JP2010284586A - Method for forming multi-layer coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming a multi-layer coating firm which exhibits subdued slippage between an electrodeposition coating film and an undercoat coating film and shows excellent adhesive properties of the undercoat. <P>SOLUTION: This method of forming a multi-layer coating film comprises a process to form an electrodeposition coating film by applying a cationic electrodeposition coating material containing a cationic epoxy resin, an isocyanate curing agent, an acrylic surface modifier and a pigment to the surface of an object to be coated and baking the coating material, and a process to form an undercoat coating film by applying an acrylic sol-based undercoat coating material containing microparticles of an acryl copolymer, a plasticizer, a filler, a block urethane resin and a mineral turpentine to the coated surface of the electrodeposition coating film, and baking the acrylic sol-based undercoat coating material. In addition, the cationic electrodeposition coating material further contains 200 to 1,000 ppm of a hindered phenol compound. The undercoat coating material contains 5 to 11 mass% of mineral turpentine for the whole coating material liquid. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for forming a multilayer coating film. More specifically, the present invention relates to a method for forming a multilayer coating film having an electrodeposition coating film and a rust-preventing protective coating film.

  A technique is known in which an anti-corrosion protective paint for vehicle bodies (hereinafter referred to as an undercoat) is applied to an object on which an electrodeposition coating is formed to form a multilayer coating (see, for example, Patent Document 1). ). However, in such a technique, there is a problem that slippage occurs between the electrodeposition coating film and the undercoat during the heat curing of the multilayer coating film, and the coating film appearance becomes poor.

JP 7-118893 A (paragraph 0014)

  The present invention has been made in order to solve the above-described conventional problems. The object of the present invention is to reduce slip between the electrodeposition coating film and the undercoat coating film, and to adhere to the undercoat film. An object of the present invention is to provide a method for forming a multi-layer coating film excellent in the above.

In the method for forming a multilayer coating film of the present invention, a cationic electrodeposition coating material containing a cationic epoxy resin, an isocyanate curing agent, an acrylic surface modifier and a pigment is applied on an object to be coated, and baked. A step of forming a coating film, and an acrylic sol-based undercoat paint containing acrylic polymer fine particles, a plasticizer, a filler, a block-type urethane resin and a mineral turpentine is applied to the coating surface of the electrodeposition coating film, Baking to form an undercoat coating film. The cationic electrodeposition paint further contains 200 ppm to 1000 ppm of a hindered phenol compound, and the undercoat paint contains 5% by mass to 11% by mass of mineral terpenes with respect to the entire coating liquid.
In a preferred embodiment, the electrodeposition coating film after baking has a methylene iodide contact angle of 42 ° or less.
In a preferred embodiment, the plasticizer is diisononyl phthalate, the filler is calcium carbonate, and the undercoat coating film in the vicinity of the electrodeposition coating film is 100 μm after coating the undercoat paint and before baking. It contains 1.0 to 3.2 diisononyl phthalate in an IR peak intensity ratio with respect to calcium carbonate.
In a preferred embodiment, in the undercoat paint, the blending amount of the diisononyl phthalate is 15% by weight to 40% by weight with respect to the entire coating liquid, and the blending amount of the calcium carbonate is 20% with respect to the entire coating liquid. % To 45% by mass.
In a preferred embodiment, in the undercoat paint, a mass ratio of the acrylic polymer fine particles / the block type urethane resin is 90/10 to 15/85.
In a preferred embodiment, the article to be coated is an automobile body.

  According to the present invention, an electrodeposition coating film and an undercoat coating film can be obtained by containing a specific amount of a hindered phenol compound in the electrodeposition paint and containing a specific amount of mineral turpent in the undercoat paint. It is possible to provide a method for forming a multi-layer coating film that is less slippery and has excellent undercoat adhesion.

  The multilayer coating film forming method of the present invention comprises a step of coating a cationic electrodeposition coating material on an object to be coated and baking it to form an electrodeposition coating film. Applying and baking a sol-based undercoat paint to form an undercoat coating film. First, the article to be coated and each paint used in the present invention will be described, and then the method for forming a multilayer coating film of the present invention will be described.

A. Objects to be coated As the objects to be coated, any base material capable of electrodeposition coating can be mentioned. Examples of such a substrate include iron, steel, aluminum, tin, zinc, and alloys containing these metals, and plating or vapor deposition products of these metals. Specific examples include car bodies and parts of automobiles such as passenger cars, trucks, motorcycles, and buses manufactured using these metal members. Conductive treatment was applied to resins such as polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer resin, polyamide resin, acrylic resin, vinylidene chloride resin, polycarbonate resin, polyurethane resin, epoxy resin, and various FRPs. A plastic material can also be used as a substrate.

  In the multilayer coating film forming method of the present invention, the substrate may be used as it is, or pretreatment such as degreasing and chemical conversion treatment may be performed before electrodeposition coating.

B. Cationic electrodeposition paint The cationic electrodeposition paint used in the method for forming a multilayer coating film of the present invention contains a cationic epoxy resin, an isocyanate curing agent, an acrylic surface modifier and a pigment. In the multilayer coating film forming method of the present invention, the cationic electrodeposition paint further contains 200 ppm to 1000 ppm of a hindered phenol compound.

B-1. Cationic Epoxy Resin As the cationic epoxy resin, any appropriate resin can be adopted. Representative examples of the cationic epoxy resin include an epoxy resin modified with an amine. Specific examples of the cationic epoxy resin are described in, for example, JP-A Nos. 54-4978 and 56-34186.

  Typically, the cationic epoxy resin is formed by opening all of the epoxy rings of the bisphenol type epoxy resin with an active hydrogen compound capable of introducing a cationic group, or some epoxy rings with other active hydrogens. It is produced by opening a ring with a compound and opening the remaining epoxy ring with an active hydrogen compound capable of introducing a cationic group.

  Typical examples of the bisphenol type epoxy resin include bisphenol A type or bisphenol F type epoxy resin. As commercial products of bisphenol A type epoxy resin, Epicoat 828 (manufactured by Yuka Shell Epoxy, epoxy equivalent 180-190), Epicoat 1001 (same, epoxy equivalent 450-500), Epicoat 1010 (same, epoxy equivalent 3000-4000). ). As a commercial product of bisphenol F type epoxy resin, Epicoat 807 (same as above, epoxy equivalent 170) can be mentioned.

  As the cationic epoxy resin, the following formula described in JP-A-5-306327 is shown.

[Wherein R represents a residue excluding the glycidyloxy group of the diglycidyl epoxy compound, R ′ represents a residue excluding the isocyanate group of the diisocyanate compound, and n represents a positive integer. You may use the oxazolidone ring containing epoxy resin represented by this. This is because a coating film excellent in heat resistance and corrosion resistance can be obtained.

  As a method for introducing an oxazolidone ring into an epoxy resin, for example, a blocked isocyanate curing agent blocked with a lower alcohol such as methanol and a polyepoxide are heated and kept in the presence of a basic catalyst, and a by-product lower alcohol is formed in the system. It is obtained by further distilling off.

  A particularly preferable epoxy resin is an oxazolidone ring-containing epoxy resin. This is because a coating film having excellent heat resistance and corrosion resistance and further excellent impact resistance can be obtained.

  It is known that an epoxy resin containing an oxazolidone ring can be obtained by reacting a bifunctional epoxy resin with a diisocyanate blocked with a monoalcohol (ie, bisurethane). Specific examples and production methods of this oxazolidone ring-containing epoxy resin are described in, for example, [0012] to [0047] of JP-A No. 2000-128959.

  These epoxy resins may be modified with suitable resins such as polyester polyols, polyether polyols, and monofunctional alkylphenols. In addition, the epoxy resin can be chain-extended using a reaction between an epoxy group and a diol or dicarboxylic acid.

  These epoxy resins preferably have an amine equivalent of 0.3 to 4.0 meq / g after ring opening, and more preferably 5 to 50% of them are occupied by primary amino groups. Ring opening with compound.

  Examples of the active hydrogen compound capable of introducing a cationic group include primary amines, secondary amines, tertiary amine acid salts, sulfides and acid mixtures. In order to prepare primary, secondary and / or tertiary amino group-containing epoxy resins, acid salts of primary amines, secondary amines and tertiary amines are used as active hydrogen compounds capable of introducing cationic groups. .

  Specific examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N, N-dimethylethanolamine acetate, diethyl disulfide / acetic acid mixture, etc. In addition, there are secondary amines in which primary amines such as aminoethylethanolamine ketimine and diethylenetriamine diketimine are blocked. A plurality of amines may be used in combination.

  The cationic epoxy resin is contained in an amount of preferably 20 to 80 parts by mass, more preferably 30 to 60 parts by mass with respect to 100 parts by mass of the solid content of the cationic electrodeposition paint.

B-2. Isocyanate curing agent Any appropriate isocyanate can be adopted as the isocyanate curing agent as long as the effects of the present invention are obtained. A typical example is a blocked isocyanate. In the present specification, the blocked isocyanate refers to a product in which a reactive group of a polyisocyanate is blocked so as to react only when heated at a high temperature. Polyisocyanate refers to a compound having two or more isocyanate groups in one molecule. The polyisocyanate may be any of aliphatic, alicyclic, aromatic and aromatic-aliphatic, for example.

  Specific examples of polyisocyanates include aromatic diisocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate, and naphthalene diisocyanate; hexamethylene diisocyanate (HDI), 2,2,4- C3-C12 aliphatic diisocyanates such as trimethylhexane diisocyanate and lysine diisocyanate; 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4′-dicyclohexylmethane diisocyanate (hydrogenated MDI) Methylcyclohexane diisocyanate, isopropylidene dicyclohexyl-4,4'-diisocyanate, and 1,3-diisocyanatomethyl Carbon such as rhohexane (hydrogenated XDI), hydrogenated TDI, 2,5- or 2,6-bis (isocyanatomethyl) -bicyclo [2.2.1] heptane (also referred to as norbornane diisocyanate). Aliphatic diisocyanates having a number of 5 to 18; aliphatic diisocyanates having an aromatic ring such as xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); modified products of these diisocyanates (urethanes, carbodiimides, Uretdione, uretonimine, burette and / or isocyanurate modified product); and the like. These may be used alone or in combination of two or more.

  An adduct or prepolymer obtained by reacting a polyisocyanate with a polyhydric alcohol such as ethylene glycol, propylene glycol, trimethylolpropane, hexanetriol or the like at an NCO / OH ratio of 2 or more may also be used as a block isocyanate curing agent.

  A blocking agent that blocks polyisocyanate is added to an isocyanate group and is stable at room temperature, but can regenerate a free isocyanate group when heated to a temperature higher than the dissociation temperature. Specific examples of the blocking agent include phenol, acid amide, lactam, lower monohydric alcohol, cellosolve, caprolactam, and oximes.

  The isocyanate curing agent needs a sufficient amount to react with active hydrogen-containing functional groups such as primary, secondary amino groups, and hydroxyl groups in the cationic epoxy resin during curing to give a good cured coating film. Is done. The amount of the isocyanate curing agent is preferably 90/10 to 50/50, more preferably 80/20, expressed as a solid mass ratio of the cationic epoxy resin and the isocyanate curing agent (cationic epoxy resin / curing agent). ~ 65/35. By adjusting the solid content ratio between the cationic epoxy resin and the blocked isocyanate curing agent, the fluidity and curing speed of the coating film (deposition film) during film formation are improved, and the smoothness of the coating film is improved. Moreover, the smoothness of an electrodeposition coating film can be improved more by using what the surface-treated steel plate has a smaller roughness.

B-3. Acrylic Surface Modification Agent The acrylic surface modification agent is typically used for preventing repellency of an electrodeposition coating film. In one embodiment, the acrylic surface modifier is (a) 5 to 90% by mass of a hydroxyl group-containing acrylic monomer, (b) 5 to 40% by mass of an amino group-containing acrylic monomer, (c) 1 to 40 It has a monomer composition comprising an ether group-containing acrylic monomer having no hydroxyl group in mass% and (d) the remaining other monomers. The acrylic surface modifier is obtained by copolymerizing the components (a) to (d).

  Arbitrary appropriate things can be used as said hydroxyl-containing acrylic monomer (a). Examples include alkylene diol mono (meth) acrylates and (meth) acrylamides. Preferable specific examples of alkylene (diol) mono (meth) acrylates include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, and 4-hydroxybutyl (meth). Examples include acrylate, polypropylene glycol mono (meth) acrylate, and 1,6-hexanediol mono (meth) acrylate. Preferable specific examples of (meth) acrylamides include N-hydroxyethyl (meth) acrylamide and N-hydroxypropyl (meth) acrylamide. In addition, a reaction product of hydroxyalkyl mono (meth) acrylate and ε-caprolactone or a reaction product of hydroxyalkyl mono (meth) acrylate and a six-membered ring carbonate can be suitably used.

  The compounding quantity of a hydroxyl-containing acrylic monomer (a) becomes like this. Preferably it is 5-90 mass%, More preferably, it is 10-60 mass%, Most preferably, it is 15-40 mass%. If the amount of the hydroxyl group-containing acrylic monomer (a) is too small, the adhesion to the undercoat film may be insufficient. If it is too large, the appearance, water resistance and corrosion resistance of the resulting multilayer film will be reduced. May decrease. When the blending amount is in the above range, the above characteristics can be satisfied at the same time.

  Any appropriate amino group-containing acrylic monomer (b) may be used. Specific examples include N, N-dimethylaminoethyl methacrylate, N, N-diethylaminoethyl methacrylate, N, N-dimethylaminopropyl methacrylate, N, N-diethylaminopropyl methacrylate and the like.

  Moreover, after copolymerizing a monomer having an epoxy group such as glycidyl (meth) acrylate, a secondary amine may be reacted with the epoxy group. In this case, it is possible to obtain substantially the same effect as that obtained by previously synthesizing the amine adduct of glycidyl (meth) acrylate and then synthesizing the copolymer. Secondary amines that can be used for the reaction with the epoxy group include diethylamine, dibutylamine, dicyclohexylamine, morpholine, diethanolamine, N-methylethanolamine and the like. In particular, an amine having a hydroxyl group and a secondary amino group in the molecule is preferable. In addition, methyl isobutyl ketone diketiminate of diethylenetriamine and methyl isobutyl ketone monoketiminate of 2- (2-aminoethylamino) ethanol can be used. The amine is reacted stoichiometrically with the epoxy groups in the copolymer.

  The amount of the amino group-containing acrylic monomer (b) is preferably 5 to 40% by mass, more preferably 5 to 20% by mass. If the amount of the amino group-containing acrylic monomer (b) is too small, the adhesion with the undercoat film may be insufficient. If it is too large, the water resistance and corrosion resistance of the resulting multilayer film will be adversely affected. Or the water-solubility of the resin becomes too high, which may cause a problem in precipitation during electrodeposition.

  As the ether group-containing acrylic monomer (c) having no hydroxyl group, any appropriate one can be used. Specific examples include 2-methoxyethyl (meth) acrylate, 4-methoxybutyl (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, ethoxyethyl (meth) acrylate, propoxyethyl (meth) acrylate, hexylbutyloxyethyl ( Examples include meth) acrylate, 2-ethylhexyloxybutyl (meth) acrylate, and furfuryl (meth) acrylate. By using an ether group-containing acrylic monomer having no hydroxyl group, the occurrence of repelling can be prevented without impairing the adhesion with the undercoat coating film.

  The amount of the ether group-containing acrylic monomer (c) having no hydroxyl group is preferably 1 to 40% by mass, more preferably 3 to 20% by mass. When the blending amount of the ether group-containing acrylic monomer (c) having no hydroxyl group is too small, the adhesion to the undercoat film may be insufficient, and when it is too large, the occurrence of repellency cannot be sufficiently prevented. There is. When the blending amount is in the above range, the above characteristics can be satisfied at the same time.

  As said other monomer (d), an ethylenically unsaturated monomer is mentioned typically. Specific examples include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, and cyclohexyl (meth) acrylate. , 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, styrene, vinyltoluene, α-methylstyrene, (meth) acrylonitrile, (meth) acrylamide, and vinyl acetate.

  As a polymerization initiator when copolymerizing the monomer components, organic peroxides such as benzoyl peroxide, t-butyl perbenzoate, t-butyl hydroperoxide, di-t-butyl peroxide, and t-butyl peroctate are used. Oxides or azo compounds such as azobisisobutyronitrile and azoisobutyric acid nitrile can be used. A polymerization initiator may be used independently and may be used in combination of 2 or more types. It is preferable that the addition amount of a polymerization initiator is 0.1-15 mass% with respect to a monomer mixture.

  Solvents used for copolymerization include aromatic hydrocarbons such as toluene and xylene, ketones such as methyl isobutyl ketone, cyclohexanone and isophorone, esters such as ethyl acetate and butyl acetate, and n-butanol Alcohols such as ethyl cellosolve, butyl cellosolve, methoxypropanol, diethylene glycol monobutyl ether and the like can be used. A solvent may be used independently and may be used in combination of multiple.

  The reaction temperature during copolymerization is preferably 50 ° C to 170 ° C, more preferably 80 ° C to 150 ° C.

  The number average molecular weight of the copolymer constituting the acrylic surface modifier is preferably 1000 to 50000 as a conversion value using a styrene standard by GPC. If the molecular weight is too small, the effect of suppressing the occurrence of repelling may be insufficient, and if it is too large, the smoothness of the resulting multilayer coating film surface may be impaired. When the molecular weight is in the above range, the above characteristics can be satisfied at the same time. The molecular weight may be adjusted by adjusting the conditions for copolymerization, or by using a chain transfer agent such as dodecyl mercaptan or 2-ethylhexyl thioglycolate.

  The acrylic surface modifier is required in an amount sufficient to suppress generation of cissing during electrodeposition baking and to provide a good cured coating film. The content of the acrylic surface modifier is preferably 1 to 20% by mass, more preferably 2 to 18% by mass, based on the resin solid content of the cationic electrodeposition paint. If the content is less than 1% by mass, the effect of suppressing the occurrence of repelling may be insufficient. If the content is more than 20% by mass, the corrosion resistance may be lowered.

B-4. Pigment As the pigment, any appropriate pigment that can be used for an electrodeposition paint can be adopted. A representative example of the pigment is an inorganic pigment. Specific examples of inorganic pigments include colored pigments such as titanium white, carbon black and bengara; extender pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica and clay; zinc phosphate, iron phosphate, phosphorus Anti-corrosive pigments such as aluminum oxide, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, aluminum phosphomolybdate, and aluminum zinc phosphomolybdate .

  When the pigment is used as a component of the electrodeposition coating, typically, the pigment is dispersed in advance in an aqueous solvent at a high concentration to form a paste. This is because the pigment is in a powder form, and it is difficult to disperse in a single step in a low concentration uniform state used in the electrodeposition coating composition. Such a paste is generally called a pigment dispersion paste. The pigment dispersion paste is prepared by dispersing a pigment in an aqueous solvent together with a pigment dispersion resin varnish. As the pigment dispersion resin, a cationic polymer such as a cationic or nonionic low molecular weight surfactant or a modified epoxy resin having a quaternary ammonium group and / or a tertiary sulfonium group is generally used. As the aqueous solvent, ion-exchanged water or water containing a small amount of alcohol is used. In general, the pigment dispersion resin is used in an amount of 20 to 100 parts by mass with respect to 100 parts by mass of the pigment. After the pigment dispersion resin varnish and the pigment are mixed, the pigment is dispersed using a normal dispersing device such as a ball mill or a sand grind mill until the particle size of the pigment in the mixture reaches a predetermined uniform particle size. A dispersion paste can be obtained.

  The pigment is preferably used in a proportion of 3 to 30 parts by mass, more preferably 10 to 25 parts by mass with respect to 100 parts by mass of the solid content of the cationic electrodeposition paint.

B-5. Hindered phenolic compound The hindered phenolic compound preferably has a bulky bulky substituent such as a t-butyl group at the ortho position of the phenol. This is because chain movement of trapped radicals hardly occurs and stability is increased. More preferably, the hindered phenol-based compound has a bulky substituent at both ortho positions.

  Examples of hindered phenol compounds include 2,6-di-t-butylphenol, 2,4-di-t-butylphenol, 2-t-butyl-4,6-di-methylphenol, and 2,6-di. -T-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,4,6-tri-t-butylphenol, 2,6-t-butyl-4-hydroxymethylphenol 2,6-di-t-butyl-2-dimethylamino-p-cresol, n-octadecyl-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate, styrenate phenol 2,2′-methylene-bis- (4-methyl-6-tert-butylphenol), 2,2′-methylene-bis- (6-cyclohexyl-4-methylphenol), 2,2′-butylidene-bis -(2-t- Til-4-methylphenol), 4,4′-methylene-bis- (2,6-di-t-butylphenol), 1,6-hexanediol-bis- [3- (3,5-di-t-) Butyl-4-hydroxyphenyl) propionate], tris (3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate, tetrakis- {methylene-3- (3 ', 5'-di-t-butyl- 4'-hydroxyphenyl) propionate} methane, 1,1,3-tris (2-methyl-4-hydroxy-5-t-butylphenyl) butane, 3,9-bis [1,1-di-methyl-2 -{Β- (3-tert-butyl-4-hydroxy-5-methylphenyl) propionyloxy} ethyl] -2,4,8,10-tetraoxaspiro [5,5] undecane, pentaerythritol-tetrakis- [ 3- (3,5-di-t-butyl-4 -Hydroxyphenyl) propionate], thiodiethylene-bis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], N, N'-hexane-1,6-diylbis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionamide], benzenepropanoic acid-3,5-bis (1,1-dimethylethyl) -4-hydroxyC7-C9 side chain alkyl ester, 3 , 3 ′, 3 ″, 5,5 ′, 5 ″ -hexa-t-butyl-a, a ′, a ″-(methylene-2,4,6-triyl) tri-p-cresol, calcium diethylbis [ Blend of [[3,5-bis- (1,1-dimethylethyl) -4-hydroxyphenyl] methyl] phosphonate] (50%) and polyethylene wax (50%), 4,6-bis- (octyl (Luthiomethyl) -o-cresol and the like.

  Preferred hindered phenol compounds are 2,6-di-t-butylphenol, 2-t-butyl-4,6-di-methylphenol, 2,6-di-t-butyl-4-methylphenol, 2, 6-di-t-butyl-4-ethylphenol, 2,4,6-tri-t-butylphenol, styrenate phenol, 2,2'-methylene-bis- (4-methyl-6-t-butylphenol), 2,2′-methylene-bis- (6-cyclohexyl-4-methylphenol), 4,4′-methylene-bis- (2,6-di-tert-butylphenol), tetrakis- {methylene-3- (3 ', 5'-di-t-butyl-4'-hydroxyphenyl) propionate} methane, 3,3', 3 ", 5,5 ', 5" -hexa-t-butyl-a, a', a " -(Methylene-2,4,6-triyl) tri-p-alkyl Sol, pentaerythritol-tetrakis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate], octadecyl-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate It is.

  Commercially available hindered phenolic compounds include, for example, Sumitizer BHT, Sumilizer S, Sumilizer BP-76, Sumilizer MDP-S, Summarizer BP-101, Sumilizer GA-80, and Sumitizer BBM-S manufactured by Sumitomo Chemical Co., Ltd. , Sumilyzer WX-R, Sumilyzer NW, Sumilyzer GM, Sumilyzer GS, Adeka Stub AO-20, Adeka Stub AO-30, Adeka Stub AO-40, Adeka Stub AO-50, Adeka Stub AO-60, Adeka Stub AO-75 , ADK STAB AO-80, ADK STAB AO-330, ADK STAB AO-616, ADK STAB AO-635, ADK STAB AO-658, ADK STAB AO-15, ADK STAB AO-18, ADK STAB 328, ADK STAB AO-37, manufactured by Ciba Specialty Chemicals Irganox 1010, Irganox 1035, Irganox 1076, Irganox 1098, Irganox 1135, Irganos 13300, Irganox 1425WL, Irganox 1520L.

  The hindered phenol compound is contained in a proportion of 200 ppm to 1000 ppm as described above, preferably in a proportion of 300 ppm to 600 ppm, based on the solid content of the cationic electrodeposition paint. By using the hindered phenol compound in such a range, the contact angle of methylene iodide of the cured electrodeposition coating film can be controlled within a desired range. This is presumed to be related to the fact that the hindered phenolic compound suppresses thermal decomposition of the coating film during baking. In addition, even if it exceeds the upper limit of the preferable range, there are many cases where the effect of the increase cannot be obtained.

B-6. Other Components The above cationic electrodeposition paint is typically a dispersion in an aqueous medium, and therefore contains an aqueous medium. The cationic electrodeposition coating composition may contain, for example, a dissociation catalyst, a neutralizing acid, and an organic solvent in addition to the above components.

  The dissociation catalyst is for promoting dissociation of the blocking agent of the isocyanate curing agent. As such a dissociation catalyst, organic tin compounds such as dibutyltin laurate, dibutyltin oxide and dioctyltin oxide, amines such as N-methylmorpholine, metal salts such as strontium, cobalt and copper can be used. The concentration of the dissociation catalyst is 0.1 to 6 parts by mass with respect to 100 parts by mass (solid content) of the cationic epoxy resin and the isocyanate curing agent in the cationic electrodeposition paint.

  The neutralizing acid is typically contained in the aqueous medium and neutralizes the cationic epoxy resin to improve the dispersibility of the binder resin emulsion. Examples of the neutralizing acid include inorganic acids such as hydrochloric acid, nitric acid, and phosphoric acid, and organic acids such as formic acid, acetic acid, and lactic acid. The amount of neutralizing acid used is preferably in the range of a lower limit of 10 mg equivalent and an upper limit of 30 mg equivalent with respect to 100 g of binder resin solid content containing a cationic epoxy resin and a blocked isocyanate curing agent. The lower limit is more preferably 15 mg equivalent, and the upper limit is more preferably 29 mg equivalent. When the amount of the neutralizing acid is less than 10 mg equivalent, the affinity for water is not sufficient and the dispersion in water cannot be carried out or the stability is often insufficient. In many cases, the amount increases, the precipitation of paint solids decreases, and the throwing power is inferior.

  The organic solvent is necessary as a solvent when synthesizing resin components such as a cationic epoxy resin, an isocyanate curing agent, and a pigment-dispersed resin, and complicated operations are required for complete removal. Moreover, when the organic solvent is contained in binder resin, the fluidity | liquidity of the coating film at the time of film forming will be improved, and the smoothness of a coating film will improve. Examples of the organic solvent that can be usually contained in the cationic electrodeposition paint include, for example, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monoethyl hexyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, propylene glycol monophenyl ether. Is mentioned.

  In addition to the above, the cationic electrodeposition paint may further contain any appropriate paint additive depending on the purpose. Specific examples of paint additives include plasticizers, surfactants, and ultraviolet absorbers. The cationic electrodeposition paint may contain an amino group-containing acrylic resin, an amino group-containing polyester resin, and the like.

B-7. Preparation of Cationic Electrodeposition Paint The cationic electrodeposition paint used in the present invention is prepared by dispersing each of the above components in an aqueous solvent.

C. Acrylic sol-based undercoat paint The acrylic sol-based undercoat paint used in the method for forming a multilayer coating film of the present invention contains acrylic polymer fine particles, a plasticizer, a filler, a block type urethane resin, and a mineral turpentine. In the present invention, the undercoat paint contains 5% by mass to 11% by mass of mineral terpenes with respect to the entire undercoat paint liquid.

C-1. Acrylic polymer fine particles As the acrylic polymer fine particles, any appropriate polymer fine particles that can be used in an acrylic sol can be used. For example, a homopolymer or copolymer of a monomer selected from alkyl acrylate, alkyl methacrylate and the like can be used. Specific examples of these monomers include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, and isobutyl methacrylate.

  The acrylic polymer fine particles are preferably core-shell type fine particles composed of a core portion and a shell portion. When core-shell type acrylic polymer fine particles are used, the storage stability of the acrylic sol is further improved, and the viscosity increase when coated as an undercoat and the bleed generation after heat curing are further suppressed. There is. Further, when the acrylic polymer fine particles are of a core-shell type, it is preferable that the core portion is composed of a plasticizer affinity polymer and the shell portion is composed of a plasticizer non-affinity polymer. The polymer in the shell portion that is poorly compatible with the plasticizer coats the compatible core portion, thereby suppressing an increase in the viscosity of the acrylic sol during storage and further improving the storage stability. Further, since the polymer of the shell portion is compatible with the plasticizer by heating to an appropriate temperature, no bleeding occurs after the heat curing.

  The core part is preferably composed of a polymer containing 50% by mass or more of a homopolymer or copolymer of methacrylate selected from n-butyl methacrylate, isobutyl methacrylate, ethyl methacrylate and the like. Thus, by making the core part highly compatible with the plasticizer, the occurrence of bleeding after heat curing can be suppressed. In particular, from the viewpoint of imparting flexibility to the coating film, the core portion is preferably mainly composed of a copolymer of butyl methacrylate and isobutyl methacrylate.

  The shell part is preferably composed of a polymer containing 50% by mass or more of a homopolymer or copolymer of a monomer selected from methyl methacrylate, benzyl methacrylate, styrene and the like. Thus, by making a shell part low compatibility with a plasticizer, the viscosity increase of the acrylic sol during storage is suppressed and storage stability improves more. In particular, from the viewpoint of further improving the storage stability, the shell part is preferably mainly composed of methyl methacrylate.

  The composition ratio of the core part to the shell part is preferably 25/75 to 70/30 in terms of the mass ratio of the polymer. When the core component is less than 25/75 as the mass ratio, the possibility that bleed occurs after heat curing is higher than that in the above desired range. In addition, when the core component exceeds 70/30 as the mass ratio, the coating of the core component of the shell component may be insufficient as compared with the desirable range described above, which affects the storage stability. There are things to do.

  The molecular weight of the acrylic polymer fine particles is preferably 100,000 to several million in terms of weight average molecular weight from the viewpoints of chipping resistance, storage stability and the like. The average particle diameter of the acrylic polymer fine particles is preferably 0.1 to 10 μm from the viewpoint of diffusibility into a plasticizer and storage stability.

C-2. Plasticizer Any appropriate compound can be used as the plasticizer as long as the effects of the present invention are obtained. Specific examples of the plasticizer include phthalic acid plasticizers such as diisononyl phthalate, di- (2-ethylhexyl) phthalate, diisodecyl phthalate, butyl benzyl phthalate, di- (2-ethylhexyl) adipate, di-n-decyl adipate, Fatty acid ester plasticizers such as di- (2-ethylhexyl) azelate, dibutyl sebacate, di- (2-ethylhexyl) sebacate, phosphoric acid such as tributyl phosphate, tri- (2-ethylhexyl) phosphate, 2-ethylhexyl diphenyl phosphate Examples include ester plasticizers, epoxy plasticizers such as epoxidized soybean oil, other polyester plasticizers, and benzoic acid plasticizers. These plasticizers may be used alone or in combination of two or more. A preferred plasticizer is diisononyl phthalate. It is cheap and easy to obtain.

  In one embodiment, the plasticizer is blended in an amount of preferably 50 parts by mass to 500 parts by mass with respect to 100 parts by mass of the acrylic polymer fine particles from the viewpoints of chipping resistance, sprayability, and the like. In one embodiment, a plasticizer is mix | blended in the ratio of preferably 15 mass%-40 mass% with respect to the whole undercoat coating liquid, More preferably, 20 mass%-35 mass%.

C-3. Filler As the above-mentioned filler, any appropriate one can be used as long as the effects of the present invention can be obtained. Specific examples of fillers include calcium carbonate, mica, talc, kaolin clay, silica, barium sulfate, etc., and fiber fillers such as glass fiber, wollastonite, alumina fiber, ceramic fiber, and various whiskers. be able to. A preferred filler is calcium carbonate. It is cheap and easy to obtain.

  In one embodiment, the filler is preferably blended at a ratio of 50 parts by mass to 800 parts by mass with respect to 100 parts by mass of the acrylic polymer fine particles from the viewpoint of chipping resistance, foamability, cost, and the like. . In one embodiment, the filler is blended in a proportion of preferably 20% by mass to 45% by mass, and more preferably 25% by mass to 40% by mass with respect to the entire undercoat coating liquid.

C-4. Block type urethane resin As the block type urethane resin, for example, a urethane resin obtained by blocking a urethane resin obtained by reacting an α-polyol and an isocyanate with a blocking agent can be used. Examples of the α-polyol include polyether polyol and polyester polyol. Examples of the isocyanate include diphenylmethane diisocyanate (MDI), tolylene diisocyanate (TDI), and hexamethylene diisocyanate (HDI). The blocking agent is as described in the above section B-2. As the urethane resin, a urethane resin synthesized from polypropylene glycol (PPG) and tolylene diisocyanate (TDI) is particularly preferable because of its versatility.

  When the undercoat paint is applied to the underfloor part or the wheel house part of an automobile, the undercoat (acrylic sol) is gelled by using the heating during baking, which is a subsequent process. At this time, the under floor part and the wheel house part of the automobile are hardly heated to a high temperature because they are located below the heating device (specifically, they are only heated to about 140 ° C.). Accordingly, it is necessary to dissociate the blocking agent at a temperature of about 140 ° C. and promote the reaction with the curing agent described later. From the viewpoint of low dissociation temperature, oximes and amines are preferable as the blocking agent in the undercoat paint. Specific examples include methyl ethyl ketone oxime and 3,5-dimethylpyrazole. In particular, 3,5-dimethylpyrazole is preferable from the viewpoint of dissociation at a lower temperature.

  The blending amount of the block type urethane resin is a mass ratio of acrylic polymer fine particles / block type urethane resin, and is preferably 90/10 to 15/85. When the blending amount is less than 90/10, the chipping performance, the cold resistance, and the adhesion of the coating film to the substrate are often insufficient as compared with those in the above desirable range. When the blending amount exceeds 15/85, the viscosity of the prepared acrylic sol is higher than that in the above desired range, which often affects workability during coating.

C-5. Mineral Turpen The mineral turpent is used as a diluent for the undercoat paint. Mineral turpen is another name for liquid paraffin and is also called mineral spirit, white mineral oil, petroleum spirit, white kerosene, white spirit and the like. In the present invention, the mineral terpenes are contained in the undercoat paint in a proportion of 5% by mass to 11% by mass, preferably 7% by mass to 9% by mass with respect to the entire coating liquid. By using the mineral terpen in such an amount for the undercoat paint, it is possible to satisfactorily prevent slippage between the electrodeposition paint film and the undercoat paint film.

C-6. Curing agent The undercoat paint typically contains a curing agent. Examples of the curing agent include solid aliphatic polyamines, aromatic polyamines, hydrazide compounds, and the like. From the viewpoint of curing at a relatively low temperature and extremely good storage stability, a solid hydrazine-based curing agent is preferable. Examples of the solid hydrazine-based curing agent include adipic acid dihydrazide (ADH) and sebacic acid dihydrazide (SDH). Since these hydrazide compounds are cured at a relatively low temperature, dissociation of the blocking agent bonded to the block type urethane resin is promoted when the undercoat paint is heated to a temperature of about 140 ° C. as described above. Reacts with urethane resin. As a result, the hydrazide compound forms a urea bond with the urethane resin, and this urea bond improves the durability of the acrylic sol coating film and the adhesion between the coating film and the substrate. In addition, these solid hydrazine-based curing agents are dispersed in a liquid urethane resin under a temperature condition for storing an acrylic sol, and have a high melting point, so compared with the case where a liquid curing agent is used, The storage stability is significantly improved. In particular, adipic acid dihydrazide (ADH) is preferable because it is a general-purpose hydrazide compound. The curing agent is preferably added in an amount necessary for curing the urethane resin so that the active hydrogen equivalent of the hydrazide compound is the same as the isocyanate equivalent of the urethane resin.

C-7. Other Components The acrylic sol-based undercoat paint can further contain any appropriate paint additive depending on the purpose. Specific examples of paint additives include colorants, antioxidants, and ultraviolet absorbers.

C-8. Preparation of undercoat paint The acrylic sol-based undercoat paint can be prepared by any appropriate method. For example, it can prepare by fully mixing and stirring each said component using arbitrary appropriate mixers. Examples of the mixer include a planetary mixer, a kneader, a glen mill, and a roll.

D. Multilayer coating film forming method The multilayer coating film forming method of the present invention comprises a step of coating the cationic electrodeposition coating material on the object to be coated and baking it to form an electrodeposition coating film. Applying the acrylic sol-based undercoat paint on the coated surface of the paint film and baking it to form an undercoat paint film.

D-1. Formation of Electrodeposition Coating Electrodeposition is performed by immersing the article to be coated described in the above section A in a bath of the cationic electrodeposition paint described in the above section B, and using the article to be coated as a cathode and an anode. Applying a voltage between the two to deposit a film. The bath temperature of the electrodeposition paint is typically 10 ° C to 45 ° C. The applied voltage is typically 50V to 450V. When the applied voltage is less than 50V, electrodeposition is often insufficient, and when it exceeds 450V, the coating film is often destroyed and an abnormal appearance is obtained. The time for applying the voltage can be appropriately set according to the electrodeposition conditions. The application time is typically 2 minutes to 4 minutes.

  The film thickness of the electrodeposition coating film is preferably 5 μm to 25 μm, more preferably 10 μm to 20 μm. When the film thickness is less than 5 μm, the rust prevention property is often insufficient, and when it exceeds 25 μm, the paint is wasted.

  A cured electrodeposition coating film is formed by baking the electrodeposition coating film obtained as described above as it is or after washing with water. The baking temperature is preferably 120 ° C to 260 ° C, more preferably 140 ° C to 220 ° C. The baking time can be appropriately set according to the baking temperature or the like. The baking time is typically 10 minutes to 30 minutes. Any appropriate heating means can be adopted as the heating means at the time of baking. For example, a gas furnace or an electric furnace may be used.

  The contact angle of methylene iodide of the electrodeposition coating film (cured electrodeposition coating film) after baking is preferably 42 ° or less, and more preferably 40 ° or less. By setting the contact angle of methylene iodide within such a range, slippage of the undercoat can be remarkably prevented. By adjusting the composition of the cationic electrodeposition paint (particularly, by setting the content of the hindered phenol compound within the predetermined range), the contact angle of methylene iodide can be controlled to 42 ° or less. .

D-2. Formation of undercoat coating film Next, the acrylic sol-based undercoat paint is applied to the coating surface of the cured electrodeposition coating film. Any appropriate method can be adopted as the coating method. Specific examples include brush coating, roller coating, air spray coating, and airless spray coating. Note that airless spray coating, which can obtain a thick film in a short time, is most suitable if an intended coating film thickness is to be obtained on the side of a coating line for automobile manufacture. In this specification, airless spray coating includes electrostatic airless spray coating and air-assisted airless spray coating.

  As described above, in one embodiment, the plasticizer contained in the undercoat paint is diisononyl phthalate and the filler is calcium carbonate. In this case, preferably, after the undercoat paint is applied and before baking, the undercoat paint film in the vicinity of 100 μm of the electrodeposition paint film is diisononyl phthalate having an IR peak intensity ratio of 1.0 to 3.2 with respect to calcium carbonate. Containing. In other words, the undercoat coating film contains diisononyl phthalate at a higher concentration in the vicinity of the electrodeposition coating film. This is presumed to be because the amount of migration of the mineral terpene contained in the undercoat paint to the coating film interface increases, and the amount of diisononyl phthalate transferred to the coating film interface increases accordingly. Therefore, by suppressing the undercoat paint to a predetermined range as described above, adhesion between the undercoat and the electrodeposition coating film can be ensured, and slippage of the undercoat can be remarkably prevented.

  Next, the painted undercoat paint is baked to form an undercoat coating film. Arbitrary appropriate conditions can be employ | adopted for the heating temperature at the time of baking, a heating time, and a heating means. For example, the heating may be performed at a constant temperature, may be performed by continuously changing the temperature, or may be performed by changing the temperature stepwise. The heating time can be appropriately set according to the heating temperature. Examples of the heating means include a hot air circulation drying furnace. In one embodiment, baking is performed in two stages from 90 ° C. to 100 ° C. for 5 minutes to 10 minutes and then from 120 ° C. to 140 ° C. for 15 minutes to 30 minutes.

  As described above, a multilayer coating film having an electrodeposition coating film and an undercoat coating film (rust-preventing protective coating film) is formed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples. In addition, the evaluation method in an Example is as follows. Unless otherwise specified, “parts” and “%” in the examples are based on mass.

(I) Contact angle of methylene iodide 0.4 ml of methylene iodide was dropped on the cured electrodeposition coating film, and the contact angle immediately after the dropping was measured using a solid-liquid interface analysis device DropMaster500 (trade name) manufactured by Kyowa Interface Science Co., Ltd. (Measurement temperature: 25 ° C.).
(II) Slip of undercoat An undercoat paint was applied in a beat shape having a diameter of 10 mm and a length of 10 cm to a plate on which a cured electrodeposition coating film was formed (electrodeposition coating plate) to prepare a test piece. This test piece was suspended vertically and allowed to stand for 30 minutes. Next, the test piece was baked at 95 ° C. for 8 minutes and then baked at 130 ° C. for 20 minutes. The sliding (sagging) length of the undercoat coating film of this test piece was measured. The evaluation criteria for slip are as follows.
○: Sliding (sagging) length is 3 mm or less ×: Sliding (sagging) length exceeds 3 mm (III) IR peak intensity ratio After applying the undercoat paint to the electrodeposition coated plate with a 20 MIL doctor blade, 60 Left for a minute. Then, the upper layer part of the undercoat coating film was scraped off with a 4MIL doctor blade. Samples were taken from the undercoat coating film to remove the upper layer portion, using a JASCO Corp. FT / IR-4100, the peak area intensity ratio of diisononyl phthalate (1667 cm ^-1) to calcium carbonate (848cm ^-1) Calculated.
(IV) Adhesion of the undercoat The undercoat paint was applied to the end of the electrodeposition coating plate of 100 × 25 × 0.8 mm, and the spacer was sandwiched and crimped so that the area of the adhesive portion was 25 × 25 mm and the thickness was 3 mm. . In this state, after baking at 130 ° C. for 20 minutes, it was allowed to stand at room temperature for 12 hours. Thereafter, the spacer was removed and pulled in the shear direction at a pulling speed of 50 mm / min to evaluate the adhesion. The evaluation criteria are as follows.
○: Cohesive failure ×: Interface failure

(Production Example 1: Preparation of cationic electrodeposition paint)
(1) Production of amine-modified epoxy resin In a flask equipped with a stirrer, a cooling tube, a nitrogen introducing tube, a thermometer and a dropping funnel, 940 parts of liquid epoxy, 61.4 parts of methyl isobutyl ketone (hereinafter abbreviated as MIBK) and 24.4 parts of methanol were charged. The reaction mixture was heated from room temperature to 40 ° C. with stirring, and then 0.01 part of dibutyltin laurate and 21.75 parts of tolylene diisocyanate (hereinafter abbreviated as TDI) were added. The reaction was continued for 30 minutes at 40-45 ° C. The reaction was continued until the absorption based on the isocyanate group disappeared in the measurement of IR spectrum. To the reaction product, 82.0 parts of polyoxyethylene bisphenol A ether and 125.0 parts of diphenylmethane-4,4′-diisocyanate were added. The reaction was performed at 55 ° C. to 60 ° C. and continued until absorption based on the isocyanate group disappeared in the measurement of IR spectrum. Subsequently, the temperature was raised, 2.0 parts of N, N-dimethylbenzylamine was added at 100 ° C., held at 130 ° C., methanol was fractionated using a fractionating tube and reacted, and the epoxy equivalent was 284. It was. Thereafter, the reaction mixture was diluted with MIBK until the nonvolatile content became 95%, the reaction mixture was cooled, and 268.1 parts of bisphenol A and 93.6 parts of 2-ethylhexanoic acid were added. The reaction was performed at 120 ° C. to 125 ° C., and when the epoxy equivalent reached 1320, the reaction mixture was diluted with MIBK until the nonvolatile content was 85%, and the reaction mixture was cooled. 93.6 parts of MIBK-blocked primary amine of diethylenetriamine and 65.2 parts of N-methylethanolamine were added and reacted at 120 ° C. for 1 hour. Thereafter, an oxazolidone ring-containing modified epoxy resin having a cationic group (resin solid content: 85%) was obtained.

(2) Manufacture of isocyanate curing agent 1330 parts of crude MDI and 585.6 parts of MIBK were charged into a reaction vessel and heated to 85-95 ° C., and then 456.6 parts of butyl diglycol ether was added dropwise over 2.5 hours. did. After completion of dropping, the mixture was kept warm for 1 hour. Thereafter, 194.8 parts of MIBK was added, cooled to 50 ° C., and 532 parts of propylene glycol was added dropwise. After completion of dropping, the mixture was heated to 60 ° C. and kept for 1 hour. After adding 0.4 parts of dibutyltin laurate, the temperature was raised and the temperature was kept at 70 ° C. for 1 hour, and in the measurement of IR spectrum, it was confirmed that the absorption based on the isocyanate group disappeared, and the block polyisocyanate curing agent was Obtained.

(3) Manufacture of acrylic-based surface modifier A reactor equipped with a stirrer, thermometer, decanter, reflux condenser, nitrogen inlet tube and dropping funnel was charged with 1500 parts of butyl cellosolve and heated to 120 ° C. while introducing nitrogen gas. The temperature was raised, 627 parts of methyl methacrylate, 191 parts of lauryl methacrylate, 182 parts of hydroxyethyl methacrylate, 300 parts of 2-methoxyethyl acrylate, 200 parts of dimethylaminoethyl methacrylate and t-butylperoxy-2-ethyl A mixture of 50 parts of hexanoate was added dropwise at a constant rate over 3 hours. After the completion of dropping, the reaction was further allowed to proceed at 120 ° C. for 3 hours, followed by cooling to obtain an acrylic surface modifier. The obtained resin had a nonvolatile content of 50% and a number average molecular weight of 10,000.

(4) Production of pigment-dispersed resin Into a reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe and a thermometer, 2220 parts of isophorone diisocyanate (hereinafter abbreviated as IPDI) and 342.1 parts of MIBK were charged, and the temperature was raised to 50 At 60 ° C., 2.2 parts of dibutyltin laurate was added, and at 87 ° C., 878.7 parts of methyl ethyl ketone oxime (hereinafter abbreviated as MEK oxime) was charged. Thereafter, the mixture was kept at 60 ° C. for 1 hour, and it was confirmed that the NCO equivalent was 348, and 890 parts of dimethylethanolamine was added. After incubating at 60 ° C. for 1 hour and confirming that the NCO peak disappeared by IR, 1872.6 parts of 50% lactic acid and 495 parts of deionized water were added while cooling so as not to exceed 60 ° C. A grader was obtained.
In another reaction vessel, 870 parts of TDI and 49.5 parts of MIBK were charged, and 667.2 parts of 2-ethylhexanol was added dropwise over 2.5 hours so as not to exceed 50 ° C. After the completion of dropping, 35.5 parts of MIBK was added and kept warm for 30 minutes. Then, it confirmed that NCO equivalent was 330-370, and obtained the half block polyisocyanate.
Further, 940.0 parts of epoxy was charged into another reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe and a thermometer, diluted with 38.5 parts of methanol, and then 0.1 part of dibutyltin dilaurate was added. It was. After raising the temperature to 50 ° C., 87.1 parts of TDI was added, and the temperature was further raised. 1.4 parts of N, N-dimethylbenzylamine was added at 100 ° C., and the mixture was kept at 130 ° C. for 2 hours. At this time, methanol was fractionated by a fractionation tube. This was cooled to 115 ° C., and MIBK was charged to a solid content of 90%. Then, 270.3 parts of bisphenol A and 39.2 parts of 2-ethylhexanoic acid were added, and heated and stirred at 125 ° C. for 2 hours. The above-mentioned half-block polyisocyanate 516.4 parts was added dropwise over 30 minutes, and then heated and stirred for 30 minutes. 1506 parts of polyoxyethylene bisphenol A ether was gradually added and dissolved. After cooling to 90 ° C., the above-described quaternizing agent was added and maintained at 70 to 80 ° C., and an acid value of 2 or less was confirmed to obtain a pigment dispersion resin. The resin solid content of this pigment dispersion resin was 30%.

(5) Production of pigment dispersion paste 106.9 parts of the pigment dispersion resin obtained in (4) above, 1.6 parts of carbon black, 40 parts of kaolin, 55.4 parts of titanium dioxide, and aluminum phosphomolybdate in a sand grind mill. 3 parts and 13 parts of deionized water were added and dispersed until the particle size became 10 μm or less to obtain a pigment dispersion paste (solid content 60%).

(6) Preparation of Cationic Electrodeposition Coating The amine-modified epoxy resin obtained in (1) above and the blocked isocyanate curing agent obtained in (2) above become uniform at a solid content ratio of 70/30. Mixed. Furthermore, hindered phenol compounds (3,3 ′, 3 ″, 5,5 ′, 5 ″ -hexa-t-butyl-a, a ′, a ″-(methylene-2,4,6-triyl) tri -P-cresol, antioxidant, manufactured by Ciba Specialty Chemicals, product name "Irganox 1330") is mixed with 400 ppm of resin solids, and the acrylic surface modifier obtained in (3) above is mixed with resin solids. 3% mixed for minutes. Thereafter, the mixture was neutralized with glacial acetic acid so that the milligram equivalent of acid per 100 g of resin solids was 27, and further diluted slowly with ion-exchanged water. By removing MIBK under reduced pressure, a blocked isocyanate-containing amine-modified bisphenol-type epoxy resin emulsion having a solid content of 36% was obtained.
1730 parts of this emulsion, 295 parts of the pigment dispersion paste obtained in the above (5), 1970 parts of ion-exchanged water, 40 parts of 10% aqueous cerium acetate solution and 10 parts of dibutyltin oxide were mixed to obtain a solid content of 20 weight. % Cationic electrodeposition coating composition A was obtained.

(Production Example 2: Preparation of cationic electrodeposition paint)
As a hindered phenol compound, pentaerythritol-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] (antioxidant, manufactured by Ciba Specialty Chemicals, product name “Irganox 1010”) ]) Was used in the same manner as in Production Example 1 except that 800 ppm was used.

(Production Example 3: Preparation of cationic electrodeposition paint)
As hindered phenol compound, octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate (antioxidant, manufactured by Ciba Specialty Chemicals, product name “Irganox 1076”) for 400 ppm A cationic electrodeposition paint C was prepared in the same manner as in Production Example 1, except that

(Production Example 4: Preparation of cationic electrodeposition paint)
Cationic electrodeposition paint D was prepared in the same manner as in Production Example 1 except that the hindered phenol compound was not used.

(Production Example 5: Preparation of undercoat paint)
Core-shell type acrylic polymer fine particles having a core part mainly composed of a copolymer of butyl methacrylate and isobutyl methacrylate and a shell part mainly comprising a methyl methacrylate polymer were prepared by a usual method. The average particle size of the fine particles was 60 μm. On the other hand, a block type urethane resin obtained by blocking a urethane resin obtained by reacting TDI and polypropylene glycol (PPG) with 3,5-dimethylpyrazole as an amine blocking agent was obtained.
Next, 90 parts by mass of the acrylic polymer fine particles, 10 parts by mass of the block type urethane resin, 0.8 parts by mass of a curing agent (adipic acid dihydrazide), 150 parts by mass of a plasticizer (diisononyl phthalate), and a foaming agent 3 parts by mass of (azodicarbonamide), 180 parts by mass of filler (calcium carbonate) and 40 parts by mass of mineral terpenes were blended and mixed and dispersed by a kneader to obtain an undercoat paint E. Here, content with respect to the whole undercoat coating liquid of a mineral turpent was 8.4 mass%.

(Production Example 6: Preparation of undercoat paint)
Undercoat paint F was obtained in the same manner as in Production Example 5, except that the content of the mineral turpent under the coating liquid was 12.4% by mass.

Example 1
An SPCC steel plate subjected to zinc phosphate treatment was prepared as a base material. This substrate was electrodeposited using the cationic electrodeposition paint A. The bath temperature of the electrodeposition paint A was 30 ° C., the applied voltage was 200 V, and the application time was 3 minutes. The coating film thus formed was baked to obtain a cured electrodeposition coating film. Baking was performed at 170 ° C. for 20 minutes using a gas furnace. The thickness of the obtained electrodeposition coating film was 15 μm.
Next, the undercoat paint E was applied to the coated surface of the electrodeposition coating film, baked at 95 ° C. for 8 minutes, and then baked at 130 ° C. for 20 minutes. The obtained undercoat coating film was 10 cm in length and 10 mm in diameter in the form of a beat (saddle shape). As described above, the multilayer coating film a was formed. About the electrodeposition coating material used by the present Example, the characteristic of the multilayer coating film a was evaluated by said (I)-(III).
Further, the undercoat paint E was applied to the coated surface of the electrodeposition coating film, and baked at 130 ° C. for 20 minutes. The thickness of the obtained undercoat coating film was 3 mm. The multilayer coating film b was formed as described above. About the electrodeposition coating material used in the present Example, the characteristic of the multilayer coating film b was evaluated by said (IV). The results are shown in Table 1 below.

(Examples 2-3 and Comparative Examples 1-2)
A multilayer coating film was formed using the electrodeposition paint and the undercoat paint shown in Table 1 above. The obtained coating film was subjected to the same evaluation as in Example 1. The results are shown in Table 1 above.

  As is apparent from Table 1, the contact angle of methylene iodide can be reduced by including a hindered phenol compound in the electrodeposition paint. On the other hand, by adjusting the content of mineral terpenes in the undercoat paint, the strength ratio of diisononyl phthalate (DINP) / calcium carbonate can be reduced in the undercoat paint film before baking with the upper layer portion removed. In other words, the diisononyl phthalate content of the undercoat coating film in the vicinity of the electrodeposition coating film can be controlled to be small. By combining these, slippage of the undercoat can be prevented.

  The method for forming a multilayer coating film of the present invention is suitably used in the paint field, and can be particularly suitably used for painting automobile bodies.

Claims (6)

Coating a cationic electrodeposition coating containing a cationic epoxy resin, an isocyanate curing agent, an acrylic surface modifier and a pigment on the object to be coated, and baking to form an electrodeposition coating;
An acrylic sol-based undercoat paint containing acrylic polymer fine particles, plasticizer, filler, block-type urethane resin and mineral turpentine is applied to the coating surface of the electrodeposition coating film and baked to form an undercoat coating film. The cationic electrodeposition coating composition further comprises 200 ppm to 1000 ppm of a hindered phenol compound,
A method for forming a multilayer coating film, wherein the undercoat paint contains 5% by mass to 11% by mass of a mineral terpene with respect to the entire coating liquid.   The method for forming a multilayer coating film according to claim 1, wherein a contact angle of methylene iodide of the electrodeposition coating film after baking is 42 ° or less. The plasticizer is diisononyl phthalate and the filler is calcium carbonate;
The undercoat coating film at 100 μm in the vicinity of the electrodeposition coating film contains diisononyl phthalate having an IR peak intensity ratio with respect to calcium carbonate of 1.0 to 3.2 after the undercoat coating is applied and before baking. Or the method for forming a multilayer coating film according to 2;   In the undercoat paint, the blending amount of the diisononyl phthalate is 15% by weight to 40% by weight with respect to the entire coating liquid, and the blending amount of the calcium carbonate is 20% by weight to 45% by weight with respect to the entire coating liquid. The method for forming a multilayer coating film according to claim 3.   The method for forming a multilayer coating film according to any one of claims 1 to 4, wherein in the undercoat paint, a mass ratio of the acrylic polymer fine particles / the block type urethane resin is 90/10 to 15/85. The multilayer coating film forming method according to claim 1, wherein the object to be coated is an automobile body.