JPH1176933A - Coating of metal sheet and coated metal sheet with excellent contamination preventing property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve performances, appearance and contamination preventing property of a coating film by forming an upper color base coating film on a primer coating film by a wet-on-wet process and using an upper clear coating material containing a specified contamination preventing agent in a specified film forming resin component. SOLUTION: The upper color base coating film is formed on a primer coating film by a wet-on-wet process. The upper clear coating material consists of a film forming resin component comprising (A) 60 to 95 pts.wt. of a hydroxyl group-contg. org. resin and (B) 5 to 40 pts.wt. of a crosslinking agent comprising a melamine resin and/or block polyisocyanate compd., and contains a contamination preventing agent comprising a silicon compd. expressed by SiO(4(a+b))/2 (OR<1> )a (OR<2> )b having 1 to 20 silicon atoms in one molecule and/or photocatalytic active titanium oxide. In the formula R<1> is a specified alkyl group, R<2> is a specified hydrocarbon group, (a) is 0.10 to 4, (b) is 0 to 1.95 and a+b<=4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an uncured topcoat base coating film, which is coated with a topcoat clear paint containing a hydrophilicity-imparting agent and baked, and is applied on a continuously moving metal plate for a short period of time. The present invention relates to a coating method capable of forming a top-coat multilayer coating film whose surface can become hydrophilic over time.

[0002]

2. Description of the Related Art In the field of precoated metal sheets, a one-coat, one-bake method in which a topcoat is applied to a continuously moving long metal sheet and baked is used. A two-coat two-bake method is generally used in which a primer paint is applied and baked, and then a top-coat paint is applied and baked on the primer coating film. Among them, the two-coat two-bake method occupies a large part. I have. In addition, a part is painted with a primer paint on a metal plate and baked, then an intermediate coat is applied on the primer coat and baked, and then a top coat is applied and baked.
The coat 3 baking method is performed.

[0003] By forming three coating films, it is possible to achieve a coating film performance and a coating appearance that cannot be achieved with one or two layers as a total coating film. The number of times of coating and the number of times of baking of the coating film are each one more than those of the two-coat baking method, and there is a problem in that the burden on equipment is increased.

In recent years, building materials used as outdoor walls, roofs and the like have a problem that dirt such as dust, soot and rainwater drips the appearance of the building materials. There was a need for a painted metal plate for use.

Therefore, the present inventors have proposed a coating method capable of forming a two-layered top coating film on a long metal plate that moves continuously and with a small load on equipment. As a result of intensive research on the method of coating a metal plate that can form a top coat that has excellent coating appearance and can prevent dirt, a primer coating film and a top coating base coating film are wet-on-wet on the metal plate. This is a method in which a top coat clear paint is applied and baked on the top coat colored base coating film after baking and baking, and a specific top coat clear paint containing a stain preventing agent is used as the top coat clear paint. As a result, the present inventors have found that the above objects can be achieved by doing so, and have completed the present invention.

[0006]

That is, according to the present invention, a primer paint film and a top coat base paint film are formed on a continuously moving long metal plate and baked. A method of applying a top coat clear paint on a top coat base coat and baking to form a laminated coat of a primer coat, a top coat base coat and a top coat clear coat on a metal plate, comprising a top coat base paint The coating film is formed on the primer coating film by wet-on-wet, and the top clear coating material is selected from (A) 60 to 95 parts by weight of a hydroxyl group-containing organic resin and (B) a melamine resin and a blocked polyisocyanate compound. At least one crosslinking agent
(C) The following general formula [1] SiO _{(4- (a + b)) / 2} (OR ¹ ) _a (OR ² ) _b [1] R ¹ is the same or different and represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R ² is a cycloalkyl group, an aryl group or an aralkyl group having 5 to 12 carbon atoms, or an ether bond and / or Or a monovalent hydrocarbon group having an ester bond and having 4 to 24 carbon atoms, a represents a number of 0.10 to 4.00, b represents a number of 0 to 1.95, and the sum of a and b Is 4 or less) characterized in that the clear coating composition contains at least one antifouling agent selected from the group consisting of a silicon compound having 1 to 20 silicon atoms in one molecule and a photocatalytically active titanium oxide. And a method for coating a metal plate.

[0007] The present invention also provides a coated metal plate coated by the above coating method.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the method of the present invention will be described in more detail.

The metal plate to be coated in the method of the present invention is a long metal plate used in the field of a precoated steel sheet, and may be made of a cold-rolled steel sheet, a galvanized steel sheet, an aluminum sheet, a stainless steel sheet or the like. Can be mentioned. As the galvanized steel sheet, hot-dip galvanized steel sheet, electric galvanized steel sheet,
Examples thereof include iron-zinc alloy-plated steel sheets, nickel-zinc alloy-plated steel sheets, and aluminum-zinc alloy-plated steel sheets (for example, plated steel sheets having trade names of “galvalume” and “galfan”). These metal plates may be those whose surfaces have been subjected to a chemical conversion treatment such as a phosphate treatment, a composite oxide film treatment, and a chromate treatment.

[0010] In the primer present invention method, on the metal plate of the long, directly or via a primer cured film, painting a topcoat colored base paint and topcoat clear coating.

The primer-cured coating film can be formed, for example, by applying a primer on a metal plate by a roll coater or the like so as to have a dry film thickness of about 1 to 15 μm, and baking and drying. The primer coating film is formed for the purpose of improving the corrosion resistance of the resulting coated metal plate, improving the adhesion of the coating film, and the like. Examples of the primer include polyester primers and epoxy primers.These primers contain rust preventive pigments such as strontium chromate, calcium chromate, zinc chromate, barium chromate, and zinc phosphate. You may.

The top clear coating used in the method of the present invention is a coating for forming the uppermost coating film of a coated metal plate, and comprises a hydroxyl group-containing organic resin (A), a crosslinking agent (B) and a hydrophilicity imparting agent. Agent (C)
It contains. An overcoat clear paint film is formed on the baked overcoat colored base film. The cured top clear coating film has a surface that becomes hydrophilic over time and can exhibit a stain preventing effect.

The hydroxyl-containing organic resin (A) includes:
Any hydroxyl group-containing organic resin capable of forming a coating film that can withstand outdoor use can be used without particular limitation. For example, one or more kinds of polyester resin, silicone polyester resin, acrylic resin, fluorine resin and the like can be used. And mixtures thereof.

Among the above-mentioned base resins, a polyester resin can be suitably used. Examples of the polyester resin include an oil-free polyester resin, an oil-modified alkyd resin, and modified products of these resins, such as a urethane-modified polyester resin, a urethane-modified alkyd resin, an epoxy-modified polyester resin, and an acryl-modified polyester resin.

The above-mentioned oil-free polyester resin comprises an esterified product of a polybasic acid component and a polyhydric alcohol component. As the polybasic acid component, for example, one or more selected from phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, succinic acid, fumaric acid, adipic acid, sebacic acid, maleic anhydride, and the like Dibasic acids and lower alkyl esterified products of these acids are mainly used, and if necessary, monobasic acids such as benzoic acid, crotonic acid, pt-butylbenzoic acid, trimellitic anhydride, methylcyclohexenetricarboxylic acid And tribasic or higher polybasic acids such as pyromellitic anhydride. Examples of the polyhydric alcohol component include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol,
Dihydric alcohols such as neopentyl glycol, 3-methylpentanediol, 1,4-hexanediol, and 1,6-hexanediol are mainly used, and if necessary, glycerin, trimethylolethane, trimethylolpropane, and pentane. A trihydric or higher polyhydric alcohol such as erythritol can be used in combination. These polyhydric alcohols can be used alone or in combination of two or more. The esterification or transesterification of both components can be carried out by a method known per se. As the acid component, isophthalic acid, terephthalic acid,
And lower alkyl esters of these acids are particularly preferred.

The alkyd resin is obtained by reacting an oil fatty acid by a method known per se in addition to the acid component and the alcohol component of the above-mentioned oil-free polyester resin. Soybean oil fatty acids, linseed oil fatty acids, safflower oil fatty acids, tall oil fatty acids, dehydrated castor oil fatty acids, kiri oil fatty acids and the like can be mentioned. The oil length of the alkyd resin is preferably 30% or less, particularly preferably about 5 to 20%.

As the urethane-modified polyester resin,
The oil-free polyester resin, or a low-molecular-weight oil-free polyester resin obtained by reacting an acid component and an alcohol component used in the production of the oil-free polyester resin, reacts with a polyisocyanate compound by a method known per se. Examples include: Further, the urethane-modified alkyd resin, the alkyd resin, or a low molecular weight alkyd resin obtained by reacting each component used in the production of the alkyd resin was reacted with a polyisocyanate compound by a method known per se. Things are included. Examples of polyisocyanate compounds that can be used when producing urethane-modified polyester resin and urethane-modified alkyd resin include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate,
4,4'-methylenebis (cyclohexyl isocyanate), 2,4,6-triisocyanatotoluene, and the like. In general, as the urethane-modified resin, those having a modification degree in which the amount of the polyisocyanate compound forming the urethane-modified resin is 30% by weight or less based on the urethane-modified resin can be suitably used.

As the epoxy-modified polyester resin,
Using a polyester resin produced from each component used in the production of the above polyester resin, a reaction product of a carboxyl group of this resin and an epoxy group-containing resin, and a hydroxyl group in the polyester resin and a hydroxyl group in the epoxy resin are polyisocyanate. Reaction products such as addition, condensation, and grafting of a polyester resin and an epoxy resin, such as a product bonded via a compound, can be given.
In general, the degree of modification in such an epoxy-modified polyester resin is preferably such that the amount of the epoxy resin is 0.1 to 30% by weight based on the epoxy-modified polyester resin.

As the acrylic-modified polyester resin,
Using a polyester resin produced from each component used in the production of the polyester resin, a group having a reactivity with these groups to a carboxyl group or a hydroxyl group of the resin, such as an acrylic resin containing a carboxyl group, a hydroxyl group or an epoxy group. And a reaction product obtained by graft polymerization of (meth) acrylic acid or (meth) acrylic ester on a polyester resin using a peroxide-based polymerization initiator. The degree of modification in such an acrylic-modified polyester resin is generally
It is preferable that the amount of the acrylic resin is 0.1 to 50% by weight based on the acrylic-modified polyester resin.

Among the polyester resins described above, a preferable one is an oil-free polyester resin. Among the polyester resins, from the viewpoints of curability, workability of the resulting coating film, etc., among others, the number average molecular weight is from 1,500 to 25,000, preferably from 3,000 to 2
000, glass transition temperature (Tg point) 0 to 70 ° C, preferably 10 ° C to 60 ° C, hydroxyl value 8 to 100 mg KO
Polyester resins having H / g, preferably 15 to 60 mg KOH / g, are suitable.

In the present invention, the glass transition temperature (Tg)
Is measured by differential thermal analysis (DTA), and the number average molecular weight is measured by gel permeation chromatography (GPC) using a standard polystyrene calibration curve.

Crosslinking agent (B) used for clear top coating
Examples include at least one crosslinking agent selected from a melamine resin and a blocked polyisocyanate compound.

Examples of the melamine resin include a methylolated amino resin obtained by reacting melamine with an aldehyde. Examples of the aldehyde used in the above reaction include formaldehyde, paraformaldehyde, acetaldehyde, benzaldehyde and the like. Further, those obtained by etherifying the above methylolated amino resin with an appropriate alcohol can also be used as the melamine resin. Examples of alcohols used for etherification include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethylbutanol,
2-ethylhexanol and the like.

The blocked polyisocyanate compound is a compound obtained by blocking free isocyanate groups of a polyisocyanate compound with a blocking agent.

Examples of the polyisocyanate compound before blocking include aliphatic diisocyanates such as hexamethylene diisocyanate or trimethylhexamethylene diisocyanate; cycloaliphatic diisocyanates such as hydrogenated xylylene diisocyanate or isophorone diisocyanate; tolylene diisocyanate Or an organic diisocyanate itself such as an aromatic diisocyanate such as 4,4'-diphenylmethane diisocyanate, or an adduct of each of these organic diisocyanates with a polyhydric alcohol, a low molecular weight polyester resin or water; Cyclic polymers among diisocyanates, and isocyanate / biuret compounds are also included.

Examples of the blocking agent for blocking the isocyanate group include phenols such as phenol, cresol and xylenol; ε-caprolactam;
Lactams such as valerolactam, γ-butyrolactam, β-propiolactam; methanol, ethanol, n-
Or i-propyl alcohol, n-, i- or t-butyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether,
Alcohols such as benzyl alcohol; oximes such as formamidoxime, acetoaldoxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, benzophenone oxime, cyclohexane oxime; dimethyl malonate, diethyl malonate, ethyl acetoacetate, ethyl acetoacetate, methyl acetoacetate, acetylacetone A blocking agent such as an active methylene type can be suitably used.

The free isocyanate group of the polyisocyanate compound can be easily blocked by mixing the polyisocyanate compound with the blocking agent.

The crosslinking agent may be composed of one kind of crosslinking agent or a mixture of two or more kinds of crosslinking agents.

The top clear coating composition contains a stain-preventing agent (C) in addition to the film-forming resin component comprising the resin (A) and the crosslinking agent (B).

The stain-preventing property imparting agent (C) represented by the following general formula _{[1] SiO (4- (a} + b)) / 2 (OR 1) a (OR 2) b [1] ( In the formula, R ¹ Are the same or different and represent a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, and R ² is a cycloalkyl group, an aryl group or an aralkyl group having 5 to 12 carbon atoms, or an ether bond and / or an ester. Represents a monovalent hydrocarbon group having 4 to 24 carbon atoms containing a bond, a represents a number of 0.10 to 4.00, b represents a number of 0 to 1.95, and the sum of a and b is 4 A silicon compound (c-1) having 1 to 20 silicon atoms in one molecule represented by the following formula:
And photocatalytically active titanium oxide (c-2).

When the silicon compound (c-1) as the stain preventing agent (C) is incorporated into the top clear coating, it is hydrolyzed with time in the top clear coating to form a surface of the top clear coating. Can be made hydrophilic.

The above formula [1] representing the silicon compound (c-1)
In the formula, (OR ¹ ) is a group directly bonded to a silicon atom, and is a hydroxyl group or a group having 1 to 10 carbon atoms, preferably 1 to 10 carbon atoms.
3 represents an alkoxyl group. Examples of the alkoxyl group having 1 to 10 carbon atoms include methoxy, ethoxy, n-propoxy, isopropoxy, butoxy, hexyloxy, octyloxy, and decyloxy groups. The alkoxyl group directly bonded to the silicon atom generates a silanol group by hydrolysis.

In the above formula [1], specific examples of the group represented by (OR ² ) include a cycloalkoxyl group such as cyclohexyloxy; an aryloxy group such as phenoxy and tolyloxy; and an aralkyl group such as benzyloxy and phenethyloxy. An oxy group; butoxyethyloxy,
Phenoxyethyl _{oxy, - (C 2 H 4 O} ) 2 -C 4 H
A group represented by ₉ ; acetoxyethyloxy, methoxypoly (oxyethylene) oxy group, ethoxypoly (oxyethylene) oxy group, methoxypoly (oxypropylene) oxy group, ethyloxycarbonylpentoxy group,
An ethyl-poly (oxycarbonylpentyl) oxy group and the like can be mentioned.

The silicon compound (c-1) includes a silicon monomer represented by the following formula [2] (hereinafter abbreviated as “silicon monomer”); one or more (co) Condensate; the above silicon monomer or its condensate and 1
Reaction products obtained by ether exchange reaction with a valent hydroxyl group-containing hydrocarbon; and a ring-opening addition product of the above-mentioned silicon monomer or its condensate with a lactone compound.

(R ¹ O) _m —Si— (OR ² ) _n [2] (wherein, R ¹ is the same or different and has the same meaning as described above, and R ² is the same or different and is the same as above) M represents an integer of 2 to 4, n represents an integer of 0 to 2, and the sum of m and n is 4. As specific examples of the silicon monomer represented by the above formula [2], In the formula [2], examples of the tetraalkoxysilane in which m is 4 include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, dimethoxydiethoxysilane, and dimethoxydipropoxysilane, and m is 2 or As three, triethoxyphenoxysilane, trimethoxyphenoxysilane,
Triethoxybenzyloxysilane, triethoxy (phenethyloxy) silane, diethoxydiphenoxysilane, dimethoxy-di (butoxyethyloxy) silane,
Diethoxy-di (butoxyethyloxy) silane, trimethoxy (butoxyethyloxy) silane, triethoxy (butoxyethyloxy) silane, dimethoxy-di (phenoxyethyloxy) silane, diethoxy-di (phenoxyethyloxy) silane, trimethoxy (phenoxyethyl) Oxy) silane, triethoxy (phenoxyethyloxy) silane, dimethoxy-di [2- (acetoxy) pentyloxy] silane, diethoxy-di [2- (acetoxy) pentyloxy] silane, trimethoxy-2- (acetoxy) pentyloxysilane , Triethoxy-2- (acetoxy) pentyloxysilane, dimethoxy-di [2- (ethoxy) pentyloxy] silane, diethoxy-di [2- (ethoxy) pentyloxy] silane, trimethy Shi-2- (ethoxy) pentyl oxy silane, triethoxy-2- (ethoxy)
Pentyloxysilane and the like can be mentioned.

As a condensate of tetraalkoxysilane, a commercially available product such as “Colcoat ES40” (trade name, manufactured by Colcoat Co., Ltd .;
Monomer, average about pentamer), "Colcoat MS51" (manufactured by Colcoat, trade name, 1 to 10 of tetramethoxysilane)
Monomer, average about tetramer) and the like.

The (co) condensation reaction for obtaining one or more (co) condensates of the silicon monomer can be carried out by a conventional method. For example, hydrolysis and condensation can be performed by heating in the presence of a predetermined amount of water.

The silicon monomer and / or its (co) condensate are transesterified with a monovalent hydroxyl group-containing hydrocarbon represented by the formula R ² OH (which may contain an ether bond and / or an ester bond). (Si-O
Part of the R ¹ ) bond can be replaced with a (Si—OR ² ) bond.

Examples of the monovalent hydroxyl-containing hydrocarbon represented by R ² OH include cycloalkanols having 5 to 12 carbon atoms such as cyclopentyl alcohol and cyclohexanol; and C 6 to C 6 carbon atoms such as phenol and cresol.
A hydroxyl group-containing aryl compound having 12 carbon atoms; an aralkyl alcohol having 7 to 12 carbon atoms such as benzyl alcohol and phenethyl alcohol; and a hydroxyl group at one end of (poly) alkylene glycol such as (poly) ethylene glycol and (poly) propylene glycol. 1-8 atoms
A hydroxyl group-containing etherified product obtained by etherifying with a monovalent alkanol, phenol or aralkyl alcohol; a hydroxyl group at one end of an alkylene glycol such as ethylene glycol or propylene glycol having 2 carbon atoms.
A hydroxyl group-containing esterified product obtained by esterification with a monocarboxylic acid of -8 to 8; a hydroxyl group-containing esterified product obtained by ring-opening a lactone such as ε-caprolactone using an alcohol as an initiator and polymerizing as necessary; A hydroxyl-containing compound having an ether bond and an ester bond obtained by esterifying a hydroxyl group at one end of a glycol with a monocarboxylic acid having 2 to 8 carbon atoms can be exemplified.

The reaction between the silicon monomer and / or (co) condensate thereof and the above-mentioned monovalent hydroxyl-containing hydrocarbon is carried out by a conventional method, for example, by mixing these components or dropping the latter to the former. Optionally, in the presence of an ether exchange catalyst such as tris (acetylacetonato) aluminum, n-butyltin trioctanoate, etc.
It can be carried out by heating and reacting at 00 ° C.

The silicon monomer and / or its (co) condensate is subjected to a ring-opening addition reaction with a lactone compound to provide an ester group (Si-OR ² ) in which a part of the (Si-OR ¹ ) bond is opened. ) Can be replaced by a bond.
This ring-opening addition reaction can be carried out by a conventional method by mixing the two or while adding the latter dropwise to the former.

The lactone compounds include δ-valerolactone, ε-caprolactone, ξ-enalactone, η
-Caprylolactone, [gamma] -valerolactone, [delta] -caprolactone, [epsilon] -enalactone, [Delta] -capryrolactone, and the like, with [epsilon] -caprolactone being preferred.

When the photocatalytically active titanium (c-2) as the stain-preventing agent (C) is incorporated in the top clear coating, it is cleared when exposed to light such as sunlight in the top clear coating. The surface of the coating film can be made hydrophilic by oxidation or the like, and the surface of the clear top coating film can be made hydrophilic over time to impart antifouling properties.

As the titanium oxide having photocatalytic activity, any titanium oxide having photocatalytic activity can be used without any particular limitation. Preferred examples of titanium oxide having photocatalytic activity include, for example, Ishihara Techno Co., Ltd. Made of
ST-01, ST-21, ST-31, ST-41; and AMT-600 manufactured by Teica Corporation.

The above-mentioned titanium oxide having photocatalytic activity can be used as ultraviolet light such as sunlight or artificial illumination light, for example, at a wavelength of 400 n.
m or less, preferably at a wavelength of 285 nm to 380 n.
excited by absorbing the light of m
And OH radicals, which show strong oxidizing performance, oxidize the clear coating surface and oxidize nitrogen oxides by contact with nitrogen oxides in the air to convert them to nitric acid, It can be converted to sulfuric acid by contact with a sulfur compound such as sulfurous acid gas. The generated nitric acid or sulfuric acid oxidizes the coating film surface, and when the clear coating film contains the silicon compound (c-1), hydrolyzes the alkoxysilyl groups in the silicon compound (c-1). It acts as a catalyst and can effectively hydrolyze alkoxysilyl groups into silanol groups early in outdoor use.

The coating film surface can be made hydrophilic by oxidation of the coating film surface or hydrolysis of alkoxysilyl groups in the coating film to silanol groups. Stains are easily washed away by rainwater or the like, and the coating film can exhibit good exposure contamination resistance.

In the top clear coating composition, both the silicon compound (c-1) and the photocatalytically active titanium oxide (c-2) are used alone as the stain preventing agent (C). Compared to the case, the overall stain prevention performance can be further improved.

The mixing ratio of the hydroxyl group-containing organic resin (A) and the crosslinking agent (B) in the top clear coating composition is the sum of the component (A) and the component (B).
In parts by weight, it is in the following range. (A): 60 to 95 parts by weight, preferably 65 to 90 parts by weight, (B): 5 to 40 parts by weight, preferably 10 to 35 parts by weight.

When the amount of component (A) is less than 60 parts by weight (when the amount of component (B) exceeds 40 parts by weight), the processability of the resulting coating film tends to decrease, On the other hand, if the blending amount of the component (A) exceeds 95 parts by weight [the blending amount of the component (B) becomes less than 5 parts by weight], the curability of the coating film decreases and the durability decreases.

The amount of the antifouling agent (C) in the top clear paint is within the following range with respect to 100 parts by weight of the film-forming resin component from the viewpoint of antifouling properties and coating film performance. Is appropriate.

That is, when the stain-preventing agent (C) is a silicon compound (c-1), it is preferably from 0.1 to 0.1%.
100 parts by weight, more preferably 1 to 50 parts by weight. On the other hand, when the stain preventing agent (C) is photocatalytically active titanium oxide (c-2), it is preferably 0.1 to 50 parts by weight.
Suitably, it is 30 parts by weight, more preferably 0.5 to 15 parts by weight.

The top clear coating composition contains a hydroxyl group-containing organic resin (A), a crosslinking agent (B) and an antifouling agent (C) as essential components, and usually contains a solvent. And a curing catalyst; and may further contain a lubricating agent, a UV stabilizer, an antifoaming agent, a coating surface adjuster, and inorganic fine powder such as silica fine powder, which are known per se for coatings.

The above-mentioned solvent may be added as necessary to improve the coatability of the top clear paint, and may be one which can dissolve or disperse the above-mentioned resin component. , Xylene, hydrocarbon solvents such as high-boiling petroleum hydrocarbons, ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, etc. Ester solvents, alcohol solvents such as methanol, ethanol, butanol, etc., ether alcohol solvents such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, And the like can be illustrated, which may be used alone or in combination of two or more.

The curing catalyst is blended as needed to promote the curing reaction between the hydroxyl group-containing resin (A) and the crosslinking agent (B), and is appropriately selected depending on the type of the crosslinking agent. Can be used.

When the crosslinking agent is a melamine resin, in particular a low molecular weight melamine resin of methyl etherification or a mixed etherification of methyl ether and butyl ether, an acid catalyst such as a sulfonic acid compound or sulfonic acid is used as a curing catalyst. An amine neutralized product of the compound is preferably used. Representative examples of the sulfonic acid compound include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. The amine in the amine-neutralized sulfonic acid compound may be any of a primary amine, a secondary amine and a tertiary amine. Among them, an amine neutralized product of p-toluenesulfonic acid and / or an amine neutralized product of dodecylbenzenesulfonic acid are preferable from the viewpoint of the stability of the coating material, the effect of accelerating the reaction, and the properties of the obtained coating film.

When the crosslinking agent is a blocked polyisocyanate compound, a curing catalyst capable of accelerating the dissociation of the blocking agent of the blocked polyisocyanate compound as a crosslinking agent is suitable.
Organics such as tin octylate, dibutyltin di (2-ethylhexanoate), dioctyltin di (2-ethylhexanoate), dibutyltin dilaurate, dibutyltin oxide, dioctyltin oxide, and lead 2-ethylhexanoate Metal catalysts and the like can be mentioned.

Topcoat-colored base paint The topcoat-colored base paint used in the method of the present invention is:
A paint for forming a topcoat base coat formed between the topcoat clear coat and the primer coat, wherein the topcoat basecoat is formed on the primer coat in a wet-on-wet manner. .

The top coating base paint contains a film-forming resin and a coloring pigment. The base resin serving as the film-forming resin is not particularly limited, and may be one or a mixture of two or more of polyester resin, silicone polyester resin, acrylic resin, and fluororesin. May optionally contain a crosslinking agent.

Among the above-mentioned base resins, polyester resins can be suitably used. The polyester resin preferably contains a hydroxyl group. Examples of the polyester resin include the polyester resins described as one of the hydroxyl-containing organic resins (A) in the clear top coating. Among the polyester resins, preferred are oil-free polyester resins. Among them, from the viewpoint of processability, curability, etc., the number average molecular weight is from 1,500 to 35,000, preferably 3,
000-25,000, glass transition temperature (Tg point) -3
0-60 ° C, preferably -20 ° C to 35 ° C, hydroxyl value 3
１００100 mg KOH / g, preferably 8 to 70 mg KO
Polyester resins having an H / g are preferred.

The above-mentioned top coating base paint may contain a cross-linking agent which reacts with the base resin and cross-links. Representative examples of the cross-linking agent are described as representative examples of the cross-linking agent in the above-mentioned clear top coating. Melamine resins and blocked polyisocyanate compounds.

The crosslinking agent may be composed of one kind of crosslinking agent or a mixture of two or more kinds of crosslinking agents. The amount of the crosslinking agent is not particularly limited, but is usually preferably in the range of 5 to 40 parts by weight based on 100 parts by weight of the total solid content of the base resin and the crosslinking agent. is there.

The top coating base paint contains a coloring pigment in addition to the base resin and, if necessary, a crosslinking agent, and usually further contains a solvent.

Examples of the above-mentioned coloring pigments include coloring pigments generally used in the field of coatings, for example, white pigments such as titanium white and zinc white; blue pigments such as cyanine blue and indathrene blue; green pigments such as cyanine green and patina. Pigments;
Organic red pigments such as azo and quinacridone, red pigments such as bengalara; organic yellow pigments such as benzimidazolone, isoindolinone, isoindoline and quinophthalone; yellow pigments such as titanium yellow and graphite; carbon Black pigments such as black, graphite, and pine smoke; glitter pigments such as aluminum powder, copper powder, nickel powder, mica powder coated with titanium oxide, mica powder coated with iron oxide, and glitter graphite; The compounding amount of the color pigment is not particularly limited as long as it is in a range in which good cosmetics can be exhibited, and is usually based on 100 parts by weight of the total film-forming resin component of the base resin and the crosslinking agent. It is used in the range of 1 to 120 parts by weight.

The solvent blended in the top-colored base paint is
It is a compound that is blended as needed for improving paintability and the like, which can dissolve or disperse the film-forming resin component can be used, for example, the same as those exemplified as the solvent that can be used for the top clear paint. Solvents can be used.

The top-colored base paint can consist essentially of the base resin, the optional cross-linking agent, the color pigment and the optional solvent, but may further comprise, if necessary, Catalysts: extender pigments such as talc, clay, silica, mica, and alumina; rust preventive pigments such as strontium chromate, calcium chromate, zinc chromate, barium chromate, and zinc phosphate; antifoaming agents known per se for paints And an additive such as a coating surface adjuster.

The above-mentioned curing catalyst is blended as needed in order to promote the curing reaction between the base resin and the crosslinking agent, and can be appropriately selected and used according to the type of the crosslinking agent. The same curing catalyst that can be used for the top clear coating material can be used.

In the method of the present invention, particularly preferred top-colored base paint is the above-mentioned oil-free polyester resin 6
The resin component contains 0 to 95 parts by weight, 5 to 40 parts by weight of at least one kind of a crosslinking agent selected from the melamine resin and the blocked polyisocyanate compound, and further contains the coloring pigment.

Coating Method of the Present Invention In the coating method of the present invention, the primer coating film and the top coating base coating film are formed on the continuously moving long metal plate and baked, and then the top coating clear is formed. The paint is applied and baked to form a topcoat clear coat on the topcoat base coat. The topcoat base paint film is formed wet-on-wet on the primer paint film.

In the method of the present invention, baking is performed after the uncured primer coating film and the uncured topcoat coloring base coating film are applied to a metal plate so as to come into contact with each other in a wet-on-wet manner.

As a method of applying the primer coating film and the top-coating colored base coating film so as to be in contact with each other on a wet-on-wet basis, there is a method such as a roll coater or a curtain coating machine on a long metal plate which moves continuously. A coating method in which a top coat coloring base paint is formed into a curtain shape by a roller curtain coating machine or a slit type curtain coating machine on a primer coating film formed by a coating machine, and a primer coating layer and a coloring base coating layer are formed in two layers. And a coating method by a die coating method in which two layers, a primer coating layer and a top coating base coating layer, are simultaneously formed on the substrate by discharging from a die.

Here, the coating method using roller curtain coating is a method in which a coating film formed on a rotating roll is scraped off by a doctor and the coating film is dropped on a moving object to be coated in a curtain shape to apply the coating. It is. As the roller curtain coating machine, for example, the coating machines described in JP-A-6-7724 and JP-A-6-134385 can be used.

The coating method using the slit-type curtain coating is a method in which the coating material is dropped from an elongated slit onto a moving object to be coated in a curtain shape and coated.

Further, as a die coating machine which can be used in the above-mentioned die coating method, for example, a die coating machine described in JP-A-4-100570 can be mentioned. FIG. 1 is a conceptual sectional view of a representative example of a die in a die coating machine.
In FIG. 1, a die 1 is composed of three blades in which an upper blade 2, a middle blade 3 and a lower blade 4 are superimposed, and side plates (not shown) on both sides. The upper blade 2 and the lower blade 4 are provided with paint holding parts 5 and 6, respectively, and paint supply ports 7 and 8 for supplying paint to the paint holding parts. The gap between the upper blade 2 and the middle blade 3 and the gap between the lower blade 4 and the middle blade 3 form slots 9 and 10, respectively. The upper and lower intervals of each slot define the thickness of the paint passing through the slot,
Usually, in the range of 5 to 500 µm, preferably 10 to 120 µm, the paint is set so that the pressure loss of the paint in the slot does not become too large and the paint flows at a uniform flow rate over the entire width of the slot.

The intersection angle a between the slots 9 and 10 is an acute angle, usually 30 degrees or less, so that the upper layer paint flowing through the slot 9 and the lower layer paint flowing through the slot 10 smoothly merge to form an overlapping paint layer. In particular, it is preferable to be within the range of 5 to 25 degrees. The tip of the middle blade 3 does not need to be a knife edge.

It is preferable that the slots 9 and 10 join together inside the die as shown in FIG. 1 to form a single slot 11, but they may be independently opened to the outside. The die is provided with a mechanism for supporting the die 1 and adjusting the positions of the lips 12 and 13 of the die, a mechanism for supplying a constant amount of paint to the die, and the like, which are shown in FIG. Absent.

In the die coating method, the paint discharged from the die may be directly applied onto the object to be coated, or may be once received by a rotating roll and then applied to the object from the rotating roll. FIG. 2 shows an example of an apparatus in a case where the paint discharged from the die is directly applied to the object to be coated in FIG. 2, in which the object to be coated 15 is supported by a rotating support roll 14, The slit of the die is arranged perpendicular to the object 15, that is, toward the center of the support roll 14.

When coating is performed using the apparatus shown in FIG.
The lower layer paint is supplied from the paint supply port 8 and the upper layer paint is supplied from the paint supply port 7 to the die by the metering pump according to the application amount while the article 15 is traveling in the direction of the arrow. The paint flows into the paint holding units 5 and 6, flows through the slots 9 and 10, merges in a layer at the slots 11, and is discharged. The paint discharged in two layers adheres uniformly to the object 15 while maintaining the layered state, and forms a coating film having a predetermined thickness.

In the method of the present invention, the thickness of the primer coating film to be formed is not particularly limited.
Usually, the thickness is preferably in the range of 1 to 15 μm in dry film thickness. The thickness of the formed top-coating base coating film is not particularly limited, but is generally preferably in the range of 5 to 25 μm in dry film thickness.

In the coating method in which the top coating base paint is applied in the form of a curtain on the primer coating film, the viscosity is 40 to 150 seconds (Ford Cup # 4, 25).
It is preferable to apply a top-coating base paint having a viscosity of 40 to 150 seconds on the primer coating film having a viscosity of 40 to 150 seconds.

In the coating method using the die coater, a primer coating film having a viscosity of 40 to 250 seconds and a
It is preferable that the top coat and the base paint film are overlaid for 0 to 250 seconds and then discharged from the die.

The conditions for curing the two paint layers, the primer paint film and the top-colored base paint film, can be appropriately selected from the baking conditions under which the two paint layers are cured. Maximum temperature reached (PMT) 160-250
C. in the range of 15 to 180 seconds, especially PMT 180 to 23
Conditions within the range of 20 to 120 seconds at 0 ° C. are preferred.

In the coating method of the present invention, the above-mentioned clear top coating is applied on the baked top-colored base coating film obtained as described above. The coating method is not particularly limited, and the coating can be performed by a roll coater, a curtain coating machine, a spray coating machine, or the like. The thickness of the formed clear top coating film is not particularly limited, but is generally preferably in the range of 5 to 25 μm in dry film thickness. The curing conditions for the top clear coating film can be appropriately selected from among the baking conditions under which the top clear coating film cures, and usually, the maximum material temperature (PMT) is 160 to 250 ° C. for 15 to 180 seconds. Conditions within the range, particularly within the range of 20 to 120 seconds at a PMT of 180 to 230 ° C, are suitable.

The coating method of the present invention can form a coating film having excellent anti-stain properties on a metal plate.
Vol.21 No.4, written by Mr. Katawaki et al., Public Works Research Institute of the Ministry of Construction, showed the highest correlation with visual sensory evaluation is the measurement result of lightness difference, and quantitatively It is described that the lightness difference (ΔL ^* ) is the best as a method of evaluation.

[0084]

The present invention will be described more specifically with reference to the following examples. Hereinafter, “parts” and “%” are based on weight.

Manufacturing Examples 1 to 13 of Top Coating Clear Coatings Coatings were prepared according to the composition shown in Table 1 below to obtain respective top coating clear coatings.

The amounts of the hydroxyl group-containing organic resin and the cross-linking agent in Table 1 are indicated by the weight of solid content, and the amount of the curing catalyst is expressed by weight in terms of the amount of the sulfonic acid compound for the sulfonic acid-based curing catalyst. , And tin-based catalysts are indicated by solid content weight. Cyclohexanone / Solvesso 150 ("Solvesso 150" is Esso Oil Co., Ltd.
Made, aromatic petroleum-based high boiling point solvent) = 40/60 (weight ratio)
Is used to adjust the viscosity of the paint, and the viscosity of the clear top paint is reduced to 110 seconds (Ford Cup # 4, 25).
° C).

[0087]

[Table 1]

(Note) in Table 1 has the following meanings, respectively.

The polyester resins indicated by (* 1) to (* 3) in Table 1 are all polyester resins manufactured by Toyobo Co., Ltd. and have the property values shown in Table 2 below.

[0090]

[Table 2]

(* 4) Lumiflon LF-200: trade name, fluororesin, manufactured by Asahi Glass Co., Ltd. (* 5) Acrylic resin A: styrene /
Acrylic resin having n-butyl methacrylate / 2-hydroxyethyl methacrylate / methyl methacrylate / n-butyl acrylate = 30/30/10/10/10/20 (weight ratio of solid content) and a number average molecular weight of about 30,000.

(* 6) Cymel 303: trade name, manufactured by Mitsui Cytec Co., Ltd., having a low molecular weight methyl etherified melamine resin and a content of hexakis (methoxymethyl) melamine of 60% by weight or more. (* 7) Desmodur BL-3175: manufactured by Sumitomo Bayer Urethane Co., Ltd., trade name, blocked polyisocyanate compound.

(* 8) Nail cure 5225: manufactured by King Industries Co., USA, trade name: solution of dodecylbenzenesulfonic acid in a secondary amine equimolar neutral solution, curing catalyst. (* 9) Formate TK-1: a curing catalyst which is an organic tin solution manufactured by Takeda Pharmaceutical Co., Ltd., a trade name, a dissociation catalyst for a blocked polyisocyanate compound.

(* 10) Ethyl silicate 48: trade name, manufactured by Nippon Colcoat Co., Ltd., a low condensate of orthoethyl silicate.

(* 11) ST-41: Ishihara Techno Co., Ltd.
Made, trade name, photocatalytically active titanium dioxide. (* 12) AMT-600: manufactured by Teika Co., Ltd., trade name
Photocatalytically active titanium dioxide.

Examples 1 to 26 and Comparative Examples 1 to 6 Preparation of Test Coated Plates Method of preparing test coated plates (1): Examples 1 to 13 and Comparative Examples 1 to 3 Chromate-treated 0.5 mm thick melt KP color 863 manufactured by Kansai Paint Co., Ltd. on galvanized steel sheet
No primer (epoxy-modified polyester-based primer for pre-coated steel sheet, Tg point of the coating film is 52 ° C.) so as to have a dry film thickness of about 4 μm, and this uncured primer coating film is shown in Table 3 below. Each of the overcoated base paints was applied by a roller curtain coating method so as to have a dry film thickness of about 18 m. Next, the steel sheets were baked for 70 seconds so that the maximum temperature at which the material reached reached 220 ° C. to obtain each of the overcoated and colored base-coated steel sheets. Viscosity of the primer coating film immediately before applying the top-colored base paint (measurement temperature 2 with Ford cup # 4)
Measured at 5 ° C., viscosity was measured in the same manner as described below).
00 seconds, and the coating viscosity of each top-colored base paint was 10
0 seconds.

Then, the top clear coating composition obtained in the above-mentioned Preparation Examples 1 to 13 was adjusted to a viscosity of 60 seconds on the top colored base coating film of each baked coated plate, and the dried film thickness was determined by a roll coater. The coated metal plate was coated to a thickness of about 7 μm and baked for 50 seconds so that the maximum temperature of the material reached 235 ° C. to obtain each coated metal plate. Table 3 shows combinations of the top-coating colored base coating type and the top-coat clear coating type in each of Examples and Comparative Examples. Various tests were performed on each of the obtained coated metal plates. The test results are shown in Table 3 below. In Example 9, the top clear paint was applied by a slit curtain coating method instead of the roller curtain coating method.

Method for preparing test coated plate (2): Examples 14 to
26 and Comparative Examples 4 to 6 KP color 863 manufactured by Kansai Paint Co., Ltd. on a 0.5 mm thick hot-dip galvanized steel sheet subjected to chromate treatment.
No. 0 primer and each of the top coating base paints shown in Table 4 to be described later were overlaid in two layers by a die coat coating method, extruded from a die, and painted on a galvanized steel sheet so that the primer paint film faced.

The coating film thickness of each paint was about 4 μm in dry film thickness of the primer paint film, and about 18 μm in dry film thickness of the top coating base paint layer. The viscosity of the paint extruded from the die was 90 seconds for the primer paint and 1 for the top-colored base paint.
00 seconds. After the coating, baking was performed for 70 seconds so that the maximum temperature at which the material reached reached 220 ° C. to obtain each overcoated colored base coated plate.

Then, the top clear coating composition obtained in the above Production Examples 1 to 13 was adjusted to a viscosity of 60 seconds on the top coated base coating film of each baked coated plate, and the dried film thickness was determined by a roll coater. The coated metal plate was coated to a thickness of about 7 μm and baked for 50 seconds so that the maximum temperature of the material reached 235 ° C. to obtain each coated metal plate. Table 4 shows combinations of the top-coating colored base coating type and the top-coat clear coating type in each of Examples and Comparative Examples. Various tests were performed on each of the obtained coated metal plates. The test results are shown in Table 4 below.

In Tables 3 and 4, the designations of the top-colored base paints have the following meanings, respectively.

B-1: Kansai Paint Co., Ltd., trade name “ARESTEC # 100 White”, polyester-melamine resin-based white topcoat. B-2: manufactured by Kansai Paint Co., Ltd., trade name "KP Color #
1573 white ", a polyester-melamine resin-based white topcoat. B-3: manufactured by Kansai Paint Co., Ltd., trade name "KP Color #
7516 White ", a white finish of silicone polyester resin. B-4: manufactured by Kansai Paint Co., Ltd., trade name "Huccaron #
3000 white ", a fluororesin-based white topcoat.

[0103]

[Table 3]

[0104]

[Table 4]

The tests in Tables 3 and 4 were performed according to the following test methods.

Test Method Appearance of Coated Surface: The appearance of the coating film of the test coated plate was visually evaluated.
も の indicates that the coating film was good without any abnormality, and x indicates that the coating film showed abnormality.

Pencil hardness: For the coating film of the test coated plate, JI
SK-5400 A pencil scratch test specified in 8.4.2 (1990) was performed to evaluate the coating film for breakage.

Solvent resistance: In a room at 20 ° C., about 1 kg was applied to the painted surface with gauze impregnated with methyl ethyl ketone.
/ Cm ² was applied and reciprocated for a length of about 5 cm, and the number of reciprocations until the primer coating film was visible was recorded.
Those in which the primer coating film was not visible after 50 reciprocations were indicated as 50 <. The greater the number, the better the curability of the coating film.

Adhesion: JIS K-5400 8.5.
2 (1990) According to the crosscut-tape method, 11 parallel vertical and horizontal straight lines are drawn at 1 mm intervals on the surface of the coating film of the test plate so as to reach the substrate with a cutter knife, and the distance is 1 mm. 100 × 1 mm squares were created. A cellophane adhesive tape was adhered to the surface, and the degree of peeling of the squares when the tape was abruptly peeled was observed and evaluated according to the following criteria. ：: No peeling of the coating film was observed at all. ○: The coating film was slightly peeled, but 90 or more squares remained. Δ: The coating film was peeled, and the number of remaining squares was 50 or more and 90.
Less than pieces.

X: The coating film was peeled off, and the number of remaining squares was 50.
Less than pieces.

[0111] Antifouling property: One month and 6 months when the test plate was left standing with the coated surface upright at the outdoor exposure site of the Kansai Paint Co., Ltd. Development Center in Hiratsuka, Kanagawa Prefecture.
The stains on the painted surface after a month were visually observed and the lightness difference from the initial painted plate (the value of ΔL ^* in the CIE color system) was described. Normally, the smaller the ΔL ^* , the less the stain on the painted surface. The visual evaluation was performed according to the following criteria.

：: The coated surface is hardly soiled Δ: The coated surface is slightly soiled X: The coated surface is extremely dirty.

[0113]

According to the coating method of the present invention, a primer coating film and a top coating base coating film are formed on a continuously moving long object by wet-on-wet, and then both coating films are formed. Simultaneous baking and curing, then applying and clearing the top clear coating, so that the number of baking is not increased compared to the 2-coat 2-bake method, and more coating layers are formed without increasing the burden on equipment. Can be. The coating film formed by the coating method of the present invention is excellent in antifouling property in outdoor exposure since the clear top coating film can form a hydrophilic coating film in a short time by the action of the antifouling agent. In the coating method of the present invention, since the primer coating film and the top coating base coating film are coated wet-on-wet, the interlayer adhesion between the two layers is excellent, and the coating properties such as hardness, curability, etc. And a coating film having excellent coating film appearance can be formed.

[Brief description of the drawings]

FIG. 1 is a conceptual sectional view showing an example of a die of a die coating machine used in the present invention.

FIG. 2 is a diagram showing an example of a positional relationship between a die coating machine and a workpiece when a coating material is applied in two layers in the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Die 2 Upper blade of die 3 Middle blade of die 4 Lower blade of die 5, 6 Paint holding part of die 7, 8 Paint supply port to die 9, 10, 11 Slot of die 12, 13 Lip of die 14 Support Roll 15 substrate

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B05D 7/24 303 B05D 7/24 303E C09D 5/14 C09D 5/14 175/00 175/00 // C09D 127/12 127 / 12 133/08 133/08 167/00 167/00 183/04 183/04

Claims (7)

[Claims] Claims: 1. A primer coating film and a top coating base paint film are formed on a continuously moving long metal plate and baked, and then a top coating clear paint is applied on the top coating base film. Painted and baked, primer coating on a metal plate,
A method of forming a laminated coating film of a top-coating colored base coating film and a top-coat clear coating film, wherein the top-coating colored base coating film is formed on a primer coating film in a wet-on-wet manner, and , (A) hydroxyl-containing organic resin 6
0 to 95 parts by weight and (B) a film-forming resin component comprising 5 to 40 parts by weight of at least one kind of crosslinking agent selected from a melamine resin and a blocked polyisocyanate compound; SiO _{(4- (a + b)) / 2} (OR ¹ ) _a (OR ² ) _b [1] (wherein R ¹ is the same or different and is a hydrogen atom, an alkyl group having 1 to 10 carbon atoms) Wherein R ² represents a cycloalkyl group having 5 to 12 carbon atoms, an aryl group or an aralkyl group, or a monovalent hydrocarbon group having 4 to 24 carbon atoms containing an ether bond and / or an ester bond. A represents a number from 0.10 to 4.00, b represents a number from 0 to 1.95, and the sum of a and b is 4 or less.) At least one type of fouling selected from silicon compounds and photocatalytically active titanium oxide; Method of coating a metal plate, which is a clear paint containing anti-imparting agent. 2. The coating method according to claim 1, wherein the top coating base paint is a polyester resin, an acrylic resin, a silicone polyester resin or a fluororesin, and optionally contains a crosslinking agent. 3. The clear top coating composition contains 0.1 to 30 parts by weight of a photocatalytically active titanium oxide based on 100 parts by weight of a film-forming resin component comprising a resin (A) and a crosslinking agent (B). The coating method according to claim 1, wherein 4. The top clear coating composition contains 0.1 to 100 parts by weight of a silicon compound represented by the general formula [1] based on 100 parts by weight of a film-forming resin component comprising a resin (A) and a crosslinking agent (B). The coating method according to claim 1, wherein the coating method is contained. 5. The coating method according to claim 1, wherein the top coating color base coating and the top coating clear coating are applied by being extruded in two layers from a die of a die coater. 6. A top coat clear paint is applied on an uncured top coat coloring base paint layer which has been coated on a continuously moving long metal plate, with or without a primer-cured coating film. The coating method according to any one of claims 1 to 4, wherein the coating is performed in a curtain shape by a roller curtain. 7. A coated metal plate coated by the coating method according to claim 1.