Method of forming multi-layered coating film

This invention provides a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) onto cured coating film of a specific electrodeposition paint (A) of low weight loss under heating, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously.

Skip to: Description  ·  Claims  · Patent History  ·  Patent History
Description
TECHNICAL FIELD

This invention relates to a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint, second coloring paint and clear paint onto cured coating film of an electrodeposition paint, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously.

BACKGROUND ART

Coating of metallic shaped articles such as automobile bodies, metallic parts of two-wheeled vehicles, house electric appliances, steel furnitures and the like is normally conducted by the steps of applying an electrodeposition paint, baking the resulting coating film, applying an intermediate coat thereon and baking the same, and then applying a top coat and baking the same to form a multi-layered coating film. However, when baking is performed after application of each of those paints, not only high energy costs are incurred for the baking but also great labor and costs are required for operation and maintenance of the baking facilities. Hence development of coating film-forming method capable of reducing use of volatile organic compounds and achieving energy-saving and step-reduction has been in demand.

For example, JP Sho 61(1986)-141969A disclosed a metallic finishing method comprising applying onto a substrate an organic solvent-based or non-aqueous dispersion type thermosetting paint, a thermosetting, water-based metallic paint and a transparent thermosetting paint, by the order stated, and heating the three-layered coating film to cure the multiple layers simultaneously. This method, however, is subject to a problem that once finishing quality of the electrocoated film which is the undercoat applied before the three layers are formed thereon degrades, that of the multi-layered coating film surface also degrades as influenced thereby.

JP Hei 3(1991)-181369A disclosed a method for forming high quality multi-layered coating film by 3C1B process, comprising applying an intermediate coat, top base coat and top clear coat, by the order stated, onto a laser-treated dull steel plate onto which an undercoat has been applied by cationic electrocoating and subsequently baked, at least one of the three paints being a non-aqueous dispersion type paint; and baking the multi-layered coating film. This method, however, is subject to a problem that costly laser-treated dull steel plate-made shaped articles must be used to secure good finishing quality of perpendicular and horizontal parts, for forming a high quality three-layered coating film by single baking.

Furthermore, JP 2002-273322A disclosed a method for forming multi-layered coating film having high decorative effect and visual appearance, comprising successively applying a first water-borne base paint, second water-borne base paint and clear paint onto an object to be coated and simultaneously heat-curing all the uncured coating films, the first water-borne base paint and/or the second water-borne base paint containing effect pigment and the solid content of uncured first base paint coating film being 40-95% by weight. While this method can provide multi-layered coating film of good appearance and free of foaming or pinholing, there remains a problem that the multi-layered coating film surface becomes uneven, as affected by unevenness on the electrocoated film surface.

JP 2002-282773A disclosed a method of forming laminated coating film, comprising a step of forming an uncured intermediate coat by forming a cationic electrocoated film and applying onto the cured electrocoated film a water-borne intermediate paint; step of forming an uncured base coating film by applying a water-borne base paint onto the uncured intermediate coating film; a step of applying a clear paint on the uncured base coating film; and simultaneously heat-curing those uncured intermediate coat, base coat and clear coat; said cationic electrodeposition paint containing a binder resin which contains cationic epoxy resin and blocked isocyanate curing agent, neutralizing acid, organic solvent and metallic catalyst. However, because the method as described in the Official Patent Gazette uses, as the cationic electrodeposition paint, a paint containing cationic epoxy resin and blocked isocyanate curing agent, undulation or unevenness result on the electrocoated surface due to volatilization of the blocking agent during the baking for curing the electrocoated film, and when the multi-layered coating film is formed on such an electrocoated film surface by 3-coat-1-bake system of the first coloring paint, second coloring paint and clear paint, the finally formed multi-layered coating film shows degraded appearance, as affected by the unevenness of the electrocoated film.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method of forming multi-layered coating film excelling in appearance, corrosion resistance, chipping resistance and so on, while coping with environ mental problems by reducing generation of volatile organic compound in coating metallic objects.

We have engaged in concentrative studies to now discover that the above object can be accomplished by successively applying a first coloring paint, second coloring paint and clear paint onto cured coating film surface of specific electrodeposition paint of little weight loss under heating, and curing the three-layered coating film by single baking (which procedures may hereafter be referred to as 3-coat-1-bake or 3C1B) and completed the present invention.

Thus, the invention provides a method of forming multi-layered coating film which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) successively wet-on-wet, onto a cured coating film of an electrodeposition paint (A) showing a heat loss (X) of not more than 5% by weight, said heat loss being calculated according to the following equation.
Heat loss (X)=[(Y−Z)/Y]×100

    • [wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film, which is obtained by electrocoating the electrodeposition paint (A), by heating at 105° C. for 3 hours; and
    • Z is the weight of the cured coating film after heating the dry coating film at 170° C. for 20 minutes].

According to the method of the present invention, coated articles with little generation of volatile organic compound and exhibiting excellent appearance, corrosion resistance, chipping resistance and so on can be obtained. Still in addition, because the method of the invention adopts 3C1B system, it can contribute to energy saving and space reduction.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a model diagram showing frequency characteristics of a power spectrum.

In FIG. 1, 1 shows short wavelength region, 2 shows middle wavelength region and 3, long wavelength region.

FIG. 2 shows frequency characteristics of power spectrum of an electrodeposited coating film according to the present invention.

FIG. 3 shows frequency characteristics of power spectrum of a conventional electrodeposition coating film.

Hereinafter the method of forming multi-layered coating film of the present invention is explained in further details.

Substrate

The substrate or object to which the method of the present invention is applicable is subject to no particular limitation so long as it is of an electrocoatable material. As such materials, for example, metals such as stainless steel, iron, steel, copper, zinc, tin, aluminium, alumite and the like; alloys of such metals, sheets which are plated with such metals and laminated sheets of such metals can be named, which can be given surface treatment, primer treatment or the like where necessary, to be imparted with improved corrosion resistance and adherability. For example, stainless steel can be given a chromium surface treatment. As the coating object, automobile bodies are preferred, and the steel sheet to serve as the substrate can be advancedly zinc phosphate-treated as customarily practiced.

Electrodeposition Paint (A)

The above-described substrate is electrocoated with said electrodeposition paint (A). In the present invention, as the electrodeposition paint (A), one having a heat loss (X)(note 1) of its electrocoating film not more than 5 wt %, preferably not more than 4 wt %, inter alia, not more than 3.5 wt %, is used. When an electrodeposition paint forming an electrocoated film having a heat loss (X) more than 5 wt % is used, degradation in surface finish of 3C1B multi-layered coating film composed of the electrocoated film, first colored coating film, second colored coating film and clear coating film may take place, which is undesirable.

    • (Note 1) Heat loss (X) is calculated according to the following equation:
      Heat loss (X)=[(Y−Z)/Y]×100
    • [wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film, which is obtained by electrocoating the electrodeposition paint (A) by heating at 105° C. for 3 hours; and
    • Z is the weight of the cured film after heating the dry coating film at 170° C. for 20 minutes].

Furthermore, the electrodeposition paint (A) to be used in the present invention is preferably such that it forms cured electrocoated film having an average power spectral value (note 2) within a wavelength region of 0.02-1 mm of generally not higher than 70, in particular, 20-49, inter alia, 33-45, and/or an integration value of the power spectral values within the wavelength region of 0.02-1 mm of generally not higher than 1.7×105, in particular, 5×104-1.2×105, inter alia, 8×104-1.1×105, said values being obtained by power spectrum frequency analyses which comprise measuring surface roughness of an electrocoated film which has been cured at 170° C. for 20 minutes, over a measuring length of 50 mm at 10 μm intervals with a surface roughness meter and then Fourier transforming the so obtained measurement data.

FIG. 1 is a model diagram of the frequency characteristics of power spectral values over a range less than 0.1 mm (short wavelength region), 0.1-1 mm (middle wavelength region) and 1 mm-10 mm (long wavelength region).

FIG. 2 is the frequency characteristics diagram of a 20 μm-thick cured coating film which is formed by electrocoating electrodeposition paint No. 1 of the present invention as obtained in later appearing Production Example 16 and heating the film at 170° C. for 20 minutes; and FIG. 3 shows the frequency characteristics diagram of a 20 μm-thick cured coating film which is formed by electrocoating a conventional electrodeposition paint as obtained in later appearing Production Example 23 and heating the film at 170° C. for 20 minutes.

As is clear upon comparing FIG. 2 with FIG. 3, the electrodeposition paint (A) used in the present invention forms cured electrocoated film excelling in power spectral values over from the short wavelength region to middle wavelength region.

    • (Note 2) Power spectral values:
      • The values obtained by measuring roughness of cured surface of an electrocoated film of an electrodeposition paint with a surface roughness measuring instrument (SURFCOM 130A, tradename, Tokyo Seimitsu Co., Ltd.) over a measuring length of 50 mm at 10 μm-intervals, and Fourier transforming the obtained data.

As the electrodeposition paint (A) to be used in the method of the present invention, particularly those comprising base resin (a) which is obtained by reacting epoxy resin (a1), amine compound (a2) and phenolic compound (a3); and epoxy resin (b) as crosslinking agent are preferred.

As the epoxy resin (a1) used for the production of the base resin (a), particularly the epoxy resins having at least two epoxy-containing functional groups per molecule which are represented by the following formula (1)
are preferred.

Said epoxy resin (a1) can be those known per se, for example, those which are described in JP Sho 60(1985)-170620A, JP Sho 62(1987)-135467A, JP Sho 60(1985)-166675A, JP Sho 60(1985)-161973A and JP Hei 2(1990)-265975A can be used.

The epoxy resin (a1) also includes those with their termini bonded to residual groups of polymerization initiating component, i.e., active hydrogen-containing organic compound residues. As active hydrogen-containing organic compounds which are the precursors thereof, for example, alcohols such as aliphatic monohydric alcohol, aromatic monohydric alcohol, aliphatic or alicyclic polyhydric alcohol and the like; phenols; fatty acids; aliphatic, alicyclic or aromatic polybasic acids; oxy acid; polyvinyl alcohol, partial hydrolyzate of polyvinyl acetate, starch, cellulose, cellulose acetate, cellulose acetate butylate, hydroxyethyl cellulose, allylpolyol resin, styrene-allyl alcohol copolymer resin, alkyd resin, polyester polyol resin, polycaprolactonepolyol resin and the like can be named. These active hydrogen-containing organic compounds may also have a skeletal structure in which unsaturated double bond is epoxidated, concurrently with the active hydrogen.

The epoxy resin (a1) can be prepared, for example, by conducting a polymerization reaction using above-described active hydrogen-containing organic compound as the initiating agent, in the presence of 4-vinylcyclohexene-1-oxide alone or concurrent presence therewith of another epoxy-containing compound, said polymerization being induced by the epoxy groups contained in the named compounds, to form polyether resin, and then epoxidating the vinyl groups present in its side chains with oxidizing agent such as peracids or hydroperoxides.

4-Vinylcyclohexene-1-oxide can be prepared, for example, by partially epoxidating vinylcyclohexene, which is formed through dimerization reaction of butadiene, with peracetic acid.

As other epoxy-containing compound copolymerizable therewith, any compounds having epoxy groups can be used without particular limitation, while those containing one epoxy group per molecule are preferred from the standpoint of ease of production. More specifically, for example, oxides of unsaturated compounds such as ethylene oxide, propylene oxide, butylenes oxide, α-olefin epoxides represented by the following formula (2)

[in which n is an integer of 2-25], styrene oxide and the like; glycidyl ethers of compounds having hydroxyl groups such as allyl glycidyl ether, 2-ethylhexyl glycidyl ether, methyl glycidyl ether, butyl glycidyl ether, phenyl glycidyl ether and the like; and glycidyl esters of organic acid such as fatty acid.

The ring-opening (co)polymerization of epoxy groups in the presence of 4-vinylcyclohexene-1-oxide alone or in concurrent presence of other epoxy-containing compound is preferably conducted in the presence of a catalyst. As the catalyst, for example, amines such as methylamine, ethylamine, propylamine, piperazine and the like; organic bases such as pyridines, imidazoles and the like; organic acids such as formic acid, propionic acid and the like; inorganic acids such as sulfuric acid, hydrochloric acid and the like; alkalai metal alcoholates such as sodium methylate and the like; alkalies such as KOH, NaOH and the like; Lewis acids such as BF3SnCl2, AlCl3, SnCl4 and the like and complexes thereof; and organometal compounds such as triethylaluminium, diethylzinc and the like can be named.

Such catalyst can be used normally within a range of 0.001-10 wt %, preferably 0.1-5 wt %, to the reactants. The ring-opening (co)polymerization reaction can be conducted generally at temperatures ranging −70° C.-200° C., preferably −30° C.-100° C. This reaction is preferably conducted in a solvent, and as the solvent ordinary organic solvent having no active hydrogen can be used.

Thus obtained polyether resin (ring-opened (co)polymer) can then be converted to an epoxy resin (a1) having the functional groups of the formula (1), by epoxidating the vinyl groups (—CH═CH2) directly bound to the carbon atoms in the alicyclic structure of side chains thereof.

The epoxidation can be effected using peracids or hydroperoxidies. As peracids, for example, performic acid, peracetic acid, perbenzoic acid trifluoroperacetic acid and the like can be used, and as hydroperoxides, for example, hydrogen peroxide, tert-butyl peroxide, cumene peroxide and the like can be used. The epoxidation reaction can be practiced in the presence of a catalyst, where necessary.

The functional groups of the formula (1) are formed as the vinyl groups in 4-vinylcyclohexene-1-oxide in the ring-opened (co)polymer are epoxidated. In this epoxidation reaction, where an alicyclic oxyrane-containing compound as afore-named is concurrently present as the epoxy-containing compound, vinyl groups in said compound may occasionally be also epoxidated, however to result in a structure different from the functional group of the formula (1).

Use or non-use of a solvent or the temperature of the epoxidation reaction can be suitably adjusted according to the apparatus or properties of the starting materials used. Depending on the epoxidation reaction conditions, a substituent of the following formula (3)
in the starting material(s) and/or the substituent of the formula (1) as formed in the reaction may side-react with epoxidation agent used, simultaneously with the epoxidation of vinyl groups in the starting polymer, to form modified substituents which come to be concurrently present in the epoxy resin (a1). The ratio of content of these modified substituents differs depending on the kind of epoxidation agent used, molar ratio between the epoxidation agent and the vinyl groups and the reaction conditions.

Commercial products may also be used as such epoxy resin (a1), for example, EHPE 3150 (tradename, Daicel Chemical Industries, Ltd.), in which vinyl groups in ring-opened polymer of 4-vinylcyclohexene-1-oxide are epoxidated, having an average degree of polymerization ranging 15-25.

It is sufficient that at least two epoxy-containing functional groups of the formula (1) are present per molecule of the epoxy resin (a1) which can generally have an epoxy equivalent within a range of 140-1,000, preferably 170-300, and a number-average molecular weight (note 3) generally within a range of 200-50,000, preferably 1,000.

    • (Note 3) number-average molecular weight:
      • A value determined by following JIS K0124-83, using as the separation columns TSK GEL 4000 HXL+G3000 HXL+G2500 HXL+G2000 HXL (tradename, Tosoh Corporation) and as the eluent tetrahydrofuran for GPC and measuring at 40° C. at a flow rate of 1.0 ml/min.; and calculating referring to the chromatogram obtained with RI refractometer and polystyrene calibration line.

Amine compound (a2) to be reacted with the epoxy resin (a1) is a cationic property-imparting component for introducing amino groups into the epoxy resin (a1) to cationize the same. As the amino compound (a2), one having at least one hydrogen to react with epoxy group is used.

As such amine compound (a2), for example, mono- or di-alkylamines such as monomethylamine, dimethylamine, monoethylamine, diethylamine, monoisopropylamine, diisopropylamine, monobutylamine, dibutylamine and the like; alkanolamines such as monoethanolamine, diethanolamine, mono(2-hydroxylpropyl)amine, di(2-hydroxypropyl)amine, tri(2-hydroxypropyl)amine, monomethylaminoethanol, monoethylaminoethanol and the like; alkylenepolyamines such as ethylenediamine, propylenediamine, butylenediamine, hexamethylenediamine, tetraethylenepentamine, pentaethylenehexamine, diethylaminopropylamine, diethylenetriamine, triethylenetetramine and the like and ketimination products of these polyamines; alkyleneimines such as ethyleneimine, propyleneimine and the like; and cyclic amines such as piperazine, morpholine, pyrazine and the like can be named. Of these, primary or secondary amine compounds having primary hydroxyl groups are particularly preferred.

As the phenolic compound (a3), one having at least one phenolic hydroxyl group per molecule can be used. More specifically, for example, polyhydric phenolic compounds such as 2,2-bis(p-hydroxyphenyl)propane, 4,4′-dihydroxybenzophenone, 1,1-bis(p-hydroxyphenyl)ethane, 1,1-bis(p-hydroxyphenyl)isobutane, 2,2-bis(4-hydroxy-3-tert-butylphenyl)propane, bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, 1,1,2,2-tetra(p-hydroxyphenyl)ethane, 4,4-dihydroxydiphenyl ether, 4,4-dihydroxydiphenylsulfone, phenol novolak, cresol novolak and the like can be named.

Monophenolic compounds such as phenol, nonylphenol, α- or β-naphthol, p-tert-octylphenol, o- or p-phenylphenol and the like may also be used.

For forming a coating film exhibiting better corrosion resistance, use of bisphenol compound such as bisphenol A [2,2-bis(p-hydroxyphenyl)propane], bisphenol F[bis(p-hydroxyphenyl)methane] and the like as the phenolic compound (a3) is particularly preferred.

Of those bisphenol compounds, particularly those represented by the following formula (4)

    • [in the formula, n is a number 0-8, and R6 stands for a residue of an active hydrogen compound]
      having a number-average molecular weight of at least 200, preferably about 800-about 3,000 and on the average not more than 2, preferably 0.8-1.2, phenolic hydroxyl group(s) per molecule are suitable.

As the active hydrogen-containing compound which is a precursor of R6 in the above formula (4), for example, compounds such as amines like secondary amines; phenols like nonylphenol; organic acids such as fatty acid; thiols; alcohols such as alkyl alcohol, cellosolve, butylcellosolve, carbitol and the like; and inorganic acids can be named. Of these, the most preferred are dialkanolamines which are secondary amines having primary hydroxyl groups and monophenols such as nonylphenol, phenylphenol and phenol. In particular, use of a secondary amine having primary hydroxyl group(s) improves curability.

The above formula (4) shows a structure in which R6− and —OH are bound respectively to its two terminals, while it is permissible that molecules of the structure whose two terminals are bound to either R6— or —OH alone are mixed.

Base resin (a) can be obtained by reacting above-described epoxy resin (a1) with amine compound (a2) and phenolic compound (a3). Such a base resin (a) is advantageous over conventional base resins whose base component is bisphenol A type epoxy resin, in that the former excels in corrosion resistance and in electrodeposition coating ability for alloyed zinc-plated steel sheet.

The reaction ratio of the epoxy resin (a1), amine compound (a2) and phenolic compound (a3) is subject to no particular limitation and can be suitably selected according to intended utility of the resulting resin for paint. Whereas, it is generally preferred to use them at such a ratio, per mol of the epoxy-containing functional group of epoxy resin (a1), that the primary or secondary amino group in the amine compound (a2) is within a range of 0.1-1 mol, in particular, 0.4-0.9 mol, and the phenolic hydroxyl group in the phenolic compound (a3), within a range of 0.02-0.4 mol, in particular, 0.1-0.3 mol; and to make the sum of above mol numbers of the amine compound (a2) and phenolic compound (a3) fall within a range of 0.75-1.5 mols, in particular, 0.8-1.2 mols, per mol of the epoxy-containing functional group in the epoxy resin (a1).

The reaction using these components can be conducted, for example, at temperatures ranging from 50-300° C., in particular, 70-200° C. The order in the reaction is not particularly limited. All of the components may be simultaneously charged in a reactor to initiate the reaction, or the components other than epoxy resin (a1) can be added to the resin (a1) by optional order to cause successive reactions.

The base resin (a) preferably has, in general terms, an amine value within a range of 20-150 mgKOH/g, in particular, 35-100 mgKOH/g; a hydroxyl value within a range of 300-1,000 mgKOH/g, in particular, 350-700 mgKOH/g; and a number-average molecular weight (cf. note 3) within a range of 800-15,000, in particular, 1,000.

The base resin (a) may further be reacted with a cationizing agent during or after its preparation, where necessary. As the cationizing agent, for example, tertiary amines such as triethylamine, triethanolamine, N,N-dimethylethanolamine, N,N-methyldiethanolamine, N,N-diethylethanolamine, N-ethyldiethanolamine and the like can be used. They may be protonated with acid in advance, and reacted with epoxy groups to be converted to quaternary salts.

On the other hand, as the epoxy resin (b) to be used as the curing agent for the base resin (a), polyepoxide compound having, on the average, at least two epoxy-containing functional groups formed of alicyclic skeletal structure bound to epoxy group(s), and glycidyl etherified novolak resin can be named. Specifically, epoxy resins (b-1), (b-2) or (b-3) having the specific structures explained hereinafter are preferred.

Epoxy Resin (b-1):

Polyepoxide compounds having recurring units of the following formula (5)
more specifically, including those as explained in relation to the epoxy resin (a1). As commercial products, EHPE3150 (tradename, Daicel Chemical Industries, Ltd.) can be named. Polyepoxide compounds (b-1) can contain 3-30, preferably 3-15, recurring units of above formula (5), per molecule.

Epoxy Resin (b-2):

Polyepoxide polymer having the recurring units of a formula (6)

    • [in which R7 is hydrogen or methyl]
      and a number-average molecular weight of 3,000, in particular, 4,000. This polymer can be prepared, for example, by polymerizing at least one monomer of the following formula (7)
    • [in which R7 is hydrogen or methyl]
      or at least one of such monomers with another polymerizable monomer.

As examples of the monomers of the formula (7), 3,4-epoxycyclohexylmethyl acrylate, 3,4-epoxycyclohexylmethyl methacrylate and the like can be named. As commercial products, for example, CYCLOMER A400 and CYCLOMER M100 (tradenames, Daicel Chemical Industries, Ltd.) can be named.

Epoxy Resin (b-3):

Epoxy resins represented by the following formula (8)

    • [in the formula,
      • R1 and R2 are same or different, and each stands for hydrogen, C1-C8 alkyl, aryl, aralkyl or halogen; R3 stands for hydrogen, C1-C10 alkyl, aryl, aralkyl, allyl or halogen; R4 and R5 are same or different and each stands for hydrogen, C1-C4 alkyl or glycidyloxyphenyl; R5 stands for hydrogen, C1-C10 alkyl, aryl, aralkyl, allyl or halogen; and n is an integer of 1-38].

In the above formula (8), “alkyl” is of linear or branched chain, examples of which including methyl, ethyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isoamyl, hexyl, heptyl, octyl, 2-ethylhexyl, nonyl and decyl. “Aryl” may be either monocyclic or polycyclic, examples of which including phenyl and naphthyl, phenyl being particularly preferred. “Aralkyl” signifies aryl-substituted alkyl, examples of which including benzyl and phenethyl, benzyl being particularly preferred.

“Halogen” includes fluorine, chlorine, bromine and iodine.

Furthermore, “glycidyloxyphenyl” standing for R4 and/or R6 of the formula (8) is a group represented by the following formula (9):

    • [wherein W stands for hydrogen or C1-C10 alkyl].

In the above formula (8), as R1 and R2, hydrogen, methyl, chlorine and bromine are convenient, hydrogen, methyl and bromine being particularly preferred. As R3 and R5, methyl, tert-butyl, nonyl, phenyl, chlorine and bromine are preferred, methyl, tert-butyl, phenyl and bromine being particularly advantageous. Furthermore, R4 and R6 are preferably hydrogen, and particularly preferred n is 1-8.

It is generally preferred for the polyepoxide compound (b-3) to have a number-average molecular weight within a range of about 400-about 8,000, in particular, 600-2,000.

The polyepoxide compound (b-3) can be those known per se, for example, those disclosed in JP Hei 5(1993)-295321A, JP Hei 6(1994)-122850A and JP Hei 6(1994)-248203A can be used. As specific examples of commercial products, polyglycidyl etherified cresol novolak resins such as EPICRON N-695 (tradename, Dainippon Ink & Chemicals, Inc.) and ESCN-195-XL (tradename, Sumitomo Chemical Co.) can be named.

The use rate of above-described epoxy resin (b) is suitably variable according to the kind of individual resins. In general terms, the base resin (a) can be within a range of 50-90 mass parts, preferably 60-85 mass parts, inter alia, 65-80 mass parts; and the epoxy resin (b), 50-10 mass parts, preferably 40-15 mass parts, inter alia, 35-20 mass parts, per 100 mass parts of combined solid content of the base resin (a) and epoxy resin (b).

The electrodeposition paint (A) according to the present invention may contain, besides above-described epoxy resin (b), other curing agent known per se. As such concurrently useful curing agent, for example, blocked polyisocyanate compound which is an addition reaction product of polyisocyanate compound and isocyanate blocking agent can be named.

As the polyisocyanate compound, for example, aromatic, alicyclic or aliphatic polyisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, phenylene diisocyanate, bis(isocyanatomethyl)cyclohexane, tetramethylene diisocyanate, hexamethylene diisocyanate, methylene diisocyanate, isophorone diisocyanate and the like; and end-isocyanate-containing prepolymers which are obtained by reacting excessive amount of these isocyanate compounds with low molecular weight, active hydrogen-containing compounds such as ethylene glycol, propylene glycol, trimethylolpropane, hexanetriol, castor oil and the like can be named.

The isocyanate blocking agent adds to isocyanate groups in the polyisocyanate compounds to block the same. It is important that the blocked polyisocyanate compounds formed upon the addition should be stable at normal temperature but when they are heated to above their dissociation temperature, the blocking agent be dissociated to regenerate free isocyanate groups.

In particular, in order to make the heat loss (cf. note 1) in electrocoated film of electrodeposition paint (A) under 170° C.-20 minutes' heating not more than 5% by weight, it is preferred to use low molecular weight compound having a molecular weight not higher than 130 as the blocking agent. As specific examples, phenolic blocking agent such as phenol, cresol, xylenol, chlorophenol, ethylphenol and the like; lactam blocking agent such as ε-caprolactam, δ-valerolactam, γ-butyrolactam, β-propiolactam and the like; active methylene blocking agent such as ethyl acetoacetate, acetyl acetone and the like; alcohol blocking agent such as methanol, ethanol, propanol, butanol, amyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether, benzyl alcohol, methyl glycolate, butyl glycolate, diacetone alcohol, methyl lactate, ethyl lactate and the like; oxime blocking agent such as formamidoxime, acetaldoxime, acetoxime, methylethylketoxine, diacetylmonoxime, cyclohexanoxime and the like; mercaptan blocking agent such as butyl mercaptan, hexyl mercaptan, t-butyl mercaptan, thiophenol, methylthiophenol, ethylthiophenol and the like; acid amide blocking agent such as acetic acid amide, benzamide and the like; imide blocking agent such as succinimide, maleimide and the like; amine blocking agent such as xylidine, aniline, butylamine, dibutylamine and the like; imdazole bloking agent such as imidazole, 2-ethylimidazole and the like; and imine blocking agent such as ethyleneimine, propyleneimine and the like can be named. Of these, oxime blocking agent such as methylethyl ketoxime and the like are particularly convenient for well balanced paint stability and coating film curability.

The use rate of these blocked polyisocyanate compounds is suitably variable according to their kind. In general terms, the base resin (a) can be used within a range of 50-90 mass parts, preferably 60-85 mass parts, inter alia, 65-80 mass parts; the epoxy resin (b), 35-5 mass parts, preferably 28-12 mass parts, inter alia, 25-19 mass parts; and the blocked polyisocyanate compound, 15-5 mass parts, preferably 12-3 mass parts, inter alia, 10-1 mass parts; per 100 mass parts of combined solid content of the three components, i.e., the base resin (a), epoxy resin (b) and blocked polyisocyanate compound.

It is furthermore preferred that the electrodeposition paint (A) should contain a catalyst, for improving low temperature curability of the coating film. As the catalyst, hydroxides of metallic elements of atomic numbers ranging 25-30 or 40-42, i.e., hydroxides of at least one metal selected from Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb and Mo can be named. In particular, copper (II) hydroxide, cobalt hydroxide and zinc hydroxide are preferred.

The electrodeposition paint (A) can also suitably contain metal salt of carboxylic acid as a curing catalyst. As the metal salt of carboxylic acid, for example, bismuth carboxylate, iron carboxylate, titanium carboxylate, vanadium carboxylate, zirconium carboxylate, calcium carboxylate, potassium carboxylate, barium carboxylate, manganese carboxylate, cerium carboxylate, aluminum carboxylate and the like can be named. Of these, bismuth carboxylate and zirconium carboxylate are particularly preferred, specific examples including bismuth (III) octanoate, bismuth (III) 2-ethylhexanoate, bismuth (III) oleate, bismuth (III) neodecanoate, bismuth (III) versate, bismuth (III) naphthenate, zirconyl (IV) 2-ethylhexanoate, zirconyl (IV) versate, zirconyl (IV) oleate, zirconyl (IV) naphthenate and the like can be named. Of these, bismuth (III) octanoate is particularly preferred for improving curability and corrosion resistance.

As the use rate (solid content) of such a catalyst, generally a range of 0.1-20 wt %, in particular, 0.3-10 wt %, inter alia, 0.1-5 wt %, based on the combined solid weight of the base resin (a) and epoxy resin (b) is preferred in respect of paint stability.

The electrodeposition paint (A) may further contain imidazole compound as a curing catalyst. As the imidazole compound, those having a molecular weight of 68-300 per imidazole ring are suitable, i.e., where the compound contains one imidazole ring, its molecular weight is desirably within a range of 68-300 and when it contains, for example, two imidazole rings, within a range of 136-600.

Specific examples of the imidazole compound include compounds having one imidazole ring per molecule, such as imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 2-ethylimidazole, 2-isopropylimidazole, 2-phenyl-4-benzylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-isopropylimidazole, 1-cyanoethyl-2-phenylimidazole, 2,4-diamino-6-[2′-methylimidazolyl-(1)′]-ethyl-s-triazine, 2,4-diamino-6-[2′-ethyl-4-methylimidazolyl-(1)′]-ethyl-s-triazine, 2,4-diamino-6-[2′-undecylimidazolyl-(1)′]-ethyl-s-triazine, 2-methylimidazolium-isocyanuric acid adduct, 2-phenylimidazolium-isocyanuric acid adduct, 1-aminoethyl-2-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4-benzyl-5-hydroxymethylimidazole and the like; and compounds containing 2 or more imidazole rings per molecule which are obtained by dehydrating above-named hydroxymethyl-containing imidazole compounds such as 2-phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole and 2-phenyl-4-benzyl-5-hydroxymethylimidazole; and condensing them by deformaldehyde reaction, e.g., 4,4′-methylene-bis-(2-ethyl-5-methylimidazole) and the like, can be named.

These imidazole compounds can be blended in the electrodeposition paint (A) at a ratio generally within a range of 0.01-10 wt %, preferably 0.05-5 wt %, inter alia, 0.1-3 wt %, based on the combined solid weight of the base resin (a) and epoxy resin (b).

Corrosion resistance of the coating film can be further improved by blending inorganic bismuth compound in the electrodeposition paint (A). As the useful inorganic bismuth compound, for example, basic bismuch carbonate, bismuth oxide carbonate, bismuth nitrate, bismuth hydroxide nitrate, basic bismuth nitrate, bismuth oxide, bismuth hydroxide and bismuth sulfate can be named, bismuth hydroxide being particularly preferred.

Blend ratio of such an inorganic bismuth compound can be generally within a range of 0.1-20 wt %, preferably 0.5-10 wt %, inter alia, 1-5 wt %, based on the combined solid weight of the base resin (a) and epoxy resin (b).

When blocked polyisocyanate compound is concurrently used in addition to the epoxy resin (b) as the crosslinking agent, the paint may further contain tin compound as a curing catalyst. As the tin compound, for example, organotin compound such as dibutyltin oxide, dioctyltin oxide and the like; and aliphatic or aromatic carboxylates of dialkyltin such as dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, dioctyltin dibenzoate, dibutyltin dibenzoate and the like can be named.

The electrodeposition paint (A) may further contain, where necessary, for example, coloring pigment such as titanium oxide, carbon black and the like; extender such as clay, baryta, calcium carbonate, silica and the like; and anti-rust pigment such as zinc phosphate, iron phosphate, zinc flower and the like. In particular, it is found that use of a rutile-type fine particulate titanium dioxide composition formed by coating particulate surfaces of the rutile-type fine particles of titanium dioxide with 0.5-8.0 wt % (based on TiO2) of zirconium oxide in terms of ZrO2 (which composition may hereafter be referred to as zirconium-coated titanium white) as the pigment can improve appearance of the multi-layered coating film formed by the method of this invention.

The zirconium-coated titanium white content in the electrodeposition paint (A) can generally be within a range of 0.1-100 mass parts, preferably 3-50 mass parts, inter alia, 5-30 mass parts, per 100 mass parts of the combined solid content of the base resin (a) and epoxy resin (b), from the standpoint of appearance of the multi-layered coating film and paint stability.

Again, of the above extenders, use of flat pigment particles can improve chipping resistance of the multi-layered coating film. Flat pigment particles signify thin and flat-shaped pigment like scales, which are assumed to laminarly overlap each other with other pigment particles in a coating film to function to alleviate internal stress or external stress. As specific examples, talc, aluminium flake, mica flake and the like can be named.

Pigment content (note 4) of the electrodeposition paint (A) can be generally within a range of 5-30 wt %, preferably 10-25 wt %, inter alia, 15-less than 23 wt % The method of the present invention is applicable even in environments easily causing cissing, because the electrodeposition paint (A) allows to secure appearance of coated surfaces even when it has high pigment content ranging 20-30 wt %.

    • (Note 4) Pigment content is calculated by the following equation: Pigment ⁢   ⁢ content ⁢   ⁢ ( % ) = weight ⁢   ⁢ of ⁢   ⁢ pigment ⁢   ⁢   ⁢ component solid ⁢   ⁢ content ⁢   ⁢ of ⁢   ⁢ electrodeposition ⁢   ⁢ paint × 100  
    • [in the equation, the weight of the pigment component signifies the weight of the ash content remaining after heating a pigment dispersion paste at 800-1,000° C. for 180 minutes, and the solid content of the electrodeposition paint signifies the weight of the residue remaining after heating 2 g of an electrodeposition paint at 105° C. for 3 hours to volatilize the water and organic solvent off].

Where necessary, the electrodeposition paint (A) can further contain alcoholic or ether organic solvent, pigment dispersant such as tertiary amino-containing acid-neutralization type epoxy resin or onium salt type epoxy resin, surface regulating agent, surfactant, and neutralizing agent such as acetic acid or formic acid.

The electrodeposition paint (A) can be formulated following customary method, by adding a pigment dispersion paste to an emulsion comprising a base resin (a), epoxy resin (b), catalyst and so on, and diluting the system with an aqueous medium.

Thus formulated electrodeposition paint (A) can be applied onto substrates as earlier described, by electrocoating.

Electrocoating of the electrodeposition paint (A) can be generally performed using an electrocoating bath in which an electrodeposition paint (A) is diluted with deionized water or the like to a solid concentration of about 5-about 40 wt % and the pH is adjusted to fall within a range of 3.0-9.0, under such conditions of the bath temperature normally ranging 15-35° C. and applied voltage of 10-400 V.

The thickness of the coating film formed of the electrodeposition paint (A) is not particularly limited, but generally preferred range is 10-40 μm in terms of the cured film. The electrocoated film can be washed with water such as UF filtrate, water for industrial use or pure water. Suitable baking temperature of the coating film generally ranges about 120-about 200° C., preferably about 140-about 180° C., as the surface temperature of the coated substrate, and the baking time can range about 5-about 60 minutes, preferably about 10-about 30 minutes.

First Coloring Paint (B)

According to the method of the present invention, the first coloring paint (B) is applied onto the cured coating film of the electrodeposition paint (A) which is formed as described in the above. As the first coloring paint (B), either of organic solvent-based coloring paint or water-based coloring paint can be used, while use of water-based coloring paint is preferred for reducing volatile organic compound which is one of the objects of the present invention. Hereafter water-based coloring paint is explained.

As the base resin for first coloring paint (B), polyester resin, acrylic resin, urethane resin, alkyd resin, epoxy resin and the like, which have hydrophilic groups (e.g., carboxyl, hydroxyl, methylol, amino, sulfonate, polyoxyethylene bond and the like) of an amount sufficient to make the resin water-soluble or water-dispersible; and functional groups (e.g. hydroxyl) which are crosslinking reactable with crosslinking agent, can be named.

As the polyester resin, those obtained by polycondensation reaction of at least one polybasic acid selected from alicyclic polybasic acids and other polybasic acids, with at least one polyhydric alcohol selected from alicyclic polyhydric alcohols and other polyhydric alcohols. Of these polyester resins, those which are obtained using alicyclic polybasic acid and/or alicyclic polyhydric alcohol as the essential reacting components have the effect of improving chipping resistance, when they are used as the base resin.

Such alicyclic polybasic acid include compounds having at least one alicyclic structure (mainly 4- to 6-membered ring) and at least two carboxyl groups per molecule. As specific examples, cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, hexahydrotrimellitic acid, methylhexahydrophthalic acid and anhydrides of these acids can be named. Of these, cyclohexane-1,4-dicarboxylic acid is particularly preferred.

Other polybasic acids include compounds having at least two carboxyl groups per molecule. Specific examples include phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, diphenylmethane-4,4′-dicarboxylic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, HET acid, maleic acid, fumaric acid, itaconinc acid, trimellitic acid, pyromellitic acid and anhydrides thereof.

Alicyclic polyhydric alcohol includes the compounds having at least one alicyclic structure (mainly 4- to 6-membered ring) and at least two hydroxyl groups per molecule, specific examples including 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, tricyclodecanedimethanol, hydrogenated bisphenol A, hydrogenated bisphenol F, spiroglycol, dihydroxymethyltricyclodecane and the like. Of these, particularly 1,4-cyclohexanedimethanol is preferred.

Among other polyhydric alcohols, as those having two hydroxyl groups per molecule, for example, glycols such as ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, tetraethylene glycol, triethylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol, tetramethylene glycol, 3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol, neopentyl glycol, hydroxypivalic acid-neopenthyl glycol ester and the like; polylactonediols formed by adding lactones such as ε-caprolactone to these glycols; and polyester diols such as bis(hydroxyethyl)terephthalate can be named. Also as polyhydric alcohols having three or more hydroxyl groups per molecule, for example, glycerine, trimethylolpropane, trimethylolethane, diglycerine, triglycerine, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, mannitol and the like can be named.

The content(s) of alicyclic polybasic acid and/or alicyclic polyhydric alcohol in the polyester resin is generally within a range of 20-70 wt %, in particular, 30-60 wt %, inter alia, 35-50 wt %, based on the total weight of the monomers constituting the polyester resin, from the viewpoint of improving chipping resistance.

Above polyester resin can generally have a weight-average molecular weight ranging 1,000,000, preferably 2,000; a hydroxyl value ranging 30-200 mgKOH/g, preferably 50-180 mgKOH/g; and an acid value ranging 5-100 mgKOH/g, preferably 10-60 mgKOH/g.

As the acrylic resin, those produced by copolymerizing hydroxyl-containing radical-polymerizable unsaturated monomers and other radical-polymerizable unsaturated monomers by customary method (e.g., solution polymerization, emulsion polymerization and the like) can be named. The resulting acrylic resin can generally have a number-average molecular weight ranging 1,000, in particular, 2,000; a hydroxyl value ranging 20-200 mgKOH/g, in particular, 50-150 mgKOH/g; and an acid value ranging 3-100 mgKOH/g, in particular, 20-70 mgKOH/g.

As the hydroxyl-containing, radical-polymerizable unsaturated monomer, for example, besides hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hyroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate and the like, PLACCEL FM1, PLACCEL FM2, PLACCEL FM3, PLACCEL FA1, PLACCEL FA2, PLACCEL FA3 (tradename, Daicel Chemical Industries Ltd., caprolactone-modified (meth)acrylic acid hydroxyesters) can be used.

As other radical-polymerizable unsaturated monomers, for example, carboxyl-containing radical-polymerizable unsaturated monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid and the like; C1-C22 alkyl or cycloalkyl esters of (meth)acrylic acid such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate; aromatic vinyl monomers such as styrene; (meth)acrylamide such as (meth)acrylic acid amide, N-butoxymethyl (meth)acrylamide, N-methylol (meth)acrylamide and derivatives thereof; and (meth)acrylonitrile can be named.

Above-described polyester resin or acrylic resin may be used concurrently with “urethane-modified polyester resin” or “urethane-modified acrylic resin” which are prepared by extending polyisocyanate compound from a part of hydroxyl groups in the resin by urethanation reaction to impart to the resin high molecular weight.

As the polyisocyanate compound, for example, aliphatic polyisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimeric acid diisocyanate, lysine diisocyanate and the like; biuret-type adducts and isocyanurate ring adducts of these polyisocyanates; alicyclic diisocyanates such as isophorone diisocyanate, 4,4′-methylenebis(cyclohexylisocyanate), methylcyclohexane-2,4- or -2,6-diisocyanate, 1,3- or 1,4-di(isocyanatomethyl)cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate and the like biuret-type adducts and isocyanurate ring adducts of these polyisocyanate; aromatic diisocyanate compounds such as xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4′-toluydine diisocyanate, 4,4′-diphenyl ether diisocyanate, m- or p-phenylene diisocyanate, 4,4′-biphenylene diisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate, bis(4-isocyanatophenyl)-sulfone, isopropylidenebis (4-phenylisocyanate) and the like and biuret type adducts and isocyanurate ring adducts of these polyisocyanates; polyisocyanates having three or more isocyanate groups per molecule, such as triphenylmethane-4,4′,4″-triisocyanate, 1,3,5-triisocyanato-benzene, 2,4,6-triisocyanatotoluene, 4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate and the like and biuret type adducts and isocyanurate ring adducts of these polyisocyanates; and urethanation adducts formed by reacting hydroxyl groups of polyols such as ethylene glycol, propylene glycol, 1,4-butylene glycol, dimethylolpropionic acid, polyalkylene glycol, trimethylolpropane, hexanetriol and the like with polyisocyanate compounds at such ratios as will render isocyanate groups excessive, and biuret type adducts and isocyanurate ring adducts of these polyisocyanates; can be named.

These base resins can be rendered water-soluble or water-dispersible, for example, by neutralization with basic substance or acid, depending on the kind of existing hydrophilic groups. It is also possible to make the base resins water-soluble or water-dispersible, by emulsion polymerization of monomeric components in the presence of surfactant or water-soluble high molecular weight compound, in the occasion of preparing the base resins by polymerization.

As the crosslinking agent which may be contained in the first coloring paint, for example, melamine resin and blocked polyisocyanate compound can be named. As the melamine resin, for example, methylolated melamine resin formed by methylolating melamine with formaldehyde; alkylated melamine resin formed by etherifying the methylol group with monohydric alcohol; methylolated or alkylated melamine resin having imino groups; and the like can be named. Mixed alkylated melamine resin obtained by using two or more monohydric alcohols in the occasion of etherifying the methylol groups can also be used. As useful monohydric alcohol, for example, methyl alcohol, ethyl alcohol, n-propyl alcohol, i-propyl alcohol, n-butyl alcohol, i-butyl alcohol, 2-ethylbutanol, 2-ethylhexanol and the like can be named.

As specific melamine resins, methylated melamine resin, imino-containing methylated melamine resin, methylated-butylated melamine resin, imino-containing methylated-butylated melamine resin and the like are preferred, imino-containing methylated melamine resin being particularly preferred.

As commercially available products of these melamine resins, for example, Cymel 202, Cymel 232, Cymel 235, Cymel 238, Cymel 254, Cymel 266, Cymel 267, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325, Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, Cymel 736, Cymel 738, Cymel 771, Cymel 1141, Cymel 1156, Cymel 1158, and the like (tradename, Nihon Cytec Industries, Inc., Ltd.); U-Van 120, U-Van 20HS, U-Van 2021, U-Van 2028, U-Van 2061 and the like (tradename, Mitsui Chemicals, Inc.); Melan 522 and the like (tradename: Hitachi Chemical) and the like, can be named.

Blocked polyisocyanate compounds are polyisocyanates having at least two free isocyanate groups per molecule, whose isocyanate groups are blocked with blocking agent. As the polyisocyanate compounds, for example, aliphatic polyisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimeric acid diisocyanate, lysine diisocyanate and biuret type adducts or isocyanurate ring adducts of these polyisocyanates; alicyclic diisocyanates such as isophorone diisocyanate, 4,4′-methylenebis-(cyclohexylisocyanate), methylcyclohexane-2,4-(or-2,6-)diisocyanate, 1,3-(or 1,4-)di(isocyanatomethyl)cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cycopentane diisocyanate, 1,2-cyclohexane diisocyanate and biuret type adducts or isocyanurate ring adducts of these polyisocyanates; aromatic diisocyanates such as xylylene diisocyanate, meta-xylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4′-toluydine diisocyanate, 4,4′-diphenylether diisocyanate, (m- or p-)phenylene diisocyanate, 4,4′-biphenylene diisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate, bis(4-isocyanatephenyl)sulfone, isopropyridenebis(4-phenylisocyanate) and biuret type adducts or isocyanurate ring adducts of these polyisocyanates; polyisocyanates having three or more isocyanate groups per molecule such as triphenylmethane-4,4′,4″-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanato-toluene, 4,4′-dimethyldiphenylmethane-2,2′, 5,5′-tetraisocyanate and biuret type adducts or isocyanurate ring adducts of these polyisocyanates; urethanation adducts formed by reacting polyisocyanate compounds with polyols such as ethylene glycol, propylene glycol, 1,4-butylene glycol, dimethylolpropionic acid, polyalkylene glycol, trimethylolpropane, hexanetriol and the like, at such ratios that the isocyanate groups are in excess of the hydroxyl groups of such polyols, and biuret type adducts and isocyanurate ring adducts of these polyisocyanates, and the like can be named.

Blocking agent is to block free isocyanate groups. Formed blocked polyisocyanate compounds are stable at normal temperature but when heated, for example, to 100° C. or above, preferably to 130° C. or above, the blocking agent is dissociated to regenerate the free isocyanate groups which can readily react with hydroxyl groups.

The blocking agent may be of glycolic acid ester type, alcohol type, oxime type, active methylene type, mercaptan type, acid amide type, amine type, imidazole type, carbamic acid ester type or sulfite type. Besides those, 3,5-dimethylpyrazole, 3-methylpyrazole, 4-nitro-3,5-dimethylpyrazole and 4-bromo-3,5-dimethylpyrazole and the like can also be used as the blocking agent.

As the blocking agent for blocking isocyanate groups of polyisocyanate compounds, hydroxycarboxylic acid having at least one hydroxyl group and at least one carboxyl group, such as hydroxypivalic acid, dimethylolpropionic acid and the like can also be used. By neutralizing the carboxyl group(s) in the hydroxycarboxylic acid, blocked polyisocyanate compound imparted with water dispersibility can be obtained. As commercially available blocked polyisocyanate compounds, for example BAYHYDROL BL 5140 (tradename, Sumitomo Bayer Urethane, Ltd.) can be named.

The first coloring paint (B) can further be suitably blended with aqueous dispersion of urethane resin which includes the products obtained by reacting aliphatic and/or alicyclic diisocyanate, at least one diol selected from polyether diols, polyester diols and polycarbonate diols having number-average molecular weight ranging 500-5,000, low molecular weight polyhydroxy compound and dimethylolalkane acid in an aqueous medium. The urethane resin can have a number-average molecular weight (cf. note 3) generally within a range of 2,000, preferably 4,000, inter alia, 5,000, for favorable chipping resistance and surface smoothness of the coating film.

As commercially available aqueous dispersions of such urethane resins, for example, U-COAT UX497, U-COAT UX4300, U-COAT UX5000, U-COAT UX8100 (tradename, Sanyo Chemical Industries), NEOREZ R-940, R-941, R-960, R-962, R-966, R-967, R-962, R-9603, R-9637, R-9618, R-9619, XR-9624, VONDIC 1310NSC (trademames, ICI), HYDRAN HW-310, HW-311, HW-312B, HW-301, HW-111, HW-140, HW-333, HW-340, HW-350, HW-910, HW-920, HW-930, HW-935, HW940, HW-960, HW-970, HW-980, AP-10, AP-20, AP-30, AP-40, AP-60, AP-70 and AP-60LM (tradename, Dainippon Ink & Chemicals, Inc.) can be named.

Where necessary, the first coloring paint can be further suitably blended with coloring pigment, iridescent pigment, extender, dispersant, antisettling agent, organic solvent, urethanation reaction accelerating catalyst (e.g., organotin compound or the like), catalyst for accelerating the crosslinking reaction of hydroxyl groups in the base resin with melamine resin (e.g., acid catalyst), defoaming agent, thickener, anti-rusting agent, UV absorber, surface regulating agent and the like.

The first coloring paint (B) can be formulated by dissolving or dispersing those components as described in the foregoing in aqueous medium by per se known means. After adjusting the viscosity to 50 seconds Ford Cup #4 at 20° C., and the solid concentration, within a range of 20-70 wt %, preferably 35-60 wt %, the paint (B) can be applied onto cured electrocoated film of the electrodeposition paint (A).

The first coloring paint (B) can be applied by per se known means, for example, by air spray, airless spray, electrostatic coating and the like, and the coated film thickness can be normally within a range of 10-100 μm, preferably 10-35 μm, in terms of dry coating film thickness.

The coated article is normally pre-heated either directly or indirectly in a drying oven at about 60-about 120° C., preferably about 70-about 110° C., for about 1-60 minutes or the coated surface can be set in an atmosphere of ambient temperature or from about 25° C.-about 80° C.

Second Coloring Paint (C)

According to the method of the present invention, then a second coloring paint (C) is applied onto an uncured coating film of the first coloring paint (B). As the second coloring paint (C), either of organic solvent-based coloring paint or water-based coloring paint can be used, while from the standpoint of reducing volatile organic compound, water-based coloring paint is preferred.

As the second coloring paint (C), for example, those comprising base resin such as polyester resin, acrylic resin, alkyd resin, urethane resin, epoxy resin and the like which have crosslinkable functional groups such as carboxyl, hydroxyl and the like and which are similar to those described as to the first coloring paint (B); crosslinking agent such as optionally blocked polyisocyanate compound, melamine resin, urea resin and the like, similar to those described as to the first coloring paint (B); and optionally pigment, defoaming agent, thickener, anti-rusting agent, UV absorber, surface regulating agent and the like, where necessary, can be used.

The second coloring paint (C) can be applied by per se known means, for example, by air spray, airless spray, electrostatic coating and the like, and the coated film thickness can be normally within a range of 5-40 μm, preferably 10-30 μm, in terms of dry coating film thickness.

The coating film after application can be suitably pre-heated and/or set. The pre-heating can be normally conducted by heating the coated article either directly or indirectly in a drying oven at about 60-about 120° C., preferably about 70-about 110° C., for about 1-60 minutes. The setting of the coated surface can be conducted in an atmosphere of ambient temperature or from about 25° C.-about 80° C.

Clear Paint (D)

On the coating film of the second coloring paint (C) formed as above, further a clear paint (D) is coated. As the clear paint (D), for example, organic solvent-based or water-based clear paint (D) which are customarily used for coating automobile bodies can be used.

More specifically, organic solvent-based paints or water-based paints which contain base resin such as acrylic resin, polyester resin, alkyd resin, urethane resin, epoxy resin and the like, having crosslinkable functional groups such as hydroxyl, carboxyl, epoxy and the like; and crosslinking agent such as melamine resin, urea resin, optionally blocked polyisocyanate compound, carboxyl-containing compound or resin, epoxy-containing compound or resin and the like, can be used.

The clear paint (D) may contain, where necessary, coloring pigment and/or iridescent pigment to an extent as will not impair transparency of the coating film. Furthermore, extender, UV absorber, and the like may also be suitably contained.

The clear paint (D) can be applied onto the second coloring paint (C)-coated surface by the means known per se, for example, electrostatic coating, airless spraying or air spraying, to a dry coating film thickness within a range of 10-60 μm, preferably 25-50 μm.

Baking of Coating Film

The multi-layered coating film composed of the three layers of uncured coating films of the first coloring paint (B), second coloring paint (C) and clear paint (D) can be simultaneously cured by ordinary coating film-baking means, for example, heating by hot air heating, IR ray heating or induction heating, at about 80-about 170° C., preferably about 120-about 160° C., for about 20-40 minutes, whereupon providing multi-layered film excelling in appearance, corrosion resistance, anti-chipping property and the like.

Furthermore, also by 4-coat-1-bake system in which the electrocoated film of the electrodeposition paint (A) is not baked and dried, but left uncured and subjected to setting, air-blowing or pre-heating, and onto which the first coloring paint (B), second coloring paint (C) and clear paint (D) are successively applied, and the four-layered coating film composed of those of the electrodeposition paint (A), first coloring paint (B), second coloring paint (C) and clear paint (D) is given single time baking, better appearance and corrosion resistance than those of the conventional cases of using amine-added epoxy resin-blocked isocyanate crosslinking type electrodeposition paint can be secured.

EXAMPLES

Hereinafter the invention is explained more specifically, referring to working Examples, it being understood that the invention is not limited to these Examples only. In the Examples, “part” and “%” are by weight.

Preparation of Electrodeposition Paint

Production Example 1 Preparation of Base Resin No. 1

A flask equippel with a stirrer, thermometer, dropping funnel and a reflux condenser was charged with 155 parts of EHPE-3150 (note 5), 70 parts of diethanolamine and the whole amount of the following phenolic hydroxyl-containing product, which were reacted at 160° C. for 5 hours. Thereafter 692 parts of methyl propanol was added to provide base resin No. 1 having a hydroxyl equivalent of 443, amine value of 63 mgKOH/g and a solid content of 60%.

Phenolic Hydroxyl-Containing Compound:

    • A product obtained by mixing 475 parts of bisphenol A diglycidyl ether having an epoxy equivalent of 190, 285 parts of bisphenol A, 53 parts of diethanolamine and 80 parts of carbitol, dissolving the mixture by heating and causing the reactants to react by maintaining them at 130° C. for 3 hours.
    • (Note 5) EHPE-3150: tradename, Daicel Chemical Industries, Ltd., an epoxy resin having at least two epoxy-containing functional groups per molecule on the average, in which epoxy groups are bound to alicyclic skeletal structure: epoxy equivalent=180.

Production Example 2 Preparation of Curing Agent No. 1

Two (2) parts of azobisdimethylvaleronitrile was dissolved in 33.4 parts of CYCLOMER M100 (3,4-epoxycyclohexylmethyl methacrylate). The solution was dropped into a 100° C. mixed solvent of 10 parts of methyl isobutyl ketone and 10 parts of ethylene glycol monobutyl ether over 2 hours, aged for an hour, given a temperature raise to 125° C. and aged for an additional hour to provide curing agent No. 1 having an epoxy equivalent of 196 and solid content of 60%.

Production Example 3 Preparation of Base Resin No. 2 (for Comparative Example)

To 1010 parts of EPICOAT 828 EL (tradename, Japan Epoxy Resin Co., an epoxy resin), 390 parts of bisphenol A and 0.2 part of dimethylbenzylamine were added, and reacted at 130° C. until the epoxy equivalent reached 800.

Then 74 parts of dimethylolbutyric acid, 63 parts of diethanolamine and 95 parts of ketiminated product of diethylenetriamine were added and reacted at 120° C. for 4 hours, followed by addition of 330 parts of ethylene glycol monobutyl ether. Thus base resin No. 2 having an amine value of 43 mgKOH/g and solid content of 80% was obtained.

Production Example 4 Preparation of Curing Agent No. 2 (for Comparative Example)

To 270 parts of COSMONATE M-200 (tradename, Mitsui Chemicals, Inc., crude MDI), 46 parts of methyl isobutyl ketone was added and heated to 70° C. Further 281 parts of diethylene glycol monobutyl ether was slowly added and the temperature was raised to 90° C. While maintaining this temperature, the reaction was continued until disappearance of absorption by unreacted isocyanate was confirmed by infrared absorption spectrum measurement of the samples taken with time at time intervals. Adjusting the amount of the organic solvent, curing agent No. 2 having a solid content of 60% which was a blocked polyisocyanate compound was obtained.

Production Example 5 Preparation of Zirconium-Treated Titanium Dioxide

Into 95 parts of hydrated titanium dioxide slurry (equivalent to 10 parts of TiO2) which was obtained by hydrolyzing titanyl sulfate solution by heating, filtering and washing according to accepted practice, 7.3 parts of 48% aqueous caustic soda solution was thrown under stirring, followed by 2 hours' aging at 95° C. Then the caustic soda-treated product was washed, and into the resulting 205 parts of slurry, 48 parts of 35% hydrochloric acid was thrown under stirring, followed by 2 hours' aging under heating at 95° C. to provide titania sol. So obtained titania sol was filtered, washed and dried at 150° C. for 30 minutes to provide rutile type fine particulate titanium dioxide having an average particle size of 0.015 μm. Into 100 parts of water, 9.7 parts of above fine particulate titanium dioxide was thrown and further 2.8 parts of an aqueous zirconium sulfate solution containing 15% of zirconium sulfate as converted to ZrO2 was thrown thereinto under stirring. By neutralizing the system with 25% aqueous ammonia, 1.0% as converted to ZrO2 (based on TiO2) of oxide hydrate of zirconium was deposited on surface of the fine particulate titanium dioxide, followed by heating at 80° C. for 30 minutes, filtration, washing, and drying at 150° C. for 30 minutes. Thus obtained dry product was calcined at 500° C. for 3 hours and pulverized in an energy mill to provide 9.0 parts of rutile type fine particulate titanium dioxide composition having an average particle size of 0.028 μm with their surface coated by 4.0% as converted to ZrO2 (based on TiO2) of zirconium oxide.

Production Example 6 Preparation of Emulsion No. 1 (for Comparative Example)

The base resin No. 1 as obtained in Production Example 1 (117 parts; solid content, 70 parts), 37.5 parts (solid content, 30 parts) of EHPE-3150 (cf. note 5) having a solid content of 80% as dissolved in ethylene glycol monobutyl ether, and 7 parts of 10% formic acid were mixed and stirred to homogeneity, into which 132.5 parts of deionized water was dropped over about 15 minutes under violent stirring, to provide emulsion No. 1 having a solid content of 34%.

Production Example 7 Preparation of Emulsion No. 2 (for Comparative Example)

The base resin No. 1 as obtained in Production Example 1 (117 parts; solid content, 70 parts), 37.5 parts (solid content, 30 parts) of EHPE-3150 (cf. note 5) having a solid content of 80% as dissolved in ethylene glycol monobutyl ether, 6 parts (as solid content) of Nikka Octics Bismuch (note 6) and 7 parts of 10% formic acid were mixed and stirred to homogeneity, into which 142.0 parts of deionized water was dropped over about 15 minutes under violent stirring, to provide emulsion No. 2 having a solid content of 34%.

    • (Note 6) Nikka Octics Bismuth: tradename, Nippon Kagaku Sangyo Co., Ltd., bismuth octanoate

Production Example 8 Preparation of Emulsion No. 3 (for Comparative Example)

The base resin No. 1 as obtained in Production Example 1 (117 parts; solid content, 70 parts), 50 parts (solid content, 30 parts) of the curing agent No. 1 as obtained in Production Example 2 and 7 parts of 10% formic acid were mixed and stirred to homogeneity, into which 120 parts of deionized water was dropped over about 15 minutes under violent stirring, to provide emulsion No. 3 having a solid content of 34%.

Production Example 9 Preparation of Emulsion No. 4

The base resin No. 2 as obtained in Production Example 3 (87.5 parts; solid content, 70 parts), 50 parts (solid content, 30 parts) of curing agent No. 2 as obtained in Production Example 4 and 7 parts of 10% formic acid were mixed and stirred to homogeneity, into which 149.5 parts of deionized water was dropped over about 15 minutes under violent stirring to provide emulsion No. 4 having a solid content of 34%.

The blended compositions of the emulsion Nos. 1-4 are collectively shown in Table 1.

TABLE 1 Production Production Production Production Example 6 Example 7 Example 8 Example 9 Emulsion No. 1 No. 2 No. 3 No. 4 Base base resin No. 1 117 117 117 Resin solid content 60% (70) (70) (70) base resin No. 2 87.5 solid content 80% (70) Cross- EHPE-3150 (cf. note 5) 37.5 37.5 linking solid content 80% (30) (30) Agent curing agent No. 1 50 solid content 60% (30) curing agent No. 2 50 solid content 60% (30) Catalyst Nikka Octics Bismuth (note 6) 8.5 solid content 70%  (6) Neutralizing 10% formic acid 7 7 7 7 agent Deionized water 132.5 142.0 120.0 149.5 34% Emulsion 294 312 294 294 (100)  (106)  (100)  (100) 

Production Example 10 Preparation of Pigment Dispersion Paste No. 1

To 8.33 parts (solid content, 5 parts) of base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of JR-600E (note 7), 1 part of Carbon MA-7 (note 8), 10 parts of Hydride PXN (note 9), 1 part of copper hydroxide, 3 parts of bismuth hydroxide and 7.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 1 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 1 was 80 wt %.

    • (note 7) JR-600E: tradename, Tayca Corporation, titanium white; Al2O3 coating, 3.8 mass %
    • (note 8) Carbon MA-7: tradename, Mitsubishi Chemical Co. Ltd., carbon black
    • (note 8) Hydride PXN: tradename, Georgia Kaolin Co., kaolin

Production Example 11 Preparation of Pigment Dispersion Paste No. 2

To 8.33 parts (solid content, 5 parts) of base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of JR-603 (note 10), 1 part of Carbon MA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 1 part of copper hydroxide, 3 parts of bismuth hydroxide and 7.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 2 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 2 was 80 wt %.

(note 10) JR-603: tradename, Tayca Corporation, titanium white, ZrO2 0.5 mass %, Al2O3 coating, 4.6 mass %.

Production Example 12 Preparation of Pigment Dispersion Paste No. 3

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of zirconium-treated titanium dioxide as obtained in Production Example 5, 1 part of Carbon MA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 1 part of copper hydroxide, 3 parts of bismuth hydroxide and 7.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 3 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 3 was 80 wt %.

Production Example 13 Preparation of Pigment Dispersion Paste No. 4

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of JR-600E (cf. note 7), 1 part of Carbon MA-7 (cf. note 8), 7 parts of Hydride PXN (cf. note 9), 3 parts of Talc MV (note 11), 1 part of copper hydroxide, 3 parts of bismuth hydroxide and 7.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 4 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 4 was 80 wt %.

(note 11) Talc MV: tradename, United Siera Divi. Co.; talc

Production Example 14 Preparation of Pigment Dispersion Paste No. 5

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of JR-600E (cf. note 7), 1 part of Carbon MA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 3 parts of bismuth hydroxide and 6.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 5 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 5 was 80 wt %.

Production Example 15 Preparation of Pigment Dispersion Paste No. 6

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 as obtained in Production Example 1, 4.4 parts of 10% formic acid was added, and to which 15 parts of deionized water was added under stirring. Further 10 parts of JR-600E(cf. note 7), 1 part of Carbon MA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 3 parts of bismuth hydroxide 1 part of dioctyltin oxide and 7.3 parts of deionized water were added and mixed, and dispersed in a ball mill for 24 hours to provide pigment dispersion paste No. 6 having a solid content of 50.0 wt %. The pigment content (cf. note 4) of the pigment dispersion paste No. 6 was 80 wt %.

The blended compositions of the pigment dispersion paste Nos. 1-6 of Production Examples 10-15 are collectively shown in Table 2.

TABLE 2 Production Production Production Production Production Production Example 10 Example 11 Example 12 Example 13 Example 14 Example 15 Pigment Dispersion Paste No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Pigment-Dispersing solid content 80% 8.33 8.33 8.33 8.33 8.33 8.33 Resin base resin No. 1  (5)  (5)  (5)  (5)  (5)  (5) Neutralizer 10% formic acid 4.4 4.4 4.4 4.4 4.4 4.4 deionized water 15 15 15 15 15 15 JR-600E (cf. note 7) 10 10 10 10 JR-603 (cf. note 10) 10 Zirconium-treated titanium 10 dioxide Carbon MA-7 (note 8) 1 1 1 1 1 1 Hydride PXN (note 9) 10 10 7 7 10 10 Talc MV (note 11) 3 3 copper hydroxide 1 1 1 1 bismuth hydroxide 3 3 3 3 3 3 dioctyltin oxide 1 deionized water 7.3 7.3 7.3 7.3 6.3 7.3 50% Pigment Dispersion Paste 60 60 60 60 58 60 (30) (30) (30) (30) (29) (30) Pigment Content % cf. note 4) 80 80 80 80 79 80

Production Example 16 Preparation of Electrodeposition Paint No. 1

To 294 parts (solid content, 100 parts) of 34% emulsion No. 1 as obtained in Production Example 6, 60 parts (solid content, 30 parts) of 50% pigment dispersion paste No. 1 as obtained in Production Example 10 and 296 parts of deionized water were added to provide electrodeposition paint No. 1 having a solid content of 20%. The pigment content (cf. note 4) of the electrodeposition paint No. 1 was 18.5%.

Production Examples 17-24 Preparation of Electrodeposition Paint Nos. 2-9

Electrodeposition paint Nos. 2-9 were prepared each having the blended composition as shown in the following Table 3, in the same manner as Production Example 16.

TABLE 3 Production Production Production Production Production Production Production Production Production Example Example Example Example 16 Example 17 Example 18 Example 19 Example 20 Example 21 22 23 24 Electrodeposition Paint No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 Paint Emulsion No. 1 294 294 294 294 294 Blend (100) (100) (100) (100) (100)   Emulsion No. 2 312 (106) Emulsion No. 3 294 (100) Emulsion No. 4 294 294 (100) (100)   Pigment Dispersion 60 60 90.4 Paste No. 1  (30)  (30)  (45.2) Pigment Dispersion 60 Paste No. 2  (30) Pigment Dispersion 60 Paste No. 3  (30) Pigment Dispersion 60 Paste No. 4  (30) Pigment Dispersion 58 Paste No. 5  (29) Pigment Dispersion 60 90.4 Paste No. 6  (30)  (45.2) Deionized Water 296 590 296 296 617 296 341.6 296 341.6 20% Bath 650 650 650 650 675 650 726 650 726 (130) (130) (130) (130) (135) (130) (145.2) (130) (145.2) Pigment Content (%) of 18.5 18.5 18.5 18.5 18.5 18.5 25 18.5 25 Electrodeposition Paint (note 4)

Production Example 25 Preparation of Aqueous Acrylic Resin Dispersion

A four-necked flask equipped with a stirrer, thermometer, reflux tube and a nitrogen-inlet tube was charged with 40 parts of deionized water and 0.8 part of an anionic surfactant (tradename: Newcol 707SF, Japan Nyukazai Co. Ltd.; non-volatile component=30%), which were stirred after nitrogen substitution of the atmosphere while being maintained at 82° C. Into the flask, first a mixture of 5 parts of an emulsified “monomeric mixture” as identified in the following and 0.3 part of ammonium perfulfate which were dissolved in 3 parts of deionized water was added, and in 20 minutes thereafter the remaining “monomeric mixture” and 0.3 part of ammonium persulfate as dissolved in 3 parts of deionized water was dropped over 4 hours to carry out emulsion polymerization.

“Monomeric Mixture”:

    • A monomeric mixture formed by stirring and emulsifying 54 parts of deionized water, 0.5 part of Newcol 707SF, 45 parts of ethyl acrylate, 48 parts of methyl methacrylate, 5 parts of hydroxyethyl acrylate, 1 part of acrylic acid and 1 part of allyl methacrylate.

After the dropping was terminated, the emulsion polymerization was continued for further 2 hours while maintaining the temperature of 82° C., and then the temperature inside the flask was dropped to 40° C. Adjusting the pH to 8.5 with aqueous ammonia, an acrylic resin emulsion having a solid content of 50 wt % was obtained. The average particle size of the emulsified resin was 0.15 μm, and the resin had an acid value of 7.8 mgKOH/g.

Production Example 26 Preparation of Water-Based Clear Paint

To 39 parts of the aqueous acrylic resin dispersion as obtained in Production Example 25, 15 parts of Cymel 325 (note 12), 9 parts of ethylene glycol monobutyl ether and 1.4 parts of N,N-dimethylaminoethanol were added and mixed by stirring. Then 3.2 parts (active component, 0.8 part) of Nacure 4167 (tradename, King Industries Co.; a phosphoric acid-derived acid catalyst; active component, 25%) was added and mixed by stirring to homogeneity, and deionized water was slowly added under continued stirring to provide a water-based clear paint having a solid content of 40%.

    • (note 12) Cymel 325: tradename, an imino-containing methylated melamine resin, Nippon Cytec Industries, Ltd.
      Test Sheet

Cold-drawn steel sheet (150 mm-long, 70 mm-wide and 0.8 mm-thick) was given a chemical treatment (PALBOND #3020, tradename, Nihon Parkerizing Co.; a zinc phosphate treating agent) to serve as the test sheet.

Examples and Comparative Examples Example 1 Formation of Multi-Layered Coating Film No. 1

A multi-layered coating film No. 1 was prepared by the following steps.

Step 1: The test sheet was maintained horizontally and electrocoated with the electrodeposition paint No. 1. The resulting coating film was heated at 170° C. for 20 minutes, to provide a coated sheet having a 20 μm-thick electrocoated film in terms of cured film thickness.

Step 2: WP-300T (tradename, Kansai Paint, a water-based first coloring paint) was spray-coated onto the electrocoated sheet to a thickness as would provide cured coating film thickness of 30 μm, allowed to stand at room temperature for 3 minutes, and thereafter pre-heated at 80° C. for 10 minutes.

Step 3: WBC-713T (tradename, Kansai Paint; a water-based second coloring paint) was further spray-coated on the above coating film surface to a thickness as would provide a cured coating film thickness of 15 μm, allowed to stand at room temperature for 3 minutes, and thereafter pre-heated at 80° C. for 10 minutes.

Step 4: Successively, onto the above coated surface KINO #1200 TW (tradename, Kansai Paint; an organic solvent-based clear paint) was spray-coated to a thickness as would provide a cured coating film thickness of 35 μm, and allowed to stand at room temperature for 5 minutes. Then the three-layered coating film as formed in above Steps 2-4 was baked at 140° C. for 30 minutes to provide cured multi-layered coating film No. 1.

Example 2-10

Multi-layered coating film Nos. 2-10 of Examples 2-10 were obtained through similar operations to those of Example 1 (cured film thickness was same for all runs), except that the steps were changed as indicated in the following Table 4.

The results of performance evaluation of the multi-layered coating films as obtained in Examples 1-10 are concurrently shown in Table 4.

Comparative Example 1

A multi-layered coating film No. 11 was prepared by the following steps.

Step 1: The test panel was maintained horizontally and electrocoated with the electrodeposition paint No. 8. The resulting coating film was heated at 170° C. for 20 minutes, to provide a coated sheet having a 20 μm-thick electrocoated film in terms of cured film thickness.

Step 2: WP-300T (tradename, Kansai Paint; a water-based first coloring paint) was spray-coated onto the electrocoated sheet to a thickness as would provide cured coating film thickness of 30 μm, allowed to stand at room temperature for 3 minutes, and thereafter pre-heated at 80° C. for 5 minutes.

Step 3: WBC-713T (tradename, Kansai Paint; a water-based second coloring paint) was further spray-coated on the above coating film surface to a thickness as would provide a cured coating film thickness of 15 μm, allowed to stand at room temperature for 3 minutes, and thereafter pre-heated at 80° C. for 5 minutes.

Step 4: Successively, onto the above coated surface KINO #1200 TW (tradename, Kansai Paint; an organic solvent-based clear paint) was spray-coated to a thickness as would provide a cured coating film thickness of 35 μm, and allowed to stand at room temperature for 5 minutes. Then the three-layered coating film as formed in above Steps 2-4 was baked at 140° C. for 30 minutes to provide cured multi-layered coating film No. 11.

Comparative Examples 2-4

Comparative Example 1 was repeated except that the steps were changed in each run as shown in the following Table 5, to provide multi-layered coating film Nos. 12-14.

The results of performance evaluation of the multi-layered coating films as obtained in Comparative Examples 1-4 are concurrently shown in Table 5.

TABLE 4 Example 1 Example 2 Example 3 Example 4 Example 5 Multi-layered Coating Film No. 1 No. 2 No. 3 No. 4 No. 5 Step 3C1B 3C1B 3C1B 3C1B 3C1B Step Step 1 Electrodeposition paint No. 1 No. 2 No. 3 No. 4 No. 5 Baking or setting 170° C. 170° C. 170° C. 170° C. 170° C. 20 min. 20 min. 20 min. 20 min. 20 min. Step 2 First coloring paint WP-300T WP300T WP-300T WP-300T WP-300T Pre-heating 80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 3 Second coloring paint WBC-713T WBC-713T WBC-713T WBC-713T WBC-713T Pre-heating 80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 4 Clear paint KINO # KINO # KINO # KINO # KINO # 1200TW 1200TW 1200TW 1200TW 1200TW Baking (temperature-time) 140° C. 140° C. 140° C. 140° C. 140° C. 30 min. 30 min. 30 min. 30 min. 30 min. Electrodeposited Heat loss of electrocoated film (note 14) 2.7 2.7 2.7 2.7 2.7 Coating Power spectral value(note 15) average 47 36 35 47 40 Film value Alone integrated 1.2 × 105 9.1 × 104 8.9 × 104 1.2 × 105 1.0 × 105 value Multi- Appearance of multi- (note 16) ◯ ◯ layered layered coating film coating Corrosion resistance (note 17) ◯ ◯ ◯ ◯ ◯ film Anti-chipping property (note 18) ◯ ◯ ◯ ◯ Example 6 Example 7 Example 8 Example 9 Example 10 Multi-layered Coating Film No. 6 No. 7 No. 8 No. 9 No. 10 Step 3C1B 3C1B 3C1B 3C1B 4C1B Step Step 1 Electrodeposition paint No. 6 No. 7 No. 1 No. 1 No. 1 Baking or setting 170° C. 170° C. 170° C. 170° C. 25° C. 20 min. 20 min. 20 min. 20 min. 20 min. Step 2 First coloring paint WP-300T WP-300T WP-300T TP-65-2 WP-300T (note 13) Pre-heating 80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 3 Second coloring paint WBC-713T WBC-713T WBC-713T WBC-713T WBC-713T Pre-heating 80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 4 Clear paint KINO # KINO # water-based KINO # KINO # 1200TW 1200TW clear paint 1200TW 1200TW Production Example 26 Baking (temperature-time) 140° C. 140° C. 140° C. 140° C. 140° C. 30 min. 30 min. 30 min. 30 min. 30 min. Electrodeposited Heat loss of electrocoated film (note 14) 2.8 2.0 2.7 2.7 2.7 Coating Power spectral value(note 15) average 52 60 47 47 47 Film value Alone integrated 1.3 × 105 01.5 × 105 1.2 × 105 1.2 × 105 1.2 × 105 value Multi- Appearance of multi- (note 16) ◯ ◯ ◯ ◯ ◯ layered layered coating film coating Corrosion resistance (note 17) ◯ ◯ ◯ ◯ film Anti-chipping property (note 18) ◯ ◯ ◯ ◯ ◯

(note 13) TP-65-2: tradename, Kansai Paint, an organic solvent based first coloring paint

TABLE 5 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Multi-layered Coating Film No. 11 No. 12 No. 13 No. 14 Step 3C1B 3C1B 3C2B 4C1B Step Step 1 Electrodeposition No. 8 No. 9 No. 8 No. 8 paint Baking or setting 170° C. 170° C. 170° C. 25° C. 20 min. 20 min. 20 min. 20 min. Step 2 First coloring paint WP-300T WP300T WP-300T WP-300T Pre-heating 80° C. 80° C. 140° C. 80° C. (temperature-time) 5 min. 5 min. 30 min. 5 min. Step 3 Second coloring WBC-713T WBC-713T WBC-713T WBC-713T paint Pre-heating 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. Step 4 Clear paint KINO # KINO # KINO # KINO # 1200TW 1200TW 1200TW 1200TW Baking 140° C. 140° C. 140° C. 140° C. (temperature-time) 30 min. 30 min. 30 min. 30 min. Electrodeposited Heat loss of electrocoated 8.1 7.8 8.1 8.1 Coating Film film (note 14) Alone Power spectral average 180 200 180 180 value (note 15) value integrated 4.5 × 105 5.1 × 105 4.5 × 105 4.5 × 105 value Multi-layered Appearance of (note 16) Δ X Δ X coating film multi-layered coating film Corrosion (note 17) ◯ ◯ ◯ Δ resistance Anti-chipping (note 18) ◯ Δ ◯ Δ property
    • (note 14) Heat loss of electrocoated film: (cf. note 1)
    • (note 15) Power spectral value:
      • Following the measuring method as described in (note 2), the values measured with SURFCOM 130A (tradename, Tokyo Seimitsu Co. Ltd.) at a measuring length 50 mm and data-sampling intervals of 10 μm
    • (note 16) Appearance of multi-layered coating film:
      • Wavescan Plus (tradename, BYK Gardner Co.) was used. While specular gloss meters measure images, Wavescan Plus focuses on coating film surface, which applies laser beam generated from the measuring instrument to coating film surface and minutely detects intensity of reflected light. Through the intensity data of reflection light, optical unevenness of coating film surface can be observed at a level close to visual observation.
      • As the wavelength structures, two kinds of long wave values (LW) of long wavelength structure and short wave values (SW) of short wavelength structure can be measured. Lower numeral values signify more favorable level of multi-layered coating film appearance:
      • {circle around (•)} short wave value (SW) less than 12
      • ◯ short wave value (SW) 12-less than 15
      • Δ short wave value (SW) 15-less than 20
      • x short wave value (SW) exceeds 20.
    • (note 17) Corrosion resistance:
      • Formed multi-layered coating film was cross-cut to the depth reaching the substrate surface, and was given a saline solution spray test for 840 hours following JIS Z-2371. Corrosion resistance was evaluated by the following standard according to width of rust and blister development from the knife cuts:
    •  {circle around (•)}: the maximum width of rusting and blistering from the cuts was less than 1.5 mm (single side);
    •  ◯: the maximum width of rusting and blistering from the cuts was no less than 1.5 mm but less than 2.5 mm (single side);
    •  Δ: the maximum width of rusting and blistering from the cuts was no less than 2.5 mm but less than 3.5 mm (single side);
    •  x: the maximum width of rusting and blistering from the cuts was 3.5 mm or more (single side).
    • (note 18) Anti-chipping property:
      • The test sheet on which a multi-layered coating film was formed was fixed on test piece-holding stand of a chipping tester (Flying Stone Tester, JA-400 Model, Suga Tester Co.) with its coated surface positioned at a right angle with the mouth of the stone nozzle. Fifty (50) g of granite rubbles of grain size No. 7 was blown at the coated surface at −20° C., with compressed air of 0.294 MPa (3 kgf/cm2). Thereafter an adhesive fabric tape (Fuji Industries) was stuck on the coated surface and rapidly peeled off. The extent of cracks occurring on the coating film was visually observed and anti-chipping property was evaluated according to the following standard.
        • {circle around (•)}: The crack size was considerably small, which occurred at a part of the second coloring coating film.
        • ◯: The crack size was small and a part of the first coloring coating film was exposed.
        • Δ: The crack size was large, a part of the first coloring coating film was broken off, and the electrodeposited coating film or the steel sheet was exposed.
        • x: The crack size was considerably large, the first coloring coating film was notably exposed, or the first coloring coating film was broken off to expose the electrodeposited coating film or the steel sheet, markedly imparing in appearance.

Claims

1. A method of forming multi-layered coating film which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) successively wet-on-wet, onto a cured coating film of an electrodeposition paint (A) showing a heat loss (X) of not more than 5% by weight, said heat loss being calculated according to the following equation: heat loss (X)=[(Y−Z)/Y]×100

[wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film obtained by electrocoating the electrodeposition paint (A), by heating at 105° C. for 3 hours; and
Z is the weight of the cured film after heating the dry coating film at 170° C. for 20 minutes];
and heat-curing the so formed three-layered coating film simultaneously.

2. A method as set forth in claim 1, in which the heat loss (X) of the electrodeposition paint (A) is not more than 4% by weight.

3. A method as set forth in claim 1, in which the electrodeposition paint (A) comprises base resin (a) obtained through reaction of epoxy resin (a1), amine compound (a2) and phenolic compound (a3), and epoxy resin (b) as a crosslinking agent.

4. A method as set forth in claim 2, in which the epoxy resin (a1) is an epoxy resin of the following formula (1) having at least two epoxy-containing functional groups per molecule.

5. A method as set forth in claim 2, in which the epoxy resin (a1) has an epoxy equivalent within a range of 140-1,000 and a number-average molecular weight within a range of 200-50,000.

6. A method as set forth in claim 2, in which the amine compound (a2) is a primary or secondary amine compound containing primary hydroxyl group(s).

7. A method as set forth in claim 2, in which the phenolic compound (a3) contains at least one phenolic hydroxyl group per molecule.

8. A method as set forth in claim 7, in which the phenolic compound (a3) is a bisphenolic compound.

9. A method as set forth in claim 2, in which the base resin (a) has an amine value within a range of 20-150 mgKOH/g; hydroxyl value within a range of 300-1,000 mgKOH/g; and a number-average molecular weight within a range of 800-15,000.

10. A method as set forth in claim 2, in which the epoxy resin (b) is polyepoxide compound containing at least two epoxy-containing functional groups per molecule on the average, said functional group being formed of epoxy group(s) binding to alicyclic skeletal structure, or glycidyl etherified product of novolak resin.

11. A method as set forth in claim 2, in which the epoxy resin (b) is selected from the group consisting of polyepoxide compounds having recurring units of the following formula (5) polyepoxide polymers having recurring units of the following formula (6)

[in which R7 is hydrogen or methyl]
and a number-average molecular weight within a range of 3,000; and epoxy resins of the following formula (8)
[in the formula, R1 and R2 are same or different, and each stands for hydrogen, C1-C8 alkyl, aryl, aralkyl or halogen; R3 stands for hydrogen, C1-C10 alkyl, aryl, aralkyl, allyl or halogen; R4 and R5 are same or different and each stands for hydrogen, C1-C4 alkyl or glycidyloxyphenyl; R5 stands for hydrogen, C1-C10 alkyl, aryl, aralkyl, allyl or halogen; and n is an integer of 1-38].

12. A method as set forth in claim 2, in which the electrodeposition paint (A) contains 0.1-20 mass % of bismuth octanoate, based on the combined solid weight of the base resin (a) and epoxy resin (b).

13. A method as set forth in claim 2, in which the electrodeposition paint (A) contains a rutile type fine particulate titanium dioxide composition which is formed by coating surface of rutile type fine particulate titanium dioxide with 0.5-8.0% by weight (based on TiO2) of zirconium oxide as converted to ZrO2.

14. A method as set forth in claim 1, in which the electrodeposition paint (A) forms a cured electrocoated film having an average power spectral value not greater than 70, said value being obtained by power spectral frequency analysis comprising measuring surface roughness of an electrocoated film which has been cured by heating at 170° C. for 20 minutes, with a surface roughness meter over a measuring length of 50 mm at 10 μm intervals, and then Fourier converting the obtained data.

15. A method as set forth in claim 1 in which the electrodeposition paint (A) forms a cured electrocoated film having an integrated spectral power value within a wavelength range 0.02-1 mm of not greater than 1.7×105, said integrated value being obtained by power spectral frequency analysis comprising measuring surface roughness of the electrocoated film which has been cured by heating at 170° C. for 20 minutes, with a surface roughness meter over a measuring length of 50 mm at 10 μm intervals, and then Fourier converting the obtained data.

16. Articles on which multi-layered coating film is formed by any of the methods as described in claims 1-15.

Patent History
Publication number: 20070023288
Type: Application
Filed: Jul 31, 2006
Publication Date: Feb 1, 2007
Inventors: Eiji Kuwano (Hiratsuka-shi), Koji Kamikado (Yokohama-shi), Tadayoshi Hiraki (Odawara-shi)
Application Number: 11/495,602
Classifications
Current U.S. Class: 204/486.000
International Classification: C25D 13/00 (20060101);