Low temperature curable one-component clear coating composition

Abstract

The present invention relates to a low-temperature curable one-component clear coating composition comprising: a first acrylic resin; a second acrylic resin; a blocked isocyanate-based curing agent; and a solvent, wherein the first acrylic resin has a hydroxyl value of 160-250 mg KOH / g and a weight-average molecular weight of 15,000-60,000 g / mol, and the second acrylic resin has a hydroxyl value of 100-150 mg KOH / g and a weight-average molecular weight of 5,000-10,000 g / mol.

Description

Technical Field

[0001] The present invention relates to a one-pack clear coating composition which can be cured at a low temperature of 130° C. or lower. Background Art

[0002] Various coating processes are applied to vehicle bodies, such as electrophoretic coating, midcoat coating, basecoat, and clearcoat, to improve appearance characteristics and protect the surface from the external environment. Specifically, after the application and curing of electrophoretic paint and the application and curing of midcoat paint are generally performed on the vehicle body, a basecoat and a clearcoat are successively applied on the midcoat, followed by drying and curing. As described above, in order to form the basecoat and the clearcoat, a basecoat composition is generally applied and dried, followed by application of a clearcoat composition, and the clearcoat and basecoat are cured together at 140-150°C. However, recently, there has been an increasing demand for clearcoat compositions that can shorten the process and can be cured at low temperatures to reduce the cost of the painting process.

[0003] As an alternative, various clearcoat compositions that can be used under low-temperature baking conditions (particularly heat-curing conditions at 140°C or lower) have been proposed. Specifically, Korean Patent Publication No. 2016-0120110 (Patent Document 1) discloses a low-temperature curable, one-shot clearcoat composition for automotive applications. The composition comprises: an acrylic resin having both double-bond oligomers and hydroxyl groups; a thermal radical initiator for crosslinking via polymerization of the double bonds; a thermal curing agent for crosslinking the hydroxyl groups; and additives. Conventional clearcoat compositions capable of low-temperature baking, such as the composition of Patent Document 1, produce coating films with insufficient mechanical properties and are limited in their application as automotive body paints.

[0004] Therefore, there is a need to research and develop a one-component clear coating composition that can be cured at a low temperature of 130° C. or lower and whose resulting coating film has excellent mechanical properties and is therefore suitable for application in vehicle body painting. Summary of the Invention

[0005] Technical issues

[0006] Thus, the present invention provides a one-component type clear coating composition which can be cured at a low temperature of 130° C. or lower and can produce a coating film having excellent mechanical properties.

[0007] Technical Solution

[0008] The present invention provides a low-temperature curable one-shot clear coating composition, comprising: a first acrylic resin, a second acrylic resin, a blocked isocyanate-based curing agent, and a solvent.

[0009] wherein the first acrylic resin has a hydroxyl value of 160 to 250 mg KOH / g and a weight average molecular weight of 15,000 to 60,000 g / mol, and

[0010] The second acrylic resin has a hydroxyl value of about 100 to about 150 mg KOH / g and a weight average molecular weight of about 5,000 to about 10,000 g / mol.

[0011] Beneficial effects

[0012] Because the low-temperature, one-shot, clearcoat composition according to the present invention can be cured at temperatures of 130°C or lower and can reduce coating process costs, it is economical. Furthermore, because the composition can be used to integrally coat a vehicle body and attached material components, it can prevent the problem of the painted component and the vehicle body having a different color after painting. Furthermore, because the low-temperature, one-shot, clearcoat composition according to the present invention can be applied to both the vehicle body and the component in a single coating process, process inconvenience and cost losses can be reduced. Furthermore, because the resulting coating film has excellent mechanical properties such as hardness, adhesion, water resistance, scratch resistance, and solvent resistance, it can be used for vehicle body painting. DETAILED DESCRIPTION

[0013] Mode for Carrying Out the Invention

[0014] Hereinafter, the present invention will be described in detail.

[0015] In the present invention, the "weight average molecular weight" of the resin may be measured by a method known in the art, and may represent, for example, a value measured by a gel permeation chromatography (GPC) method.

[0016] In addition, in the present invention, the "glass transition temperature (Tg)" of the resin can be measured by methods well known in the art, such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), mechanical method, volumetric method, etc., and Tg can, for example, represent a value measured by the differential scanning calorimetry (DSC) method.

[0017] In the present invention, functional group values ​​such as "acid value", "hydroxyl value" and "unreacted NCO group content" can be measured by methods well known in the art, and for example, may be values ​​measured by titration or the like.

[0018] In addition, in this specification, "(meth)acrylic acid" means "acrylic acid" and / or "methacrylic acid", and "(meth)acrylate" means "acrylate" and / or "methacrylate".

[0019] The low-temperature curable, one-shot clear coating composition according to the present invention comprises a first acrylic resin, a second acrylic resin, a blocked isocyanate-based curing agent, and a solvent, wherein the first acrylic resin has a higher hydroxyl value and a higher weight-average molecular weight than the second acrylic resin. As described above, when two acrylic resins having different hydroxyl values ​​and weight-average molecular weights are used, the drying speed and crosslinking density of the coating film are adjusted to improve the appearance characteristics and mechanical properties of the coating film.

[0020] First acrylic resin

[0021] The first acrylic resin is used to improve the low temperature curing property of the composition.

[0022] The first acrylic resin may include repeating units derived from an ethylenically unsaturated monomer and repeating units derived from a (meth)acrylic acid monomer containing a hydroxyl group. That is, the first acrylic resin may be produced from an ethylenically unsaturated monomer and a (meth)acrylic acid monomer containing a hydroxyl group. For example, the first acrylic resin may be produced from a (meth)acrylic acid monomer containing an alkyl group and a (meth)acrylate monomer containing a hydroxyl group.

[0023] The ethylenically unsaturated monomer may be selected from styrene and its derivatives, butadiene, (meth) acrylic acid, C 1-12 Alkyl esters (C 1-12 alkyl(meth)acryl) and (meth)acrylic acid C 1-12 Alkyl esters (C 1-12 One or more of alkyl(meth)acrylic acidester).

[0024] The hydroxyl group-containing (meth)acrylic monomer may be, for example, one or more selected from hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, cardura acrylate, cardura methacrylate, caprolactone acrylate, caprolactone methacrylate, 2,3-dihydroxypropyl acrylate, 2,3-dihydroxypropyl methacrylate, polypropylene-modified acrylate, polypropylene-modified methacrylate, 4-hydroxymethylcyclohexyl-methacrylate, and 4-hydroxymethylcyclohexyl-methyl methacrylate.

[0025] The first acrylic resin may have a glass transition temperature (Tg) of 40 to 70° C., 45 to 65° C., or 50 to 60° C. When the glass transition temperature of the first acrylic resin is within this range, the initial hardness of the resulting coating film may be improved. Furthermore, when the glass transition temperature of the first acrylic resin is lower than this range, the drying speed and crosslinking density of the coating film decrease to the extent that the hardness and water resistance deteriorate, while when it is higher than this range, the coating film becomes brittle and its appearance and crack resistance may be reduced.

[0026] In addition, the first acrylic resin may have a hydroxyl value (OHv) of 160 to 250 mg KOH / g, 170 to 200 mg KOH / g, or 175 to 195 mg KOH / g. When the hydroxyl value of the first acrylic resin is within this range, the curability of the composition comprising the resin can be improved. In addition, when the hydroxyl value of the first acrylic resin is below this range, the crosslinking density is reduced to such an extent that the mechanical properties deteriorate, while when it is above this range, over-curing occurs, making the coating film brittle and losing elasticity, the composition lacking water resistance and weather resistance, and the viscosity increases to such an extent that the workability deteriorates.

[0027] The first acrylic resin may have an acid value (Av) of 7 mg KOH / g or less, 0.5 to 7 mg KOH / g, or 1 to 5 mg KOH / g. When the acid value of the first acrylic resin is within this range, the reactivity of the composition containing the resin can be adjusted to improve the appearance characteristics of the coating film produced therefrom. In addition, when the acid value of the first acrylic resin is below this range, the curing reaction rate is reduced to such an extent that the appearance of the resulting coating film deteriorates, while when it is above this range, the viscosity of the composition increases due to increased cohesion of the resin, resulting in deterioration of workability and storage at room temperature.

[0028] In addition, the first acrylic resin may have a weight average molecular weight of 15,000 to 60,000 g / mol, 15,000 to 55,000 g / mol, or 15,000 to 30,000 g / mol. When the weight average molecular weight of the first acrylic resin is within this range, the appearance characteristics and physical properties of the resulting coating are suitable. In addition, when the weight average molecular weight of the first acrylic resin is below this range, the molecular weight is too low, so that the acid resistance, water resistance, and cold crack resistance of the resulting coating may deteriorate. When it is above this range, the molecular weight increases, the anti-sagging property decreases, thereby deteriorating the appearance and mechanical properties.

[0029] The first acrylic resin has a higher hydroxyl value than the second acrylic resin. For example, the hydroxyl value of the first acrylic resin is 30 to 60 mg KOH / g higher than that of the second acrylic resin.

[0030] When using a resin in which the first acrylic resin has a higher hydroxyl value than the second acrylic resin, the crosslinking density increases, thereby enhancing mechanical properties. Furthermore, when the difference in hydroxyl values ​​between the first acrylic resin and the second acrylic resin is below this range, the crosslinking density decreases, deteriorating the appearance of the resulting coating film. When the difference is above this range, overcuring occurs, making the coating film brittle and losing elasticity, and the water resistance and weather resistance of the composition may also deteriorate.

[0031] In addition, the weight average molecular weight of the first acrylic resin is higher than that of the subsequent second acrylic resin. For example, the weight average molecular weight of the first acrylic resin may be 10,000 to 30,000 g / mol higher than that of the second acrylic resin.

[0032] When a resin in which the first acrylic resin has a higher weight average molecular weight than the second acrylic resin is used, the crosslinking density increases to improve mechanical properties. In addition, when the difference in weight average molecular weight between the first acrylic resin and the second acrylic resin is below this range, the reactivity increases to the point where the appearance and mechanical properties of the resulting coating film deteriorate, and when the difference is above this range, the reactivity and crosslinking density decrease to the point where water resistance and chemical resistance deteriorate.

[0033] In addition, the first acrylic resin may be included in an amount of 30 to 60 wt%, 35 to 55 wt%, or 40 to 50 wt% based on the total weight of the paint composition. When the content of the first acrylic resin is below this range, the drying property is reduced, resulting in poor gloss, water resistance, and impact resistance of the coating film produced by the paint composition. When it is above this range, the coating composition dries quickly, resulting in insufficient coating workability and anti-sagging properties, and poor appearance and cold chipping resistance of the coating film.

[0034] The first acrylic resin and the subsequent second acrylic resin may be included in the composition at a weight ratio of 2 to 7:1 or a weight ratio of 3 to 6:1. When the weight ratio between the first acrylic resin and the second acrylic resin is below this range, that is, when a relatively small amount of the first acrylic resin is included, the crosslinking density may be reduced to deteriorate water resistance and chemical resistance, and when the weight ratio is above this range, that is, when an excessive amount of the first acrylic resin is included, the viscosity of the composition increases, so that the coating workability and anti-sagging properties of the composition may be insufficient, resulting in deterioration of the appearance and mechanical properties of the coating film.

[0035] Second acrylic resin

[0036] The second acrylic resin is used to control the sag resistance of the composition.

[0037] The second acrylic resin may include repeating units derived from an ethylenically unsaturated monomer and repeating units derived from a (meth)acrylic acid monomer containing a hydroxyl group. That is, the second acrylic resin may be produced from an ethylenically unsaturated monomer and a (meth)acrylic acid monomer containing a hydroxyl group. For example, the second acrylic resin may be produced from a (meth)acrylic acid monomer containing an alkyl group and a (meth)acrylate monomer containing a hydroxyl group.

[0038] As another example, the second acrylic resin may be produced by reacting an ethylenically unsaturated monomer, a (meth)acrylic monomer containing a hydroxyl group, and benzylamine, and may have a chemical structure in which a needle-shaped bisurea group is present.

[0039] The ethylenically unsaturated monomer may be selected from styrene and its derivatives, butadiene, (meth) acrylic acid, C 1-12 Alkyl esters and (meth) acrylic acid C 1-12 One or more of alkyl esters.

[0040] The hydroxyl group-containing (meth)acrylic monomer may be, for example, one or more selected from hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, cardura acrylate, cardura methacrylate, caprolactone acrylate, caprolactone methacrylate, 2,3-dihydroxypropyl acrylate, 2,3-dihydroxypropyl methacrylate, polypropylene-modified acrylate, polypropylene-modified methacrylate, 4-hydroxymethylcyclohexyl-methacrylate, and 4-hydroxymethylcyclohexyl-methyl methacrylate.

[0041] The second acrylic resin may have a hydroxyl value (OHv) of 100 to 150 mg KOH / g, 120 to 150 mg KOH / g, or 130 to 150 mg KOH / g. When the hydroxyl value of the second acrylic resin is within this range, the sag resistance of the composition containing the resin can be adjusted to improve workability. In addition, when the hydroxyl value of the second acrylic resin is below this range, the crosslinking density is reduced to such an extent that the appearance characteristics and glossiness of the resulting coating film are deteriorated, while when it is above this range, over-curing occurs, making the coating film brittle and losing elasticity, and the water resistance and weather resistance of the composition may be deteriorated.

[0042] In addition, the second acrylic resin may have an acid value (Av) of 5 mg KOH / g or less, 0.5 to 4.5 mg KOH / g, or 1 to 4 mg KOH / g. When the acid value of the second acrylic resin is within this range, the sag resistance of the composition containing the resin can be adjusted to improve workability. Furthermore, when the acid value of the second acrylic resin is below this range, the curing reaction rate is reduced to the point where the appearance of the resulting coating film is deteriorated, while when it is above this range, the viscosity of the composition is increased to the point where workability and sag resistance are deteriorated.

[0043] The second acrylic resin may have a glass transition temperature (Tg) of 30 to 70° C., 35 to 65° C., or 40 to 60° C. When the glass transition temperature of the second acrylic resin is within this range, the appearance can be improved, and the appearance and hardness of the resulting coating film can be improved. Furthermore, when the glass transition temperature of the second acrylic resin is below this range, the drying speed of the coating is slowed, resulting in a deterioration in appearance, while when it is above this range, the coating film becomes brittle, resulting in a deterioration in appearance characteristics and glossiness.

[0044] In addition, the second acrylic resin may have a weight average molecular weight (Mw) of 5,000 to 10,000 g / mol, 6,000 to 9,500 g / mol, or 7,000 to 9,000 g / mol. When the weight average molecular weight of the second acrylic resin is within this range, the appearance characteristics and physical properties of the resulting coating are suitable. In addition, when the weight average molecular weight of the second acrylic resin is below this range, the molecular weight is too small, so that the acid resistance and chemical resistance of the resulting coating are deteriorated. When it is above this range, the anti-sagging and leveling properties are reduced due to the increase in molecular weight, thereby deteriorating the appearance and mechanical properties.

[0045] The second acrylic resin may be included in an amount of 1 to 20 wt%, 3 to 15 wt%, or 5 to 13 wt% based on the total weight of the paint composition. When the content of the second acrylic resin is below this range, drying properties are reduced to the extent that the gloss and water resistance of the paint composition containing the resin deteriorate, while when it is above this range, drying is rapid, resulting in deterioration in coating workability, anti-sagging properties, and the appearance of the composition.

[0046] Blocked isocyanate-based curing agents

[0047] The blocked isocyanate-based curing agent is used to crosslink with the components in the composition to cure the composition to form a coating film. Herein, the blocked isocyanate-based curing agent refers to a curing agent whose reactivity is adjusted by treating the terminal isocyanate group (NCO group) with a blocking agent.

[0048] The blocking agent may include, for example, one or more selected from reactive methylene-based compounds, polyalkylene polyol compounds, and pyrazole-based compounds. That is, the blocked isocyanate-based curing agent is one or more blocking agents selected from reactive methylene-based compounds, polyalkylene polyol compounds, and pyrazole-based compounds, and has a blocked terminal isocyanate group.

[0049] The blocking agent blocks the curing reaction of the blocked isocyanate curing agent and then dissociates within a certain temperature range (eg, curing temperature of the paint composition: 100 to 130° C.), allowing the curing agent to perform its original function.

[0050] The coating composition of the present invention is a one-shot type, but can exhibit stable stability and curing reactivity by the blocking agent, thereby improving the workability of the coating composition.

[0051] The blocked isocyanate-based curing agent may have a viscosity of 5,000 to 10,000 cps, 6,000 to 9,000 cps, or 7,000 to 8,000 cps at 25°C. Furthermore, the blocked isocyanate-based curing agent may have a solid content of 60 to 80 wt% or 65 to 75 wt% based on the total weight of the curing agent. When the viscosity of the blocked isocyanate-based curing agent at 25°C is below this range, sag resistance is reduced to the point where the appearance and mechanical properties of the resulting coating film deteriorate, while when it is above this range, coating workability may be deteriorated.

[0052] In addition, the blocked isocyanate-based curing agent may have an unreacted NCO group content of 5 to 10 wt%, 6 to 9 wt%, or 6.5 to 8.5 wt%, based on the total weight of the curing agent. When the unreacted NCO group content of the blocked isocyanate-based curing agent is below this range, the crosslinking density is reduced to deteriorate the impact resistance, water resistance, and cold crack resistance of the coating film, while when it is above this range, the curing rate is increased and the resin agglomerates to deteriorate the appearance characteristics, glossiness, and mechanical properties of the coating film.

[0053] The blocked isocyanate-based curing agent may have a weight average molecular weight (Mw) of 1,500 to 4,000 g / mol, 2,000 to 3,500 g / mol, or 2,000 to 3,000 g / mol. When the weight average molecular weight of the blocked isocyanate-based curing agent is below this range, the molecular weight is low, resulting in insufficient reactivity of the composition, thereby deteriorating the appearance and mechanical properties. When it is above this range, handling of the blocking agent becomes difficult due to the increased molecular weight, which may make reactivity adjustment and coating processability difficult and lead to deterioration of mechanical properties.

[0054] In addition, the content of the blocked isocyanate-based curing agent may be 10 to 30 wt%, 10 to 25 wt%, or 13 to 23 wt%, based on the total weight of the paint composition. When the content of the blocked isocyanate-based curing agent is below this range, the reactivity of the composition is insufficient, thereby deteriorating the crosslinking density of the resulting coating film and thus deteriorating the mechanical properties. When it is above this range, unreacted products may be generated in the cured coating film, thereby deteriorating the appearance, adhesion, and cold crack resistance of the coating film.

[0055] solvent

[0056] The solvent is used to adjust the viscosity of the coating composition to improve the workability of the composition.

[0057] The solvent may include one or more selected from hydrocarbon-based solvents, acetate-based solvents, alcohol-based solvents, and carboxylic acid-based solvents. For example, the solvent may include aromatic hydrocarbon-based 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, dimethyl glutarate, dimethyl succinate, dimethyl adipate, methyl glutarate, methyl succinate, and methyl adipate; alcohol-based solvents such as n-butanol, propanol, and 1-methoxy-2-propanol; and the like. Commercially available hydrocarbon-based solvents may include Kocosol #100, Kocosol #150, and the like. Furthermore, commercially available acetate-based solvents may include Rhodiasolv RPD available from Solvay, and the like.

[0058] The solvent may be included in an amount of 10 to 45 wt %, 13 to 30 wt %, or 15 to 20 wt % based on the total weight of the paint composition. When the solvent is included within this content range, the paint composition including the solvent has excellent workability during preparation and coating. When the solvent content is below this range, the viscosity of the composition is high, resulting in insufficient workability, while when it is above this range, it may take a long time to remove the solvent during curing.

[0059] additive

[0060] The clear coating composition may further comprise one or more additives selected from UV absorbers, curing catalysts, leveling agents, and defoaming agents.

[0061] The additives may be included in an amount of 1 to 10 wt % or 2 to 6 wt % based on the total weight of the paint composition.

[0062] UV absorbers absorb UV light to prevent discoloration of the coating composition and are used to prevent swelling, delamination, and loss of gloss of the coating film produced by the coating composition. The UV absorber is not particularly limited as long as it is a UV absorber commonly used in coating compositions, and may include, for example, benzotriazole-based compounds such as hydroxyphenylbenzotriazole. Commercially available UV absorbers include Tinuvin 384 and Tinuvin 5151, available from BASF.

[0063] The curing catalyst is used to prevent incomplete curing of the coating composition, thereby improving the mechanical properties of the coating film produced therefrom. In addition, the curing catalyst may include dibutyltin dilaurate, triethylamine, diethylenetriamine, bismuth carboxylate, zirconium chelate, dinonylnaphthalene disulfonic acid, etc.

[0064] The leveling agent imparts smoothness to the coating composition to improve the appearance characteristics of the coating composition produced therefrom and is used to suppress the appearance of orange peel. In addition, the leveling agent is not particularly limited as long as it is a leveling agent commonly used in coating compositions, and may include, for example, ether-modified polysiloxane.

[0065] The defoaming agent is used to suppress the occurrence of bubbles during the preparation of the coating composition and to suppress or eliminate phenomena, pinholes, or popping caused during the formation of the coating film. The defoaming agent is not particularly limited as long as it is a defoaming agent commonly used in coating compositions, and may include, for example, BYK-011, BYK-015, BYK-072, and BYKETOL-OK available from BYK, DF-21 available from Air Products, agitan 281 available from Munzing, Foamster-324 available from Sannopco, and the like as commercially available products.

[0066] One-component paint composition

[0067] The clear coating composition is a one-component type coating composition comprising two acrylic resins and a blocked isocyanate-based curing agent.

[0068] The clear coating composition may have a solid content (NV) of 40 to 65 wt %, 45 to 60 wt %, or 40 to 55 wt %. When the clear coating composition has a solid content within this range, coating workability of the composition may be improved.

[0069] The clearcoat composition can be cured at 100°C to 130°C, 105°C to 125°C, or 110°C to 120°C. Since the clearcoat composition can be cured within this temperature range, the cost required for the coating process can be reduced, thus being economical. Furthermore, since the composition can be used to integrally coat the vehicle body and the material components attached thereto, the problem of the painted component having a different color from the vehicle body can be avoided. Furthermore, since the clearcoat composition can be applied to both the vehicle body and the component in a single coating process, inconvenience and cost losses during the process can be reduced.

[0070] The clear coating composition may have a viscosity of 40 to 70 seconds, 45 to 65 seconds, or 50 to 60 seconds based on Ford Cup No. 4. When the viscosity of the clear coating composition is lower than this range, problems such as downward flow on a vertical plane may occur, while when it is higher than this range, the viscosity of the composition is so high that the appearance characteristics of the resulting coating film deteriorate or a load is imposed on the coating machine to cause a malfunction of the coating machine.

[0071] Because the clearcoat composition according to the present invention, as described above, can be cured at low temperatures of 130°C or lower and can reduce coating process costs, it is economical. Furthermore, because the composition can be used to integrally coat a vehicle body and attached material components, it can prevent the problem of the painted component and the vehicle body having a different color after painting. Furthermore, because the clearcoat composition according to the present invention can be applied to both the vehicle body and the component in a single coating process, process inconvenience and cost losses can be reduced. Furthermore, because the coating film produced by the clearcoat composition has excellent mechanical properties, such as appearance characteristics, adhesion, water resistance, scratch resistance, and solvent resistance, it can be used for vehicle body painting.

[0072] Best Mode for Carrying Out the Invention

[0073] Below, the present invention will be described in more detail by the following examples. However, the following examples are only used to help understand the present invention, and the scope of the present invention is not limited thereto in any sense.

[0074] Examples 1 to 13 and Comparative Examples 1 to 10: Preparation of Clear Coating Compositions

[0075] Components of the compositions shown in Tables 1 to 3 below were stirred and mixed, respectively, to prepare clear coating compositions having a viscosity of 55 seconds based on Ford Cup No. 4.

[0076] [Table 1]

[0077]

[0078]

[0079] [Table 2]

[0080]

[0081]

[0082] [Table 3]

[0083]

[0084] The manufacturers, product names and physical properties of the components used in the comparative examples and examples are as follows:

[0085] [Table 4]

[0086]

[0087] Test Example: Measurement of physical properties of coating films produced from paint compositions

[0088] A topcoat (manufacturer: KCC, product name: WT3900) was applied to the sample in a bell shape to a dry film thickness of 15 μm, and the remaining water in the paint was evaporated while blowing hot air at 80°C for 3 minutes to form a topcoat film. The clearcoat compositions of Examples and Comparative Examples were then applied over the topcoat film and cured at 120°C for 25 minutes to form a clearcoat film having a thickness of 40 μm.

[0089] The physical properties of the samples described above were measured, and the results are shown in Table 5.

[0090] Specifically, a manual spray gun (nozzle diameter: 1.5 mm, air pressure: kept constant at 4.5 kgf / cm 2 The clear coating composition was applied while keeping the distance between the nozzle opening and the sample constant at 30 cm and moving horizontally at a speed of 40-50 cm / second.

[0091] (1) Coating processability

[0092] When the paint was sprayed out with a manual spray gun, the clear paint compositions of Examples and Comparative Examples were painted back and forth twice, and the degree of atomization, smoothness and buildup of the paint during painting were compared.

[0093] Specifically, upon visual inspection, if the paint spreadability on the surface is good and the buildup is excellent, the coating workability is evaluated as excellent (◎); upon visual inspection, if the buildup is good but the paint spreadability is insufficient, the coating workability is evaluated as good (○); upon visual inspection, if both the buildup and the paint spreadability are insufficient and there are less than 3 paint foreign objects, the coating workability is evaluated as fair (△), and if there are 3 or more paint foreign objects and shrinkage cavities occur, the coating workability is evaluated as poor (×).

[0094] (2) Appearance

[0095] The gloss (LU), sharpness (SH), and orange peel (OP) of the painted samples were measured using a Wave Scan DOI (BYK Gardner) automotive exterior instrument, and the comprehensive appearance evaluation value (CF) was calculated by the following Equation 1 using the measured physical properties.

[0096] [Mathematical formula 1]

[0097] CF=LU×0.15+SH×0.35+OP×0.5

[0098] At this time, the CF is measured and calculated in the transverse and longitudinal directions.

[0099] When the calculated CF was 65 or higher, the appearance was evaluated as excellent (◎), when the CF was 60 or higher and less than 65, the appearance was evaluated as good (○), when the CF was 55 or higher and less than 60, the appearance was evaluated as fair (△), and when the CF was less than 55, the appearance was evaluated as poor (×).

[0100] (3) Glossiness

[0101] The 20° glossiness of the painted samples was measured using a gloss meter, and a 20° glossiness of 90 or more was evaluated as excellent (◎), a 20° glossiness of 85 or more and less than 90 was evaluated as good (○), a 20° glossiness of 80 or more and less than 85 was evaluated as fair (△), and a 20° glossiness of less than 80 was evaluated as poor (×).

[0102] (4) Recoating adhesion

[0103] The topcoat film and the clearcoat film were all removed, the topcoat and clearcoat compositions were applied in the same manner as above, and the recoat adhesion was evaluated by the checkerboard method.

[0104] Specifically, the checkerboard method is a method of measuring adhesion by cutting the surface of a clear coating film into 100 squares of 2 mm width and 2 mm height with a knife and peeling the squares with tape. At this time, when 100% of the 100 squares are completely adhered, the measured adhesion is evaluated as excellent (◎), when 70% or more and less than 100% of the squares remain, the measured adhesion is evaluated as good (○), when 50% or more and less than 70% of the squares remain, the measured adhesion is evaluated as fair (△), and when less than 50% of the squares remain, the measured adhesion is evaluated as poor (×).

[0105] (5) Impact resistance

[0106] The impact resistance of the coating sample was evaluated according to ASTM D2794. At this time, a DuPont impact tester was used, and a weight of 500 g was dropped onto the sample while the drop height of the weight was changed from 30 cm to 50 cm to observe the appearance of the coating film. As a result of observing the appearance of the coating film, when no cracks or delamination occurred on the lower paint coating film at a drop height of 50 cm, the impact resistance was evaluated as excellent (◎), when the height was 40 cm or more and less than 50 cm, the impact resistance was evaluated as good (○), when the height was 30 cm or more and less than 40 cm, the impact resistance was evaluated as general (△), and when the height was less than 30 cm, the impact resistance was evaluated as poor (×).

[0107] (6) Water resistance

[0108] The coated sample was immersed in a constant temperature water bath at 40°C for 240 hours, allowed to stand at room temperature for 1 hour, and then evaluated for adhesion in the same manner as the checkerboard method in item (4), and the evaluation criteria were applied identically.

[0109] (7) Anti-sagging

[0110] A sample having a topcoat coating film formed on a steel plate having a hole having a diameter of 5 mm was hung vertically, the clear coating compositions of Examples and Comparative Examples were applied in the same manner as described above, dried and cured, and the surface of the resulting coating film was observed to evaluate the anti-sagging property of the composition.

[0111] Specifically, for sag resistance, the presence of paint formation at the bottom of the hole was observed, and the thickness (μm) of the coating film at the starting point where flow was observed was recorded as the flow limit film thickness. The lower the value of the measured flow limit film thickness, the less sag resistance was considered. When the flow limit film thickness was 40 μm or greater, sag resistance was evaluated as excellent (◎); when the flow limit film thickness was 35 μm or greater and less than 40 μm, sag resistance was evaluated as good (○); when the flow limit film thickness was 30 μm or greater and less than 35 μm, sag resistance was evaluated as fair (△); and when the flow limit film thickness was less than 30 μm, sag resistance was evaluated as poor (×).

[0112] (8) Resistance to cold cracking

[0113] A sample having a size of 150 mm×70 mm was left to stand at −20±3° C. for 3 hours, and cold chipping resistance was measured using a chipping resistance tester (manufacturer: Stone Impact Tester, model name: SAE J 400).

[0114] Specifically, the cold chipping resistance is measured by spraying 50g of crushed stone (diameter: 4mm) at a pressure of 4bar at an angle of 45° on the clear coating film, removing the sample, and removing foreign matter such as the peeling coating remaining on the coating film with cellophane tape. At this time, when the damaged area of ​​the finished coating film is 1mm or less, the cold chipping resistance is evaluated as excellent (◎), when the damaged area of ​​the finished coating film is greater than 1mm and less than or equal to 2mm, the cold chipping resistance is evaluated as good (○), when the damaged area of ​​the finished coating film is greater than 2mm and less than or equal to 3mm, the cold chipping resistance is evaluated as fair (△), and when the damaged area of ​​the finished coating film is greater than 3mm, the cold chipping resistance is evaluated as poor (×).

[0115] [Table 5]

[0116]

[0117] As can be seen from Table 5, the clear coating compositions of the Examples have excellent coating workability, and the coating films produced from the compositions are all excellent in appearance, gloss, recoat adhesion, impact resistance, water resistance, sag resistance, and cold chip resistance.

[0118] Meanwhile, the coating films produced by Comparative Examples 1 and 2, which comprised only one acrylic resin, exhibited insufficient gloss and water resistance. In particular, Comparative Example 1, which comprised only the first acrylic resin, also exhibited insufficient coating workability. Furthermore, Comparative Example 2, which comprised only the second acrylic resin, also exhibited insufficient impact resistance and cold crack resistance.

[0119] In addition, Comparative Example 3, which includes the first acrylic resin-2 having a low hydroxyl value, and Comparative Example 4, which includes the first acrylic resin-3 having a high hydroxyl value, have insufficient impact resistance and water resistance. In addition, the composition of Comparative Example 4 has insufficient coating workability, and the resulting coating film has insufficient cold crack resistance.

[0120] Comparative Example 5, which included the first acrylic resin-4 having a low weight-average molecular weight, and Comparative Example 6, which included the first acrylic resin-5 having a high weight-average molecular weight, had insufficient water resistance and cold crack resistance. Furthermore, Comparative Example 5 also had insufficient recoating adhesion. Furthermore, the coating film of Comparative Example 6 also had insufficient appearance and sag resistance.

[0121] In addition, the coating film appearance characteristics of Comparative Example 7 containing the second acrylic resin-2 having a low hydroxyl value and Comparative Example 8 containing the second acrylic resin-3 having a high hydroxyl value were insufficient. Furthermore, the gloss and sag resistance of Comparative Example 7 were also insufficient, and the coating workability of Comparative Example 8 was also insufficient, and the impact resistance, water resistance, and cold crack resistance of the resulting coating film were also insufficient.

[0122] Furthermore, Comparative Example 9, which includes the second acrylic resin-4 having a low weight average molecular weight, and Comparative Example 10, which includes the second acrylic resin-5 having a high weight average molecular weight, have insufficient water resistance and cold crack resistance. Furthermore, Comparative Example 9 also has insufficient impact resistance, and Comparative Example 10 also has insufficient coating workability and anti-sagging properties.

Claims

1. A low-temperature curable one-shot clear coating composition comprising: a first acrylic resin, a second acrylic resin, a blocked isocyanate-based curing agent, and a solvent. wherein the first acrylic resin has a hydroxyl value of 160 to 250 mg KOH / g, an acid value of 7 mg KOH / g or less, a weight average molecular weight of 15,000 to 60,000 g / mol, and a glass transition temperature of 40 to 70° C., the second acrylic resin having a hydroxyl value of 100 to 150 mg KOH / g, an acid value of 5 mg KOH / g or less, a weight average molecular weight of 5,000 to 10,000 g / mol, and a glass transition temperature of 30 to 70° C.; The blocked isocyanate-based curing agent has a viscosity at 25° C. of 5,000 to 10,000 cps, an unreacted NCO group content of 5 to 10 wt %, and a weight average molecular weight of 1,500 to 4,000 g / mol; and comprising the first acrylic resin and the second acrylic resin in a weight ratio of 2 to 7:1; Based on the total weight of the coating composition, the coating composition comprises 30 to 60 wt% of the first acrylic resin, 1 to 20 wt% of the second acrylic resin, 10 to 30 wt% of the blocked isocyanate-based curing agent, and 10 to 45 wt% of the solvent.

2. The low-temperature curable one-shot type clear coating composition according to claim 1 , wherein the blocked isocyanate-based curing agent is one or more blocking agents selected from reactive methylene-based compounds, polyalkylene polyol compounds, and pyrazole-based compounds, and has a blocked terminal isocyanate group.

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