Low temperature curable transparent coating compositions

Abstract

The present invention relates to a low temperature curable transparent coating composition comprising: an acrylic resin; a first polyester resin; a second polyester resin; and an isocyanate based curing agent, wherein the first polyester resin has a hydroxyl value of 240 to 320 mg KOH / g, and the second polyester resin has a hydroxyl value of 100 to 180 mg KOH / g.

Description

TECHNICAL FIELD

[0001] The present invention relates to a low-temperature curable transparent coating composition which can be cured at a low temperature, is low in manufacturing cost, and produces a coating film having excellent appearance properties. BACKGROUND

[0002] Generally, the outer panel of a vehicle body should not have a deteriorated coating film and rust, and should have durability to maintain the gloss or color of the coating film. Therefore, the coating process of a vehicle is generally performed by electrocoating the vehicle body after a pretreatment process, performing intermediate coating to improve adhesion and smoothness, and performing base coating on the intermediate-coated vehicle body to improve the appearance of the vehicle body. Thereafter, in order to protect the color of the base coating film, improve the appearance thereof, and protect the base coating film from external influences, a transparent coating film is generally coated.

[0003] As a conventional transparent coating paint for automobiles, a thermosetting coating composition including a resin having a hydroxyl group and an amino resin is widely used. However, when the conventional thermosetting coating composition is used, a component composed of a plastic material such as a bumper and a rearview mirror can have appearance damage or deformation when cured under a high-temperature curing condition, and thus there is inconvenience in that the component is separated from the vehicle body and a separate coating process is performed. In addition, the color of the plastic material component which is separately coated from the vehicle body is different from the color of the vehicle body. Therefore, a low-temperature curable transparent coating composition having a reduced curing temperature so as to perform a coating process in the case in which a plastic material component is integrated into a vehicle body is attracting attention.

[0004] As an alternative thereto, Korean Patent Registration No. 1,655,621 (Patent Document 1) discloses a transparent coating composition including two kinds of acrylic polyol resins, a polyester polyol resin, a reactive silicon additive, and an isocyanate curing agent. However, since the coating film produced by the conventional low-temperature curable transparent coating composition such as the transparent coating composition of Patent Document 1 has insufficient mechanical properties, it can be applied only to the repair or coating of a component, and has limitations in the application to vehicle body coating.

[0005] Therefore, there is a need to research and develop a transparent coating composition which can be cured at a low temperature of 120°C or less and which produces a coating film having excellent mechanical properties, and thus is suitable for application to vehicle body coating. SUMMARY

[0006] Technical problem

[0007] The present invention provides a transparent coating composition which can be cured at a low temperature of 120°C or less and which produces a coating film having excellent mechanical properties.

[0008] Technical scheme

[0009] The present application provides a low-temperature curable transparent coating composition, which comprises: an acrylic resin, a first polyester resin, a second polyester resin, and an isocyanate-based curing agent,

[0010] wherein the first polyester resin has a hydroxyl value of 240 to 320 mg KOH / g, and

[0011] the second polyester resin has a hydroxyl value of 100 to 180 mg KOH / g.

[0012] Beneficial effects

[0013] Since the transparent coating composition according to the present application can be cured at a low temperature of 120°C or less and can reduce the energy cost of the coating process, it is economical, and since it can perform integrated coating of a vehicle body and a material part attached to the vehicle body, it can prevent a problem of color mismatch between the coated part and the vehicle body after coating. In addition, since both the vehicle body and the part can be coated by one coating process, inconvenience and cost loss in the process can be reduced. Furthermore, since the coating film produced from the transparent coating composition has excellent mechanical properties such as hardness, adhesion, water resistance, acid resistance, scratch resistance, and solvent resistance, it can be used for vehicle body coating. DETAILED DESCRIPTION

[0014] Mode for carrying out the invention

[0015] Hereinafter, the present application will be explained in detail.

[0016] The "weight average molecular weight" and "number average molecular weight" used in the present specification are measured by a conventional method known in the art, and can be measured, for example, by a method of gel permeation chromatography (GPC).

[0017] In addition, the "glass transition temperature (Tg)" is measured by a method known in the art, and can be measured, for example, by a method of differential scanning calorimetry (DSC).

[0018] The functional group values such as "acid value" and "hydroxyl value" can be measured by a method well known in the art, and can be represented, for example, by a value measured by titration, etc.

[0019] In addition, in the present specification, "(meth)acrylic acid" means "acrylic acid" and / or "methacrylic acid", and "(meth)acrylate" means "acrylate" and / or "methacrylate".

[0020] The low-temperature curable transparent coating composition according to the present application comprises an acrylic resin, a first polyester resin, a second polyester resin, and an isocyanate-based curing agent.

[0021] Acrylic resin

[0022] The acrylic resin is used to impart film-forming properties and mechanical properties to the composition.

[0023] As the acrylic resin, a product directly synthesized according to a known method can be used, or a commercially available product can be used. For example, the acrylic resin can be prepared by polymerizing one or more of a vinyl-based monomer and a (meth)acrylate-based monomer.

[0024] The kind of the vinyl-based monomer is not particularly limited, and for example, one or more selected from styrene, methylstyrene, dimethylstyrene, fluorostyrene, ethoxystyrene, methoxystyrene, phenylenevinyl ketone, vinyl tert-butylbenzoate, vinyl cyclohexanoate, vinyl acetate, vinyl pyrrolidone, chlorovinyl, vinyl alcohol, acetoxy styrene, tert-butylstyrene, and vinyltoluene can be used.

[0025] The (meth)acrylate-based monomer can include one or more selected from a (meth)acrylate-based monomer not containing a hydroxyl group and a (meth)acrylate monomer containing a hydroxyl group.

[0026] The (meth)acrylate monomer not containing a hydroxyl group can include, for example, one or more selected from (meth)acrylic acid, methyl(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, amyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, isooctyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, isobornyl (meth)acrylate, lauryl (meth)acrylate.

[0027] The (meth)acrylate monomer containing a hydroxyl group can be, for example, a hydroxyalkyl-containing (meth)acrylate, and specifically, can include one or more selected from 2-hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate.

[0028] For example, the acrylic resin can be produced using a radical polymerization method, and physical properties, i.e., weight average molecular weight (Mw), hydroxyl value (OHv), acid value (Av), etc. can be adjusted according to an initiator and a polymerization time.

[0029] In addition, the acrylic resin can have a weight average molecular weight (Mw) of 10,000 to 40,000 g / mol, 15,000 to 35,000 g / mol, or 20,000 to 30,000 g / mol. When the weight average molecular weight of the acrylic resin is within this range, the hardness of the resulting coating film can be excellent. In addition, when the weight average molecular weight of the acrylic resin is lower than this range, its molecular weight is too small to provide the resulting coating film with sufficient water resistance and chemical resistance, and when the weight average molecular weight is higher than this range, the viscosity increases as the molecular weight increases, and the processability of the coating composition including the acrylic resin is poor and the leveling property is not good, so that it can be difficult to produce a coating film with an excellent appearance.

[0030] In addition, the acrylic resin can have a hydroxyl value (OHv) of 100 to 200 mg KOH / g or 145 to 170 mg KOH / g. When the hydroxyl value of the acrylic resin is within this range, it has the effect of improving the weather resistance of the coating film. In addition, when the hydroxyl value of the acrylic resin is lower than this range, the coating film formation by cross-linking reaction with the curing agent is insufficient, so that the hardness, water resistance, etc. of the coating film are deteriorated, and when it is higher than this range, over-curing occurs, so that the coating film becomes brittle and loses elasticity, and the appearance and weather resistance of the resulting coating film can be deteriorated.

[0031] The acrylic resin can have an acid value (Av) of 5 to 15 mg KOH / g or 8 to 11 mg KOH / g. When the acid value of the acrylic resin is within this range, the water dispersibility and storage stability of the coating composition can be improved and the water resistance can be improved. In addition, when the acid value of the acrylic resin is lower than this range, the curing reaction rate decreases to deteriorate the appearance of the resulting coating film and the water dispersion stability of the resin, and when it is higher than this range, the viscosity of the composition increases to deteriorate the processability and water resistance of the coating film.

[0032] The acrylic resin can have a glass transition temperature (Tg) of 20 to 100°C, 40 to 70°C, or 52 to 58°C. When the glass transition temperature of the acrylic resin is within this range, the coating film formability and defoaming property of the composition and the gloss characteristics of the coating film are improved. In addition, when the glass transition temperature of the acrylic resin is lower than this range, the drying speed and cross-linking density of the coating film decrease to deteriorate the impact resistance and water resistance of the resulting coating film, and when it is higher than this range, the coating film becomes brittle and its appearance and hardness can decrease.

[0033] The acrylic resin can have a solid content (NV) of 60 to 80 wt%, 65 to 75 wt%, or 68 to 73 wt% relative to the total weight of the resin. When the solid content of the acrylic resin is within this range, the storage stability of the resin and the storage stability of the coating composition can be improved and the processability can be excellent. In addition, when the solid content of the acrylic resin is lower than this range, the viscosity is too low such that the processability of the coating composition including the resin becomes insufficient, while when it is higher than this range, the viscosity of the acrylic resin is too high such that the stability during the reaction is poor and the dispersion stability is deteriorated, so that agglomeration can occur over time.

[0034] In addition, the content of the acrylic resin included can be 15 to 35 wt% or 20 to 30 wt% based on the total weight of the coating composition. When the acrylic resin included is within this content range, the adhesion and impact resistance of the coating film can be improved. In addition, when the content of the acrylic resin is lower than this range, the drying property is reduced, thereby deteriorating the adhesion strength and durability of the coating film, while when it is higher than this range, the paint viscosity increases to deteriorate the coating processability, paint flowability, and water resistance.

[0035] First polyester resin

[0036] The first polyester resin serves to form the appearance properties and mechanical properties of the coating film produced by chemical bonding with the isocyanate-based curing agent.

[0037] As the first polyester resin, a product directly synthesized according to a known method can be used, or a commercially available product can be used. For example, the first polyester resin can be produced by reacting a first carboxylic acid and a first polyol.

[0038] Herein, the first carboxylic acid can be one or more selected from adipic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), an alicyclic acid, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride (HHPA), hexahydrophthalic anhydride, and derivatives thereof.

[0039] The first polyol can be, for example, one or more selected from methoxypolyethylene glycol, 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), trimethylolpropane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylene glycol.

[0040] The first polyester resin can have a hydroxyl value (OHv) of 240 to 320 mg KOH / g or 260 to 290 mg KOH / g. When the first polyester resin has a hydroxyl value within this range, the spreadability of the coating film is improved, and the chemical resistance through urethane reaction is improved. In addition, when the hydroxyl value of the first polyester resin is lower than this range, the crosslinking density with the isocyanate-based curing agent is insufficient, thereby deteriorating the durability and chemical resistance of the resulting coating film, and when it is higher than this range, over-curing occurs, such that the coating film becomes brittle, and the impact resistance and cold chipping resistance are deteriorated.

[0041] In addition, the first polyester resin can have a number average molecular weight (Mn) of 100 to 1,000 g / mol, 200 to 900 g / mol, or 300 to 800 g / mol. When the number average molecular weight of the first polyester resin is within this range, there is an effect of imparting smoothness to the paint and forming a soft coating film. When the number average molecular weight of the first polyester resin is lower than this range, the molecular weight is small, such that the mechanical properties of the resulting coating film are deteriorated, and when it is higher than this range, the flowability is deteriorated due to the increase in molecular weight, such that the appearance is deteriorated and the coating film becomes brittle, thereby deteriorating the cold chipping resistance and impact resistance.

[0042] The first polyester resin can have an acid value (Av) of 5 to 35 mg KOH / g or 15 to 25 mg KOH / g. When the acid value of the first polyester resin is within this range, rapid curing through heat treatment is prevented, thereby preventing appearance defects of the coating film, and reducing the occurrence of popping. When the acid value of the first polyester resin is lower than this range, the curing reaction rate is reduced to deteriorate the hardness and appearance properties of the resulting coating film, and when it is higher than this range, the hydrophilicity is increased to deteriorate the water resistance.

[0043] In addition, the first polyester resin can have a solid content (NV) of 50 to 80 wt% or 60 to 70 wt%. When the solid content of the first polyester resin is within this range, the content of total volatile organic compounds (TVOC) can be reduced due to the high solid content. When the solid content of the first polyester resin is lower than this range, the curing reactivity is reduced due to the reduced solid content of the composition, and when it is higher than this range, poor processability of the resulting paint can occur, thereby deteriorating the appearance.

[0044] In addition, the content of the included first polyester resin can be 5 to 15 wt% or 8 to 12 wt% based on the total weight of the coating composition. When the included first polyester resin is within this content range, the appearance, scratch resistance, and coating film smoothness can be improved. When the content of the first polyester resin in the composition is lower than this range, the crosslinking density is reduced to deteriorate the mechanical properties and appearance, and when it is higher than this range, the viscosity of the composition can become excessively high, thereby deteriorating the processability and drying properties.

[0045] The composition can include the first polyester resin and the second polyester resin in a weight ratio of 1.5:1 to 2.5:1 or 1.8:1 to 2.2:1. When the mixed weight ratio of the first polyester resin and the second polyester resin is within the range, the durability and scratch resistance of the resulting coating film are improved. When the mixed weight ratio of the first polyester resin and the second polyester resin is lower than the range, i.e., the content of the first polyester resin is lower than that of the second polyester resin, the crosslinking density of the resulting coating film is reduced to deteriorate the durability and scratch resistance, and when it is higher than the range, i.e., the content of the first polyester resin is higher than that of the second polyester resin, the viscosity of the composition is increased to deteriorate the drying property and curing reactivity, thereby deteriorating the mechanical properties of the resulting coating film.

[0046] Second polyester resin

[0047] The second polyester resin serves to increase the solid content of the composition and improve the appearance, smoothness, scratch resistance, and slipperiness of the resulting coating film.

[0048] As the second polyester resin, a product directly synthesized according to a known method can be used, or a commercially available product can be used. For example, the second polyester resin can be produced by reacting a second carboxylic acid and a second polyol.

[0049] Herein, the second carboxylic acid can be one or more selected from adipic acid (AA), isophthalic acid (IPA), trimellitic anhydride (TMA), an alicyclic acid, phthalic anhydride, isophthalic acid, terephthalic acid, succinic acid, adipic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride (HHPA), isononyl acid (INA), and derivatives thereof.

[0050] The second polyol can be one or more selected from 1,6-hexanediol (1,6-HD), pentaerythritol, sorbitol, methoxypolyethylene glycol, neopentyl glycol (NPG), trimethylolpropane (TMP), ethylene glycol, propylene glycol, diethylene glycol, butanediol, 1,4-hexanediol, and 3-methylene glycol.

[0051] The second polyester resin can have a hydroxyl value (OHv) of 100 to 180 mg KOH / g or 130 to 150 mg KOH / g. When the second polyester resin has a hydroxyl value within the range, the spreadability of the coating film is improved, and the chemical resistance through urethane reaction is improved. In addition, when the hydroxyl value of the second polyester resin is lower than the range, the crosslinking density with the isocyanate-based curing agent is insufficient, thereby deteriorating the appearance, durability, and chemical resistance of the resulting coating film, and when it is higher than the range, overcuring occurs, such that the coating film becomes brittle, and the coating workability and cold-cracking resistance are deteriorated.

[0052] In addition, the second polyester resin can have a number average molecular weight (Mn) of 1,200 to 5,000 g / mol, 1,300 to 4,500 g / mol, or 1,500 to 4,000 g / mol. When the number average molecular weight of the second polyester resin is within this range, there are effects of coating smoothness and formation of a soft coating film. When the number average molecular weight of the second polyester resin is lower than this range, the molecular weight is small, so that the mechanical properties of the resulting coating film become poor, and when it is higher than this range, the flowability becomes poor due to the increase in molecular weight, so that the appearance becomes poor and the coating film becomes hard, thereby making the cold crack resistance poor.

[0053] In addition, the second polyester resin can have an acid value (Av) of 10 to 40 mg KOH / g or 20 to 30 mg KOH / g. When the acid value of the second polyester resin is within this range, rapid curing by heat treatment is avoided, so that appearance defects of the coating film are prevented, and the occurrence of popping is reduced. When the acid value of the second polyester resin is lower than this range, the curing reaction rate decreases to make the hardness and appearance characteristics of the resulting coating film poor, and when it is higher than this range, the hydrophilicity increases to make the water resistance poor.

[0054] The second polyester resin can have a solid content (NV) of 60 to 80 wt% or 65 to 75 wt%. When the solid content of the second polyester resin is within this range, the content of total volatile organic compounds (TVOC) can be reduced due to the high solid content. When the solid content of the second polyester resin is lower than this range, the solid content of the composition decreases to make the curing reactivity poor, thereby making the mechanical properties of the resulting coating film poor, and when it is higher than this range, the processability of the resulting coating paint is poor, thereby making the appearance characteristics of the coating film poor.

[0055] In addition, the content of the second polyester resin included can be 1 to 10 wt% or 3 to 7 wt% based on the total weight of the coating composition. When the second polyester resin included is within this content range, the appearance, scratch resistance, and coating film smoothness can be improved. When the content of the second polyester resin in the composition is lower than this range, the crosslinking density of the coating film decreases to make the cold crack resistance and scratch resistance poor, and when it is higher than this range, the viscosity of the composition can become too high to make the processability and drying property poor, thereby making the appearance poor.

[0056] Isocyanate-based curing agent

[0057] The isocyanate-based curing agent reacts with the hydroxyl groups of the resin as described above to form urethane bonds, thereby curing the composition, thereby forming a coating film.

[0058] The type of isocyanate-based curing agent is not particularly limited, for example, the isocyanate-based curing agent can include an aliphatic polyisocyanate compound having no yellowness. Specifically, the isocyanate-based curing agent can include hexamethylene diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, tetramethylxylene diisocyanate, or a mixture thereof.

[0059] In addition, the isocyanate-based curing agent can have a content of unreacted isocyanate groups (NCO%) of 15 to 30 wt% or 19 to 24 wt% with respect to the total weight of the curing agent. When the NCO% of the isocyanate-based curing agent is lower than this range, the crosslinking density of the coating film is reduced to deteriorate the impact resistance, water resistance, and cold shatter resistance of the coating film, and when it is higher than this range, the curing rate is increased and the resin is agglomerated to deteriorate the appearance properties, gloss, and mechanical properties of the coating film.

[0060] The content of the isocyanate-based curing agent included in the composition can be 20 to 35 wt% or 24 to 29 wt% based on the total weight of the coating composition. When the content of the isocyanate-based curing agent is lower than this range, the reactivity of the composition is insufficient, thereby deteriorating the crosslinking density, so that the mechanical properties are deteriorated, and when it is higher than this range, unreacted isocyanate groups appear in the coating film after curing, thereby deteriorating the appearance and processability of the coating film.

[0061] The transparent coating composition can further include a solvent.

[0062] Solvent

[0063] The solvent serves to adjust the viscosity of the composition, improve the drying property, and improve the appearance properties and spreadability of the produced coating film.

[0064] The solvent is not particularly limited as long as it is commonly used in the transparent coating composition, and for example, can include one or more selected from the group consisting of an aromatic solvent, an acetic acid ester-based solvent, an alcohol-based solvent, and a propionic acid ester-based solvent. Specifically, the solvent can include an aromatic solvent such as toluene and xylene; an acetic acid ester-based solvent such as 1-methoxy-2-propyl acetate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methyl glutarate, methyl succinate, methyl adipate, dimethyl glutarate, dimethyl succinate, dimethyl adipate, propylene glycol methyl ether acetate (PMA), butyl carbitol acetate, and butyl cellosolve acetate; an alcohol-based solvent such as n-butanol, propanol, 1-methoxy-2-propanol, and 2-butoxy ethanol; a ketone-based solvent such as acetone, methyl ethyl ketone, methyl butyl ketone, and methyl isobutyl ketone; a propionic acid ester-based solvent such as ethoxyethyl propionate, and the like. In addition, commercially available products of the aromatic solvent can include Kocosol #100, Kocosol #150, and the like.

[0065] In addition, the solvent included in the composition can be included in an amount of 15 to 45 wt% or 25 to 35 wt% based on the total weight of the coating composition. When the solvent is included in this range, the viscosity of the composition is properly adjusted to improve processability and dryability.

[0066] Additive

[0067] The transparent coating composition according to the present application can further include additives such as a curing catalyst, a UV absorber, a surface smoothing agent, and a flowability adjusting agent. Herein, the additives are not particularly limited as long as they are generally added to the coating composition.

[0068] In addition, the content of the additives is not particularly limited as long as it is generally included in the transparent coating composition. For example, the content of the additives included in the coating composition can be 1 to 10 wt% or 3 to 7 wt% based on the total weight of the coating composition.

[0069] Two-pack type coating composition

[0070] The transparent coating composition can be a two-part type coating composition including a main part and a curing agent part. For example, the transparent coating composition can include a main part including an acrylic resin, a first polyester resin, a second polyester resin, a solvent, and an additive, and a curing agent part including an isocyanate-based curing agent. In addition, the transparent coating composition can be used after mixing the main part and the curing agent part before application.

[0071] The transparent coating composition can have a solid content of 40 to 60 wt% or 45 to 55 wt%. When the solid content of the transparent coating composition is in this range, the coating processability of the composition can become suitable.

[0072] The transparent coating composition can be cured at 60 to 120℃, 70 to 110℃, or 80 to 110℃. Since the transparent coating composition can be cured in this temperature range, the cost required for the coating process can be reduced, thus being economical, and since it can perform integrated coating of the vehicle body and a material part attached thereto, the problem of color mismatch between the part and the vehicle body after coating can be prevented. In addition, since both the vehicle body and the part can be coated by one coating process, inconvenience and cost loss in the process can be reduced.

[0073] The clear coating composition can have a viscosity of 40 to 70 seconds or 50 to 60 seconds based on Ford Cup No. 4 at 25°C. When the viscosity of the clear coating composition at 25°C is lower than this range, problems such as a downward flow on a vertical plane can occur, and when it is higher than this range, the viscosity of the composition is too high so that the appearance properties of the coating film resulting therefrom are deteriorated or the load applied to the coater causes a failure of the coater.

[0074] Since the clear coating composition according to the present application as described above can be cured at a low temperature of 120°C or less and can reduce the cost of the painting process, it is economical, and since it can integrally paint the vehicle body and the material parts attached to the vehicle body, it can prevent the problem of the color of the parts after painting from being different from that of the vehicle body. In addition, since both the vehicle body and the parts can be painted by one painting process, inconvenience in the process and cost loss can be reduced. Furthermore, since the coating film resulting from the clear coating composition has excellent mechanical properties such as hardness, adhesion, water resistance, acid resistance, scratch resistance, and solvent resistance, it can be used for vehicle body painting.

[0075] Best mode for carrying out the invention

[0076] Hereinafter, the present application will be described in more detail by the following examples. However, the following examples are for the purpose of helping the understanding of the present application only and the scope of the present application is in no way limited thereto.

[0077] Examples 1 to 15 and Comparative Examples 1 to 7: Preparation of Clear Coating Composition

[0078] The components were mixed in the amounts in Tables 1 to 3 to prepare the clear coating composition, which had a viscosity of 55 seconds based on Ford Cup No. 4 at 25°C.

[0079] [Table 1]

[0080]

[0081] [Table 2]

[0082]

[0083]

[0084] [Table 3]

[0085]

[0086] Hereinafter, the manufacturers, product names, component names, etc. used in the comparative examples and examples are shown in Table 4.

[0087] [Table 4]

[0088]

[0089]

[0090] Experimental Example: Evaluation of Coating Film Properties

[0091] A top coat (manufacturer: KCC, product name: WT3090) was applied to the samples and dried to form a top coat film having a thickness of 15 μm. Then, the transparent coating compositions produced in the examples and comparative examples were applied to the top coat film, and cured at 100°C for 25 minutes to form a transparent coating film having a thickness of 40 μm, thereby producing a finished coating film. The physical properties of the samples were measured, and the results are shown in Table 5.

[0092] Specifically, the transparent coating film was applied using a manual spray gun (nozzle aperture: 1.5 mm, air pressure: maintained at 4.5 kgf / cm 2 to the left and right) while maintaining the distance between the nozzle opening and the sample constant at 30 cm and moving horizontally at a speed of 40-50 cm / sec.

[0093] (1) Coating workability

[0094] When the coating material was sprayed with the manual spray gun, the transparent coating compositions of the examples and comparative examples were applied back and forth twice, and the degree of atomization of the coating material during application, smoothness, and thick paste application ability were compared.

[0095] Specifically, the coating material spread well on the surface and had excellent thick paste application ability by visual inspection, and the application workability was evaluated as excellent (◎); the thick paste application ability was good but the coating material spread poorly by visual inspection, and the application workability was evaluated as good (0); the thick paste application ability and the coating material spread were both poor and the number of times that coating material foreign matter appeared was less than 3 by visual inspection, and the application workability was evaluated as fair (Δ), and the number of times that coating material foreign matter appeared was 3 or more and shrinkage holes occurred, and the application workability was evaluated as poor (X).

[0096] (2) Appearance

[0097] The gloss (LU), sharpness (SH), and orange peel (OP) of the finished coating film produced were measured using an automotive exterior appearance meter Wave Scan DOI (BYK Gardner), and using the measured physical properties, the comprehensive appearance evaluation value (CF) was calculated by Equation 1 below.

[0098] [Equation 1]

[0099] CF = LU x 0.15 + SH x 0.35 + OP x 0.5

[0100] At this time, the lateral and longitudinal measurements and the CF are calculated.

[0101] When the calculated CF is 65 or more, the appearance is evaluated as excellent ( ), when the CF is 60 or more and less than 65, the appearance is evaluated as good (0), when the CF is 55 or more and less than 60, the appearance is evaluated as fair (△), and when the CF is less than 55, the appearance is evaluated as poor (x).

[0102] (3) Gloss

[0103] The 20° gloss of the painted sample is measured using a gloss meter, and 20° gloss of 90 or more is evaluated as excellent ( ), 20° gloss of 85 or more and less than 90 is evaluated as good (0), 20° gloss of 80 or more and less than 85 is evaluated as fair (△), and 20° gloss of less than 80 is evaluated as poor (x).

[0104] (4) Overcoating adhesion

[0105] The topcoat paint film and the clear paint film are removed, the topcoat paint and the clear coat composition are painted in the same manner as described above, and the recoat adhesion is evaluated by the checkerboard method.

[0106] Specifically, in the checkerboard method, the surface of the clear paint film is cut into 100 squares of 2 mm width x 2 mm height with a knife, and the squares are peeled off using a tape to measure the adhesion. At this time, when 100% of the squares are completely attached, 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 (0), 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 (x).

[0107] (5) Impact resistance

[0108] The impact resistance of the painted sample is evaluated according to ASTM D2794. At this time, using a DuPont impact tester, a 500g weight is dropped onto the sample while the drop height of the weight is changed from 30 cm to 50 cm, and the appearance of the paint film is observed. As the observation result, when no cracks and peeling occur on the topcoat paint film at a weight drop height of 50 cm, the impact resistance is evaluated as excellent ( ), when the drop height is 40 cm or more and less than 50 cm, the impact resistance is evaluated as good (0), when the drop height is 30 cm or more and less than 40 cm, the impact resistance is evaluated as fair (△), and when the drop height is less than 30 cm, the impact resistance is evaluated as poor (x).

[0109] (6) Water resistance

[0110] The finished paint film was immersed in a constant temperature water tank 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 crosshatch method of item (4), with the same evaluation criteria applied.

[0111] (7) Paint flowability

[0112] The sample on which the topcoat paint film was formed on a steel plate having a hole with a diameter of 5 mm was hung vertically, the transparent coating composition of the examples and comparative examples was coated in the same manner as described above, and dried and cured, and the surface of the resulting paint film was observed to evaluate the paint flowability of the composition.

[0113] Specifically, the paint film thickness at which the onset of flow was observed was recorded as the flow limit film thickness. At this time, the lower the value of the measured flow limit film thickness, the less the paint flowability was considered to be. At this time, when the flow limit film thickness was 40 μm or more, the paint flowability was evaluated as excellent (◎), when the flow limit film thickness was 35 μm or more and less than 40 μm, the paint flowability was evaluated as good (O), when the flow limit film thickness was 30 μm or more and less than 35 μm, the paint flowability was evaluated as fair (Δ), and when the flow limit film thickness was less than 30 μm, the paint flowability was evaluated as poor (X).

[0114] (8) Cold shatter resistance

[0115] The coated sample with a size of 150 mm x 70 mm was allowed to stand at -20 ± 3°C for 3 hours, and the cold chipping resistance was measured using a chipping resistance tester (manufacturer: Anti Stone Tester, model name: SAE J 400). Specifically, 50 g of chippings (diameter: 4 mm) were sprayed at an angle of 45°C at a pressure of 4 bar on the finished paint. Then, the sample was removed, and the foreign matter remaining in the paint film, such as peeled paint film, was removed with a glassine tape.

[0116] At this time, when the damage area of the finished paint film was 1 mm or less, the cold chipping resistance was evaluated as excellent (◎), when the damage area was greater than 1 mm and less than or equal to 2 mm, the cold chipping resistance was evaluated as good (O), when the damage area was greater than 2 mm and less than or equal to 3 mm, the cold chipping resistance was evaluated as fair (Δ), and when the damage area was greater than 3 mm, the cold chipping resistance was evaluated as poor (X).

[0117] [Table 5]

[0118]

[0119]

[0120] As seen from Table 5, the composition of the examples has better paint workability and paint flow than the composition of the comparative examples, and the coating film produced from the composition of the examples has appearance, gloss, re-coat adhesion, impact resistance, and cold shatter resistance superior to those of the coating film of the comparative examples.

[0121] In addition, the coating film of Comparative Example 1 containing the first polyester resin-4 having a low hydroxyl value and the coating film of Comparative Example 3 containing the second polyester resin-4 having a low hydroxyl value are insufficient in impact resistance, water resistance, and cold shatter resistance. In particular, the coating film of Comparative Example 3 is insufficient in appearance properties.

[0122] In addition, the coating film of Comparative Example 2 containing the first polyester resin-5 having a high hydroxyl value and the coating film of Comparative Example 4 containing the second polyester resin-5 having a high hydroxyl value are insufficient in appearance properties, gloss, and cold shatter resistance. In particular, the coating film of Comparative Example 2 is also insufficient in impact resistance, while the composition of Comparative Example 4 is insufficient in paint workability.

[0123] The coating film of Comparative Example 5 containing no acrylic resin is insufficient in appearance properties, re-coat adhesion, impact resistance, water resistance, and cold shatter resistance.

[0124] In addition, the composition of Comparative Example 5 containing only one second polyester resin without a first polyester resin is insufficient in paint flow, and the coating film produced therefrom is insufficient in appearance properties, impact resistance, and cold shatter resistance.

[0125] The coating film of Comparative Example 7 containing only one first polyester resin without a second polyester resin is insufficient in gloss, impact resistance, water resistance, and cold shatter resistance.

Claims

1. A low temperature curable transparent coating composition comprising: an acrylic resin, a first polyester resin, a second polyester resin and an isocyanate based curing agent, wherein the acrylic resin having a weight average molecular weight of 10,000 to 40,000 g / mol, a hydroxyl value of 100 to 200 mg KOH / g, an acid value of 5 to 15 mg KOH / g and a glass transition temperature of 20 to 100 °C, wherein the first polyester resin has an acid value of 5 to 35 mg KOH / g, a hydroxyl value of 260 to 290 mg KOH / g and has a number average molecular weight of 300 to 800 g / mol, the second polyester resin has an acid value of 10 to 40 mg KOH / g, a hydroxyl value of 130 to 150 mg KOH / g and has a number average molecular weight of 1,500 to 4,000 g / mol, wherein the first polyester resin and the second polyester resin are comprised in a weight ratio of 1.5:1 to 2.5:1, wherein the isocyanate based curing agent has an unreacted isocyanate group content of 15 to 30 wt% relative to the total weight of the curing agent, and wherein the acrylic resin is comprised in an amount of 15 to 35 wt%, the first polyester resin is comprised in an amount of 5 to 15 wt%, the second polyester resin is comprised in an amount of 1 to 10 wt% and the isocyanate based curing agent is comprised in an amount of 20 to 35 wt% based on the total weight of the coating composition.

Images