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 acrylic resin, a second acrylic resin, a polyester resin, a silicone-modified polyester resin, and a melamine resin, wherein the first acrylic resin has a glass transition temperature of 25 to 55°C and the second acrylic resin has a glass transition temperature of 1 to 20°C.

Description

Technical Field

[0001] This invention relates to a transparent coating composition having excellent appearance properties, antifouling properties, adhesion properties during recoating, and solvent resistance of the resulting coating film. Background Technology

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

[0003] Furthermore, conventional transparent coating compositions are based on urethane curing systems, including polyester resins, acrylic resins, melamine curing agents, or block isocyanate-based curing agents. Moreover, the reason why conventional transparent coating compositions, as described above, are widely used is that the resulting coatings exhibit excellent appearance properties and excellent scratch resistance. Specifically, Korean Patent No. 1655621 (Patent Document 1) discloses a transparent coating composition comprising two types of acrylic polyol resins, polyester polyol resins, reactive silicone additives, and a two-component isocyanate curing agent.

[0004] However, the conventional transparent coating composition in Patent Document 1 lacks the antifouling properties of the resulting coating film, thus causing oil and / or dirt to adhere to the surface due to external and / or process contamination, resulting in limitations such as poor appearance characteristics and lack of smoothness in the film portion.

[0005] Therefore, there is a need to research and develop a transparent coating composition that can produce a coating with excellent smoothness and scratch resistance in the thin film portion, excellent anti-fouling properties due to low surface tension, and excellent slip properties, thus exhibiting excellent anti-fouling properties in processes such as sanding powder. Summary of the Invention

[0006] [Technical Issues]

[0007] The present invention aims to provide a transparent coating composition capable of preparing a coating having excellent smoothness and scratch resistance in the thin film portion of the coating, excellent antifouling properties against external contaminants due to low surface tension, and excellent smoothness, thereby exhibiting excellent anti-contamination properties in processes such as sandblasting.

[0008] [Technical Solution]

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

[0010] The glass transition temperature of the first acrylic resin is between 25°C and 55°C.

[0011] The glass transition temperature of the second acrylic resin is 1°C to 20°C.

[0012] [Beneficial Effects]

[0013] The transparent coating composition according to the invention comprises a silicone-modified polyester resin and two types of acrylic resins with different glass transition temperatures, thus improving the smoothness and scratch resistance of the thin film portion of the manufactured coating. Furthermore, the coating produced from the transparent coating composition exhibits excellent oil resistance to external contaminants due to its low surface tension and excellent smoothness, thus providing excellent resistance to contaminants that may arise during processes such as sandblasting. Detailed Implementation

[0014] The present invention will be described in detail below.

[0015] The “weight-average molecular weight” used in this specification is measured by methods commonly known in the art, and can be measured by methods such as GPC (gel permeation chromatography).

[0016] Furthermore, the glass transition temperature is measured using methods commonly known in the art, and can be measured, for example, by differential scanning calorimetry (DSC).

[0017] Functional group values ​​such as "acid value" and "hydroxyl value" can be measured by methods known in the art, and can be measured by, for example, titration.

[0018] Furthermore, as used in this specification, the term "(meth)acrylic acid" refers to "acrylic acid" and / or "methacrylic acid", while the term "(meth)acrylate" refers to "acrylate" and / or "methacrylate".

[0019] The transparent coating composition of the present invention comprises a first acrylic resin, a second acrylic resin, a polyester resin, a silicone-modified polyester resin, and a melamine resin. The first and second acrylic resins have different glass transition temperatures. Therefore, the transparent coating composition of the present invention, comprising two types of acrylic resins with different glass transition temperatures, has the effect of improving appearance by controlling the drying speed of the coating film. Furthermore, the transparent coating composition of the present invention, by using both polyester resin and silicone-modified polyester resin together and thereby controlling surface tension, has the effect of improving appearance properties and scratch resistance. In addition, the transparent coating composition of the present invention contains a polysiloxane surface conditioner, thus having the effect of improving oil resistance to external contaminants due to the low surface tension of the manufactured coating film, and improving resistance to dirt during processes such as sandblasting due to improved smoothness.

[0020] First acrylic resin

[0021] Firstly, acrylic resins play a role in improving the physical properties of coatings, such as durability and appearance.

[0022] The first acrylic resin can be synthesized directly using known methods, or it can be a commercially available product. Specifically, the first acrylic resin can be prepared by polymerizing a first vinyl monomer and a first (meth)acrylate monomer.

[0023] There is no particular limitation on the type of the first vinyl monomer, but for example, at least one selected from the group consisting of: styrene, methylstyrene, dimethylstyrene, fluorostyrene, ethoxystyrene, methoxystyrene, phenyl vinyl ketone, tert-butylbenzoate, vinyl cyclohexanoate, vinyl acetate, vinylpyrrolidone, vinyl chloride, vinyl alcohol, acetoxystyrene, tert-butylstyrene, and vinyltoluene.

[0024] The first (meth)acrylate monomer may include at least one selected from the group consisting of (meth)acrylate monomers without hydroxyl groups and (meth)acrylate monomers containing hydroxyl groups.

[0025] For example, a hydroxyl-free (meth)acrylate monomer may include at least one selected from the group consisting of: (meth)acrylic acid, meth(meth)acrylic acid, (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, isooctyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecanyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, isobornyl (meth)acrylate, and lauryl (meth)acrylate.

[0026] The hydroxyl-containing (meth)acrylate monomer can be, for example, a hydroxyalkyl-containing (meth)acrylate, and specifically can include at least one selected from the group consisting of 2-hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and 2-hydroxybutyl (meth)acrylate.

[0027] For example, free radical polymerization can be used to manufacture the first acrylic resin, and the physical properties of the first acrylic resin, such as weight-average molecular weight (Mw), hydroxyl value (OHv), and acid value (Av), can be adjusted according to the initiator and polymerization time.

[0028] The first acrylic resin and the second acrylic resin have different glass transition temperatures. As described above, by including two types of acrylic resins with different glass transition temperatures, the transparent coating composition has the effect of maintaining an appropriate drying rate, thereby improving the appearance properties of the manufactured coating film.

[0029] For example, the first acrylic resin may have a higher glass transition temperature than the second acrylic resin. Specifically, the first acrylic resin may have a glass transition temperature that is 20°C to 40°C or 24°C to 35°C higher than that of the second acrylic resin. If the difference between the glass transition temperatures of the first and second acrylic resins is less than the above range, that is, if the glass transition temperature of the first acrylic resin is the same as or similar to that of the second acrylic resin, there is a problem of reduced drying speed and deteriorated mechanical properties of the resulting coating. Furthermore, if the difference between the glass transition temperatures of the first and second acrylic resins exceeds the above range, that is, if the glass transition temperature of the first acrylic resin is much higher than that of the second acrylic resin, there is a problem of increased drying and reaction rates, resulting in a brittle coating and reduced appearance and scratch resistance.

[0030] The glass transition temperature (Tg) of the first acrylic resin can be between 25°C and 55°C, or between 35°C and 45°C. If the glass transition temperature of the first acrylic resin is within this range, it improves the gloss properties of the coating film. However, if the glass transition temperature of the first acrylic resin is below this range, the drying speed increases, resulting in a brittle coating film and insufficient scratch resistance. If the glass transition temperature of the first acrylic resin exceeds this range, the drying speed is too slow, and the appearance and hardness of the coating film may become insufficient.

[0031] Furthermore, the first acrylic resin may have a hydroxyl value (OHv) of 50 to 100 mg KOH / g or 75 to 95 mg KOH / g. If the hydroxyl value of the first acrylic resin is within the above range, the curability of the composition containing this resin can be improved. However, if the hydroxyl value of the first acrylic resin is less than the above range, there may be a problem of insufficient coating formation through the crosslinking reaction with the melamine resin as a curing agent, and therefore the mechanical properties of the coating (such as hardness) deteriorate. If the hydroxyl value of the first acrylic resin exceeds the above range, there may be problems such as: the coating becoming brittle due to over-curing, thereby reducing elasticity; insufficient water resistance and weather resistance of the composition; and increased viscosity, thereby reducing processability.

[0032] The first acrylic resin can have a weight-average molecular weight (Mw) of 6000 to 10000 g / mol, 7000 to 9000 g / mol, or 7500 to 8500 g / mol. If the weight-average molecular weight of the first acrylic resin is within the above range, the long-term physical properties of the manufactured coating, such as durability and weather resistance, can be excellent. However, if the weight-average molecular weight of the first acrylic resin is less than the above range, there may be a problem of low molecular weight, resulting in insufficient acid resistance, water resistance, and cold cracking resistance of the manufactured coating. If the weight-average molecular weight of the first acrylic resin exceeds the above range, there may be a problem that the fluidity of the paint decreases with increasing molecular weight, leading to a decrease in the appearance and mechanical properties of the coating.

[0033] Furthermore, the acid value (Av) of the first acrylic resin can be 7 mg KOH / g or less, 5 mg KOH / g or less, or 0.1 to 5 mg KOH / g. If the acid value of the first acrylic resin is within the above range, the appearance properties of the resulting coating can be improved by controlling the reactivity of the composition containing the resin. If the acid value of the first acrylic resin is less than the above range, there may be a problem of reduced curing reaction rate and deterioration of the appearance of the resulting coating. If the acid value of the first acrylic resin exceeds the above range, there may be a problem that the viscosity of the composition increases with the increase of resin cohesion, thereby reducing processability and storage properties at room temperature.

[0034] Based on the total weight of the resin, the first acrylic resin can have a solids content (NV) of 60 to 80 wt%, 65 to 75 wt%, or 68 to 73 wt%. If the solids content of the first acrylic resin is within the above ranges, the storage stability of the resin and the coating composition can be improved, and the processability is excellent. Furthermore, if the solids content of the first acrylic resin is less than the above ranges, the viscosity may become too low, resulting in insufficient processability of the transparent coating composition containing it. If the solids content of the first acrylic resin exceeds the above ranges, the viscosity of the first acrylic resin becomes too high, thereby reducing stability during the reaction, potentially worsening dispersion stability, and therefore causing aggregation over time.

[0035] Furthermore, based on the total weight of the transparent coating composition, the content of the acrylic resin can be 10 to 20 wt%, or 12 to 18 wt%. If the content of the first acrylic resin is less than the above range, there may be problems with reduced drying performance and reduced gloss, water resistance, and impact resistance of the coating film made from the transparent coating composition containing it. If the content of the first acrylic resin exceeds the above range, there may be problems with rapid drying, and therefore insufficient coating processability and paint flow of the composition, resulting in reduced appearance and scratch resistance of the coating film.

[0036] Second acrylic resin

[0037] The second acrylic resin plays a role in improving the smoothness and scratch resistance of the manufactured coating.

[0038] The second acrylic resin can be synthesized directly using known methods, or it can be a commercially available product. For example, the second acrylic resin can be prepared by polymerizing at least one of a second vinyl monomer and a second (meth)acrylate monomer.

[0039] There are no particular restrictions on the type of the second vinyl monomer, but for example, at least one selected from the group consisting of: styrene, methylstyrene, dimethylstyrene, fluorostyrene, ethoxystyrene, methoxystyrene, phenyl vinyl ketone, tert-butylbenzoate, vinyl cyclohexanoate, vinyl acetate, vinylpyrrolidone, vinyl chloride, vinyl alcohol, acetoxystyrene, tert-butylstyrene, and vinyltoluene.

[0040] The second (meth)acrylate monomer may include at least one selected from the group consisting of (meth)acrylate monomers without hydroxyl groups and (meth)acrylate monomers containing hydroxyl groups.

[0041] For example, a hydroxyl-free (meth)acrylate monomer may include at least one selected from the group consisting of: (meth)acrylic acid, meth(meth)acrylic acid, (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, isooctyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecanyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, isobornyl (meth)acrylate, and lauryl (meth)acrylate.

[0042] The hydroxyl-containing (meth)acrylate monomer can be, for example, a hydroxyalkyl-containing (meth)acrylate, and specifically can include at least one selected from the group consisting of 2-hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and 2-hydroxybutyl (meth)acrylate.

[0043] For example, free radical polymerization can be used to manufacture a second acrylic resin, and the physical properties of the second acrylic resin, such as weight-average molecular weight (Mw), hydroxyl value (OHv), and acid value (Av), can be adjusted according to the initiator and polymerization time.

[0044] The glass transition temperature (Tg) of the second acrylic resin can be between 1 and 20°C, or between 5 and 15°C. If the glass transition temperature of the second acrylic resin is within this range, it improves the smoothness and scratch resistance of the coating film. Conversely, if the glass transition temperature of the second acrylic resin is below this range, the resulting coating film will have insufficient shatter resistance. If the glass transition temperature of the second acrylic resin exceeds this range, the appearance properties and hardness of the coating film may become insufficient.

[0045] Furthermore, the second acrylic resin may have an acid value (Av) of 3 to 10 mg KOH / g or 5 to 8 mg KOH / g. If the acid value of the second acrylic resin is within the above range, the appearance properties of the resulting coating can be improved by controlling the reactivity of the composition containing the resin. If the acid value of the second acrylic resin is less than the above range, there may be a problem of reduced curing reaction rate and deterioration of the appearance of the resulting coating. If the acid value of the second acrylic resin exceeds the above range, there may be a problem that the viscosity of the composition increases with the increase of resin cohesion, thereby reducing processability and storage properties at room temperature.

[0046] The second acrylic resin can have a hydroxyl value (OHv) of 110 to 200 mg KOH / g or 140 to 160 mg KOH / g. If the hydroxyl value of the second acrylic resin is within the above range, it improves the weather resistance of the coating film. However, if the hydroxyl value of the second acrylic resin is less than the above range, the coating film formed through the crosslinking reaction with the melamine resin as a curing agent may be insufficient, resulting in poor mechanical properties of the coating film, such as hardness. If the hydroxyl value of the second acrylic resin exceeds the above range, the coating film may become brittle due to over-curing, thus reducing elasticity, and the resulting coating film may have insufficient appearance, water resistance, and scratch resistance.

[0047] The second acrylic resin can have a weight-average molecular weight (Mw) of 1000 to 4000 g / mol, 1500 to 3500 g / mol, or 2000 to 3000 g / mol. If the weight-average molecular weight of the second acrylic resin is within the above range, the long-term physical properties of the manufactured coating, such as durability and weather resistance, can be excellent. However, if the weight-average molecular weight of the second acrylic resin is less than the above range, there may be a problem of insufficient weather resistance and scratch resistance in the manufactured coating due to the low molecular weight. If the weight-average molecular weight of the second acrylic resin exceeds the above range, there may be a problem that the fluidity decreases with increasing molecular weight, resulting in poor processability of the coating composition containing the resin, and difficulty in preparing a coating with excellent appearance due to poor leveling properties.

[0048] Based on the total weight of the resin, the second acrylic resin can have a solids content (NV) of 50 to 80 wt% or 60 to 70 wt%. If the solids content of the second acrylic resin is within the above range, the storage stability of the resin and the coating composition can be improved, and the processability will be excellent. Furthermore, if the solids content of the second acrylic resin is less than the above range, the viscosity may become too low, resulting in insufficient processability of the coating composition containing this resin. If the solids content of the second acrylic resin exceeds the above range, the viscosity of the second acrylic resin is too high, thereby reducing stability during the reaction, potentially worsening dispersion stability, and therefore causing aggregation over time.

[0049] Furthermore, based on the total weight of the transparent coating composition, the content of the second acrylic resin can be 10 to 20 wt%, or 12 to 18 wt%. If the content of the second acrylic resin is less than the above range, there may be problems with reduced drying performance and reduced gloss, water resistance, oil resistance, dirt resistance, and impact resistance of the coating film made from the transparent coating composition containing the second acrylic resin. If the content of the second acrylic resin exceeds the above range, there may be problems with rapid drying, resulting in insufficient processability and paint flow of the transparent coating composition, thereby reducing the coating appearance and scratch resistance.

[0050] Polyester resin

[0051] Polyester resins play a role in improving the appearance properties of coatings and enhancing scratch resistance.

[0052] Polyester resins can be synthesized directly using known methods or are commercially available products. For example, polyester resins can be made from carboxylic acids and polyols. As another example, polyester resins can be unmodified with organosilicones, and specifically, unmodified with organopolysiloxanes.

[0053] In this case, the carboxylic acid can be, for example, at least one selected from the group consisting of: caprolactone, adipic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), cyclic aliphatic acids, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride (HHPA), hexahydrophthalic anhydride, and derivatives thereof.

[0054] The polyol may be, for example, at least one selected from the group consisting of: cyclohexanediol (CHDM), methoxy polyethylene glycol, 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylpentanediol.

[0055] Polyester resins can 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 polyester resin is within this range, rapid hardening caused by heat treatment can be prevented, which helps prevent poor appearance of the coating and reduces blistering (cracking). If the acid value of the polyester resin is less than the above range, there may be a problem of reduced hardness and appearance properties of the manufactured coating due to a slower curing reaction rate. If the acid value of the polyester resin exceeds the above range, there may be problems such as increased brittleness of the coating, decreased scratch resistance, and decreased water resistance with increased hydrophilicity.

[0056] Furthermore, the polyester resin can 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 polyester resin is within the above range, it improves the spreadability of the coating film and enhances chemical resistance due to the urethane reaction. Conversely, if the hydroxyl value of the polyester resin is less than the above range, insufficient crosslinking density due to inadequate reactivity with the melamine resin may occur, resulting in reduced durability, adhesion during recoating, and solvent resistance of the manufactured coating film. If the hydroxyl value of the polyester resin exceeds the above range, over-curing and brittleness of the coating film may occur, leading to a deterioration in its appearance and scratch resistance.

[0057] The glass transition temperature (Tg) of polyester resin can be between 15 and 30°C, or between 18 and 25°C. If the glass transition temperature of the polyester resin is within this range, it exhibits excellent processability (spray feel) and imparts flexibility to the coating film. If the glass transition temperature of the polyester resin is below this range, there may be issues with reduced drying speed and deterioration of the coating film's mechanical properties. If the glass transition temperature of the polyester resin exceeds this range, there may be problems such as decreased appearance with increasing drying speed, and reduced scratch resistance as the coating film becomes more brittle.

[0058] Furthermore, the weight-average molecular weight (Mw) of the polyester resin can be between 800 and 1500 g / mol, or between 1000 and 1300 g / mol. If the weight-average molecular weight of the polyester resin is within the above range, the smoothness of the coating is improved, and it has the effect of forming a soft coating film. If the weight-average molecular weight of the polyester resin is less than the above range, there may be a problem that the mechanical properties of the manufactured coating film deteriorate due to the low molecular weight. If the weight-average molecular weight of the polyester resin exceeds the above range, there may be a problem that as the molecular weight increases, the fluidity of the coating decreases, the coating film hardens, thereby reducing smoothness and scratch resistance.

[0059] Based on the total weight of the resin, the polyester resin can have a solids content (NV) of 60 to 85 wt% or 70 to 80 wt%. If the solids content of the polyester resin is within the above range, the high solids content has the effect of reducing the total volatile organic compound (TVOC) content. If the solids content of the polyester resin is less than the above range, there is a problem of reduced curing reactivity due to the reduced solids content of the composition. If the solids content of the polyester resin exceeds the above range, there may be problems with poor processability and poor appearance of the manufactured coating.

[0060] Furthermore, the viscosity of the polyester resin at 25°C can be 700 cps to 1900 cps, 750 cps to 1850 cps, or 800 cps to 1800 cps. If the viscosity of the polyester resin at 25°C is within the above range, it has the effect of increasing the processability of the coating composition. However, if the viscosity of the polyester resin at 25°C is less than the above range, the viscosity of the composition is too low to form a coating film, resulting in reduced adhesion and shatter resistance of the coating film. If the viscosity of the polyester resin at 25°C exceeds the above range, the appearance characteristics of the manufactured coating film may be insufficient due to insufficient processability of the composition.

[0061] Based on the total weight of the transparent coating composition, the content of polyester resin can be 1 to 10 wt%, or 3 to 8 wt%. If the content of polyester resin is within the above range, it has the effect of improving the coating appearance, scratch resistance, and coating smoothness. If the content of polyester resin in the composition is less than the above range, there may be problems such as reduced crosslinking density, deteriorated mechanical properties, and poor appearance. If the content of polyester resin in the composition exceeds the above range, there may be a problem of excessively increased viscosity of the composition, leading to poor pinhole characteristics, which reduces processability and drying performance.

[0062] Organosilicon modified polyester resin

[0063] Organosilicon-modified polyester resin improves the appearance and smoothness of the coating by preventing pinholes.

[0064] Organosilicon-modified polyester resins can be synthesized directly using known methods or are commercially available products. Specifically, organosilicon-modified polyester resins can be prepared by modifying polyester resins made from carboxylic acids and polyols with organopolysiloxanes.

[0065] In this case, the carboxylic acid can be, for example, at least one selected from the group consisting of: caprolactone, adipic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), cyclic aliphatic acids, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride (HHPA), hexahydrophthalic anhydride, and derivatives thereof.

[0066] The polyol may be, for example, at least one selected from the group consisting of: cyclohexanediol (CHDM), methoxy polyethylene glycol, 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylpentanediol.

[0067] Organopolysiloxanes may include functional groups and non-functional organic groups. For example, organopolysiloxanes may include at least one functional group selected from silanols and alkoxy groups, and may include at least one non-functional organic group selected from methyl, propyl, and phenyl groups. The alkoxy group may be, for example, a methoxy, ethoxy, or butoxy group.

[0068] The acid value (Av) of the silicone-modified polyester resin can be 10 mg KOH / g or less, 8 mg KOH / g, or 1 to 8 mg KOH / g. If the acid value of the silicone-modified polyester resin is less than the above range, there may be a problem of reduced appearance properties of the coating film due to the high surface tension of the composition. If the acid value of the silicone-modified polyester resin exceeds the above range, there may be a problem of deteriorated mechanical properties of the coating film due to excessively low surface tension.

[0069] Furthermore, silicone-modified polyester resins can have a hydroxyl value (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, insufficient crosslinking density due to insufficient reactivity with melamine resin may lead to reduced durability and chemical resistance of the manufactured coating. If the hydroxyl value of the silicone-modified polyester resin exceeds the above range, over-curing of the paint may occur, resulting in a brittle coating and consequently poor appearance and scratch resistance.

[0070] The glass transition temperature (Tg) of silicone-modified polyester resin can be -20 to 0°C or -15 to -5°C. If the glass transition temperature of the silicone-modified polyester resin is below the above range, the hardness of the coating film may decrease due to increased elasticity, and the appearance properties of the coating film may be reduced due to poor drying performance. If the glass transition temperature of the silicone-modified polyester resin exceeds the above range, the appearance properties of the coating film may deteriorate due to reduced fluidity of the coating film, and the elasticity of the coating film may decrease, resulting in reduced adhesion and shatter resistance.

[0071] Furthermore, the weight-average molecular weight (Mw) of the silicone-modified polyester resin can be 1300 to 2500 g / mol, 1500 to 2200 g / mol, or 1700 to 2000 g / mol. If the weight-average molecular weight of the silicone-modified polyester resin is within the above range, the smoothness of the coating is improved, and it has the effect of forming a soft coating film. If the weight-average molecular weight of the silicone-modified polyester resin is less than the above range, there is a problem that the mechanical properties of the manufactured coating film deteriorate due to the small molecular weight. If the weight-average molecular weight of the silicone-modified polyester resin exceeds the above range, there may be a problem that as the molecular weight increases, the fluidity of the coating decreases, thereby hardening the coating film and thus reducing smoothness and scratch resistance.

[0072] The viscosity of silicone-modified polyester resin at 25°C can be 5000 to 10000 cps, or 6000 to 9000 cps. If the viscosity of the silicone-modified polyester resin at 25°C is less than the above range, the paint viscosity may be too low, thus failing to form a film, resulting in reduced adhesion and scratch resistance of the film. If the viscosity of the silicone-modified polyester resin at 25°C exceeds the above range, the appearance characteristics of the manufactured film may deteriorate due to insufficient processability of the paint.

[0073] Furthermore, based on the total weight of the resin, the solids content (NV) of the silicone-modified polyester resin can be 80 wt% or higher, or 85 to 95 wt%. 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 decrease in the solids content in the coating. If the solids content of the silicone-modified polyester resin exceeds the above range, there may be problems of poor processability and deteriorated appearance properties in the coating.

[0074] Based on the total weight of the transparent coating composition, the content of the silicone-modified polyester resin can be 1 to 10 wt%, or 3 to 8 wt%. If the content of the silicone-modified polyester resin is within the above range, it has the effect of improving the appearance, scratch resistance, and smoothness of the coating film. If the content of the silicone-modified polyester resin in the composition is less than the above range, there may be a decrease in crosslinking density, resulting in a reduction in the appearance and scratch resistance of the coating film. If the content of the polyester resin in the composition exceeds the above range, there may be a problem that the viscosity of the composition becomes too high, which reduces processability and drying performance, leading to a deterioration in the appearance and mechanical properties of the coating film.

[0075] melamine resin

[0076] Melamine resin is a curing agent that plays a role in the curing of the composition by crosslinking with each component of the transparent coating composition and in increasing the hardness of the coating film.

[0077] Melamine resin can be an alkylated melamine resin, which can be synthesized directly using known methods or is a commercially available product. For example, melamine resin may contain at least one selected from the group consisting of: methoxymethyl melamine, methyl melamine, butyl melamine, isobutoxy melamine, butoxy melamine, hexahydroxymethyl melamine, hexamethoxymethyl melamine, hexamethoxybutoxymethyl melamine, and iminomethoxymethyl melamine.

[0078] Furthermore, the melamine resin may include two resins with different viscosities and weight-average molecular weights at 25°C. That is, the transparent coating composition may include a first melamine resin and a second melamine resin with different viscosities and weight-average molecular weights at 25°C. As described above, if the transparent coating composition of the present invention includes two types of melamine resins with different viscosities and weight-average molecular weights at 25°C, it has the effect of improving the mechanical properties of the coating film by controlling the curing speed.

[0079] For example, compared to the second melamine resin, the first melamine resin has a lower viscosity and a lower weight-average molecular weight at 25°C. Specifically, the viscosity of the first melamine resin at 25°C can be 3000 to 5000 cps lower than that of the second melamine resin. Furthermore, the weight-average molecular weight of the first melamine resin can be 300 to 700 g / mol lower than that of the second melamine resin.

[0080] The weight-average molecular weight (Mw) of the first melamine resin can be 500 to 800 g / mol or 550 to 750 g / mol. If the weight-average molecular weight of the first melamine resin is within the above range, the crosslinking density is increased, thus improving the adhesion and hardness of the manufactured coating. However, if the weight-average molecular weight of the first melamine resin is less than the above range, the weather resistance and scratch resistance of the coating decrease due to the reduced crosslinking density. If the weight-average molecular weight of the first melamine resin exceeds the above range, the molecular weight may increase, resulting in a deterioration in the appearance properties of the coating.

[0081] The viscosity of the first melamine resin at 25°C may be between 2000 and 4000 cps, or between 2400 and 3800 cps. If the viscosity of the first melamine resin at 25°C is within this range, the appearance characteristics of the coating film are excellent. However, if the viscosity of the first melamine resin at 25°C is less than this range, the viscosity of the paint is too low to form a film, resulting in reduced adhesion and scratch resistance. If the viscosity of the first melamine resin at 25°C exceeds this range, the appearance characteristics of the manufactured coating film may deteriorate due to insufficient processability of the paint.

[0082] Furthermore, the acid value (Av) of the first melamine resin can be 1 mg KOH / g or less, or 0.1 to 1 mg KOH / g. If the acid value of the first melamine resin exceeds the above range, there may be problems such as increased reaction rate and decreased adhesion and scratch resistance of the manufactured coating.

[0083] Based on the total weight of the resin, the solids content (NV) of the first melamine resin can be 90 to 100 wt%, or 95 to 100 wt%. If the solids content of the first melamine resin is within the above range, the storage stability of the resin and the coating composition can be improved, and the processability will be excellent. Furthermore, if the solids content of the first melamine resin is less than the above range, there are problems with excessively low viscosity and insufficient processability of the coating composition containing this resin. If the solids content of the first melamine resin exceeds the above range, the viscosity of the melamine resin is too high, thus reducing stability during the curing reaction and potentially worsening dispersion stability, leading to aggregation over time.

[0084] The weight-average molecular weight (Mw) of the second melamine resin can be 900 to 1500 g / mol or 1000 to 1300 g / mol. If the weight-average molecular weight of the second melamine resin is within the above range, it has the effect of improving the adhesion and hardness of the manufactured coating film by increasing the crosslinking density. Furthermore, if the weight-average molecular weight of the second melamine resin is less than the above range, there is a problem that the weather resistance and scratch resistance of the coating film decrease as the crosslinking density decreases. If the weight-average molecular weight of the second melamine resin exceeds the above range, there may be a problem that the molecular weight increases, resulting in a deterioration in the appearance properties of the coating film.

[0085] Furthermore, the viscosity of the second melamine resin at 25°C can be 5000 to 9000 cps, or 7000 to 8000 cps. If the viscosity of the second melamine resin at 25°C is within the above range, the appearance characteristics of the coating film are excellent. If the viscosity of the second melamine resin at 25°C is less than the above range, the viscosity of the paint may be too low to form a film, resulting in reduced adhesion and scratch resistance of the coating film. If the viscosity of the second melamine resin at 25°C exceeds the above range, the appearance characteristics of the manufactured coating film may deteriorate due to insufficient processability of the paint.

[0086] The acid value (Av) of the second melamine resin can be 1 mg KOH / g or less, or 0.1 to 1 mg KOH / g. If the acid value of the second melamine resin exceeds the above range, there may be problems such as increased reaction rate and reduced adhesion and scratch resistance of the manufactured coating.

[0087] Furthermore, based on the total weight of the resin, the solids content (NV) of the second melamine resin can be 75 to 95 wt%, or 80 to 90 wt%. If the solids content of the second melamine resin is within the above range, the storage stability of the second melamine resin and the coating composition can be improved, and the processability is excellent. However, if the solids content of the second melamine resin is less than the above range, the viscosity becomes too low, resulting in insufficient processability of the coating composition containing this resin. If the solids content of the second melamine resin exceeds the above range, the viscosity of the melamine resin is too high, resulting in low stability during the curing reaction and potentially poor dispersion stability, leading to aggregation over time.

[0088] Based on the total weight of the transparent coating composition, the melamine resin content can be 10 to 25 wt%, or 12 to 23 wt%. If the melamine resin content is within the above range, it has the effect of improving the adhesion and hardness of the manufactured coating film by increasing the crosslinking density. Furthermore, if the melamine resin content is less than the above range, there is a problem that the mechanical properties and chemical resistance of the coating film deteriorate due to reduced curing. If the melamine resin content exceeds the above range, the coating film may become brittle due to over-curing, thus potentially reducing adhesion and shatter resistance.

[0089] Specifically, the melamine resin may include a first melamine resin and a second melamine resin in a weight ratio of 1:0.1 to 1.0 or 1:0.2 to 0.8. If the weight ratio of the first melamine resin and the second melamine resin is less than the above range, that is, if the weight of the first melamine resin contains a small amount of second melamine resin, there is a problem of increased curing speed, resulting in a brittle coating and reduced appearance and scratch resistance. If the weight ratio of the first melamine resin and the second melamine resin exceeds the above range, that is, if the weight of the first melamine resin contains an excessive amount of second melamine resin, there may be a problem of reduced crosslinking density and deteriorated mechanical properties of the coating.

[0090] Transparent coating compositions may include solvents.

[0091] solvent

[0092] Solvents play a role in controlling the viscosity of the composition, improving drying performance, and improving the appearance and spreadability of the manufactured coating.

[0093] The solvent is not particularly limited, as long as it is generally used in the transparent coating composition, and may, for example, be at least one selected from the group consisting of aromatic, acetate-based, alcohol-based, 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, propylene glycol methyl ether acetate (PMA), butyl carbitol acetate, butyl cellosolve acetate, 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.

[0094] Furthermore, based on the total weight of the transparent coating composition, the solvent content can be 5 to 40 wt%, or 10 to 35 wt%. If the solvent content is within the above range, the viscosity of the coating can be appropriately adjusted, thereby improving processability and drying performance. Conversely, if the solvent content in the coating composition is less than the above range, there may be a problem of insufficient processability due to high solids content in the composition. If the solvent content exceeds the above range, the solids content of the coating is low, resulting in insufficient appearance and adhesion of the manufactured coating film.

[0095] additive

[0096] The transparent coating composition according to the invention may further include additives such as curing catalysts, light stabilizers, ultraviolet absorbers, leveling agents, anti-flow agents, and surface conditioners. In this case, there are no particular limitations on the additives, as long as they can be added to the transparent coating composition.

[0097] For example, the surface conditioner can be a polysiloxane surface conditioner, which improves the appearance properties of the coating, improves leveling performance, enhances resistance to dirt during processes such as sandblasting, and reduces surface tension to improve resistance to external oils.

[0098] Furthermore, there are no particular limitations on the amount of additives, as long as they fall within the range typically included in a clear coating composition. For example, based on the total weight of the clear coating composition, the amount of additives can be 5 to 45 wt%, or 10 to 40 wt%.

[0099] The transparent coating composition can be a one-part type, comprising a main part and a curing agent part.

[0100] Furthermore, the clear coating composition can have a solids content of 50 to 70 wt% or 55 to 65 wt%. If the solids content of the clear coating composition is within the above range, the composition has the advantage of suitable coatability. However, if the solids content of the coating composition is less than the above range, there is a problem of reduced curing reactivity due to the decrease in the solids content of the composition. If the solids content of the coating composition exceeds the above range, there may be problems with poor processability of the manufactured paint and deterioration of the appearance properties of the coating film.

[0101] The viscosity of the clear coat composition based on Ford Cup No. 4 at 25°C can be 20 to 50 seconds, or 25 to 45 seconds. If the viscosity of the clear coat composition at 25°C is less than the above range, dripping from vertical surfaces may occur. If the viscosity of the clear coat composition at 25°C exceeds the above range, the appearance characteristics of the coating film made from it may be reduced due to the high viscosity of the composition, or it may impose a load on the sprayer, leading to sprayer failure.

[0102] As described above, the transparent coating composition according to the present invention comprises a silicone-modified polyester resin and two types of acrylic resins with different glass transition temperatures, thus improving the smoothness and scratch resistance of the thin film portion of the manufactured coating. Furthermore, the coating film manufactured from the transparent coating composition has low surface tension, thus exhibiting excellent oil resistance to external contaminants and excellent smoothness, thereby providing excellent resistance to contaminants (such as abrasive powder) that may occur during processing.

[0103] The invention will be described in more detail below by way of examples. However, these examples are intended only to aid in understanding the invention, and the scope of the invention is not limited in any way to these examples.

[0104] Examples 1 to 21 and Comparative Examples 1 to 8. Preparation of transparent coating compositions.

[0105] A transparent coating composition based on Ford Cup No. 4 with a viscosity of 35 seconds at 25°C was prepared by mixing the same amounts of the components shown in Tables 1 to 3.

[0106] [Table 1]

[0107]

[0108]

[0109]

[0110] [Table 2]

[0111]

[0112]

[0113] [Table 3]

[0114]

[0115]

[0116] Below, Table 4 shows the manufacturer, product name, or ingredient name of each component used in the comparative examples and embodiments:

[0117] [Table 4]

[0118]

[0119]

[0120] Experimental Example: Evaluation of Coating Properties

[0121] A primer (manufacturer: KCC, product name: WT3062) was bell-shaped applied to the sample, and hot air was blown at 80°C for 3 minutes to evaporate any water remaining in the primer, thus forming a primer layer with a thickness of 15 μm. Then, the transparent coating composition prepared in the examples and comparative examples was applied to the primer layer and cured at 140°C for 20 minutes to form a transparent coating film with a thickness of 40 μm, thus preparing the finished coating film. The physical properties of the above samples were measured as follows, and the results are shown in Table 5.

[0122] Specifically, by using a handheld spray gun (nozzle diameter: 1.5mm, air pressure: kept constant at 4.5kgf / cm²), 2 Apply a transparent coating by moving horizontally at a speed of 40 to 50 cm / s while maintaining a constant distance of 30 cm between the nozzle inlet and the sample.

[0123] (1) Appearance

[0124] The gloss (LU), sharpness (SH), and orange peel (OP) of the manufactured finished coating were measured using the automotive exterior measurement instrument Wave Scan DOI (BYK Gardner), and the overall appearance evaluation value (CF) was calculated using Equation 1 below based on the measured physical properties.

[0125] [Equation 1]

[0126] CF=LU×0.15+SH×0.35+OP×0.5

[0127] In this case, CF is measured and calculated in both the horizontal and vertical directions.

[0128] 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 (O); if the calculated CF is 55 or greater but less than 60, it is rated as average (△); if the calculated CF is less than 55, it is rated as poor (X).

[0129] (2) Oil resistance

[0130] Apply a primer to a 300mm x 300mm coated sample, then spray 5ml of fragrance material (polyoxyethylene) with oil particles, and apply a clear coat. Then measure the number of pits on the coating surface.

[0131] Specifically, if the number of pit shapes relative to the total surface area of ​​the sample is 3 or fewer, it is rated as excellent (◎); if the number is 4 or more but less than 6, it is rated as good (○); if the number is 7 or more but less than 9, it is rated as average (△); and if the number is 10 or more, it is rated as poor (×).

[0132] (3) Adhesion properties during recoating

[0133] The primer and clear coat were peeled off, and the primer and clear coat were applied in the same manner as described above. The adhesion properties during recoating were then evaluated using the checkerboard method.

[0134] Specifically, in the checkerboard method, 100 squares, each 2 mm wide and 2 mm high, are made on the surface of the finished coating with a knife. These squares are then removed with tape to measure adhesion. In this case, if all 100 squares are 100% completely 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 average (△); and if less than 50% of the remaining squares remain, the measured adhesion is rated as poor (X).

[0135] (4) Solvent resistance

[0136] Place a cotton cloth fully soaked in xylene solvent on the surface of the finished coating, then scratch the surface four times per minute with a fingernail at a force of 2 kgf, and measure the time it takes for the base coat to be exposed.

[0137] As a result of the measurement, if it is 15 minutes or longer, it is rated as excellent (◎); if it is 7 minutes or longer but less than 15 minutes, it is rated as good (○); if it is 5 minutes or longer but less than 7 minutes, it is rated as average (△); and if it is less than 5 minutes, it is rated as poor (×).

[0138] (5) Scratch resistance

[0139] The initial 20° gloss of the finished coating was measured using a polishing machine, and the 20° gloss was measured after 10 cycles of surface treatment while simultaneously spraying quartz powder and water using an Amtec Kistler (a type of car wash machine). The gloss retention rate after surface treatment was calculated based on the gloss before surface treatment.

[0140] In this case, if the gloss retention rate is 70% or higher, it is rated as excellent (◎); if the gloss retention rate is 60% or higher but less than 70%, it is rated as good (O); if the gloss retention rate is 55% or higher but less than 60%, it is rated as average (△); and if the gloss retention rate is less than 55%, it is rated as poor (X).

[0141] (6) Impact resistance

[0142] The impact resistance of the finished coating was evaluated according to ASTM D2794. In this case, a DuPont impact tester was used, and the appearance of the coating was observed when a 500g weight was dropped onto the sample while the drop height was changed from 30cm to 50cm.

[0143] The results of the observation are as follows: if no cracks or peeling occur in the coating when the weight is dropped from a height of 50cm, it is rated as excellent (◎); if cracks appear in the coating when the weight is dropped from a height of 30cm or higher but less than 50cm, it is rated as good (○); if cracks appear in the coating when the weight is dropped from a height of 20cm or higher but less than 30cm, it is rated as average (△); and if cracks appear in the coating when the weight is dropped from a height of less than 20cm, it is rated as poor (×).

[0144] Table 5:

[0145]

[0146]

[0147] As shown in Table 5, the coatings prepared by the compositions of Examples 1 to 21 were found to be superior to the coatings prepared by the compositions of the comparative examples in terms of appearance properties, oil resistance, adhesion properties during recoating, solvent resistance, scratch resistance and impact resistance.

[0148] Meanwhile, Comparative Example 1, which does not contain the first acrylic resin, and Comparative Example 3, which does not contain the polyester resin, have insufficient appearance properties, solvent resistance, and impact resistance. In particular, Comparative Example 3 also has insufficient scratch resistance.

[0149] Furthermore, Comparative Example 2, which does not contain the second acrylic resin, and Comparative Example 4, which does not contain the silicone-modified polyester resin, exhibit insufficient appearance properties, oil resistance, and impact resistance. In particular, Comparative Example 2 shows insufficient solvent resistance, and Comparative Example 4 shows insufficient scratch resistance.

[0150] Comparative Example 5, which includes a first acrylic resin-4 with a low glass transition temperature, Comparative Example 6, which includes a first acrylic resin-5 with a high glass transition temperature, Comparative Example 7, which includes a second acrylic resin-4 with a low glass transition temperature, and Comparative Example 8, which includes a second acrylic resin-5 with a high glass transition temperature, exhibit insufficient adhesion, scratch resistance, and impact resistance during recoating. Furthermore, Comparative Example 5 shows insufficient solvent resistance, and Comparative Examples 6 and 8 exhibit insufficient appearance properties.

Claims

1.A transparent coating composition, comprising: a first acrylic resin, a second acrylic resin, a polyester resin, a silicone-modified polyester resin, and a melamine resin, wherein the first acrylic resin has a glass transition temperature of 25 to 55℃, a weight average molecular weight of 6000 to 10000 g / mol, and the second acrylic resin has a glass transition temperature of 1 to 20℃, a weight average molecular weight of 1000 to 4000 g / mol, wherein the transparent coating composition comprises 10 to 20 wt% of the first acrylic resin, 10 to 20 wt% of the second acrylic resin, 1 to 10 wt% of the polyester resin, 1 to 10 wt% of the silicone-modified polyester resin, and 10 to 25 wt% of the melamine resin, based on the total weight of the composition. 2.The transparent coating composition according to claim 1, wherein the first acrylic resin has a hydroxyl value of 50 to 100 mgKOH / g. 3.The transparent coating composition according to claim 1, wherein the second acrylic resin has an acid value of 3 to 10 mgKOH / g, a hydroxyl value of 110 to 200 mgKOH / g. 4.The transparent coating composition according to claim 1, wherein the polyester resin has an acid value of 15 to 25 mgKOH / g, a hydroxyl value of 200 to 300 mgKOH / g, a glass transition temperature of 15 to 30℃, a weight average molecular weight of 800 to 1500 g / mol. 5.The transparent coating composition according to claim 1, wherein the silicone-modified polyester resin has an acid value of 10 mgKOH / g or less, a hydroxyl value of 150 to 250 mgKOH / g, a glass transition temperature of -20 to 0℃, a weight average molecular weight of 1300 to 2500 g / mol.

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