WO2006137569A1 - Water-borne clear coating composition and method of forming clear coating film - Google Patents

Water-borne clear coating composition and method of forming clear coating film Download PDF

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Publication number
WO2006137569A1
WO2006137569A1 PCT/JP2006/312840 JP2006312840W WO2006137569A1 WO 2006137569 A1 WO2006137569 A1 WO 2006137569A1 JP 2006312840 W JP2006312840 W JP 2006312840W WO 2006137569 A1 WO2006137569 A1 WO 2006137569A1
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WO
WIPO (PCT)
Prior art keywords
coating film
coating composition
water
clear coating
borne
Prior art date
Application number
PCT/JP2006/312840
Other languages
French (fr)
Inventor
Takashi Irie
Hisaki Tanabe
Shinichi Ikehara
Original Assignee
Nippon Paint Co., Ltd.
Honda Motor Co., Ltd.
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Filing date
Publication date
Application filed by Nippon Paint Co., Ltd., Honda Motor Co., Ltd. filed Critical Nippon Paint Co., Ltd.
Publication of WO2006137569A1 publication Critical patent/WO2006137569A1/en

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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D4/00—Coating compositions, e.g. paints, varnishes or lacquers, based on organic non-macromolecular compounds having at least one polymerisable carbon-to-carbon unsaturated bond ; Coating compositions, based on monomers of macromolecular compounds of groups C09D183/00 - C09D183/16
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
    • B05D3/02—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by baking
    • B05D3/0209—Multistage baking
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G18/00—Polymeric products of isocyanates or isothiocyanates
    • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
    • C08G18/28—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen characterised by the compounds used containing active hydrogen
    • C08G18/40—High-molecular-weight compounds
    • C08G18/62—Polymers of compounds having carbon-to-carbon double bonds
    • C08G18/6216—Polymers of alpha-beta ethylenically unsaturated carboxylic acids or of derivatives thereof
    • C08G18/625—Polymers of alpha-beta ethylenically unsaturated carboxylic acids; hydrolyzed polymers of esters of these acids
    • C08G18/6254—Polymers of alpha-beta ethylenically unsaturated carboxylic acids and of esters of these acids containing hydroxy groups
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G18/00—Polymeric products of isocyanates or isothiocyanates
    • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
    • C08G18/70—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen characterised by the isocyanates or isothiocyanates used
    • C08G18/72—Polyisocyanates or polyisothiocyanates
    • C08G18/77—Polyisocyanates or polyisothiocyanates having heteroatoms in addition to the isocyanate or isothiocyanate nitrogen and oxygen or sulfur
    • C08G18/78—Nitrogen
    • C08G18/79—Nitrogen characterised by the polyisocyanates used, these having groups formed by oligomerisation of isocyanates or isothiocyanates
    • C08G18/791—Nitrogen characterised by the polyisocyanates used, these having groups formed by oligomerisation of isocyanates or isothiocyanates containing isocyanurate groups
    • C08G18/792—Nitrogen characterised by the polyisocyanates used, these having groups formed by oligomerisation of isocyanates or isothiocyanates containing isocyanurate groups formed by oligomerisation of aliphatic and/or cycloaliphatic isocyanates or isothiocyanates
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G18/00—Polymeric products of isocyanates or isothiocyanates
    • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
    • C08G18/70—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen characterised by the isocyanates or isothiocyanates used
    • C08G18/72—Polyisocyanates or polyisothiocyanates
    • C08G18/77—Polyisocyanates or polyisothiocyanates having heteroatoms in addition to the isocyanate or isothiocyanate nitrogen and oxygen or sulfur
    • C08G18/78—Nitrogen
    • C08G18/79—Nitrogen characterised by the polyisocyanates used, these having groups formed by oligomerisation of isocyanates or isothiocyanates
    • C08G18/798—Nitrogen characterised by the polyisocyanates used, these having groups formed by oligomerisation of isocyanates or isothiocyanates containing urethdione groups
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D133/00—Coating compositions based on homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Coating compositions based on derivatives of such polymers
    • C09D133/04—Homopolymers or copolymers of esters
    • C09D133/06—Homopolymers or copolymers of esters of esters containing only carbon, hydrogen and oxygen, the oxygen atom being present only as part of the carboxyl radical
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D175/00—Coating compositions based on polyureas or polyurethanes; Coating compositions based on derivatives of such polymers
    • C09D175/04—Polyurethanes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
    • B05D3/02—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by baking
    • B05D3/0254—After-treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
    • B05D3/06—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by exposure to radiation
    • B05D3/061—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by exposure to radiation using U.V.
    • B05D3/065—After-treatment
    • B05D3/067—Curing or cross-linking the coating
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
    • B05D7/50—Multilayers
    • B05D7/56—Three layers or more
    • B05D7/57—Three layers or more the last layer being a clear coat
    • B05D7/576—Three layers or more the last layer being a clear coat each layer being cured, at least partially, separately
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
    • B05D7/50—Multilayers
    • B05D7/56—Three layers or more
    • B05D7/57—Three layers or more the last layer being a clear coat
    • B05D7/577—Three layers or more the last layer being a clear coat some layers being coated "wet-on-wet", the others not
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L75/00—Compositions of polyureas or polyurethanes; Compositions of derivatives of such polymers
    • C08L75/04—Polyurethanes

Definitions

  • the present invention relates to a water-borne clear coating composition and a method of forming a clear coating film.
  • a top coating film consisting of a base coating film and a clear coating film
  • a clear coating film constitutes the outermost layer in a coating film, of automotive bodies and the like, properties such as high appearance, water resistance andweather resistance are required, and as a clear coating composition having such properties, various clear coating compositions are used.
  • the water-borne clear coating composition it is desired to control adequately a sagging. Since the occurrence of the sagging causes projections and depressions at the surface of a coating film and thereby the smoothness of a coating film is deteriorated and a defective appearance is produced, it is a very important issue that to prevent the sagging from taking place. Particularly, it was extremely difficult to control the smoothness of a coating film since an automotive body has a complicated structure. Such a starrises also in a solvent-borne clear coating composition, but the deterioration of an appearance due to such a problem is relatively slight in the solvent-borne clear coating composition since the viscosity of a coating composition is increased due to volatilization of a solvent in the solvent-borne clear coating composition. However, in the water-borne clear coating composition, since vaporization of water is slow, such a problem becomes more significant.
  • a close-to-horizontal plane of a substrate to be coated it is not preferred to coat the surface with a coating composition having excessively high viscosity in order to attain the smoothness of the surface .
  • a coating film is formed from a coating composition having low viscosity, a smooth surface is formed spontaneously, and contrarily, when the coating film is formed from a coating composition of high viscosity, the coating composition is not fluidized andtherefore the surface is less prone to being leveled.
  • a coating composition of low viscosity readily run down, and therefore the coating filmrequires ameasure of viscosity.
  • a method of forming a coating film comprising the steps of applying a melamine-based coating composition containing a active energy-ray curable compound to a substrate to be coated, irradiating active energy-rays to pre-curing a coating film, and heating to cure a coating film completely in order to control sagging in an intermediate coating composition .
  • Japanese Kokai Publication 2003-245606 there is described a method of forming a heat cured coating film, comprising the steps of applying a coating composition containing a photo-curable composition, a thermosetting resin composition, a photopolymerization initiator, and a photoacid generator, irradiating light to the coating film to increase the viscosity of the coating film, and heating and curing the coating film.
  • these methods are a method of forming a coating film by curing a coating composition containing an organic solvent and a method of coating a water-borne clear coating composition, in which a problem of deteriorating an appearance due to sagging is remarkable, is not described.
  • the photoacid generator when used, there is a possibility that properties such as the water resistance and the weather resistance of a coating film are deteriorated, and therefore it has been required to form a coating film having higher water resistance and higher weather resistance.
  • the active energy-ray curable compound having an unsaturatedbond is preferably a compoundhaving a (meth) acrylate group .
  • the polyisocyanate compound is preferably a blocked polyisocyanate.
  • the present invention pertains to a method of forming a clear coating film, comprising steps of applying the water-borne clear coating composition mentioned above to a substrate to be coated (step 1), irradiating active energy-rays to a not-yet-cured coating film obtained by the step 1 from an approximately vertical plane (step 2) , and heating the substrate to be coated, which has been subjected to the step 2, to give a cured coating film (step 3) .
  • the viscosity of the coating film which has been subj ected to the step 2 is preferably 2000 to 500000 mPa » s.
  • said method preferably comprises the step (A-I) of applying an intermediate coating composition onto a cured electrodeposition coating film and heat curing the intermediate coating composition applied to form a multilayer coating film, and the step (A-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1.
  • said method preferably comprises the step (B-I) of applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating film to form a multilayer coating film, and the step (B-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1.
  • said method preferably comprises a step (C) of applying a overcoat clear coating composition to the substrate after the step 3.
  • the present invention pertains to a coated substrate, wherein a substrate to be coated is an automotive body or an automotive assemblypart and said coated substrate is coated by the method mentioned above.
  • the water-borne clear coating composition and the method of forming a clear coating film of the present invention is characterized by using an active energy-ray curable compound having an unsaturated bond and a photopolymerization initiator in combination with a water-soluble thermosetting resin component.
  • active energy-rays are irradiated after applying a coating composition containing the above-mentioned active energy-ray curable compound having an unsaturated bond and the above-mentioned photopolymerization initiator to a substrate to be coated, a polymerization reaction of the above active energy-ray curable compound having an unsaturated bond takes place due to the irradiation of the active energy-ray and an increase in the viscosity of the coating film takes place.
  • the present invention pertains to the water-borne clear coating composition contains an active energy-ray curable compound having an unsaturated bond, a photopolymerization initiator, a water-borne acrylic resin and a polyisocyanate compound.
  • the above-mentioned active energy-ray curable compound having an unsaturated bond is preferably a compound having two ormore ⁇ , ⁇ -unsaturated carbonyl groups in amolecule .
  • the above-mentioned ⁇ , ⁇ -unsaturated carbonyl group is a functional group in which a double bond exists between ⁇ carbon and ⁇ carbon on a carbonyl group and examples of the functional groups can include a methacrylate group, an acrylate group, a maleate group and a fumarate group.
  • the above active energy-ray curable compound has only one ⁇ , ⁇ -unsaturated carbonyl group, it is not preferred in that active energy-ray curability is not adequately exhibited.
  • Number of the above ⁇ , ⁇ -unsaturated carbonyl groups in a molecule is preferably 10 or less and most preferably 6 or less.
  • the above active energy-ray curable compound having an unsaturatedbond is not particularly limited, and it can include, for example, polyol (meth) acrylate ester, unsaturatedpolyester polymer containing ⁇ , ⁇ -unsaturated dicarboxylic acid such as fumaric acid and maleic acid as an acid component, epoxy polymer (meth) acrylate, a (meth) acryloyl group-containing urethane compound, an acrylic polymer containing an ⁇ , ⁇ -unsaturated carbonyl group, a (meth) acryloyl group-containing polyether polymer and a (meth) acryloyl group-containing silicone oligomer.
  • polyol (meth) acrylate ester unsaturatedpolyester polymer containing ⁇ , ⁇ -unsaturated dicarboxylic acid such as fumaric acid and maleic acid as an acid component
  • epoxy polymer (meth) acrylate a (meth) acryloyl group-containing
  • the above polyol (meth) acrylate ester is not particularly limited, and it can include, for example, (meth) acrylate esters of low molecular weight polyol such as ethylene glycol di (meth) acrylate, neopentyl glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, pentaerithritol tetra (meth) acrylate, dipentaerithritol hexa (meth) acrylate, 1, 4-cyclohexyldimethanol di (meth) acrylate, 4,4' -isopropylidenedicyclohexanol di (meth) acrylate, bis (hydroxymethyl) tricyclo [5, 2,1,0] decane di (meth) acrylate and 1, 3, 5-tris (2-hydroxyethyl) cyanurate tri (meth) acrylate; and (meth) acrylate esters of a hydroxyl group-
  • the above-mentioned unsaturated polyester polymer is not particularly limited, and it can include, for example, polymer obtained by polycondensation of an acid component consisting of ⁇ , ⁇ -unsaturated dicarboxylic acid such as maleic acid anhydride and fumaric acid and another polyhydric carboxylic acid used as required, and polyol having two or more hydroxyl groups .
  • Polyol used for the above unsaturated polyester polymer is not particularly limited, and it can include, for example, ethylene glycol, diethylene glycol, propylene glycol, tetramethylene glycol, 1, 6-hexanediol, neopentyl glycol, trimethylolpropane, glycerin, pentaerythritol, 1, 4-cyclohexanedimethanol,
  • Another polyhydric carboxylic acid which can be used for the above unsaturated polyester polymer, is not particularly limited, and it can include, for example, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methyltetrahydrophthalic acid, isophthalic acid, terephthalic acid, maleic acid, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, dodecenylsuccinic acid and 1, 4-cyclohexanedicarboxylic acid.
  • the above-mentioned epoxy polymer (meth) acrylate can include, for example, a polymer obtained by a ring-opening addition reaction of an epoxy polymer such as a bisphenol type epoxy polymer and a novolak type epoxy polymer and (meth) acrylic acid.
  • the above-mentioned (meth) acryloyl group-containing urethane compound can include, for example, a compound obtained by an addition reaction of a polyisocyanate compound such as isophorone diisocyanate, trilenediisocyanate, diphenylmethane diisocyanate and hexamethylene diisocyanate or urethane prepolymer thereof, and 2-hydroxyethyl (meth) acrylate.
  • the above-mentioned acrylic polymer containing an ⁇ , ⁇ -unsaturated carbonyl group can include, for example, an acrylic polymer having a (meth) acrylate group on a side chain, which is obtained by reacting an acrylic polymer formed by copolymerizing glycidyl (meth) acrylate with (meth) acrylic acid; and an acrylic polymer having a (meth) acrylate group on a side chain, which is obtained by reacting a carboxyl group-containing acrylic polymer with an epoxy group-containing acrylic monomer such as glycidyl (meth) acrylate .
  • the above-mentioned (meth) acryloyl group-containing polyether polymer can include, for example, a compound obtained by reacting polyether having a hydroxyl group at a terminal with 2- (meth) acryloyloxyethyl isocyanate .
  • the above active energy-ray curable compound having an unsaturated bond may have a plurality of hydroxyl groups and the like in addition to the ⁇ , ⁇ -unsaturated carbonyl group .
  • the above-mentioned active energy-ray curable compounds having an unsaturated bond may be used alone or in combination of two or more kinds .
  • the active energy-ray curable compound having an unsaturated bond contained in the coating composition of the present invention is preferably polyol (meth) acrylate ester because polyol (meth) acrylate ester is highly reactive and has excellent weather resistance and compatibility and a high gloss .
  • a number-average molecular weight (Mn) of the active energy-ray curable compoundhaving an unsaturatedbond contained in the coating composition of the present invention is preferably within a range of 200 (lower limit) to 5000 (upper limit) .
  • a commercially available article of the above active energy-ray curable compound having an unsaturated bond can include A-TMM-3L (trade name, produce by SHIN-NAKAMURA CHEMICAL Co., Ltd.), M-400 (trade name, produced by TOAGOSEI Co. , Ltd.), TMPTA: (producedby TOAGOSEI Co., Ltd. ), and D-330: (trade name, produced by Nippon Kayaku Co., Ltd.) .
  • the content of the above active energy-ray curable compound having an unsaturated bond is preferably 1 weight % (lower limit) to 35 weight % (upper limit) in 100 weight % of the total resin solid matter of the water-borne acrylic resin and the polyisocyanate compound.
  • this content is less than 1 weight %, an effect may not be developed.
  • the content is more than 35 weight %, this may have an adverse effect on coating film properties.
  • the above content more preferably has a lower limit of 5 weight % and an upper limit of 30 weight %.
  • the water-borne clear coating composition of the present invention further has a photopolymerization initiator.
  • a publicly known initiator can be employed, and specific examples of the initiators can include benzoin and benzoin alkyl ethers such as benzoin, benzoin methyl ether, benzoin ethyl ether and benzoin propyl ether; acetophenones such as acetophenone, 2, 2-dimethoxy-2-phenylacetophenone, 2, 2-diethoxy-2-phenylacetophenone and
  • 2, 4, 6-trimethylbenzoyldiphenylphosphine oxide may be used in combination of two or more kinds and may be further used in combination with tertiary amine such as triethanolamine or a photopolymerization initiator such as ethyl dimethylaminobenzoate .
  • a commercially available article of the above ⁇ photopolymerization initiator can include Darocur 1173, Irgacure 184, Irgacure 500, Irgacure 2959 (trade name, every produced by Ciba Specialty Chemicals K. K.).
  • An amount of the above photopolymerization initiator to be mixed is not particularly limited and it is appropriately established in accordance with a ratio of a heat curing reaction and a photocuring reaction. Further, the amount of the above photopolymerization initiator to be mixed can be adjusted in accordance with a degree of weather resistance required, kinds and amounts of additives such as an ultraviolet absorber component.
  • An amount of the above photopolymerization initiator to be generally mixed in a coating composition containing a curable binder used in the present invention is, for example, 0.01 to 10 weight % with respect to 100 weight % of the above active energy-ray curable compound having an unsaturated bond.
  • the water-borne clear coating composition of the present invention further contains the water-borne acrylic resin. Since an acrylic resin is highly resistant to hydrolysis, by using this resin, the coating composition of the present invention becomes a water-borne clear coating composition which can attain a coating composition which is excellent in long-term storage stability.
  • the above-mentioned water-borne acrylic resin is a copolymer which is predominantly composed of acrylic acid and/or methacrylic acid
  • a monomer used for polymerizing the above water-borne acrylic resin is not particularly limited and can include, for example, ethylenic unsaturated carboxylic acid monomers such as acrylic acid, methacrylic acid, maleic acid, itaconic acid and the like; ethylenic unsaturated carboxylic alkyl ester monomers such as methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate and the like; ethylenic unsaturated dicarboxylic monoester monomers such as ethyl maleate, butyl maleate, ethyl itaconate, butyl itaconate and the like; hydroxyl group-containing ethylenic unsaturated carboxylic
  • a mixture of the above monomers may be in the form of using the above monomers alone or in combination of two or more components .
  • a polymerization method for obtaining the above acrylic resin is not particularly limited and publicly known methods such as solution polymerization, dispersion polymerization and emulsion polymerization can be employed.
  • the above acrylic resin may be one obtained by further grafting an acrylic resin obtained from the mixture of the above monomers.
  • By grafting the above acrylic resin it is possible to change a structure of the acrylic resin for a structure which is resistant to hydrolysis. And, when the above acrylic resin is grafted, the viscosity of a coating composition resists increasing even when the acrylic resin is contained in high concentrations in the water-borne coating composition, and therefore it is possible to coat at a high solid content and improve coating efficiency.
  • a method of grafting described above is not particularly limited and publicly known methods such as solution polymerization, emulsion polymerization and the like can be employed.
  • a number-average molecular weight of the water-borne acrylic resin of the present invention is preferably within a range of 1000 (lower limit) to 10000 (upper limit) .
  • the above-mentioned number-average molecular weight of an acrylic resin is less thanlOOO, amolecularweight is too lowandtherefore the weather resistance and the curability as a clear coating film are low, and basic properties as a clear coating film may be impaired.
  • the number-average molecular weight is more than 8000, since the viscosity of the water-borne coating composition becomes too large, the coating efficiency may be deteriorated.
  • the above lower limit is more preferably 1500 and furthermore preferably 2000.
  • the above upper limit is more preferably 7500 and furthermore preferably 6000.
  • a hydroxyl group value of resin is more preferably within a range of 30 mg KOH/g (lower limit) to 150 mg KOH/g (upper limit) .
  • the hydroxyl group value is less than 30 mg KOH/g, a crosslinking property becomes insufficient and there may be cases where adequate coating film performance cannot be secured.
  • it is more than 150 mg KOH/g, the water resistance and the weather resistance of a coating film of a coating composition obtained by using this resin may be deteriorated.
  • the above hydroxyl group value more more preferably has a lower limit of 40 mg KOH/g and furthermore preferably has a lower limit of 50 mg KOH/g.
  • the above hydroxyl group value more preferably has an upper limit of 130 mg KOH/g and furthermore preferably has an upper limit of 100 mg KOH/g.
  • an acid value of resin is preferably within a range of 10 mg KOH/g (lower limit) to 100 mg KOH/g (upper limit) .
  • the acid value is less than 10 mg KOH/g, water solubility may be deteriorated to impair stability.
  • it is more than 100 mg KOH/g a hydrophilic property of a resinmaybecome too large and therefore water resistance and weather resistance of a coating film may be deteriorated.
  • the above acid value more preferably has a lower limit of 20 mg KOH/g and furthermore preferably has a lower limit of 30 mg KOH/g.
  • the above acid value more preferably has an upper limit of 80 mg KOH/g and furthermore preferably has an upper limit of 60 mg KOH/g.
  • the above water-borne acrylic resin is preferably contained in an amount 50 weight % (lower limit) to 95 weight % (upper limit) with respect to the total resin solid matter contained in the water-borne clear coating composition. When this amount is less than 50 weight %, it may become difficult to secure the water resistance, the weather resistance and the appearance of a coating film to be obtained. When this amount is more than 95 weight %, a crosslinking property becomes insufficient and there maybe cases where adequate coating film performance cannot be secured.
  • the above lower limit is more preferably 65weight % andthe aboveupper limit isr ⁇ orepreferably 90 weight %.
  • the water-based clear coating material composition of the present invention further contains a polyisocyanate compound.
  • the above-mentioned polyisocyanate compound is not particularly limited as long as it is a compound having two or more isocyanate groups, and it can include, for example, aliphatic isocyanates such as trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HDI) and trimethylhexamethylene diisocyanate; aliphatic cyclic isocyanates such as 1, 3-cyclopentane diisocyanate, 1, 4-cyclohexane diisocyanate and 1, 2-cyclohexane diisocyanate; aromatic isocyanates such as xylylene diisocyanate (XDI), 2, 4-trilene diisocyanate (TDI) and 2,6-trilene diisocyanate; and alicyclic isocyanates such as isophorone diiso
  • the above-mentioned polyisocyanate compound is preferably a blocked polyisocyanate.
  • the above-mentioned blocked polyisocyanate can include a substance obtained by adding a blocking agent having active oxygen to the above polyisocyanate compound, in which the blocking agent is dissociated by heating to produce an isocyanate group and this isocyanate group reacts with a functional group in the above water-borne acrylic resin to cure the coating composition.
  • the above-mentioned blocking agent can include, for example, halogenated hydrocarbons, aliphatic, aromatic or heterocyclic alcohols such as methanol, ethanol, n-propanol, isopropanol, furfuryl alcohol, alkyl group substituted furfuryl alcohol andbenzyl alcohol, andoximes such as methyl ethyl ketone oxime, methyl isobutyl ketone oxime, acetone oxime and cyclohexane oxime, and in addition caprolactam.
  • the above-mentioned water-borne clear coating material composition preferably contains a film forming resin having a hydroxyl group.
  • an ultraviolet absorber In the above water-borne clear coating composition, an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, a crosslinking resin particle, and a surface control agent in addition to the above polymers may be mixed.
  • the crosslinking resinparticle is preferablymixed in an amount 0.01 weight % (lower limit) to 10 weight % (upper limit) with respect to the resin solidmatter of the water-borne clear coating composition of the present invention.
  • the above lower limit is more preferably 0.1 weight % and the above upper limit is more preferably 5 weight %.
  • a method of producing the above water-borne clear coating composition is not particularly limited and methods, which are well known to those skilled in the art, such as amethod of kneading and dispersing a formulation of the above polymers and the like with a kneader or a roller, and the like can be employed.
  • the coating composition of the present invention is water-borne, and it does not need to take the volatilization portion of a solvent into consideration in adjusting viscosity, can control sagging suitably and can reduce the environmental loads.
  • a solution used in the above water-borne coating composition is not particularly limited, and it can include, for example, well known solutions such as Byketol-Special, Byketol-OK (trade names, both produced by BYK-Chemie Japan KK) and the like.
  • it is preferred to adjust the viscosity in applying the coating composition so as to be 20 to 50 seconds at 20°C with a Ford cup No. 4.
  • a coating film after coating has excellent smoothness and sagging can be suitably prevented.
  • the present invention also pertains to a method of forming a clear coating film, which is constituted by using the water-borne clear coating composition described above.
  • the method of forming a clear coating film of the present invention comprises steps of applying the water-borne clear coating composition described above to a substrate to be coated (step 1), irradiating active energy-rays to a not-yet-cured coating film obtained by the step 1 from an approximately vertical plane (step 2) , and heating the coated substrate, which has been subjected to the step 2, to cure the not-yet-cured coating film (step 3) .
  • the method of forming a clear coating film of the present invention is amethod in which a coating film canbe formedwithout causing sagging even though a coated plane is approximately vertical by employing the water-borne clear coating composition described above and coating by the coating steps described above .
  • the above-mentioned step 1 is a step of applying the water-borne clear coating composition described above to a substrate to be coated.
  • a method of applying a coating composition containing a curable binder is not particularly limited, and it can include, for example, brush application, roller application, air spray application, airless spray application, immersion application, flow coating and the like.
  • the substrate to be coated is an automotive body or an automotive part, electrostatic air spray coating or rotary atomization type electrostatic coating is preferred.
  • the viscosity of the above-mentioned coating composition containing a curable binder can be appropriately adjusted with water in accordance with a coating method.
  • the above-mentioned step 2 is a step of irradiating active energy-rays to a not-yet-cured coating film obtainedby the above step 1 from an approximately vertical plane. That is, in the step 2, by irradiating active energy-rays to approximately vertical plane on which sagging is particularly apt to occur, the viscosity of the coating composition is increased to inhibit sagging, and thereby the smoothness of a coating film is improved and a surface appearance of a coating film becomes better.
  • the above step 2 is a step in which since the active energy-rays are irradiated froman approximatelyvertical plane, sagging on a vertical plane or a close-to-vertical place, on which sagging is particularly apt to occur, is inhibited and thereby projections and depressions at the surface of a coating film are inhibited and the surface appearance of a coating film is improved.
  • the irradiation of the active energy-rays from an approximately vertical plane refers to irradiating from a plane approximately perpendicular to a horizontal plane, but the direction of irradiation have not to be precisely perpendicular and it means that the active energy-rays are irradiated at such an angle that a sufficient amount of active energy-rays can be irradiated to a vertical plane or a close-to-vertical plane on which sagging is apt to occur.
  • the active energy-rays are irradiated at least from the approximately vertical plane, but it may be simultaneously irradiated from a horizontal plane.
  • the irradiation of the active energy-ray in the above step 2 can include irradiation of active energy-rays such as ultraviolet light, sun light, visible light, a microwave and an electron beam, but among others, the irradiation of the active energy-ray is particularly preferably performed by the irradiation of ultraviolet light.
  • the above-mentioned irradiation of ultraviolet light can be performedby irradiating using a carbon arc lamp, an electrodeless lamp, a mercury-vapor lamp, a xenon lamp, a fluorescent lamp and argon glow discharge as an ultraviolet source .
  • the electrodeless lamp is preferred because it can irradiate light uniformly to a substrate to be coated, which has a complicated structure.
  • the above step 2 may be performed just after the step 1, or a preheating step is performed after the step 1 and the above step 2 maybe performed after the preheating step, but the above step 2 is preferably performed just after the step 1.
  • the irradiation intensity of ultraviolet light in the step 2 is preferably 200 to 2000 mJ/cm 2 and the irradiation time is preferably several seconds.
  • the composition of a water-borne clear coating composition and the conditions of irradiating the active energy-rays in such a way that the viscosity of the coating film is 2000 to 500000 mPa'S.
  • the viscosity of a coating film is less than 2000 mPa*s, there is a possibility that sagging cannot be adequately inhibited.
  • it is more than 500000 i ⁇ Pa*s this may cause a problem that smoothness of a coating film is lost and a defective appearance such as an orange peel and the like is produced.
  • the viscosity of a coating film more preferably has a lower limit of 5000 mPa*s and an upper limit of 100000 mPa»s.
  • the above-mentioned step 3 is a step of heating a coated substrate, which has been subjected to the step 2, to cure a coating film.
  • the conditions of the above step 3 varies with the composition of a coating composition to be used, and they can be appropriately established by those skilled in the art, but a heating temperature is generally 80 to 200°C, preferably 100 to 18O 0 C and a heating time is preferably 10 to 40 minutes.
  • the method of forming a clear coating film of the present invention may have a step (step 4) of further irradiating active energy-rays to a coating film which has been subjected to the above step 3.
  • the method of forming a clear coating film of the present invention can be suitably used particularly for a coating method of a substrate to be coated, having a complicated shape, such as automotive bodies, automotive assemblyparts and special-purpose vehicles .
  • the method of forming a clear coating film of the present invention may further includes the step (A-I) of applying an intermediate coating composition onto a cured electrodeposition coating filmandheat curing the intermediate coating composition applied to form a multilayer coating film, and the step (A-2) of applying a base coating composition onto the above-mentioned multilayer coating film, prior to the step 1.
  • the method of forming a clear coating film of the present invention may further includes the step (B-I) of applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating filmto formamultilayer coating film, and the step (B-2) of applying a base coating composition onto the above-mentioned multilayer coating film, prior to the step 1.
  • step (A-I) is a step of applying an intermediate coating composition onto a electrodeposition coating film formed on a substrate to be coated and heat curing the intermediate coating compositionappliedto formamultilayer coating film.
  • the method of forming a clear coating film of the present invention is preferably applied to metal molded articles onwhich cationic electrocoating can be performed.
  • the above-mentioned substrate to be coated is preferably chemical conversion treated on its surface. Further, the substrate to be coated may be provided with an electrodeposition coating film.
  • a coating composition used in the above electrocoating a cationic and an anionic coating compositions can be used, but a cationic electrodeposition coating composition is preferred from the viewpoint of an anti-corrosive property.
  • the above-mentioned base coating composition is not particularly limited and it may include, for example, a film forming resin, a curing agent, an organic or an inorganic or a convinced material-containing coloringpigment and an extender .
  • the formof the above base coating composition is not particularly limited and it can include a water-borne base coating composition or an organic solvent type base coating composition.
  • a method of coating the substrate, on which the above multilayer coating film is formed, with the above base coating composition is not particularly limited and it can include spray coating and rotary atomization type coating, and a multi-stage coating using these methods or a coating method of combining these methods is preferred from the viewpoint of improving an appearance.
  • a film thickness of the above base coating composition applied is preferably within a range of 10 ⁇ m (lower limit) to 20 ⁇ m (upper limit) in terms of a dried film thickness .
  • a clear coating film of the present invention when the abovebase coating composition is water-borne, it is desired to heat a base not-yet-cured coating film at 40°C to 100°C for 2 to 10 minutes before applying the above water-borne clear coating composition in order to obtain a good finished coating film.
  • the multilayer coating film can be formed by performing the above steps 1 to 3 after heat curing the above base not-yet-cured coating film to form a clear coating film.
  • the sagging can be suitably controlled and a coating film, which can satisfy properties such as appearance performance required in a clear coating film, can be formed. That is, it is possible to improve the sagging while maintaining properties as a clear coating film, and thereby it is possible to obtain a clear coating film having an excellent appearance performance.
  • the coating composition of the present invention has no problem of VOC and can reduce the environmental loads because of being water-borne.
  • ⁇ N part(s)” and “%” refer to "part (s) byweight” and ⁇ % byweight” in Examples, unless otherwise specified.
  • DPM dipropyleneglycol monomethyl ether
  • the obtained polyisocyanate resin solution had a solid content of 100 weight %, average viscosity of 150 mPa » s, and an average
  • TINUVIN 384 ultraviolet absorber produce by CIBA-GEIGY
  • the obtained curable coating composition product was diluted with ion-exchange water in such a way that the viscosity of the coating composition by Ford cup No. 4 becomes 40 seconds at 20°C.
  • the coated test plate was honed with a No. 1000 sand paper and wiped with a cloth impregnated with petroleum benzin to degrease to obtain a coated test plate .
  • This coated test plate was placed with a longitudinal direction vertical, and the above-mentioned coating composition prepared was air sprayed onto an upper area of the test plate, which is 70 mm in height, in such a way that a dried film thickness is about 40 ⁇ m, and immediately ultraviolet rays were irradiated to the test plate in the conditions of a bulb type of an H bulb, a conveyer speed of 4 m/minute and a lamp distance of 10 cm using an electrodeless lamp "F600 (240 W/cm) " manufactured by Fusion UV Systems Japan K.K. After irradiation, the test plate was placed with a longitudinal direction vertical and heat cured at 140°C for 30 minutes to obtain a test plate. (Examples 2 to 10) Based on the formulations in Table 1, each curable coating compositionwas prepared in the predeterminedheating conditions in the same manner as in Example 1 and a test plate was obtained.
  • each curable coating composition was preparedin thepredeterminedheating conditions in the same manner as in Example 1 and a test plate was obtained. (Method of evaluation)
  • Example 2 The same procedure as in Example was performed except for using a tin plate in place of the above coated test plate, and a coating film on the tin plate was scraped off before and after ultraviolet rays were irradiated, respectively, and the viscosity of each coat at 25 0 C was measured with an E type viscometer (VISCONIC EMD viscometer manufactured by TOKI SANGYO Co., LTD.) .
  • E type viscometer VISCONIC EMD viscometer manufactured by TOKI SANGYO Co., LTD.
  • the distance (mm) by which the coating composition at the coated area sagged in an uncoated area of 30 mm in width in the lower portion of the above coated test plate after heat curing, was measured.
  • Pencil hardness was measured on the obtained coating film according to JIS K 5400.8.4.2.
  • A-TMM-3L (pentaerithritol triacrylate produced by SHIN-NAKAMURA CHEMICAL Co., Ltd.)
  • Darocur 1173 (photopolymerization initiator produced by Ciba Specialty Chemicals K. K.)
  • Irgacure 184 (photopolymerization initiator produced by Ciba Specialty Chemicals K. K.)
  • the water-borne clear coating composition and the method of forming a clear coating film of the present invention are suitable for coating of automotive bodies, automotive parts and special-purpose vehicles, and thereby sagging can be inhibited while maintaining various properties required in a clear coating composition and thereby a clear coating film having an excellent appearance can be formed.

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Abstract

It is an object of the present invention to provide a water-borne clear coating composition and a method of forming a clear coating film, which can control a sagging suitably and form a coating film having a good appearance and in which consideration is given to environments. A water-borne clear coating composition, containing an active energy-ray curable compound having an unsaturated bond, a photopolymerization initiator, a water-borne acrylic resin and a polyisocyanate compound.

Description

DESCRIPTION
WATER-BORNE CLEAR COATING COMPOSITIONANDMETHOD OF FORMING CLEAR
COATING FILM
TECHNICAL FIELD
The present invention relates to a water-borne clear coating composition and a method of forming a clear coating film.
BACKGROUND ART
In coating of automotive bodies, the formation of a top coating film consisting of a base coating film and a clear coating filmis in widespreaduse. Particularly, since the clear coating film constitutes the outermost layer in a coating film, of automotive bodies and the like, properties such as high appearance, water resistance andweather resistance are required, and as a clear coating composition having such properties, various clear coating compositions are used.
On the other hand, in the field of automotive coatings, an effort is under way to reduce amounts of organic solvents to be used from the viewpoint of consideration to environments. In order to achieve such an object, it is studied to convert a solvent-borne coating composition to a water-borne coating composition, and in an intermediate coating composition and a base coating composition, this conversion has already become commercially practical. But, in a clear coating composition, this conversionhas been delayed since it was difficult to satisfy all of required properties described above.
Particularly, in the water-borne clear coating composition, it is desired to control adequately a sagging. Since the occurrence of the sagging causes projections and depressions at the surface of a coating film and thereby the smoothness of a coating film is deteriorated and a defective appearance is produced, it is a very important issue that to prevent the sagging from taking place. Particularly, it was extremely difficult to control the smoothness of a coating film since an automotive body has a complicated structure. Such a problemarises also in a solvent-borne clear coating composition, but the deterioration of an appearance due to such a problem is relatively slight in the solvent-borne clear coating composition since the viscosity of a coating composition is increased due to volatilization of a solvent in the solvent-borne clear coating composition. However, in the water-borne clear coating composition, since vaporization of water is slow, such a problem becomes more significant.
For example, as for a close-to-horizontal plane of a substrate to be coated, it is not preferred to coat the surface with a coating composition having excessively high viscosity in order to attain the smoothness of the surface . When a coating film is formed from a coating composition having low viscosity, a smooth surface is formed spontaneously, and contrarily, when the coating film is formed from a coating composition of high viscosity, the coating composition is not fluidized andtherefore the surface is less prone to being leveled. On the other hand, on a close-to-vertical plane, a coating composition of low viscosity readily run down, and therefore the coating filmrequires ameasure of viscosity. However, since it is not practical to switch the kinds of a coating composition to be used between the close-to-horizontal plane and the close-to-verticalplane, it is required to achieve the smoothness on the above-mentioned horizontal plane and the smoothness on the above-mentioned vertical plane simultaneously with the same coating composition.
In Japanese Kokai Publication Heill-300272, there is described a method of forming a coating film, comprising the steps of applying a melamine-based coating composition containing a active energy-ray curable compound to a substrate to be coated, irradiating active energy-rays to pre-curing a coating film, and heating to cure a coating film completely in order to control sagging in an intermediate coating composition . In Japanese Kokai Publication 2003-245606, there is described a method of forming a heat cured coating film, comprising the steps of applying a coating composition containing a photo-curable composition, a thermosetting resin composition, a photopolymerization initiator, and a photoacid generator, irradiating light to the coating film to increase the viscosity of the coating film, and heating and curing the coating film. However, these methods are a method of forming a coating film by curing a coating composition containing an organic solvent and a method of coating a water-borne clear coating composition, in which a problem of deteriorating an appearance due to sagging is remarkable, is not described. Further, when the photoacid generator is used, there is a possibility that properties such as the water resistance and the weather resistance of a coating film are deteriorated, and therefore it has been required to form a coating film having higher water resistance and higher weather resistance.
DISCLOSURE OF THE INVENTION In view of the above state of the art, it is an object of the present invention to provide a water-borne clear coating composition and a method of forming a clear coating film, which can control a sagging suitably and form a coating film having a good appearance and in which consideration is given to environments.
The present invention pertains to a water-borne clear coating composition, containing an active energy-ray curable compound having an unsaturated bond, a photopolymerization initiator, a water-borne acrylic resin and a polyisocyanate compound.
The active energy-ray curable compound having an unsaturatedbondispreferably a compoundhaving a (meth) acrylate group .
The polyisocyanate compound is preferably a blocked polyisocyanate. The present invention pertains to a method of forming a clear coating film, comprising steps of applying the water-borne clear coating composition mentioned above to a substrate to be coated (step 1), irradiating active energy-rays to a not-yet-cured coating film obtained by the step 1 from an approximately vertical plane (step 2) , and heating the substrate to be coated, which has been subjected to the step 2, to give a cured coating film (step 3) .
The viscosity of the coating film which has been subj ected to the step 2 is preferably 2000 to 500000 mPa»s.
In the method of forming a clear coating film of the present invention, said method preferably comprises the step (A-I) of applying an intermediate coating composition onto a cured electrodeposition coating film and heat curing the intermediate coating composition applied to form a multilayer coating film, and the step (A-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1. In the method of forming a clear coating film of the present invention, said method preferably comprises the step (B-I) of applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating film to form a multilayer coating film, and the step (B-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1.
In the method of forming a clear coating film of the present invention, said method preferably comprises a step (C) of applying a overcoat clear coating composition to the substrate after the step 3.
The present invention pertains to a coated substrate, wherein a substrate to be coated is an automotive body or an automotive assemblypart and said coated substrate is coated by the method mentioned above.
Hereinafter, the present invention will be described in detail.
The water-borne clear coating composition and the method of forming a clear coating film of the present invention is characterized by using an active energy-ray curable compound having an unsaturated bond and a photopolymerization initiator in combination with a water-soluble thermosetting resin component. When active energy-rays are irradiated after applying a coating composition containing the above-mentioned active energy-ray curable compound having an unsaturated bond and the above-mentioned photopolymerization initiator to a substrate to be coated, a polymerization reaction of the above active energy-ray curable compound having an unsaturated bond takes place due to the irradiation of the active energy-ray and an increase in the viscosity of the coating film takes place. When the increase in the viscosity of the coating film takes place, sagging becomes hard to occur and an appearance of a clear coating film can be controlled. By employing such a coating composition and applying the above-mentioned method, an excellent effect of inhibiting sagging can be attained even in the case of a water-borne clear coating composition and an environmental load of the clear coating composition can be reduced. The present invention pertains to the water-borne clear coating composition contains an active energy-ray curable compound having an unsaturated bond, a photopolymerization initiator, a water-borne acrylic resin and a polyisocyanate compound. The above-mentioned active energy-ray curable compound having an unsaturated bond is preferably a compound having two ormore α, β-unsaturated carbonyl groups in amolecule . The above-mentioned α, β-unsaturated carbonyl group is a functional group in which a double bond exists between α carbon and β carbon on a carbonyl group and examples of the functional groups can include a methacrylate group, an acrylate group, a maleate group and a fumarate group. When the above active energy-ray curable compound has only one α, β-unsaturated carbonyl group, it is not preferred in that active energy-ray curability is not adequately exhibited. Number of the above α, β-unsaturated carbonyl groups in a molecule is preferably 10 or less and most preferably 6 or less.
The above active energy-ray curable compound having an unsaturatedbond is not particularly limited, and it can include, for example, polyol (meth) acrylate ester, unsaturatedpolyester polymer containing α, β-unsaturated dicarboxylic acid such as fumaric acid and maleic acid as an acid component, epoxy polymer (meth) acrylate, a (meth) acryloyl group-containing urethane compound, an acrylic polymer containing an α, β-unsaturated carbonyl group, a (meth) acryloyl group-containing polyether polymer and a (meth) acryloyl group-containing silicone oligomer.
The above-mentioned polyol (meth) acrylate ester is ester of polyol having two or more hydroxyl groups and acrylic acid. The above-mentioned compound having two or more hydroxyl groups may be a low molecular weight compound or may be a polymer. The above polyol (meth) acrylate ester is not particularly limited, and it can include, for example, (meth) acrylate esters of low molecular weight polyol such as ethylene glycol di (meth) acrylate, neopentyl glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, pentaerithritol tetra (meth) acrylate, dipentaerithritol hexa (meth) acrylate, 1, 4-cyclohexyldimethanol di (meth) acrylate, 4,4' -isopropylidenedicyclohexanol di (meth) acrylate, bis (hydroxymethyl) tricyclo [5, 2,1,0] decane di (meth) acrylate and 1, 3, 5-tris (2-hydroxyethyl) cyanurate tri (meth) acrylate; and (meth) acrylate esters of a hydroxyl group-containingpolymer such as (meth) acrylate of an acrylic polymer having a hydroxyl group, (meth) acrylate of polyester polyol, (meth) acrylate of polyether polyol, (meth) acrylate of epoxy polyol, (meth) acrylate of polyurethane polyol and poly (meth) acrylate of silicone polyol. In the present specification, (meth) acrylate refers to acrylate and methacrylate.
The above-mentioned unsaturated polyester polymer is not particularly limited, and it can include, for example, polymer obtained by polycondensation of an acid component consisting of α, β-unsaturated dicarboxylic acid such as maleic acid anhydride and fumaric acid and another polyhydric carboxylic acid used as required, and polyol having two or more hydroxyl groups . Polyol used for the above unsaturated polyester polymer is not particularly limited, and it can include, for example, ethylene glycol, diethylene glycol, propylene glycol, tetramethylene glycol, 1, 6-hexanediol, neopentyl glycol, trimethylolpropane, glycerin, pentaerythritol, 1, 4-cyclohexanedimethanol,
4, 4' -isopropylidenedicyclohexanol, bis (hydroxymethyl) tricyclo [5, 2,1,0] decane,
1, 3, 5-tris (2-hydroxyethyl) cyanuric acid, isopropylidenebis (3, 4-cyclohexanediol) , and ethylene oxide adduct, propylene oxide adduct and/or caprolactone adduct thereof.
Another polyhydric carboxylic acid, which can be used for the above unsaturated polyester polymer, is not particularly limited, and it can include, for example, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methyltetrahydrophthalic acid, isophthalic acid, terephthalic acid, maleic acid, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, dodecenylsuccinic acid and 1, 4-cyclohexanedicarboxylic acid. The above-mentioned epoxy polymer (meth) acrylate can include, for example, a polymer obtained by a ring-opening addition reaction of an epoxy polymer such as a bisphenol type epoxy polymer and a novolak type epoxy polymer and (meth) acrylic acid. The above-mentioned (meth) acryloyl group-containing urethane compound can include, for example, a compound obtained by an addition reaction of a polyisocyanate compound such as isophorone diisocyanate, trilenediisocyanate, diphenylmethane diisocyanate and hexamethylene diisocyanate or urethane prepolymer thereof, and 2-hydroxyethyl (meth) acrylate. The above-mentioned acrylic polymer containing an α, β-unsaturated carbonyl group can include, for example, an acrylic polymer having a (meth) acrylate group on a side chain, which is obtained by reacting an acrylic polymer formed by copolymerizing glycidyl (meth) acrylate with (meth) acrylic acid; and an acrylic polymer having a (meth) acrylate group on a side chain, which is obtained by reacting a carboxyl group-containing acrylic polymer with an epoxy group-containing acrylic monomer such as glycidyl (meth) acrylate . The above-mentioned (meth) acryloyl group-containing polyether polymer can include, for example, a compound obtained by reacting polyether having a hydroxyl group at a terminal with 2- (meth) acryloyloxyethyl isocyanate .
The above-mentioned (meth) acryloyl group-containing silicone oligomer can include, for example, polyorganosiloxane having 3- (meth) acryloyloxypropyl groups at both terminals.
The above active energy-ray curable compound having an unsaturated bond may have a plurality of hydroxyl groups and the like in addition to the α, β-unsaturated carbonyl group . The above-mentioned active energy-ray curable compounds having an unsaturated bond may be used alone or in combination of two or more kinds .
The active energy-ray curable compound having an unsaturated bond contained in the coating composition of the present invention is preferably polyol (meth) acrylate ester because polyol (meth) acrylate ester is highly reactive and has excellent weather resistance and compatibility and a high gloss . A number-average molecular weight (Mn) of the active energy-ray curable compoundhaving an unsaturatedbond contained in the coating composition of the present invention is preferably within a range of 200 (lower limit) to 5000 (upper limit) . When the above-mentionednumber-averagemolecularweight (Mn) is less than 200, the solvent resistance, the water resistance and the weather resistance of a coating film may be deteriorated due to vaporization during heat curing, reduction in the hardness of a coating film and reduction in the curability of a coating film. When the above number-average molecular weight (Mn) is more than 5000, the viscosity of the active energy-ray curable compound having an unsaturated bond itself increases and a content of a solution in a diluted coating composition upon applying a coating composition may become large in some cases. The above lower limit is more preferably 250. The above upper limit is more preferably 3000. In addition, the number-average molecular weight (Mn) is a number-average molecular weight on the polystyrene equivalent basis measuredby GPC (gel permeation chromatography) in the present specification.
A double bond equivalent of the above-mentioned active energy-ray curable compound having an unsaturated bond is preferably within a range of 50 (lower limit) to 1500 (upper limit) . When the above double bond equivalent is less than 50, an unreacted (meth) acrylate group remains in a coating film to be obtained and therefore the weather resistance of a coating film may be deteriorated or a coating film to be obtained may become hard and brittle. And, when the above double bond equivalent is more than 1500, a crosslinking density of a coating film to be obtained becomes low and therefore coating film properties and performance may be deteriorated. In addition, the double bond equivalent in the present specification means a molecular weight per a double bond. The above double bond equivalent more preferably has a lower limit of 70 and an upper limit of 1000.
A commercially available article of the above active energy-ray curable compound having an unsaturated bond can include A-TMM-3L (trade name, produce by SHIN-NAKAMURA CHEMICAL Co., Ltd.), M-400 (trade name, produced by TOAGOSEI Co. , Ltd.), TMPTA: (producedby TOAGOSEI Co., Ltd. ), and D-330: (trade name, produced by Nippon Kayaku Co., Ltd.) .
The content of the above active energy-ray curable compound having an unsaturated bond is preferably 1 weight % (lower limit) to 35 weight % (upper limit) in 100 weight % of the total resin solid matter of the water-borne acrylic resin and the polyisocyanate compound. When this content is less than 1 weight %, an effect may not be developed. When the content is more than 35 weight %, this may have an adverse effect on coating film properties. The above content more preferably has a lower limit of 5 weight % and an upper limit of 30 weight %.
The water-borne clear coating composition of the present invention further has a photopolymerization initiator.
As the above-mentioned photopolymerization initiator, a publicly known initiator can be employed, and specific examples of the initiators can include benzoin and benzoin alkyl ethers such as benzoin, benzoin methyl ether, benzoin ethyl ether and benzoin propyl ether; acetophenones such as acetophenone, 2, 2-dimethoxy-2-phenylacetophenone, 2, 2-diethoxy-2-phenylacetophenone and
1, 1-dichloroacetophenone; aminoacetophenones such as 2-methyll- [4- (methylthio) phenyl] -2-morpholinopropanone-l, 2- benzyl-2-dimethylamino-l- (4-morpholinophenyl) -butanone-1 and N,N-dimethylaminoacetophenone; anthraquinones such as 2-methylanthraquinone, 2-ethylanthraquinone,
2-tert-butylanthraquinone and 1-chloroanthraquinone; thioxanthones such as 2, 4-dimethylthioxanthone, 2, 4-diethylthioxanthone, 2-chlorothioxanthone and 2, 4-diisopropylthioxanthone; ketals such as acetophenonedimethylketal and benzyldimethylketal; benzophenones or xanthones such as benzophenone and 4, 4' -bisdiethylaminobenzophenone; and
2, 4, 6-trimethylbenzoyldiphenylphosphine oxide. These may be used in combination of two or more kinds and may be further used in combination with tertiary amine such as triethanolamine or a photopolymerization initiator such as ethyl dimethylaminobenzoate .
A commercially available article of the above ■photopolymerization initiator can include Darocur 1173, Irgacure 184, Irgacure 500, Irgacure 2959 (trade name, every produced by Ciba Specialty Chemicals K. K.).
An amount of the above photopolymerization initiator to be mixed is not particularly limited and it is appropriately established in accordance with a ratio of a heat curing reaction and a photocuring reaction. Further, the amount of the above photopolymerization initiator to be mixed can be adjusted in accordance with a degree of weather resistance required, kinds and amounts of additives such as an ultraviolet absorber component. An amount of the above photopolymerization initiator to be generally mixed in a coating composition containing a curable binder used in the present invention is, for example, 0.01 to 10 weight % with respect to 100 weight % of the above active energy-ray curable compound having an unsaturated bond. The water-borne clear coating composition of the present invention further contains the water-borne acrylic resin. Since an acrylic resin is highly resistant to hydrolysis, by using this resin, the coating composition of the present invention becomes a water-borne clear coating composition which can attain a coating composition which is excellent in long-term storage stability.
The above-mentioned water-borne acrylic resin is a copolymer which is predominantly composed of acrylic acid and/or methacrylic acid, and a monomer used for polymerizing the above water-borne acrylic resin is not particularly limited and can include, for example, ethylenic unsaturated carboxylic acid monomers such as acrylic acid, methacrylic acid, maleic acid, itaconic acid and the like; ethylenic unsaturated carboxylic alkyl ester monomers such as methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate and the like; ethylenic unsaturated dicarboxylic monoester monomers such as ethyl maleate, butyl maleate, ethyl itaconate, butyl itaconate and the like; hydroxyl group-containing ethylenic unsaturated carboxylic alkyl ester monomers such as 2-hydroxyethyl (meth) acrylate,
2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, a reactant of 2-hydroxyethyl (meth) acrylate and ε-caprolactone and the like; ethylenic unsaturated carboxylic aminoalkyl ester monomers such as aminoethyl (meth) acrylate, dimethylaminoethyl (meth) acrylate, butylaminoethyl (meth) acrylate and the like; ethylenic unsaturated carboxylic acid aminoalkylamide monomers such as aminoethyl (meth) acrylamide, dimethylaminomethyl (meth) acrylamide, methylaminopropyl (meth) acrylamide and the like; another amide group-containing ethylenic unsaturated carboxylic acid monomers such as acrylamide, methacrylamide, N-methylolacrylamide, methoxybutylacrylamide, diacetoneacrylamide and the like; unsaturated fatty acid glycidyl ester monomers such as glycidyl acrylate, glycidyl methacrylate and the like; vinyl cyanide monomers such as
(meth) acrylonitrile, α-chloroacrylonitrile and the like; saturated aliphatic carboxylic acid vinyl ester monomers such as vinyl acetate, vinyl propionate and the like; styrenic monomers such as styrene, α-methyl styrene, vinyl toluene and the like. A mixture of the above monomers may be in the form of using the above monomers alone or in combination of two or more components .
A polymerization method for obtaining the above acrylic resin is not particularly limited and publicly known methods such as solution polymerization, dispersion polymerization and emulsion polymerization can be employed.
The above acrylic resin may be one obtained by further grafting an acrylic resin obtained from the mixture of the above monomers. By grafting the above acrylic resin, it is possible to change a structure of the acrylic resin for a structure which is resistant to hydrolysis. And, when the above acrylic resin is grafted, the viscosity of a coating composition resists increasing even when the acrylic resin is contained in high concentrations in the water-borne coating composition, and therefore it is possible to coat at a high solid content and improve coating efficiency.
A method of grafting described above is not particularly limited and publicly known methods such as solution polymerization, emulsion polymerization and the like can be employed.
A number-average molecular weight of the water-borne acrylic resin of the present invention is preferably within a range of 1000 (lower limit) to 10000 (upper limit) . When the above-mentioned number-average molecular weight of an acrylic resinis less thanlOOO, amolecularweight is too lowandtherefore the weather resistance and the curability as a clear coating film are low, and basic properties as a clear coating film may be impaired. And, when the number-average molecular weight is more than 8000, since the viscosity of the water-borne coating composition becomes too large, the coating efficiency may be deteriorated. The above lower limit is more preferably 1500 and furthermore preferably 2000. The above upper limit is more preferably 7500 and furthermore preferably 6000.
In the above-mentioned water-borne acrylic resin, a hydroxyl group value of resin is more preferably within a range of 30 mg KOH/g (lower limit) to 150 mg KOH/g (upper limit) . When the hydroxyl group value is less than 30 mg KOH/g, a crosslinking property becomes insufficient and there may be cases where adequate coating film performance cannot be secured. When it is more than 150 mg KOH/g, the water resistance and the weather resistance of a coating film of a coating composition obtained by using this resin may be deteriorated. The above hydroxyl group value more preferably has a lower limit of 40 mg KOH/g and furthermore preferably has a lower limit of 50 mg KOH/g. The above hydroxyl group value more preferably has an upper limit of 130 mg KOH/g and furthermore preferably has an upper limit of 100 mg KOH/g.
In the above-mentioned water-borne acrylic resin, an acid value of resin is preferably within a range of 10 mg KOH/g (lower limit) to 100 mg KOH/g (upper limit) . When the acid value is less than 10 mg KOH/g, water solubility may be deteriorated to impair stability. When it is more than 100 mg KOH/g, a hydrophilic property of a resinmaybecome too large and therefore water resistance and weather resistance of a coating film may be deteriorated. The above acid value more preferably has a lower limit of 20 mg KOH/g and furthermore preferably has a lower limit of 30 mg KOH/g. The above acid value more preferably has an upper limit of 80 mg KOH/g and furthermore preferably has an upper limit of 60 mg KOH/g. The above water-borne acrylic resin is preferably contained in an amount 50 weight % (lower limit) to 95 weight % (upper limit) with respect to the total resin solid matter contained in the water-borne clear coating composition. When this amount is less than 50 weight %, it may become difficult to secure the water resistance, the weather resistance and the appearance of a coating film to be obtained. When this amount is more than 95 weight %, a crosslinking property becomes insufficient and there maybe cases where adequate coating film performance cannot be secured. The above lower limit is more preferably 65weight % andthe aboveupper limit isrαorepreferably 90 weight %.
The water-based clear coating material composition of the present invention further contains a polyisocyanate compound. The above-mentioned polyisocyanate compound is not particularly limited as long as it is a compound having two or more isocyanate groups, and it can include, for example, aliphatic isocyanates such as trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HDI) and trimethylhexamethylene diisocyanate; aliphatic cyclic isocyanates such as 1, 3-cyclopentane diisocyanate, 1, 4-cyclohexane diisocyanate and 1, 2-cyclohexane diisocyanate; aromatic isocyanates such as xylylene diisocyanate (XDI), 2, 4-trilene diisocyanate (TDI) and 2,6-trilene diisocyanate; and alicyclic isocyanates such as isophorone diisocyanate (IPDI) and norbornane diisocyanatomethyl; polymers of abiuret type andan isocyanurate type thereof, and mixtures thereof.
The above-mentioned polyisocyanate compound is preferably a blocked polyisocyanate. When the blocked polyisocyanate is used, it is preferred since the storage stability of a coating composition becomes better. The above-mentioned blocked polyisocyanate can include a substance obtained by adding a blocking agent having active oxygen to the above polyisocyanate compound, in which the blocking agent is dissociated by heating to produce an isocyanate group and this isocyanate group reacts with a functional group in the above water-borne acrylic resin to cure the coating composition.
The above-mentioned blocking agent can include, for example, halogenated hydrocarbons, aliphatic, aromatic or heterocyclic alcohols such as methanol, ethanol, n-propanol, isopropanol, furfuryl alcohol, alkyl group substituted furfuryl alcohol andbenzyl alcohol, andoximes such as methyl ethyl ketone oxime, methyl isobutyl ketone oxime, acetone oxime and cyclohexane oxime, and in addition caprolactam. The above-mentioned water-borne clear coating material composition preferably contains a film forming resin having a hydroxyl group. A ratio of the isocyanate compound to be mixed to the above film forming resin can be variously selected in accordance with objects, but in the clear coating composition used in the present invention, it is preferred to construct in such a way that the equivalence ratio (NCO/OH) of an isocyanate group (NCO) to a hydroxyl group (OH) is within a range of 0.2/1 (lower limit) to 5/1 (upper limt) . When the above equivalence ratio (NCO/OH) is less than the lower limit, the curability of a coating composition become insufficient, and when it is more than the upper limit, a cured coat becomes too hard and brittle. The above equivalence ratio more preferably has a lower limit of 0.5/1 and an upper limit of 2/1.
In the above water-borne clear coating composition, an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, a crosslinking resin particle, and a surface control agent in addition to the above polymers may be mixed. When the above-mentioned crosslinking resin particle is used, the crosslinking resinparticle is preferablymixed in an amount 0.01 weight % (lower limit) to 10 weight % (upper limit) with respect to the resin solidmatter of the water-borne clear coating composition of the present invention. The above lower limit is more preferably 0.1 weight % and the above upper limit is more preferably 5 weight %. When an amount of the above crosslinking resinparticle to be added is more than 10 weight %, an appearance of a coating film to be obtained is deteriorated, and when it is less than 0.01 weight %, an effect of controlling rheology cannot be attained.
A method of producing the above water-borne clear coating composition is not particularly limited and methods, which are well known to those skilled in the art, such as amethod of kneading and dispersing a formulation of the above polymers and the like with a kneader or a roller, and the like can be employed. The coating composition of the present invention is water-borne, and it does not need to take the volatilization portion of a solvent into consideration in adjusting viscosity, can control sagging suitably and can reduce the environmental loads. A solution used in the above water-borne coating composition is not particularly limited, and it can include, for example, well known solutions such as Byketol-Special, Byketol-OK (trade names, both produced by BYK-Chemie Japan KK) and the like. In the water-borne clear coating composition, it is preferred to adjust the viscosity in applying the coating composition so as to be 20 to 50 seconds at 20°C with a Ford cup No. 4. By limiting the viscosity of a coating composition within this range, a coating film after coating has excellent smoothness and sagging can be suitably prevented.
The present invention also pertains to a method of forming a clear coating film, which is constituted by using the water-borne clear coating composition described above. The method of forming a clear coating film of the present invention comprises steps of applying the water-borne clear coating composition described above to a substrate to be coated (step 1), irradiating active energy-rays to a not-yet-cured coating film obtained by the step 1 from an approximately vertical plane (step 2) , and heating the coated substrate, which has been subjected to the step 2, to cure the not-yet-cured coating film (step 3) .
The method of forming a clear coating film of the present invention is amethod in which a coating film canbe formedwithout causing sagging even though a coated plane is approximately vertical by employing the water-borne clear coating composition described above and coating by the coating steps described above .
The above-mentioned step 1 is a step of applying the water-borne clear coating composition described above to a substrate to be coated. In the above step 1, a method of applying a coating composition containing a curable binder is not particularly limited, and it can include, for example, brush application, roller application, air spray application, airless spray application, immersion application, flow coating and the like. Particularly, when the substrate to be coated is an automotive body or an automotive part, electrostatic air spray coating or rotary atomization type electrostatic coating is preferred. The viscosity of the above-mentioned coating composition containing a curable binder can be appropriately adjusted with water in accordance with a coating method. A film thickness of a coating composition applied is not particularly limited and it can be appropriately established in accordance with the applications of a coated substrate to be obtained. A method of coating the above clear coating composition can include, specifically, a method of coating by an electrostatic coating apparatus of rotary atomization type, referredto as aMicroMicrobell, or aMicrobell . Afilmthickness of the above clear coating composition applied in the above step 1 is preferably within a range of 25 μm (lower limit) to 45 μm (upper limit) in terms of a dried film thickness.
The above-mentioned step 2 is a step of irradiating active energy-rays to a not-yet-cured coating film obtainedby the above step 1 from an approximately vertical plane. That is, in the step 2, by irradiating active energy-rays to approximately vertical plane on which sagging is particularly apt to occur, the viscosity of the coating composition is increased to inhibit sagging, and thereby the smoothness of a coating film is improved and a surface appearance of a coating film becomes better. The above step 2 is a step in which since the active energy-rays are irradiated froman approximatelyvertical plane, sagging on a vertical plane or a close-to-vertical place, on which sagging is particularly apt to occur, is inhibited and thereby projections and depressions at the surface of a coating film are inhibited and the surface appearance of a coating film is improved. Herein, the irradiation of the active energy-rays from an approximately vertical plane refers to irradiating from a plane approximately perpendicular to a horizontal plane, but the direction of irradiation have not to be precisely perpendicular and it means that the active energy-rays are irradiated at such an angle that a sufficient amount of active energy-rays can be irradiated to a vertical plane or a close-to-vertical plane on which sagging is apt to occur. In the above step 2, the active energy-rays are irradiated at least from the approximately vertical plane, but it may be simultaneously irradiated from a horizontal plane.
The irradiation of the active energy-ray in the above step 2 can include irradiation of active energy-rays such as ultraviolet light, sun light, visible light, a microwave and an electron beam, but among others, the irradiation of the active energy-ray is particularly preferably performed by the irradiation of ultraviolet light. The above-mentioned irradiation of ultraviolet light can be performedby irradiating using a carbon arc lamp, an electrodeless lamp, a mercury-vapor lamp, a xenon lamp, a fluorescent lamp and argon glow discharge as an ultraviolet source . Among others, the electrodeless lamp is preferred because it can irradiate light uniformly to a substrate to be coated, which has a complicated structure. The above step 2 may be performed just after the step 1, or a preheating step is performed after the step 1 and the above step 2 maybe performed after the preheating step, but the above step 2 is preferably performed just after the step 1. The irradiation intensity of ultraviolet light in the step 2 is preferably 200 to 2000 mJ/cm2 and the irradiation time is preferably several seconds.
After subjecting the coating film to the step 2, it is preferred to adjust the composition of a water-borne clear coating composition and the conditions of irradiating the active energy-rays in such a way that the viscosity of the coating film is 2000 to 500000 mPa'S. When the viscosity of a coating film is less than 2000 mPa*s, there is a possibility that sagging cannot be adequately inhibited. When it is more than 500000 iαPa*s, this may cause a problem that smoothness of a coating film is lost and a defective appearance such as an orange peel and the like is produced. The viscosity of a coating film more preferably has a lower limit of 5000 mPa*s and an upper limit of 100000 mPa»s.
The above-mentioned step 3 is a step of heating a coated substrate, which has been subjected to the step 2, to cure a coating film. The conditions of the above step 3 varies with the composition of a coating composition to be used, and they can be appropriately established by those skilled in the art, but a heating temperature is generally 80 to 200°C, preferably 100 to 18O0C and a heating time is preferably 10 to 40 minutes. The method of forming a clear coating film of the present invention may have a step (step 4) of further irradiating active energy-rays to a coating film which has been subjected to the above step 3. If an active energy-ray curable compound having an unsaturated bond, which has not reacted in the above step 2, remains, there is a possibility that properties of a coating film such as weather resistance may be deteriorated, and if the above step 4 is carried out, it is possible to attain higher crosslinking density and thereby impulse-resistant property of a coating film may be improved. The above-mentioned step 4 is a step by which a carbon-carbon unsaturated double bond in the above active energy-ray curable compound is cured through a radical polymerization reaction, and specifically a step of irradiating ultraviolet rays using a carbon arc lamp, an electrodeless lamp, a mercury-vapor lamp, a xenon lamp, a fluorescent lamp and argon glow discharge as an ultraviolet source. Among others, the electrodeless lamp is preferred because it can irradiate light uniformly to a substrate to be coated, which has a complicated structure.
A substrate to be coated used in the method of forming a clear coating film of the present invention can include various base materials such as metal molded articles, plastic molded articles, foam substances and the like, and more specifically it can include molded articles of metals such as iron, aluminum and alloys thereof, anodized aluminum, brass, bronze, galvanized steel sheet, tin-plated steel sheet, nickel-plated steel sheet and chromium-plated steel sheet, and plastic molded articles. Themethodof forming a clear coating filmof the present invention is particularly effective in the case where the substrate to be coated has both of an approximately horizontal plane and an approximately vertical plane. Accordingly, the method of forming a clear coating film of the present invention can be suitably used particularly for a coating method of a substrate to be coated, having a complicated shape, such as automotive bodies, automotive assemblyparts and special-purpose vehicles . The method of forming a clear coating film of the present invention may further includes the step (A-I) of applying an intermediate coating composition onto a cured electrodeposition coating filmandheat curing the intermediate coating composition applied to form a multilayer coating film, and the step (A-2) of applying a base coating composition onto the above-mentioned multilayer coating film, prior to the step 1.
And, the method of forming a clear coating film of the present inventionmay further includes the step (B-I) of applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating filmto formamultilayer coating film, and the step (B-2) of applying a base coating composition onto the above-mentioned multilayer coating film, prior to the step 1.
The above-mentioned step (A-I) is a step of applying an intermediate coating composition onto a electrodeposition coating film formed on a substrate to be coated and heat curing the intermediate coating compositionappliedto formamultilayer coating film.
The method of forming a clear coating film of the present invention is preferably applied to metal molded articles onwhich cationic electrocoating can be performed. The above-mentioned substrate to be coated is preferably chemical conversion treated on its surface. Further, the substrate to be coated may be provided with an electrodeposition coating film. As a coating composition used in the above electrocoating, a cationic and an anionic coating compositions can be used, but a cationic electrodeposition coating composition is preferred from the viewpoint of an anti-corrosive property.
And, it is possible that the intermediate coating film has been formed on the electrodeposition coating film. The intermediate coating composition is used for forming the intermediate coating film. The above intermediate coating composition is not particularly limited and it can include a water-borne intermediate coating composition or an organic solvent type intermediate coating compositionwell known to those skilled in the art. When the above intermediate coating composition is applied, the above electrodeposition coating film may be a not-yet-cured one. That is, the multilayer coating film may be formed by applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating film (step (B-I) ) .
As a method of heat curing the above intermediate coating composition, a publicly known method can be employed.
The above-mentioned steps (A-2) and (B-2) are a step of applying a base coating composition onto a multilayer coating film consisting of the electrodeposition coating film and the intermediate coating film.
The above-mentioned base coating composition is not particularly limited and it may include, for example, a film forming resin, a curing agent, an organic or an inorganic or a glorious material-containing coloringpigment and an extender . The formof the above base coating composition is not particularly limited and it can include a water-borne base coating composition or an organic solvent type base coating composition.
A method of coating the substrate, on which the above multilayer coating film is formed, with the above base coating composition is not particularly limited and it can include spray coating and rotary atomization type coating, and a multi-stage coating using these methods or a coating method of combining these methods is preferred from the viewpoint of improving an appearance.
The method of forming a clear coating film of the present invention may be a method of coating by 2 coating processes and 1 baking process in which a base coating composition is applied to a substrate to be coated to form a base not-yet-cured coating film, and then the above water-borne clear coating composition is applied onto the above base not-yet-cured coating film according to the above steps 1 to 3, and the above base not-yet-cured coating film and a clear not-yet-cured coating film are simultaneously heated and cured, or may be a method of coating by 2 coating processes and 2 baking processes in which the above base not-yet-cured coating film is heated and cured, and then the above water-borne clear coating composition is applied to form a clear not-yet-cured coat, and this clear not-yet-cured coating film is heated and cured. When coating is performed by the 2 coating processes and
1 baking processes, a film thickness of the above base coating composition applied is preferably within a range of 10 μm (lower limit) to 20 μm (upper limit) in terms of a dried film thickness .
In the method of forming a clear coating film of the present invention, when the abovebase coating compositionis water-borne, it is desired to heat a base not-yet-cured coating film at 40°C to 100°C for 2 to 10 minutes before applying the above water-borne clear coating composition in order to obtain a good finished coating film. The base not-yet-cured coating film formed by the above-mentioned method together with the clear not-yet-cured coating film obtained by the above step 1 and the above step
2 in the method of forming a clear coat of the present invention are simultaneously heated and cured in the above step 3, and thereby a multilayer coating film can be formed.
And, when a multilayer coating film is formed by the above-mentioned 2 coating processes and 2 baking processes, the multilayer coating film can be formed by performing the above steps 1 to 3 after heat curing the above base not-yet-cured coating film to form a clear coating film.
A heating temperature in heat curing the above base not-yet-cured coating film is preferably within a range of 100°C (lower limit) to 180°C (upper limit) . And, the heating temperature more preferably has a lower limit of 120°C and an upper limit of 160°C. A heat curing time varies with a curing temperature, but it is proper to heat cure in 10 to 30 minutes when heat curing is carried out at the above heat curing temperature .
The method of forming a clear coating film of the present invention may also include a step (C) of further applying a overcoat clear coating composition to a coating film which has been subj ected to the above step 3. The above-mentioned overcoat clear coating composition is intended for forming two or more layers of clear coating films when higher appearance is attained or a particle size of pigment in a base coating film is large. A method of forming those can include, for example, a method described in Japanese Kokai Publication Heill-253877.
In accordance with the water-borne clear coating composition and the method of forming a -clear coating film of the present invention, the sagging can be suitably controlled and a coating film, which can satisfy properties such as appearance performance required in a clear coating film, can be formed. That is, it is possible to improve the sagging while maintaining properties as a clear coating film, and thereby it is possible to obtain a clear coating film having an excellent appearance performance. And, the coating composition of the present invention has no problem of VOC and can reduce the environmental loads because of being water-borne.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples. In addition, λNpart(s)" and "%" refer to "part (s) byweight" and λλ% byweight" in Examples, unless otherwise specified.
(Production Example 1 Production of acrylic resin)
In a flask equipped with a condenser, a solvent recovery unit, a stirrer, a thermometer and a nitrogen gas inlet pipe,
40 parts of dipropyleneglycol monomethyl ether (DPM) was put, and the DPM was heated to 130°C and kept at this temperature, and then to this, a mixture of 15 parts of acrylic acid, 16 parts of hydroxyethyl methacrylate, 39 parts of ethyl acrylate, 5 parts of styrene and 5 parts of MSD-100 as monomers, and 20 parts of NK Ester M-90G (produced by SHIN-NAKAMURA CHEMICAL Co . , Ltd. ) , 13 parts of an initiator Kayaester 0 (t-butylperoxyhexanoate, produced by Kayaku Akzo Corporation) and 10 parts of DPM was added dropwise over 3 hours. Then, the resulting mixture was stirred at 130°C for 30 minutes and to this, a solution formed by dissolving 0.3 part of Kayaester 0 in 5 parts of DPM was added dropwise over 30 minutes . After this, a temperature was reduced to 110°Candto this, 20 parts of CARDURAElOP (tradename, produced by Japan Epoxy Resins Co., Ltd.) and 2.6 parts of TBAH (tetrabutylammonium hydroxide) were added and the mixture was stirred at 1100C to graft themixture . After an acidvalue reached 58.8, all of the solvent was eliminated and the solution was neutralized with 100% DMEA and then water-solbilized by adding deionized water . A solid content of the resulting acrylic resin solution was 44.8 weight % and a number-average molecular weight by GPC was 3000. (Production Example 2 Preparation of polyisocyanate resin) Uretodion containing two terminal NCO functional groups, i.e., dimerized hexamethylene diisocyanate 70 parts and 30 parts of N, N' ,N"-tris- ( 6-isocyanatohexyl) -isocyanurate were mixed.
The obtained polyisocyanate resin solution had a solid content of 100 weight %, average viscosity of 150 mPa»s, and an average
NCO content of 22.5%. (Example 1)
TINUVIN 384 (ultraviolet absorber produce by CIBA-GEIGY
Corporation) 2 parts, 2 parts of TINUVIN 292 (light stabilizer produced by CIBA-GEIGY Corporation) , and 2 parts of an acrylic surface control agent (solid content 50 weight %) produced by
NIPPON PAINT Co . , Ltd. were mixed to obtain an additive solution . The acrylic resin, obtained in Production Example 1, 178 parts, 22 parts of the polyisocyanate resin obtained in Production Example 2, 10 parts of A-TMM-3L (special acrylate produced by SHIN-NAKAMURA CHEMICAL Co., Ltd.) , and 2 parts of Darocur 1173 (photopolymerization initiator produced by CIBA-GEIGY Corporation) were stirred with Disper and mixed uniformly, and then to this, the above-mentioned additive solution obtained was added, and the resulting mixture was stirred to prepare a curable coating composition product. The obtained curable coating composition product was diluted with ion-exchange water in such a way that the viscosity of the coating composition by Ford cup No. 4 becomes 40 seconds at 20°C. A cationic electrodeposition coating POWERTOP PU-50
(trade name) producedbyNIPPON PAINT Co ., Ltd. was electrocoated on a dull steel sheet having a size of 300 mm in length, 100 mm in width and 0.8 mm in thickness, treated with zinc phosphate, in such a way that a dried film thickness is about 25 μm, and onto this, Orga P-2 Sealer (trade name) produced by NIPPON PAINT
Co . , Ltd. was air sprayed in such a way that a dried film thickness is about 40 μm and heat cured at 1400C for 30 minutes. To this coated test plate, a black colored water-borne base coating composition was air sprayed in such a way that a dried film thickness is 16 μm and the coated test plate was heat cured at
1400C for 30 minutes. Further, the coated test plate was honed with a No. 1000 sand paper and wiped with a cloth impregnated with petroleum benzin to degrease to obtain a coated test plate .
This coated test plate was placed with a longitudinal direction vertical, and the above-mentioned coating composition prepared was air sprayed onto an upper area of the test plate, which is 70 mm in height, in such a way that a dried film thickness is about 40 μm, and immediately ultraviolet rays were irradiated to the test plate in the conditions of a bulb type of an H bulb, a conveyer speed of 4 m/minute and a lamp distance of 10 cm using an electrodeless lamp "F600 (240 W/cm) " manufactured by Fusion UV Systems Japan K.K. After irradiation, the test plate was placed with a longitudinal direction vertical and heat cured at 140°C for 30 minutes to obtain a test plate. (Examples 2 to 10) Based on the formulations in Table 1, each curable coating compositionwas prepared in the predeterminedheating conditions in the same manner as in Example 1 and a test plate was obtained.
(Comparative Examples 1 to 4) Based on the formulations in Table 2, each curable coating compositionwas preparedin thepredeterminedheating conditions in the same manner as in Example 1 and a test plate was obtained. (Method of evaluation)
On the test plates obtained as described above, each coating film of the test plates obtained was evaluated according to the following evaluation method. The results of evaluations are shown in Tables 1 and 2.
(1) Measurement of viscosity
The same procedure as in Example was performed except for using a tin plate in place of the above coated test plate, and a coating film on the tin plate was scraped off before and after ultraviolet rays were irradiated, respectively, and the viscosity of each coat at 250C was measured with an E type viscometer (VISCONIC EMD viscometer manufactured by TOKI SANGYO Co., LTD.) .
(2) Evaluation of sagging
The distance (mm) , by which the coating composition at the coated area sagged in an uncoated area of 30 mm in width in the lower portion of the above coated test plate after heat curing, was measured.
(3) Pencil hardness
Pencil hardness was measured on the obtained coating film according to JIS K 5400.8.4.2.
(4) Appearance of coating film The appearance of the cured coating filmobtainedwas rated visually. Rating criteria are as follows.
©: There is not the occurrence of wrinkling or cratering.
O: There are few occurrence of wrinkling or cratering.
Δ: There are a few occurrences of wrinkling or cratering. X: There are many occurrences of wrinkling or cratering. Table 1
OO
Figure imgf000029_0001
Figure imgf000030_0001
Numerical value in a parenthesis in Table represents parts by weight (solid matter) .
A-TMM-3L: (pentaerithritol triacrylate produced by SHIN-NAKAMURA CHEMICAL Co., Ltd.)
M-400: (mixture of dipentaerithritol pentaacrylate and hexaacrylate, produced by TOAGOSEI Co., Ltd.)
D-330: (acrylate oligomer produced by Nippon Kayaku Co., Ltd.; Mn 584)
TMPTA: (trimethylolpropane triacrylate)
Darocur 1173: (photopolymerization initiator produced by Ciba Specialty Chemicals K. K.)
Irgacure 184: (photopolymerization initiator produced by Ciba Specialty Chemicals K. K.)
Table 2
O
Figure imgf000031_0001
From Tables 1 and 2, it was found that in the clear coating films obtained by the water-borne clear coating compositions of Examples, sagging was adequately inhibited and appearance performance was excellent. On the other hand, in the clear coating films obtained by the water-borne clear coating compositions of Comparative Examples, sagging was not adequately inhibited.
INDUSTRIAL APPLICABILITY The water-borne clear coating composition and the method of forming a clear coating film of the present invention are suitable for coating of automotive bodies, automotive parts and special-purpose vehicles, and thereby sagging can be inhibited while maintaining various properties required in a clear coating composition and thereby a clear coating film having an excellent appearance can be formed.

Claims

1. A water-borne clear coating composition, containing an active energy-ray curable compound having an unsaturated bond, a photopolymerization initiator, a water-borne acrylic resin and a polyisocyanate compound.
2. The water-borne clear coating composition according to Claim 1, wherein the active energy-ray curable compound having an unsaturated bond is a compound having a (meth) acrylate group.
3. The water-borne clear coating composition according to Claim 1 or 2, wherein the polyisocyanate compound is a blocked polyisocyanate .
4. A method of forming a clear coating film, comprising steps of applying the water-borne clear coating composition according to any one of Claims 1 to 3 to a substrate to be coated (step 1) , irradiating active energy-rays to a not-yet-cured coating film obtained by the step 1 from an approximately vertical plane
(step 2) , and heating the substrate to be coated, which has been subjected to the step 2, to give a cured coating film (step 3) .
5. The method of forming a clear coating film according to Claim 4, wherein the viscosity of the coating film which has been subjected to the step 2 is 2000 to 500000 mPa«s.
6. The method of forming a clear coating film according to Claim 4 or 5, wherein said method comprises the step (A-I) of applying an intermediate coating composition onto a cured electrodeposition coating film and heat curing the intermediate coating composition applied to form a multilayer coating film, and the step (A-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1.
7. The method of forming a clear coating film according to Claim 4 or 5, wherein said method comprises the step (B-I) of applying an intermediate coating composition onto a not-yet-cured electrodeposition coating film and heat curing the intermediate coating composition applied and the not-yet-cured electrodeposition coating filmto formamultilayer coating film, and the step (B-2) of applying a base coating composition onto said multilayer coating film, prior to the step 1.
8. The method of forming a clear coating film according to any one of Claims 4 to I1 comprising a step (C) of applying a overcoat clear coating composition to the substrate after the step 3.
9. A coated substrate, wherein a substrate to be coated is an automotive body or an automotive assemblypart and said coated substrate is coated by the method according to any one of Claims 4 to 8.
PCT/JP2006/312840 2005-06-22 2006-06-21 Water-borne clear coating composition and method of forming clear coating film WO2006137569A1 (en)

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