METHOD FOR FORMING MULTILAYER COATING FILM

Abstract

Provided is a method for forming a multilayer coating film, which is capable of forming a multilayer coating film that exhibits excellent curability at low temperatures, while having excellent hardness and excellent finish appearance. A method for forming a multilayer coating film, which sequentially performs: (1) a step for forming an uncured first color coating film by applying a two-package type aqueous first color coating material (X) onto an object to be coated; (2) a step for forming an uncured second color coating film by applying a one-package type aqueous second color coating material (Y) onto the uncured first color coating film obtained in the step (1); (3) a step for forming an uncured clear coating film by applying a two-package type clear coating material (Z) onto the uncured second color coating film obtained in the step (2); and (4) a step for curing the uncured first color coating film, the uncured second color coating film and the uncured clear coating film, which are formed in the steps (1)-(3), at the same time by heating these coating films. This method for forming a multilayer coating film is configured such that the one-package type aqueous second color coating material (Y) contains a specific hydroxyl group-containing acrylic resin (Y1), a specific hydroxyl group-containing polyester resin (Y2), and a blocked polyisocyanate compound and/or a melamine resin (Y3).

Claims

1. A multilayer coating film-forming method that includes carrying out, in the following order: (1) a step of applying an aqueous two-component type first pigmented coating material (X) on an article to be coated to form an uncured first pigmented coating film, (2) a step of applying an aqueous one-component type second pigmented coating material (Y) onto the uncured first pigmented coating film obtained in step (1) to form an uncured second pigmented coating film, (3) a step of applying a two-component type clear coating material (Z) onto the uncured second pigmented coating film obtained in step (2) to form an uncured clear coating film, and (4) a step of heating the uncured first pigmented coating film, the uncured second pigmented coating film and the uncured clear coating film formed in steps (1) to (3) to simultaneously cure the coated films, wherein: the aqueous one-component type second pigmented coating material (Y) contains a hydroxyl group-containing acrylic resin (Y1), a hydroxyl group-containing polyester resin (Y2) and a blocked polyisocyanate compound and/or melamine resin (Y3), the hydroxyl group-containing acrylic resin (Y1) includes a core-shell type water-dispersible hydroxyl group-containing acrylic resin (Y11), the core-shell type water-dispersible hydroxyl group-containing acrylic resin (Y11) has a core-shell type structure, and the content ratios of each of the monomers in the total amount of the monomer components composing the core section and shell section of the core-shell type structure, based on the total mass of the monomer components composing the core section and shell section, are: 40 to 85 mass % of a polymerizable unsaturated monomer (a-1) with a homopolymer glass transition temperature of no higher than 20 C., 0.1 to 15 mass % of a hydroxyl group-containing polymerizable unsaturated monomer (a-2), 0.1 to 10 mass % of a carboxyl group-containing polymerizable unsaturated monomer (a-3) and 0 to 59.8 mass % of a polymerizable unsaturated monomer (a-4) other than the polymerizable unsaturated monomers (a-1) to (a-3), and the hydroxyl group-containing polyester resin (Y2) includes a polyester resin (Y21) having a hydroxyl value of 100 to 200 mgKOH/g and an acid value of 10 to 30 mgKOH/g.

2. The multilayer coating film-forming method according to claim 1, wherein the hydroxyl group-containing polyester resin (Y21) includes an alicyclic polybasic carboxylic acid (which includes acid anhydrides) (b-1) as the acid component of the starting material, the content of the alicyclic polybasic carboxylic acid (which includes acid anhydrides) (b-1) being at least 30 mol % based on the total amount of the acid component, and includes a diol (b-2) having a branched alkylene group and at least 5 carbon atoms, as the alcohol component of the starting material, the content of the diol (b-2) having a branched alkylene group and at least 5 carbon atoms being at least 40 mol % based on the total amount of the alcohol component.

3. The multilayer coating film-forming method according to claim 1, wherein the aqueous two-component type first pigmented coating material (X) contains a hydroxyl group-containing resin (X1) and a polyisocyanate compound (X2) at 1.1 to 2.0 equivalents of isocyanate groups in the polyisocyanate compound (X2) with respect to 1 equivalent of hydroxyl groups in the hydroxyl group-containing resin (X1).

4. The multilayer coating film-forming method according to claim 1, wherein the two-component type clear coating material (Z) contains a hydroxyl group-containing acrylic resin (Z1) and a polyisocyanate compound (Z2) at 1.1 to 2.0 equivalents of isocyanate groups in the polyisocyanate compound (Z2) with respect to 1 equivalent of hydroxyl groups in the hydroxyl group-containing acrylic resin (Z1).

Description

EXAMPLES

[0144] The present invention will now be explained in greater detail using examples and comparative examples. However, it is to be understood that the invention is not limited only to these examples. The parts and % values are all based on mass.

Production of Aqueous First Pigmented Coating Material (X)

Production Example 1

[0145] A pigment-dispersed paste (P-1)*.sup.2 at a solid content of 110.6 parts, an acryl-modified polyester resin aqueous dispersion (X1-1)*.sup.1 at a solid content of 10 parts, a hydroxyl group-containing acrylic resin aqueous dispersion (X1-2)*.sup.3 at a solid content of 15 parts, UCOAT UX-310 (trade name of Sanyo Chemical Industries, Ltd., polycarbonate-based aqueous polyurethane resin) at a solid content of 20 parts, 2-(dimethylamino)ethanol and deionized water added for adjustment of the pH, and BAYHYDUR XP-2655 (trade name of Sumika Covestro Urethane Co., Ltd., polyisocyanate compound) at 35 parts (NCO/OH ratio: 1.55), were uniformly mixed to obtain an aqueous first pigmented coating material (X-1) having a solid content of 51% and a viscosity of 20 seconds with a Ford cup No. 4 at 20 C.

Production Example 2

[0146] An aqueous first pigmented coating material (X-2) was obtained in the same manner as Production Example 1, except that the 35 parts of BAYHYDUR XP-2655 in Production Example 1 was changed to 43 parts of BAYHYDUR 3100 (trade name of Sumika Covestro Urethane Co., Ltd., polyisocyanate compound) (NCO/OH ratio: 1.55).

[0147] *1: Acryl-modified polyester resin aqueous dispersion (X1-1): After charging 85 parts of hexahydrophthalic anhydride, 51.6 parts of adipic acid, 39.1 parts of 1,6-hexanediol, 34.8 parts of neopentyl glycol, 35.3 parts of 2-butyl-2-ethyl-1,3-propanediol, 2.16 parts of maleic anhydride and 30.1 parts of trimethylolpropane in a reactor equipped with a thermometer, thermostat, stirrer, heating apparatus and rectification column, the temperature was raised to 160 C. while stirring. The temperature of the contents was then gradually increased from 160 C. to 240 C. over a period of 4 hours, and the condensation water that was generated was distilled off through the rectification column After continuing the reaction at 240 C. for 90 minutes, the rectification column was replaced with a water separator, approximately 15 parts of toluene was added to the contents, and the water and toluene were azeotropically distilled off to remove the condensation water. Measurement of the acid value was initiated 1 hour after addition of the toluene, terminating the heating upon confirming that the acid value had fallen below 3.5, and removing the toluene under reduced pressure, after which the mixture was cooled and 58 parts of dipropyleneglycol monomethyl ether was added. The mixture was cooled to 130 C., and a mixture of 9 parts of styrene, 12.6 parts of acrylic acid, 23.4 parts of 2-ethylhexyl acrylate and 2.2 parts of t-butylperoxy-2-ethyl hexanoate was added dropwise over a period of 2 hours. It was then kept at 130 C. for 30 minutes, after which 0.44 part of t-butylperoxy-2-ethyl hexanoate as an additional catalyst was added and the mixture was aged for 1 hour. After then cooling to 85 C., it was neutralized with dimethylethanolamine, deionized water was added, and the mixture was dispersed in water to obtain an acryl-modified polyester resin aqueous dispersion (X1-1) with a solid content of 35%. The obtained acryl-modified polyester resin had an acid value of 35 mgKOH/g, a hydroxyl value of 110 mgKOH/g and a number-average molecular weight of 2300.

[0148] *2: Pigment-dispersed paste (P-1): After placing the aforementioned acryl-modified polyester resin aqueous dispersion (X1-1)*.sup.1 at a solid mass of 20 parts, JR-806 (trade name of Tayca Corp., rutile titanium dioxide) at 60 parts, BLANC FIXE micro (trade name of Sachtleben Chemie Co., barium sulfate, particle size: 0.7 m) at 30 parts, Mitsubishi Carbon Black MA-100 (trade name of Mitsubishi Chemical Corp., carbon black) at 0.6 part and deionized water at 30 parts in a stirring mixing vessel, they were uniformly mixed and 2-(dimethylamino)ethanol was further added for adjustment to pH 8.0. Next, the obtained liquid mixture was placed in a wide-mouth glass bottle, glass beads of approximately 1.3 mm diameter were added as a dispersion medium, the bottle was sealed, and the mixture was dispersed for 4 hours with a paint shaker to obtain a pigment dispersion (P-1).

[0149] *3: Hydroxyl group-containing acrylic resin aqueous dispersion (X1-2): Aqueous dispersion of a core-shell type hydroxyl group-containing acrylic resin with a hydroxyl value of 65 mgKOH/g, an acid value of 13 mgKOH/g and a solid concentration of 30%.

Production of Water-Dispersible Hydroxyl Group-Containing Acrylic Resin (Y1)

Production Example 3

[0150] After charging 120 parts of deionized water and 0.52 part of ADEKA REASOAP SR-1025 (trade name of Adeka Corp., emulsifying agent, active ingredient: 25%) into a reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube and dropper, the mixture was stirred under a nitrogen stream and heated to 80 C. Next, 1% of the total monomer emulsion (1) described below and 2.2 parts of a 6% ammonium persulfate aqueous solution were introduced into the reactor, and the mixture was kept at 80 C. for 15 minutes. The remainder of the monomer emulsion (1) was then added dropwise into the reactor kept at the same temperature over a period of 3 hours, and upon completion of the dropwise addition the mixture was aged for 1 hour. Next, the monomer emulsion (2) described below was added dropwise over a period of 1 hour and aged for 1 hour, and then cooled to 30 C. while gradually adding 40 parts of a 5% 2-(dimethylamino)ethanol water-soluble solution to the reactor, to obtain a water-dispersible hydroxyl group-containing acrylic resin dispersion (Y1-1) having a mean particle diameter of 195 nm (measured at 20 C. using a COULTER N4 submicron particle size distribution analyzer (product of Beckman Coulter, Inc.) after dilution with deionized water) and a solid concentration of 30%. The obtained water-dispersible acrylic resin had a hydroxyl value of 24 mgKOH/g and an acid value of 8 mgKOH/g.

[0151] Monomer emulsion (1): Monomer emulsion (1) was obtained by mixing and stirring 54 parts of deionized water, 6.9 parts of ADEKA REASOAP SR-1025, 49 parts of 2-ethylhexyl acrylate, 23 parts of ethyl acrylate, 13 parts of methyl methacrylate, 4.5 parts of 2-hydroxyethyl methacrylate and 0.2 part of allyl methacrylate.

[0152] Monomer emulsion (2): Monomer emulsion (2) was obtained by mixing and stirring 12 parts of deionized water, 0.8 part of ADEKA REASOAP SR-1025, 0.2 part of ammonium persulfate, 3.1 parts of methyl methacrylate, 5 parts of ethyl acrylate, 1 part of 2-hydroxyethyl methacrylate and 1.2 parts of methacrylic acid.

Production Examples 4 to 8

[0153] Water-dispersible hydroxyl group-containing acrylic resin dispersions (Y1-2) to (Y1-6) were obtained by the same procedure as Production Example 3, except that the monomer compositions for monomer emulsions (1) and (2) were changed as shown in Table 1.

TABLE-US-00001 TABLE 1 Production Example 3 4 5 6 7 8 Water-dispersible hydroxyl group-containing acrylic resin aqueous dispersion name Y1-1 Y1-2 Y1-3 Y1-4 Y1-5 Y1-6 Deionized water 120 110 110 110 110 110 ADEKA REASOAP SR-1025 0.52 0.52 0.52 0.52 0.52 0.52 6% Ammonium persulfate aqueous solution 2.2 2.2 2.2 2.2 2.2 2.2 Monomer Deionized water 54 54 54 54 54 54 emulsion ADEKA REASOAP SR-1025 6.9 4.7 4.7 4.7 4.7 4.7 for core Monomer Polymerizable unsaturated Ethyl 23 39 49 46 section (1) components monomer (a-1) with acrylate homopolymer glass transition 2-Ethylhexyl 49 27 temperature of 20 C. acrylate n-Butyl 34 acrylate Hydroxyl group-containing 2-Hydroxyethyl 4.5 10 2.3 polymerizable unsaturated methacrylate monomer (a-2) Carboxyl group-containing Acrylic acid 0.5 polymerizable unsaturated monomer (a-3) Polymerizable unsaturated Methyl 13 30 19 12 15 monomer (a-4) other than methacrylate monomers (a-1) to (a-3) Styrene 7 7 7 7 45 n-Butyl 20 methacrylate 1,6-HDDA 1.5 1.5 1.5 1.5 Allyl 0.2 methacrylate Monomer Deionized water 12 39 39 39 39 39 emulsion Ammonium persulfate 0.2 0.15 0.15 0.15 0.15 0.15 for shell ADEKA REASOAP SR-1025 0.8 1.38 1.38 1.38 1.38 1.38 section (2) Monomer Polymerizable unsaturated Ethyl 5 9 15 15 components monomer (a-1) with acrylate homopolymer glass transition n-Butyl 9 temperature of 20 C. acrylate 2-Ethylhexyl 8.2 acrylate Hydroxyl group-containing 2-Hydroxyethyl 1 4.8 4.8 3.5 2 polymerizable unsaturated methacrylate monomer (a-2) Hydroxyethyl 1.9 acrylate Carboxyl group-containing Acrylic acid 1 polymerizable unsaturated Methacrylic 1.2 1.6 1.6 1.6 1.6 monomer (a-3) acid Polymerizable unsaturated Styrene 2 2 2 2 14 monomer (a-4) other than n-Butyl 4 monomers (a-1) to (a-3) methacrylate Methyl 3.1 8 3 methacrylate 5% 2-(Dimethylamino)ethanol aqueous solution 40 4 4 4 4 4 Total amount of polymerizable unsaturated monomers [parts] 100 100 100 100 100 100 Total amount of monomer component for formation of core section [parts] 89.7 77.5 77 77 78 75 Total amount of monomer component for formation of shell section [parts] 10.3 22.5 23 23 23 25 Proportion of each Polymerizable unsaturated monomer (a-1) with homo- 77 48 64 61 43 35 monomer based on total polymer glass transition temperature of 20 C. amount of polymerizable Hydroxyl group-containing polymerizable 5.5 1.9 4.8 14.8 3.5 2 unsaturated monomers unsaturated monomer (a-2) [mass %] Carboxyl group-containing polymerizable 1.2 1.6 1.6 1.6 1.6 2 unsaturated monomer (a-3) Polymerizable unsaturated monomer (a-4) 16.3 48.5 29.6 22.6 51.9 63 other than monomers (a-1) to (a-3) Solid content [%] 30 32 32 32 32 32 Acid value [mgKOH/g] 8 11 10 10 10 9 Hydroxyl value [mgKOH/g] 24 9 21 64 15 19

Production of Hydroxyl Group-Containing Polyester Resin (Y2)

Production Examples 9 to 12

[0154] After charging the acid components and alcohol components in the molar ratios listed in Table 1 into a four-necked flask equipped with a heating apparatus, stirrer, thermometer, reflux condenser and rectification column and raising the temperature to 160 C., the temperature was increased from 160 C. to 230 C. over a period of 3 hours while distilling off the generated condensation water using a rectification column, and then reaction was conducted at 230 C. for 2 hours.

[0155] The rectification column was then switched to a water separator and toluene was appropriately added, maintaining a state of circulation at 230 C., and condensation reaction was conducted while separating and distilling off the condensation water with the water separator.

[0156] When the resin acid value reached 7, the toluene was removed under reduced pressure and the mixture was cooled to 170 C. Trimellitic anhydride was added to the reaction product in the molar ratios listed in Table 1, and after addition reaction for 60 minutes at 170 C., propyleneglycol monomethyl ether was added at 10% with respect to the reaction product and the temperature was adjusted to 85 C., and then neutralization was performed with N,N-dimethylethanolamine and deionized water was further added in a gradual manner to form an aqueous dispersion, thus obtaining polyester resins (Y2-1) to (Y2-4) with solid contents of 45% (all adjusted to pH 8.5). Table 2 shows the acid values, hydroxyl values and number-average molecular weights of each of the obtained hydroxyl group-containing polyester resins (Y2-1) to (Y2-4).

TABLE-US-00002 TABLE 2 Production Example 9 10 11 12 Polyester resin (Y2) name Y2-1 Y2-2 Y2-3 Y2-4 1,2-Cyclohexane- 0.27 0.53 0.55 0.55 dicarboxylic anhydride Adipic acid 0.58 0.32 0.25 0.3 Trimethylolpropane 0.35 0.2 0.45 0.4 Neopentyl glycol 0.15 2-Butyl-2-ethyl-1,3- 0.65 0.8 0.4 0.6 propanediol Trimellitic anhydride 0.028 0.028 0.02 0.0165 (addition acid, acid component) DBR 0.85 0.85 0.8 0.85 mol % (b-1) in acid 31 60 67 63 component Hydroxyl value 142 105 195 151 (mgKOH/g) Acid value (mgKOH/g) 19 19 16 12 Number-average molecular 1400 1500 1000 1500 weight

Production of Active Methylene Blocked Polyisocyanate Compounds

Production Example 13

[0157] After charging 360 parts of SUMIDUR N-3300 (trade name of Sumika Bayer Urethane Co., Ltd., polyisocyanate containing an isocyanurate structure derived from hexamethylene diisocyanate, solid content: approximately 100%, isocyanate group content: 21.8%), 60 parts of UNIOX M-550 (product of NOF Corp., polyethyleneglycol monomethyl ether, average molecular weight: approximately 550) and 0.2 part of 2,6-di-tert-butyl-4-methylphenol into a reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, dropper and simple trap for the removed solvent, the contents were thoroughly mixed and heated under a nitrogen stream at 130 C. for 3 hours. Next, 110 parts of ethyl acetate and 252 parts of diisopropyl malonate were charged in, 3 parts of a 28% methanol solution of sodium methoxide was added while stirring under a nitrogen stream, and stirring was continued at 65 C. for 8 hours.

[0158] The isocyanate content in the obtained resin solution was 0.12 mol/Kg. After adding 683 parts of 4-methyl-2-pentanol, the solvent was distilled off over a period of 3 hours under reduced pressure conditions while maintaining a system temperature of 80 C. to 85 C., to obtain 1010 parts of a blocked polyisocyanate compound solution (Y3-1). The simple trap for the removed solvent included 95 parts of isopropanol. The solid concentration of the obtained blocked polyisocyanate compound solution (Y3-1) was approximately 60%.

Production Example 14

[0159] After charging 360 parts of SUMIDUR N-3300, 50 parts of UNIOX M-400 (product of NOF Corp., polyethyleneglycol monomethyl ether, average molecular weight: approximately 400), 5 parts of PEG#600 (product of NOF Corp., polyethylene glycol, average molecular weight: approximately 600) and 0.2 part of 2,6-di-tent-butyl-4-methylphenol into a reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube, dropper and simple trap for the removed solvent, the contents were thoroughly mixed and heated at 130 C. for 3 hours under a nitrogen stream. Next, 110 parts of ethyl acetate and 247 parts of diisopropyl malonate were charged in, 3 parts of a 28% methanol solution of sodium methoxide was added while stirring under a nitrogen stream, and stirring was continued at 65 C. for 8 hours. The isocyanate content in the obtained resin solution was 0.11 mol/Kg. After adding 670 parts of 4-methyl-2-pentanol, the solvent was distilled off over a period of 3 hours under reduced pressure conditions while maintaining a system temperature of 80 C. to 85 C., to obtain 1010 parts of a blocked polyisocyanate compound solution (Y3-2). The simple trap for the removed solvent included 92 parts of isopropanol. The solid concentration of the obtained blocked polyisocyanate compound solution (Y3-2) was approximately 60%.

Production of Aqueous Second Pigmented Coating Material (Y)

Production Example 15

[0160] After thoroughly mixing 120 parts of the water-dispersible hydroxyl group-containing acrylic resin dispersion (Y1-1) obtained in Production Example 3 (solid resin content: 36 parts), 71.4 parts of the hydroxyl group-containing polyester resin solution (Y2-1) obtained in Production Example 9 (solid resin content: 27 parts), 26.3 parts of UX-3945 (trade name of Sanyo Chemical Industries, Ltd., aqueous polyurethane resin) (solid resin content: 10 parts), 63 parts of a lustrous pigment concentrate (P-2)*.sup.4, 16.6 parts of the blocked polyisocyanate compound solution (Y3-1) obtained in Production Example 13 (solid resin content: 10 parts) and 11.1 parts of a melamine resin (CYMEL 327, trade name of Allnex Co.) (solid resin content: 10 parts), there were further added PRIMAL ASE-60 (trade name of Rohm & Haas, thickening agent), 2-(dimethylamino)ethanol and deionized water, to obtain an aqueous second pigmented coating material (Y-1) having a pH of 8.0, a coating material solid content of 25%, and a viscosity of 40 seconds with a Ford cup No. 4 at 20 C.

[0161] *4: Lustrous pigment concentrate (P-2): In a stirring and mixing container there were uniformly mixed 20 parts of an aluminum pigment paste (GX-180A, trade name of Asahi Kasei Metals Co., Ltd., metal content: 74%), 35 parts of 2-ethyl-1-hexanol, 8 parts of a phosphate group-containing resin solution*.sup.5 and 0.2 part of 2-(dimethylamino)ethanol, to obtain a lustrous pigment dispersion.

[0162] *5: Phosphate group-containing resin solution: A mixed solvent comprising 27.5 parts of methoxypropanol and 27.5 parts of isobutanol was placed in a reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube and dropper, and heated to 110 C. Next, 121.5 parts of a mixture comprising 25 parts of styrene, 27.5 parts of n-butyl methacrylate, 20 parts of a branched higher alkyl acrylate (trade name: Isostearyl Acrylate, product of Osaka Organic Chemical Industry, Ltd.), 7.5 parts of 4-hydroxybutyl acrylate, 15 parts of a phosphate group-containing polymerizable monomer*.sup.6, 12.5 parts of 2-methacryloyloxyethyl acid phosphate, 10 parts of isobutanol and 4 parts of t-butyl peroxyoctanoate was added to the mixed solvent over a period of 4 hours, and then a mixture of 0.5 part of t-butyl peroxyoctanoate and 20 parts of isopropanol was added dropwise over a period of one hour. The mixture was subsequently aged while stirring for 1 hour to obtain a phosphate group-containing resin solution with a solid concentration of 50%. The acid value due to the phosphate groups of the phosphate group-containing resin was 83 mgKOH/g, the hydroxyl value was 29 mgKOH/g and the weight-average molecular weight was 10,000.

[0163] *6: Phosphate group-containing polymerizable monomer solution: After placing 57.5 parts of monobutylphosphoric acid and 41 parts of isobutanol in a reactor equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube and dropper and heating them to 90 C., 42.5 parts of glycidyl methacrylate was added dropwise over a period of 2 hours. After then ageing the mixture while stirring for 1 hour, 59 parts of isopropanol was added to obtain a phosphate group-containing polymerizable monomer solution with a solid concentration of 50%. The acid value due to the phosphate groups in the obtained monomer was 285 mgKOH/g.

Production Examples 16 to 25

[0164] Aqueous second pigmented coating materials (Y-2) to (Y-11) were obtained in the same manner as Production Example 15, except that the composition in Production Example 15 was as shown in Table 3.

TABLE-US-00003 TABLE 3 Production Example 15 16 17 18 19 20 21 22 23 24 25 Aqueous second pigmented Y-1 Y-2 Y-3 Y-4 Y-5 Y-6 Y-7 Y-8 Y-9 Y-10 Y-11 coating material name Lustrous pigment concentrate 19 19 19 19 19 19 19 19 19 19 19 (P-2) (*4) Water-dispersible Y1-1 36 36 36 36 36 36 hydroxyl group- Y1-2 36 containing Y1-3 36 acrylic resin Y1-4 36 Y1-5 36 Y1-6 36 Hydroxyl group- Y2-1 27 27 27 27 27 27 27 27 containing Y2-2 27 polyester resin Y2-3 27 Y2-4 27 Polyurethane resin 10 10 10 10 10 10 10 10 10 10 Blocked polyisocyanate 10 10 10 10 10 20 20 10 10 10 10 Melamine resin 10 10 10 10 10 10 10 10 10 10

Production of Hydroxyl Group-Containing Acrylic Resin (Z1)

Production Example 26

[0165] After charging 31 parts of ethoxyethyl propionate into a four-necked flask equipped with a stirrer, thermometer, condenser tube and nitrogen gas inlet, the temperature was raised to 155 C. under a nitrogen gas stream. After reaching 155 C., aeration with nitrogen gas was stopped and a monomer mixture with a composition comprising 30 parts of styrene, 37.5 parts of n-butyl acrylate, 30 parts of 2-hydroxypropyl acrylate, 2.5 parts of acrylic acid and 4 parts of di-tertiary amyl peroxide (polymerization initiator) was added dropwise over a period of 4 hours. After 30 minutes, a polymerization initiator solution comprising 0.5 part of 2,2-azobis(2,4-dimethylvaleronitrile) (polymerization initiator) dissolved in 3 parts of SWASOL 1000 (hydrocarbon-based solvent) was added dropwise over a period of one hour. After then ageing for 2 hours while streaming nitrogen gas at 155 C., the mixture was cooled to 100 C. and diluted with 29 parts of butyl acetate to obtain a solution of a hydroxyl group-containing acrylic resin (Z1-1) with a solid content of 60%. The hydroxyl group-containing acrylic resin (Z1-1) had a hydroxyl value of 129 mgKOH/g, an acid value of 19 mgKOH/g and a weight-average molecular weight of approximately 12,000.

Production Example 27

[0166] A hydroxyl group-containing acrylic resin (Z1-2) solution with a solid content of 60% was obtained by production in the same manner as Production Example 26, except that the monomer mixture having a composition comprising 30 parts of styrene, 37.5 parts of n-butyl acrylate, 30 parts of 2-hydroxypropyl acrylate, 2.5 parts of acrylic acid and 4 parts of di-tertiary amyl peroxide (polymerization initiator) in Production Example 26 was changed to a monomer mixture having a composition comprising 30 parts of styrene, 40.5 parts of n-butyl acrylate, 27 parts of 2-hydroxyethyl acrylate, 2.5 parts of acrylic acid and 4 parts of di-tertiary amyl peroxide (polymerization initiator). The obtained hydroxyl group-containing acrylic resin (Z1-2) had a hydroxyl value of 129 mgKOH/g, an acid value of 19 mgKOH/g and a weight-average molecular weight of approximately 12,000.

Production of Clear Coating Material (Z)

Production Example 28

[0167] After uniformly mixing 100 parts of the hydroxyl group-containing acrylic resin (Z1-1) solution obtained in Production Example 26 (solid content: 60 parts), 45 parts of DESMODUR N3900 (trade name of Sumika Covestro Urethane Co., Ltd., polyisocyanate compound) (NCO/OH ratio: 1.6), 0.2 part of BYK-300 (trade name of Byk-Chemie Corp., surface control agent, active ingredient: 52%), 2.0 parts of TINUVIN900 (trade name of B.A.S.F. Corp., benzotriazole-based ultraviolet absorber, active ingredient: 100%) and 1.0 part of TINUVIN292 (trade name of B.A.S.F. Corp., hindered amine-based light stabilizer, active ingredient: 100%), SWASOL 1000 (trade name of Cosmo Oil Co., Ltd., hydrocarbon-based solvent) was further added to obtain a clear coating material (Z-1) having a viscosity of 25 seconds with a Ford cup No. 4 at 20 C.

Production Example 29

[0168] A clear coating material (Z-2) was obtained in the same manner as Production Example 28, except for using the hydroxyl group-containing acrylic resin (Z1-2) solution obtained in

[0169] Production Example 27 in the process of Production Example 28 instead of the hydroxyl group-containing acrylic resin (Z1-1) solution obtained in Production Example 26.

Production Example 30

[0170] A clear coating material (Z-3) was obtained in the same manner as Production Example 28, except that the DESMODUR N3900 in Production Example 28 was changed to 45 parts of DURANATE TLA-100 (trade name of Asahi Kasei Chemicals Corp., polyisocyanate compound) (NCO/OH ratio: 1.6).

Fabrication of Test Coating Articles

[0171] A cold-rolled steel sheet that had been subjected to zinc phosphate chemical conversion treatment was electrodeposited with ELECRON GT-10 (trade name of Kansai Paint Co., Ltd., cationic electrodeposition coating) to a dry film thickness of 20 pm, and was heated at 170 C. for 30 minutes for curing to produce a test coating article (1).

[0172] A polypropylene sheet (degreasing treated) was also air spray coated with the primer SOFLEX 3100 (trade name of Kansai Paint Co., Ltd., polyolefin-containing conductive organic solvent-based coating material) to a dry film thickness of 10 m, and heat cured at 80 C. for 30 minutes to obtain a test coating article (2).

Fabrication of Test Coated Sheets

Example 1

[0173] The test coating article (1) was electrostatically coated with the aqueous first pigmented coating material (X-1) obtained in Production Example 1 to a dry film thickness of 20 m using a rotary atomizing electrostatic coater, and allowed to stand for 2 minutes, after which the uncured first pigmented coating film was electrostatically coated with the aqueous second pigmented coating material (Y-1) obtained in Production Example 15 to a dry film thickness of 10 m using a rotary atomizing electrostatic coater and allowed to stand for 2 minutes, and then preheated at 80 C. for 3 minutes.

[0174] The base coating film was electrostatically coated with the clear coating material (Z-1) obtained in Production Example 28 to a dry film thickness of 35 m, and allowed to stand for 7 minutes. It was subsequently heated at 95 C. for 20 minutes (keep time) and the 3-layer coated film was heat cured to prepare a test coated sheet.

Examples 2 to 17 and Comparative Examples 1 and 2

[0175] Test coated sheets were prepared in the same manner as Example 1, except that the article to be coated, aqueous first pigmented coating material, aqueous second pigmented coating material and clear coating material in Example 1 were changed to the combinations listed in Table 4.

Evaluation Tests

[0176] Each of the test sheets obtained in Examples 1 to 17 and Comparative Examples 1 and 2 were evaluated by the following test methods. The evaluation results are summarized in Table 4.

(Test Methods)

[0177] Pencil hardness: Following the procedure specified by JIS K 5600-5-4, the core of a pencil was placed against the test coated sheet surface at an angle of about 45, and was moved forward approximately 10 mm at a uniform speed while firmly pressing it against the test coated sheet surface without breaking the core. The hardness number of the hardest pencil with which the coated film did not tear was recorded as the pencil hardness.

[0178] Smoothness: Evaluation was conducted using the Wc value measured with a Wave Scan DOI (trade name of BYK Gardner). The Wc value is an index of the amplitude of surface roughness with a wavelength of about 1 to 3 mm, with a smaller measured value representing higher smoothness of the coating surface.

[0179] Sharpness: Evaluation was conducted using the Wa value measured with a Wave Scan DOI. The Wa value is an index of the amplitude of surface roughness with a wavelength of about 0.1 to 0.3 mm, with a smaller measured value representing higher sharpness of the coating surface.

[0180] Chipping resistance: The test sheet was set on the sample holding stage of a chipping tester (trade name Q-G-R Gravelometer by Q-Panel Co.), and at a temperature of 20 C., 100 g of crushed granite of grain size #6 was impacted onto the test sheet at an angle of 90 using compressed air at 480 to 520 kPa, from a location 30 cm distant from the test sheet. The obtained test sheet was then washed and dried, and cloth adhesive tape (product of Nichiban Co., Ltd.)

[0181] was attached to the coating surface. The tape was peeled off, and the area where the electrodeposition surface and the underlying member were exposed (detached surface area) was used to evaluate the percentage (%) of the detached surface area with respect to the test coated sheet area.

[0182] Acceptability was judged to be a detached surface area percentage of less than 5% with respect to the test coated sheet area.

TABLE-US-00004 TABLE 4 Aqueous Aqueous first second pigmented pigmented Clear Evaluation Article to coating coating coating Pencil Chipping be coated material material material hardness Smoothness Sharpness resistance Example 1 (1) X-1 Y-1 Z-1 B 6 9 3 Example 2 (1) X-1 Y-2 Z-1 B 5 10 4 Example 3 (1) X-1 Y-3 Z-1 B 6 10 3 Example 4 (1) X-1 Y-4 Z-1 B 6 10 4 Example 5 (1) X-1 Y-5 Z-1 B 6 9 3 Example 6 (1) X-1 Y-6 Z-1 B 5 9 3 Example 7 (1) X-1 Y-7 Z-1 B 5 10 3 Example 8 (1) X-1 Y-8 Z-1 B 6 9 3 Example 9 (1) X-1 Y-9 Z-1 B 6 10 4 Example 10 (1) X-1 Y-10 Z-1 B 6 10 4 Example 11 (1) X-2 Y-1 Z-1 B 5 9 4 Example 11 (1) X-1 Y-1 Z-2 B 6 9 3 Example 12 (1) X-1 Y-1 Z-3 B 6 9 3 Example 13 (2) X-1 Y-1 Z-1 4B 6 8 3 Example 14 (2) X-1 Y-2 Z-1 4B 5 10 4 Example 15 (2) X-1 Y-4 Z-1 4B 6 10 4 Example 16 (2) X-1 Y-5 Z-1 4B 6 9 3 Example 17 (2) X-1 Y-1 Z-2 4B 6 9 4 Comp. Example 1 (1) X-1 Y-11 Z-1 B 6 17 6 Comp. Example 2 (2) X-1 Y-11 Z-1 4B 6 18 6