WO2007013684A2

WO2007013684A2 - Aqueous clear coating composition and method for forming multilayer topcoat film

Info

Publication number: WO2007013684A2
Application number: PCT/JP2006/315435
Authority: WO
Inventors: Seiji Wada; Yuta Baba; Koki Chiga; Takashi Noguchi; Takato Adachi; Hiroyuki Onoyama; Hideo Sugai
Original assignee: Kansai Paint Co., Ltd.
Priority date: 2005-07-29
Filing date: 2006-07-28
Publication date: 2007-02-01
Also published as: JP2009503122A; WO2007013684A3

Abstract

The present invention provides an aqueous clear coating composition comprising (A) an aqueous dispersion of a hydroxy group- and acid group-containing particulate resin having a hydroxy value of 30 to 200 mg KOH/g, an acid value of 5 to 50 mg KOH/g, and a mean particle size of 50 to 300 nm, and (B) a crosslinking agent, the composition having a viscosity such that the lowest viscosity value of the composition as measured over the temperature range of 30°C to 150°C at a frequency of 0.1 Hz at a solids content of at least 90 mass % is 30 Pa·s or less; and a method for forming a multilayer topcoat film using the aqueous clear coating composition.

Description

DESCRIPTION

AQUEOUS CLEAR COATING- COMPOSITION AND METHOD FOR FORMING MULTILAYER TOPCOAT FILM

TECHNICAL FIELD

The present invention relates to an aqueous clear coating composition and a method for forming a multilayer topcoat film using the aqueous clear coating composition.

BACKGROUND OF THE INVENTION

In recent years, environmental problems have become an issue of great concern on the global scale. In the automotive industry, environmental protection measures in the manufacturing process have been actively promoted. In the automotive manufacturing process, there is an urgent necessity to reduce the amount of volatile organic compounds (VOCs) discharged in the coating process .

To impart corrosion resistance and beautiful appearance, the outer panels, etc. of automobile bodies are usually coated with coating films comprising an undercoat, an intermediate coat and a topcoat. Of these coats, the undercoat is formed by applying an aqueous coating composition such as a cationic electrodeposition coating composition, etc. To reduce the amount of VOCs, the use of aqueous intermediate and top coating compositions for forming intermediate and top coats, respectively, has been actively promoted.

A multilayer topcoat film can be formed, for example, by a two-coat one-bake method comprising applying a colored base coating composition to a substrate, further applying a clear coating composition without heat-curing the base coating layer, and then curing the two coating layers simultaneously. When aqueous coating compositions are used as such top coating compositions, aqueous compositions, in particular, aqueous clear coating compositions for forming the uppermost layer, are required to provide good coating appearance such as smoothness with no ^' abnormalities such as foaming on the coated surface, so as to improve the appearance of automobile bodies, etc.

U.S. Patent No. 5,075,370 discloses a two-component coating composition comprising a polyol component containing hydroxy and carboxylate groups, and a polyisocyanate component which is present in emulsified form in the polyol component. However, this two-component coating composition is such that compared to conventional organic solvent-based coating compositions, abnormalities such as foaming on the coated surface tend to occur and coating appearance such as smoothness are unsatisfactory.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide an aqueous clear coating composition capable of providing good coating appearance such as smoothness with no abnormalities such as foaming on the coated surface, the composition being suitable for- use as a top coating composition in methods of application such as two-coat one-bake methods for coating an automotive vehicle body, etc.

Another object of the present invention is to provide a method for forming a multilayer topcoat film using the aqueous clear coating composition.. The present inventors carried out extensive resea'rch on aqueous clear coating compositions, focusing attention on the influence of viscosity of the aqueous coating composition on the occurrence of abnormalities such as foaming on the coated surface. As a result, the inventors found that when using an aqueous coating composition comprising an aqueous dispersion of a specific hydroxy group- and acid group-containing particulate resin and a crosslinking agent and having a viscosity within a specific range, the above^' objects can be achieved. The present invention has been accomplished based on this finding. The present invention provides the following aqueous clear coating compositions and method for forming a multilayer topcoat^' film.

1. An aqueous clear coating composition comprising:

(A) an aqueous dispersion of a hydroxy group- and acid group- containing particulate resin having a hydroxy value of 30 to 200 mg KOH/g, an acid value of 5 to 50 mg KOH/g and a mean particle size of 50 to 300 nm, and

(B) a crosslinking agent; the composition having a viscosity such that the lowest viscosity value of the composition as measured over the temperature range of 30⁰C to 15O⁰C at a frequency of 0.1 Hz at a solids content of at least 90 mass % is 30 Pa-s or less.

2. The aqueous clear coating composition according to item 1 wherein the melt viscosity of the hydroxy group- and acid group- containing particulate resin in dispersion (A) as measured at a solids content of at least 96 mass % at a shear rate of 564 s^~ at 14O⁰C is 1 to 12 Pa-s:

3. The aqueous clear coating composition according to item 1 wherein the hydroxy group- and acid group-containing particulate resin in dispersion (A) has a weight average molecular weight of 3,000 to 30,000.

4. The aqueous clear coating composition according to item 1 wherein the hydroxy group- and acid group-containing particulate resin in dispersion (A) has a glass transition temperature within the range of from -30⁰C to +40°C.

5. The aqueous clear coating composition according to item 1 wherein crosslinking agent (B) is at least one crosslinking agent selected from the group consisting of polyisocyanate compounds, blocked polyisocyanate compounds and melamine resins. 6. The aqueous clear coating composition according to item 5 wherein crosslinking agent (B) is at least one polyisocyanate compound and/or at least one blocked polyisocyanate compound.

7. The aqueous clear coating composition according to item 5 wherein the equivalent ratio (NCO/OH) of isocyanate groups of the polyisocyanate compound and/or blocked isocyanate compound to hydroxy groups of the hydroxy group- and acid group-containing particulate resin in dispersion (A) is 0.5 to 2.0.^'

8. The aqueous clear coating composition according to item 5 wherein crosslinking agent (B) is at least one melamine resin. 9. The aqueous clear coating composition according to item 8 wherein the proportions of hydroxy group- and acid group- containing particulate resin in dispersion (A) and melamine resin, based on the total weight of these two resins, are 50 to 90 wt.% of the former and 50 to 10 wt.% of the latter. 10. The aqueous clear coating composition according to item 1 having a solids content of 35 to 65 mass %.

11. A method for forming a multilayer topcoat film, comprising forming on a substrate one or two color base coat layers and one or two clear coat layers, the uppermost clear coat layer being formed by using the aqueous clear coating composition of item 1.

The aqueous clear coating composition and the method of forming a multilayer topcoat film of the invention are described below in detail.

Aqueous clear coating composition The aqueous clear coating composition comprises an aqueous dispersion (A) of a specific hydroxy group- and acid group-containing particulate resin, and a crosslinking agent (B) , and has a viscosity such that the lowest viscosity value of the coating composition as measured over the temperature range of 30°C to 150°C at a frequency of 0.1 Hz at a solids' content of at least 90 mass % is 30 Pa-s or less.

Aqueous hydroxy group- and acid group-containing particulate resin dispersion (A)

The aqueous dispersion (A) is prepared by dispersing in water a hydroxy group- and acid group-containing resin with a hydroxy value of about 30 to about 200 mg KOH/g and an acid value of about 5 to about 50 mg KOH/g to form an aqueous dispersion of a particulate resin with a mean particle size of about 50 to about 300 nm. In view of lowering the melt viscosity, the hydroxy group- and acid group-containing resin preferably has a weight average molecular weight of about 3,000 to about 30,000, and/or a glass transition temperature of about -30 to about +40°C.

The hydroxy group- and acid group-containing resin dispersed in water may be any resin that has a hydroxy value of about 30 to about 200 mg KOH/g and an acid value of about 5 to about 50 mg KOH/g. The hydroxy group mainly acts as a functional group for the reaction of the resin in dispersion (A) with crosslinking agent (B) . The acid group mainly imparts water dispersibility to the resin and also acts as an internal catalyst for the crosslinking reaction of the resin in dispersion (A) with crosslinking agent (B) .

Examples of resins of hydroxy group- and acid group- containing resins include acrylic resins, polyester resins, polyether resins, polycarbonate resins, polyurethane resins and the like. Examples of acid groups include carboxy, sulfonate, phosphate and like groups. Among these, hydroxy group- and acid group-containing acrylic resins, hydroxy group-^' and carboxy group-containing polyester resins, hydroxy group- and carboxy group-containing polyurethane resins and the like are preferable. Hydroxy group- and acid group-containing acrylic resins can be produced by copolymerizing at least one hydroxy group- containing vinyl monomer (M-I) , at least one acid group- containing vinyl -monomer (M-2) and at least one other copolymerizable vinyl monomer (M-3) according to usual methods. The hydroxy group-containing vinyl monomer (M-I) Is a compound containing one hydroxy group and one polymerizable unsaturated bond per molecule. Examples of such monomers include monoesterified products of (meth) acrylic acid with a dihydric alcohol having 2 to 10 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl

(meth) acrylate and the like; compounds obtained by ring-opening polymerization of ε-caprolactone; etc. Commercially available products can be used as such compounds obtained by ring-opening polymerization of ε-caprolactone. Examples of such commercially available products include "PLACCEL FA-I", "PLACCEL FA-2", "PLACCEL FA-3", "PLACCEL FA-4", "PLACCEL FA-5", "PLACCEL FM-I", "PLACCEL FM-2", "PLACCEL FM-3", "PLACCEL FM-4", "PLACCEL FM-5" (trade names, products of Daicel Chemical Industries, Ltd.).

Such monomers (M-I) can be used singly or in combination of two or more. The amount of monomer (M-I) may be any amount such that the resulting hydroxy group- and acid group- containing acrylic resin has a hydroxy value of about 30 to about 200 mg KOH/g.

In this specification, " (meth) acrylate" means acrylate and/or methacrylate . " (Meth) acrylic acid" means acrylic acid and/or methacrylic acid. " (Meth) aeryloyl" means acryloyl and/or methacryloyl.

The acid group-containing vinyl monomer (M-2) is a compound having one acid group and one polymerizable unsaturated bond per molecule. Examples of such monomers include carboxy group-containing unsaturated monomers such as (meth) acrylic acid, crotonic acid, itaconic acid, maleic acid, and maleic anhydride; sulfonic acid group-containing unsaturated monomers such as vinylsulfonic acid and sulfoethyl (meth) acrylate; acid phosphate unsaturated monomers such as 2- (meth) acryloyloxyethyl acid phosphate, 2- (meth) acryloyloxypropyl acid phosphate, 2- (meth) acryloyloxy-3-chloropropyl acid phosphate and 2- methacrolyloxyethylphenyl phosphate; and the like.

Such monomers (M-2) can be used singly or in combination of two or more. The amount of monomer (M-2) may be any amount such that the resulting hydroxy group- and acid group- containing acrylic resin has an acid value of about 5 to about 50 mg KOH/g.

The other copolymerizable vinyl monomer (M-3) is a compound other than the above hydroxy group-containing vinyl monomer (M-I) and acid group-containing vinyl monomer (M-2) and has one polymerizable unsaturated bond per molecule. Specific examples of monomer (M-3) ^' are listed in (I)- (6) below.

(1) Monoesterified products of acrylic or methacrylic acid and a monohydric alcohol having 1 to 20 carbon atoms: for example, methyl (meth) aerylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, cyclohexyl (meth) acrylate, lauryl (meth) acrylate, isobornyl (meth) acrylate, tridecyl (meth) acrylate, stearyl (meth) acrylate and the like.

(2) Aromatic vinyl monomers: for example, styrene, α- methylstyrene, vinyltoluene and the like.

(3) Glycidyl group-containing vinyl monomers: compounds having one glycidyl group and one polymerizable unsaturated bond per molecule, such as glycidyl acrylate, glycidyl methacrylate, and the like.

(4) Polymerizable unsaturated bond-containing amide compounds: for example, acrylamide, methacrylamide, dimethylacrylamide, N,N- dimethylpropylacrylamide, N-butoxymethylacrylamide, N- methylolacrylamide, N-methylolmethacrylamide, diacetoneacrylamide and the like.

(5) Other vinyl compounds: for example, vinyl¹ acetate, vinyl propionate, vinyl chloride, vinyl versatates and the like. Examples of vinyl versatates include commercially available products such as "VEOVA9" and "VEOVAlO" (trade names, products of Japan Epoxy Resin Co., Ltd.) and the like. '

(6) Polymerizable unsaturated bond-containing nitrile compounds: for example, acrylonitrile, methacrylonitrile and the like.

Such other vinyl monomers (M-3) can be used singly or in combination of two or more. ^!

A preferable polymerization method for producing the hydroxy group- and acid group-containing acrylic resin is solution polymerization of a monomer mixture in the presence of a solvent. Such solution polymerization may be single-stage or multistage polymerization.

Single-stage polymerization is a method comprising adding a monomer mixture and a polymerization initiator dropwise^' together to allow a polymerization reaction to proceed. Multistage polymerization is a method comprising adding monomers to a reaction vessel in stages to allow a multistage polymerization to proceed.

To provide an aqueous hydroxy group- and acid group- containing acrylic resin dispersion • (A) with excellent dispersion stability, and a coating composition thereof with excellent dispersion stability, a hydroxy group-' and acid group-containing acrylic resin obtained by multistage polymerization, i.e., two or more stage polymerization is preferably used. More specifically, for example, a hydroxy group- and acid group-containing acrylic resin obtained by a two-stage polymerization method comprising dropwise addition and polymerization of a monomer mixture not containing or substantially not containing any acid group- containing monomers, and dropwise addition and polymerization of a monomer mixture containing an acid group-containing monomer, has good dispersion stability and thus can be advantageously used. ^' The hydroxy group- and acid group-containing acrylic resin has a hydroxy value of about 30 to about 200 mg KOH/g, and preferably about 50 to about 180 mg KOH/g. When the hydroxy value is less than 30 mg KOH/g, the composition may have poor curability. When the hydroxy value exceeds 200 mg KOH/g, the coating film may have insufficient water resistance.

The hydroxy groups of the hydroxy group- and acid group-containing acrylic resin may be primary or secondary. Primary hydroxy groups are preferable for excellent curability of the composition. The presence of both the primary and secondary hydroxy groups is advantageous in terms of improving antifdarning properties of the coating film. 2-Hydroxyisopropyl (meth) acrylate, etc. can be mentioned as examples of the hydroxy group-containing vinyl monomer (M-I) that provides secondary hydroxy groups.

The hydroxy group- and acid group-containing acrylic resin has an acid value of about 5 to about 50 mg KOH/g, and preferably about 10 to about 40 mg KOH/g. When the acid value is less than 5 mg KOH/g, the resulting aqueous dispersion may have poor dispersion stability. When the acid value exceeds 50 mg KOH/g, the obtained coating film may have insufficient water resistance. To provide a coating film with excellent acid resistance and surface smoothness, the hydroxy group- and acid group-containing acrylic resin preferably has a weight average molecular weight of about 3,000 to about 30,000, and more preferably about 5,000 to about 20,000-.

In this specification, the weight average molecular weight of the resins was determined by GPC (gel permeation chromatography) relative to polystyrene standards. In the Production Examples, etc. described later, measurements were made using a GPC apparatus "HLC8120GPC" (trade name, product of Tosoh Corporation) together with four columns "TSKgel G-4000 HXL", "TSKgel G-3000 HXL", "TSKgel G-2500 HXL" and "TSKgel G-2000 HXL" (trade names, products of Tosoh Corporation) under the following conditions: mobile phase: tetrahydrofuran; measurement temperature: 40⁰C; flow rate: 1 cc/min.; and detector: RI.

To provide a coating film with excellent hardness and surface smoothness, the hydroxy group- and acid group-containing acrylic resin preferably has a glass transition temperature of within the range of from about -30⁰C to about +40⁰C, and more preferably about -20⁰C to about +20⁰C.

In this specification, the glass transition temperature was determined at a temperature increase rate of 10°C/min. by DSC (differential scanning calorimetry) according to JIS K7121 (method of measuring the transition temperature of plastics) . In the Production Examples, etc. described later, measurements'¹ were made using a DSC apparatus "SSC5200" (trade name, product of Seiko Instruments, Inc.) after a specific amount of sample was placed on a sample tray .and dried at 13O⁰C for 3 hours.

The hydroxy group- and carboxy group-containing polyester resin can be prepared, for example, by esterification of a polybasic acid with a polyhydric alcohol according to the usual method.

The polybasic acid is a compound having at least two carboxy groups per molecule, and examples thereof include phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid, fumaric acid, itaconic acid, trimellitic acid, pyromellitic acid-,- and like polybasic acids; and anhydrides thereof. The polyhydric alcohol is a compound having at least two hydroxy groups per' molecule, and examples thereof include ethylene glycol, propylene glycol, 1,3- propanediol, 1, 4-butanediol, 1,5-pentanediol, 1, 6-hexanediol, 2,2-diethyl-l,3-propanediol, neopentylglycol, 1, 9-nonanediol, 1, 4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-l,3-propanediol, 3-methyl-l,5-pentanediol, 2,2,4- trimethylpentanediol, hydrogenated bisphenol A, and like diols; trimethylolpropane, trimethylolethane, glycerin, pentaerythritol and like at least trihydric polyols; 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2- dimethylolhexanoic acid, 2,2-dimethyloloctanoic acid and like hydroxycarboxylic acids; etc.

Alternatively, a monoepoxy compound such as propylene oxide, butylene oxide or like α-olefin epoxide, "Cardura ElO"

(trade name, product of Japan Epoxy Resin Co., Ltd., a synthetic highly branched saturated fatty acid glycidyl ester) or the like may be reacted with an acid and the resulting compound may be introduced into the polyester resin.

The introduction of carboxy groups into the polyester resin can be made, for example, by addition of an acid anhydride to a hydroxy group-containing polyester for half-esterifica'-tion.

Examples of usable acid anhydrides include trimellitic anhydride and the like.

The hydroxy group- and carboxy group-containing polyester resin has a hydroxy value of about 30 to about 200 mg KOH/g, and preferably about 50 to about 180 mg KOH/g. When the hydroxy value is less than 30 mg KOH/g, the resulting composition may have poor curability. When the hydroxy value exceeds 200 mg ^'

KOH/g, the obtained coating film may have insufficient water resistance. The hydroxy group- and carboxy group-containing polyester resin has an acid value of about 5 to about 50 mg KOH/g, and preferably about 10 to about 40 mg KOH/g. When the acid value is less than 5 mg KOH/g, the resulting aqueous dispersion may have poor dispersion stability. When the acid value exceeds 50 mg KOH/g, the obtained coating film may have insufficient water resistance.

To provide a coating film with excellent acid resistance and surface smoothness, the hydroxy group- and carboxy group-containing polyester resin preferably has a weight average molecular weight of about 3,000 to about 30,000, and more preferably about 5,000 to about 20,000.

To provide a coating film with excellent hardness and surface smoothness, the hydroxy group- and carboxy group- containing polyester resin preferably has a glass transition temperature within the range of from about -3O⁰C to about +4O⁰C, and more preferably about -2O⁰C to about +20⁰C.

Hydroxy group- and carboxy group-containing polyurethane resins include resins known to be usable for aqueous coating compositions, such as resins having carboxy groups introduced into the molecule that are obtained by reacting a polyol mixture comprising a carboxyl group-containing polyol with a diisocyanate .

The polyol mixture preferably comprises a carboxy group-containing polyol and a polyol containing no carboxy groups. Examples of polyols containing no carboxy groups ^l include low molecular weight polyols such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, hexamethylene glycol and like dihydric alcohols; trimethylolpropane, glycerol, pentaerythritol and like trihydric alcohols; and higher molecular weight polyols such as polyether polyols, polyester polyols, acrylic polyols, epoxy polyols and the like. Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like. Examples of polyester polyols include polycondensates of a dihydric alcohol such as dipropylene glycol, 1,4-butanediol, 1,6- hexanediol, neopentyl glycol or the like with a dibasic acid such as adipic acid, azelaic acid, sebacic acid or the like; ^' polyols obtained by ring-opening polymerization of a lactone such as polycaprolactone; polycarbonate diols; etc. Examples of carboxy group-containing polyols include

2,2-dimethylolpropionic acid, 2,2-dimethlolbutanoic acid and the like. Among these, 2,2-dimethylolpropionic acid is particularly preferable. When using such a carboxy group-containing polyol, a small quantity of solvent such as N-methylpyrrolidone can be used to accelerate the reaction.

Examples of polyisocyanates to be reacted with such polyols .include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, lysine diisocyanate and the like; biuret adducts and isocyanurate ring adducts of such aliphatic diisocyanates; alicyclic diisocyanates such as isophorone diisocyanate, 4,4'- methylenebis (cyclohexylisocyanate) , methylcyclohexane-2, 4- diisocyanate, methylcyclohexane-2, 6-diisocyanate, 1,3- di (isocyanatomethyl)cyclohexane, 1,4- di (isocyanatomethyl)cyclohexane, 1, 4-cyclohexane diisocyanate, 1, 3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate and the like; biuret adducts and isocyanurate ring adducts of such alicyclic diisocyanates; aromatic diisocyanate compounds such as xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,^;4'- diphenylmethane diisocyanate, 1, 5-naρhthalene diisocyanate, 1,4- naphthalene diisocyanate, 4 , 4' -diphenylether diisocyanate, m- phenylene diisocyanate, p-phenylene diisocyanate, 4,4'- biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis (4-isocyanatophenyl) sulfone, isopropylidene bis (4-phenylisocyanate) and the like; biuret adducts and isocyanuric ring adducts of such aromatic diisocyanates; polyisocyanates having at^' least three isocyanate groups per molecule, such as triphenylmethane-4, 4' ,4"-triisocyanate, 1,3,5- triisocyanatobenzene, 2, 4, 6-triisocyanatotoluene, 4,4'- dimethyldiphenylmethane-2, 2' , 5, 5' -tetraisocyanate and the like; biuret adducts and isocyanurate ring adducts of such polyisocyanates; etc.

The hydroxy group- and carboxy group-containing polyurethane resin has a hydroxy value of about 30 to about 200 mg KOH/g, and preferably about 50 to about 180 mg KOH/g. When the hydroxy value is less than 30 mg KOH/g, the resulting composition may have poor curability. When the hydroxy value exceeds 200 mg KOH/g, the obtained coating film may have insufficient water resistance.

The hydroxy group- and carboxy group-containing polyurethane resin has an acid value of about 5 to about 50 mg KOH/g, and preferably about 10 to about 40 mg KOH/g. When the acid value is less than 5 mg KOH/g, the resulting aqueous dispersion may have poor dispersion stability. When the acid value exceeds 50 mg KOH/g, the obtained coating film may have insufficient water resistance.

To provide a coating film with excellent acid resistance and surface smoothness, the hydroxy group- and carboxy group-containing polyurethane resin preferably has a weight average molecular weight of about 3,000 to about 30,000, and more preferably about 5,000 to about 20,000.

To provide a coating film with excellent hardness and surface smoothness, the hydroxy group- and carboxy group- containing polyurethane resin preferably has a glass transition temperature of about -30 to about +4O⁰C, and more preferably about -20 to about +20⁰C.

The aqueous hydroxy group-containing and acid group- containing particulate resin dispersion (A) is obtained by dispersing a hydroxy group- and acid group-containing resin in water in such a manner that the particulate resin has a mean particle size of about 50 to about 300 nm. The dispersion of the hydroxy group- and acid group-containing resin in water can be made, for example, in the following manner. A hydroxy group- and acid group-containing resin is usually obtained in the form of an organic solvent solution by various synthetic reactions and the solvent is distilled off under reduced pressure to a solids content of about 95 mass %. The temperature during the reduced pressure distillation is set to an optimal temperature according to the kind of solvent used in the preparation of the resin. To reduce the amount of VOCs, the organic solvent is preferably distilled off as much as possible. After the organic solvent is distilled off under reduced pressure, for example, a neutralizing agent is added at about 90⁰C to neutralize the resin solution and then a specific amount of deionized water is added dropwise with stirring at about 80⁰C, thus giving an aqueous hydroxy group- containing and acid group-containing particulate resin dispersion (A) .

Examples of preferable neutralizing agents include ammonia, ethylamine, isopropylamine, cyclohexylamine, dipropylamine, dibutylamine, triethylamine, tributylamine, ethylenediamine, morpholine, pyridine, isopropanolamine, methylisopropanolamine, dimethylethanolamine, aminomethylpropanol, diisopropanolamine, diethanolamine, triethanolamine, diethylethanolamine and like amine compounds.

The amount of neutralizing agent can be suitably selected. To provide excellent dispersion stability, the amount of neutralizing agent is preferably about 0.4 to about 0.9 equivalents, and particularly preferably about 0.5 to about 0.8 equivalents, per acid group of the hydroxy group- and acid¹group- containing resin. In the aqueous dispersion, if necessary, emulsifiers may be used to improve dispersibility.

In the aqueous clear coating composition of the invention, it is necessary to disperse the hydroxy group- and acid group-containing resin in water in such a manner that the particulate resin in the aqueous dispersion (A) has a mean particle size of about 50 to about 300 nm, preferably about 100 to about 250 nm, and more preferably about 100 to about 200 nm. When the mean particle size of the dispersed resin particles is less than 50 nm, the aqueous dispersion (A) may have high viscosity and poor antifoaming properties, etc. When the mean particle size exceeds 300 nm, the coated surface may have insufficient smoothness.

In this specification, the mean particle size of the particulate resin is a value obtained by measurement at 2O⁰C using a submicron particle size distribution analyzer after the resin dispersion is diluted with deionized water according to the usual method. Examples of submicron particle size distribution analyzers include "COULTER N4" (trade name, product of Beckman Coulter, Inc. ) .

To provide excellent surface smoothness, application workability and hardness of the coating film, the melt viscosity of the hydroxy group- and acid group-containing particulate resin in dispersion (A) as measured at a solids content of at least 96 mass % at a shear rate of 564 s^"1 at 140⁰C is preferably in the range of about 1 Pa*s to about 12 Pa-s, more preferably about 1 Pa-s to about- 8 Pa* s, and further preferably about 1 Pa* s to about 6 Pa ^• s . ^■ ■

Melt viscosities as measured at 120°C and 16O⁰C under the same conditions as in the measurement at 140⁰C are preferably within the following ranges, respectively. ^"The melt viscosity as measured at 120⁰C is preferably about 6 Pa* s to about 72 Pa* s, more preferably about 6 Pa*s to about 48 Pa* s, and further preferably about 6 Pa*s to about 36 Pa* s. The melt viscosity as measured at 16O⁰C is preferably about 0.2 Pa* s to about 2.4^! Pa* s, more preferably about 0.2 Pa*s to about 1.6 Pa* s, and further preferably about 0.2 Pa*s to about 1.2 Pa* s.

The melt viscosity of the hydroxy group- and acid group-containing particulate resin at a solids content of at least 96 mass % means that the hydroxy group- and acid group- containing particulate resin has a melt viscosity within the above-mentioned range when measured at any concentration not lower than 96 mass %, on a solids basis.

In this specification, the melt viscosity of the hydroxy group- and acid group-containing particulate resin at a solids- content of at least 96 mass % was determined by applying the dispersion (A) to a glass plate using a 4-mil applicator, drying at 110⁰C for 3 hours to a solids content of at least 96 mass % and then measuring the viscosity at 14O⁰C at a shear rate of 564 s^~ using a cone-and-plate viscosity meter. In the

Production Examples described later, measurements were made using a cone-and-plate viscosity meter "VISCONE CV-I" (trade name, product of Misec Corporation) together with a IOOP rotor (cone diameter: 14.5 mm, corn angle: 2°). The hydroxy group- and acid group-containing particulate resin solids content was determined by scraping off the uncured coating film formed by drying at 110⁰C for 3 hours by the above process, placing about 2.0 g of the uncured coating film in an aluminum foil cup with a diameter of about 5 cm, heating at 110⁰C for 1 hour and measuring the amount of residue (g) .

The aqueous dispersion (A) is not particularly limited as long as the dispersion is obtained by dispersing the hydroxy group- and acid group-containing resin in water. Such dispersions can be used singly or in combination of two or more. Dispersions obtained by dispersing a hydroxy group- and acid group-containing acrylic resin or a hydroxy group- and carboxy group-containing polyester resin in water are preferably used as aqueous dispersions (A) . -

To provide an aqueous dispersion with excellent stability, the aqueous dispersion (A) preferably has a solids content of about 35 to about 55 mass %. In the Production Examples, etc. described later, the solids content was calculated by placing about 2.0 g of the aqueous dispersion in an aluminum foil cup with a diameter of about 5 cm, heating at HO⁰C for 1 hour and measuring the amount of residue (g) .

To provide an aqueous dispersion with excellent stability, the aqueous dispersion (A) preferably has a Brookfield viscosity of about 400 to about 1,000 Pa* s, and more preferably about 500 to about 900 Pa- s. In the Production Examples, etc. described later, the viscosity was measured at 20⁰C at 60 rpm using a Brookfield viscometer.

To provide an aqueous dispersion with excellent stability, the aqueous dispersion (A) preferably has a pH of about 6.0 to about 8.5, and more preferably about 6.5 to about 8.0. In the Production Examples, etc. described later, the pH was measured using a pH meter. Examples of pH meters include "F-22" (trade name, product of Horiba Ltd. ) . Crosslinking agent (B)

The crosslinking agent (B) of the aqueous clear coating composition of the invention is not limited as long as it is capable of crosslinking with the hydroxy groups of the hydroxy group- and acid group-containing resin in the aqueous dispersion (A) . Examples of usable crosslinking agents include polyisocyanate compounds, blocked polyisocyanate compounds, melamine resins and the like. Among these, polyisocyanate compounds and blocked polyisocyanate compounds are preferable.

Polyisocyante compounds have at least two free isocyanate groups per molecule. Examples of polyisocyanate compounds include those known for use in the production of polyurethanes, such as aliphatic polyisocyanates, alicyclic polyisocyanates, aliphatic-aromatic polyisocyanates, aromatic polyisocyantates, and derivatives thereof.

Examples of aliphatic polyisocyanates include trimethylene diisocyanate,. tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, ' 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3- butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- trimethylhexamethylene diisocyanate, 2,2, 4-trimethylhexamethylene diisocyanate, 2, 6-diisocyanatomethylcaproate and like aliphatic diisocyanates; lysine ester triisocyanates, 1, 4, 8-triisocyanato octane, 1, 6, 11-triisocyanato undecane, 1, 8-diisocyanato-4- isocyanato methyloctane, 1,3, 6-triisocyanato hexane, 2,5,7- trimethyl-1, 8-diisocyanate-5-isocyanato methyloctane and like aliphatic triisocyanates; etc. Examples of alicyclic polyisocyanates include 1,3- cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3- cyclohexane diisocyanate, 3-isocyanatomethyl-3, 5,5- trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate) , 4, 4' -methylenebis (cyclohexylisocyanate) , methyl- 2, 4-cyclohexane diisocyanate, methyl-2, 6-cyclohexane diisocyanate, 1, 3-bis (isocyanatomethyl) cyclohexane, 1,4- bis (isocyanatomethyl) cyclohexane, norbornane diisocyanate and like alicyclic diisocyanates; 1,3,5- triisocyanato cyclohexane, 1, 3, 5-trimethylisocyanato cyclohexane, 2- (3-isocyanatopropyl) -2, 5-di (isocyanatomethyl) - bicyclo[2.2.1] heptane, 2- (3-isocyanatopropyl) -2, 6- " di (isocyanatomethyl)-bicyclo[2.2.1] heptane, 3- (3- isocyanatopropyl) -2, 5-di (isocyanatomethyl) -bicyclo [2.2.1] heptane, 5- (2-isocyanatoethyl) -2-isocyantemethyl-3- (3-isocyanatopropyl) - bicyclo [2.2.1] heptane, 6- (2-isocyanatoethyl) -2-isocyanatomethyl- 3- (3-isocyanatopropyl) -bicyclo [2.2.1] heptane, 5- (2- isocyanatoethyl) -2-isocyanatomethyl-2- (3-isocyanatopropyl) - bicyclo [2.2.1] heptane, 6- (2-isocyanatoethyl) -2-isocyanatemethyl- ^■ 2- (3-isocyanatopropyl) -bicyclo [2.2.1] heptane and like alicyclic triisocyanates; etc.

Examples of aliphatic-aromatic polyisocyanates include 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate, ω, ω' - diisocyanato-1, 4-diethylbenzene, 1, 3-bis (1-isocyanato-l- methylethyl)benzene (common name: tetramethylxylylene diisocyanate), 1, 4-bis (1-isocyanato-l-methylethyl) benzene ('common name: tetramethylxylylene diisocyanate) and like aliphatic- aromatic diisocyanates; and 1,3,5-triisocyanate methylbenzene and like aliphatic-aromatic triisocyanates; and the like. Examples of aromatic polyisocyanates include m-phenylene diisocyanate, p-phenylene diisocyanate, 4, 4' -diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2, 4' -diphenylmethane diisocyanate, 4, 4' -diphenylmethane diisocyante, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4, 4' -toluidine diisocyanate, 4, 4' -diphenylether diisocyanate and like aromatic diisocyanates; triphenylmethane-4, 4' ,4' ' -triisocyanate, 1,3,5- triisocyanato benzene, 2, 4, 6-triisocyanato toluene and like aromatic triisocyanates; 4,4' -diphenylmethane-2, 2' ,b,b^r - tetraisocyanate and like aromatic tetraisocyanates; etc.

Examples of polyisocyanate derivatives include dimers, trimers, biurets, allophonates, carbodiimides, urethodiones, urethoimines, isocyanurates, oxadiazinediones and like-modified derivatives of such polyisocyanate compounds.

Such polyisocyanates can be used singly or in combination of two or more. Among the above polyisocyantes, aliphatic diisocyanates, alicyclic diisocyanates and derivatives thereof are preferably used in view of excellent weatherability, etc. of the obtained cured coating film.

Blocked polyisocyanate compounds are obtained by blocking isocyanate groups of such polyisocyanate compounds with a blocking agent.

A blocking agent is used to block the free isocyanate groups. When the blocked isocyante groups are heated, for example, at 100⁰C or more, preferably 130⁰C or more, the blocking agent is dissociated from the isocyanate groups and the recovered isocyanate groups can then easily react with hydroxy groups.

Examples of such blocking agents include phenol, cresol, xylenol, nitrophenol, ethylphenol, hydroxydiphenyl, butylphenol, isopropylphenol, -nonylphenol, octylphenol, methyl hydroxybenzoate and like phenols; ε-caprolactam, δ-valerolactam, γ-butyrolactam, β-propiolactam and like lactams; methanol, ethanol, propyl ' alcohol, butyl alcohol, amyl alcohol, lauryl alcohol and like aliphatic alcohols; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, methoxymethanol and like ethers; benzyl alcohol; glycόlic acid; methyl glycolate, ethyl glycolate, butyl glycolate and like glycolates; lactic acid, methyl lactate, ethyl lactate, butyl lactate and like lactates; methylol urea, methylol melamine, diacetone alcohol, 2- hydroxyethyl acrylate, 2-hydroxyethyl methacrylate and like alcohols; formamide oxime, acetamide oxime, acetoxime, methyl ethyl ketoxime, diacetyl monoxime, benzophenone oxime, cyclohexane oxime and like oximes; dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, acetylacetone and like active methylene group-containing compounds; butyl mercaptan, t-butyl mercaptan, hexyl mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol, methylthiophenol, ethylthiophenol and like mercaptans; acetanilide, acetanisidide, acetotoluide, acrylamide, methacrylamide, acetamide, stearamide, benzamide and like acid amides; succinimide, phthalimide, maleimide and like imides; diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine, butylphenylamine and like amines; imidazole, 2-ethylimidazole and like imidazoles; 3,5- dimethylpyrazole and like pyrazoles; urea, thiourea, ethylene urea, ethylenethiourea, diphenylurea and like urea derivatives; phenyl N-phenylcarbamate and like carbamates; ethyleneimine, propyleneimine and like imines; sodium bisulfite, potassium bisulfite and like sulfites; etc. If necessary, solvents may be used in reacting such a polyisocyanate compound with a blocking agent to form a blocked polyisocyanate compound. Examples of solvents preferably used include solvents ^■ that are not reactive to isocyanate groups . Examples of such solvents include acetone, methyl ethyl ketone and like ketones; ethyl acetate and like esters; N-methyl ⁽ pyrrolidone (NMP) ; etc.

Preferable compounds used as crosslinking agents in the aqueous clear coating composition of the invention to improve water dispersiblity and reduce the amount of VOCs are hydrophilic polyisocyanate compounds and hydrophilic blocked polyisocyanate compounds obtained by modifying polyisocyanate compounds and blocked polyisocyanate compounds as mentioned above to hydrophilic forms.

Hydrophilic polyisocyanate compounds can be obtained, for example, by reacting polyisocyanate compounds and hydrophilic polyether alcohols such as polyethylene glycol monomethyl ether.

Hydrophilic polyisocyanate compounds can also be obtained, for example, by reacting the isocyanate groups of a polyisocyanate compound and active hydrogen group (s) of an active-hydrogen-group-containing compound having anionic group (s)

Active-hydrogen-group-containing compounds having anionic group (s) have anionic group (s) such as carboxy, sulfonyl, phosphate, sulfobetaine and like betaine structure-containing group (s) and contain active hydrogen group (s) capable of reacting with isocyanate groups, such as hydroxy, amino and like groups.

Examples of such active-hydrogen-group-containing compounds having anionic group (s) include compounds having one anionic group and two or more active hydrogen groups . More specifically, examples of active-hydrogen-group-containing compounds having carboxy group (s) include dihydroxycarboxylic acids such as 2,2-dimethylolacetic acid, 2,2-dimethylollactic acid, 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, dimethylheptanoic acid, dimethylolnonanoic acid, 2,2- dimethylolbutyric acid, 2,2-dimethylolvaleric acid and the like; and diaminocarboxylic acids such as l-carboxy-1,5- pentylenediamine, dihydroxybenzoic acid, 3, 5-diaminobenzoic acid, lysine, arginine and the like; half-esterified products of polyoxypropyene triol with maleic anhydride or phthalic anhydride; etc. Examples of active-hydrogen-group-containing compounds having sulfonyl group (s) include N,N-bis (2-hydroxyethyl) -2- aminoethanesulfonic acid, l,3-phenylenediamine-4, 6-disulfonic acid, diaminobutanesulfonic acid, 3, 6-diamino-2-toluenesulfonic acid, 2, 4-diamino-5-toluenesulfonic acid and the like. Examples of active-hydrogen-group-containing compounds having phosphate group (s) include^~2,3-dihydroxypropylphenyl phosphate .

Examples of active-hydrogen-group-containing compounds having betaine structure-containing group (s) include sulfobetaine group-containing compounds obtained by reacting a tertiary amine such as N-methyldiethanolamine with 1,3-propanedisulfonic acid.

Examples of active-hydrogen-group-containing compounds having anionic group (s) further include alkyleneoxide-modified products obtained by addition of an alkyleneoxide such as ethyleneoxide, propylenoxide or the like, to anionic group- containing compounds.

Such active-hydrogen-group-containing compounds having anionic group (s) may be used singly or in combination of two or more. Active-hydrogen-group-containing compounds having sulfonyl group (s) and active-hydrogen-group-containing compounds having phosphate group (s) are particularly preferable.

Polyisocyanate compounds for modification into hydrophilic forms may be polyisocyante compounds mentioned above. Preferable examples thereof include hexamethylene diisocyanate (HMDI) and derivatives thereof, isophorone diisocyanate (IPDI) and derivatives thereof. ₍

Hydrophilic blocked isocyanate compounds can be synthesized by, for example, blocking isocyante groups of such ^■ hydrophilically modified isocyanate compounds with a blocking agent.

Examples of usable blocking agents are as mentioned in the description of blocked polyisocyanate compounds. The blocking reaction can be carried out in the same manner as mentioned in the description of blocked polyisocyanate compounds. When a polyisocyanate compound or- blocked '^■ polyisocyanate compound is used as crosslinking agent (B) , organic tin compounds can be used as curing catalysts .

Examples of melamine resins include dimethylolmelamine^', trimethylolmelamine, tetramethylolmelamine, pentamethylolmelamine, hexamethylolmelamine and like methylolmelamines; alkyl-etherified products of methylolmelamines witfi an alcohol; alkyletherified methylolmelamine condensates; etc. Examples of usable alcohols include methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol, 2-ethylhexyl alcohol and the like. Examples of preferable melamine resins include melamine resins having an average of at least three methyl-etherfied methylol groups per triazine nucleus; hydrophilic imino group- containing alkyl-etherified melamine resins with a weight average molecular weight of about 500 to about 1,000; etc.

Examples of usable melamine resins include commercially available products, which are available, for example, under the ^' tradenames "Cymel 303", "Cymel 323", "Cymel 325"', "Cymel 327", "Cymel 350", "Cymel 370", "Cymel 380", "Cymel 385" and "Cymel 254" (products of Japan Cytec Industries, Inc.); "Resimene 735", "Resimene 740", "Resimene 741", "Resimene 745", "Resimene 746" and "Resimene 747" (products of Monsanto Chemical Co., Ltd.), "SUMIMAL M55", "SUMIMAL M30W" and "SUMIMAL M50W" (products of Sumitomo Chemical Co., Ltd.); "U-VAN 20SE" (product of Mitsui Chemicals, Inc.); etc.

Such melamine resins can be used singly or in combination of two or more.

When a melamine resin is used as crosslinking agent (B) , curing catalysts can be used. Examples of such curing catalysts include para-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid and like sulfonic acids; amine neutralization salts of such sulfonic acids; and amine neutralization salts of phosphate compounds; etc.

At least one crosslinking agent selected from the group consisting of polyisocyanate compounds, blocked polyisocyanate compounds, and melamine resins, is preferably used as crosslinking agent (B) .

When a polyisocyanate compound and/or blocked polyisocyanate compound is used as crosslinking agent (B) in the aqueous clear coating composition of the invention, in view of excellent curability and stability of the coating composition it is preferable that the equivalent ratio (NCO/OH) of isocyanate ^' groups of said polyisocyanate compound and/or blocked polyisocyanate compound to hydroxy groups of the hydroxy group- and acid group-containing particulate resin in dispersion (A) be within the range of about 0.5 to about 2.0, and more preferably about 0.8 to about 1.5.

When a melamine resin is used as crosslinking agent (B) in the aqueous clear coating composition of the invention, in view of excellent curability of the composition it is preferable that the weight proportions of hydroxy group- and acid group- containing particulate resin in dispersion (A) and melamine resin, based on the total weight of these two resins, be 50 to 90 wt.% of particulate resin and 50 to 10 wt.% of melamine resin, and more preferably 60 to 80 wt.% of particulate resin and 40 to 20 wt.% of melamine resin.

If necessary, the aqueous clear coating composition of the invention may contain UV absorbers. Examples of usable UV absorbers include benzotriazole UV absorbers_/ triazine UV absorbers and the like.

Examples of benzotriazole UV absorbers include 2-(2'- hydroxy-5' -methylphenyl) benzotriazole, 2- [2' -hydroxy-3' , 5' -di (1, 1-dimethylbenzyl) phenyl] -2H-benzotriazole, 2- (2' -hydroxy-3' , 5' - di-tert-butylphenyl) benzotriazole, 2- (2' -hydroxy-3' -tert-butyl- 5' -methylphenyl) benzotriazole, 2- (2' -hydroxy-3' , 5' -di-tert- amylphenyl)benzotriazole, 2- (2' -hydroxy-3' , 5' -di-tert- butylphenyl) -5-chlorobenzotriazole, 2- (2' -hydroxy-3' , 5' -di- isoamylphenyl) benzotriazole, 2- (2' -hydroxy-5' -tert- butylphenyl) benzotriazole .and the like. Examples of triazine UV absorbers include 2- (4,6-' diphenyl-1, 3, 5-triazin-2-yl) -5 (hexyloxy) -phenol (e.g. , "TINUVIN 1577FF", trade name, product of Ciba Specialty Chemicals) , a mixture of 2- [4- [6 (2-hydroxy-3-dodecyloxypropyl) oxy] -2- hydroxyphenyl]-4, 6-bis (2,4-dimethylphenyl)-l,3, 5-triazine and 2- [4- [6 (2-hydroxy-3-dodecyloxypropyl) oxy] -2-hydroxyphenyl] -4,6- bis (2, 4-dimethylphenyl) -1,3, 5-triazine (e.g., "TINUVIN 400", trade name, product of Ciba Specialty Chemicals), 2,4-bis(2,4- dimethylphenyl) -6- (2-hydroxy-4-iso-octyloxyphenyl) -s-triazine (e.g., "TINUVIN 411L", trade name, product of Ciba Specialty Chemicals) , 2, 4-bis (2, 4-dimethylphenyl) -6- (2-hydroxy-4- octyloxyphenyl)-l,3,5-triazine (e.g., "Cyagard UV-Il64L", trade name, product of Mitsui-Cytec Ltd.), and the like.

Such UV absorbers can be used singly or in combination of two or more. The amount of UV absorber is preferably about 0.1 to about 10 parts by weight, more preferably about 0.5 to about 5 parts by weight, and further preferably about 0.8 to about 3 parts by weight, per 100 parts by weight of the total amount of aqueous hydroxy group- and acid group-containing particulate resin dispersion (A) and crosslinking agent (B) , on a solids basis.

If necessary, the aqueous clear coating composition of the invention may contain light stabilizers. Examples of light stabilizers include hindered amine derivatives. Specific examples thereof include bis (2,2' , 6, 6'-tetramethyl-4-piperidinyl) sebacate, 4-benzoyloxy-2,2' ,6, 6' -tetramethylpiperidine and the like. Such light stabilizers can be used singly or in combination of two or more. The amount of light stabilizer is preferably about 0.1 to about 10 parts by weight, more preferably about 0.5 to about 5 parts by_.weight, and further preferably about 0.8 to about 3 parts by weight, per 100 parts by weight of the total amount of aqueous hydroxy containing- and acid group- containing particulate resin dispersion (A) and crosslinking agent (B), on a solids basis.

To facilitate dispersion of such UV absorbers and light stabilizers in water to prepare the aqueous coating composition, for example, a method comprising dissolving such additives in a solvent such as toluene, followed by dispersion in water using an emulsifier; a method comprising dissolving such additives in the hydroxy group- and acid group-containing resin solution finally obtained in the resin synthesis and dispersing the additives and the resin in water at the same time; a method comprising adding such additives to a monomer mixture used as the starting material for the synthesis of a hydroxy group- and acid group-containing resin, followed by polymerizing the monomer mixture and dispersing the additives together with the resin in water. The aqueous clear coating composition of the invention may contain curing catalysts, rheology control agents, surface conditioners, coloring pigments, metallic pigments, light interference pigments, extender pigments and like additives, if required. The amount of coloring pigments, metallic pigments, light interference pigments, extender pigments, etc. may be within the range that transparency of the coating film is not impaired.

When a polyisocyanate compound and/or a hydrophilic polyiso^'cyante compound, which are capable of easily crosslinking with hydroxy groups at room temperature, are used as crosslinking agent (B) in the aqueous clear coating composition, it is preferable that the aqueous hydroxy group- and acid group- containing particulate resin dispersion (A) and crosslinnking agent (B) are separate as a two package composition and mixed just before application. _ In this^' case, it is usually preferable that the UV absorbers, light stabilizers and other additives be incorporated as components of aqueous dispersion (A) . Mixing can be performed by using known mixing devices such as agitators, homogenizers and the like.

The aqueous clear coating composition of the invention is mainly characterized in having a viscosity such that the lowest viscosity value of the composition as measured over the temperature range of 30⁰C -to 150°C at a frequency of 0.1 Hz at a solids content of at least 90 mass % is 30 Pa-s or less. The lowest viscosity value is preferably 20 Pa^«s or less, and more preferably 15 Pa-s or less. The lowest viscosity value means that when the viscosity of the composition is measured over the temperature range of 30⁰C to 150°C at a frequency of 0.1 Hz at any solids content not lower than 90 mass %, the lowest viscosity value falls within the above-mentioned range.

In the aqueous clear coating composition of the present invention, the lowest viscosity value of the composition as measured over the temperature range of 30⁰C to 150⁰C refers to the lowest complex viscosity during the melting and flowing of the composition by heating after application of the composition to a substrate. The temperature during the melting and flowing of the agueous clear coating composition by heating is usually in the range of 3O⁰C to 150⁰C. In this specification, the lowest viscosity was determined using a viscoelasticity meter. In the Examples, etc. described later, the viscosity was measured. using a viscoelasticity meter "Rheostress RS-150" (trade name, product of HAAKE) . More specifically, the aqueous clear coating composition whose viscosity as measured at 20⁰C using Ford Cup No. 4 had been adjusted to 15 to 60 seconds was applied by air spraying to the surface of a tin plate (300 x 450 x 0.3 mm) degreased with isopropanol to a film thickness of 35 μm (when dried) and heated at 6O⁰C for about 10 minutes. The uncured coating film formed on the tin plate was scraped off and collected into a sample bottle, which was sealed immediately by closing the cap. Using 1.0 g of this sample, the dynamic viscoelasticity was measured under strain control (frequency: 0.1 Hz, strain: 1.0, temperature increase rate: 6°C/min., sensor: parallel plate (φ = 20 mm), gap: 0.5 mm) over the temperature range of 30⁰C to 150⁰C to determine the lowest complex viscosity.

The solids content of the coating composition during ^■ the viscosity measurement was calculated by placing about 2.0 g of the sample in an aluminum foil cup with- a diameter of 5 cm, heating at 110⁰C for 1 hour and then measuring the amount of residue (g) .

When the lowest viscosity exceeds 30 Pa- s, the aqueous clear coating composition when heated after application has an increased solids content and thus has low thermal flowability, so that problems such as poor surface smoothness, foaming, etc. of the obtained coating film result. Herein, measurements were made at a solids content of at least 90 mass % because aqueous clear coating compositions when heated to flow after application usually have a high solids content of at least 90 mass %. The viscosity mainly depends on the properties of the hydroxy group- and acid group-containing particulate resin in dispersion (A) . The lowest viscosity value can be adjusted to 30 Pa-s or less by, for example, using a hydroxy group- and acid group-containing particulate resin in dispersion (A) whose melt viscosity as measured at a solids content of at least 96 mass % at a shear rate of 564 s^"1 at 140⁰C is about 1 to about 12 Pa-s. The aqueous hydroxy group- and acid group-containing particulate resin having such a melt viscosity preferably has a weight average molecular weight of about 3,000 to about 30,000 and/or a glass transition temperature within the range of about -30⁰C to about +40⁰C.

Method of preparing the coating composition

The aqueous clear coating composition of the invention can be prepared by mixing the aqueous hydroxy group- and acid group-containing particulate resin dispersion (A) and crosslinking agent (B) , optionally together with additives such as UV absorbers, light stabilizers and other additives, according to known methods. When the composition of the invention is a two package composition, it is preferable to mix the components just before use. When applied, the composition may be diluted with deionized water, if necessary, and the viscosity is preferably adjusted to about 20 to about 60 seconds as measured at 20⁰C using Ford Cup No.. 4, and more preferably be about 30 to 50 seconds. In this case, the solids content is preferably about 35 to about 65 mass %, and preferably about 40 to about 60 mass %.

The amount of VOCs in the aqueous clear coating composition of the invention is preferably 0 to 300 g/1, and more preferably 0 to 150 g/1. Herein, VOCs refers to volatile organic substances classified as "highly volatile organic compounds" or "volatile organic compounds" by the World Health Organization (WHO) .

Application methods

The aqueous clear coating composition of the invention can be advantageously used in various application methods described below. Substrates

Examples of substrates to be coated include automobiles, motorcycles and like vehicle bodies and parts thereof. Examples of substrates include those that constitute such vehicle bodies and the like, such as cold rolled steel sheets and plates, galvanized steel sheets and plates, zinc alloy-plated steel sheets and plates, stainless steel sheets and plates, tinned steel sheets and plates and like steel sheets and plates, aluminum sheets and plates, aluminum alloy sheets and plates and like metal substrates; plastic substrates; and the like.

Such vehicle bodies, parts and metal substrates whose metal surfaces have been subjected to a chemical conversion treatment, such as phosphate treatment or chromate treatment, are also usable as substrates. Such vehicle bodies, metal substrates and the like onto which an undercoat of a cationic electrodeposition coating composition, etc. and/or an intermediate coat have been formed are also usable as substrates. Application and curing methods

The method of applying the coating composition of the invention is not particularly limited. For example, air spray coating, airless spray coating, rotary atomization coating, curtain coating and like application methods can be used to form a wet coating film. In air spray coating, airless spray coating and rotary atomization coating, an electrostatic charge may be applied, if necessary. The coating composition is preferably applied to a film thickness of about 10 to about 80 urn (when cured) , and more preferably about 20 to about 60 urn (when cured) .

Heating can be performed by known heating means. For example, drying furnaces such as hot air furnaces, electric furnaces, infrared heating furnaces and the like can be used.

The heating temperature^"is usually within the range of from about 80⁰C to about 18O⁰C, and preferably about 100⁰C to about 160⁰C. The heating time is usually about 10 to about 40 minutes . Before the heating, if necessary, preheating at about 50⁰C to about 8O⁰C for about 3 to about 10 minutes may be carried out to promote vaporization of volatile components such as water. Multilayer topcoat formation method

The coating composition of the invention can form a coating film with excellent properties- in terms of coating appearance such as surface smoothness, water resistance, antifoaming properties, etc. Therefore, the composition of the invention is advantageously used as a clear coating composition for forming a top clear coat in a method of forming a multilayer topcoat^' film on a substrate.

Thus, the multilayer topcoat formation method of the invention is a method for forming on a substrate one or two colored base coat layers and one or two clear coat layers, the uppermost clear coat layer being formed by using the coating composition of the invention.

Especially preferable substrates to which the multilayer topcoat formation method of the invention can be applied are automobile bodies and parts thereof.

Specific examples of the multilayer topcoat formation method of the invention include the following methods (a) to (c) wherein the clear coating composition of the invention is used to form the top clear coat.

Method -'(a): a two-coat method for forming a multilayer topcoat film, wherein a colored base coat and a top clear coat are formed in that order on a substrate. . ^• i

Method (b) : a three-coat method for forming a multilayer topcoat film, wherein a colored base coat, a clear coat and a top clear coat are formed in that order on a substrate.

Method (c) : a three-coat method for forming a multilayer topcoat film, wherein a first colored base coat, a second colored base coat and a top clear coat are formed in that order on a substrate.

The steps for forming a topcoat film in methods (a) , (b) and (c) are described below in detail. In method (a) , a known colored coating composition can be used as the coating composition for forming a colored base coat.

A coating composition for..automobile bodies or the like is preferably used as the colored base coating composition. The colored base coating composition is an organic solvent-based or aqueous coating composition comprising a base resin, a crosslinking agent, and a pigment such as a coloring pigment, a metallic pigment, a light interference pigment, an extender pigment or the like. To reduce the amount of VOCs, an aqueous coating composition is preferably used as the colored base coating composition.

Examples of base resins include acrylic resins, vinyl resins, polyester resins, alkyd resins, urethane resins and the like. Such resins can be used singly or as a mixture. Such base resins have crosslinkable functional groups such as hydroxy, epoxy, carboxy, alkoxysilyl and like groups. Examples of crosslinking agents include alkyl-etherified melamine resins, urea resins, guanamine resins, polyisocyanate compounds, blocked polyisocyanate compounds, epoxy compounds, carboxy-containing compounds and the like. Such crosslinking agents can be used singly or as a mixture. The proportions of base resin and crosslinking agent are preferably 50% to 90% by weight of base resin and 50% to- 10% by weight of crosslinking agent, based on the total amount of these components. In the above methods, the colored base coating < composition and the clear coating composition can be applied by application methods such as airless spray coating, air spray coating, rotary atomization coating, etc. In such application ' methods, an electrostatic charge may be applied, if necessary. In method (a) , the colored base coating composition is. applied to a substrate to a film thickness of about 10 to about 50 μm (when cured) . The applied base coating composition is either cured by heating at about 100⁰C to about 180⁰C, preferably at about 120⁰C to about 160⁰C, for about 10 to about 40 minutes, or is not cured, with the coated substrate being left to stand at room temperature for several minutes or being preheated at about 40⁰C to about 100⁰C for about 1 to about 20 minutes.

Subsequently, the clear coating composition of the invention is applied to a film thickness of about 10 to about 70 μm (when cured) to form a top clear coat and then heated to form a cured multilayer coating film. The heating is performed at about 100⁰C to about 180⁰C, preferably at about 120⁰C to about 160⁰C, for about 10 to about 40 minutes.

Of the above two-coat methods, the method comprising applying a base coating composition, applying a clear coating composition without heat-curing the base coating layer, and then curing the resulting two coating layers simultaneously is referred to as a two-coat one-bake method. The method of applying and heat-curing a base coating composition and then applying and curing a clear coating composition is referred to as a two-coat two-bake method.

In method (b) , examples of usable colored base coating compositions are the same as described in method (a) . The first clear coating composition for forming a clear coat may be any coating composition for forming a clear coat. Examples of usable coating compositions include those obtained -by completely or almost completely removing pigments from such known colored base coating compositions as mentioned above. The coating composition of the invention is used as the second clear coating composition for forming a top clear coat. By using the clear coating ' composition of the invention as the first clear coating composition, both the clear and top clear coats may be formed from the clear coating composition of the invention.

In method (b) , as in method (a) , a colored base coating composition is applied to the substrate and either cured by heating, or not cured, with the coated substrate being left to stand at room temperature for several minutes or being preheated. Thereafter, a first clear coating composition is applied to the colored base coat surface to a film thickness of about 10 to about 50 μm (when cured) , and is either cured by heating at about 100⁰C to about 18O⁰C, preferably at about 12O⁰C to about 160⁰C, for about 10 to about 40 minutes, or is not cured, with the coated substrate being left to stand at room temperature for several minutes or being preheated. Subsequently, the coating composition of the invention is applied as a second clear coating composition to a film thickness of about 10 to about 50 μm (when cured) and then heated to form a cured multilayer coating film. The heating conditions are as in method (a) . The method of applying a base coating composition, applying a first clear coating composition without heat-curing the base coating layer, applying a second clear coating composition without curing the first clear coating layer, and then curing the resulting three coating layers simultaneously is referred to as a three-coat one-bake method. The method of applying a base coating composition, applying a first clear coating composition without heat-curing the base coating layer, curing the resulting two coating layers simultaneously and then applying and curing a second clear coating composition is referred to as a three-coat two-bake method. The method of applying and heat-curing a base coating composition, applying and curing a first clear coating composition, and applying and curing a second clear coating composition is referred to as a three-coat three-bake method. ■ In method (c) , examples of colored base coating ' compositions usable as the first colored base coating composition in method (c) are the same as described in method (a) .

In method (c) ,. as in method (a) , a first colored base coating composition is applied to the substrate and either cured by heating, or not cured, with the coated substrate being left to stand at room temperature for several minutes or being preheated. The second colored base coating composition is then applied to the first colored base coat surface to a film thickness of about 10 to about 50 μm (when cured) and is either cured by heating at about 100⁰C to about 180°C, preferably at about 12O⁰C to about 160⁰C, for about 10 to about 40 minutes, or is not cured, with the coated substrate being left to stand at room temperature for several minutes or being preheated.

Subsequently, the coating composition of the invention is applied as a top clear coating composition to a film thickness of about 10 to about 50 um (when cured) and heated to form a cured multilayer coating film. The heating conditions are as in method (a) .

The method of applying a first base coating composition, applying a second base coating composition without heat-curing the first base coating layer, applying a clear coating composition without curing the second base coating layer, and then curing the resulting three coating layers simultaneously is referred to as a three-coat one-bake method. The method of applying and heat-curing a first base coating composition, applying a second base coating composition, applying a clear coating composition without curing the second base coating layer and then curing the resulting two coating layers simultaneously is referred to as a three-coat two-bake method. The method of applying and heat-curing a first base coating composition, applying and curing a second base coating composition, and applying and curing a clear coating composition is referred to as a three-coat three-bake method.

EFFECTS OF THE INVENTION The aqueous clear coating composition and the method of forming a multilayer topcoat film using the composition of the invention can achieve the following remarkable effects.

(1) The coating composition of the invention can form on a substrate a coating film with excellent properties in terms of appearance such as surface smoothness, with no abnormalities such as foaming on the coated surface. Therefore, the coating composition of the invention is suitable for use as an aqueous clear coating composition for forming a top clear coat in methods of forming a multilayer topcoat film such as two-coat one-bake methods for coating automobile bodies, etc. The composition of the invention is capable of forming a coating film with excellent appearance, antifoaming properties, etc. presumably because the applied • coating composition has improved thermal flowability and thus has good releasability of volatile component bubbles, which cause foaming during the coating film curing process, and bubble traces formed on the coating film disappear due to excellent thermal flow.

Furthermore, the obtained coating film has improved surface smoothness presumably because the hydroxy group- and acid group-containing particulate resin in dispersion (A) of the coating composition of the invention has a mean particle size of 50 to 300 nm, which optimizes the viscosity of the composition and particle size of the particulate resin.

(2) Since the coating composition of the invention is an aqueous coating composition, the amount of VOCs discharged during the process of application to automobile bodies, etc. can be easily reduced.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described below in more detail with reference to Production Examples, Examples, and Comparative Examples. However, the present invention is not limited to these examples. In the examples, parts and percentages are expressed on a mass basis, unless otherwise specified. Thicknesses of coating films are based on the cured coating films. ^l

Production of aqueous hydroxy group- and carboxy group- containing particulate acrylic resin dispersion Production Example 1

Thirty parts of propylene glycol monopropyl ether was placed in a four-necked flask equipped with a heater, a stirrer, a thermometer, a reflux condenser and a water separator and was heated to 145⁰C while supplying nitrogen gas. After stopping the nitrogen gas flow, as a first stage, 15 parts of styrene, 31 parts of n-butyl acrylate, 29 parts of 2-hydroxyethyl methacrylate and 2.6 parts of di-t-butylperoxide were added dropwise over a period of 4 hours and then maintained at the above temperature for 30 minutes. As a second stage, 15.5 parts of n-butyl acrylate, 6.0 parts of 2-hydroxyethyl methacrylates, 3.5 parts of acrylic acid and 0.88 parts of di-t-butyl peroxide were added dropwise over a period of 30 minutes and then aged for 1 hour. The solvent was distilled off under reduced pressure until the solids content of the mixture became 95%. After the mixture was cooled to 9O⁰C, 3 parts of dimethylethanolamine was added and then 105 parts of deionized water was added dropwise at 80⁰C to give an aqueous dispersion (A-I) of a hydroxy group- and carboxy group-containing particulate acrylic resin.

The aqueous dispersion (A-I) had a solids content of 47%, a viscosity of 570 mPa-s (as measured at 20⁰C at 60 rpm using a Brookfield viscometer) and a pH of 7.08, and the dispersed resin particles had a mean particle size of 140 nm.

The melt viscosity obtained by drying the aqueous dispersion (A_^-I) at 110⁰C for 3 hours to a solids content of 96 mass % and measuring the viscosity at a shear rate of 564 s at 140⁰C was 4.0 Pa- s. The solids content was determined by applying the dispersion (A-I) to a glass plate using a 4-mil applicator, drying at 110⁰C for 3 hours, scraping off the resulting uncured coating film, placing about 2.0 g of the uncured coating film in an aluminum foil - cup with a diameter of about 5 cm, heating at 110⁰C for 1 hour and measuring the amount of residue (g) . This hydroxy group- and carboxy group-containing > particulate acrylic resin had a hydroxy value of 150 mg KOH/g, and an acid value of 27 mg KOH/g, a weight average molecular weight of 15,000, and a glass transition temperature (Tg) of O⁰C. Production Examples 2 to 6 Aqueous dispersions (A-2) to (A-6) of hydroxy group- and carboxy group-containing particulate acrylic resins were prepared in a manner similar to Production Example 1 using the components and proportions shown in Table 1.

Table 1 also shows properties of the carboxy group- containing particulate acrylic resins and aqueous dispersions.

. Production of aqueous dispersion of hydroxy group- and carboxy group-containing particulate polyurethane resin Production Example 7

Five parts of N-methylpyrrolidone, 10 parts of methyl ethyl ketone, 21.84 parts of neopentylglycol adipate (molecular weight: 1,000, hydroxy value: 116 mg KOH/g), 3.82 parts of neopentylglycol and 4.91 parts of 2,2-dimethylolbutanoic acid were placed into a four-necked flask equipped with a heater, a stirrer, a thermometer, a reflux condenser and a water separator and heated to 7O⁰C to dissolve the mixture. Subsequently, 18.36 parts of isophorone diisocyanate and 24.24 parts of hexamethylene diisocyanate were added dropwise over a period of 2 hours and allowed to react at 70⁰C for 3 hours. After distilling off methyl ethyl ketone, 2.86 parts of dimethylethanolamine was added and then 108 parts of deionized water was added dropwise at 60⁰C to give an aqueous dispersion (A-7) of a hydroxy group- and carboxy group-containing particulate polyurethane resin. The aqueous dispersion (A-7) had a solids content of

47%, a viscosity of 460 mPa-s (as measured at 20⁰C at 60 rpiti using a Brookfield viscometer) and a pH of 7.98, and the dispersed resin particles had a mean particle size of 115 nm.

The melt viscosity obtained by drying the aqueous dispersion (A-7) at HO⁰C for 3 hours to a solids content of 96 mass % and measuring the_r viscosity at a shear rate of 564 s^~ at 14O⁰C was 4.8 Pa-s. The solids content was determined by applying the dispersion (A-7) to a glass plate using a 4-mil applicator, drying at 110⁰C for 3 hours, scraping off the resulting uncured coating film, placing about 2.0 g of the uncured coating film in an aluminum foil cup with a diameter of about 5 cm, heating at

110⁰C for 1 hour and measuring the amount of residue (g) . This hydroxy group- and carboxy group-containing particulate polyurethane resin had a hydroxy value of 150 mg KOH/g, an acid value of 20 mg KOH/g, a weight average molecular weight of 6,000 and a glass transition temperature (Tg) of 10⁰C. Production of aqueous clear coating composition Examples 1 to 12 and Comparative Examples 1 to 2

Aqueous clear coating compositions 1 to 14 were obtained by mixing with stirring aqueous dispersions of hydroxy group- and acid group-containing particulate resins (A-I) to (A-7) obtained in Production Examples 1 to 7 and crosslinking agents (B-I) to (B-5) in the proportions shown in Table 2 using a agitator. In Table 2, the proportions of the components in the coating compositions are expressed as mass ratios of the components on a solids basis. Crosslinking agents (B-I) to (B-5) in Table 2 refer to the following products.

(B-I) : "XP-2570" (trade name, product of Sumika Bayer ϋrethane Co. , Ltd. , a water-dispersible polyisocyanate compound) , (B-2) : "N-3100" (trade name, product of Sumika Bayer Urethane Co., Ltd., a hydrophilic group-containing polyisocyanate compound) ,

(B-3) : "XP-2410" (trade name, product of Sumika Bayer ϋrethane Co., Ltd., a low-viscosity polyisocyanate compound) , (B-4) : "LS2253" (trade name, product of Sumika Bayer Urethane Co., Ltd., an amine-blocked polyisocyanate compound),

(B-5): "Cymel 325" (trade name, product of Mitsui Cytec Ltd., an imino group-containing methyl-etherified melamine resin) .

By adding water, the aqueous clear coating compositions 1 to 14 obtained in Examples 1 to 12 and Comparative Examples 1 to 2 were adjusted to a viscosity of 45 seconds as measured at 20⁰C using Ford Cup No. 4.

The solids content (%) , the amount of VOCs (g/1) and the lowest viscosity value of each of the viscosity-adjusted aqueous clear coating compositions were determined.

Solids content: About 2 g of the viscosity-adjusted aqueous clear , coating composition was placed into an aluminum foil cup ^■ with a diameter of about 5 cm, dried at 110⁰C for 1 hour and then the weight was measured to, calculate the solids content (%) of the composition. <

VOC amount: The VOC amount of the viscosity-adjusted aqueous clear coating composition was calculated from the solids content, specific gravity and moisture content of the composition according to equation (1) shown below. The specific gravity was measured by the specific gravity cup method according to JIS K-

5400 4.6.2. The moisture content ^"was measured by the Karl Fischer method using an automatic moisture meter "KF-100" (trade name, product of Mitsui Chemical, Inc.).

VOC amount (g/1) = ([100 - (S+W) ] X p) / [100 - (W x p) ] (1) In equation (1), S represents the solids content (%) of the coating composition, W the moisture content (%) of the coating composition, and p the specific gravity (g/1) of the coating composition.

The lowest viscosity value: The viscosity-adjusted aqueous clear coating composition was applied by air spraying to a tin plate (300 x 450 x 0.3 mm) having been degreased with isopropanol, to a film thickness of 35 μm (when dried) and heated at 60⁰C for about 10 minutes. The uncured coating film formed on the tin plate was scraped off, collected into a sample bottle and sealed therein as a sample by closing the cap. Using a "Rheostress RS- 150" (trade name, product of HAAP(E, a viscoelasticity meter) , the dynamic viscoelasticity of 0.1 g of this sample was measured under strain control at a frequency of 0.1 Hz, a strain of 1.0 and a temperature increase rate of 6°C/min using a parallel plate (φ = 20 mm) sensor with a gap of 0.5 mm. The lowest viscosity value over the temperature range of 30⁰C to 150°C was determined.

Solids content (%) at the time of viscosity measurement: About 2.0 g of the sample was placed in an aluminum foil cup with a diameter of about 5 cm and heated at 110⁰C for 1 hour to measure the amount of residue (g) . The amount of residue after the heating for 1 hour was divided by the original weight of the sample to calculate the solids content (%) of the coating ^■ composition after applied and heated at 6O⁰C for about 10 minutes. Table 2 shows the proportions of the components, solids content, VOC amount, lowest viscosity value, and solids content at the time of viscosity measurement of each aqueous clear coating composition.

Performance test of the aqueous clear coating composition Preparation of test plates

(1) By adding water, the aqueous clear coating ^l compositions 1 to 14 obtained in Examples 1 to 12 and Comparative Examples 1 to 2 were adjusted to a viscosity of 45 seconds as measured at 20⁰C using Ford Cup No. 4.

(2) A thermosetting epoxy resin cationic electrodeposition coating composition (trade name "Elecron GT-10", product of Kansai Paint Co., Ltd.) was applied by electrodeposition to a zinc phosphate-treated cold rolled steel plate treated using "Palbond #3020" (trade name, product of Nihon Parkerizing Co., Ltd.) to a film thickness of about 20 μm (when cured) and heat-cured at 170⁰C for 30 minutes. Subsequently, a polyester resin/melamine resin intermediate coating composition for automobiles (trade name "Amilac TP-65-2", product of Kansai Paint Co., Ltd.) was applied to the . electrodeposition coat surface by air spraying to a film thickness of about 35 μm (when cured) and heat-cured at 140⁰C for 30 minutes. The steel plate having the electrodeposition coat and the intermediate coat formed thereon was used as a substrate.

(3) An acrylic resin/melamine resin base coating composition for automobiles (trade name "Aqueous metallic base coat WBC 710T #1E7", product of Kansai Paint Co., Ltd.) was applied to the substrate by air spraying to a film thickness of about 15 um (when cured) , allowed to stand at room temperature for 5 minutes and pre-heated at 80⁰C for 10 minutes. Each of the above aqueous clear coating compositions 1 to 14 with a viscosity as adjusted in (1) was applied on the uncured base coating layer by air spraying to a film thickness of about 35 μm (when cured) . The coated substrate was allowed to stand at room temperature for 10 minutes, preheated at 6O⁰C for 10 minutes and then heated at 140⁰C for 20 minutes to cure the resulting two coating layers simultaneously. Thus, test plates comprising a substrate and a multilayer topcoat film consisting of a base coat and a clear coat formed on the substrate were prepared by a two-coat one-bake method.

Performance Tests The obtained test plates were tested for coating i performance in terms of appearance, Tukon hardness and water resistance. The coating film thickness (μm) at which foaming occurred during the application was also examined. The test methods are as follows: Appearance: The test plate was observed by the naked eye to evaluate the surface smoothness a^"nd gloss according to the following criteria:

A: Good in both smoothness and gloss B: Poor in either smoothness or gloss C: Poor in both smoothness and gloss Tukon hardness: After the test plate was left in a thermostatic room at 20°C for 24 hours, Tukon hardness of the coating film was measured^' using a Tukon Microhardness Tester (trade name, product of American Chain & Cable Company) . Tukon hardness, also called "Knoop Hardness Number (KHN)", is a value expressing the hardness of a coating film determined by- pressing a square pyramidal diamond indenter with a specific load in the surface of a test material and measuring the size of the diamond-shaped indentation in the surface. The greater is the Tukon hardness value, the higher is the hardness.

Water resistance: The test plate was left in a thermostatic - room at 20⁰C for 24 hours, immersed at 80⁰C in warm water for 5 hours and then gradually cooled from 80⁰C to room temperature while kept immersed. The test plate was removed from the water and the surface state of the test plate was evaluated according to the following criteria: A: Good gloss; B: Poor gloss;

C: Poor gloss and white cloudy coated surface. Coating film thickness at which foaming occurred: Test plates were prepared in the same manner as the above except that each aqueous coating composition was applied to form a coating film ^• with gradient thicknesses of from 20 um to 60 μm. The test plate was observed for air bubble traces on the coated surface due to foaming, and the minimum coating film thickness of the portάons at which air bubble traces were observed was defined as the coating film thickness (μm) at which foaming occurred. The greater is the value, the higher is the antifoaming property.

Table 3 shows the above performance test results . In the table, ">60" indicates that the coating film thickness at which foaming occurred exceeds 60^~'μm. Table 3

I en

I

Claims

1. An aqueous clear coating composition comprising:

(B) a crosslinking agent; the composition having a viscosity such that the lowest viscosity value of the composition as measured over the temperature range of 30°C to 150°C at a frequency of 0.1 Hz at a solids content of at least 90 mass % is 30 Pa- s or less.

2. The aqueous clear coating composition according to claim 1 wherein the melt viscosity of the hydroxy group- and acid group-containing particulate resin in dispersion (A) as measured at a solids content of at least 96 mass % at a shear rate of 564 s^"1 at 140⁰C is 1 to 12 Pa- s.

3. The aqueous clear coating composition according to claim 1 wherein the hydroxy group- and acid group-containing particulate resin in dispersion (A) has a weight average molecular weight of 3,000 to 30,000.

4. The aqueous clear coating composition according to claim 1 wherein the hydroxy group- and acid group-containing particulate resin in dispersion (A) has a glass transition temperature within the range of from -30⁰C to^' +40⁰C. ^l

5. The aqueous clear coating composition according to claim 1 wherein crosslinking agent (B) is at least one crosslinking agent selected from the group consisting of polyisocyanate compounds, blocked polyisocyanate compounds and melamine resins.

6. The aqueous clear coating composition according to claim 5 wherein crosslinking agent (B) is at least one polyisocyanate compound and/or at least one blocked polyisocyanate compound.

7. The aqueous clear coating composition according to claim 5 wherein the equivalent ratio (NCO/OH) of isocyanate . groups of the polyisocyanate compound and/or blocked isocyanate compound to hydroxy groups of the hydroxy group- and acid group- containing particulate resin in dispersion (A) is 0.5 to 2.0.

8. The aqueous clear coating composition according to claim 5 wherein crosslinking agent (B) is at least one melamine resin.

9. The aqueous clear coating composition according to claim 8 wherein the proportions of hydroxy group- and acid group- containing particulate resin in dispersion (A) and melamine resin, based on the total weight of these two resins, are 50 to 90 wt.% of the former and 50 to 10 wt.% of the latter.

10. The aqueous clear coating composition according to claim 1 having a solids content of 35 to 65 mass _.%.

11. A method for forming a multilayer topcoat film, comprising forming on a substrate one or two color base coat layers and one or two clear coat layers, the uppermost clear coat layer being formed by using the aqueous clear coating composition of claim 1.