Clear coat composition

Abstract

The present invention relates to a clear coat composition capable of low-temperature curing. The clear coat composition of the present invention comprises: a polyester resin; a first acrylic resin containing styrene units; a second acrylic resin having an acid value of 1.6-3.4 mg KOH / g; an isocyanate-based curing agent; and a curing catalyst.

Description

Clear coat composition

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2024-0060101, filed May 7, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a clear coat composition, and more particularly, to a clear coat composition that can be cured at low temperature and thus can be applied to various car body materials.

[0003] During automobile manufacturing, multiple coating processes, such as electrodeposition painting, intermediate painting, base painting, and clear painting, are performed to improve the exterior characteristics of the body and protect the body surface from the external environment. The coating process is generally performed by applying an electrodeposition paint to the body and curing it to form an electrodeposition film, applying an intermediate paint on the electrodeposition film and curing it to form an intermediate film, and then successively applying a base coat and a clear coat on the intermediate film, followed by drying and curing.

[0004] Meanwhile, automobile bodies are made of steel, while plastic is primarily used for parts. Because steel and plastic materials exhibit different degrees of thermal deformation, different paints are used in the painting process. Specifically, high-temperature curing paints are used for steel, which exhibits minimal thermal deformation, while low-temperature curing paints are used for plastic, which is easily deformed by heat. When different paints are used for steel and plastic, the painting process for the body and parts must be performed separately, which prolongs the process time, consumes significant energy during the coating curing process, and increases CO2 emissions.

[0005] The present invention is intended to solve the above problems, and provides a clear coat composition that can be cured at low temperatures even in steel sheet materials, can be applied to various body materials, and thus can integrate the body and parts painting process, and can reduce energy consumption and CO2 emissions in the painting process.

[0006] According to one embodiment, the present invention provides a clear coat composition comprising: a polyester resin; a first acrylic resin including a styrene unit; a second acrylic resin having an acid value of 1.6 mgKOH / g to 3.4 mgKOH / g; an isocyanate-based curing agent; and a curing catalyst, wherein the first acrylic resin is included in an amount of 5 to 15 parts by weight based on 100 parts by weight of the total weight of the clear coat composition.

[0007] Specifically, the clear coat composition may include a polyester resin, a first acrylic resin, a second acrylic resin, an isocyanate-based curing agent, and a curing catalyst in a weight ratio of 100:12.5 to 75:2.5 to 25:25 to 200:0.025 to 10.

[0008] The above polyester resin may have a hydroxyl value of 250 mgKOH / g to 300 mgKOH / g, an acid value of 5 mgKOH / g to 50 mgKOH / g, and a weight average molecular weight of 1,000 g / mol to 5,000 g / mol.

[0009] Meanwhile, the styrene unit may be included in an amount of 10 to 25 parts by weight based on 100 parts by weight of the first acrylic resin.

[0010] In addition, the first acrylic resin may have a hydroxyl value of 130 mgKOH / g to 150 mgKOH / g, an acid value of 10 mgKOH / g to 30 mgKOH / g, a weight average molecular weight of 30,000 g / mol to 70,000 g / mol, and a glass transition temperature of 20°C to 50°C.

[0011] When a polyester resin with excellent reactivity with a hardener is used in a clear coat composition, fast-curing and low-temperature curing characteristics can be imparted. However, as the content of the polyester resin increases, the coating properties such as coating adhesion, impact resistance, water resistance, flowability, and cold-chip resistance can deteriorate. Therefore, in the present invention, a first acrylic resin containing a styrene unit capable of maintaining the coating film hard after curing is used in combination with the polyester resin.

[0012] The above second acrylic resin is intended to improve the flowability of the clear coat composition. In the present invention, by applying an acrylic resin having an acid value (Av) of 1.6 mgKOH / g to 3.4 mgKOH / g, the effect of improving the flowability can be obtained by temporarily forming hydrogen bonds before curing after painting.

[0013] The above second acrylic resin may have a hydroxyl group of 130 mgKOH / g to 150 mgKOH / g, a weight average molecular weight of 6,000 g / mol to 10,000 g / mol, a viscosity of 3 Pa·s to 12 Pa·s, a particle size of 15 μm or less, a glass transition temperature of 1°C to 20°C, and an amine content of 6.0 mmol / kg or less.

[0014] The above curing agent includes an isocyanate curing agent. Conventionally, melamine curing agents have been commonly used as curing agents for clear coat compositions. However, low-temperature curing is difficult when melamine curing agents are used. Therefore, the present invention uses an isocyanate curing agent instead of a melamine curing agent, thereby improving low-temperature curing performance. Since the isocyanate curing agent has a faster reactivity than the melamine curing agent, the clear coat composition can be completely cured at 90°C when the physical properties of the polyester resin, the first acrylic resin, and the second acrylic resin satisfy the above ranges.

[0015] The clear coat composition according to the present invention may take 20 minutes or less to be completely cured at 90°C.

[0016] The clear coat composition according to the present invention improves low-temperature curing properties by applying a polyester resin, and simultaneously implements excellent coating quality even during low-temperature curing by using two specific acrylic resins in combination. Accordingly, the clear coat composition according to the present invention enables low-temperature baking (90°C, 20 minutes) and can be applied to both steel and plastic materials. Therefore, by using the clear coat composition according to the present invention, a body made of steel and parts made of plastic can be painted using a single paint and a single process, thereby improving the painting process.

[0017] In addition, since the clear coat composition according to the present invention can be fired at a lower temperature than before, the amount of energy required for painting and the amount of CO2 generated are reduced, thereby enabling the clear coat composition to respond to strengthened fuel efficiency and CO2 regulations.

[0018] Hereinafter, the present invention will be described in more detail.

[0019] In this specification, “weight average molecular weight” is measured by a conventional method known in the art, and can be measured by a method such as GPC (gel permeation chromatography).

[0020] Additionally, the “glass transition temperature” is measured by a conventional method known in the art, and can be measured by, for example, differential scanning calorimetry (DSC).

[0021] Functional values ​​such as “acid value” and “hydroxyl value” can be measured by methods well known in the art, for example, by titration.

[0022] Additionally, in this specification, “(meth)acrylic” means “acrylic” and / or “methacrylic”, and “(meth)acrylate” means “acrylate” and / or “methacrylate”.

[0023]

[0024] A clear coat composition according to the present invention comprises a polyester resin, a first acrylic resin containing a styrene unit, a second acrylic resin having an acid value of 1.6 mgKOH / g to 3.4 mgKOH / g, an isocyanate-based curing agent, and a curing catalyst, and may further comprise additives such as a UV absorber, a leveling agent, an antifoaming agent, a viscosity modifier, etc., as needed.

[0025]

[0026] Hereinafter, each component constituting the clear coat composition according to the present invention will be described.

[0027]

[0028] (1) Polyester resin

[0029] Polyester resin is used to improve curing properties. Because polyester resin exhibits excellent reactivity with curing agents, increasing the polyester resin content enables low-temperature curing.

[0030] The polyester resin may be synthesized directly using a known method, or a commercially available product may be used. For example, the polyester resin may be manufactured by reacting a carboxylic acid with a polyol. At this time, the carboxylic acid may be at least one selected from the group consisting of dimer acid, hexahydrophthalic anhydride (HHPA), adipic acid (AA), isophthalic acid (IPA), trimaletic anhydride (TMA), phthalic anhydride, terephthalic acid, succinic acid, fumaric acid, maleic anhydride, tetrahydrophthalic anhydride, and derivatives thereof. In addition, the polyol may be at least one selected from the group consisting of trimethylol propane (TMP), propylene glycol methyl ether acetate (PMA glycol ether), methoxypolyethylene glycol, 1,6-hexanediol (1,6-HD), neopentyl glycol (NPG), ethylene glycol, propylene glycol, diethylene glycol, butanediol 1,4-hexanediol, and 3-methylpentanediol.

[0031] Specifically, the polyester resin may be manufactured by reacting at least one of dimer acid and hexahydrophthalic anhydride (HHPA) with at least one of trimethylolpropane (TMP) and propylene glycol methyl ether acetate (PMA glycol ether).

[0032] More specifically, it may be manufactured by mixing dimer acid, hexahydrophthalic anhydride (HHPA), trimethylolpropane (TMP), and propylene glycol methyl ether acetate (PMA glycol ether) to form a reaction mixture, and then polymerizing the reaction mixture.

[0033] At this time, the dimer acid may be mixed in an amount of 30 to 50 parts by weight, preferably 35 to 50 parts by weight, and more preferably 35 to 45 parts by weight, based on 100 parts by weight of the reaction mixture, and the hexahydrophthalic anhydride may be mixed in an amount of 25 to 40 parts by weight, preferably 30 to 40 parts by weight, and more preferably 30 to 35 parts by weight, based on 100 parts by weight of the reaction mixture.

[0034] In addition, the trimethylolpropane may be mixed in an amount of 1 to 10 parts by weight, preferably 3 to 10 parts by weight, and more preferably 3 to 8 parts by weight, based on 100 parts by weight of the reaction mixture, and the propylene glycol methyl ether acetate may be included in an amount of 5 to 20 parts by weight, preferably 10 to 20 parts by weight, and more preferably 10 to 15 parts by weight, based on 100 parts by weight of the reaction mixture.

[0035] Meanwhile, the polyester resin can be manufactured by additionally adding a diluent for viscosity control, etc. after the polymerization reaction of the reaction mixture is completed.

[0036]

[0037] The above polyester resin may have a hydroxyl value (OH Value) of 250 mgKOH / g to 300 mgKOH / g, preferably 260 mgKOH / g to 280 mgKOH / g, and more preferably 270 mgKOH / g to 280 mgKOH / g. When the hydroxyl value of the polyester resin satisfies the above range, the reactivity with the curing agent is improved, the pot life is shortened, and the low-temperature curing characteristics are excellent. When the hydroxyl value of the polyester resin is less than 250 mgKOH / g, the effect of improving the low-temperature curing characteristics is minimal, and when it exceeds 300 mgKOH / g, the workability of the coating film is reduced, and the quality of the manufactured coating film may be deteriorated.

[0038] In addition, the polyester resin may have an acid value of 5 mgKOH / g to 50 mgKOH / g, preferably 5 mgKOH / g to 35 mgKOH / g, and more preferably 10 mgKOH / g to 30 mgKOH / g. When the acid value of the polyester resin satisfies the above range, the coating workability and coating film quality are excellent. If the acid value of the polyester resin is too low, the viscosity of the composition may increase, resulting in poor workability or reduced compatibility with the acrylic resin, and if it is too high, the storability of the composition may be reduced or the coating film adhesion may be reduced due to an additional reaction with an additive.

[0039] The above polyester resin may have a weight average molecular weight of 1,000 g / mol to 5,000 g / mol, preferably 1,000 g / mol to 4,000 g / mol, and more preferably 1,500 g / mol to 3,000 g / mol. When the weight average molecular weight of the polyester resin satisfies the above range, the reactivity with the curing agent is excellent, so that the low-temperature curing performance is improved and the coating film properties are excellent. Specifically, if the weight average molecular weight of the polyester resin is too small, the physical properties such as the coating film durability are deteriorated, and if it is too large, the mobility is reduced, so that the reactivity with the curing agent may be reduced.

[0040] The above polyester resin may have a solid content (NV) of 50 wt% to 80 wt%, or 60 wt% to 70 wt%, based on the total weight of the polyester resin. When the solid content of the polyester resin satisfies the above range, the curing reactivity is excellent, and the coating workability and coating appearance quality are excellent. Specifically, when the solid content of the polyester resin is too small, the content of volatile organic compounds (TVOC) increases, the curing reactivity is reduced, and the low-temperature curing improvement effect is minimal, and when the solid content is too large, the workability is reduced, and the appearance of the final coating film may deteriorate.

[0041] The above polyester resin may have a viscosity of V to Y as measured using a Gardner Viscometer at 25°C. When the viscosity of the polyester resin satisfies the above range, the paint workability is improved, and the film adhesion and appearance characteristics are excellent.

[0042] Meanwhile, the polyester resin may be included in an amount of 20 to 40 parts by weight, preferably 25 to 35 parts by weight, and more preferably 27 to 33 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition. When the content of the polyester resin satisfies the above range, excellent coating film properties can be realized even when baking at low temperatures. Specifically, when the content of the polyester resin is less than 20 parts by weight, the effect of improving low-temperature, fast-curing characteristics is minimal, and when it exceeds 40 parts by weight, the problem of deterioration of the coating film properties may occur.

[0043]

[0044] (2) First acrylic resin

[0045] The first acrylic resin is an acrylic resin containing styrene units, which is intended to improve the coating film properties, particularly durability, appearance characteristics, and curing properties.

[0046] When a polyester resin with excellent reactivity with a hardener is used in a clear coat composition, fast-curing and low-temperature curing characteristics can be imparted. However, as the content of the polyester resin increases, the physical properties of the coating, such as coating adhesion, impact resistance, water resistance, flowability, and cold-chip resistance, deteriorate. In order to solve this problem, the present invention uses a first acrylic resin containing a styrene unit capable of maintaining the coating film hard after curing in combination with a polyester resin.

[0047] The above first acrylic resin can be synthesized directly according to a known method, or a commercially available product can be used.

[0048] Specifically, the first acrylic resin can be manufactured by polymerizing at least one of a styrene-based monomer and a first (meth)acrylate-based monomer. For example, the first acrylic resin can be manufactured by mixing an initiator, a styrene-based monomer, and a first (meth)acrylate monomer in a polymerization solvent and then performing a radical polymerization reaction. By controlling the monomer mixing ratio, the type of initiator, the polymerization time, etc., the physical properties such as the weight average molecular weight (Mw), hydroxyl value (OHv), acid value (Av), glass transition temperature (Tg), etc. can be controlled.

[0049] As the above styrene-based monomer, for example, one or more selected from the group consisting of styrene, methylstyrene, dimethylstyrene, fluorostyrene, ethoxystyrene, methoxystyrene, acetoxystyrene, and t-butylstyrene may be used, but is not limited thereto.

[0050] The above first (meth)acrylate monomer may include at least one selected from the group consisting of a hydroxyl group-free (meth)acrylate monomer and a hydroxyl group-containing (meth)acrylate monomer.

[0051] The above hydroxyl group-free (meth)acrylate monomers include, for example, carboxyl group-containing (meth)acrylate monomers such as (meth)acrylic acid, methyl(meth)acrylic acid, etc.; Aliphatic (meth)acrylate monomers such as (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (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, lauryl (meth)acrylate, etc.; It may include at least one selected from the group consisting of alicyclic (meth)acrylate monomers such as isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, etc. Preferably, the hydroxyl group-free (meth)acrylate monomer may include a carboxyl group-containing (meth)acrylate monomer and an alicyclic (meth)acrylate monomer, and more preferably, (meth)acrylic acid and isobornyl (meth)acrylate.

[0052] The above hydroxyl group-containing (meth)acrylate monomer may be, for example, a hydroxyalkyl-containing (meth)acrylate, and specifically, may include at least one selected from the group consisting of 2-hydroxy methyl (meth)acrylate, 2-hydroxy ethyl (meth)acrylate, 2-hydroxy propyl (meth)acrylate, and 2-hydroxy butyl (meth)acrylate.

[0053] Meanwhile, the styrene unit derived from the styrene monomer may be included in an amount of 10 to 35 parts by weight, 10 to 25 parts by weight, or 15 to 25 parts by weight, based on 100 parts by weight of the first acrylic resin to be finally manufactured. When the content of the styrene unit in the first acrylic resin satisfies the above range, the coating strength increases, thereby improving the durability and impact resistance of the coating.

[0054] In addition, the first (meth)acrylic unit derived from the first (meth)acrylate monomer may be included in an amount of 25 to 50 parts by weight or 25 to 45 parts by weight based on 100 parts by weight of the first acrylic resin to be finally manufactured. When the amount of the first (meth)acrylic unit in the first acrylic resin satisfies the above range, the reactivity with the curing agent is excellent, and the coating film can be prevented from becoming excessively brittle.

[0055] The above first acrylic resin may include, in addition to a copolymer of a styrene monomer and a first (meth)acrylate monomer, a solvent, a diluent, and / or a polymerization initiator.

[0056]

[0057] Preferably, the first acrylic resin may include a copolymer prepared by polymerizing a reaction mixture containing styrene, isobornyl acrylate, acrylic acid, 2-hydroxyethyl methacrylate (2-HEMA, 2-Hydroxyethyl methacrylate), a polymerization initiator, and a reaction solvent. At this time, styrene may be included in an amount of 20 parts by weight to 40 parts by weight or 25 parts by weight to 35 parts by weight, isobornyl acrylate in an amount of 10 parts by weight to 30 parts by weight or 15 parts by weight to 25 parts by weight, acrylic acid in an amount of 1 part by weight to 10 parts by weight or 1 part by weight to 5 parts by weight, and 2-hydroxyethyl methacrylate (2-HEMA) in an amount of 10 parts by weight to 30 parts by weight or 15 parts by weight to 25 parts by weight, based on 100 parts by weight of the total weight of the reaction mixture.

[0058] Meanwhile, the first acrylic resin can be manufactured by additionally adding a diluent for viscosity control, etc. after the polymerization reaction of the reaction mixture is completed.

[0059] The first acrylic resin may have a hydroxyl value (OHv) of 130 mgKOH / g to 150 mgKOH / g, or 135 mgKOH / g to 145 mgKOH / g. When the hydroxyl value of the first acrylic resin satisfies the above range, the reactivity with an isocyanate-based curing agent is improved, resulting in better low-temperature curing performance.

[0060] The above first acrylic resin may have an acid value (Av) of 10 mgKOH / g to 30 mgKOH / g, 10 mgKOH / g to 20 mgKOH / g, or 12 mgKOH / g to 17 mgKOH / g. When the acid value of the first acrylic resin satisfies the above range, the composition exhibits excellent storage properties and low-temperature curing properties. If the acid value of the first acrylic resin is too low, the curing reaction speed decreases, resulting in poor fast-curing properties. If the acid value is too high, the cohesiveness of the resin increases, resulting in increased viscosity of the composition, which may result in poor workability and low-temperature storability.

[0061] The above first acrylic resin may have a weight average molecular weight of 30,000 g / mol to 70,000 g / mol, 40,000 g / mol to 60,000 g / mol, or 45,000 g / mol to 55,000 g / mol. When the weight average molecular weight of the first acrylic resin satisfies the above range, the coating film durability and appearance characteristics are excellent. Specifically, if the weight average molecular weight of the first acrylic resin is too small, the coating film appearance characteristics may be improved but the coating film durability may be reduced, and if the weight average molecular weight is too large, the coating film durability may be improved but the coating film appearance characteristics may be reduced.

[0062] In addition, the first acrylic resin may have a solid content of 50 wt% to 80 wt%, 60 wt% to 80 wt%, or 65 wt% to 75 wt%. When the solid content of the first acrylic resin satisfies the above range, the storage stability of the resin and the storage stability of the clear coat composition are improved, and workability is excellent. If the solid content of the first acrylic resin is too low, the viscosity becomes excessively low, which reduces workability, and if it is too high, the dispersibility deteriorates, which may cause agglomeration over time.

[0063] In addition, the first acrylic resin may have a viscosity of Z2 to Z4 as measured using a Gardner viscometer at 25°C. When the viscosity of the first acrylic resin satisfies the above range, the coating film appearance quality and workability are excellent.

[0064] The above first acrylic resin may have a glass transition temperature (Tg) of 20°C to 50°C, 25°C to 45°C, or 30°C to 40°C. When the glass transition temperature of the first acrylic resin satisfies the above range, the coating film properties are excellent. Specifically, if the glass transition temperature of the first acrylic resin is too low, the coating film formation through crosslinking reaction with the curing agent is insufficient, resulting in a decrease in the mechanical properties such as the coating film hardness, and if it is too high, the coating film may become brittle due to over-curing, and the water resistance and durability may be reduced.

[0065] Meanwhile, the first acrylic resin may be included in an amount of 5 to 15 parts by weight, preferably 5 to 12 parts by weight, and more preferably 5 to 10 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition. When the content of the first acrylic resin satisfies the above range, the coating film properties are excellent. Specifically, if the content of the first acrylic resin is too small, the paint flowability and cold chipping resistance may deteriorate, and conversely, if it is too large, the water resistance and impact resistance of the final coating film may deteriorate.

[0066]

[0067] (3) Second acrylic resin

[0068] The second acrylic resin is intended to improve the flowability of the clear coat composition and is an acrylic resin having an acid value (Av) of 1.6 mgKOH / g to 3.4 mgKOH / g. When the acid value of the second acrylic resin satisfies the above range, the effect of improving flowability can be obtained by temporarily forming hydrogen bonds before curing after coating.

[0069] The second acrylic resin may be synthesized directly using a known method, or a commercially available product may be used. For example, the second acrylic resin may be manufactured by polymerizing at least one of a second vinyl monomer and a second (meth)acrylate monomer. Specifically, the second acrylic resin may be manufactured using a radical polymerization method, and its physical properties, such as the weight average molecular weight (Mw), hydroxyl value (OHv), and acid value (Av), may be controlled depending on the initiator and polymerization time.

[0070] The type of the second vinyl monomer is not particularly limited, but for example, at least one selected from the group consisting of styrene, methylstyrene, dimethylstyrene, fluorostyrene, ethoxystyrene, methoxystyrene, phenylene vinyl ketone, vinyl t-butyl benzoate, vinyl cyclohexanoate, vinyl acetate, vinyl pyrrolidone, vinyl chloride, vinyl alcohol, acetoxystyrene, t-butylstyrene, and vinyltoluene may be used.

[0071] The above second (meth)acrylate monomer may include at least one selected from the group consisting of a hydroxyl group-free (meth)acrylate monomer and a hydroxyl group-containing (meth)acrylate monomer.

[0072] The above hydroxyl group-free (meth)acrylate monomers include, for example, (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, t-butyl (meth)acrylate, pentyl (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, and It may include at least one selected from the group consisting of lauryl (meth)acrylate.

[0073] The above hydroxyl group-containing (meth)acrylate monomer may be, for example, a hydroxyalkyl-containing (meth)acrylate, and specifically, may include at least one selected from the group consisting of 2-hydroxy methyl (meth)acrylate, 2-hydroxy ethyl (meth)acrylate, 2-hydroxy propyl (meth)acrylate, and 2-hydroxy butyl (meth)acrylate.

[0074] More specifically, the second acrylic resin may include a copolymer prepared by polymerizing a monomer mixture including styrene, butylacrylate, methyl methacrylate, and 2-hydroxyethyl methacrylate.

[0075] At this time, based on 100 parts by weight of the second acrylic resin finally manufactured, styrene may be included in an amount of 5 to 10 parts by weight, butyl acrylate in an amount of 15 to 20 parts by weight, methyl methacrylate in an amount of 5 to 10 parts by weight, and 2-hydroxyethyl methacrylate (2-HEMA) in an amount of 15 to 20 parts by weight.

[0076] The above second acrylic resin may include, in addition to a copolymer of a second vinyl monomer and a second (meth)acrylate monomer, a solvent, a diluent, and / or a polymerization initiator.

[0077] The second acrylic resin may have a hydroxyl value (OHv) of 130 mgKOH / g to 150 mgKOH / g, or 135 mgKOH / g to 145 mgKOH / g. When the hydroxyl value of the first acrylic resin satisfies the above range, the reactivity with an isocyanate-based curing agent is improved, resulting in better low-temperature curing performance.

[0078] Meanwhile, the second acrylic resin may have a hydroxyl group content of 2% to 6% by weight. When the hydroxyl group content satisfies the above range, the hydroxyl value of the second acrylic resin is appropriately controlled, resulting in excellent low-temperature curing performance.

[0079] Additionally, the second acrylic resin may have an amine content of 6.0 mmol / kg or less, preferably 2 mmol / kg to 4 mmol / kg. When the amine content satisfies the above range, the coating film exhibits excellent physical properties, such as adhesion and water resistance.

[0080] Meanwhile, the second acrylic resin may have a weight average molecular weight of 6,000 g / mol to 10,000 g / mol, 7,000 g / mol to 9,000 g / mol, or 7,500 g / mol to 8,500 g / mol. When the weight average molecular weight of the second acrylic resin satisfies the above range, the appearance and durability of the coating film are excellent. If the weight average molecular weight of the second acrylic resin is too small, the long-term physical properties of the coating film deteriorate, and if it is too large, the appearance deteriorates due to decreased compatibility.

[0081] Additionally, the second acrylic resin may have a solid content of 40 wt% to 60 wt%, 45 wt% to 80 wt%, or 50 wt% to 60 wt%. When the solid content of the second acrylic resin satisfies the above range, an appearance improvement effect can be obtained.

[0082] In addition, the second acrylic resin may have a viscosity of 3 Pa·s to 12 Pa·s, preferably 3 Pa·s to 8 Pa·s, as measured using a Brookfield viscometer at 25°C. When the viscosity of the second acrylic resin satisfies the above range, it is effective in improving the appearance.

[0083] The second acrylic resin may have a glass transition temperature (Tg) of 1°C to 20°C, 5°C to 20°C, or 10°C to 20°C. When the glass transition temperature of the second acrylic resin satisfies the above range, the coating film properties are excellent. Specifically, if the glass transition temperature of the second acrylic resin is too low, the paint flowability may deteriorate, and if it is too high, the appearance properties may deteriorate.

[0084] The above second acrylic resin may have an average particle size of 15 μm or less, 8 μm to 12 μm, or 10 μm to 12 μm. In this case, the average particle size may be a value measured using a PSA (Particle Size Analysis) particle size analyzer. When the average particle size of the second acrylic resin particles satisfies the above range, the effects of improving appearance and controlling paint flow can be obtained.

[0085] Meanwhile, the second acrylic resin may be included in an amount of 1 to 5 parts by weight, preferably 1 to 4 parts by weight, and more preferably 2 to 4 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition. When the content of the second acrylic resin satisfies the above range, the film flowability and film properties are excellent. Specifically, if the content of the second acrylic resin is too small, the effect of improving the paint flowability is minimal, and if it is too large, the appearance quality may deteriorate.

[0086]

[0087] (4) Isocyanate curing agent

[0088] The clear coat composition according to the present invention includes an isocyanate curing agent. Conventionally, melamine curing agents have been commonly used as curing agents in clear coat compositions. However, low-temperature curing is difficult when using melamine curing agents. Therefore, the present invention utilizes an isocyanate curing agent instead of a melamine curing agent, thereby improving low-temperature curing performance.

[0089] As the above isocyanate-based curing agent, various isocyanate curing agents known in the art can be used, and the type thereof is not particularly limited. For example, the above isocyanate-based curing agent includes aliphatic diisocyanates such as hexamethylene diisocyanate and trimethyl hexamethylene diisocyanate; cyclic aliphatic diisocyanates such as hydrogenated xylene diisocyanate and isophorone diisocyanate; aromatic diisocyanates such as tolylene diisocyanate and 4,4´-diphenylmethane diisocyanate; Organic polyisocyanates themselves, such as polyisocyanate compounds having three or more isocyanate groups, such as methane-4,4´,4´´-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotriene, and 4,4´-dimethyldiphenylmethane-2,2´,5,5´-tetraisocyanate, or adducts of each of these organic polyisocyanates with polyhydric alcohols, low molecular weight polyester resins, or water, or mixtures thereof, may be used. Specifically, the isocyanate-based curing agent may be hexamethylene diisocyanate trimer.

[0090] The above curing agent may be included in an amount of 10 to 40 parts by weight, preferably 15 to 35 parts by weight, and more preferably 20 to 30 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition. When the content of the curing agent satisfies the above range, the clear coat composition exhibits excellent low-temperature curing and fast-curing properties.

[0091]

[0092] (5) Curing catalyst

[0093] The curing catalyst is intended to increase the curing speed of the clear coat composition, and various curing catalysts known in the art can be used, and the type thereof is not particularly limited.

[0094] For example, the curing catalyst may be dibutyltin dilaurate, dibutylbis[(1-oxododecyl)oxy]stannane (DBTDL, dibutylbis[(1-oxododecyl)oxy]stannane), dibutylbis(dodecylthio)stannane (DABCO), bismuth octoater, bismuth neodecanoate, tin octoate, zinc octoate, or a mixture thereof.

[0095] The curing catalyst may be included in an amount of 0.01 to 2 parts by weight, or 0.1 to 1 part by weight, based on 100 parts by weight of the total weight of the clear coat composition. Conventional clear coat compositions typically use curing catalysts in an amount of about 0.005 parts by weight. However, the present invention further improves low-temperature curing characteristics by including a curing catalyst in a higher amount than before.

[0096] The clear coat composition according to the present invention further improves low-temperature curing characteristics by including the polyester resin, the first acrylic resin, the second acrylic resin, the isocyanate-based curing agent, and the curing catalyst in a weight ratio of 100:12.5 to 75:2.5 to 25:25 to 200:0.025 to 10.

[0097]

[0098] (6) Additives

[0099] The clear coat composition according to the present invention may further include additives such as a defoaming agent, an ultraviolet absorber, a leveling agent, a flow inhibitor, a viscosity modifier, etc., in addition to the above components, in order to improve the coating film properties. The additives may be any additive used in the art for clear coat compositions, and the type thereof is not particularly limited.

[0100] The content of the above additive is not particularly limited as long as it is within a range that can be typically included in a clear coat composition. For example, the additive may be included in an amount of 1 to 20 parts by weight, or 1 to 10 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition.

[0101]

[0102] (7) Solvent

[0103] The clear coat composition according to the present invention may further include a solvent to control the viscosity of the composition and improve the appearance characteristics and spreadability of the produced coating film.

[0104]

[0105] The solvent is not particularly limited as long as it is a solvent commonly used in a clear coat composition, and may include, for example, at least one selected from the group consisting of aromatic solvents, acetate solvents, alcohol solvents, and propionate solvents. Specifically, the solvent may include aromatic solvents such as toluene and xylene; acetate solvents such as 1-methoxy-2-propyl acetate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, methyl glutarate, methyl succinate, methyl adipate, dimethyl glutarate, dimethyl succinate, dimethyl adipate, propylene glycol methyl ether acetate (PMA), butyl carbitol acetate, butyl cellosolve acetate, and trimethyl-o-acetate; alcohol solvents such as n-butanol, propanol, 1-methoxy-2-propanol, and 2-butoxyethanol; It may include ketone solvents such as acetone, methyl ethyl ketone, methyl butyl ketone, and methyl isobutyl ketone; and propionate solvents such as ethyl ethoxypropionate; etc. In addition, commercially available products of the aromatic solvents include Cocosol #100, Cocosol #150, etc.

[0106]

[0107] The solvent may be included in an amount of 5 to 40 parts by weight, or 20 to 30 parts by weight, based on 100 parts by weight of the total weight of the clear coat composition. When the solvent is included within the above range, the viscosity of the composition is appropriately adjusted, thereby improving workability and drying properties. In addition, when the content of the solvent in the coat composition is less than the above range, the solid content in the composition is high, which may cause a problem of insufficient workability of the composition, and when it exceeds the above range, the solid content of the manufactured paint may be low, which may cause a problem of insufficient appearance and adhesion of the manufactured coating film.

[0108]

[0109] The clear coat composition according to the present invention comprises a polyester resin having excellent low-temperature curing properties, a first acrylic resin containing styrene units to improve coating durability, and a second acrylic resin having a low acid value to improve paint flowability, thereby enabling excellent coating quality even during low-temperature curing.

[0110] Specifically, the clear coat composition of the present invention can be applied to both steel and plastic materials, as it takes 20 minutes or less to fully cure at 90°C. Therefore, using the clear coat composition of the present invention, a body made of steel and parts made of plastic can be painted using a single paint and a single process, thereby improving the painting process.

[0111]

[0112] Hereinafter, the present invention will be described in more detail through specific examples. However, the following examples are provided solely to aid understanding of the present invention and are not intended to limit the scope of the present invention to these examples.

[0113]

[0114] Examples and Comparative Examples

[0115] A clear coat composition was prepared by mixing each component according to the composition described in [Table 1] below.

[0116] Ingredients (weight parts) Example Comparative Example 1 2 3 4 5 1 2 3 4 5 6 Polyester resin 29.8 2 5 2 6 3 2 9.8 5 0 1 9 3 4 8 4 0 2 9.8 2 9.8 Acrylic resin 19.6 1 5 12.3 6.9--2 0 4 6--9.6 Acrylic resin 22.9 2.3 4 3.4 2.9 2.9 3.3 2.9 2.9 2.9 Acrylic resin 3------9.6 Acrylic resin 4-------2.9 Acrylic resin 59.6 Isocyanate hardener 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 25 26.4 26.4 Curing Catalyst0.040.040.040.040.040.040.040.040.040.040.040.040.04UV absorber1.61.61.61.61.61.61.61.61.61.6Leveling agent0.50.50.50.50.50.50.50.50.50.50.50.50.50.5Solvent29.1629.1629.1629.1618.9629.1629.1629.9629.1629.16

[0117] The specific specifications of each component used in the above examples and comparative examples are as follows.

[0118]

[0119] (1) Polyester resin: A polyester resin synthesized by the following method was used. 320 parts by weight of dimer acid, 250 parts by weight of hexahydrophthalic anhydride (HHPA), 50 parts by weight of trimethylolpropane (TMP), and 100 parts by weight of PMA glycol ether (Propylene Glycol Methyl Ether Acetate) were charged into a four-necked flask equipped with a thermometer, a stirring device, a condenser, a packed column, and a separator, and the temperature was raised to 230°C while removing water of condensation under a nitrogen atmosphere. The packed column was removed when the acid value was 25 mgKOH / g, and the reaction was continued until it was cooled to an acid value of 20 mgKOH / g, and diluted with 100 parts by weight of PMA glycol ether (Propylene Glycol Methyl Ether Acetate) at 130°C.

[0120] The polyester resin manufactured as described above had a solid content (NV) of 65 wt%, a Gardner viscosity of VIS V ~ Y, a hydroxyl value (OH value) of 275 mgKOH / g, an acid value of 20 mgKOH / g, and a weight average molecular weight of 2,500 g / mol.

[0121]

[0122] (2) Acrylic resin 1: Acrylic resin synthesized by the following method was used.

[0123] 155 parts by weight of Cocosol #100 and 150 parts by weight of isobornyl acrylate were charged into a four-necked synthetic flask equipped with a thermometer, a stirrer, a condenser, and a heating device, and the temperature was raised to 150°C. After the isotherm became stable, 200 parts by weight of styrene monomer, 150 parts by weight of hydroxyethyl methacrylate, 15 parts by weight of acrylic acid, and 5 parts by weight of ditertiary butyl peroxide were uniformly mixed, added dropwise evenly to the flask over 300 minutes, and then the isotherm was maintained for 120 minutes. After the maintenance was completed, the reactant was cooled to 80°C and diluted with 100 parts by weight of butyl acetate and 100 parts by weight of Cocosol #100.

[0124] The acrylic resin manufactured as described above had a solid content (NV) of 70.2 wt%, a viscosity of VIS Z2 to Z4, a hydroxyl value (OH value) of 140 mgKOH / g, an acid value of 14.2 mgKOH / g, a weight average molecular weight of 50,000 g / mol, and a glass transition temperature of 35°C.

[0125]

[0126] (3) Acrylic resin 2:

[0127] Allnex's SETALUX-81198 SS-55 was used, which had a solid content of 54.5 wt%, an acid value of 2.8 mgKOH / g, a hydroxyl value of 140 mgKOH / g, a weight average molecular weight of 8,511 g / mol, a hydroxyl content of 4 wt%, a Brookfield viscosity of 7 Pa·s, an average particle size of 12 μm, a glass transition temperature of 13°C, and an amine content of 5.8 mmol / kg.

[0128]

[0129] (4) Acrylic resin 3:

[0130] Allnex's SETALUX-91756 VS-60, which does not contain styrene units, has a solid content (NV) of 58 to 61 wt%, a viscosity of 10 to 18 Pa.s, a hydroxyl value (OH value) of 0 mgKOH / g, an acid value of 4 mgKOH / g, a weight average molecular weight of 8,200 g / mol, and a glass transition temperature of 11°C, was used.

[0131]

[0132] (5) Acrylic resin 4:

[0133] Allnex's SETALUX-61230 B-60, which has a solid content (NV) of 59 to 61 wt%, a viscosity of 1.5 to 2.9 Pa.s, a hydroxyl value (OH value) of 3 mgKOH / g, an acid value of 4 mgKOH / g, a weight average molecular weight of 7,800 g / mol, and a glass transition temperature of 11°C, was used.

[0134]

[0135] (6) Acrylic resin 5: Acrylic resin synthesized by the following method was used.

[0136] 155 parts by weight of Cocosol #100 and 150 parts by weight of isobornyl acrylate were charged into a four-necked synthetic flask equipped with a thermometer, a stirrer, a condenser, and a heating device, and the temperature was raised to 150°C. After the isotherm became stable, 300 parts by weight of styrene monomer, 150 parts by weight of hydroxyethyl methacrylate, 15 parts by weight of acrylic acid, and 5 parts by weight of ditertiary butyl peroxide were uniformly mixed, added dropwise evenly to the flask over 300 minutes, and then the isotherm was maintained for 120 minutes. After the maintenance was completed, the reactant was cooled to 80°C and diluted with 100 parts by weight of butyl acetate and 100 parts by weight of Cocosol #100.

[0137] The acrylic resin manufactured as described above had a solid content (NV) of 70.5 wt%, a viscosity of VIS Z2 to 4, a hydroxyl value (OH value) of 140 mgKOH / g, an acid value of 14.8 mgKOH / g, a weight average molecular weight of 50,000 g / mol, and a glass transition temperature of 35°C.

[0138]

[0139] (7) Isocyanate-based curing agent: Hexamethylene diisocyanate trimer (HMDI Trimer) with a solid content of 100 wt% and an isocyanate group content of 21.5 wt% was used.

[0140]

[0141] (8) Curing catalyst: Dibutyltin dilaurate with a solid content of 100 wt% was used.

[0142]

[0143] (9) UV absorber: Decanedioic acid bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ester with a solid content of 100 wt% and methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate were mixed and used at a weight ratio of 66:34.

[0144]

[0145] (10) Leveling agent: BYK-325 (polyether-modified polymethylalkylsiloxane) from BYK was used.

[0146]

[0147] (11) Solvent: GS Caltex's KOKOSOL #100, Huchem's RHODIASOLV RPDE, xylene, and butyl acetate were mixed and used in a weight ratio of 25:20:20:35.

[0148]

[0149] Experimental example

[0150] After drying the primer coat composition on the painted steel plate electrodeposited surface, bell coating was applied so that the film thickness was 30 to 50 μm, and then cured in a general oven at 140°C for 20 to 30 minutes, and then the base coat composition was dried, bell coating was applied so that the film thickness was 10 to 20 μm, and hot air was blown at 80°C for 3 minutes to evaporate the water remaining in the paint to form a base coat. Then, the clear coat compositions of the examples and comparative examples were dried on the base coat, bell coating was applied so that the film thickness was 30 to 50 μm, and then cured in a general oven at 90°C for 20 minutes to form a final film. The appearance, workability, gloss, adhesion, impact resistance, water resistance, paint flowability, and cold chipping resistance of the final film were measured and evaluated as follows, and the results are shown in [Table 2] below.

[0151]

[0152] (1) Appearance of the coating

[0153] The final CF value was measured using the Wave Scan DOI (BYK Gardner), an automotive exterior measuring device. Based on the measurement results, the coating appearance was evaluated according to the following criteria.

[0154] ◎-Excellent (CF 70 or higher)

[0155] ○-Good (CF 69∼65)

[0156] △-Normal (CF 64∼60)

[0157] ×-defective (CF less than 60)

[0158]

[0159] (2) Painting workability

[0160] After completing the intermediate / top / clear coating on the surface (steel plate), the sandability and workability of the surface were evaluated based on the following criteria.

[0161] ◎-Excellent: No sanding marks after final painting

[0162] ○-Good: Ultra-fine sanding marks (less than 1 cm) appear after final painting.

[0163] △-Normal: Fine marks (more than 1 cm and less than 3 cm) may appear after the final painting.

[0164] ×-Defective: Marks exceeding 3cm occur after final painting.

[0165]

[0166] (3) Gloss

[0167] The gloss of the final coating was measured using a BYK micro gloss meter according to the following criteria.

[0168] ◎-Excellent: 20 degree gloss of 95 or higher

[0169] ○-Good: 20 degree gloss of 85 or more but less than 95

[0170] △-Normal: 20 degree gloss of 75 or more but less than 85

[0171] ×-Defective: 20 degree gloss less than 75

[0172]

[0173] (4) Adhesion

[0174] According to the ASTM D3359 tape adhesion test method, 100 squares measuring 2 mm wide × 2 mm long were drawn on the coating film with a blade, and then the tape was used to remove them, and the adhesion was measured according to the following criteria.

[0175] ◎-Excellent: When 100 squares are 100% completely connected

[0176] ○-Good: If the remaining squares are 95% or more but less than 100%

[0177] △-Normal: When the remaining squares are 90% or more but less than 95%

[0178] ×-Defective: If the remaining squares are 85% or more but less than 90%

[0179]

[0180] (5) Impact resistance

[0181] The impact resistance was measured by dropping a 500g weight on the film from a height of 30cm or more according to the following criteria.

[0182] ◎-Excellent: No cracks or marks are observed on the coating.

[0183] ○-Good: No cracks, but fine marks less than 1 cm in size

[0184] △-Normal: No cracks occurred, but a mark larger than 1 cm occurred.

[0185] ×-Defective: If a crack occurs

[0186]

[0187] (6) Water resistance

[0188] After the completed coating was immersed in a 40℃ constant temperature bath for 10 days, the presence of coating peeling and discoloration was measured based on the following criteria.

[0189] ◎-Excellent: When no peeling or discoloration of the coating is observed

[0190] ○-Good: No peeling of the coating, but very slight discoloration is observed.

[0191] △-Normal: No peeling of the coating, but partial discoloration is observed.

[0192] ×-Defective: If peeling off of the coating or extensive discoloration is observed

[0193]

[0194] (7) Fluidity of the paint

[0195] The specimens that had been coated with a coating were hung vertically, coated with the clear coat compositions of the examples and comparative examples, and then cured. The surface of the coating was observed to evaluate the paint flowability based on the following criteria.

[0196] ◎-Excellent: When the film thickness at the starting point where flow occurrence is observed is 45㎛ or more.

[0197] ○-Good: When the film thickness at the starting point where flow occurrence is observed is 40㎛ or more and less than 45㎛.

[0198] △-Normal: When the film thickness at the starting point where flow occurrence is observed is 35㎛ or more and less than 40㎛.

[0199] ×-Defective: When the film thickness at the starting point where flow occurrence is observed is less than 35㎛

[0200]

[0201] (8) Cold-resistant chipping resistance

[0202] After leaving the coating film at -20℃ for 3 hours, a 50g chipping stone was used to push out a 50g chipping stone at a pressure of 4 bar to strike the surface of the coating film, and then the cold chipping resistance was evaluated using the following criteria.

[0203] ◎-Excellent: If there are 10 or fewer damages of 1 mm or less in size

[0204] ○-Good: If there are 10 or fewer damages exceeding 1 mm and 2 mm in size.

[0205] △-Normal: If there are 10 or fewer damages measuring 2 mm or more but less than 3 mm in size.

[0206] ×-Defective: If there are more than 10 damages measuring 2 mm or more but less than 3 mm in size.

[0207]

[0208] Evaluation ItemsExample 1Example 2Example 3Example 4Example 5Comparative Example 1Comparative Example 2Comparative Example 3Comparative Example 4Comparative Example 5Comparative Example 6Coating Appearance◎○○○○○○◎◎◎◎Painting Workability◎○○○△○◎○△◎◎Gloss◎◎◎◎○○○△△△Adhesion◎○○○△×△△×△△Impact Resistance◎○○○△××○××××Water Resistance◎○○○△××△○×△Paint Flow◎◎◎◎◎×△×△×○Cold Chipping Resistance◎○○○△×△×△○△Evaluation: ◎-Excellent, ○-Good, △- Average, × - Poor

[0209] As shown in Table 2 above, when the clear coat compositions of Examples 1 to 5 comprising a polyester resin, a first acrylic resin containing a styrene unit, and a second acrylic resin having an acid value of 1.6 mgKOH / g to 3.4 mgKOH / g are applied, it can be confirmed that all physical properties are excellent or better even when a coating film is formed through low-temperature baking. In contrast, when the clear coat compositions of Comparative Examples 1, 4, and 5 not containing the first acrylic resin containing a styrene unit are used, or the clear coat composition of Comparative Example 6 in which an acrylic resin having an acid value outside the range of the present invention is applied as the second acrylic resin is used, it was found that the adhesion and / or impact resistance of the coating film is deteriorated.

[0210] In addition, when the clear coat compositions of Comparative Examples 2 and 3, in which the content of the first acrylic resin is outside the scope of the present invention, were used, the physical properties of the coating film, such as impact resistance, water resistance, paint flow resistance, and cold chipping resistance, were found to be deteriorated.

Claims

1. Polyester resin; A first acrylic resin containing a styrene unit; A second acrylic resin having an acid value of 1.6 mgKOH / g to 3.4 mgKOH / g; Isocyanate curing agent; and Contains a curing catalyst, A clear coat composition comprising the first acrylic resin in an amount of 5 to 15 parts by weight based on 100 parts by weight of the total weight of the clear coat composition.

2. In claim 1, A clear coat composition comprising the polyester resin, the first acrylic resin, the second acrylic resin, the isocyanate curing agent, and the curing catalyst in a weight ratio of 100:12.5 to 75:2.5 to 25:25 to 200:0.025 to 10.

3. In claim 1, A clear coat composition in which the styrene unit is included in an amount of 10 to 25 parts by weight based on 100 parts by weight of the first acrylic resin.

4. In claim 1, The above polyester resin is a clear coat composition having a hydroxyl value of 250 mgKOH / g to 300 mgKOH / g, an acid value of 5 mgKOH / g to 50 mgKOH / g, and a weight average molecular weight of 1,000 g / mol to 5,000 g / mol.

5. In claim 1, A clear coat composition in which the first acrylic resin has a hydroxyl value of 130 mgKOH / g to 150 mgKOH / g, an acid value of 10 mgKOH / g to 30 mgKOH / g, a weight average molecular weight of 30,000 g / mol to 70,000 g / mol, and a glass transition temperature of 20°C to 50°C.

6. In claim 1, The above second acrylic resin is a clear coat composition having a hydroxyl group of 130 mgKOH / g to 150 mgKOH / g, a weight average molecular weight of 6,000 g / mol to 10,000 g / mol, a viscosity of 3 Pa·s to 12 Pa·s, a particle size of 15 μm or less, a glass transition temperature of 1°C to 20°C, and an amine content of 6.0 mmol / kg or less.

7. In claim 1, The above clear coat composition is a clear coat composition that takes 20 minutes or less to be completely cured at 90°C.