Water-soluble topcoat composition

Abstract

The present invention relates to a water-soluble topcoat paint composition comprising an acrylic emulsion resin, a melamine resin, a polyester resin, and a urethane resin, wherein the acrylic emulsion resin has a weight-average molecular weight of 200,000 to 300,000, the melamine resin has a glass transition temperature (Tg) of -80 to -30 ℃, the polyester resin has a hydroxyl value of 50 to 150 mgKOH / g, the urethane resin has a weight-average molecular weight of 20,000 to 50,000 and a glass transition temperature (Tg) of -55 to -15 ℃, and the solid content is 9 to 17 weight% based on the total weight of the paint composition.

Description

Technology Field

[0001] The present invention relates to a water-soluble topcoat paint composition, and more specifically, to a water-soluble topcoat paint composition for automobiles that has improved ejection properties during a coating process using inkjet technology. Background Technology

[0002] The spraying process using a spray gun or bell sprayer is a process of applying paint by atomizing it with relatively high-pressure air.

[0003] Conventional paint compositions used in such spray processes have a highly thixotropic nature, making it difficult to ensure jetting through fine inkjet nozzles of approximately 100 µm. Furthermore, even if jetting is achieved and the paint is applied to the substrate, there is a limitation in that it is difficult to satisfy the required appearance level by filling the gaps formed between the paint lines applied in the direction of application from each nozzle and ensuring leveling. Prior art literature

[0004] Republic of Korea Registered Patent No. 10-1277446 (June 17, 2013) Republic of Korea Registered Patent No. 10-1857857 (May 8, 2018) The problem to be solved

[0005] One objective of the present invention is to provide a water-soluble topcoat paint composition capable of ensuring ejection through a nozzle plate composed of a nozzle of approximately 100 μm, which is a component of an inkjet application, and enabling the realization of an appropriate film with only a single path. means of solving the problem

[0006] In one embodiment, the present invention provides a water-soluble topcoat paint composition comprising an acrylic emulsion resin, a melamine resin, a polyester resin, and a urethane resin, wherein the acrylic emulsion resin has a weight-average molecular weight of 200,000 to 300,000, the melamine resin has a glass transition temperature (Tg) of -80 to -30 ℃, the polyester resin has a hydroxyl value of 50 to 150 mgKOH / g, the urethane resin has a weight-average molecular weight of 20,000 to 50,000 and a glass transition temperature (Tg) of -55 to -15 ℃, and the solid content is 9 to 17 weight% based on the total weight of the paint composition. Effects of the invention

[0007] The water-soluble topcoat paint composition according to the present invention ensures ejection through a fine nozzle in a coating process using inkjet technology, and enables the realization of an appropriate coating film with only a single path. Specific details for implementing the invention

[0008] The present invention will be described in detail below.

[0009] In this specification, "weight-average molecular weight (Mw)" and "number-average molecular weight (Mn)" are measured by methods commonly known in the art to which the present invention belongs, and can be measured, for example, by methods such as GPC (gel permeation chromatograph).

[0010] In this specification, "glass transition temperature (Tg)" is measured by a method commonly known in the art to which the present invention belongs, such as differential scanning calorimetry (DSC).

[0011] In this specification, functional groups such as "acid value (Av)" and "hydroxyl value (OHv)" are measured by methods commonly known in the art to which this invention belongs, and may be measured, for example, by methods such as titration.

[0012] A water-soluble topcoat paint composition according to one embodiment of the present invention comprises an acrylic emulsion resin, a melamine resin, a polyester resin, and a urethane resin.

[0013] The water-soluble topcoat paint composition according to the present invention has a solid content of 9 to 17 weight% or 9 to 15 weight% based on the total weight of the paint composition.

[0014] The water-soluble topcoat paint composition according to the present invention comprises an acrylic emulsion resin having a weight-average molecular weight of 200,000 to 300,000, and the manufactured paint film may have excellent long-term physical properties such as durability and weather resistance. If the weight-average molecular weight of the acrylic emulsion resin is below the above range, the weather resistance and scratch resistance of the manufactured paint film become insufficient due to the low molecular weight; if it exceeds the above range, the flowability decreases due to the increase in molecular weight, resulting in poor workability of the paint composition containing it and poor surface smoothness, which may cause problems making it difficult to manufacture a paint film having an excellent appearance.

[0015] Meanwhile, to compensate for the poor extrusion performance caused by the high weight-average molecular weight of the acrylic emulsion resin, melamine resin, polyester resin, and urethane resin having relatively low weight-average molecular weight were mixed to control the viscosity balance and reduce the solid content of the paint composition to 9 to 17 weight%, thereby securing extrusion performance for applying the paint composition to inkjet applications.

[0016] In addition, the coating film may be manufactured by including a melamine resin with a glass transition temperature (Tg) of -80 to -30 ℃ and may have excellent appearance, impact resistance, and adhesion. If the glass transition temperature (Tg) is below the above range, drying is slow, resulting in incomplete drying under limited HAB (Hot Air Blow) intermediate drying process conditions, which may impair the final appearance or cause problems with reduced hardness due to a soft coating film. If the glass transition temperature (Tg) exceeds the above range, the coating film becomes excessively brittle, which may cause problems such as reduced impact resistance or chipping resistance, reduced interlayer adhesion, or reduced recoating adhesion.

[0017] In addition, when a polyester resin with a hydroxyl value of 50 to 150 mgKOH / g is included, the reactivity with the melamine resin is improved, which increases the crosslinking density of the coating film and has the effect of improving mechanical properties, appearance, and chemical resistance. If the hydroxyl value of the polyester resin is below the above range, there is a problem in that the durability and chemical resistance of the manufactured coating film are reduced due to insufficient reactivity with the melamine resin and insufficient crosslinking density, and if it exceeds the above range, there may be a problem in that the coating film becomes brittle due to over-curing, which reduces appearance and scratch resistance.

[0018] In addition, a paint composition containing a urethane resin with a weight-average molecular weight of 20,000 to 50,000 has the effect of improving smoothness and forming a smooth film, resulting in excellent gloss and appearance characteristics of the film. If the weight-average molecular weight of the urethane resin is below the above range, there is a problem in that mechanical properties and weather resistance are reduced due to the small molecular weight, and if it exceeds the above range, smoothness and flowability decrease due to the increase in molecular weight, resulting in reduced appearance characteristics and a problem in that the film becomes brittle, leading to reduced scratch resistance.

[0019] In addition, when a urethane resin with a glass transition temperature (Tg) of -55 to -15 ℃ is included, the flexibility of the coating film increases, which has the effect of improving the gloss characteristics and scratch resistance of the coating film. If the glass transition temperature of the urethane resin is below the above range, the elasticity of the coating film increases, the hardness decreases, and the drying properties of the paint composition are poor, resulting in a deterioration of the appearance characteristics of the coating film; if it exceeds the above range, the appearance characteristics decrease due to a decrease in the flowability of the paint composition, and the elasticity of the coating film decreases, which may cause problems such as reduced adhesion and scratch resistance.

[0020] Acrylic Emulsion Resin

[0021] The acrylic emulsion resin serves as the main resin of the paint composition of the present invention, and plays a role in forming a coating film and improving appearance characteristics such as durability and gloss of the manufactured coating film.

[0022] The acrylic emulsion resin may be one that is directly synthesized according to known methods or a commercially available product may be used. The acrylic emulsion resin may be, for example, one prepared by polymerizing one or more of vinyl monomers and (meth)acrylate monomers.

[0023] The acrylic emulsion resin may have an acid value of 10 to 30 mgKOH / g or 10 to 20 mgKOH / g. If the acid value of the acrylic emulsion resin is within the above range, the reactivity of the composition containing it can be controlled to improve the appearance characteristics of the coating film produced therefrom. If the acid value of the acrylic emulsion resin is below the above range, the curing reaction rate decreases, causing a problem of deterioration in the appearance of the produced coating film; if it exceeds the above range, the viscosity of the composition increases due to increased resin cohesion, which may cause a problem of reduced workability and room temperature storage stability.

[0024] The acrylic emulsion resin may have a weight-average molecular weight (Mw) of 200,000 to 300,000 or 230,000 to 280,000. When the weight-average molecular weight of the acrylic emulsion resin is within the above range, the long-term physical properties of the manufactured coating film, such as durability and weather resistance, may be excellent. When the weight-average molecular weight of the acrylic emulsion resin is below the above range, the low molecular weight results in insufficient weather resistance and scratch resistance of the manufactured coating film. When it exceeds the above range, the flowability decreases due to the increase in molecular weight, which may lead to poor workability of the paint composition containing it and poor surface smoothness, making it difficult to manufacture a coating film with an excellent appearance.

[0025] The acrylic emulsion resin may have a glass transition temperature (Tg) of -10 to 5 ℃ or -5 to 5 ℃. When the glass transition temperature of the acrylic emulsion resin is within the above range, the hardness and solvent resistance of the coating film are improved. When the glass transition temperature of the acrylic emulsion resin is below the above range, the drying speed and crosslinking density of the coating film decrease, resulting in a problem where the hardness and solvent resistance of the manufactured coating film are insufficient. When it exceeds the above range, the coating film becomes brittle, which may result in a lack of appearance characteristics and chipping resistance of the manufactured coating film.

[0026] The acrylic emulsion resin may have a hydroxyl value of 1 to 10 mgKOH / g or 5 to 10 mgKOH / g. When the hydroxyl value of the acrylic emulsion resin is within the above range, there is an effect of improving the durability and weather resistance of the coating film. When the hydroxyl value of the acrylic emulsion resin is below the above range, the formation of the coating film by the cross-linking reaction with the curing agent is insufficient, and mechanical properties such as the hardness of the coating film are reduced. When it exceeds the above range, it is over-cured, causing the coating film to become brittle, resulting in reduced elasticity and insufficient appearance characteristics of the manufactured coating film.

[0027] The acrylic emulsion resin includes a core-shell structure. Here, the volume ratio of the core to the shell may be 4:6 to 6:4. When the volume ratio of the core to the shell is within the above range, a crosslinking reaction suitable for the required specifications can be achieved, and appropriate mechanical properties can be achieved. If the volume ratio of the core is below the above range, thermal storage stability may be reduced, and there may be problems with the deterioration of properties related to the flexibility of the coating film, such as chipping resistance and impact resistance. On the other hand, if the volume ratio of the core exceeds the above range, the overall mechanical properties may be degraded due to a decrease in the crosslinking reaction, and in particular, there may be problems with the pencil hardness property deteriorating due to a decrease in the relative Tg.

[0028] Acrylic emulsion resin may be included in an amount of 4 to 12 weight percent or 7 to 11 weight percent based on the total weight of the paint composition. When acrylic emulsion resin is included within the above content range, it has the effect of improving the adhesion and durability of the paint film. If the content of acrylic emulsion resin is below the above range, drying performance is reduced, which causes problems such as reduced adhesion, gloss, and durability of the paint film; if it exceeds the above range, drying proceeds too quickly, resulting in insufficient paint workability and paint flowability of the composition, which may lead to a reduction in the appearance and scratch resistance of the paint film.

[0029] Melamine resin

[0030] Melamine resin crosslinks with each component of the paint composition to cure the composition and improve the hardness of the manufactured paint film.

[0031] The melamine resin may be an alkylated melamine resin, and may be one synthesized directly according to known methods or a commercially available product. The melamine resin may include, for example, one or more selected from the group consisting of methoxymethyl melamine, methyl melamine, butyl melamine, isobutoxymelamine, butoxymelamine, hexamethylol melamine, hexamethoxymethyl melamine, hexabutoxymethyl melamine, hexamethoxybutoxymethyl melamine, and aminomethoxymethyl melamine. In one embodiment, a methylated melamine formaldehyde resin was used as the melamine resin.

[0032] The weight-average molecular weight of the melamine resin may be 100 to 800 or 300 to 800. When the weight-average molecular weight of the melamine resin is within the above range, it has the effect of improving the adhesion and hardness of the manufactured coating film by improving the crosslinking density. When the weight-average molecular weight of the melamine resin is below the above range, the crosslinking density of the coating film decreases, which causes problems such as reduced chemical resistance and scratch resistance. When it exceeds the above range, problems such as reduced appearance characteristics of the coating film may occur as the molecular weight increases.

[0033] Melamine resin may have a glass transition temperature (Tg) of -80 to -30 ℃ or -70 to -30 ℃. If the glass transition temperature (Tg) is below the above range, drying is slowed down, resulting in incomplete drying under limited HAB (Hot Air Blow) intermediate drying process conditions, which may impair the final appearance or cause problems with reduced hardness due to a soft coating film. If the glass transition temperature (Tg) exceeds the above range, the coating film becomes excessively brittle, which may lead to problems such as reduced impact resistance or chipping resistance, reduced interlayer adhesion, or reduced recoating adhesion.

[0034] Melamine resin may be included in an amount of 0.1 to 3 weight% or 0.5 to 2 weight% based on the total weight of the paint composition. When the content of melamine resin is within the above range, it has the effect of improving the adhesion and hardness of the manufactured coating film by improving the crosslinking density. If the content of melamine resin is below the above range, there is a problem of reduced hardness and appearance characteristics due to reduced curability, and if it exceeds the above range, the coating film may become brittle due to excessive curability, resulting in reduced adhesion, impact resistance, and scratch resistance.

[0035] Polyester resin

[0036] Polyester resin plays a role in imparting flexibility to the paint composition, improving crosslinking density, and enhancing mechanical properties of the manufactured coating film, such as cold chipping resistance, impact resistance, and scratch resistance.

[0037] Polyester resin may be used that is directly synthesized according to known methods, or commercially available products may be used. Polyester resin may be prepared, for example, from a condensation reaction between a polyol monomer and a carboxyl group-containing monomer. Polyester resin may be prepared using a condensation reaction between an acid and an alcohol, and physical properties, such as weight-average molecular weight (Mw), hydroxyl value (OHv), and acid value (Av), can be controlled depending on the input ratio of the acid and alcohol.

[0038] The polyester resin may have an acid value of 1 to 30 mgKOH / g or 1 to 15 mgKOH / g. When the acid value of the polyester resin is within the above range, there is an effect of improving the weather resistance of the coating film. If the acid value of the polyester resin is below the above range, the formation of the coating film by the cross-linking reaction with the curing agent is insufficient, and mechanical properties such as the hardness of the coating film are reduced. If it exceeds the above range, it is over-cured, causing the coating film to become brittle and lose elasticity, and problems may arise such as insufficient appearance characteristics, water resistance, and scratch resistance of the manufactured coating film.

[0039] The polyester resin may have a weight-average molecular weight of 3,000 to 10,000 or 5,000 to 8,000. When the weight-average molecular weight of the polyester resin is within the above range, the smoothness of the paint composition is improved, and a soft film is formed, which has the effect of improving scratch resistance and hardness. When the weight-average molecular weight of the polyester resin is below the above range, there is a problem that the mechanical properties of the manufactured film are degraded due to the small molecular weight, and when it exceeds the above range, the flowability decreases due to the increase in molecular weight, causing the film to become brittle, which may result in reduced smoothness and reduced scratch resistance.

[0040] The polyester resin may have a glass transition temperature (Tg) of -60 to -20 ℃ or -40 to -20 ℃. When the glass transition temperature of the polyester resin is within the above range, the gloss characteristics and hardness of the coating film are improved. If the glass transition temperature of the polyester resin is below the above range, the drying speed of the paint composition is delayed, resulting in a problem where the solvent resistance and chipping resistance of the manufactured coating film are insufficient. If it exceeds the above range, the coating film may become brittle, resulting in a lack of appearance characteristics and hardness of the coating film.

[0041] The polyester resin may have a hydroxyl value of 50 to 150 mgKOH / g or 50 to 100 mgKOH / g. When the hydroxyl value of the polyester resin is within the above range, it improves reactivity with the melamine resin, thereby increasing the crosslinking density of the coating film and improving mechanical properties, appearance, and chemical resistance. When the hydroxyl value of the polyester resin is below the above range, there is a problem where the durability and chemical resistance of the manufactured coating film are reduced due to insufficient reactivity with the melamine resin and insufficient crosslinking density. When it exceeds the above range, it may cause over-curing, resulting in a brittle coating film and reduced appearance and scratch resistance.

[0042] Polyester resin may be included in an amount of 0.1 to 5 weight% or 1 to 4 weight% based on the total weight of the paint composition. When the polyester resin is included within the above content range, it has the effect of improving appearance, scratch resistance, and film smoothness. If the content of polyester resin in the composition is below the above range, the crosslinking density decreases, which may cause problems such as reduced appearance and scratch resistance; if it exceeds the above range, the viscosity of the composition becomes excessively high, which may cause problems such as reduced workability and drying properties, and reduced appearance and mechanical properties.

[0043] Urethane resin

[0044] Urethane resin serves to impart elasticity to the manufactured coating film and improve gloss, smoothness, and scratch resistance.

[0045] Urethane resins may be used that are directly synthesized according to known methods, or commercially available products may be used. Urethane resins may be manufactured, for example, from one or more of alcohol-based monomers and isocyanate-based compounds.

[0046] Urethane resin may be manufactured using an addition reaction between an isocyanate-based compound and an alcohol-based monomer, and physical properties, such as weight-average molecular weight (Mw) and glass transition temperature (Tg), can be controlled depending on the reaction ratio.

[0047] The acid value of the urethane resin may be 1 to 50 mgKOH / g or 1 to 30 mgKOH / g. If the acid value of the urethane resin exceeds the above range, there may be a problem with reduced water resistance.

[0048] The weight-average molecular weight of the urethane resin may be 20,000 to 50,000 or 30,000 to 40,000. When the weight-average molecular weight of the urethane resin is within the above range, the smoothness of the paint composition is improved and a smooth film is formed, resulting in excellent gloss and appearance characteristics of the film. When the weight-average molecular weight of the urethane resin is below the above range, there is a problem where mechanical properties and weather resistance are reduced due to the small molecular weight. When it exceeds the above range, the smoothness and flowability decrease due to the increase in molecular weight, resulting in reduced appearance characteristics and a problem where the film becomes brittle, leading to reduced scratch resistance.

[0049] The urethane resin may have a glass transition temperature (Tg) of -55 to -15 ℃ or -35 to -15 ℃. When the glass transition temperature of the urethane resin is within the above range, the flexibility of the coating film increases, which has the effect of improving the gloss characteristics and scratch resistance of the coating film. When the glass transition temperature of the urethane resin is below the above range, the elasticity of the coating film increases, the hardness decreases, and the drying properties of the paint composition are poor, which degrades the appearance characteristics of the coating film. When it exceeds the above range, the appearance characteristics degrade due to the reduced flowability of the paint composition, and the elasticity of the coating film decreases, which may cause problems such as reduced adhesion and scratch resistance.

[0050] Urethane resin may be included in an amount of 0.5 to 10 weight percent or 1 to 5 weight percent based on the total weight of the paint composition. When the content of urethane resin is within the above range, the flexibility and appearance characteristics of the manufactured film are excellent. If the content of urethane resin is below the above range, the crosslinking density of the film decreases, leading to problems such as reduced scratch resistance and appearance. If it exceeds the above range, the viscosity of the paint composition increases, resulting in poor workability and reduced drying properties, which may lead to problems such as reduced hardness of the film.

[0051] Colorants

[0052] The paint composition of the present invention may include a colorant.

[0053] Colorants serve to impart color to the manufactured paint film.

[0054] Colorants are not particularly limited as long as they are pigments commonly used in paint compositions; for example, inorganic pigments and organic pigments may be used. Other examples of pigments include metal oxides such as titanium dioxide, carbon black, zinc oxide, ultramarine, and red iron oxide; sulfides of lithopone, lead, cadmium, iron, cobalt, and aluminum, and their hydrochlorides or sulfates; azo pigments; and copper phthalocyanine pigments.

[0055] The colorant may be included in an amount of 0.1 to 3 weight percent or 1 to 2 weight percent based on the total weight of the paint composition. If the colorant content 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 colorant can be prevented.

[0056] <Additives>

[0057] The paint composition of the present invention may include, as additives, a neutralizing agent for adjusting the pH of the paint (e.g., AMP-95®), an ultraviolet absorber for absorbing ultraviolet rays and suppressing radical chain reactions caused by ultraviolet rays (e.g., TINUVIN® 1130), an acid catalyst for promoting the reaction and controlling the curing speed (e.g., dodecylbenzenesulfonic acid type acid catalyst, NACURE XP-221, King Industries), an antifoaming agent for suppressing bubble generation (e.g., SURFYNOL 107L), and a surface modifier for oriented toward the surface of the paint film during the drying process to homogenize the surface tension of the paint film and prevent color stains or craters (e.g., Hydropalat® WE 3323).

[0058] The content of additives is not specifically limited as long as it falls within the range that can typically be included in the paint composition. For example, based on the total weight of the paint composition, a neutralizing agent may be included in an amount of 0.1 to 10 weight% or 2 to 5 weight%, a UV absorber in an amount of 0.1 to 3 weight% or 0.1 to 1 weight%, an acid catalyst in an amount of 0.1 to 3 weight% or 0.1 to 1 weight%, an antifoaming agent in an amount of 0.1 to 3 weight% or 0.5 to 1.5 weight%, and a surface modifier in an amount of 0.1 to 3 weight% or 2 to 3 weight%.

[0059] Solvent

[0060] The solvent plays a role in maintaining the thickness of the manufactured coating film and improving workability by controlling the viscosity of the paint composition. The solvent may include one or more selected from the group consisting of organic solvents and water. For example, the solvent may include a mixture of water and an organic solvent, but may be appropriately selected and used depending on the characteristics or volatilization rate of the resin in the composition. In one embodiment, deionized water was used as the solvent.

[0061] Examples of water include deionized water (DIW) and distilled water, and it serves to reduce the generation of volatile organic compounds (VOCs) in the composition. Organic solvents are not specifically limited as long as they are compatible with water, and examples include isopropanol, propylene glycol, normal propyl ether, propylene glycol normal butyl ether, dipropylene glycol methyl ether, dipropylene glycol normal butyl ether, dibasic ether, propanediol diacetate, and mixtures thereof.

[0062] The solvent may be included in the remainder amount or 45 to 65 weight percent based on the total weight of the paint composition.

[0063] Jo Yong-je

[0064] A co-solvent is used to contribute to the volatilization of distilled water, which is the main solvent of the present invention, to prevent popping of the coating film and to affect its smoothness, and to facilitate the atomization and fine-graining of the paint during the painting process. The co-solvent plays a role in controlling the drying speed of the paint composition and controlling resin swelling.

[0065] Specifically, the present invention ensures ejectability by reducing the solid content of the paint composition. However, the problem of deterioration in appearance and physical properties during the film drying process caused by this is resolved by applying a specific solvent mixture.

[0066] The co-working agent mixture applied in the present invention uses a mixture of a paraffin-based co-working agent, an alcohol-based co-working agent, and a pyrrolidone-based co-working agent.

[0067] Specifically, in one embodiment of the present invention, a mixture of an iso-paraffinic co-agent marketed under the trade name Exxsol™ D60 as a paraffinic co-agent, n-propanol as an alcoholic co-agent, and N-ethylenepyrrolidone-2 as a pyrrolidone co-agent was used.

[0068] The paint composition of the present invention comprises, based on the total weight of the paint composition, 0.1 to 3 weight% or 0.1 to 2 weight% of a paraffin-based co-modifier, 0.1 to 10 weight% or 4 to 10 weight% of an alcohol-based co-modifier, and 1 to 15 weight% or 5 to 10 weight% of a pyrrolidone-based co-modifier. The desired effect can be achieved within the above ranges for the content of the co-modifiers; if the content is below the above ranges, it may affect the fluidity of the paint, and if it exceeds the above ranges, there may be a problem that the drying of the paint becomes relatively slow.

[0069] Flow Control Agent

[0070] As described above, the present invention ensures dischargeability by reducing the solid content of the paint composition. At the same time, by controlling the type and content of a flow control agent (or rheology control agent) so that a uniform film surface and leveling can be secured during the flash-off process before the paint composition is introduced into the HAB (Hot Air Blow) drying process, the fluidity (or rheology) suitable for the corresponding painting process is secured.

[0071] Flow modifiers play a role in preventing the addition of each component of the paint composition.

[0072] The fluidity regulator of the present invention may be a wax emulsion or a polyurethane-based thickener. Specifically, the wax emulsion may be a non-ionic emulsion of a modified ethylene vinyl acetate (EVA) copolymer wax, commercially available under the trademark AQUATIX 8421 of BYK-Chemie GmbH. In addition, the polyurethane-based thickener may be a solvent-free polyurethane thickener, commercially available under the trademark Coapur™ XS 83.

[0073] A flow modifier may be included in an amount of 2 to 10 weight percent or 3 to 8 weight percent based on the total weight of the paint composition. If the content of the flow modifier is below the above range, it causes paint flow during painting, and if it exceeds the above range, the viscosity of the paint becomes too high and dispersibility decreases, which may result in a deterioration of the paint's appearance.

[0074] Water-soluble topcoat paint composition

[0075] As described above, the present invention ensures dischargeability by reducing the solid content of the paint composition. To this end, the paint composition of the present invention may have a solid content of 9 to 17 weight% or 9 to 15 weight% based on the total weight of the paint composition.

[0076] If the solid content is below the above range, there may be a problem that leads to a deterioration in overall physical properties, including water resistance, solvent resistance, and accelerated weathering resistance. If the solid content exceeds the above range, it results in an increase in overall viscosity and pseudoplastic behavior in terms of fluid rheology, which may make it difficult to ensure ejection through a nozzle of approximately 100 µm for the inkjet application to be applied. Additionally, due to the characteristics of the process, which requires achieving an appropriate film thickness with only single-path coating, an excessive film thickness may be induced, leading to a problem of appearance degradation within the specified drying process conditions.

[0077] Specifically, based on the total weight of the paint composition, the solid content of the acrylic emulsion resin may be 1.8 to 5.4 weight% or 3 to 5 weight%, the solid content of the melamine resin may be 0.1 to 3 weight% or 0.5 to 2 weight%, the solid content of the polyester resin may be 0.08 to 4 weight% or 0.8 to 3.2 weight%, and the solid content of the urethane resin may be 0.3 to 3.5 weight% or 0.3 to 1.8 weight%.

[0078] Various embodiments of the present invention are described below. However, these embodiments are intended only to aid in understanding the present invention and do not imply that the scope of the present invention is limited to these embodiments in any way.

[0079] [Example]

[0080] Preparation of water-soluble topcoat paint compositions of examples and comparative examples

[0081] A water-soluble topcoat paint composition was prepared by stirring and mixing each component in the amounts presented in Tables 1 to 4 below.

[0082] Ingredients (Weight%) Example 1 Example 2 Example 3 Example 4 Example 5 Acrylic emulsion resin -1 7.18 8.62 10.64 8.62 8.62 Acrylic emulsion resin -2 　 　 　 　 　 Melamine resin -1 0.86 0.86 0.86 1.83 0.86 Melamine resin -2 　 　 　 　 　 Polyester resin 1.2 1.2 1.2 1.97 1.2 Urethane resin -1 1.25 2.59 4.83 2.59 2.59 Urethane resin -2 　 　 　 　 　 pigment 1.78 1.78 1.78 1.78 1.78 corrector 3.21 3.21 3.21 3.21 3.21 UV absorber 0.48 0.48 0.48 0.48 0.48 acid catalyst 0.32 0.32 0.32 0.32 0.32 Antifoamer 1.08 1.08 1.08 1.08 1.08 Surface modifier 2.59 2.59 2.59 2.59 2.59 Flow regulator 6.56 6.56 6.56 6.56 3.37 distilled water 59.49 56.71 52.45 54.97 58.91 Jo Yong-je 1 0.54 0.54 0.54 0.54 0.54 Jo Yong-je 2 6.46 6.46 6.46 6.46 7.45 Jo Yong-je 3 7 7 7 7 7 total 100.00 100.00 100.00 100.00 100.00

[0083] Ingredients (Weight%) Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 Acrylic emulsion resin -1 8.62 5.43 11.57 8.62 5.59 5.59 Acrylic emulsion resin -2 　 　 　 　 　 　 Melamine resin -1 0.86 0.86 0.86 2.75 0.86 0.86 Melamine resin -2 　 　 　 　 　 　 Polyester resin 1.2 1.2 1.2 1.2 1.2 1.2 Urethane resin -1 2.59 0.84 7.43 0.85 7.51 7.51 Urethane resin -2 　 　 　 　 　 　 pigment 1.78 1.78 1.78 1.78 1.78 1.78 corrector 3.21 3.21 3.21 3.21 3.21 3.21 UV absorber 0.48 0.48 0.48 0.48 0.48 0.48 acid catalyst 0.32 0.32 0.32 0.32 0.32 0.32 Antifoamer 1.08 1.08 1.08 1.08 1.08 1.08 Surface modifier 2.59 2.59 2.59 2.59 2.59 2.59 Flow regulator 7.67 6.56 6.56 6.56 2.77 6.56 distilled water 57.77 61.65 48.92 56.56 58.61 60 Jo Yong-je 1 0.54 0.54 0.54 0.54 0.54 0.8 Jo Yong-je 2 4.29 6.46 6.46 6.46 6.46 3.76 Jo Yong-je 3 7 7 7 7 7 4.26 total 100.00 100.00 100.00 100.00 100.00 100.00

[0084] Ingredients (Weight%) Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Acrylic emulsion resin -1 　 0.56 　 8.62 13.1 Acrylic emulsion resin -2 0.56 　 　 　 　 Melamine resin -1 0.86 0.86 0.86 　 1.36 Melamine resin -2 　 　 　 0.86 　 Polyester resin 1.2 1.2 1.2 1.2 0.7 Urethane resin -1 13.03 　 13.1 0.93 　 Urethane resin -2 　 13.03 　 　 　 pigment 1.78 1.78 1.78 1.78 1.78 corrector 3.21 3.21 3.21 3.21 3.21 UV absorber 0.48 0.48 0.48 0.48 0.48 acid catalyst 0.32 0.32 0.32 0.32 0.32 Antifoamer 1.08 1.08 1.08 1.08 1.08 Surface modifier 2.59 2.59 2.59 2.59 2.59 Flow regulator 6.56 6.56 6.56 6.56 6.56 distilled water 61.96 61.96 54.82 58.37 54.82 Jo Yong-je 1 1.58 1.58 0.54 0.54 0.54 Jo Yong-je 2 2.38 2.38 6.46 6.46 6.46 Jo Yong-je 3 2.41 2.41 7 7 7 total 100.00 100.00 100.00 100.00 100.00

[0085] Ingredients (Weight%) Comparative Example 6 Comparative Example 7 Comparative Example 8 Comparative Example 9 Acrylic emulsion resin -1 　 　 17.74 27.25 Acrylic emulsion resin -2 8.62 8.62 　 　 Melamine resin -1 0.86 0.86 1.77 5.35 Melamine resin -2 　 　 　 　 Polyester resin 1.2 1.2 1.66 5.46 Urethane resin -1 　 　 3.55 11.87 Urethane resin -2 2.59 2.59 　 　 pigment 1.78 1.78 2.5 6.15 corrector 3.21 3.21 3.21 3.21 UV absorber 0.48 0.48 0.48 0.48 acid catalyst 0.32 0.32 0.32 0.32 Antifoamer 1.08 1.08 1.08 1.08 Surface modifier 2.59 2.59 2.59 2.59 Flow regulator 6.13 6.13 6.13 3 distilled water 57.68 54.14 44.97 21 Jo Yong-je 1 　 3.54 0.54 0.54 Jo Yong-je 2 6.46 6.46 6.46 6.46 Jo Yong-je 3 7 7 7 5.24 total 100.00 100.00 100.00 100.00

[0086] The physical properties, manufacturer and / or product name, etc. of each component used in the examples and comparative examples are shown in Table 5 below.

[0087] ingredient Physical properties, or manufacturer and product reviews Acrylic emulsion resin -1 Av 15 mg KOH / g, Mw 265453, Tg -3℃, OHv 6 mg KOH / g, Core : Shell=5 : 5* Acrylic emulsion resin -2 Av 20 mg KOH / g, Mw 16000, Tg 11℃, OHv 12 mg KOH / g, Core : Shell = 2 : 8* Melamine resin -1 Mw 589, Tg -50.16℃ Melamine resin -2 Mw 862 , Tg -3.63℃ Polyester resin Av 7 mg KOH / g, Mw 7923, Tg -38℃, OHv 89 mg KOH / g Urethane resin -1 Av 18 mg KOH / g, Mw 38840, Tg -23℃ Urethane resin -2 Av 12 mg KOH / g, Mw 19999, Tg -38.2℃ pigment Carbon Black RAVEN 5000 ULTRA III POWDER corrector AMP95 (10%) DIW Solution UV absorber Tinuvin 1130 (50%) BC Solution acid catalyst NACURE XP-221 Semi-finished Product Solution Antifoamer SURFYNOL 107L(Alkane Diol in Solvnet) Surface modifier WE 3323 Flow regulator COAPUR XS 83 solvent DIWater Jo Yong-je 1 Exxsol D60 Jo Yong-je 2 n-Propanol Jo Yong-je 3 N-Ethylpyrrolidone-2

[0088] (*Based on morphological volume ratio)

[0089] Test Example: Evaluation of characteristics of the manufactured coating film

[0090] The characteristics of the manufactured coating film were evaluated according to HMC MS 653-01 using the method presented in Table 6 below.

[0091]

[0092] The well-mixed paint is placed in a 500 mL glass bottle, sealed, and heated in an oven at 50°C for 72 hours. After removing it, when the paint temperature reaches room temperature (23±2°C), the viscosity is measured using Fordcup #4. Heat storage stability is evaluated by comparing the viscosity using Fordcup #4 before and after heat storage. If the change in viscosity is less than 30%, it is evaluated as excellent (◎); if it is between 30% and less than 40%, it is evaluated as good (○); if it is between 40% and less than 50%, it is evaluated as average (△); and if it is 50% or more, it is evaluated as poor (×).

[0093] (2) Painting workability (dischargeability)

[0094] The paint composition is applied to an inkjet application to observe jetting performance.

[0095] The evaluation criteria for the jetting performance of inkjet applications are defined as follows.

[0096] [ Excellent (◎) ] : Discharge through the nozzle is smooth and continuous, and each coating line formed by discharge through each nozzle is a smooth straight line, and there is no tendency for one coating line to encroach upon another coating line immediately after coating. After coating, within a specified flash time (3 to 4 minutes) at room temperature, each coating line connects to form a single wet film surface, and there are no gaps or irregularities on the wet film surface.

[0097] [ Good (○) ] : Each coating line is not smooth or interference occurs between coating lines, and irregularities are observed within 5% of the total moisture film surface standard, but discharge through the nozzle is smooth and continuous, and finally, a single moisture film surface without gaps is formed within the Flash Time (3 to 4 minutes).

[0098] [ Normal (△) ] : Discharge through the nozzle is smooth and continuous, but each coating line is not smooth or interference occurs between coating lines, so irregularities exceeding 5% and within 10% of the total wet film surface area are observed, or a flash time of 4 to 5 minutes is required to form a wet film surface without gaps.

[0099] [ Defect(×) ] : Cases where discharge through the nozzle is not smooth or continuous, where the unevenness of the wet film surface caused by uneven individual painting lines or interference between painting lines exceeds 10% of the total area, or where a flash time exceeding 5 minutes is required to form a gap-free painted surface, or where a gap-free painted surface cannot be formed.

[0100] (Inkjet applications: Duur EcoPaintJet and EcoPaintJet Pro, ABB PixelPaint, SAMES KREMLIN PRiNTEC, etc.)

[0101] (3) Image clarity (CF)

[0102] The CF value among the measurement results of BYK’s WAVE SCAN-DOI was used as the image clarity result. The CF value measured by WAVE SCAN-DOI was evaluated as excellent (◎) if it was 70 or higher, good (○) if it was 65 or higher but less than 70, average (△) if it was 55 or higher but less than 65, and poor (×) if it was less than 55.

[0103] (4) Gloss

[0104] The 20° gloss value from the BYK gloss meter measurement results was used as the result. The gloss meter 20° angle measurement value was evaluated as excellent (◎) if it was 90% or higher, good (○) if it was 89% or higher but less than 90%, average (△) if it was 88% or higher but less than 89%, and poor (×) if it was less than 88%.

[0105] (5) Adhesion

[0106] A crosscut with a 10x10 grid pattern was made on the specimen at 2mm intervals, and Nichiban cellophane tape was applied in full contact with the area. Then, one end of the tape was grasped and peeled off in a 90° direction within 0.5 seconds. Based on the following evaluation criteria, M-1.0 was evaluated as excellent (◎), M-1.0 to M-2.0 was evaluated as good (○), M-2.0 to M-2.5 was evaluated as average (△), and M-2.5 was evaluated as poor (×).

[0107] Judgment criteria M-1.0 to M-5.0 are defined as follows.

[0108] [ M-1.0 ] The corners of the checkerboard cross-cut shall have no more than 10 broken pieces, no peeling along straight lines, and no peeling of more than 50% of the checkerboard pattern.

[0109] [ M-2.0 ] Corner breakage of the checkerboard cross-cut may be visible throughout, but there must be no peeling along straight lines; there must be no peeled portion covering more than 50% of the checkerboard grid, and the total peeled area must be less than 5%.

[0110] [ M-3.0 ] Peeling along a straight line is permitted, but there are no peeled parts covering more than 50% of the grid area, and the total peeled area is less than 5 to 15%.

[0111] [ M-4.0 ] Significant peeling along a straight line is visible, about 5 checkerboard patterns are completely peeled off, and the total peeled area is less than 15 to 35%.

[0112] [ M-5.0 ] There is a lot of peeling along straight lines, and there are about 20 pieces where the checkerboard pattern is completely peeled off, and the total peeled area is less than 35 to 65%.

[0113] (6) Water resistance

[0114] At least 70% of the specimen is immersed in a constant temperature water bath at 40°C, and after 240 hours, it is removed, dried, and left at room temperature for 1 hour. Then, the appearance is visually inspected for any abnormalities, and an adhesion test is performed. At this time, the boundary between the immersed area and the non-immersed area is observed with a focus on comparison. Stainless steel is used for the constant temperature water bath material to avoid affecting the coating surface, and the bath is filled with pure water and replaced 100% every 7 days. When no abnormalities such as swelling or discoloration are observed in the appearance of the coating surface visually after the water resistance test, the standard adhesion test result is evaluated as Excellent (◎) if M-1.0 or higher but M-2.0 or lower, Good (○) if M-1.0 or higher but M-2.5 or lower, Average (△) if M-2.5 or higher, and Poor (×) if M-2.5 or higher. However, even if the adhesion result after the water resistance test satisfies M-2.5 or lower, if swelling, discoloration, etc. are observed in the appearance, it is evaluated as defective (×).

[0115] (7) Content

[0116] After attaching a gauze sufficiently impregnated with xylene solvent to the surface of the coating film, the coating film is scratched with a fingernail at 1-minute intervals to check whether the underlying coating surface is exposed. During the solvent resistance test, if the time it takes for the underlying coating surface to start to be exposed is 7 minutes or more, it is evaluated as excellent (◎); if it is 6 minutes or more but less than 7 minutes, it is evaluated as good (○); if it is 5 minutes or more but less than 6 minutes, it is evaluated as average (△); and if it is less than 5 minutes, it is evaluated as poor (×).

[0117] (8) Impact resistance

[0118] A DuPont-type drop weight tester is equipped with a 0.5 kgf weight and a 0.5-inch diameter indenter, and impacts are applied to specimens fixed to specimen supports while varying the drop height of the weight, and the condition of the coating film is examined. However, the indenter is lightly placed on the fixed specimen, and the weight is lifted up the guide tube to the height to be tested and then dropped onto the indenter. When the test is conducted by increasing the drop height of the weight from a low height to a high height, the maximum drop height at which there is no cracking or peeling of the coating film of the test specimen is evaluated as excellent (◎) if it is 40 cm or more and less than 40 cm, good (○) if it is 30 cm or more and less than 30 cm, average (△) if it is 20 cm or more and less than 30 cm, and poor (×) if it is less than 20 cm.

[0119] (9) Chipping

[0120] For the chipping evaluation, the specimen is placed in a -20°C freezer for 3 hours until the temperature reaches -20±3°C, then mounted on a chipping resistance tester at a 45° incline, and 50g of chipping stone No. 7 as specified in JIS A 5001 is fed to achieve 4±0.2kgf / cm² 2The specimen is sprayed with pressure. After spraying, the specimen is removed, and foreign substances such as peeled film remaining on the coating are removed using cellophane adhesive tape. Then, the degree of damage to the specimen is compared with a standard plate to determine the grade. If the grade determined by comparison with the standard plate is grade 1 or 2, it is evaluated as excellent (◎); if it is grade 2 or higher but grade 2.5, it is good (○); if it is grade 2.5 or higher but grade 3.0, it is average (△); and if it is grade 3.0 or higher but grade 6.0, it is poor (×).

[0121] (10) Pencil hardness

[0122] Sharpen a Mitsubishi UNI pencil so that a 5-6mm lead is exposed, place it vertically on sandpaper, and grind the cross-section to a flat surface. Then, fix it at a 45° angle to a pencil hardness tester positioned on top of a specimen placed on a horizontal surface. Adjust weights so that the total vertical load applied to the specimen is 1.0 kgf, and move the pencil hardness tester forward at a constant speed over the specimen to scrape the coating surface. After 30 seconds, observe the surface and determine the grade according to the pencil hardness grade standard.

[0123] - Grade 1: No traces

[0124] - Grade 2: Cases where there are no scratches and only a faint indentation at the starting point

[0125] - Grade 3: Cases where there are no scratches and only faint indentations on the lines.

[0126] - Grade 4: Cases with scratches

[0127] - Grade 5: Clearly scratched, thick

[0128] In a total of 5 tests using Mitsubishi UNI B pencils, if Grade 1 was satisfied 4 or more times, it was evaluated as Excellent (◎); if Grade 2 was satisfied, it was evaluated as Good (○); if Grade 3 was satisfied, it was evaluated as Average (△); and if it exceeded Grade 3, it was evaluated as Poor (×). However, if a pencil with a hardness greater than B was used (e.g., HB pencil), even if Grade 3 was satisfied 4 or more times in a total of 5 tests, it was evaluated as Excellent (◎).

[0129] (11) Accelerated weathering

[0130] Using a Xenon Weather-O-Meter (Lamp Filter combination: QUARTZ - BORO / SILICATE) device set to SAE J1960 / 2527 conditions, the specimen is mounted in the internal specimen holder facing the Xenon lamp and held for 1000 hours. After 1000 hours, any remaining scale and foreign matter on the specimen are removed using a neutral detergent and a soft sponge, and after drying thoroughly, adhesion, 25° / 45° / 75° color difference (ΔE), and 20° gloss retention are evaluated.

[0131] The three items evaluated in the accelerated weathering test—adhesion, color difference, and gloss retention rate—are classified as follows.

[0132] i) Adhesion

[0133] Excellent(◎): M-1.0

[0134] Good (○): Greater than M-1.0 and less than or equal to M-2.0

[0135] Normal (△): Greater than M-2.0, less than or equal to M-2.5

[0136] Defective(×): Exceeds M-2.5

[0137] ii) Color difference

[0138] Excellent (◎): ΔE must be 1.0 or less

[0139] Good (○): ΔE greater than 1.0, ΔE 2.0 or less

[0140] Normal (△): ΔE greater than 2.0, ΔE 3.0 or less

[0141] Defective(×): ΔE greater than 3.0

[0142] iii) Gloss retention rate (based on 20°)

[0143] Excellent (◎): Must be 98% or higher

[0144] Good (○): 95% or higher and less than 98%

[0145] Normal (△): 90% or more and less than 95%

[0146] Defective(×): Less than 90%

[0147] The accelerated weathering results of the paint composition are defined based on the lowest classification of excellent (◎), good (○), average (△), and poor (×) that satisfies all three of the above items.

[0148] Example) Paint Composition #1: Good adhesion after accelerated weathering (○), excellent color difference (◎), excellent gloss retention (◎) ▶ Paint Composition #1 Good accelerated weathering (○)

[0149] Example) Paint Composition #2: Adhesion after accelerated weathering is average (△), color difference is excellent (◎), gloss retention is poor (×) ▶ Paint Composition #2 Accelerated Weathering is poor (×)

[0150] (12) Recoating adhesion

[0151] After degreasing the test specimens that were finished with a topcoat on the same day, recoating was performed and left at room temperature for 24 hours, after which an adhesion test was conducted. The results of the recoating adhesion test were evaluated as excellent (◎) if M-1.0, good (○) if M-1.0 or higher and M-2.0 or lower, average (△) if M-2.0 or higher and M-2.5 or lower, and poor (×) if M-2.5 or higher, in the same way as the existing adhesion test.

[0152] The appearance characteristics and physical properties of the final coating film were measured in the manner described above, and the results are shown in Tables 7 to 10 below. In Tables 7 to 10, Excellent (◎), Good (○), Average (△), and Poor (×) are indicated.

[0153] Physical properties Example 1 Example 2 Example 3 Example 4 Example 5 Thermal storage stability ◎ ◎ ◎ ◎ ○ Painting workability (dischargeability) ◎ ◎ ◎ ◎ ◎ Exterior (CF) ◎ ◎ ○ ◎ ◎ gloss ◎ ◎ ◎ ◎ ◎ Adhesion ◎ ◎ ◎ ◎ ◎ water resistance ◎ ◎ ◎ ◎ ◎ Content ○ ◎ ◎ ◎ ◎ Impact resistance ◎ ◎ ◎ ◎ ◎ Chipping ◎ ◎ ○ ○ ◎ Pencil hardness ○ ◎ ◎ ◎ ◎ Accelerated weathering ◎ ◎ ◎ ○ ◎ Repaint adhesion ◎ ◎ ◎ ◎ ◎ NV (%) 10.54 11.64 13.35 13.24 10.89

[0154] Physical properties Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 Thermal storage stability ◎ ◎ ○ ◎ ○ ◎ Painting workability (dischargeability) ○ ○ ○ ◎ ◎ ◎ Exterior (CF) ○ ◎ △ ○ ◎ ○ gloss ◎ ◎ ◎ ◎ ◎ ○ Adhesion ◎ ◎ ◎ ◎ ◎ ◎ water resistance ◎ ○ ◎ ○ ○ ○ Content ◎ △ ◎ ○ △ △ Impact resistance ◎ ◎ ○ ○ ◎ ◎ Chipping ◎ ○ ○ △ ◎ ◎ Pencil hardness ◎ ○ △ ○ △ △ Accelerated weathering ◎ ○ ○ △ ○ ◎ Repaint adhesion ◎ ◎ ◎ ◎ ◎ ◎ NV (%) 11.75 9.61 14.68 12.93 11.26 12.01

[0155] Physical properties Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Thermal storage stability X ○ X △ △ Painting workability (dischargeability) ○ ◎ ○ ◎ ◎ Exterior (CF) X △ X △ △ gloss ○ △ X △ △ Adhesion △ X X X △ water resistance X X X X X Content X X X X X Impact resistance △ △ X △ X Chipping X △ X △ X Pencil hardness X X X X △ Accelerated weathering X X X X X Repaint adhesion X X X X X NV (%) 11.68 11.68 11.45 11.07 12.86

[0156] Physical properties Comparative Example 6 Comparative Example 7 Comparative Example 8 Comparative Example 9 Thermal storage stability X △ ○ ○ Painting workability (dischargeability) ◎ ◎ △ X Exterior (CF) △ △ X - gloss X △ △ - Adhesion △ △ △ - water resistance △ △ △ - Content X X X - Impact resistance △ △ △ - Chipping △ △ △ - Pencil hardness △ △ △ - Accelerated weathering X X X - Repaint adhesion △ △ △ - NV (%) 11.44 11.44 17.38 31.69

[0157] As shown in Tables 7 to 10, the water-soluble topcoat paint compositions of the examples exhibit good or excellent properties in most physical properties, and average properties in only some physical properties. However, the comparative examples exhibited average or poor properties in most physical properties.

[0158] Various embodiments of the present invention are described below.

[0159] (1) A water-soluble topcoat paint composition comprising an acrylic emulsion resin, a melamine resin, a polyester resin, and a urethane resin, wherein the acrylic emulsion resin has a weight-average molecular weight of 200,000 to 300,000, the melamine resin has a glass transition temperature (Tg) of -80 to -30 ℃, the polyester resin has a hydroxyl value of 50 to 150 mgKOH / g, the urethane resin has a weight-average molecular weight of 20,000 to 50,000 and a glass transition temperature (Tg) of -55 to -15 ℃, and the solid content is 9 to 17 weight% based on the total weight of the paint composition.

[0160] (2) An acrylic emulsion resin has an acid value of 10 to 30 mgKOH / g, a glass transition temperature (Tg) of -10 to 5 ℃, and a hydroxyl value of 1 to 10 mgKOH / g, and the acrylic emulsion resin has a core-shell structure, with a core:shell volume ratio of 4:6 to 6:4, and is a water-soluble topcoat paint composition.

[0161] (3) A water-soluble topcoat composition in which the melamine resin has a weight-average molecular weight of 100 to 800, the polyester resin has an acid value of 1 to 30 mgKOH / g, a weight-average molecular weight of 3,000 to 10,000, a glass transition temperature (Tg) of -60 to -20 ℃, and the urethane resin has an acid value of 1 to 50 mgKOH / g.

[0162] (4) A water-soluble topcoat paint composition comprising, based on the total weight of the paint composition, 4 to 12 weight% acrylic emulsion resin, 0.1 to 3 weight% melamine resin, 0.1 to 5 weight% polyester resin, and 0.5 to 10 weight% urethane resin.

[0163] (5) A water-soluble topcoat paint composition comprising, based on the total weight of the paint composition, 0.1 to 3 weight% of a paraffin-based solvent, 0.1 to 10 weight% of an alcohol-based solvent, and 1 to 15 weight% of a pyrrolidone-based solvent.

[0164] (6) A water-soluble topcoat paint composition further comprising 2 to 10 weight percent of a wax emulsion or a polyurethane-based thickener as a flow control agent based on the total weight of the paint composition.

Claims

Claim 1 A water-soluble topcoat paint composition comprising an acrylic emulsion resin, a melamine resin, a polyester resin, and a urethane resin, wherein the acrylic emulsion resin has a weight-average molecular weight of 200,000 to 300,000, the melamine resin has a glass transition temperature (Tg) of -80 to -30 ℃, the polyester resin has a hydroxyl value of 50 to 150 mgKOH / g, the urethane resin has a weight-average molecular weight of 20,000 to 50,000 and a glass transition temperature (Tg) of -55 to -15 ℃, the solid content is 9 to 17 weight% based on the total weight of the paint composition, and further comprising, based on the total weight of the paint composition, 0.1 to 3 weight% of a paraffin-based co-modifier, 0.1 to 10 weight% of an alcohol-based co-modifier, and 1 to 15 weight% of a pyrrolidone-based co-modifier. Claim 2 A water-soluble topcoat paint composition according to claim 1, wherein the acrylic emulsion resin has an acid value of 10 to 30 mgKOH / g, a glass transition temperature (Tg) of -10 to 5 ℃, and a hydroxyl value of 1 to 10 mgKOH / g, and the acrylic emulsion resin includes a core-shell structure, wherein the volume ratio of the core to the shell is 4:6 to 6:

4. Claim 3 A water-soluble topcoat paint composition according to claim 1, wherein the melamine resin has a weight-average molecular weight of 100 to 800, the polyester resin has an acid value of 1 to 30 mgKOH / g, a weight-average molecular weight of 3,000 to 10,000, a glass transition temperature (Tg) of -60 to -20 ℃, and the urethane resin has an acid value of 1 to 50 mgKOH / g. Claim 4 A water-soluble topcoat paint composition according to claim 1, comprising, based on the total weight of the paint composition, 4 to 12 weight% of an acrylic emulsion resin, 0.1 to 3 weight% of a melamine resin, 0.1 to 5 weight% of a polyester resin, and 0.5 to 10 weight% of a urethane resin. Claim 5 delete Claim 6 A water-soluble topcoat paint composition according to claim 1, further comprising 2 to 10 weight percent of a wax emulsion or a polyurethane-based thickener as a flow control agent based on the total weight of the paint composition.

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