JP2009155396A - Clear coating composition, and method of forming multilayered coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a clear coating composition capable of restraining color reversion of a base coating film, and capable of forming an excellent multilayered coating film. <P>SOLUTION: This clear coating composition contains an acrylic resin obtained from a monomer mixture containing a long-chain (meth)acrylate monomer of which the ester part is a 9-15C linear hydrocarbon group, a carboxylic group containing (meth)acrylic monomer, and a hydroxy group containing (meth)acrylic monomer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a clear coating composition and a method for forming a multilayer coating film.

In painting automobile bodies and the like, it is widely practiced to form a top coat film containing a base coat film and a clear coat film. In particular, since the clear coating film constitutes the outermost layer in the coating film of an automobile body or the like, various physical properties such as design properties, acid resistance, and scratch resistance are required.

Conventionally, acrylic / melamine resin thermosetting paint has been used as a clear paint for automobile painting. However, since the coating film obtained by using such a melamine resin as a curing agent is inferior in acid resistance, it is likely to be deteriorated by acid rain, and there is a risk of causing a problem in appearance.

Examples of such clear paints other than the acrylic / melamine resin thermosetting type include urethane-based paints (for example, see Patent Document 1).
However, although a urethane-based paint can obtain a coating film excellent in acid resistance and scratch resistance, a clear coating film is formed on the uncured base coating film using a urethane-based paint, and the base coating film and the clear coating film are formed. When the top coat film is formed by so-called two-coat one-bake that heat cures the film at the same time, the acrylic resin (acrylic polyol) in the clear coating composition penetrates into the uncured base paint film and affects the design properties. There was a case to give.

In particular, when the base paint composition is a metallic paint having a glitter pigment, the penetration of the components in the clear paint composition disturbs the arrangement of the glitter pigment in the uncured base coating, resulting in highlights. A phenomenon called color reversion of the base coating film in which (shade surface) becomes bright while (front surface) becomes dark may occur, and design properties may deteriorate.
Therefore, urethane-based paints are difficult to obtain coating films having excellent design properties as clear paints for automotive coatings, especially clear paints for automotive coatings for markets where high design properties are required. There was a problem.
JP-A-2005-787

In view of the above-mentioned present situation, the present invention is a clear coating composition excellent in design maintainability capable of forming a multi-layer coating film that suppresses color reversion of the base coating film and maintains the high design properties inherent in the base coating film. The object is to provide a product and a method for forming a multilayer coating film.

The present invention includes a long-chain (meth) acrylic acid ester monomer whose ester moiety is a linear hydrocarbon group having 9 to 15 carbon atoms, a carboxyl group-containing (meth) acrylic monomer, and a hydroxyl group-containing (meth) acrylic monomer. A clear coating composition comprising an acrylic resin obtained from a monomer mixture.
The clear coating composition of the present invention preferably further contains a curing agent capable of curing with a carboxyl group and / or a hydroxyl group.
The curing agent is preferably a polyisocyanate compound.

The content of the long-chain (meth) acrylic acid ester monomer in the monomer mixture is preferably 5 to 15% by mass with respect to the total amount of monomer solids.
The solid content acid value of the acrylic resin is preferably 1 to 10 mgKOH / g.

The present invention also includes a step (1) of applying a base coating to a substrate to form an uncured base coating, and a clear coating on the uncured base coating obtained in the step (1). The step (2) of forming an uncured clear coating by coating the uncured base coating and the uncured clear coating obtained in the above steps (1) and (2) are simultaneously cured by heating. A multilayer coating film forming method comprising a step (3) of forming a layer coating film, wherein the clear coating material is the above-described clear coating composition.
The multilayer coating film forming method of the present invention includes a step (P) of applying an intermediate coating or primer surfacer to the substrate to obtain an intermediate coating or primer surfacer coating before the step (1). It may be a thing.
The present invention is described in detail below.

The clear coating composition of the present invention comprises a long chain (meth) acrylic acid ester monomer, a carboxyl group-containing (meth) acrylic monomer, and a hydroxyl group-containing (meta ) An acrylic resin obtained from a monomer mixture containing an acrylic monomer is included.
The clear coating composition of the present invention is capable of forming a coating film having excellent acid resistance and scratch resistance, and further can suppress color reversion of the base coating film, and thus has excellent design properties. A coating film can be formed. Therefore, it can be suitably used as a two-pack type clear paint for automobile painting.

The reason why a coating film having an excellent appearance can be formed by the clear coating composition of the present invention is that the acrylic resin has a relatively long linear hydrocarbon group having 9 to 15 carbon atoms as a side chain. In the formation process of the top coat film by 2 coats and 1 bake, when the clear paint is applied, it is possible to prevent the acrylic resin from penetrating into the uncured base film due to the steric hindrance of the linear hydrocarbon group. This is thought to be possible. Furthermore, it can be considered that making the acrylic resin have a high acid value is effective in preventing the penetration of the acrylic resin into the uncured base coating film.

Thus, according to the clear coating composition of the present invention, it is possible to prevent the acrylic resin from penetrating into the uncured base coating film, and when the base coating composition is a metallic paint having a glitter pigment. It is presumed that since the disorder of the orientation of the glitter pigment can be prevented, the color return of the base coating film can be suppressed.

The acrylic resin comprises a long-chain (meth) acrylic acid ester monomer, a carboxyl group-containing (meth) acrylic monomer, and a hydroxyl group-containing (meth) acrylic monomer whose ester part is a linear hydrocarbon group having 9 to 15 carbon atoms. It is obtained from the monomer mixture containing.

The long chain (meth) acrylic acid ester monomer is, for example, the following general formula (A)

(R represents hydrogen or a methyl group, and n represents a natural number of 8 to 14).

The long-chain (meth) acrylic acid ester monomer is not particularly limited as long as the ester portion is a straight chain hydrocarbon group having 9 to 15 carbon atoms. For example, lauryl (meth) acrylate, (meta ) Tridecyl acrylate, decyl (meth) acrylate, and nonyl (meth) acrylate. These may use only 1 type and may use 2 or more types together.

The content of the long-chain (meth) acrylic acid ester monomer in the monomer mixture is preferably 5 to 15% by mass with respect to the total amount of monomer solids. If it is less than 5% by mass, it is difficult to sufficiently suppress the influence of the clear coating film on the base coating film side. On the other hand, when it exceeds 15 mass%, there exists a possibility that the adhesiveness in the case of repainting may fall.

The carboxyl group-containing (meth) acrylic monomer is not particularly limited, and examples thereof include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, isocrotonic acid, and maleic acid. These may use only 1 type and may use 2 or more types together. Of these, acrylic acid and methacrylic acid are preferable.
By using a carboxyl group-containing (meth) acrylic monomer, a coating film having excellent design properties can be formed. Such an effect is presumed to be because the apparent molecular weight of the acrylic resin can be increased by the association of carboxyl groups, and thus the penetration of the acrylic resin into the base coating film can be prevented.

The hydroxyl group-containing (meth) acrylic monomer is not particularly limited. For example, hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, allyl alcohol, methacryl alcohol, Examples include adducts of hydroxyethyl (meth) acrylate and ε-caprolactone. These may use only 1 type and may use 2 or more types together.

The acrylic resin can be obtained from the monomer mixture containing the long-chain (meth) acrylic acid ester monomer, the carboxyl group-containing (meth) acrylic monomer, and the hydroxyl group-containing (meth) acrylic monomer. The monomer mixture may contain other unsaturated monomers.

Examples of the other unsaturated monomer include, for example, (meth) acrylic acid ester, methyl (meth) acrylate, ethyl (meth) acrylate, i-propyl (meth) acrylate, and n-butyl (meth) acrylate. (Meth) acrylic acid alkyl ester having 1 to 8 carbon atoms in the ester moiety such as i-butyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate; (Meth) acrylic acid cyclic hydrocarbon esters such as phenyl acrylate, cyclohexyl (meth) acrylate; (meth) acrylate isobornyl; (poly) ethylene glycol mono (meth) acrylate, polyethylene glycol mono ( (Meth) acrylic acid polyalkylene glycol esters such as (meth) acrylates; and In addition to alkoxyalkyl (meth) acrylates having 1 to 3 carbon atoms, (meth) acrylamide; vinyl compounds such as styrene, α-methylstyrene, vinyl acetate, vinyl propionate, vinyl benzoate, vinyltoluene, acrylonitrile; and Examples thereof include diesters of unsaturated dibasic acids such as crotonic acid esters; maleic acid diesters and itaconic acid diesters. The other unsaturated monomers may be used alone or in combination of two or more.

The polymerization method for obtaining the acrylic resin is not particularly limited, and known methods such as solution polymerization, dispersion polymerization, and emulsion polymerization can be used.

The acrylic resin preferably has a solid content acid value of 1 to 10 mgKOH / g.
By making the acid value within the above range, the apparent molecular weight can be increased by the association of carboxyl groups, so that the acrylic resin can be prevented from penetrating into the base coating film, and the coating film has excellent design properties It is because it can form.
If the solid content acid value is less than 1 mg KOH / g, the number of points of association decreases and it becomes difficult to increase the apparent molecular weight, which may make it difficult to form a coating film having excellent design properties. . When the solid content acid value exceeds 10 mgKOH / g, the viscosity becomes too high and it may be difficult to obtain a coating composition having a high solid content. The solid content acid value is more preferably 2 to 7 mgKOH / g.

The acrylic resin preferably has a solid content hydroxyl value of 50 to 140 mgKOH / g. There exists a possibility that sclerosis | hardenability may fall that solid content hydroxyl value is less than 50 mgKOH / g. When the solid content hydroxyl value exceeds 140 mgKOH / g, the water resistance of the coating film may decrease. The solid content hydroxyl value is more preferably 80 to 140 mgKOH / g. In addition, the said acid value and hydroxyl value can be adjusted with the compounding ratio of a raw material monomer.

The acrylic resin preferably has a number average molecular weight (Mn) of 2000 to 10,000. When the number average molecular weight (Mn) is less than 2,000, the curability of the coating is not sufficient, and when it exceeds 10,000, the viscosity becomes high and it may be difficult to form a coating composition having a high solid content. The number average molecular weight of the acrylic resin is more preferably 4000 to 6000.
In addition, in this specification, a number average molecular weight (Mn) is a number average molecular weight of polystyrene conversion measured by GPC (gel permeation chromatography).

The acrylic resin preferably has a glass transition temperature (Tg value) of −30 to 50 ° C. If the Tg value is less than −30 ° C., the coating film hardness may be reduced, and the coating film thickness formed may vary due to fluctuations in the coating liquid temperature. On the other hand, when it exceeds 50 ° C., the viscosity becomes too high and it may be difficult to form a coating composition having a high solid content. The acrylic resin preferably has a Tg value of -10 to 30 ° C.

The Tg value of the acrylic resin can be calculated, for example, by calculating from the known Tg value and the composition ratio of the constituent monomer or homopolymer.

The clear coating composition preferably contains a carboxyl group and / or a hydroxyl group and a curable curing agent.
As said hardening | curing agent, a polyisocyanate compound, a polycarbodiimide compound, an oxazoline compound etc. can be mentioned, for example. Of these, polyisocyanate compounds are preferred.

The polyisocyanate compound is not particularly limited as long as it is a compound having two or more isocyanate groups. For example, trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HMDI), trimethylhexamethylene diisocyanate and the like. Aliphatic isocyanates such as 1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate, 1,2-cyclohexane diisocyanate, and the like, xylylene diisocyanate (XDI), 2,4-tolylene diisocyanate (TDI) ), Aromatic isocyanates such as 2,6-tolylene diisocyanate, isophorone diisocyanate (IPDI), norbornane diisocyanate Alicyclic isocyanates such Tomechiru, these isocyanurate body, can be given burette body, multimers of the adduct and the like and mixtures. Among these, a nurate body is preferable from the viewpoint of weather resistance.

The carbodiimide group-containing compound is not particularly limited as long as it has at least two functional groups for reacting under heating to form a crosslinked structure, but preferably has two isocyanate groups in the molecule ( 1) and those having at least two carbodiimide groups in the molecule (2). As such a carbodiimide group-containing compound, those obtained by decarboxylation of an isocyanate compound as described above can be used.

As what has two isocyanate groups in the said molecule | numerator (1), what is represented by a following formula can be mentioned.
_{OCN-R 1 - (NCN-} R 1) n-NCO
(In the formula, R ₁ is the same or different and is a residue obtained by removing an isocyanate group from an organic diisocyanate, n is an integer of 1 or more, and represents the number of carbodiimide group repeats.)

Examples of the carbodiimide group-containing carbodiimide (2) having at least two carbodiimide groups in the molecule include those represented by the following formula.
_{R 2 -NCN-R 1 - (} NCN-R 1) m-NCN-R 2
(Wherein R ₁ is the same or different and a residue obtained by removing an isocyanate group from an organic diisocyanate; R ₂ is the same or different and a residue obtained by removing an isocyanate from a monoisocyanate compound; m is an integer of 0 or 1 or more) And represents the number of carbodiimide group repeats.)

The oxazoline group-containing compound is a compound having a plurality of oxazoline groups in one molecule. The oxazoline group-containing compound is not particularly limited. For example, 2,2'-bis- (2-oxazoline), 2,2'-methylene-bis- (2-oxazoline), 2,2'-ethylene-bis -(2-oxazoline), 2,2'-trimethylene-bis- (2-oxazoline), 2,2'-tetramethylene-bis- (2-oxazoline), 2,2'-hexamethylene-bis- (2 -Oxazoline), 2,2'-octamethylene-bis- (2-oxazoline), 2,2'-ethylene-bis- (4,4-dimethyl-2-oxazoline), 2,2 '-(1,3 -Phenylene) -bis- (2-oxazoline), 2,2 '-(1,3-phenylene) -bis- (4,4-dimethyl-2-oxazoline), 2,2'-(1,4-phenylene) ) -Bis- (2-Oki Dioxazoline compounds such as zoline), bis- (2-oxazolinylcyclohexane) sulfide, bis- (2-oxazolinylnorbornane) sulfide; 2,2 ′-(1,2,4-phenylene) -tris- ( And trioxazoline compounds such as 2-oxazoline).

The said hardening | curing agent has a functional group which can be hardened | cured with the carboxyl group and / or hydroxyl group which the said acrylic resin has. It is preferable that the functional group amount ratio between the carboxyl group and / or the hydroxyl group and the curable functional group of the acrylic resin is 1 / 1.5 to 1.5 / 1. When the functional group amount ratio is less than 1 / 1.5, curability becomes insufficient, and when it is 1.5 / 1, the cured film becomes too hard and brittle. The functional group amount ratio is more preferably 1.4 / 1 to 1 / 1.4.

In the clear coating composition, an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, a crosslinked resin particle, a surface conditioner, and the like may be blended. When using the said crosslinked resin particle, it is preferable to mix | blend in the ratio of 0.01 mass% of lower limits and 10 mass% of upper limits with respect to resin solid content of the curable resin composition of this invention. The lower limit is more preferably 0.1% by mass, and the upper limit is more preferably 5% by mass. When the addition amount of the crosslinked resin particles exceeds 10% by mass, the appearance of the obtained coating film is deteriorated, and when it is less than 0.01% by mass, the rheology control effect cannot be obtained.

The clear coating composition of the present invention is preferably a two-component curable coating composition comprising the above-mentioned acrylic resin and a carboxyl group and / or hydroxyl group and a curable curing agent.

The clear coating composition of the present invention can be applied by spray coating, brush coating, dip coating, roll coating, flow coating, or the like. When the clear coating composition is a two-component curable coating composition, it can be applied by the method described above after mixing both immediately before use.

The clear coating composition of the present invention can be advantageously used for, for example, wood, metal, glass, cloth, plastic, foam and the like, particularly plastic and metal surfaces such as steel, aluminum and alloys thereof. It is particularly preferable to use it as a clear paint for automobile painting.

The multilayer coating film forming method of the present invention comprises: (1) applying a base coating to a substrate to obtain an uncured base coating; and (2) applying the clear coating on the uncured base coating. This is a multilayer coating film forming method in which a composition is applied to form an uncured clear coating film, and (3) a multilayer coating film is formed by heating and curing the uncured base coating film and the uncured clear coating film. Such a method can be suitably used particularly as an automobile coating.

As the above-mentioned base material, it can be used for, for example, metal molded products, plastic molded products, foams, etc., but as base materials for forming a multilayer coating film for automobiles, iron, aluminum, zinc-based substrates and these Examples thereof include metal molded products such as alloys, plastic molded products, and the like. It is preferable to apply to metal molded products that can be subjected to cationic electrodeposition coating. The base material is preferably subjected to a chemical conversion treatment on the surface. Furthermore, an electrodeposition coating film may be formed on the substrate. As the electrodeposition paint, the cation type and the anion type can be used. From the viewpoint of anticorrosion, a cation type electrodeposition paint is preferable.

As the base paint, a brightener base paint containing a base resin component, a bright pigment, a colored pigment, an extender pigment, a solvent and the like can be used. This base coating composition is of an aqueous or organic solvent type including an aqueous dispersion or an organic solvent dispersion. The aqueous solvent and organic solvent are not particularly limited, and conventionally known solvents can be used.

The base resin component contained in these base paints is composed of a base resin and, if necessary, a base curing agent. Using the base resin component, the glitter pigment and, if necessary, the color pigment are dispersed in the glitter base coating.

Examples of the base resin include acrylic resin, polyester resin, polyurethane resin, alkyd resin, epoxy resin, and polyether resin. Of these resins, acrylic resins, polyester resins, and polyurethane resins are preferably used. These resins may be used alone or in combination of two or more.

The base resin generally has a curable type and a lacquer type, but a curable type is preferably used. When a curable type is used, a base curing agent such as a melamine resin, a (block) isocyanate compound, an oxazoline compound and a carbodiimide compound is used in combination with the base resin. This base curing agent can be included in the base resin component to allow the curing reaction to proceed later under heating or at room temperature. Further, a curable type base resin and a non-curable type resin can be used in combination.

When a base hardening | curing agent is contained, the preferable mass ratio of the base resin and the baking hardening agent in paint solid content is 90 / 10-50 / 50, More preferably, it is 85 / 15-60 / 40. If the amount of the base curing agent is so small that the mass ratio between the base resin and the base curing agent deviates from 90/10, sufficient crosslinking in the coating film may not be obtained. On the other hand, when the amount of the base curing agent is so large that this ratio is deviated from 50/50, the storage stability of the coating material is lowered and the curing rate is increased, which may deteriorate the appearance of the coating film.

As the base curing agent, an etherified melamine resin can be used in addition to those described above. The etherified melamine resin is obtained by etherifying melamine with an alcohol such as methanol or butanol.

As a pigment contained in the base paint, a non-colored or colored scale-like metallic luster material such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide or an alloy, or a mixture thereof, or interference mica powder And bright pigments such as non-colored colored bright materials such as colored mica powder, white mica powder, graphite and glass flakes.
In addition to the glitter pigment, for example, extender pigments such as barita powder, precipitated barium sulfate, barium carbonate, gypsum, clay, silica, talc, magnesium carbonate, and alumina white, and colored pigments. Examples of coloring pigments include azo lake pigments, phthalocyanine pigments, indico pigments, berylene pigments, quinophthalone pigments, dioxazine pigments, quinacridone pigments, isoindolinone pigments, diketopyrrolopyrrole pigments, and benzimidazolones. Organic pigments such as system pigments and metal complex pigments, and inorganic pigments such as yellow lead, yellow iron oxide, bengara, titanium dioxide, and carbon black. The amount of the pigment can be arbitrarily set according to the desired performance and hue. These pigments may be used alone or in combination of two or more.

In addition to the above components, the base paint is composed of a polyamide wax which is a lubricating dispersion of an aliphatic amide, a polyethylene wax which is a colloidal dispersion mainly composed of polyethylene oxide, a curing catalyst, an ultraviolet absorber, an antioxidant, and a leveling agent. Further, a surface conditioner such as silicon or organic polymer, an anti-sagging agent, a thickener, an antifoaming agent, a lubricant, a crosslinkable polymer particle (microgel), and the like can be appropriately added. By blending these additives at a ratio of 15 parts by mass or less with respect to 100 parts by mass (based on solid content) of the base resin component, the performance of the paint or coating film can be improved.

The upper limit of the pigment concentration (PWC) with respect to the solid content of the base coating is preferably 65% by mass. The lower limit is more preferably 5% by mass. Further, the solid content concentration of the base coating is preferably in the range of a lower limit of 15% by mass and an upper limit of 60% by mass.

Specific examples of the method of applying the base paint include multi-stage coating by air electrostatic spraying or rotary atomizing electrostatic coating machine, preferably two-stage coating. By performing this coating method, the designability can be improved. Alternatively, as a method for applying the base coating material, a coating method in combination with an electrostatic electrostatic spraying machine of an air electrostatic spray rotary atomization type may be mentioned. The dry film thickness of the topcoat base coating film is preferably 5 to 50 μm, more preferably 10 to 30 μm per coat.

The obtained uncured base coating film may be preheated. By applying pre-heating after applying the base coating material to form an uncured base coating film, a multilayer coating film having a better finished appearance can be finally obtained. In the present invention, “uncured” refers to a state where the film is not completely cured, and includes a state of a coating film that has been preheated after coating. “Preheating” can be performed by leaving or heating at room temperature to 100 ° C., which is lower than the temperature used in the heat curing (baking) treatment, for 1 to 10 minutes.

Specific examples of the method for applying the clear coating composition in the multilayer coating film forming method of the present invention include a micro-microbell and a coating method using a rotary atomizing electrostatic coater called a microbell. Can do. In the case where the clear coating composition is a two-component curable type, the clear coating composition can be applied by the above method after mixing them before use. The coating film thickness of the clear coating composition in the multilayer coating film forming method of the present invention is preferably in the range of a dry film thickness of a lower limit of 20 μm and an upper limit of 60 μm.

The base uncured coating film and the clear uncured coating film formed by the above method are preferably heated and cured simultaneously to form a multilayer coating film. The heating temperature is preferably performed within a range of a lower limit of 100 ° C. and an upper limit of 180 ° C. More preferably, the lower limit is 120 ° C and the upper limit is 160 ° C. The heat-curing time varies depending on the curing temperature or the like, but when it is performed at the above-mentioned heat-curing temperature, it is appropriate to be 10 to 30 minutes.

The total film thickness of the base coating film and the clear coating film of the multilayer coating film thus obtained is preferably in the range of a lower limit of 30 μm and an upper limit of 80 μm. The multilayer coating film obtained by the above method is excellent in coating film properties such as scratch resistance, acid resistance and solvent resistance, and has high design properties.

In the multilayer coating film forming method of the present invention, before the step (1), the intermediate coating film or primer surfacer coating is applied to the substrate to obtain the intermediate coating film or primer surfacer coating film (P). May be included. That is, the multi-layer coating film forming method of the present invention can be applied to the painting at the time of repairing the automobile body. As the intermediate coating material that can be used in the step (P), a conventionally used intermediate coating material can be used.

The intermediate coating may be a water-based coating or a solvent-based coating. Examples of the components of these paints include intermediate coating resin components, colored pigments, extender pigments, and aqueous and / or organic solvents. The intermediate coating resin component is composed of an intermediate coating resin and, if necessary, an intermediate coating curing agent. As the intermediate coating resin component, intermediate coating curing agent, coloring pigment, extender pigment, various additives and solvent contained in the intermediate coating, any of those described for the above-mentioned base coating can be used.

The concentration (PWC) of the pigment with respect to the solid content of the intermediate coating is preferably in the range of a lower limit of 30% by mass and an upper limit of 65% by mass. The upper limit is more preferably 50% by mass. The solid content concentration of the intermediate coating is preferably in the range of 35% by mass to 65% by mass.

Examples of the method for applying the intermediate coating include those using a spray method, a roll coater method, and the like. Specifically, it uses air electrostatic sprays called “react guns” or rotary atomizing electrostatic coatings called “micro-micro (μμ) bell”, “micro (μ) bell”, “metabell”, etc. It is preferable to paint using a machine. Among these, it is particularly preferable to perform the coating using a rotary atomizing electrostatic coating machine. A preferable dry film thickness of the intermediate coating film is 5 to 80 μm, and more preferably 10 to 50 μm.

The uncured intermediate coating film thus formed can be baked and cured at 120 to 160 ° C. for a predetermined time to obtain a cured intermediate coating film. The base paint and the clear paint can be applied on the cured intermediate coating film thus obtained.

In addition, the above base coating and clear coating are applied wet-on-wet on the obtained uncured intermediate coating, and the uncured base coating and clear coating are applied on the uncured intermediate coating. Are preferably formed sequentially. In this way, when applying base paint and clear paint in wet-on-wet, by applying pre-heating after applying the intermediate coating, an uncured intermediate coating film is formed, and finally a better finished appearance Can be obtained.

In the present invention, a primer surfacer can be used instead of the intermediate coating. That is, the multilayer coating film forming method of the present invention can also be applied to painting during automobile repair. A conventionally known primer surfacer can be used.

ADVANTAGE OF THE INVENTION According to this invention, the multilayer coating film which suppresses the color return of a base coating film and maintains the high designability which the base coating film originally has can be formed.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited only to these examples. In the examples, “parts” means “parts by mass” unless otherwise specified.

[Synthesis Example a]
448 parts of propylene glycol monomethyl ether acetate was added to a reaction vessel equipped with a thermometer, a stirring blade, a nitrogen introduction tube, a cooling condenser and a dropping funnel, and heated to 120 ° C. in a nitrogen atmosphere. In the container, using a dropping funnel, 100 parts of propylene glycol monomethyl ether acetate, 105 parts of tert-butylperoxy 2-ethylhexanoate, 200 parts of styrene, 270 parts of isobornyl methacrylate, acrylic acid-n -A monomer mixed solution consisting of 67 parts of butyl, 100 parts of methacrylic acid-lauryl, 360 parts of acrylic acid-4-hydroxybutyl and 3 parts of methacrylic acid was added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 120 ° C. for 0.5 hour, and 10 parts of tert-butylperoxy 2-ethylhexanoate dissolved in 50 parts of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, heating was continued at 120 ° C. for 1 hour.
As a result, calculation Tg 4.8 ° C., solid content acid value 2 mg KOH / g, hydroxyl value 140 mg KOH / g, number average molecular weight (Mn) 4600, weight average molecular weight (Mw) 11600 in terms of standard polystyrene obtained using GPC, An acrylic resin copolymer a having a resin solid content of 62.5% was obtained.

[Synthesis Examples b to k]
Hydroxy group-containing acrylic resin copolymers b to k shown in Table 1 were obtained in the same manner as in Synthesis Example a except that the formulation shown in Table 1 was used.
In addition, as alkyl methacrylate, acrylate ester SL (manufactured by Mitsubishi Rayon Co., Ltd.) having a mixing ratio (mass basis) of lauryl methacrylate / tridecyl methacrylate of 4/6 was used.

[Examples 1-7, Comparative Examples 1-4]
In accordance with the formulation shown in Table 2, the hydroxy group-containing acrylic resin copolymers a to k and isocyanurate type isocyanate compounds (Dismodule N-3300, manufactured by Sumika Bayer Urethane Co., Ltd.) were formulated. Clear coating compositions of -7 and Comparative Examples 1-4 were obtained.

Cathode electrodeposition paint “Power Top U-50” and gray intermediate paint “Orga P-30” (both trade names) manufactured by Nippon Paint Co., Ltd. on the phosphated steel sheet so that the dry film thicknesses are 25 μm and 40 μm, respectively. Examples 1 to 7 were coated with a silver metallic base paint of water paint “AQUAREX AR-2000” (trade name) manufactured by Nippon Paint Co., Ltd. Each of the clear coating compositions of Comparative Examples 1 to 4 was applied and baked and dried at 140 ° C. for 30 minutes to prepare a 2-coat 1-bake coating test plate. In addition, the multilayer coating film by the said base coating material and clear coating composition was apply | coated so that the dry film thickness might be 15 micrometers and 40 micrometers, respectively. Separately, the silver metallic base paint was applied to the test plate so that the dry film thickness was 15 μm, and baked and dried at 140 ° C. for 30 minutes to produce a base single-layer coating test plate.

[Evaluation methods]
Using the coating test plates obtained from the clear coating compositions of Examples 1 to 7 and Comparative Examples 1 to 4, evaluation was performed by the following methods (1) to (4). The evaluation results are shown in Table 2.

(1) The color difference with the metallic coating film obtained by 2 coat 1 baking was measured on the basis of the metallic base single layer coating film corresponding to L value.
Measurement was performed using CM-512m3 manufactured by Minolta Co., Ltd., L values at light receiving angles of 25 ° (highlight) and 75 ° (shade) were measured, and ΔL value was calculated by the following formula.
ΔL value = (L value of multilayer coating film) − (L value of uncured base single layer coating film)

(2) Visual observation Evaluation of the design properties of the multilayer coating film by visual judgment was shown. As evaluation criteria, ◎ indicates very strong flip-flop property and very good design, ○ indicates slightly mixed layer of base coating and clear coating, but strong flip-flop and base The design characteristics inherent to the coating film are maintained, △ indicates that the base coating film and the clear coating film are mixed, the flip-flop property is weak and the design characteristics are low, and × indicates the base coating film and the clear coating It shows that the film is mixed, the flip-flop property is very weak, and the design property is very low.

(3) Primary adhesion by re-coating Apply the same aqueous base paint again on the multi-layer coating film after heat curing, then apply the same clear paint composition and bake and dry at 120 ° C for 30 minutes. Then, a test plate for adhesion evaluation was prepared. The obtained test plate was subjected to a cross cut test in accordance with JIS K 5600-5-6 to evaluate the presence or absence of peeling of the coating film. ○ indicates that there is no peeling of the coating film, and x indicates that there is peeling of the coating film.

(4) Water-resistant secondary adhesion of repainting The test plate for adhesion evaluation was immersed in ion-exchanged water maintained at 40 ° C. for 10 days. Immediately after taking out, moisture on the surface was wiped off, and a cross-cut test was performed in accordance with JIS K 5600-5-6 to evaluate the presence or absence of peeling of the coating film. ○ indicates that there is no peeling of the coating film, and x indicates that there is peeling of the coating film.

In Examples 1 to 7, it was possible to obtain a multilayer coating film in which the ΔL value was small for both the highlight and the shade, the color reversion was suppressed, and the high design property originally possessed by the base coating film was maintained.
On the other hand, in Comparative Examples 1 to 4, both the highlight and the shade had a large ΔL value, and color reversion occurred. Also, the design of the coating film was not good.

INDUSTRIAL APPLICABILITY The present invention can be suitably used as a clear paint for automobile painting, particularly as a clear paint for automobile painting for a market that requires high designability, whether for a new car or for repair. In particular, it can be suitably used as a clear paint to be applied to a metallic base coating film.

Claims (7)

Obtained from a monomer mixture containing a long-chain (meth) acrylic acid ester monomer, a carboxyl group-containing (meth) acrylic monomer, and a hydroxyl group-containing (meth) acrylic monomer in which the ester portion is a linear hydrocarbon group having 9 to 15 carbon atoms A clear coating composition characterized by containing an acrylic resin. Furthermore, the clear coating composition of Claim 1 containing the hardening | curing agent which can be hardened with a carboxyl group and / or a hydroxyl group. The clear coating composition according to claim 2, wherein the curing agent is a polyisocyanate compound. The clear coating composition according to any one of claims 1 to 3, wherein the content of the long-chain (meth) acrylic acid ester monomer in the monomer mixture is 5 to 15% by mass relative to the total amount of monomer solids. The clear coating composition according to any one of claims 1 to 4, wherein the acrylic resin has a solid acid value of 1 to 10 mgKOH / g. A step (1) of applying a base coating to the substrate to form an uncured base coating;
A step (2) of applying a clear coating on the uncured base coating obtained in the step (1) to form an uncured clear coating;
Forming a multilayer coating film comprising the step (3) of simultaneously heating and curing the uncured base coating film and the uncured clear coating film obtained in the steps (1) and (2) to form a multilayer coating film; A method,
The method of forming a multilayer coating film, wherein the clear paint is the clear paint composition according to any one of claims 1 to 5. The coating-film formation method of Claim 6 including the process (P) of apply | coating an intermediate coating material or a primer surfacer to the said base material before the said process (1), and obtaining an intermediate coating film or a primer surfacer coating film.