Aqueous coating composition and method for forming a coating film

Abstract

The objective of the present invention is to provide an aqueous coating composition which has excellent curability at low temperatures. The present invention provides an aqueous coating composition which contains an aqueous resin having a hydroxyl group and a carboxyl group (A), a water-dispersible blocked polyisocyanate compound (B), a hydrophilicized carbodiimide compound (C), and an aqueous polyurethane resin (D), wherein the aqueous resin having a hydroxyl group and a carboxyl group (A) has a hydroxyl value of 80 to 200 mgKOH/g and an acid value of 10 to 40 mgKOH/g in terms of resin solid content, the aqueous polyurethane resin (D) has a glass transition point (Tg) of 50 C. or less, and a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at 20 C.

Claims

1. An aqueous coating composition comprising: an aqueous resin having a hydroxyl group and a carboxyl group (A), a water-dispersible blocked polyisocyanate compound (B), a hydrophilicized carbodiimide compound (C), and an aqueous polyurethane resin (D), wherein the aqueous resin having a hydroxyl group and a carboxyl group (A) has a hydroxyl value of 80 to 200 mgKOH/g and an acid value of 10 to 40 mgKOH/g in terms of resin solid content, the hydrophilicized carbodiimide compound (C) is a compound represented by formula (I), (II), or (III) below, the aqueous polyurethane resin (D) has a glass transition point (Tg) of 50 C. or less, and a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at 20 C., ##STR00012## wherein X is a bifunctional organic group having at least one carbodiimide group, Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Z is a structure resulting from elimination of a hydroxyl group from a bifunctional polyol having a number-average molecular weight of 200 to 5,000, ##STR00013## wherein each X is a bifunctional organic group having at least one carbodiimide group, Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether, R.sup.0 is hydrogen, a methyl group or an ethyl group, each R.sup.1 is an alkylene group having 4 or less carbon atoms, n is 0 or 1, and each m is a number from 0 to 60, ##STR00014## wherein X is a bifunctional organic group having at least one carbodiimide group, and Y is each same or different structure resulting from elimination of a hydroxyl group from a polyalkylene glycol monoalkyl ether.

2. The aqueous coating composition according to claim 1, wherein a content of the hydrophilicized carbodiimide compound (C) is 1 to 8% by mass in terms of a resin solid content of the aqueous coating composition.

3. The aqueous coating composition according to claim 1, wherein the aqueous resin (A) comprises an acrylic emulsion having a number-average molecular weight of 10,000 to 80,000.

4. The aqueous coating composition according to claim 1, wherein the hydrophilicized carbodiimide compound (C) represented by the formula (III) above is a compound below, ##STR00015## wherein X is a bifunctional organic group having at least one carbodiimide group, and Y is each same or different structure selected from (i) and (ii) below: (i) a structure resulting from elimination of a hydroxyl group from a polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-linked to an end of a polyethylene oxide unit having a repeat number of 6 to 20, (ii) a structure resulting from elimination of a hydroxyl group from a polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-linked to an end of a polypropylene oxide unit having a repeat number of 4 to 60.

5. The aqueous coating composition according to claim 4, wherein in the hydrophilicized carbodiimide compound (C) represented by the formula (III) above, one Y is the (i) and another Y is the (ii), and a ratio of the structure (i) to the structure (ii) is within a range of (i):(ii)=1:0.7 to 1:8.

6. The aqueous coating composition according to claim 1, wherein a content of the aqueous polyurethane resin (D) is 15% by mass or more in terms of the resin solid content of the aqueous coating composition.

7. The aqueous coating composition according to claim 1, wherein a content of the aqueous polyurethane resin (D) is 30% by mass or more and 40% by mass or less in terms of the resin solid content of the aqueous coating composition.

8. The aqueous coating composition according to claim 1, wherein a content of the aqueous polyurethane resin (D) is 15% by mass or more and less than 30% by mass in terms of the resin solid content of the aqueous coating composition.

9. The aqueous coating composition according to claim 1, wherein a content of the water-dispersible blocked polyisocyanate compound (B) is 10 to 25% by mass in terms of the resin solid content of the aqueous coating composition, and a content of the hydrophilicized carbodiimide compound (C) is 1.5 to 7% by mass in terms of the resin solid content of the aqueous coating composition.

10. The aqueous coating composition according to claim 1, wherein a mass ratio of the content of the water-dispersible blocked polyisocyanate compound (B) to the content of the hydrophilicized carbodiimide compound (C) in terms of the resin solid content of the aqueous coating composition is (B):(C)=25:1 to 1.25:1.

11. A method for forming a coating film in use of an aqueous coating composition, wherein the method comprises: a coating step of applying the aqueous coating composition according to claim 1 to an object to be coated to form a coating film, and a curing step of curing the resulting coating film at 70 to 100 C.

12. The method for forming a coating film according to claim 11, wherein the coating step comprises: a first coating step of applying the aqueous coating composition, wherein a content of the aqueous polyurethane resin (D) is 30% by mass or more and 40% by mass or less in terms of the resin solid content of the aqueous coating composition, onto an object to be coated to form a first coating film, and a second application step of applying the aqueous coating composition, wherein a content of the aqueous polyurethane resin (D) is 15% by mass or more and less than 30% by mass in terms of the resin solid content of the aqueous coating composition, onto the first coating film to form a second coating film.

13. The method for forming a coating film according to claim 11, wherein the object to be coated includes a steel plate part and a resin part.

Description

EXAMPLES

(1) The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the examples. In the examples, parts and % are on a mass basis unless otherwise indicated.

Production Example 1

Production of Acrylic Emulsion Having Hydroxyl Group and Carboxyl Group

(2) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature controller, a condenser, and a dropping funnel was charged with 1,000 parts of deionized water, and was then heated to 80 C. with stirring under nitrogen atmosphere.

(3) A pre-emulsion prepared by adding to 1,000 parts of deionized water 103 part of styrene, 290 parts of n-butyl methacrylate, 280 parts of n-butyl acrylate, 302 parts of hydroxyethyl acrylate, 26 parts of acrylic acid, 3 parts of dodecyl mercaptan, and 100 parts of LATEMUL PD-104 (produced by Kao Corporation. 20% aqueous solution) as an emulsifier and then emulsifying them was dropped over 2 hours together with an aqueous initiator solution prepared by dissolving 3 parts of ammonium persulfate in 300 parts of deionized water.

(4) After the completion of the dropping, the reaction was continued at 80 C. for 1 hour, followed by cooling, and 8.2 parts of N,N-dimethylaminoethanol was added and thus an acrylic emulsion having a resin solid content of 30% by mass was obtained. The hydroxyl value of the acrylic emulsion, in terms of resin solid content, calculated from the monomer composition was 130 mgKOH/g and the acid value was 20 mgKOH/g. The acrylic resin in the resulting acrylic emulsion had a number-average molecular weight of 45,000 as determined by GPC measurement after removing water.

Production Example 2

Production of Acrylic Emulsion having Hydroxyl Group and Carboxyl Group

(5) Polymerization was carried out in the same manner as in Production Example 1 except that the amounts of monomers in the monomer mixture were changed to 90 parts for styrene, 249 parts for n-butyl acrylate, 403 parts for n-butyl methacrylate, 232 parts for hydroxyethyl acrylate, 26 parts for acrylic acid, and 3 parts for dodecyl mercaptan, and then 8.2 parts of N,N-dimethylaminoethanol was added, and thus an acrylic emulsion having a resin solid content of 30% by mass was obtained. The hydroxyl value of the acrylic emulsion, in terms of resin solid content, calculated from the monomer composition was 100 mgKOH/g and the acid value was 20 mgKOH/g. The acrylic resin in the resulting acrylic emulsion had a number-average molecular weight of 43,000 as determined by GPC measurement after removing water.

Production Example 3

Production of Aqueous Acrylic Dispersion Having Hydroxyl Group and Carboxyl Group

(6) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature controller, a condenser, and a dropping funnel was charged with 712 parts of 2-methoxy -1-propanol, and was then heated to 120 C. with stirring under nitrogen atmosphere.

(7) A monomer mixture of 50 parts of styrene, 20 parts of 2-ethylhexyl acrylate, 122 parts of 2-ethylhexyl methacrylate, 426 parts of n-butyl methacrylate, and 182 parts of hydroxyethyl methacrylate was dropped over 1.5 hours together with an initiator solution prepared by dissolving 24 parts of Kayaester O (tert-butyl peroxy-2-ethylhexanoate produced by Kayaku Akzo Corporation) in 160 parts of 2-methoxy-1-propanol.

(8) After the completion of the dropping, the temperature was held at 120 C. for 1 hour, and subsequently, a monomer mixture of 50 parts of n-butyl methacrylate, 119 parts of hydroxyethyl methacrylate, and 31 parts of acrylic acid was dropped over 1 hour together with an initiator solution prepared by dissolving 6 parts of Kayaester O in 40 parts of 2-methoxy-1-propanol.

(9) After the completion of the dropping, the temperature was held at 120 C. for 0.5 hours, and then an initiator solution prepared by dissolving 3 parts of Kayaester O in 55 parts of 2-methoxy-1-propanol was dropped over 0.5 hours as a post-shot, followed by stirring at 120 C. for 1 hour, and thus an acrylic resin having a resin solid content of 50% by mass and a number-average molecular weight of 6,000 as determined by GPC measurement was obtained.

(10) After evaporating the solvent by heating under reduced pressure, 39 parts of N,N-dimethylaminoethanol was added and further 1206 parts of deionized water was fed, followed by stirring, and thus an aqueous acrylic resin dispersion having a resin solid content of 45% by mass was obtained. The hydroxyl value of the aqueous acrylic dispersion, in terms of resin solid content, calculated from the monomer composition was 130 mgKOH/g and the acid value was 24 mgKOH/g.

Production Example 4

Production of Aqueous Polyester Dispersion Having a Hydroxyl Group and Carboxyl Group

(11) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature controller, a condenser, and a decanter was charged with 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexandicarboxylic acid, then the mixture was heated to 180 C., and then a condensation reaction was carried out until no more water distilled out. After cooling to 60 C., 120 parts of phthalic anhydride was added and the mixture was heated to 140 C. and held for 60 minutes, and thus a polyester resin having a number-average molecular weight of 2,000 as determined by GPC measurement was obtained. Fifty nine parts of dimethylaminoethanol (corresponding to 80% of the acid value of the resin (neutralization ratio: 80%)) was added at 80 C., and 1920 parts of deionized water was further added, followed by stirring, and thus an aqueous polyester dispersion having a resin solid content of 45% by mass was obtained. The hydroxyl value of the aqueous polyester dispersion in terms of resin solid content was 90 mgKOH/g and the acid value was 35 mgKOH/g.

Production Example 5

Preparation of Hydrophilicized Carbodiimide Compound (1)

(12) By reacting 700 parts of 4,4-dicyclohexylmethane diisocyanate with 7 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide at 170 C. for 7 hours, obtained was a carbodiimide compound with the structure represented by the above formula (a), the carbodiimide compound having three carbodiimide groups in one molecule and having isocyanate groups at its both ends.

(13) Next, to 180 parts of the produced 4,4-dicyclohexylmethanecarbodiimide having isocyanate ends, were added 95 parts of PTMG-1000 (polytetramethylene glycol having a number-average molecular weight of 1,000 produced by Mitsubishi Chemical; repeat number of tetramethylene oxide calculated from number-average molecular weight was 13.6) and 0.2 parts of dibutyltin dilaurate, and the mixture was then heated to 85 C. and held for 2 hours.

(14) Subsequently, 86.4 parts of Methyl Poly Glycol 130 (polyethylene glycol monomethyl ether produced by Nippon Nyukazai Co., Ltd.; repeat number of ethylene oxide calculated from hydroxyl value of 130 mgKOH/g was 9) was added and then the mixture was held at 85 C. for 3 hours. After confirming disappearance of a peak of NCO by IR measurement, the reaction was finished, followed by cooling to 60 C., and then deionized water was added, and thus an aqueous dispersion of a hydrophilicized carbodiimide compound (1) having a resin solid content of 40% by mass was obtained. The resulting hydrophilicized carbodiimide compound was a compound represented by the above formula (I).

Production Example 6

Preparation of Hydrophilicized Carbodiimide Compound (2)

(15) To 90 parts of the 4,4-dicyclohexylmethanecarbodiimide having isocyanate ends produced in Production Example 5, were added 120 parts of polypropylene glycol monobutyl ether having an average repeat number of 19, 43.2 parts of Methyl Poly Glycol 130, and 0.07 parts of dibutyltin dilaurate, and the temperature was held at 80 C. until absorption of NCO disappeared in an IR spectrum. After cooling to 60 C., deionized water was added and thus an aqueous dispersion of a hydrophilicized carbodiimide compound (2) having a resin solid content of 25% was obtained. The resulting hydrophilicized carbodiimide compound was a compound represented by the above formula (III).

(16) In the resulting hydrophilicized carbodiimide compound, the ratio of (i) a structure resulting from elimination of a hydroxyl group from polyethylene glycol monoalkyl ether and (ii) a structure resulting from elimination of a hydroxyl group from polypropylene glycol monoalkyl ether was (i):(ii)=1.0:1.0.

Production Example 7

reparation of Hydrophilicized Carbodiimide Compound (3)

(17) By reacting 393 parts of 4,4-dicyclohexylmethane diisocyanate with 8 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide at 180 C. for 16 hours, obtained was a carbodiimide compound with a structure represented by the following formula, the carbodiimide compound having four carbodiimide groups in one molecule and having isocyanate groups at both ends. Here, 130 parts of polyethylene glycol monomethyl ether having an oxyethylene group repeat number of 9 and 0.2 parts of dibutyltin dilaurate were added, followed by heating at 90 C. for 2 hours, and thus a carbodiimide compound with a structure represented by the following formula, the carbodiimide compound having an isocyanate group and a hydrophilic group at its ends was obtained. In addition, 300 parts of GP-3000 (trihydric polyol having a structure in which 17 mol, in average, of propylene oxide was added to respective three hydroxyl groups of glycerol, produced by Sanyo Chemical Industries, Ltd.) was added and was reacted at 90 C. for 6 hours. After confirming disappearance of a peak of NCO by IR measurement, the reaction was finished and thus a hydrophilicized carbodiimide compound (3) was obtained. Deionized water was added thereto and thus an aqueous dispersion of the hydrophilicized carbodiimide compound (3) having a resin solid content of 30% by mass was obtained. The resulting hydrophilicized carbodiimide compound was a compound represented by the above formula (II).

Production Example 8

Production of Coloring Pigment Paste

(18) After preliminarily mixing 9.2 parts of a commercially available dispersing agent Disperbyk 190 (produced by PYK-Chemie), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile type titanium dioxide, a bead medium was added to the mixture in a paint conditioner, and mixed and dispersed at room temperature until the particle size reached 5 m or less, and then the bead medium was removed by filtration and thus a coloring pigment paste was obtained.

Production Example 9

Production of Emulsion Resin

(19) To reaction vessel containing 194.1 parts of ion-exchanged water were added 0.2 parts of ADEKA REASOAP NE-20 (-[1-[(allyloxy)methyl]-2-(nonylphenoxy)ethyl]--hydroxyoxyethylene, produced by ADEKA Corporation, aqueous solution having a solid content of 80% by weight) and 0.2 parts of Aqualon HS-10 (polyoxyethylene alkylpropenylphenyl ether sulfate, produced by DKS Co. Ltd.), the mixture was then heated to 80 C. with mixing and stirring under a nitrogen flow. Subsequently, a monomer mixture composed of 18.5 parts of methyl acrylate, 31.7 parts of ethyl acrylate, 5.8 parts of 2-hydroxyethyl acrylate, 10.0 parts of styrene, 4.0 parts of acrylamide, 0.3 parts of ADEKA REASOAP NE-20, 0.2 parts of Aqualon HS-10, and 70 parts of ion-exchanged water as an ,-ethylenically unsaturated monomer mixture for the first step, and an initiator solution composed of 0.2 parts of ammonium persulfate and 7 parts of ion-exchanged water were dropped in parallel into the reaction vessel over 2 hours. After the completion of the dropping, aging was carried out at the same temperature for 1 hour.

(20) Further, a monomer mixture composed of 24.5 parts of ethyl acrylate, 2.5 of 2-hydroxyethyl acrylate, 3.1 parts of methacrylic acid, 0.3 parts Aqualon HS-10, and 30 parts of ion -exchanged water as an ,-ethylenically unsaturated monomer mixture for the second step, and an initiator solution composed of 0.1 parts of ammonium persulfate and 3 parts of ion-exchanged water were dropped in parallel into the reaction vessel at 80 C. over 0.5 hours. After the completion of the dropping, aging was carried out at the same temperature for 2 hours.

(21) Subsequently, the mixture was cooled to 40 C. and was filtered with a 400 mesh filter. Further, a 10% by weight aqueous dimethylaminoethanol solution was added and the pH was adjusted to 7, and thus an emulsion resin having an average particle diameter of 110 nm, a solid content of 24% by weight, a solid content acid value of 20, and a hydroxyl value of 40 was obtained. The glass transition point was calculated to be 0 C. on the basis of the whole monomer composition.

Example 1

Preparation of Intermediate Coating Composition

(22) A vessel equipped with a stirrer was charged with 119.67 parts (resin solid content: 30%) of the hydroxyl group-containing acrylic resin emulsion prepared in Production Example 1, then, 25.78 parts (resin solid content: 45%) of the hydroxy group-containing polyester resin prepared in Production Example 2, 24.19 parts of WM44-L70G (produced by Asahi Kasei Chemicals Corporation, solid concentration: 70.7%) as a water-dispersible blocked polyisocyanate, 13.5 parts (resin solid content: 40%) of the hydrophilicized carbodiimide compound prepared in Production Example 5, 100 parts (resin solid content: 30%) of an aqueous polyurethane resin, 122.11 parts of the pigment-dispersed paste portion prepared in Production Example 8, 68 parts of ion-exchanged water were added and stirred. Next, pH was adjusted to 8.0 in adding 0.01 parts of dimethylethanolamine (produced by KISHIDA CHEMICAL Co., Ltd.), and then, 1.0 part of ADEKA NOL UH-814N (urethane association type thickening agent, effective component: 30%, produced by ADEKA Corporation, trade name) was mixed and stirred, and thus an aqueous intermediate coating composition was obtained.

Preparation of Aqueous Base Coating Composition

(23) In a vessel equipped with a stirrer, 100 parts (resin solid content: 25%) of the hydroxyl group-containing acrylic resin emulsion obtained in Production Example 1 and 125 parts (resin solid content: 24%) of the acrylate emulsion resin obtained in Production Example 9 were stirred and mixed, and then 21.2 parts of WM44-L70G (produced by Asahi Kasei Chemicals Corporation, solid concentration: 70.7%) as a water-dispersible blocked polyisocyanate, 12.5 parts (resin solid content: 40%) of the hydrophilicized carbodiimide compound prepared in Production Example 5, 66.7 parts (resin solid content: 30%) of an aqueous polyurethane resin, 21 parts (solid content: 65%, PWC 12%) of Alpaste MH8801 (produced by Asahi Kasei Corporation) as a luster pigment, 5 parts of an acrylic resin containing a phosphate group, and 0.3 parts of lauryl acid phosphate were added. Further, 30 parts of 2-ethylhexanol, 3.3 parts of ADEKA NOL UH-814N (thickening agent produced by ADEKA Corporation, solid content: 30%), 0.01 parts of dimethylethanolamine (produced by KISHIDA CHEMICAL Co., Ltd.), and 150 parts of ion-exchanged water were dispersed uniformly, and thus an aqueous base coating composition was obtained.

Formation of Multilayer Coating Film

(24) Powernics 150 (trade name, cationic electrodeposition coating composition produced by Nippon Paint Automotive Coatings Co., Ltd.) was electrodeposition coated on a dull steel sheet treated with zinc phosphate such that the thickness of the dry coating film was 20 m, followed by heat-curing at 160 C. for 30 minutes and subsequent cooling, and thus a steel substrate was prepared.

(25) The aqueous intermediate coating composition was applied to the resulting substrate by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 25 m, and then the aqueous base coating composition was applied by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 15 m, followed by preheating at 80 C. for 3 minutes. The aqueous base coating composition was applied after an interval of 6 minutes from the application of the aqueous intermediate coating composition. In addition, Polyure Excel O-1200 (trade name, produced by Nippon Paint Automotive Coatings Co., Ltd., polyisocyanate compound-containing two-components acrylic urethane-based organic solvent type clear coating composition) was applied to the coated plate by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 35 m, and then was heated and cured at 80 C. for 20 minutes, and thus a specimen on which a multilayer coating film had been formed was obtained.

Examples 2 to 17

(26) Intermediate coating compositions and aqueous base coating compositions were prepared in the same manner as in Example 1 except that the components and their blending amounts were changed as shown in the following table in the preparation of the intermediate coating compositions and the aqueous base coating compositions. Using the coating compositions obtained, a multilayer coating film was formed in the same manner as in Example 1. The blending amounts shown in the following table are the resin solid content amount.

Example 18

(27) Using the intermediate coating composition and the aqueous base coating composition prepared in Example 1, a multilayer coating film was formed in the following procedure.

(28) WB-3110CB (trade name, produced by Nippon Paint Automotive Coatings Co., Ltd., non-chlorinated polyolefin-containing electroconductive coating composition) as an aqueous primer for adhesion was applied to a resin member (polypropylene) by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 15 m, and subsequently, the aqueous intermediate coating composition was applied by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 25 m, and then the aqueous base coating composition was applied by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 15 m, followed by preheating at 80 C. for 3 minutes. In the above steps, the aqueous base coating composition was applied after an interval of 6 minutes from the application of the aqueous intermediate coating composition.

(29) Next, Polyure Excel O-1200 (trade name, produced by Nippon Paint Automotive Coatings Co., Ltd., polyisocyanate compound-containing two-components acrylic urethane-based organic solvent type clear coating composition was applied to the coated plate by using a rotary atomization type electrostatic applicator such that the thickness of the dry coating film was 35 m, and then was heated and cured at 80 C. for 20 minutes, and thus a specimen on which a multilayer coating film had been formed was obtained.

Comparative Examples 1 to 7

(30) Intermediate coating compositions and aqueous base coating compositions were prepared in the same manner as in Example 1 except that the components and their blending amounts were changed as shown in the following table in the preparation of the intermediate coating compositions and the aqueous based coating compositions. Using the coating compositions obtained, a multilayer coating film was formed in the same manner as in Example 1.

(31) For the multilayer coating films obtained in the examples and the comparative examples described above as well as the raw materials used in the preparation of the intermediate coating compositions and the aqueous base coating compositions, evaluation tests were carried out as described below. The results of the evaluation are shown in the following tables.

Measurement of Elongation at Break of Aqueous Polyurethane Resin (D)

(32) Ninety-five parts (resin solid content amount) of the aqueous polyurethane resin (D) and 5 part by mass (resin solid content amount) of the hydrophilicized carbodiimide compound (C) described in Production Example 5 were mixed such that the resin solid contents thereof was 100 parts by mass in total. In a clean environment where dusts or the like do not adhere, the mixed liquid prepared was applied onto a flat polypropylene plate uniformly with a doctor blade such that the thickness of the dry coating film was 20 m. After leaving at rest at 20 C. for 10 minutes, the resulting plate was preheated at 80 C. for 3 minutes, thereby volatilizing water, and then was baked at 120 C. for 30 minutes, and thus a cured film was prepared. The cured film obtained was subjected to a tensile performance test at a testing temperature of 20 C. in accordance with JIS K7127 and an elongation ratio at the time of breaking (elongation at break) was measured. Measurement was performed 20 times and the average of 18 measurements except the maximum and the minimum values was taken as the elongation at break of the sample.

Evaluation of Chipping Resistance

(33) The test plates each with a multilayer coating film obtained in the examples and the comparative examples were subjected to a stepping stone test under the conditions shown below using a Gravelometer KSS-1 (produced by Suga Test Instruments Co., Ltd.).

(34) <Test Conditions>

(35) Stone size: 6 to 8 mm

(36) Amount of stone: 0.7 to 0.8 g/piece

(37) Distance: 35 cm

(38) Shot pressure: 0.6 kg/cm.sup.2

(39) Shot angle: 45

(40) Test temperature: 20 C.

(41) The test plates after the stepping stone test were evaluated visually according to the following criteria. Under the following criteria, when the score is 4 or more, the test plate can be used practically and is judged to be acceptable.

(42) 5: Almost no exfoliation is observed.

(43) 4: The exfoliated area is small, and almost no exfoliation is observed at the interface between the electrodeposition coating film and the intermediate coating film.

(44) 3: The exfoliated area is slightly large and exfoliation is observed at the interface between the electrodeposition coating film and the intermediate coating film.

(45) 2: The exfoliated area is large and exfoliation is observed at the interface between the electrodeposition coating film and the intermediate coating film.

(46) 1: The exfoliated area is large and the electrodeposition coating film is broken.

Evaluation of Water-Resistant Adhesion

(47) The test plates each with a multilayer coating film were immersed in warm water at 40 C. for 240 hours and removed therefrom, and were then dried at 20 C. for 24 hours. Lattice-like cuts were made in the multilayer coating film on each of the test plates with a knife to reach the base material, so that 100 crosscuts having a size of 2 mm2 mm were made. Subsequently, an Adhesive Cellophane Tape (trademark) was affixed to each of the test plates, the tape was abruptly peeled off at 20 C., and the number of remaining crosscut coating films was counted.

(48) The relative merits of the coating film can be judged from the number of peeled crosscut sections. Even if only one crosscut section peeled, the sample is judged to be difficult to be used practically.

Evaluation of Water-Resistant Shrinkage

(49) The test plates each with a multilayer coating film were immersed in warm water at 40 C. for 240 hours and removed therefrom, and were then dried at 20 C. for 24 hours. Then, each of the test plates was examined visually, and change in its appearance before and after the test was observed. Under the following criteria, test plates with rating or can be judged to be practically usable.

(50) : Almost no difference is observed in gloss and smoothness.

(51) : Slight change is observed in gloss and smoothness.

(52) : Change is observed in gloss and smoothness.

(53) x: Change is observed in both gloss and smoothness, and especially, change in gloss is remarkable.

(54) x: Remarkable difference is observed in both gloss and smoothness.

Evaluation of Moisture-Resistant Adhesion

(55) The test plates each with a multilayer coating film were exposed to an atmosphere having humidity of 99% or more at 40 C. and 50 C. for 240 hours. After the completion of the exposure and subsequent drying at 20 C. for 24 hours, lattice-like cuts were made in the multilayer coating film on each of the test plates with a knife to reach the base material, so that 100 crosscuts having a size of 2 mm2 mm were made. Subsequently, an Adhesive Cellophane Tape (trademark) was affixed to each of the test plates, the tape was abruptly peeled off at 20 C., and the number of remaining crosscut coating films was counted.

(56) The relative merits of the coating film can be judged from the number of peeled crosscut sections. Even if only one crosscut section peeled, the sample is judged to be difficult to be used practically.

Stability Evaluation of Intermediate Coating Composition and Aqueous Base Coating Composition

(57) A coating composition was prepared and left at rest at 40 C. for 3 days. Then, when a sample in which non-flowable masses were formed in the coating material liquid or the coating material liquid lost its flowability as a whole, it was judged to be unacceptable (x), and otherwise it was judged to be acceptable (o).

(58) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Primer Intermediate (B) Water-dispersible Blending 17.1 17.1 17.1 20.1 14.5 17.1 17.1 17.1 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Blending 5.4 5.4 5.4 2.4 8 5.4 5.4 5.4 amount (A) Acrylic resin Type Production Production Production Production Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 and carboxyl group Blending 35.9 25.9 35.9 35.9 35.9 35.9 35.9 40.9 amount (A) Polyester resin Blending 11.6 11.6 11.6 11.6 11.6 11.6 11.6 11.6 having hydroxyl group amount and carboxyl group (D) Aqueous Type D D D D D D D D polyurethene resin Blending 30 40 30 30 30 30 30 25 amount Tg 60 60 60 60 60 60 60 60 Elongation 610 610 610 610 610 610 610 610 at break Total of blending amounts 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 of resins Stability (40 C., 3 days) Aqueous base (B) Water-dispersible Blending 15 15 15 15 15 18 12 18 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Example 5 Blending 5 5 5 5 5 2 8 2 amount (A) Acrylic resin Type Production Production Production Production Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 and carboxyl group Blending 30 30 35 30 30 30 30 30 amount Emulsion resin of Blending 30 30 30 30 30 30 30 30 Production Example 9 amount (D) Aqueous Type D D D D D D D D polyurethene resin Blending 20 20 15 20 20 20 20 20 amount Tg 60 60 60 60 60 60 60 60 Elongation 610 610 610 610 610 610 610 610 at break Total of blending amounts of 100 100 100 100 100 100 100 100 resins Stability (40 C., 3 days) Clear coating composition (Polyure Excel O-1200) Performance Water-resistant adhesion (Number 0 0 0 0 0 0 0 0 of baking at of 100-crosscuts peeled sections) 80 C. for Water-resistant shrinkage 20 minutes Moisture-resistant adhesion 0 0 0 0 0 0 0 0 (Number of 100-crosscuts peeled sections) Chipping resistance 5 5 5 5 5 5 5 4 (5: good; 4 or more: Acceptable) Example 9 Example 10 Example 11 Example 12 Example 13 Example 14 Primer Intermediate (B) Water-dispersible Blending 17.1 17.1 17.1 17.1 17.1 17.1 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production Production Production carbodiimide compound Example 5 Example 6 Example 7 Example 5 Example 5 Example 5 Blending 5.4 5.4 5.4 5.4 5.0 5.4 amount (A) Acrylic resin Type Production Production Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 Example 1 Example 2 and carboxyl group Blending 40.9 35.9 35.9 35.9 35.9 35.9 amount (A) Polyester resin Blending 11.6 11.6 11.6 11.6 11.6 11.6 having hydroxyl group amount and carboxyl group (D) Aqueous Type D D D D D D polyurethene resin Blending 25 30 30 30 30 30 amount Tg 60 60 60 60 60 60 Elongation 610 610 610 610 610 610 at break Total of blending amounts 100.0 100.0 100.0 100.0 100.0 100.0 of resins Stability (40 C., 3 days) Aqueous base (B) Water-dispersible Blending 18 18 18 18 18 18 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 6 Example 7 Example 5 Blending 2 2 2 2 2 2 amount (A) Acrylic resin Type Production Production Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 and carboxyl group Blending 35 30 30 30 30 30 amount Emulsion resin of Blending 30 30 30 30 30 30 Production Example 9 amount (D) Aqueous Type D D D D D D polyurethene resin Blending 15 20 20 20 20 20 amount Tg 60 60 60 60 60 60 Elongation 610 610 610 610 610 610 at break Total of blending amounts of 100 100 100 100 100 100 resins Stability (40 C., 3 days) Clear coating composition (Polyure Excel O-1200) Performance Water-resistant adhesion (Number 0 0 0 0 0 0 of baking at of 100-crosscuts peeled sections) 80 C. for Water-resistant shrinkage 20 minutes Moisture-resistant adhesion 0 0 0 0 0 0 (Number of 100-crosscuts peeled sections) Chipping resistance 4 5 5 5 5 5 (5: good; 4 or more: Acceptable) Example 15 Example 16 Example 17 Example 18 Primer Aqueous primer Intermediate (B) Water-dispersible Blending 17.1 17.1 17.1 17.2 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Blending 5.4 5.4 5.4 5.4 amount (A) Acrylic resin Type Production Production Production Production having hydroxyl group Example 3 Example 1 Example 1 Example 1 and carboxyl group Blending 35.9 35.9 35.9 35.9 amount (A) Polyester resin Blending 11.6 11.6 11.6 11.6 having hydroxyl group amount and carboxyl group (D) Aqueous Type D D D D polyurethene resin Blending 30 30 30 30 amount Tg 60 60 60 60 Elongation 610 610 610 610 at break Total of blending amounts 100.0 100.0 100.0 100.0 of resins Stability (40 C., 3 days) Aqueous base (B) Water-dispersible Blending 18 18 18 15 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Blending 2 2 2 5 amount (A) Acrylic resin Type Production Production Production Production having hydroxyl group Example 1 Example 2 Example 3 Example 1 and carboxyl group Blending 30 30 30 30 amount Emulsion resin of Blending 30 30 30 30 Production Example 9 amount (D) Aqueous Type D D D D polyurethene resin Blending 20 20 20 20 amount Tg 60 60 60 60 Elongation 610 610 610 610 at break Total of blending amounts of 100 100 100 100 resins Stability (40 C., 3 days) Clear coating composition (Polyure Excel O-1200) Performance Water-resistant adhesion (Number 0 0 0 0 of baking at of 100-crosscuts peeled sections) 80 C. for Water-resistant shrinkage 20 minutes Moisture-resistant adhesion 0 0 0 0 (Number of 100-crosscuts peeled sections) Chipping resistance 5 5 5 5 (5: good; 4 or more: Acceptable)

(59) TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Comparative Example 1 example 2 Example 3 example 4 Intermediate (B) Water-dispersible Blending 22.5 17.1 17.1 17.1 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Blending 0 5.4 5.4 5.4 amount (A) Acrylic resin Type Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 and carboxyl group Blending 35.9 35.9 35.9 35.9 amount (A) Polyester resin Blending 11.6 11.6 11.6 11.6 having hydroxyl group amount and carboxyl group (D) Aqueous Type D A B C polyurethene resin Blending 30 30 30 30 amount Tg 60 10 18 60 Elongation 610 12 130 312 at break Total of blending amounts of resins 100.0 100.0 100.0 100.0 Stability (40 C., 3 days) Aqueous base (B) Water-dispersible Blending 20 20 20 20 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Example 5 Blending 0 0 0 0 amount (A) Acrylic resin Type Production Production Production Production having hydroxyl group Example 1 Example 1 Example 1 Example 1 and carboxyl group Blending 30 30 30 30 Emulsion resin of Blending 30 30 30 30 Production Example 9 amount (D) Aqueous Type D D D D polyurethene resin Blending 20 20 20 20 amount Tg 60 60 60 60 Elongation 610 610 610 610 at break Total of blending amounts of resins 100 100 100 100 Stability (40 C., 3 days) Clear coating composition (Polyure Excel O-1200) Performance Water-resistance adhesion (Number of 50 10 5 0 of baking at 100-crossscuts peeled sections) 80 C. for Water-resistance shrinkage x x x 20 minutes Moisture-resistant adhesion (Number of 70 20 10 0 100-crosscuts peeled sections) Chipping resistance 5 1 2 2 (5: Good; 4 or more: Acceptable) Comparative Comparative Comparative Example 5 Example 6 example 7 Intermediate (B) Water-dispersible Blending 22.5 22.5 22.5 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Blending 0 0 0 amount (A) Acrylic resin Type Production Production Production having hydroxyl group Example 1 Example 1 Example 1 and carboxyl group Blending 35.9 35.9 35.9 amount (A) Polyester resin Blending 11.6 11.6 11.6 having hydroxyl group amount and carboxyl group (D) Aqueous Type D D D polyurethene resin Blending 30 30 30 amount Tg 60 60 60 Elongation 610 610 610 at break Total of blending amounts of resins 100.0 100.0 100.0 Stability (40 C., 3 days) Aqueous base (B) Water-dispersible Blending 15 15 15 coating blocked polyisocyanate amount composition (C) Hydrophilicized Type Production Production Production carbodiimide compound Example 5 Example 5 Example 5 Blending 5 5 5 amount (A) Acrylic resin Type Production Production Production having hydroxyl group Example 1 Example 1 Example 1 and carboxyl group Blending 30 30 30 Emulsion resin of Blending 30 30 30 Production Example 9 amount (D) Aqueous Type A B C polyurethene resin Blending 20 20 20 amount Tg 10 18 60 Elongation 12 130 312 at break Total of blending amounts of resins 100 100 100 Stability (40 C., 3 days) Clear coating composition (Polyure Excel O-1200) Performance Water-resistance adhesion (Number of 60 55 60 of baking at 100-crossscuts peeled sections) 80 C. for Water-resistance shrinkage x x x 20 minutes Moisture-resistant adhesion (Number of 80 80 80 100-crosscuts peeled sections) Chipping resistance 2 3 3 (5: Good; 4 or more: Acceptable)

(60) The types of the aqueous polyurethane resins shown in the above tables are as follows.

(61) A: N9603 (produced by Kusumoto Chemicals, Ltd.), solid concentration: 34%, Tg: 10 C., elongation at break: 12%

(62) B: HUX-232 (produced by ADEKA Corporation), solid concentration: 30%, Tg: 18 C., elongation at break: 130%

(63) C: N800 (produced by Sanyo Chemical Industries, Ltd.), solid concentration: 38%, Tg: 60 C., elongation at break: 312%

(64) D: PERMARIN U150 (produced by Sanyo Chemical Industries, Ltd.), solid concentration: 30%, Tg: 60 C., elongation at break: 610%

(65) The multilayer coating film formed using the aqueous coating compositions of the examples were confirmed to have excellent water resistance, moisture resistance, and chipping resistance even after the multilayer coating film were subjected to baking and curing at a low-temperature condition of 80 C.

(66) On the other hand, the multilayer coating films formed using the aqueous coating compositions of the comparative examples were configured to be inferior in one or two or more of water resistance, moisture resistance, and chipping resistance.

INDUSTRIAL APPLICABILITY

(67) The aqueous coating composition of the present invention is advantageous in that a curing reaction proceeds well even under heating conditions under low temperature conditions (e.g., heating conditions at 100 C. or less), so that a cured coating film having excellent coating film properties can be obtained. By use of the aqueous coating composition of the present invention, it is possible to obtain a coating film having excellent coating film properties (water resistance, chipping resistance, etc.) under milder heating conditions. By performing coating using the aqueous coating composition of the present invention, it is possible to reduce burden on the environment, such as energy saving and reduction in CO.sub.2 emission.