Transparent coating composition A transparent coating composition comprising a binder, a crosslinker, and

Abstract

The present invention relates to a transparent coating composition comprising a first polyester resin, a second polyester resin, a silicone-modified polyester resin, and a melamine resin, wherein the first polyester resin has a hydroxyl value of 200 to 300 mg KOH / g and a glass transition temperature of 15 to 30°C, and the second polyester resin has a hydroxyl value of 80 to 180 mg KOH / g and a glass transition temperature of 1 to 10°C.

Description

Technical Field

[0001] This invention relates to a transparent coating composition that has excellent reactivity and improved pinhole properties, resulting in a coating with excellent appearance properties and excellent mechanical properties, such as scratch resistance. Background Technology

[0002] The vehicle body undergoes various coating processes, such as electrodeposition coating, intermediate coating, primer coating, and clear coat, to improve appearance properties and protect the surface from external environmental influences. Specifically, electrodeposition paint and intermediate paint are typically applied to the vehicle body and cured, followed by the continuous application of a primer coating and a clear coat over the intermediate coating layer, then drying and curing. As mentioned above, in the case of primer and clear coats, a primer composition is typically applied and dried, followed by the application of a clear coat composition, and the clear coat and primer are cured together at 140 to 150°C.

[0003] Meanwhile, conventional transparent coating compositions are based on polyurethane curing systems comprising polyester resin, acrylic resin, and melamine curing agent. Furthermore, conventional transparent coating compositions, as described above, are widely used because the resulting coatings exhibit excellent appearance properties and superior scratch resistance. Specifically, Korean Patent Publication No. 2014-0125415 (Patent Document 1) discloses a one-component coating composition comprising a polyester resin containing hydroxyl groups and a melamine resin as a crosslinking agent reacting with the hydroxyl groups.

[0004] However, when the drying time after coating is short, the distance from the drying of the transparent coating to the curing oven is short, or the distance between the vehicle body is short, conventional transparent coating compositions have poor appearance characteristics of the manufactured coating, especially problems such as pinholes and poor gloss.

[0005] Therefore, there is a need to research and develop a transparent coating composition with excellent reactivity, thereby ensuring that the manufactured coating has excellent appearance characteristics and excellent mechanical properties, such as scratch resistance, even under conditions of short drying time after coating, short distance between the transparent coating and the curing oven after drying, or short distance between vehicle bodies. Summary of the Invention

[0006] Technical issues

[0007] The present invention aims to provide a transparent coating composition with excellent reactivity, thereby ensuring that the manufactured coating has excellent appearance characteristics and excellent mechanical properties, such as scratch resistance, even under conditions of short drying time after coating, short distance from the transparent coating to the curing oven after drying, or short distance between vehicle bodies.

[0008] Technical solution

[0009] This invention provides a transparent coating composition comprising a first polyester resin, a second polyester resin, a silicone-modified polyester resin, and a melamine resin.

[0010] The first polyester resin has a hydroxyl value of 200 to 300 mg KOH / g and a glass transition temperature of 15 to 30 °C.

[0011] The second polyester resin has a hydroxyl value of 80 to 180 mg KOH / g and a glass transition temperature of 1 to 10 °C.

[0012] In addition, the present invention provides a transparent coating kit comprising a coating composition and a curing agent.

[0013] Beneficial effects

[0014] The transparent coating composition according to the invention exhibits excellent reactivity, and therefore excellent appearance properties, because the resulting coating is free of pinholes and does not suffer from a loss of gloss, even under conditions of short drying times after coating, short distances between the dried transparent coating and the curing oven, or short distances between vehicle bodies. Furthermore, the coating manufactured from the transparent coating composition possesses excellent mechanical properties, such as hardness, adhesion, and scratch resistance, thus making it effective for painting vehicle bodies. Detailed Implementation

[0015] Best mode

[0016] The present invention will now be described in detail.

[0017] The “weight-average molecular weight” used in this specification is measured by common methods known in the art and can be measured by, for example, GPC (gel permeation chromatography).

[0018] In addition, the glass transition temperature is measured by common methods known in the art, and can be measured, for example, by differential scanning calorimetry (DSC).

[0019] Functional group values ​​such as “acid value” and “hydroxyl value” can be measured by methods known in the art and can be expressed as values ​​measured by, for example, titration.

[0020] Transparent coating composition

[0021] The transparent coating composition according to the present invention comprises a first polyester resin, a second polyester resin, a silicone-modified polyester resin, and a melamine resin. In this case, the first polyester resin has a higher hydroxyl value (OHv) and glass transition temperature than the second polyester resin. Since the transparent coating composition contains two polyester resins having different acid values ​​and hydroxyl values ​​as described above, it has the effect of improving the appearance properties and mechanical properties such as hardness, adhesion, and scratch resistance of the manufactured coating by controlling the drying rate and crosslinking density of the composition.

[0022] First polyester resin

[0023] The first polyester resin is the main resin in the composition and is used to impart coating molding properties.

[0024] The first polyester resin can be synthesized directly using known methods, or it can be a commercially available product. For example, the first polyester resin can be produced by reacting a first carboxylic acid with a first polyol. That is, the first polyester resin can be an unmodified polyester resin.

[0025] In this case, the first carboxylic acid may be, for example, at least one of the following: acetic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), cyclic fatty acids, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride (HHPA), and derivatives thereof.

[0026] The first polyol may be, for example, at least one selected from methoxy polyethylene glycol, 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), trimethylolpropane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylpentanediol.

[0027] The first polyester resin may have an acid value (Av) of 15 to 25 mg KOH / g or 18 to 22 mg KOH / g. If the acid value of the first polyester resin is within the above range, rapid hardening caused by heat treatment can be prevented, which has the effect of preventing poor coating appearance and reducing the occurrence of bubbles (bursting). If the acid value of the first polyester resin is less than the above range, there may be a problem of reduced hardness and appearance characteristics of the manufactured coating due to a slower curing reaction rate. If the acid value of the first polyester resin exceeds the above range, there may be a problem that the coating becomes brittle and scratch resistance decreases with increasing curing reaction rate, while water resistance decreases with increasing hydrophilicity.

[0028] Furthermore, the first polyester resin may have a hydroxyl value (OHv) of 200 to 300 mg KOH / g or 220 to 280 mg KOH / g. If the hydroxyl value of the first polyester resin is within the above range, it has the effect of improving the spreadability of the coating and improving the chemical resistance due to the polyurethane reaction. On the other hand, if the hydroxyl value of the first polyester resin is less than the above range, there may be a problem that the crosslinking density is insufficient due to insufficient reactivity with the melamine resin, resulting in reduced durability, adhesion characteristics during recoating, and solvent resistance of the manufactured coating. If the hydroxyl value of the first polyester resin exceeds the above range, there may be problems of over-curing and brittleness of the coating, thereby deteriorating its appearance and scratch resistance.

[0029] The first polyester resin may have a glass transition temperature (Tg) of 15 to 30°C or 18 to 25°C. If the glass transition temperature of the first polyester resin is within the above range, it exhibits excellent processability (spray-like feel) and imparts flexibility to the coating. If the glass transition temperature of the first polyester resin is below the above range, there may be problems with reduced drying speed and deterioration of the coating's mechanical properties. If the glass transition temperature of the first polyester resin exceeds the above range, there may be problems such as decreased appearance with increasing drying speed and reduced scratch resistance as the coating becomes brittle.

[0030] Furthermore, the first polyester resin may have a weight-average molecular weight (Mw) of 800 to 1,500 g / mol or 1,000 to 1,300 g / mol. If the weight-average molecular weight of the first polyester resin is within the above range, the smoothness of the coating is improved and it has the effect of forming a soft coating. If the weight-average molecular weight of the first polyester resin is less than the above range, there may be a problem that the mechanical properties of the manufactured coating deteriorate due to the low molecular weight. If the weight-average molecular weight of the first polyester resin exceeds the above range, there may be a problem that as the molecular weight increases, the fluidity decreases and the manufactured coating hardens, thereby reducing smoothness and scratch resistance.

[0031] Based on the total weight of the resin, the first polyester resin may have a solid content (NV) of 60% to 85% or 70% to 80% by weight. If the solid content of the first polyester resin is within the above range, it has the effect of reducing the total volatile organic compound (TVOC) content due to the high solid content. If the solid content of the first polyester resin is less than the above range, there is a problem of reduced curing reactivity due to the reduced solid content of the composition. If the solid content of the first polyester resin exceeds the above range, there may be a problem of poor processability due to the excessively high solid content in the coating, resulting in a reduced appearance characteristic of the manufactured coating.

[0032] Furthermore, based on the total weight of the transparent coating composition, the amount of the first polyester resin included can be 20% to 30% or 22% to 28% by weight. If the included first polyester resin is within the above-mentioned content range, it has the effect of improving appearance, scratch resistance, and coating smoothness. If the content of the first polyester resin in the composition is less than the above-mentioned range, there are problems with reduced crosslinking density and deterioration of mechanical properties and appearance. If the content of the first polyester resin in the composition exceeds the above-mentioned range, the viscosity of the composition may increase excessively, resulting in poor pinhole characteristics, thereby reducing processability and drying performance.

[0033] Second polyester resin

[0034] The second polyester resin is used to improve the appearance properties of the manufactured coating.

[0035] The second polyester resin can be synthesized directly using known methods, or it can be a commercially available product. For example, the second polyester resin can be produced by reacting a second carboxylic acid with a second polyol. That is, the second polyester resin can be an unmodified polyester resin.

[0036] In this case, the second carboxylic acid may be, for example, at least one of the following: acetic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), cyclic fatty acids, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride (HHPA), isononanoic acid (INA), and derivatives thereof.

[0037] The second polyol may be selected from at least one of 1,6-hexanediol (1,6-HD), pentaerythritol, sorbitol, methoxy polyethylene glycol, neopentyl glycol (NPG), trimethylolpropane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylpentanediol.

[0038] The second polyester resin can have an acid value (Av) of 20 to 30 mg KOH / g or 20 to 25 mg KOH / g. If the acid value of the second polyester resin is within the above range, rapid curing caused by heat treatment can be prevented, which has the effect of preventing poor coating appearance and reducing the occurrence of bubbles (bursting). If the acid value of the second polyester resin is less than the above range, there may be a problem of reduced hardness and appearance characteristics of the manufactured coating due to a slower curing reaction rate. If the acid value of the second polyester resin exceeds the above range, there may be a problem of decreased water resistance due to increased hydrophilicity.

[0039] Furthermore, the second polyester resin may have a hydroxyl value (OHv) of 80 to 180 mg KOH / g or 100 to 150 mg KOH / g. If the hydroxyl value of the second polyester resin is within the above range, it has the effect of improving the spreadability of the coating and improving the chemical resistance caused by the polyurethane reaction. On the other hand, if the hydroxyl value of the second polyester resin is less than the above range, there may be a problem that the appearance, durability, and chemical resistance of the manufactured coating are reduced due to insufficient crosslinking density with the melamine resin as a curing agent. If the hydroxyl value of the second polyester resin exceeds the above range, there may be problems of over-curing and coating brittleness, thereby deteriorating its paint processability and cold chipping resistance.

[0040] The second polyester resin may have a glass transition temperature (Tg) of 1 to 10°C or 3 to 7°C. If the glass transition temperature of the second polyester resin is within the above range, it exhibits excellent processability (spray-like feel) and imparts flexibility to the coating. If the glass transition temperature of the second polyester resin is below the above range, there may be problems with reduced drying speed and deterioration of the coating's mechanical properties. If the glass transition temperature of the second polyester resin exceeds the above range, there may be problems such as decreased appearance with increasing drying speed and reduced scratch resistance as the coating becomes brittle.

[0041] Furthermore, the second polyester resin may have a weight-average molecular weight (Mw) of 800 to 1,600 g / mol or 1,000 to 1,500 g / mol. If the weight-average molecular weight of the second polyester resin is within the above range, the smoothness of the coating is improved and it has the effect of forming a soft coating. If the weight-average molecular weight of the second polyester resin is less than the above range, there may be a problem that the mechanical properties of the manufactured coating deteriorate due to the low molecular weight. If the weight-average molecular weight of the second polyester resin exceeds the above range, there may be a problem that as the molecular weight increases, the fluidity decreases and the manufactured coating hardens, thereby reducing smoothness and scratch resistance.

[0042] Based on the total weight of the resin, the second polyester resin may have a solids content (NV) of 60% to 80% or 65% to 75% by weight. If the solids content of the second polyester resin is within the above range, it has the effect of reducing the total volatile organic compound (TVOC) content due to the high solids content. If the solids content of the second polyester resin is less than the above range, there is a problem of reduced curing reactivity due to the reduced solids content of the coating. If the solids content of the second polyester resin exceeds the above range, there may be a problem of reduced appearance characteristics of the manufactured coating due to poor processability of the coating.

[0043] Furthermore, based on the total weight of the transparent coating composition, the amount of the second polyester resin included can be 15% to 25% or 17% to 23% by weight. If the included second polyester resin is within the above-mentioned content range, it has the effect of improving appearance, scratch resistance, and coating smoothness. If the content of the second polyester resin in the composition is less than the above-mentioned range, there is a problem of reduced crosslinking density of the coating, resulting in poor adhesion and scratch resistance during recoating. If the content of the second polyester resin in the composition exceeds the above-mentioned range, there is a problem that as viscosity increases excessively, processability and drying performance decrease, thus deteriorating the appearance characteristics of the manufactured coating.

[0044] Organosilicon modified polyester resin

[0045] Silicone-modified polyester resins are highly reactive and improve appearance properties by preventing pinholes in the manufactured coatings.

[0046] The silicone-modified polyester resin can be synthesized directly by known methods, or it can be a commercially available product, and it can be obtained by modifying a polyester resin prepared by reacting a third carboxylic acid and a third polyol with an organopolysiloxane.

[0047] In this case, the third carboxylic acid may be, for example, at least one of the following: acetic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), cyclic fatty acids, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride (HHPA), isononanoic acid (INA), and derivatives thereof.

[0048] The third polyol may be selected from at least one of 1,6-hexanediol (1,6-HD), pentaerythritol, sorbitol, methoxy polyethylene glycol, neopentyl glycol (NPG), trimethylolpropane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylpentanediol.

[0049] Organopolysiloxanes may contain functional groups and non-functional organic groups. For example, organopolysiloxanes may contain at least one functional group selected from silanols and alkoxy groups, and may contain at least one non-functional organic group selected from methyl, propyl, and phenyl groups. Alkoxy groups may be, for example, methoxy, ethoxy, and butoxy groups.

[0050] Silicone-modified polyester resins can have an acid value (Av) of 10 mg KOH / g or lower, 8 mg KOH / g or lower, or 1 to 8 mg KOH / g. If the acid value of the silicone-modified polyester resin is less than the above range, the appearance properties of the resulting coating may be reduced due to the high surface tension of the coating. If the acid value of the silicone-modified polyester resin exceeds the above range, the mechanical properties of the resulting coating may be deteriorated due to the excessively low surface tension of the coating.

[0051] Furthermore, silicone-modified polyester resins can have hydroxyl values ​​(OHv) of 150 to 250 mg KOH / g or 190 to 210 mg KOH / g. If the hydroxyl value of the silicone-modified polyester resin is less than the above range, there may be a problem: insufficient crosslinking density due to insufficient reactivity with melamine resin, resulting in reduced durability and chemical resistance of the manufactured coating. If the hydroxyl value of the silicone-modified polyester resin exceeds the above range, there may be problems with over-curing and coating brittleness, thereby deteriorating its appearance properties and scratch resistance.

[0052] Silicone-modified polyester resins can have glass transition temperatures (Tg) ranging from -20 to 0°C or from -15 to -5°C. If the glass transition temperature of the silicone-modified polyester resin is below these ranges, problems may arise such as decreased paint hardness due to increased elasticity, and reduced coating appearance due to poor drying performance. If the glass transition temperature of the silicone-modified polyester resin exceeds these ranges, problems may arise such as decreased paint flowability, deteriorated coating appearance, and reduced coating elasticity, leading to decreased adhesion and shatter resistance.

[0053] The viscosity of the silicone-modified polyester resin at 25°C can be 5,000 to 10,000 cps or 6,000 to 9,000 cps. If the viscosity of the silicone-modified polyester resin at 25°C is less than the above range, there may be a problem that the viscosity of the composition is too low, resulting in reduced adhesion and scratch resistance of the coating due to incomplete formation. If the viscosity of the silicone-modified polyester resin at 25°C exceeds the above range, there may be a problem that the appearance properties of the coating are deteriorated due to insufficient processability of the composition.

[0054] Furthermore, based on the total weight of the resin, the silicone-modified polyester resin can have a solids content (NV) of 80% or greater, or 85% to 95% by weight. If the solids content of the silicone-modified polyester resin is less than the above range, there may be a problem of reduced curing reactivity due to the reduced solids content of the composition. If the solids content of the silicone-modified polyester resin exceeds the above range, there may be a problem of poor processability of the manufactured coating and deterioration of the appearance properties of the coating.

[0055] Furthermore, based on the total weight of the transparent coating composition, the amount of silicone-modified polyester resin included can be 2% to 10% or 4% to 8% by weight. If the included silicone-modified polyester resin is within the above range, it improves appearance, scratch resistance, and coating smoothness. If the content of polyester resin in the composition is less than the above range, the crosslinking density may decrease, leading to a reduction in appearance and scratch resistance. If the content of polyester resin in the composition exceeds the above range, the viscosity of the composition may become too high (which reduces processability and drying performance), resulting in a deterioration in appearance and mechanical properties.

[0056] melamine resin

[0057] Melamine resin is a curing agent used to harden a composition by crosslinking with the components of a transparent coating composition.

[0058] Melamine resin can be synthesized directly using known methods, or it can be a commercially available product. For example, melamine resin can be at least one selected from methoxymethyl melamine, methyl melamine, butyl melamine, isobutoxy melamine, butoxy melamine, hexahydroxymethyl melamine, hexamethoxymethyl melamine, hexabutoxymethyl melamine, hexamethoxybutoxymethyl melamine, and iminomethoxymethyl melamine.

[0059] Furthermore, melamine resins can have a weight-average molecular weight (Mw) of 100 to 5,000 g / mol, 500 to 4,500 g / mol, or 1,000 to 4,000 g / mol. If 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 by increasing the crosslinking density. Conversely, if the weight-average molecular weight of the melamine resin is less than the above range, there may be problems with reduced crosslinking density and decreased chemical resistance and scratch resistance of the coating. If the weight-average molecular weight of the melamine resin exceeds the above range, there may be problems with the appearance properties of the coating deteriorating due to the increased molecular weight.

[0060] The viscosity of melamine resin at 25°C can be 400 to 1,200 cps, 500 to 1,100 cps, or 600 to 1,000 cps. If the viscosity of the melamine resin at 25°C is within these ranges, the resulting coating will have excellent gloss and appearance. Conversely, if the viscosity of the melamine resin at 25°C is less than these ranges, the paint viscosity may be low, preventing coating formation and reducing the coating's adhesion and scratch resistance. If the viscosity of the melamine resin at 25°C exceeds these ranges, the composition may have poor processability, resulting in insufficient appearance characteristics of the resulting coating.

[0061] Additionally, melamine resins can have an acid value (Av) of 5 mg KOH / g or lower, or 0.1 to 3 mg KOH / g. If the acid value of the melamine resin is less than the above range, there may be a decrease in the curing reaction rate, and consequently, a decrease in the hardness and appearance properties of the resulting coating. If the acid value of the melamine resin exceeds the above range, there may be a decrease in the water resistance of the coating due to increased hydrophilicity.

[0062] Based on the total weight of the resin, melamine resin can have a solids content (NV) of 50% to 70% or 55% to 60% by weight. If the solids content of the melamine resin is less than the above range, there may be a problem with poor processability of the coating. If the solids content of the melamine resin exceeds the above range, there may be a problem with reduced processability due to the high solids content during the coating production process.

[0063] Furthermore, based on the total weight of the transparent coating composition, the amount of melamine resin included can be 5% to 15% or 7% to 13% by weight. If the melamine resin included is within the above-mentioned range, it has the effect of improving the adhesion and hardness of the manufactured coating by increasing the crosslinking density. If the melamine resin content in the composition is less than the above-mentioned range, there may be problems with reduced mechanical properties and chemical resistance of the coating due to decreased curability. If the melamine resin content in the composition exceeds the above-mentioned range, there may be problems with the coating becoming brittle and having poor adhesion and crack resistance due to over-curing.

[0064] The transparent coating composition may also contain a solvent.

[0065] solvent

[0066] Solvents play a role in controlling the viscosity of the composition, improving drying performance, and enhancing the appearance and spreadability of the resulting coating.

[0067] The solvent is not particularly limited, as long as it is commonly used in the transparent coating composition, and for example, it can be at least one selected from aromatic solvents, acetate-based solvents, alcohol-based solvents, and propionate-based solvents. Specifically, the solvent may include aromatic solvents such as toluene and xylene; acetate-based solvents such as 1-methoxy-2-propyl acetate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methyl glutarate, methyl succinate, methyl adipate, dimethyl glutarate, dimethyl succinate, dimethyl adipate, butyl carbitol acetate, butyl cellosolve acetate, propylene glycol methyl ether acetate (PMA), and trimethyl o-acetate; alcohol-based solvents such as n-butanol, propanol, 1-methoxy-2-propanol, and 2-butoxyethanol; ketone-based solvents such as acetone, methyl ethyl ketone, methyl butyl ketone, and methyl isobutyl ketone; and propionate-based solvents such as ethyl ethoxypropionate. In addition, commercially available aromatic solvents include Cocosol#100, Cocosol#150, etc.

[0068] Furthermore, based on the total weight of the transparent coating composition, the amount of solvent contained can be 5% to 40% or 10% to 35% by weight. If the contained solvent is within the above range, the viscosity of the composition is appropriately adjusted, thereby improving processability and drying properties. Conversely, if the solvent content is less than the above range, there may be a problem of insufficient processability of the composition due to its high solids content. If the solvent content exceeds the above range, the solids content of the manufactured coating is low, resulting in insufficient appearance and adhesion of the manufactured coating.

[0069] additive

[0070] The transparent coating composition according to the invention may also contain additives such as anti-drip agents, ultraviolet absorbers, dispersants, surface conditioners, defoamers, and curing catalysts. In this case, there are no particular limitations on the additives, as long as they are commonly added to the coating composition.

[0071] In addition, there are no particular restrictions on the amount of additives, as long as they fall within the range that can generally be included in a transparent coating composition, and for example, the amount of additives included by weight can be 10% to 30% or 15% to 25% based on the total weight of the transparent coating composition.

[0072] The clear coating composition can have a solid content of 40% to 60% or 45% to 55% by weight. If the solid content of the clear coating composition is within this range, there is an advantage that the coating processability of the composition becomes appropriate.

[0073] Furthermore, based on Ford Cup 4, the clear coat composition can have a viscosity of 15 to 50 seconds or 20 to 35 seconds at 25°C. If the viscosity of the clear coat composition at 25°C is less than the above range, problems such as dripping from vertical surfaces may occur, resulting in insufficient processability. If the viscosity of the clear coat composition at 25°C exceeds the above range, the appearance characteristics of the coating made from it may be reduced due to the high viscosity of the composition, or it may put a load on the spraying machine, leading to spraying machine failure.

[0074] As described above, the transparent coating composition according to the present invention has excellent appearance characteristics because the resulting coating is free of pinholes and does not lose gloss, even under conditions of short drying time after coating, short distance between the dried transparent coating and the curing oven, or short distance between vehicle bodies. Furthermore, the coating made from the transparent coating composition has excellent mechanical properties, such as hardness, adhesion, and scratch resistance, and can be effectively used for painting vehicle bodies.

[0075] Transparent paint kit

[0076] Furthermore, the transparent coating kit of the present invention comprises the transparent coating composition as described above; and a curing agent. That is, the transparent coating kit of the present invention can be a two-part coating.

[0077] The clear coat kit can be used after mixing the clear coating composition and hardener before application.

[0078] curing agent

[0079] The curing agent functions to form a coating by reacting with the hydroxyl groups of the resin as described above to form urethane bonds and to harden the composition.

[0080] The curing agent may contain isocyanate-based compounds. In this case, there are no particular limitations on the type of isocyanate-based compound, and it may include, for example, aliphatic polyisocyanate compounds that do not exhibit yellowing. Specifically, the isocyanate-based compound may include hexamethylene diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, tetramethylxylene diisocyanate, or mixtures thereof.

[0081] Additionally, based on the total weight of the curing agent, the curing agent may have an unreacted isocyanate group content (NCO%) of 15% to 30% or 19% to 24% by weight. If the NCO% of the curing agent is less than the above range, there may be a decrease in the crosslinking density of the coating, thus reducing the coating's impact resistance, water resistance, and cold crack resistance. If the NCO% of the curing agent exceeds the above range, there may be an increase in curing speed and resin agglomeration, thereby reducing the coating's appearance characteristics, gloss, and mechanical properties.

[0082] A clear coat kit may contain a clear coating composition and a hardener in a weight ratio of 2 to 4:1 or 2.5 to 3.4:1. If the weight ratio of the clear coating composition to the hardener is less than the above range, i.e., if it contains a small amount of clear coating composition relative to the weight of the hardener, unreacted isocyanate groups may appear in the coating after curing, resulting in a deterioration in the physical properties of the coating. If the weight ratio of the clear coating composition to the hardener exceeds the above range, i.e., if it contains an excessive amount of clear coating composition relative to the weight of the hardener, the cured density may decrease due to a lack of reactivity in the composition.

[0083] Clear coat kits can have a solids content of 50% to 70% or 55% to 65% by weight. If the solids content of the clear coat kit falls within this range, there is an advantage that the coating processability of the composition becomes suitable.

[0084] Furthermore, based on Ford Cup 4, the clear coat kit can have a viscosity of 20 to 50 seconds or 25 to 45 seconds at 25°C. If the viscosity of the clear coat kit at 25°C is less than the above range, problems such as dripping from vertical surfaces may occur. If the viscosity of the clear coat kit at 25°C exceeds the above range, the high viscosity of the paint may reduce the appearance characteristics of the coating or may put a load on the sprayer, leading to sprayer malfunction.

[0085] As described above, the transparent coating kit according to the invention has excellent reactivity and therefore excellent appearance characteristics because no pinholes appear in the manufactured coating and the gloss is not lost, even in cases where the drying time after coating is short, the distance from the transparent coating to the curing oven is short, or the distance between vehicle bodies is short.

[0086] The invention will now be described in more detail through embodiments. However, these embodiments are intended only to aid in understanding the invention, and the scope of the invention is not in any way limited to these embodiments.

[0087] Examples 1 to 21 and Comparative Examples 1 to 11. Preparation of transparent coating compositions.

[0088] A transparent coating composition with a viscosity of 28 seconds at 25°C was prepared by mixing the components in the same amounts as shown in Tables 1 to 3.

[0089] [Table 1]

[0090]

[0091]

[0092] Table 2:

[0093]

[0094]

[0095] [Table 3]

[0096]

[0097] The manufacturers, product names, or ingredient names of the components used in the comparative examples and embodiments are shown in Table 4 below: [Table 4]

[0098]

[0099]

[0100] Reference example. Preparation of curing agent.

[0101] The curing agent was prepared by mixing 49 wt.% hexamethylene diisocyanate (HMDI) as an isocyanate-based resin, 15 wt.% Cocosol-100 as an organic solvent, 17 wt.% xylene and 19 wt.% butyl acetate.

[0102] The prepared curing agent contained 21 wt.% unreacted isocyanate groups (NCO%).

[0103] Experimental Example: Evaluation of Coating Properties

[0104] A primer paint (manufacturer: KCC, product name: FU2290) was applied to the sample and cured at 140°C for 20 minutes to form a 40μm thick primer coating. Then, a basecoat (manufacturer: KCC, product name: WT3062) was applied in a bell shape over the primer coating and heated with 80°C hot air for 3 minutes to evaporate any residual moisture in the paint, thus forming a 15μm thick basecoat.

[0105] Then, the transparent coating compositions prepared in the examples and comparative examples were mixed with the curing agent in the reference example at a weight ratio of 3:1 to prepare a transparent coating on the base layer. The transparent coating was then applied and cured at 140°C for 25 minutes to form a transparent coating with a thickness of 40 μm, thereby preparing the final coating. The physical properties of the above samples were measured as follows, and the results are shown in Table 5.

[0106] (1) Pinhole characteristics

[0107] Apply a transparent coating with a thickness of 10 to 80 μm at 5 μm intervals in the same manner as described above, and measure the point where the first pinhole appears.

[0108] Specifically, a pinhole at a thickness of 60 μm or greater is rated as excellent (◎), a pinhole at a thickness of 50 μm or greater but less than 60 μm is rated as good (○), a pinhole at a thickness of 40 μm or greater but less than 50 μm is rated as normal (△), and a pinhole at a thickness of less than 40 μm is rated as defective (×).

[0109] (2) Appearance

[0110] The gloss (LU), sharpness (SH), and orange peel (OP) of the final coating were measured using a Wave Scan DOI (BYK Gardner) automotive exterior measurement instrument, and the overall appearance evaluation value (CF) was calculated using the measured physical properties using Equation 1 below.

[0111] [Equation 1]

[0112] CF=LU×0.15+SH×0.35+OP×0.5

[0113] In this case, the CF is measured and calculated in both the transverse and longitudinal directions.

[0114] If the calculated CF is 65 or greater, it is rated as excellent (◎); if the calculated CF is 60 or greater but less than 65, it is rated as good (○); if the calculated CF is 55 or greater but less than 60, it is rated as normal (△); and if the calculated CF is less than 55, it is rated as defective (×).

[0115] (3) Gloss

[0116] For the final coating, the gloss at 20° is measured using a gloss meter. If the gloss at 20° is 91 or greater, it is rated as excellent (◎); if the gloss at 20° is 89 or greater but less than 91, it is rated as good (○); if the gloss at 20° is 87 or greater but less than 89, it is rated as normal (△); and if the gloss at 20° is less than 87, it is rated as defective (×).

[0117] (4) Adhesion properties during recoating

[0118] For the final coating, the base coat and clear coat were applied in the same manner as described above, and then the adhesion properties during recoating were evaluated using the checkerboard method.

[0119] Specifically, in the checkerboard method, 100 squares, each 2mm wide and 2mm high, are made on the surface of the final coating with a knife. The squares are then removed with tape to measure adhesion. In this case, if all 100 squares are 100% fully adhered, the measured adhesion is rated as excellent (◎); if more than 70% but less than 100% of the remaining squares remain, the measured adhesion is rated as good (○); if 50% or more but less than 70% of the remaining squares remain, the measured adhesion is rated as normal (△); and if less than 50% of the remaining squares remain, the measured adhesion is rated as defective (×).

[0120] (5) Scratch resistance

[0121] The initial 20° gloss of the final coating was measured using a polishing machine, and the 20° gloss was measured again after 10 cycles of surface treatment using an Amtec Kistler (car wash machine) while simultaneously spraying quartz powder and water. The gloss retention rate after surface treatment was calculated based on the gloss before surface treatment.

[0122] At this point, if the gloss retention rate is 70% or greater, it is rated as excellent (◎); if the gloss retention rate is 60% or greater but less than 70%, it is rated as good (○); if the gloss retention rate is 55% or greater but less than 60%, it is rated as normal (△); and if the gloss retention rate is less than 55%, it is rated as defective (×).

[0123] [Table 5]

[0124]

[0125]

[0126] As shown in Table 5, the coatings made from the compositions in the examples were found to have excellent pinhole characteristics, appearance characteristics, gloss, adhesion during recoating, and scratch resistance.

[0127] On the other hand, Comparative Example 1, which does not contain the first polyester resin, lacks pinhole properties and scratch resistance.

[0128] Comparative Example 2, which does not contain the second polyester resin, lacks appearance characteristics and gloss.

[0129] In addition, Comparative Example 3, which does not contain silicone-modified polyester resin, lacks pinhole characteristics and appearance characteristics.

[0130] Comparative Example 4, which contains a first polyester resin-4 with a low hydroxyl value, and Comparative Example 10, which contains a second polyester resin-8 with a low glass transition temperature, lack adhesive properties and scratch resistance during recoating.

[0131] Furthermore, Comparative Example 5, which contains a first polyester resin-5 with a high hydroxyl value, and Comparative Example 9, which contains a second polyester resin-5 with a high hydroxyl value, lacked appearance properties and scratch resistance. In particular, Comparative Example 9 also lacked gloss and adhesion properties during recoating.

[0132] Comparative Example 6, which contains a first polyester resin-8 with a low glass transition temperature, and Comparative Example 8, which contains a second polyester resin-4 with a low hydroxyl value, lacks pinhole properties, adhesion properties during recoating, and scratch resistance.

[0133] Furthermore, Comparative Example 7, which contains a first polyester resin-9 with a high glass transition temperature, and Comparative Example 11, which contains a second polyester resin-9 with a high glass transition temperature, lack pinhole characteristics, appearance characteristics, and scratch resistance.

Claims

1. A transparent coating composition, wherein, based on the total weight of the composition, it comprises 22 to 28 wt.% of a first polyester resin, 17 to 23 wt.% of a second polyester resin, 4 to 8 wt.% of a silicone-modified polyester resin, and 7 to 13 wt.% of a melamine resin. in, The first polyester resin has a hydroxyl value of 220 to 280 mg KOH / g and a glass transition temperature of 18 to 25 °C, and a weight-average molecular weight of 800 to 1,500 g / mol. The second polyester resin has a hydroxyl value of 100 to 150 mg KOH / g, a glass transition temperature of 3 to 7 °C, and a weight-average molecular weight of 800 to 1,600 g / mol.

2. The transparent coating composition according to claim 1, wherein the first polyester resin has an acid value of 15 to 25 mg KOH / g, and the second polyester resin has an acid value of 20 to 30 mg KOH / g.

3. The transparent coating composition according to claim 1, wherein the silicone-modified polyester resin has an acid value of 10 mg KOH / g or lower, a hydroxyl value of 150 to 250 mg KOH / g, a glass transition temperature of -20 to 0°C, and a viscosity of 5,000 to 10,000 cps at 25°C.

4. A clear coating kit comprising any one of the compositions of claims 1 to 3 and a curing agent.

5. The clear coating kit of claim 4, wherein it comprises the composition and the curing agent in a weight ratio of 2 to 4:1; and the curing agent comprises an isocyanate-based compound.

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