judged that the lower the measured flow limit thickness value, the poorer the paint flowability.

Specifically, if the flow limit thickness is 40 ㎛ or more, it is excellent (◎), if it is 35 ㎛ or more but less than 40 ㎛, it is good (○), if it is 30 ㎛ or more but less than 35 ㎛, it is average (△), and if it is less than 30 ㎛, it is poor (×). It was evaluated as .

(8) Cold chipping resistance

After the final film was left at -20°C for 3 hours, a 50g chipping stone was pushed out with a pressure of 4 bar to hit the surface of the final film. At this time, if the damaged area of the final coating film was 1 mm or less, it was evaluated as excellent (◎), if it was more than 1 mm and less than 2 mm, it was evaluated as good (○), if it was more than 2 mm and less than 3 mm, it was evaluated as average (△), and if it was more than 3 mm, it was evaluated as poor (×).

(9) Recoating adhesion

After peeling off the base coat and clear coat in the final film, the water-soluble base coat compositions and clear paints of Examples 1 to 21 and Comparative Examples 1 to 11 were repainted and cured in the same manner as described above to prepare a recoat film. . Afterwards, adhesion to the recoated film was measured using the checkerboard method.

Specifically, adhesion was measured using the same method and evaluation criteria as the badukmok method in item (4).

As shown in Tables 5 to 7, all physical properties of the water-soluble base coat compositions of Examples 1 to 21 were evaluated as excellent, good, or average in the property evaluation test of the produced coating film.

However, Comparative Example 1, which did not contain the first acrylic resin, had poor coating film appearance, gloss, and impact resistance, and Comparative Example 2, which did not contain the second acrylic resin, had poor impact resistance and cold chipping resistance.

Comparative Example 3, which did not contain polyurethane dispersion resin, had poor coating film appearance, adhesion, impact resistance, water resistance, cold chipping resistance, and recoating adhesion.

Comparative Example 4, in which the glass transition temperature of the first acrylic resin was below the numerical range of the present invention, had poor impact resistance, water resistance, and cold chipping resistance, and in Comparative Example 4, in which the glass transition temperature of the first acrylic resin exceeded the numerical range of the present invention, Comparative Example 5 had poor coating film appearance, gloss, and cold chipping resistance.

Comparative Example 6, in which the weight average molecular weight of the first acrylic resin was below the numerical range of the present invention, had poor adhesion, impact resistance, water resistance, and cold chipping resistance, and the weight average molecular weight of the first acrylic resin was within the numerical range of the present invention. In Comparative Example 7, which exceeded , the coating film workability, coating film appearance, gloss, and impact resistance were poor.

Comparative Example 8, in which the glass transition temperature of the second acrylic resin was below the numerical range of the present invention, had poor coating film appearance, gloss, impact resistance, water resistance, and cold chipping resistance, and the glass transition temperature of the second acrylic resin was less than that of the present invention. Comparative Example 9, which exceeded the numerical range, had poor coating film appearance, adhesion, impact resistance, and recoating adhesion.

Comparative Example 10, in which the weight average molecular weight of the second acrylic resin was below the numerical range of the present invention, had poor adhesion, impact resistance, water resistance, and cold chipping resistance, and the weight average molecular weight of the second acrylic resin was within the numerical range of the present invention. Comparative Example 11, which exceeded , had poor coating film appearance, water resistance, paint flowability, and cold chipping resistance.

Hereinafter, various embodiments of the present invention will be described.

(1) It includes a first acrylic resin, a second acrylic resin, a polyurethane dispersion resin, a polyester resin, a urethane resin, and a melamine-based curing agent, and the first acrylic resin has a glass transition temperature of 15 to 25 ℃, and the weight A water-soluble base coat composition having an average molecular weight of 11,000 to 15,000 g/mol, the second acrylic resin having a glass transition temperature of 1 to 10° C., and a weight average molecular weight of 5,000 to 10,000 g/mol.

(2) The first acrylic resin has a hydroxyl value of 11 to 20 mgKOH/g and an acid value of 1 to 10 mgKOH/g, and the second acrylic resin has a hydroxyl value of 1 to 10 mgKOH/g and an acid value of 11 to 20. mgKOH/g water-soluble base coat composition.

(3) A water-soluble base coat composition wherein the polyurethane dispersion resin has a glass transition temperature of -45 to -20°C, a weight average molecular weight of 35,000 to 60,000 g/mol, and an acid value of 15 to 30 mgKOH/g.

(4) Polyester resin has a glass transition temperature of -50 to -25 ℃, a weight average molecular weight of 6,000 to 12,000 g/mol, a hydroxyl value of 80 to 110 mgKOH/g, and an acid value of 1 to 10 mgKOH/g. Phosphorus, water-soluble base coat composition.

(5) A water-soluble base coat composition in which the urethane resin has a glass transition temperature of -40 to -10°C, a weight average molecular weight of 200 to 900 g/mol, and an acid value of 15 to 30 mgKOH/g.

(6) Based on the total weight of the water-soluble base coat composition, 5 to 15% by weight of the first acrylic resin, 5 to 15% by weight of the second acrylic resin, 0.1 to 5% by weight of polyurethane dispersion resin, and 1 to 5% by weight of polyester resin. A water-soluble base coat composition comprising, by weight, 1 to 5% by weight of a urethane resin, and 3 to 8% by weight of a melamine-based curing agent.

Claims

It includes a first acrylic resin, a second acrylic resin, a polyurethane dispersion resin, a polyester resin, a urethane resin, and a melamine-based curing agent,

The first acrylic resin has a glass transition temperature of 15 to 25 ℃ and a weight average molecular weight of 11,000 to 15,000 g/mol,

A water-soluble base coat composition wherein the second acrylic resin has a glass transition temperature of 1 to 10° C. and a weight average molecular weight of 5,000 to 10,000 g/mol.
In claim 1,

The first acrylic resin has a hydroxyl value of 11 to 20 mgKOH/g and an acid value of 1 to 10 mgKOH/g,

A water-soluble base coat composition wherein the second acrylic resin has a hydroxyl value of 1 to 10 mgKOH/g and an acid value of 11 to 20 mgKOH/g.
In claim 1,

The polyurethane dispersion resin is a water-soluble base coat composition having a glass transition temperature of -45 to -20 ℃, a weight average molecular weight of 35,000 to 60,000 g/mol, and an acid value of 15 to 30 mgKOH/g.
In claim 1,

The polyester resin is water-soluble with a glass transition temperature of -50 to -25 ℃, a weight average molecular weight of 6,000 to 12,000 g/mol, a hydroxyl value of 80 to 110 mgKOH/g, and an acid value of 1 to 10 mgKOH/g. Base coat composition.
In claim 1,

A water-soluble base coat composition wherein the urethane resin has a glass transition temperature of -40 to -10°C, a weight average molecular weight of 200 to 900 g/mol, and an acid value of 15 to 30 mgKOH/g.
In claim 1,

Based on the total weight of the water-soluble base coat composition, 5 to 15% by weight of the first acrylic resin, 5 to 15% by weight of the second acrylic resin, 0.1 to 5% by weight of polyurethane dispersion resin, 1 to 5% by weight of polyester resin, A water-soluble base coat composition comprising 1 to 5% by weight of a urethane resin and 3 to 8% by weight of a melamine-based curing agent.