JP4352399B2 - Formation method of multilayer coating film - Google Patents

Description

  The present invention relates to a method for forming a multilayer coating film, and more particularly to a method for forming an aqueous intermediate coating film and a top coating film on an automobile body using a three-coat one-bake method.

  Basically, automobile body coating is performed by sequentially laminating an electrodeposition coating, an intermediate coating, and a top coating consisting of a base coating and a clear coating on the steel sheet to be coated. . Conventionally, these coating films have been formed by applying a coating composition whose composition is adjusted according to the function of each coating film and baking and curing each coating film. When multiple coatings are applied, unless the underlying layer is completely formed and smoothed, the adjacent coating layers interfere with each other, and the unevenness of the underlying layer is reflected in the upper layer. This is because the appearance of the film is deteriorated.

  However, in order to increase work efficiency and realize energy saving, which has been particularly demanded in recent years, even in the automobile body painting industry, a method for forming a multilayer coating film in which a plurality of coatings are applied without baking and curing, and then they are simultaneously cured. Has gradually been adopted.

  In JP-A-4-2848881 (Patent Document 1), an electrodeposition coating film is formed on an object to be coated, and then a water-based undercoating paint, a water-based topcoating paint, and a clear coating are applied by wet-on-wetting. A three-coating, one-bake, multi-layer coating method is disclosed in which the coating films are simultaneously cured. However, three layers of conventional water-based paint for sequential baking are applied on the electrodeposition coating film. Such a method does not solve the problem of deterioration of the appearance of the multilayer coating film due to the reflection of the mixed phase of the coating layer and the unevenness of the underlayer.

  Japanese Patent Application Laid-Open No. 8-33865 (Patent Document 2) discloses two coats in which an electrodeposition coating film is formed on an object to be coated and two types of water-based paints are applied and two layers of coating films are cured simultaneously. A 1-bake multilayer coating formation method is described. Here, mixed phase and inversion are prevented by adjusting the neutralization value of the two layers of resin to be applied. However, it does not describe how to improve the appearance of the multilayer coating film when performing the 3-coat 1-bake method.

  In Japanese Patent Laid-Open No. 2001-170559 (Patent Document 3), an electrodeposition coating film and an intermediate coating film are formed on an object to be coated, and then a base coating, a glittering material-containing base coating, and a clear coating are wet-on-wet. And a method for forming a three-coat one-bake multi-layer coating film in which three layers are baked and cured. However, in this method, the intermediate coating film is once baked and cured before recoating the base paint or the like, and energy saving and work efficiency are not sufficient.

In JP-A-2001-205175 (Patent Document 4), after forming an electrodeposition coating film on an object to be coated, a water-based intermediate coating, a water-based metallic base coating, and a clear coating are applied repeatedly to form a three-layer coating. A three-coat one-bake multilayer coating method is described that simultaneously cures the film. Here, the aqueous intermediate coating material contains an aqueous dispersion of amide group-containing acrylic resin particles obtained by emulsion polymerization of an amide group-containing ethylenically unsaturated monomer and another ethylenically unsaturated monomer, and the coating layer The familiarity and reversal at the interface are controlled to improve the appearance of the multilayer coating film. However, even in this method, the surface smoothness of the multilayer coating film is insufficiently improved, and it is necessary to further improve the appearance of the multilayer coating film.
JP-A-4-284881 JP-A-8-33865 JP 2001-170559 A JP 2001-205175 A

  The present invention solves the above-mentioned conventional problems, and its object is to form a multilayer coating film having excellent surface smoothness by effectively preventing a mixed phase between the intermediate coating film and the base coating film. It is to provide a method.

The present invention includes (1) a step of providing an article on which an electrodeposition coating film is formed; (2) a step of forming an intermediate coating film by applying an aqueous intermediate coating on the electrodeposition coating; (3) A step of forming a base coating and a clear coating by sequentially applying a water-based base coating and a clear coating on the intermediate coating without wet curing the intermediate coating; A method of forming a multi-layer coating film, comprising: simultaneously baking and curing a coating film, a base coating film, and a clear coating film;
The intermediate coating film formed from the aqueous intermediate coating material has a coating film water absorption of 10% or less and a coating film water elution rate of 5% or less,
An acrylic resin emulsion having a glass transition temperature of −50 to 20 ° C., an acid value of 2 to 60 mg KOH / g and a hydroxyl value of 10 to 120 mg KOH / g, a urethane resin emulsion having an acid value of 5 to 50 mg KOH / g, And a method for forming a multilayer coating film characterized by containing a curing agent, whereby the above-mentioned object is achieved.

  According to the coating film forming method of the present invention, it is possible to effectively prevent a mixed phase between the intermediate coating film and the base coating film and form a multilayer coating film having excellent surface smoothness.

Aqueous intermediate coating The aqueous intermediate coating used in the method of the present invention contains an acrylic resin emulsion, a urethane resin emulsion and a curing agent in a state of being dispersed or dissolved in an aqueous medium. The aqueous intermediate coating material may further contain pigments and additives usually contained in the aqueous intermediate coating material for automobile bodies such as thickeners and fillers.

  The acrylic resin emulsion is obtained by emulsion polymerization of a monomer mixture containing (meth) acrylic acid alkyl ester (a), acid group-containing ethylenically unsaturated monomer (b), and hydroxyl group-containing ethylenically unsaturated monomer (c). Can do. In addition, you may use the compound illustrated below as a component of a monomer mixture, combining suitably 1 type (s) or 2 or more types.

  The (meth) acrylic acid alkyl ester (a) is used for constituting the main skeleton of the acrylic resin emulsion.

  Specific examples of the (meth) acrylic acid alkyl ester (a) include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, and (meth) acrylic. Isobutyl acid, t-butyl (meth) acrylate, hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, ( Examples thereof include dodecyl (meth) acrylate and stearyl (meth) acrylate.

  The acid group-containing ethylenically unsaturated monomer (b) improves various stability such as storage stability, mechanical stability and stability against freezing of the resulting acrylic resin emulsion, Used to accelerate the curing reaction with the curing agent. The acid group is preferably selected from a carboxyl group, a sulfonic acid group, a phosphoric acid group, and the like. A particularly preferred acid group is a carboxyl group from the viewpoints of the above-mentioned various stability improvements and curing reaction promoting functions.

  Examples of the carboxyl group-containing ethylenically unsaturated monomer include acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride and fumaric acid. Examples of the sulfonic acid group-containing ethylenically unsaturated monomer include p-vinylbenzenesulfonic acid, p-acrylamidepropanesulfonic acid, t-butylacrylamidesulfonic acid and the like. Examples of the phosphoric acid group-containing ethylenically unsaturated monomer include light ester PM (manufactured by Kyoeisha Chemical Co., Ltd.) such as 2-hydroxyethyl acrylate phosphate monoester and 2-hydroxypropyl methacrylate phosphate monoester.

  The hydroxyl group-containing ethylenically unsaturated monomer (c) imparts hydrophilicity based on a hydroxyl group to an acrylic resin emulsion, increases the workability and stability against freezing when this is used as a coating material, and also provides a melamine resin or isocyanate-based curing. Used to impart curing reactivity with the agent.

  Examples of the hydroxyl group-containing ethylenically unsaturated monomer (c) include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, N-methylolacrylamide, allyl alcohol, Examples include ε-caprolactone-modified acrylic monomers.

  Specific examples of the ε-caprolactone-modified acrylic monomer include “Placcel FA-1”, “Placcel FA-2”, “Placcel FA-3”, “Placcel FA-4”, “Placcel” manufactured by Daicel Chemical Industries, Ltd. FA-5 "," Placcel FM-1 "," Placcel FM-2 "," Placcel FM-3 "," Placcel FM-4 "," Placcel FM-5 "and the like.

  The monomer mixture may contain, as an optional component, at least one monomer selected from the group consisting of a styrene monomer, (meth) acrylonitrile, and (meth) acrylamide. Examples of the styrenic monomer include α-methylstyrene in addition to styrene.

  The monomer mixture may contain a crosslinkable monomer such as a carbonyl group-containing ethylenically unsaturated monomer, a hydrolyzable polymerizable silyl group-containing monomer, and various polyfunctional vinyl monomers. In that case, the resulting acrylic resin emulsion is self-crosslinkable.

  Examples of the carbonyl group-containing monomer include acrolein, diacetone (meth) acrylamide, acetoacetoxyethyl (meth) acrylate, formylstyrene, alkyl vinyl ketone having 4 to 7 carbon atoms (for example, methyl vinyl ketone, ethyl vinyl ketone, And monomers containing keto groups such as butyl vinyl ketone. Of these, diacetone (meth) acrylamide is preferred. When such a carbonyl group-containing monomer is used, a hydrazine-based compound is added as a crosslinking aid in the acrylic resin emulsion so that a crosslinked structure is formed when the coating film is formed.

  Examples of the hydrazine-based compound include succinic acid dihydrazide, malonic acid dihydrazide, glutaric acid dihydrazide, succinic acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, and the like; Monoolefinic unsaturated dicarboxylic acid dihydrazide such as dihydrazide, fumaric acid dihydrazide, itaconic acid dihydrazide; phthalic acid dihydrazide, terephthalic acid dihydrazide, isophthalic acid dihydrazide, pyromellitic acid dihydrazide, trihydrazide or tetrahydrazide; nitrilotritrihydrazide, Hydrazide, 1,2,4-benzenetrihydrazide, ethylenediaminetetraacetic acid tetrahydrazide, 1,4,5,8-naphthoic acid tetrahydrazide, Polyhydrazide obtained by reacting a low polymer having a lower alkyl ester group with hydrazine or hydrazine hydrate (hydrazine hydride); dihydrazide carbonate, bissemicarbazide; diisocyanate such as hexamethylene diisocyanate or isophorone diisocyanate or derived therefrom Examples thereof include an aqueous polyfunctional semicarbazide obtained by excessively reacting the polyisocyanate compound with a hydrazine compound or the above-exemplified dihydrazide.

  Examples of the hydrolyzable silyl group-containing monomer include γ- (meth) acryloxypropylmethyldimethoxysilane, γ- (meth) acryloxypropylmethyldiethoxysilane, γ- (meth) acryloxypropyltriethoxysilane, and the like. And a monomer containing an alkoxysilyl group.

  A polyfunctional vinyl monomer is a compound having two or more radically polymerizable ethylenically unsaturated groups in the molecule, such as divinylbenzene, ethylene glycol di (meth) acrylate, hexanediol di (meth) acrylate, Polyethylene glycol di (meth) acrylate, allyl (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanedi (meth) acrylate, neopentyl glycol di (meth) acrylate, pentaerythritol di ( Examples thereof include divinyl compounds such as (meth) acrylate, and pentaerythritol tri (meth) acrylate, trimethylolpropane tri (meth) acrylate, dipentaerythritol hexa (meth) acrylate, and the like.

  Emulsion copolymerization can be carried out by heating the monomer mixture in an aqueous liquid with stirring in the presence of a radical polymerization initiator and an emulsifier. The reaction temperature is about 30 to 100 ° C., for example, and the reaction time is preferably about 1 to 10 hours. It is good to do.

  As the radical polymerization initiator, known initiators usually used in emulsion polymerization of acrylic resins can be used. Specifically, as a water-soluble free radical polymerization initiator, for example, a persulfate such as potassium persulfate, sodium persulfate, or ammonium persulfate is used in the form of an aqueous solution. In addition, so-called redox initiators in which an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate or hydrogen peroxide and a reducing agent such as sodium bisulfite, sodium thiosulfate, Rongalite or ascorbic acid are combined are aqueous solutions. Used in the form of

  As an emulsifier, an anionic system selected from a micelle compound having a hydrocarbon group having 6 or more carbon atoms and a hydrophilic moiety such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule Alternatively, a nonionic emulsifier is used. Among these, as anionic emulsifiers, alkali metal salts or ammonium salts of alkylphenols or sulfuric alcohol half esters; alkyl or allyl sulfonate alkali metal salts or ammonium salts; polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl ethers Or the alkali metal salt or ammonium salt of the sulfuric acid half ester of polyoxyethylene allyl ether is mentioned. Examples of nonionic emulsifiers include polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, and polyoxyethylene allyl ether. In addition to these general-purpose anionic and nonionic emulsifiers, the molecule has a radically polymerizable unsaturated double bond, that is, acrylic, methacrylic, propenyl, allyl, allyl ether, maleic acid. Various anionic and nonionic reactive emulsifiers having a group such as are also used singly or in combination of two or more.

  In the emulsion polymerization, the use of an auxiliary agent (chain transfer agent) for adjusting the molecular weight such as a mercaptan compound or a lower alcohol promotes the smooth and uniform formation of the coating film from the viewpoint of promoting the emulsion polymerization. From the viewpoint of improving the adhesion to the material, there are many cases where it is preferable, and it is carried out depending on the situation as appropriate.

  Emulsion polymerization can be performed by any one of the usual one-step continuous monomer uniform dropping method, core-shell polymerization method which is a multi-stage monomer feed method, and power feed polymerization method which continuously changes the monomer composition fed during the polymerization. Can be legal.

  Thus, the acrylic resin emulsion used in the present invention is prepared. Although the weight average molecular weight of the obtained acrylic resin is not particularly limited, it is generally about 50,000 to 1,000,000, for example, about 100,000 to 800,000.

  The glass transition temperature (Tg) of the acrylic resin is −50 ° C. to 20 ° C., preferably −40 ° C. to 10 ° C., more preferably −30 ° C. to 0 ° C. By setting the Tg of the resin in this range, when an aqueous intermediate coating containing an acrylic resin emulsion is used in a wet-on-wet system, the affinity and adhesion with the undercoat and the topcoat are improved, and the wet state Familiarity at the interface with the upper coating is good and no reversal occurs. Moreover, the moderate softness | flexibility of the coating film finally obtained is acquired, and chipping resistance is improved. As a result, a multilayer coating film having a very high appearance can be formed. If the Tg of the resin is less than −50 ° C., the mechanical strength of the coating film is insufficient and the chipping resistance is weak. On the other hand, if the Tg of the resin exceeds 20 ° C., the coating film becomes hard and brittle, so that it lacks impact resistance and weakens chipping resistance. The type and amount of each monomer component are selected so that the Tg of the resin falls within the above range.

  The acid value of the acrylic resin is 2 to 60 mgKOH / g, preferably 5 to 50 mgKOH / g. By setting the acid value of the resin within this range, various stability such as storage stability, mechanical stability and stability against freezing of the resin emulsion and the water-based intermediate coating material using the resin emulsion are improved. A curing reaction with a curing agent such as a melamine resin at the time of formation sufficiently occurs, and various strengths, chipping resistance, and water resistance of the coating film are improved. When the acid value of the resin is less than 2 mg KOH / g, the above-mentioned various stability is inferior, and the curing reaction with a curing agent such as a melamine resin is not sufficiently performed, so that various strengths, chipping resistance and water resistance of the coating film are inferior. On the other hand, when the acid value of the resin exceeds 60 mgKOH / g, the polymerization stability of the resin is deteriorated, the above-mentioned various stability is deteriorated, or the water resistance of the obtained coating film is inferior. The type and amount of each monomer component are selected so that the acid value of the resin falls within the above range. As described above, it is important to use a carboxyl group-containing monomer among the acid group-containing ethylenically unsaturated monomers (b), and among the monomers (b), the carboxyl group-containing monomer is preferably 50% by weight or more. More preferably 80% by weight or more is contained.

  The hydroxyl value of the acrylic resin is 10 to 120 mgKOH / g, preferably 20 to 100 mgKOH / g. By setting the hydroxyl value of the resin within this range, the resin has moderate hydrophilicity, and when used as a coating composition containing a resin emulsion, the workability and stability against freezing are increased, and the melamine resin or isocyanate type is used. The curing reactivity with the curing agent is also sufficient. When the hydroxyl value is less than 10 mg KOH / g, the curing reaction with the curing agent is insufficient, the mechanical properties of the coating film are weak, chipping resistance is poor, and water resistance and solvent resistance are also poor. On the other hand, when the hydroxyl value exceeds 120 mgKOH / g, the water resistance of the obtained coating film is decreased, the compatibility with the curing agent is poor, the coating film is distorted, and the curing reaction is uneven. As a result, the various strengths of the coating film, particularly chipping resistance, solvent resistance and water resistance are inferior. The type and amount of each monomer component are selected so that the hydroxyl value of the resin falls within the above range.

  A basic compound is added to the obtained acrylic resin emulsion in order to neutralize part or all of the carboxylic acid and maintain the stability of the acrylic resin emulsion. As these basic compounds, ammonia, various amines, alkali metals and the like are usually used, and they are also used appropriately in the present invention.

  The urethane emulsion of the present invention has an acid value from the viewpoints of curability and stability, like the acrylic emulsion. The acid value of the urethane emulsion is 5 to 50 mgKOH / g. When the acid value exceeds 50 mgKOH / g, the coating film performance decreases, and when it falls below 5 mgKOH / g, the stability decreases. The urethane emulsion can have a hydroxyl value. The hydroxyl value is preferably 100 mgKOH / g or less. When it exceeds 100 mgKOH / g, the coating film performance is deteriorated.

  The urethane resin emulsion is not particularly limited, but can be produced, for example, as follows. That is, first, a diisocyanate is reacted with a glycol having a glycol and a carboxylic acid group to produce a urethane prepolymer. Next, this prepolymer is neutralized and chain-extended, and distilled water is added to obtain a urethane resin emulsion.

  The diisocyanate used in making the urethane prepolymer is not particularly limited, and examples thereof include aliphatic, alicyclic, and aromatic diisocyanates. Specifically, 2,4-tolylene diene is used. Isocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, m-phenylene diisocyanate, xylylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate ester, 1,4-cyclohexylene diisocyanate, 4,4 '-Dicyclohexylmethane diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 1,5-naphtha Data diisocyanate, 1,5-tetrahydronaphthalene diisocyanate, isophorone diisocyanate, and the like.

  Examples of the glycols include, but are not limited to, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, trimethylene glycol, 1,3-butylene glycol, tetramethylene glycol, hexamethylene. Low molecular weight glycols such as glycols, hydrogenated bisphenol A, ethylene oxide or propylene oxide adducts of bisphenol A, or polyethers such as polyethylene glycol and polypropylene glycol polyols, ethylene glycol and adipic acid, hexanediol and adipic acid Polyesters which are condensates of ethylene glycol and phthalic acid, polycaprolactone, and the like.

  In addition, the glycol having a carboxylic acid group is not particularly limited, and examples thereof include 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, 2,2-dimethylolvaleric acid, and the like. The neutralizing agent used for obtaining a urethane resin emulsion by neutralizing and chain-extending the urethane prepolymer obtained by reacting the raw materials described above and adding distilled water is not particularly limited. Although not, for example, amines such as dimethylethanolamine, trimethylamine, triethylamine, tri-n-propylamine, tributylamine, triethanolamine, sodium hydroxide, potassium hydroxide, ammonia and the like can be mentioned.

  Furthermore, the chain extender is not particularly limited, but examples thereof include polyols such as ethylene glycol and propylene glycol, ethylenediamine, propylenediamine, hexamethylenediamine, tolylenediamine, xylylenediamine, diphenyldiamine, and diamino. Aliphatic, alicyclic, or aromatic diamines such as diphenylmethane, diaminocyclohexylmethane, piperazine, 2-methylpiperazine, and isophoronediamine, and water are exemplified.

  Commercially available urethane resin emulsions that can be used in the present invention are not particularly limited. For example, “Bondic” manufactured by Dainippon Ink, “Hydran” series, “Superflex” manufactured by Daiichi Kogyo Seiyaku. Asahi Denka's “Adekabon titer” series and the like.

  The urethane resin emulsion may be used alone or in combination of two or more.

  Furthermore, as a urethane resin emulsion, what made diisocyanate and polyether or polyester react can be used. As the diisocyanate, those described above can be used.

  The polyether contains at least two active hydrogens, and representative examples thereof include polyoxypropylene glycols, adducts of polyoxypropylene and glycerin, polyoxypropylene and trimethylolpropane, An adduct of polyoxypropylene and 1,2,6-hexanetriol, an adduct of polyoxypropylene and pentaerythritol, an adduct of polyoxypropylene and sorbit, methylene-bis-phenyl diisocyanate, And polytetrafuran polyether chain-extended with hydrazine and derivatives thereof.

  Also, polyester containing at least two active hydrogens is used. Typical examples include adipic acid or phthalic anhydride, ethylene glycol, propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, diethylene glycol, 1,2,6-hexanetriol, trimethylolpropane, or 1 , 1,1-trimethylolethane reaction products.

  This emulsion is obtained by dispersing a reaction product of a polyether or polyester as described above with an excess of a diisocyanate in water using a cationic, nonionic or anionic surfactant to form a primary diamine ( For example, it can be obtained by chain extension with ethylenediamine, m-tolylenediamine, etc.) or 1,2-bis (2-cyanoethylamino) ethane.

  In addition, when using the above-mentioned polyether having 3 or more hydroxyl groups in one molecule, an excess isocyanate compound is added to the polyether and reacted, and then the remaining NCO group is not treated with phenol or the like. It can also be activated to produce a blocked isocyanate compound, which can be dispersed in water in the presence of, for example, a nonionic surfactant to obtain a target urethane resin emulsion.

  The curing agent is not particularly limited as long as it generates a curing reaction with an acrylic resin or urethane resin contained as an emulsion and can be blended in an aqueous intermediate coating, for example, a melamine resin, an isocyanate resin, an oxazoline compound. Or a carbodiimide type compound etc. are mentioned. These 1 type (s) or 2 or more types are used in combination as appropriate.

  It does not specifically limit as a melamine resin, What is normally used as a hardening | curing agent can be used.

  For example, an alkyl etherified melamine resin obtained by alkyl etherification is preferable, and a melamine resin substituted with a methoxy group and / or a butoxy group is more preferable. As such melamine resins, those having a methoxy group alone, Cymel 325, Cymel 327, Cymel 370, Mycoat 723; those having both a methoxy group and a butoxy group, Cymel 202, Cymel 204, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 254, Cymel 266, Cymel 267 (all trade names, manufactured by Mitsui Cytec); My Coat 506 (trade name, Mitsui Cytech, Inc.) having a butoxy group alone Product), Uban 20N60, Uban 20SE (both trade names, manufactured by Mitsui Chemicals) and the like. These may be used alone or in combination of two or more. Of these, Cymel 325, Cymel 327, and My Coat 723 are more preferable.

  The isocyanate resin is obtained by blocking a diisocyanate compound with an appropriate blocking agent. The diisocyanate compound is not particularly limited as long as it has two or more isocyanate groups in one molecule. For example, aliphatic diisocyanates such as hexamethylene diisocyanate (HMDI) and trimethylhexamethylene diisocyanate (TMDI); isophorone Alicyclic diisocyanates such as diisocyanate (IPDI); aromatic-aliphatic diisocyanates such as xylylene diisocyanate (XDI); aromatic diisocyanates such as tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI) Hydrogenated diisocyanates such as dimer acid diisocyanate (DDI), hydrogenated TDI (HTDI), hydrogenated XDI (H6XDI), hydrogenated MDI (H12MDI), Adducts of beauty more diisocyanates and isocyanurates thereof, and the like. Furthermore, these 1 type (s) or 2 or more types can be used in combination as appropriate.

  The blocking agent for blocking the diisocyanate compound is not particularly limited. For example, oximes such as methyl ethyl ketoxime, acetoxime, and cyclohexanone oxime; phenols such as m-cresol and xylenol; butanol, 2-ethylhexanol, cyclohexanol, ethylene Alcohols such as glycol monoethyl ether; lactams such as ε-caprolactam; diketones such as diethyl malonate and acetoacetate; mercaptans such as thiophenol; ureas such as thiouric acid; imidazoles; Can be mentioned. Of these, oximes, phenols, alcohols, lactams and diketones are preferred.

  The oxazoline-based compound is preferably a compound having two or more 2-oxazoline groups, and examples thereof include the following oxazolines and oxazoline group-containing polymers. These 1 type (s) or 2 or more types can be used in combination. The oxazoline compound is obtained by using a method of dehydrating cyclization by heating amide alcohol in the presence of a catalyst, a method of synthesizing from alkanolamine and nitrile, or a method of synthesizing from alkanolamine and carboxylic acid. .

  Examples of the oxazolines include 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'-p-phenylene-bis- (2-oxazoline) ), 2,2′-m-phenylene-bis- (2-oxazoline), 2,2′-m-phenylene-bis- (4,4′-dimethyl-2-oxazoline), bis- (2-oxazoline) Nylcyclohex Down) sulfide, bis - (2-oxazolinyl sulfonyl norbornane) sulfide, and the like. These 1 type (s) or 2 or more types can be used in combination as appropriate.

  The oxazoline group-containing polymer is obtained by polymerizing an addition polymerizable oxazoline and at least one other polymerizable monomer as required. Examples of the addition polymerizable oxazoline include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, Examples include 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, and the like. These 1 type (s) or 2 or more types are used in combination as appropriate. Of these, 2-isopropenyl-2-oxazoline is preferred because it is easily available industrially.

  The amount of addition polymerizable oxazoline used is not particularly limited, but is preferably 1% by weight or more in the oxazoline group-containing polymer. If the amount is less than 1% by weight, the degree of curing tends to be insufficient, and durability, water resistance and the like tend to be impaired.

  The other polymerizable monomer is not particularly limited as long as it is a monomer that can be copolymerized with an addition-polymerizable oxazoline and does not react with the oxazoline group. For example, methyl (meth) acrylate, (meth) (Meth) acrylic acid esters such as butyl acrylate and 2-ethylhexyl (meth) acrylate; Unsaturated nitriles such as (meth) acrylonitrile; Unsaturated amides such as (meth) acrylamide and N-methylol (meth) acrylamide Vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogenated α and β such as vinyl chloride, vinylidene chloride and vinyl fluoride -Unsaturated monomers: α, β-unsaturated aromatics such as styrene and α-methylstyrene Mer, etc. can be mentioned. These 1 type (s) or 2 or more types can be used in combination as appropriate.

  The oxazoline group-containing polymer can be produced by adding an addition-polymerizable oxazoline and, if necessary, at least one other polymerizable monomer by a conventionally known polymerization method such as suspension polymerization, solution polymerization, emulsion polymerization and the like. Examples of the supply form of the oxazoline group-containing compound include organic solvent solutions, aqueous solutions, non-aqueous dispersions, and emulsions, but are not particularly limited to these forms.

  As carbodiimide compounds, those produced by various methods can be used, but basically those obtained by synthesizing isocyanate-terminated polycarbodiimides by condensation reactions involving decarbonization of organic diisocyanates should be mentioned. Can do. More specifically, in the production of a polycarbodiimide compound, a polycarbodiimide compound containing at least two isocyanate groups in one molecule, and a polyol having a hydroxyl group at the molecular end, the molar amount of the isocyanate group of the polycarbodiimide compound. Hydrophilization obtained by the step of reacting at a ratio exceeding the molar amount of hydroxyl group of the polyol and the step of reacting the reaction product obtained in the above step with a hydrophilizing agent having active hydrogen and a hydrophilic moiety. A modified carbodiimide compound can be mentioned as a preferable one.

  The carbodiimide compound containing at least two isocyanate groups in one molecule is not particularly limited, but is preferably a carbodiimide compound having isocyanate groups at both ends from the viewpoint of reactivity. A method for producing a carbodiimide compound having isocyanate groups at both ends is well known by those skilled in the art. For example, a condensation reaction involving decarbonization of an organic diisocyanate can be used.

  The aqueous intermediate coating material used in the present invention can further contain the following components. For example, additional resin components, pigment dispersion pastes, thickeners, other additive components, and the like.

  Although it does not specifically limit as said additional resin component, For example, a polyester resin, an acrylic resin, a carbonate resin, an epoxy resin etc. can be mentioned.

  The pigment dispersion paste is obtained by previously dispersing a pigment and a pigment dispersant in a small amount of an aqueous medium. The solid content of the pigment dispersant does not contain any volatile basic substances or contains 3% by weight or less. In the aqueous intermediate coating composition used in the present invention, by using such a pigment dispersant, the amount of volatile basic substances in the coating film formed from the aqueous intermediate coating composition is reduced, and the resulting multilayer coating is obtained. Yellowing of the film can be suppressed. Accordingly, it is preferable that the solid content of the pigment dispersant contains more than 3% by weight of the volatile basic substance because the resulting multilayer coating film tends to yellow and the finished appearance tends to deteriorate. Absent.

  The volatile basic substance means a basic substance having a boiling point of 300 ° C. or less, and examples thereof include inorganic and organic nitrogen-containing basic substances. As an inorganic basic substance, ammonia etc. are mentioned, for example. Examples of the organic basic substance include linear or branched alkyl having 1 to 20 carbon atoms such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, and dimethyldodecylamine. Group-containing primary to tertiary amines; monoethanolamine, diethanolamine, 2-amino-2-methylpropanol and other linear or branched hydroxyalkyl group-containing primary to tertiary amines having 1 to 20 carbon atoms; dimethylethanolamine A primary or tertiary amine containing a linear or branched alkyl group having 1 to 20 carbon atoms and a linear or branched hydroxyalkyl group having 1 to 20 carbon atoms such as diethylethanolamine; diethylenetriamine , Substitution of 1 to 20 carbon atoms such as triethylenetetramine Unsubstituted chain polyamine; substituted or unsubstituted cyclic monoamine having 1 to 20 carbon atoms such as morpholine, N-methylmorpholine, N-ethylmorpholine; piperazine, N-methylpiperazine, N-ethylpiperazine, N, N-dimethylpiperazine Examples thereof include amines such as substituted or unsubstituted cyclic polyamines having 1 to 20 carbon atoms.

The aqueous intermediate coating composition used in the present invention may contain a volatile basic substance in components other than the pigment dispersant. Accordingly, it is more preferable that the weight of the volatile basic substance contained in the pigment dispersant is suppressed. That is, it is preferable to disperse using a pigment dispersant that does not substantially contain a volatile basic substance. Further, it is more preferable not to use an amine-neutralized pigment dispersion resin generally used conventionally. And it is preferable to use a pigment dispersant so that the volatile basic substance per unit area 1 mm < ² > may be 7 * 10 < ^-6 > mmol or less at the time of multilayer coating film formation.

  The pigment dispersant is a resin having a structure including a pigment affinity part and a hydrophilic part. Examples of the pigment affinity portion and the hydrophilic portion include nonionic, cationic and anionic functional groups. The pigment dispersant may have two or more of the above functional groups in one molecule.

  Examples of the nonionic functional group include a hydroxyl group, an amide group, a polyoxyalkylene group, and the like. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. Examples of the anionic functional group include a carboxyl group, a sulfonic acid group, and a phosphoric acid group. Such pigment dispersants can be produced by methods well known to those skilled in the art.

  The pigment dispersant is not particularly limited as long as it does not contain a volatile basic substance in the solid content or has a content of 3% by weight or less. What can disperse | distribute a pigment is preferable. For example, commercially available products (hereinafter, trade names) can also be used. Specifically, Disperbyk 190, Disperbyk 181, Disperbyk 182 (polymer co-polymers), which are anionic and nonionic dispersants manufactured by Big Chemie, Inc., can be used. Polymer), Disperbyk 184 (polymer copolymer), EFKAPOLYMER 4550, an anionic / nonionic dispersant manufactured by EFKA, Solsperse 27000, a nonionic dispersant manufactured by Abyssia, Solsperse 41000, an anionic dispersant, And Solsperse 53095.

  The number average molecular weight of the pigment dispersant is preferably a lower limit of 1000 and an upper limit of 100,000. If it is less than 1000, the dispersion stability may not be sufficient, and if it exceeds 100,000, the viscosity may be too high and handling may be difficult. More preferably, the lower limit is 2000 and the upper limit is 50,000, and even more preferably, the lower limit is 4000 and the upper limit is 50,000.

  The pigment dispersion paste is obtained by mixing and dispersing a pigment dispersant and a pigment according to a known method. The ratio of the pigment dispersant during the production of the pigment dispersion paste is preferably 1% by weight lower limit and 20% by weight upper limit with respect to the solid content of the pigment dispersion paste. If it is less than 1% by weight, it is difficult to stably disperse the pigment, and if it exceeds 20% by weight, the physical properties of the coating film may be inferior. Preferably, the lower limit is 5% by weight and the upper limit is 15% by weight.

  Although it will not specifically limit if it is a pigment used for a normal water-based paint, A colored pigment is preferable from the point which improves a weather resistance and ensures concealment property. In particular, titanium dioxide is more preferable because it is excellent in color concealment and is inexpensive.

  Examples of pigments other than titanium dioxide include azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, and isoindolinone. Organic pigments such as pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments and metal complex pigments; inorganic pigments such as yellow lead, yellow iron oxide, Bengala and carbon black. These pigments may be used in combination with extender pigments such as calcium carbonate, barium sulfate, clay and talc.

  As the pigment, a standard gray paint having carbon black and titanium dioxide as main pigments can also be used. In addition, it is also possible to use a paint in which the top coat paint is combined with lightness or hue or a combination of various color pigments.

  The pigment preferably has a pigment weight content (PWC) ratio of 10 to 60% by weight based on the total solid content of the resin and the total weight of the pigment contained in the aqueous intermediate coating. If it is less than 10% by weight, the concealability may be lowered. If it exceeds 60% by weight, an increase in viscosity at the time of curing may be caused, the flowability may be lowered, and the coating film appearance may be lowered.

  The content of the pigment dispersant is preferably 0.5% by weight lower limit and 10% by weight upper limit with respect to the weight of the pigment. If it is less than 0.5% by weight, the pigment dispersion stability may be inferior due to the small amount of the pigment dispersant. If it exceeds 10% by weight, the physical properties of the coating film may be inferior. Preferably, the lower limit is 1% by weight and the upper limit is 5% by weight.

  Although it does not specifically limit as a thickener, Cellulose type | system | groups, such as viscose, methylcellulose, ethylcellulose, hydroxyethylcellulose, and what is marketed, such as Tyroze MH and Tyroze H (all are the Hoechst company make, brand name), etc. Things; Sodium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, and commercially available products (all are trade names below) include Primal ASE-60, Primal TT-615, Primal RM-5 (all manufactured by Rohm & Haas) ), Alkali thickening type such as Euker Polyphobe (manufactured by Union Carbide); polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter referred to as trade names) include Adecanol UH-420 and Adecanol UH- 4 2, Association type such as Adecanol UH-472, UH-540, Adecanol UH-814N (Asahi Denka Kogyo Co., Ltd.), Primal RH-1020 (Rohm & Haas), Kuraray Poval (Kuraray Co., Ltd.) be able to. These may be used alone or in combination of two or more.

  By containing the thickener, the viscosity of the aqueous intermediate coating can be increased, and the occurrence of dripping can be suppressed when the aqueous intermediate coating is applied. Moreover, the mixed layer between an intermediate coating film and a base coating film can be suppressed more. As a result, compared to the case where no thickener is included, the coating workability during coating is improved, and the finished appearance of the resulting coating film can be made excellent.

  The content of the thickener is 0.01 parts by weight lower limit and 20 parts by weight upper limit with respect to 100 parts by weight of the resin solid content of the aqueous intermediate coating (solid content of all resins contained in the aqueous intermediate coating). The lower limit is preferably 0.1 parts by weight, and the upper limit is more preferably 10 parts by weight. If the amount is less than 0.01 parts by weight, the thickening effect cannot be obtained, and sagging may occur during coating. If the amount exceeds 20 parts by weight, the appearance and various performances of the resulting coating film may be deteriorated. is there.

  Examples of other additives include additives usually added in addition to the above-mentioned components, such as ultraviolet absorbers; antioxidants; antifoaming agents; surface conditioners; pinhole inhibitors. These compounding amounts are within the range known to those skilled in the art.

  The aqueous intermediate coating used in the present invention is prepared by mixing the above-mentioned acrylic resin emulsion, urethane resin emulsion, curing agent and the like. The ratio of the acrylic resin emulsion to the urethane resin emulsion is set to a solid content weight ratio of 1/1 to 2/1. When this ratio is less than 1/1, the viscosity of the coating film is increased, the smoothness of the intermediate coating is lowered, and the appearance is lowered. When it exceeds 2/1, the water absorption rate and the elution rate increase, and the appearance tends to deteriorate.

  The curing agent has a lower limit of 2% by weight, an upper limit of 50% by weight, preferably a lower limit of 4% by weight and an upper limit of 40% by weight, more preferably a lower limit, based on the total amount of solids of the curing agent, acrylic resin emulsion, and urethane resin emulsion. It is used so that it may become 5 weight% and an upper limit 30 weight%. When the amount is less than 2% by weight, the water resistance of the resulting coating film tends to decrease. Moreover, when it exceeds 50 weight%, there exists a tendency for the chipping property of the coating film obtained to fall.

  Appropriate amounts of the additional resin component, pigment dispersion paste and other additives may be mixed. However, the additional resin component is preferably blended in a proportion of 50% by weight or less based on the solid content of all the resins contained in the aqueous intermediate coating composition. When the blending amount exceeds 50% by weight, it is difficult to increase the solid content concentration in the coating material, which is not preferable.

  The order of adding these components may be before or after adding the curing agent to the emulsion. The form of the aqueous intermediate coating is not particularly limited as long as it is aqueous, and may be in the form of, for example, water-soluble, water-dispersed, or aqueous emulsion.

Water-based base paint The water-based base paint used in the method of the present invention may be a paint composition usually used as a water-based intermediate coating for automobile bodies. Examples thereof include those containing a coating film-forming resin, a curing agent, a bright pigment, a pigment such as a color pigment or an extender pigment, various additives and the like in a state dispersed or dissolved in an aqueous medium. As the film-forming resin, for example, a polyester resin, an acrylic resin, a urethane resin, a carbonate resin, and an epoxy resin can be used. From the viewpoint of pigment dispersibility and workability, a combination of acrylic resin and / or polyester resin and melamine resin is preferred. Commonly used curing agents, pigments, and various additives can also be used.

  The pigment concentration (PWC) contained in the aqueous base paint is generally 0.1% by weight lower limit and 50% by weight upper limit, more preferably 0.5% by weight lower limit and 40% by weight upper limit. More preferably, the lower limit is 1% by weight and the upper limit is 30% by weight. If the pigment concentration is less than 0.1% by weight, the effect of the pigment cannot be obtained, and if it exceeds 50% by weight, the appearance of the resulting coating film may be deteriorated.

  The aqueous base paint can be prepared by the same method as that for the intermediate paint. The form of the water-based base coating is not particularly limited as long as it is water-based. For example, the form may be water-soluble, water-dispersed, water-based emulsion, or the like.

Clear paint The clear paint used in the method of the present invention may be a paint composition usually used as a clear paint for automobile bodies. For example, what contains a film-forming resin, a hardening | curing agent, and another additive in the state disperse | distributed or dissolved in the medium can be mentioned. Examples of the film-forming resin include acrylic resin, polyester resin, epoxy resin, and urethane resin. These may be used in combination with a curing agent such as an amino resin and / or an isocyanate resin. From the viewpoint of transparency or acid etching resistance, it is preferable to use a combination of an acrylic resin and / or a polyester resin and an amino resin, or an acrylic resin and / or a polyester resin having a carboxylic acid / epoxy curing system.

  The coating form of the clear paint may be any of organic solvent type, aqueous type (water-soluble, water-dispersible, emulsion), non-aqueous dispersion type, and powder type. If necessary, a curing catalyst, a surface conditioner, etc. It may be included.

Formation method of a multilayer coating film In the formation method of the multilayer coating film of this invention, the to-be-coated object in which the electrodeposition coating film was formed is provided first. The electrodeposition coating film is formed by applying an electrodeposition paint to an object to be coated and baking and curing it. The object to be coated is not particularly limited as long as it is a metal product capable of cationic electrodeposition coating. For example, iron, copper, aluminum, tin, zinc and alloys containing these metals, and plating or vapor deposition products using these metals can be used.

  The electrodeposition paint is not particularly limited, and a known cationic electrodeposition paint or anion electrodeposition paint can be used. Electrodeposition coating and baking may be performed by a method and conditions usually used for electrodeposition coating of automobile bodies.

  Next, an aqueous intermediate coating is applied on the electrodeposition coating to form an intermediate coating. Intermediate coatings are, for example, air electrostatic sprays known as “react guns”, commonly known as “micro-microbells (μμbells)”, “microbells (μbells)”, “metallic bells (metabells)”, etc. It can be applied by spraying using a rotary atomizing electrostatic coating machine.

  The coating amount is adjusted so that the film thickness of the coated film after curing is 10 to 40 μm, preferably 15 to 30 μm. If the film thickness is less than 10 μm, the appearance and chipping resistance of the resulting coating film may be reduced, and if it exceeds 40 μm, problems such as sagging during coating and pinholes during baking hardening may occur.

  The intermediate coating film is preferably dried by heating or blowing before applying the aqueous base paint. The reason is that when the drying is insufficient, the water remaining in the coating film causes bumping in the step of baking the multilayer coating film, and it is easy to generate a crack. Further, when the base is coated on the intermediate coating, it is easy to mix with the base and the appearance may be deteriorated. And it is preferable that the dried intermediate coating film is poor in water absorption. The water content of the water-based base coating that is applied over this layer is difficult to be absorbed by the intermediate coating film, so the increase in the solid content concentration of the base coating film is suppressed and the flow of the coating film is improved. This is because the smoothness is improved. As a result, it is considered that the smoothness of the surface of the multilayer coating film is also improved.

  Specifically, the waterborne intermediate coating material preferably has a coating film water absorption of 10% or less. The water absorption rate of the coating film is determined as follows. An aqueous base coating material is applied on a substrate to a thickness of 20 μm to form an initial base coating film, which is dried at 80 ° C. for 5 minutes to form a pre-dried coating film. The pre-dried coating film is immersed in water for 2 minutes. In this case, the weight fraction of the amount of water absorbed in the pre-dried coating film with respect to the initial base coating film immediately after coating is calculated.

  Moreover, it is preferable that the dry intermediate coating film has few components eluted in water. When the aqueous base paint is applied on the intermediate coating film, the water-eluting component is easily eluted by the water content of the aqueous base paint and easily transferred to the base coating film. If it does so, it will become easy to produce distortion in the interface of an intermediate coating film and a base coating film. As a result, it is considered that the appearance of the surface of the multilayer coating film is deteriorated.

  Specifically, the waterborne intermediate coating material has a coating film water elution rate of 5% or less. The coating water elution rate is determined as follows. The pre-dried coating film is immersed in water for 2 minutes. In this case, the weight fraction of the intermediate coating film component eluted is calculated with respect to the pre-dried coating film.

  Calculation of the coating film water absorption rate and the coating film water elution rate can be performed, for example, as follows. The aqueous intermediate coating is spray-coated on two aluminum foils of aluminum A and aluminum B to a film thickness of 20 μm. These are pre-dried at 80 ° C. for 5 minutes, and aluminum B is immersed in pure water for 2 minutes. Thereafter, aluminum A and B are dried at 140 ° C. for 20 minutes. In this series of steps, the weight of the aluminum foil is defined as follows.

Then, a to d are expressed as follows.

  The water absorption rate and elution rate are expressed by the following equations.

  The water absorption rate and water elution rate of the intermediate coating film can be suppressed to a low value by using the above-mentioned acrylic resin emulsion and the above-mentioned urethane resin emulsion in combination as the resin component of the aqueous intermediate coating material. . The urethane resin emulsion does not participate in curing, but tends to form a strong film by fusion. By blending the urethane resin emulsion in the intermediate coating, it is considered that a barrier is formed in the film, preventing the penetration of water and the migration of elution components.

  Next, the base coating film and the clear coating film are formed by sequentially applying an aqueous base coating and a clear coating on the intermediate coating without wet curing the wet coating. Here, wet-on-wet coating refers to coating a plurality of coating films without curing.

  The coating amount of the aqueous base paint is usually adjusted so that the film thickness after curing of the coating film is 10 to 30 μm. If the film thickness after curing is less than 10 μm, there is a risk that the underlaying will be insufficiently concealed and color unevenness may occur. If it exceeds 30 μm, sagging will occur during painting and pinholes will occur during heat curing. There is a risk of doing so.

  The coating amount of the clear paint is usually adjusted so that the film thickness after drying and curing of the coating film is 10 to 70 μm. When the film thickness after curing is less than 10 μm, the appearance such as glossiness of the multilayer coating film is deteriorated, and when it exceeds 70 μm, the sharpness is deteriorated or defects such as unevenness and flow occur at the time of coating.

  Next, the intermediate coating film, the base coating film, and the clear coating film are simultaneously baked and cured. Baking is usually performed by heating to a temperature of 110 to 180 ° C, preferably 120 to 160 ° C. Thereby, a cured coating film having a high degree of crosslinking can be obtained. If the heating temperature is less than 110 ° C, curing tends to be insufficient, and if it exceeds 180 ° C, the resulting coating film may be hard and brittle. Although the time to heat can be suitably set according to the said temperature, for example, when temperature is 120-160 degreeC, it is 10 to 60 minutes.

  EXAMPLES Hereinafter, although an Example is hung up and demonstrated in more detail about this invention, this invention is not limited only to these Examples. In the examples, “parts” means “parts by weight” unless otherwise specified.

Example 1
(A) Manufacture of water-based intermediate coating (preparation of pigment dispersion paste)
Commercially available dispersant “Disperbyk 190” (Bion Chemie nonionic anionic dispersant, trade name) 4.5 parts, Antifoam “BYK-011” (Bic Chemie antifoam) 0.5 parts, ion-exchanged water After 22.9 parts and 72.1 parts of rutile titanium dioxide were premixed, glass bead medium was added in a paint conditioner and mixed and dispersed at room temperature until the particle size became 5 μm or less to obtain a pigment dispersion paste.

(Preparation of acrylic resin emulsion)
In a reaction vessel for producing an ordinary acrylic resin emulsion equipped with a stirrer, a thermometer, a dropping funnel, a reflux condenser, a nitrogen introducing tube, etc., 445 parts of water and 5 parts of New Coal 293 (manufactured by Nippon Emulsifier Co., Ltd.) The temperature was raised to 75 ° C. while charging and stirring. The following monomer mixture (resin acid value: 18, hydroxyl value: 85, Tg: −22 ° C.), 240 parts of water and 30 parts of New Coal 293 (manufactured by Nippon Emulsifier Co., Ltd.) was emulsified using a homogenizer, The monomer pre-emulsified solution was dropped into the reaction vessel with stirring for 3 hours. Along with the dropping of the monomer pre-emulsified solution, an aqueous solution prepared by dissolving 1 part of APS (ammonium persulfate) as a polymerization initiator in 50 parts of water is uniformly dropped into the reaction vessel until the end of the dropping of the monomer pre-emulsified solution. did. After completion of the dropwise addition of the monomer pre-emulsion, the reaction was further continued at 80 ° C. for 1 hour, and then cooled. After cooling, an aqueous solution in which 2 parts of dimethylaminoethanol was dissolved in 20 parts of water was added to obtain an aqueous acrylic resin emulsion having a nonvolatile content of 40.6% by weight (Resin 1).

  The obtained acrylic resin emulsion was adjusted to pH 7.2 using a 30% aqueous dimethylaminoethanol solution.

(Preparation of water-based intermediate coating)
55.5 parts of the pigment dispersion paste obtained as described above, 83.7 parts of acrylic resin emulsion, 56.7 parts of urethane resin emulsion A (Adeka Bon titer HUX-232 manufactured by Asahi Denka Kogyo Co., Ltd.), and curing After mixing 9.3 parts of Mycoat 723 (Mitsui Cytec Co., Ltd., imino melamine resin, trade name) as an agent, Adecanol UH-814N (urethane associative thickener, active ingredient 30%, manufactured by Asahi Denka Kogyo Co., Ltd., Product name) 1.0 part of the mixture was mixed and stirred to obtain an aqueous intermediate coating.

(Measurement of water absorption and dissolution rate of coating film)
The weights of two aluminum foils A and B each having a size of 100 mm × 55 mm were precisely weighed to obtain A0 = 0.6492 g and B0 = 0.6496 g, respectively. The aqueous intermediate coating obtained in Example 1 was applied onto the two aluminum foils by air spray (film thickness 20 μm). The two aluminum foils were allowed to stand for 2 minutes, preheated at 80 ° C. for 5 minutes, and precisely weighed to obtain A1 = 0.8404 g and B1 = 0.8167 g. Thereafter, only the aluminum foil B was immersed in pure water for 2 minutes and taken out, and then lightly wiped off and precisely weighed to obtain B2 = 0.8281 g. Thereafter, the aluminum foils A and B were heated at 140 ° C. for 20 minutes and precisely weighed to obtain A3 = 0.8376 g and B3 = 0.8140 g, respectively. According to the calculation method shown in Table 2, Formula 1 and Formula 2, the water absorption rate of the intermediate coating was 7% and the dissolution rate was 0.2%.

(B) Formation of coating film On a zinc phosphate-treated dull steel plate, Powernics 110 (Cation Electrodeposition Paint, trade name, manufactured by Nippon Paint Co., Ltd.) was electrodeposited so that the dry coating thickness was 20 μm, and 160 ° C. The steel plate substrate was prepared by cooling after heat curing for 30 minutes.

  The obtained substrate was coated with 20 μm of the above water-based intermediate coating by air spray coating and preheated at 80 ° C. for 5 minutes. ) Was applied by air spray coating to 10 μm and preheated at 80 ° C. for 3 minutes. Furthermore, as a clear paint on the coated plate, Mac Flow O-1800W-2 Clear (Nippon Paint Co., Ltd. acid epoxy curable clear paint, trade name) was applied by air spray coating to 35 μm, and then heated at 140 ° C. for 30 minutes. Curing was performed to obtain a test piece.

  The finished appearance of the multilayer coating film obtained after heat curing was visually evaluated. Table 8 shows the evaluation criteria. The results are shown in Table 4.

  The aqueous intermediate coating, aqueous base coating and clear coating were diluted under the following conditions and used for coating.

Water-based intermediate coating paint thinner: ion exchange water 40 seconds / NO. 4 Ford Cup / 20 ° C
The solid content of the paint was 54% by weight.

Water-based paint thinner: ion exchange water 45 seconds / NO. 4 Ford Cup / 20 ° C

Clear paint thinner: EEP (Ethoxyethyl propionate) / S-150 (Exxon aromatic hydrocarbon solvent, trade name) = 1/1 (weight ratio) mixed solvent 30 seconds / NO. 4 Ford Cup / 20 ° C

Preparation Example Synthesis of Acrylic Emulsion Polymerization was conducted in the same manner as in the preparation of Resin 1 except that the following monomer mixture was used to obtain an acrylic resin emulsion (Resin 2) having a nonvolatile content of 40.6%.

Examples 2-9 and Comparative Examples 1-2
In the same manner as in Example 1 except that those shown in Table 5 and Tables 6 to 8 below are used as the acrylic resin emulsion and the urethane resin emulsion, an intermediate coating material and a multilayer coating film are formed and evaluated. did. The results are shown in Table 5.

Claims (3)

(1) A step of providing an article on which an electrodeposition coating film is formed; (2) A step of forming an intermediate coating film by applying a water-based intermediate coating on the electrodeposition coating; (3) A step of forming a base coating and a clear coating by sequentially applying a water-based base coating and a clear coating on the intermediate coating without wet curing the coating; (4) an intermediate coating; A step of simultaneously baking and curing a base coating film and a clear coating film;
The intermediate coating film formed from the aqueous intermediate coating material has a coating film water absorption of 10% or less and a coating film water elution rate of 5% or less,
An acrylic resin emulsion having a glass transition temperature of −50 to 20 ° C., an acid value of 2 to 60 mg KOH / g and a hydroxyl value of 10 to 120 mg KOH / g, a urethane resin emulsion having an acid value of 5 to 50 mg KOH / g, And a curing agent, and a method for forming a multilayer coating film.   The acrylic resin emulsion is obtained by emulsion polymerization of a monomer mixture containing (a) (meth) acrylic acid alkyl ester, (b) acid group-containing ethylenically unsaturated monomer, and (c) hydroxyl group-containing ethylenically unsaturated monomer. The method of claim 1, wherein The method according to claim 1, wherein a ratio of the acrylic resin emulsion to the urethane resin emulsion is a solid content weight ratio of 1/1 to 2/1.