Classifications

• • 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
  • C09D133/14—Homopolymers or copolymers of esters of esters containing halogen, nitrogen, sulfur or oxygen atoms in addition to the carboxy oxygen
• • 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
  • C09D161/00—Coating compositions based on condensation polymers of aldehydes or ketones; Coating compositions based on derivatives of such polymers
  • C09D161/20—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen
  • C09D161/26—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes with heterocyclic compounds
  • C09D161/28—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes with heterocyclic compounds with melamine
• • 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
  • C09D167/00—Coating compositions based on polyesters obtained by reactions forming a carboxylic ester link in the main chain; Coating compositions based on derivatives of such polymers
• • 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
  • C09D171/00—Coating compositions based on polyethers obtained by reactions forming an ether link in the main chain; Coating compositions based on derivatives of such polymers
  • C09D171/02—Polyalkylene oxides
• • 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
• • 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
  • C09D201/00—Coating compositions based on unspecified macromolecular compounds
  • C09D201/02—Coating compositions based on unspecified macromolecular compounds characterised by the presence of specified groups, e.g. terminal or pendant functional groups
  • C09D201/06—Coating compositions based on unspecified macromolecular compounds characterised by the presence of specified groups, e.g. terminal or pendant functional groups containing oxygen atoms
• • 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
  • C09D5/00—Coating compositions, e.g. paints, varnishes or lacquers, characterised by their physical nature or the effects produced; Filling pastes
• • 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
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/36—Successively applying liquids or other fluent materials, e.g. without intermediate treatment

Definitions

• the present invention relates to an aqueous paint composition and a method for producing a coated article.
• the most common method for forming multi-layer coatings on automobile bodies is the so-called water-based 3WET coating (3C1B), which involves applying a water-based undercoat paint composition, pre-drying, applying a water-based base paint composition, pre-drying, applying a clear paint composition, and baking, in that order.
• 3C1B water-based 3WET coating
• Patent Document 1 discloses a method for forming a multi-layer coating film using a paint containing a specific water-based polyester resin, a specific water-based acrylic resin, a water-based urethane resin, and a melamine resin.
• the coating composition of Patent Document 1 is not effective enough in suppressing the formation of the above-mentioned mixed layer, and the appearance of the resulting coating film is likely to deteriorate.
• the coating film obtained from the coating composition of Patent Document 1 does not have sufficient chipping resistance.
• the object of the present invention is to provide an aqueous paint composition that can provide a multi-layer coating film with excellent appearance and chipping resistance even when the preliminary drying step performed after applying the aqueous paint composition is omitted.
• An aqueous coating composition comprising: An aqueous polyester resin (A); An aqueous polyurethane resin (B); A melamine resin (C); The aqueous polyester resin (A), the aqueous polyurethane resin (B), and a hydroxyl group-containing resin (D) other than the melamine resin (C),
• the aqueous polyester resin (A) has a number average molecular weight of 1000 or more and 6000 or less, a glass transition temperature of -30°C or more and 20°C or less, an acid value of 10 mgKOH/g or more and 25 mgKOH/g or less, and a hydroxyl value of more than 60 mgKOH/g and 100 mgKOH/g or less, and the solid content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less relative to 100 parts by mass of the resin solid content of the aqueous coating composition,
• the aqueous polyester resin (A) has a number average mole
• the aqueous coating composition of the present invention allows a multi-layer coating film with excellent appearance and chipping resistance to be obtained even when the preliminary drying step performed after applying the aqueous coating composition is omitted.
• the aqueous coating composition according to the present disclosure contains an aqueous polyester resin (A), an aqueous polyurethane resin (B), a melamine resin (C), and a hydroxyl-containing resin (D) other than these.
• the aqueous polyester resin (A) has a number average molecular weight of 1000 to 6000, a glass transition temperature of -30°C to 20°C, an acid value of 10 mgKOH/g to 25 mgKOH/g, and a hydroxyl value of more than 60 mgKOH/g to 100 mgKOH/g.
• the solid content of the aqueous polyester resin (A) is 12.5 to 45 parts by mass relative to 100 parts by mass of the resin solid content of the aqueous coating composition.
• the solid content of the aqueous polyurethane resin (B) is 15 to 60 parts by mass relative to 100 parts by mass of the resin solid content of the aqueous coating composition.
• the aqueous coating composition according to the present disclosure contains an aqueous polyester resin (A) with a low molecular weight and a high hydroxyl value. Because the aqueous polyester resin (A) has a low molecular weight, its viscosity decreases when heated. This improves the appearance of the resulting coating film. In addition, because the aqueous polyester resin (A) has a high hydroxyl value, the crosslinking density increases, improving the chipping resistance of the resulting coating film. Furthermore, because a specific amount of aqueous polyurethane resin (B) is included, the elasticity of the resulting coating film is increased, further improving chipping resistance.
• the aqueous polyester resin (A) has a number average molecular weight of 1,000 to 6,000, a glass transition temperature (Tg) of -30°C to 20°C, an acid value of 10 mgKOH/g to 25 mgKOH/g, and a hydroxyl value of more than 60 mgKOH/g to 100 mgKOH/g.
• Tg glass transition temperature
• the viscosity of the aqueous polyester resin (A) is reduced during heat curing, improving the smoothness of the resulting coating film.
• the flexibility of the resulting coating film is increased, improving chipping resistance.
• the number average molecular weight of the aqueous polyester resin (A) may be 1,200 or more, 1,500 or more, or 2,000 or more.
• the number average molecular weight of the aqueous polyester resin (A) may be 5,000 or less, 4,000 or less, or 3,000 or less.
• the number average molecular weight and weight average molecular weight are determined by GPC using polystyrene as the standard.
• the Tg of the aqueous polyester resin (A) may be -28°C or higher, -25°C or higher, or -23°C or higher.
• the Tg of the aqueous polyester resin (A) may be 18°C or lower, 15°C or lower, or 10°C or lower.
• the hydroxyl value of the aqueous polyester resin (A) may be 95 mgKOH/g or less, or may be 90 mgKOH/g or less.
• the hydroxyl value of the aqueous polyester resin (A) may be 70 mgKOH/g or more, or may be 80 mgKOH/g or more.
• the acid value of the aqueous polyester resin (A) may be 20 mgKOH/g or less, 18 mgKOH/g or less, or 15 mgKOH/g or less.
• the acid value of the aqueous polyester resin (A) may be 11 mgKOH/g or more, or 11.5 mgKOH/g or more.
• the hydroxyl value and acid value are determined based on the mass of solids.
• the hydroxyl value and acid value can be measured by the known method described in JIS K 0070:1992.
• the hydroxyl value and acid value may be calculated from the amount of unsaturated monomer in the raw material monomer of the resin (e.g., aqueous polyester resin (A)).
• the aqueous coating composition may contain one or more (particularly two or more) aqueous polyester resins (A).
• the aqueous polyester resin (A) may be dissolved. That is, the aqueous polyester resin (A) may be a water-soluble polyester resin. In the aqueous paint composition, the aqueous polyester resin (A) may be in the form of a dispersion.
• Water-soluble polyester resin and aqueous polyester resin dispersion can be obtained, for example, by neutralizing polyester resin with a base and gradually adding water under stirring to cause phase inversion emulsification, or by using a high-speed homogenizer to forcibly emulsify polyester resin in water.
• the aqueous polyester resin (A) may have a hydroxyl group and a carboxyl group.
• the number of hydroxyl groups in one molecule of the aqueous polyester resin (A) may be two or more, and may be three or more.
• the number of carboxyl groups in one molecule of the aqueous polyester resin (A) may be two or more, and may be two.
• Polyester resins are obtained, for example, by condensation of a polyhydric alcohol component and a polybasic acid component.
• a polyhydric alcohol has two or more hydroxyl groups in one molecule.
• Examples of polyhydric alcohols include diols such as propylene glycol, neopentyl glycol, butylene glycol, hexylene glycol, octylene glycol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,12-dodecanediol, 1,2-cyclohexanediol, 1,3-cyclohexanediol, 1,4-cyclohexanediol, hydrogenated bisphenol A, caprolactone diol, and bishydroxyethyl taurine; triols such as trimethylolpropane and hexanetriol; and tetra
• polybasic acid component examples include polycarboxylic acids.
• Polycarboxylic acids have two or more carboxy groups in one molecule.
• polycarboxylic acids include aromatic dicarboxylic acids such as phthalic acid and isophthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and tetrahydrophthalic acid; and tricarboxylic acids such as trimellitic acid. These may be used alone or in combination of two or more.
• saturated fatty acids such as stearic acid, lauric acid, and myristic acid
• unsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid
• long-chain fatty acids of natural oils and fats such as castor oil, palm oil, and soybean oil, and modified products thereof may be used in combination.
• At least one of monohydric alcohols, monoepoxide compounds, lactones, and hydroxycarboxylic acids may also be used as raw materials for the polyester resin.
• Lactones have a cyclic ester bond.
• lactones include dimethylpropiolactone, ⁇ -caprolactone, ⁇ -caprolactone, ⁇ -valerolactone, and ⁇ -butyrolactone. These may be used alone or in combination of two or more.
• Hydroxycarboxylic acids have a hydroxyl group and a carboxyl group in one molecule.
• Examples of hydroxycarboxylic acids include dimethylolpropionic acid. These can be used alone or in combination of two or more.
• Carboxy groups can be introduced by modifying some or all of the hydroxyl groups of polyester resin with acid anhydrides such as phthalic anhydride, succinic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, etc.
• acid anhydrides such as phthalic anhydride, succinic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, etc.
• the solid content of the aqueous polyester resin (A) is 12.5 parts by mass or more and 45 parts by mass or less, based on 100 parts by mass of the resin solid content of the aqueous coating composition. This provides an effect of inhibiting intermixing.
• the solid content of the aqueous polyester resin (A) may be 20 parts by mass or more, or 25 parts by mass or more.
• the solid content of the aqueous polyester resin (A) may be 40 parts by mass or less, or 38 parts by mass or less, or 35 parts by mass or less.
• the resin solids mass of the aqueous coating composition is the total solids mass of the aqueous polyester resin (A), aqueous polyurethane resin (B), melamine resin (C), hydroxyl group-containing resin (D) and other resin components.
• the aqueous coating composition may contain an aqueous polyester resin (a) other than the aqueous polyester resin (A).
• the other aqueous polyester resin (a) may be water-soluble.
• the form of the other aqueous polyester resin (a) may be a dispersion.
• aqueous polyester resins (a) include, for example, low molecular weight polyester resins (a1) having a number average molecular weight of less than 1000 and a hydroxyl value of 60 mgKOH/g or less.
• low molecular weight polyester resins (a1) having a number average molecular weight of less than 1000 and a hydroxyl value of 60 mgKOH/g or less.
• the solid content of the low molecular weight polyester resin (a1) is, for example, 1 part by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition.
• the solid content of the low molecular weight polyester resin (a1) may be 2 parts by mass or more, or 4 parts by mass or more.
• the solid content of the low molecular weight polyester resin (a1) may be 8 parts by mass or less, or 7 parts by mass or less.
• the aqueous polyurethane resin (B) increases the elasticity of the coating film and improves its chipping resistance.
• Water-soluble polyurethane resin and aqueous polyurethane resin dispersion can be obtained, for example, by forcibly emulsifying polyurethane resin using a surfactant, or by neutralizing polyurethane resin with a base or acid.
• Polyurethane resins are obtained, for example, by reacting a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate, and, if necessary, a chain extender and a polymerization terminator.
• Polyol compounds contain two or more hydroxyl groups in the molecule.
• polyol compounds include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerin; polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol; polyester polyols obtained from dicarboxylic acids such as adipic acid, sebacic acid, itaconic acid, maleic anhydride, phthalic acid, and isophthalic acid and glycols such as ethylene glycol, triethylene glycol, propylene glycol, butylene glycol, tripropylene glycol, and neopentyl glycol; polycaprolactone polyols; polybutadiene polyols; polycarbonate polyols; and polythioether polyols. These may be used alone or in combination of
• Examples of compounds having an active hydrogen group and a hydrophilic group in the molecule include compounds containing active hydrogen and an anionic group, a cationic group, or a nonionic hydrophilic group.
• Anionic groups include anionic groups and anion-forming groups.
• Anion-forming groups are groups that can form an anionic group by reacting with a base, and specifically, anionic groups are formed by neutralizing with a base before, during, or after the urethane reaction.
• Examples of compounds containing active hydrogen and an anionic group are described in JP-B-42-24192 and JP-B-55-41607, and specific examples include ⁇ , ⁇ -dimethylolpropionic acid and ⁇ , ⁇ -dimethylolbutyric acid.
• Examples of compounds having active hydrogen and a cationic group are described in JP-B-43-9076.
• Examples of compounds having active hydrogen and a nonionic hydrophilic group are described in JP-B-48-41718, and specific examples include polyethylene glycol and alkyl alcohol alkylene oxide adducts.
• Organic polyisocyanates contain two or more isocyanate groups in the molecule.
• organic polyisocyanates include 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexyl-2,4-diisocyanate, methylcyclohexyl-2,6-diisocyanate, xylylene diisocyanate, 1,3-bis(isocyanato)methylcyclohexane, tetramethylxylylene diisocyanate, transcyclohexane-1,4-diisocyanate, aliphatic diisocyanates such as lysine diisocyanate, 2,4-toluylene diisocyanate
• the chain extender contains two or more active hydrogen groups in the molecule.
• chain extenders include low molecular weight polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 3-methylpentanediol, 2-ethyl-1,3-hexanediol, and trimethylolpropane; polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine, and isophoronediamine; and water. These may be used alone or in combination of two or more.
• Polymerization terminators include compounds that have one active hydrogen in the molecule (such as monoalcohols and monoamines) and monoisocyanate compounds.
• the synthesis method of the urethane resin may be a one-shot method in which each component is reacted at once, or a multi-stage method in which the components are reacted in stages.
• a portion of the active hydrogen-containing compound e.g., a polymer polyol
• a polyisocyanate to form an NCO-terminated prepolymer
• this prepolymer is reacted with the remainder of the active hydrogen-containing compound.
• the solid content of the aqueous polyurethane resin (B) is 15 parts by mass or more and 60 parts by mass or less, relative to 100 parts by mass of the total amount of resin solids in the aqueous coating composition. This improves chipping resistance without impairing the intermixing suppression effect.
• the solid content of the aqueous polyurethane resin (B) may be more than 15 parts by mass, may be 20 parts by mass or more, may be 22 parts by mass or more, or may be 25 parts by mass or more.
• the solid content of the aqueous polyurethane resin (B) may be 50 parts by mass or less, may be 40 parts by mass or less, or may be 30 parts by mass or less.
• the Tg of the aqueous polyurethane resin (B) is, for example, -90°C or higher and -10°C or lower. This further increases the flexibility of the coating film, and can further improve chipping resistance.
• the Tg of the aqueous polyurethane resin (B) may be -40°C or higher, or -35°C or higher.
• the Tg of the aqueous polyurethane resin (B) may be -15°C or lower, or -18°C or lower.
• the Tg of the aqueous polyurethane resin (B) can be measured by a differential scanning calorimeter.
• the melamine resin (C) is a curing agent that reacts mainly with the hydroxyl-containing resin (D) to form a crosslinked structure, thereby providing a coating film with high hardness.
• the melamine resin (C) may be water-soluble or water-insoluble.
• the melamine resin (C) contains a structure (-N 1 (R 1 ) (R 2 ), -N 2 (R 3 ) (R 4 ), -N 3 (R 5 ) (R 6 )) in which six substituents R 1 to R 6 are bonded to the periphery of a triazine ring ( triazine nucleus) via three nitrogen atoms N 1 to N 3.
• the hardness of the resulting coating film is further improved by the planar structure and rigid triazine ring. This allows both excellent chipping resistance and excellent hardness due to the aqueous polyester resin (A) and the aqueous polyurethane resin (B).
• the substituents R 1 to R 6 are each independently a hydrogen atom, an alkyl ether group (-CH 2 -OR 7 ), or a methylol group (-CH 2 OH).
• the substituents R 1 to R 6 and R 7 may each independently be an alkyl group having 1 to 8 carbon atoms, or an alkyl group having 1 to 4 carbon atoms.
• the alkyl group may be linear or branched.
• the alkyl group may be a methyl group, an n-butyl group, or an isobutyl group.
• the melamine resin (C) is generally composed of a polynuclear compound in which multiple triazine rings are bonded to each other.
• the melamine resin (C) may also be a mononuclear compound composed of one triazine ring.
• Examples of the melamine resin (C) include a full alkyl type having only alkyl ether groups as the substituents R 1 to R 6 ; a methylol group type having -N(-CH 2 -OR 7 ) (-CH 2 OH); an imino group type having -N(-CH 2 -OR 7 ) (H); and a methylol/imino group type having -N(-CH 2 -OR 7 ) (-CH 2 OH) and -N(-CH 2 -OR 7 ) (H).
• the melamine resin (C) may be hydrophobic or hydrophilic.
• the melamine resin (C) may be particularly hydrophobic.
• the melamine resin (C) can react with hydroxyl groups of other resins while self-aggregating to form a cluster structure.
• the cluster structure can further improve the hardness of the coating film.
• the hydrophobicity of the entire uncured coating film can be further increased, and the effect of suppressing intermixing can be further improved.
• the water resistance of the coating film is improved.
• a coating film having excellent hardness and water resistance is suitable for, for example, direct bonding of glass.
• the glass is, for example, a vehicle window. Direct bonding of glass is a method of bonding glass and a vehicle via an adhesive. When the aqueous coating composition according to the present disclosure is used for painting a vehicle, direct bonding of glass becomes easy.
• the hydrophobic melamine resin (C) is, for example, one in which the average total number of hydrogen atoms and methylol groups among the substituents R 1 to R 6 is 1 or less.
• the hydrophobic melamine resin (C) may be a full alkyl type. Specific examples of the full alkyl type melamine resin (C) include methylated melamine resin, butylated melamine resin, and isobutylated melamine resin.
• the weight average molecular weight of the melamine resin (C) may be 1000 or more and 7000 or less. This can further improve the hardness of the resulting coating film.
• the weight average molecular weight of the melamine resin (C) may be 1500 or more, or may be 2000 or more.
• the weight average molecular weight of the melamine resin (C) may be 5000 or less, or may be 4000 or less.
• melamine resins include, for example, the Cymel series (trade name) manufactured by Allnex, specifically, Cymel 202, Cymel 204, Cymel 211, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 250, Cymel 251, Cymel 254, Cymel 266, Cymel 267, Cymel 270, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325, Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, and Cymel 1141; and the U-Ban (trade name) series manufactured by Mitsui Chemicals. These may be used alone or in combination of two or more types.
• the solid content of the melamine resin (C) is, for example, 10 parts by mass or more and 40 parts by mass or less, relative to 100 parts by mass of the resin solid content of the aqueous coating composition. This makes it easier for the curing reaction to proceed, making it easier to obtain a coating film with high hardness.
• the solid content of the melamine resin (C) may be 15 parts by mass or more, or may be 17 parts by mass or more.
• the solid content of the melamine resin (C) may be 35 parts by mass or less, or may be 30 parts by mass or less.
• the aqueous coating composition may contain a curing agent other than the melamine resin (C).
• a curing agent other than the melamine resin (C) examples include blocked isocyanate compounds, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. These may be used alone or in combination of two or more.
• the hydroxyl-containing resin (D) reacts with the melamine resin (C) to form a crosslinked structure.
• the hydroxyl-containing resin (D) is a resin containing a hydroxyl group other than the aqueous polyester resin (A), the aqueous polyurethane resin (B), and the melamine resin (C).
• the hydroxyl-containing resin (D) has one or more hydroxyl groups.
• hydroxyl-containing resins include hydroxyl-containing acrylic resins (D1) and polyether polyols (D2). These may be used alone or in combination.
• the aqueous coating composition may contain one or more (particularly two or more) hydroxyl-containing acrylic resins (D1), or one or more hydroxyl-containing acrylic resins (D1) and one or more polyether polyols (D2).
• the solid content of the hydroxyl-containing resin (D) is, for example, 10 parts by mass or more and 30 parts by mass or less, based on 100 parts by mass of the resin solid content of the aqueous coating composition. This makes it easier for the curing reaction to proceed, making it easier to obtain a coating film with high hardness.
• the solid content of the hydroxyl-containing resin (D) may be 15 parts by mass or more, or may be 16 parts by mass or more.
• the solid content of the hydroxyl-containing resin (D) may be 28 parts by mass or less, or may be 25 parts by mass or less.
• the hydroxyl-containing acrylic resin (D1) may be in the form of an emulsion.
• the hydroxyl-containing acrylic resin (D1) may be dissolved. That is, the hydroxyl-containing acrylic resin (D1) may be a water-soluble acrylic resin.
• the aqueous coating composition may contain both a water-soluble acrylic resin and an aqueous acrylic resin in the form of an emulsion.
• Acrylic resin emulsions can be prepared by emulsion polymerization of ⁇ , ⁇ -ethylenically unsaturated monomers.
• ⁇ , ⁇ -ethylenically unsaturated monomers include (meth)acrylic acid esters, ⁇ , ⁇ -ethylenically unsaturated monomers having an acid group, and ⁇ , ⁇ -ethylenically unsaturated monomers having a hydroxyl group.
• the monomers can be used alone or in combination of two or more.
• (meth)acrylic acid esters examples include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and dihydrodicyclopentadienyl (meth)acrylate.
• (Meth)acrylic acid esters refer to acrylic acid esters and methacrylic acid esters.
• Examples of ⁇ , ⁇ -ethylenically unsaturated monomers having an acid group include acrylic acid, methacrylic acid, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ⁇ -carboxy-polycaprolactone mono(meth)acrylate, isocrotonic acid, ⁇ -hydro- ⁇ -((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acrylamido-2-methylpropanesulfonic acid, p-hydroxystyrene, and 2,4-dihydroxy-4'-vinylbenzophenone.
• Examples of ⁇ , ⁇ -ethylenically unsaturated monomers having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ⁇ -caprolactone.
• ⁇ , ⁇ -ethylenically unsaturated monomers may be used in combination.
• examples of other ⁇ , ⁇ -ethylenically unsaturated monomers include polymerizable amide compounds, polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, ⁇ -olefins, dienes, polymerizable carbonyl compounds, polymerizable alkoxysilyl compounds, and other polymerizable compounds.
• the method of emulsion polymerization is not particularly limited.
• an emulsifier is dissolved in an aqueous medium containing water or, if necessary, an organic solvent such as an alcohol or ether (e.g., dipropylene glycol methyl ether, propylene glycol methyl ether, etc.), and the ⁇ , ⁇ -ethylenically unsaturated monomer and polymerization initiator are added dropwise while heating and stirring.
• the ⁇ , ⁇ -ethylenically unsaturated monomer may be emulsified in advance with an emulsifier.
• the polymerization initiator and emulsifier may be those commonly used by those skilled in the art. If necessary, the molecular weight may be adjusted using a chain transfer agent such as mercaptan (e.g., lauryl mercaptan) and ⁇ -methylstyrene dimer. The reaction temperature, reaction time, etc. may be appropriately selected within the range commonly used by those skilled in the art. The obtained acrylic resin emulsion is neutralized with a base if necessary.
• the hydroxyl-containing acrylic resin (D1) (acrylic resin emulsion) obtained by emulsion polymerization may have a number average molecular weight of 3,000 or more.
• the hydroxyl group-containing acrylic resin (D1) may have a hydroxyl value (solid content hydroxyl value) of 20 mgKOH/g or more and 180 mgKOH/g or less.
• the hydroxyl group-containing acrylic resin (D1) may have an acid value (solid content acid value) of 1 mgKOH/g or more and 80 mgKOH/g or less.
• the water-soluble hydroxyl group-containing acrylic resin (D1) can be prepared, for example, by solution polymerizing the above-mentioned ⁇ , ⁇ -ethylenically unsaturated monomer and then making it water-soluble using a basic compound.
• the polyether polyol (D2) may be in the form of an emulsion. In the coating composition, the polyether polyol (D2) may be dissolved. That is, the polyether polyol (D2) may be a water-soluble polyether polyol.
• the polyether polyol (D2) can be obtained, for example, by addition polymerization of an alkylene oxide such as ethylene oxide, propylene oxide, or tetrahydrofuran to a polyhydric alcohol.
• the polyether polyol (D2) may be a polyether diol having two hydroxyl groups in one molecule.
• polyether diols examples include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol.
• the aqueous coating composition contains water as a solvent.
• the aqueous coating composition may contain an organic solvent together with the aqueous solvent.
• the proportion of water in the solvent is, for example, 50 mass % or more, 70 mass % or more, or 90 mass % or more.
• organic solvent examples include ester solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate;
• solvent examples include ether solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, methyl methoxybutanol, ethoxypropanol, ethylene glycol isopropyl ether, ethylene glycol t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, methoxybutanol, and propylene glycol monobutyl ether; alcohol solvents such as methanol, ethanol, butanol, and propyl alcohol; and ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ether
• the aqueous coating composition may further contain other components, such as pigments, film-forming assistants, surface conditioners, preservatives, antifungal agents, defoamers, light stabilizers, UV absorbers, antioxidants, and pH adjusters.
• other components such as pigments, film-forming assistants, surface conditioners, preservatives, antifungal agents, defoamers, light stabilizers, UV absorbers, antioxidants, and pH adjusters.
• the aqueous coating composition comprises an aqueous polyester resin (A) in the form of a dispersion, a low molecular weight polyester resin (a1), an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), and each of the hydroxyl group-containing acrylic resins (D1) in the form of a water-soluble and an emulsion.
• the solid content of each resin per 100 parts by mass of resin solids in the aqueous coating composition is as follows.
• Aqueous polyester resin in dispersion form (A) 20 parts by mass or more and 30 parts by mass or less.
• Low molecular weight polyester resin (a1) 2 parts by mass or more and 10 parts by mass or less.
• Aqueous polyurethane resin in dispersion form (B) 25 parts by mass or more and 35 parts by mass or less.
• Water-soluble and emulsion-form acrylic resin (D1) 5 parts by mass or more and 15 parts by mass or less, respectively.
• the aqueous coating composition comprises an aqueous polyester resin (A) in the form of a dispersion, an aqueous polyurethane resin (B) in the form of a dispersion, a melamine resin (C), water-soluble and emulsion-form hydroxyl-containing acrylic resins (D1), and a water-soluble polyether polyol (D2).
• the solid content of each resin per 100 parts by mass of resin solids in the aqueous coating composition is as follows.
• the aqueous coating composition can be prepared, for example, by stirring each component with a stirrer, etc.
• the pigment may be dispersed in a vehicle containing water, a surfactant, a dispersant, etc., using a sand grind mill, etc., and then mixed with other components.
• the aqueous coating composition according to the present disclosure can suppress the formation of a mixed layer between the aqueous coatings applied in layers even when pre-drying is omitted. Therefore, the aqueous coating composition according to the present disclosure is suitable for use in the manufacture of coated articles having a multi-layer coating film formed by a plurality of aqueous coatings. In particular, the aqueous coating composition according to the present disclosure is suitable for forming an intermediate coating film.
• the method for producing a coated article according to the present disclosure comprises the steps of applying the aqueous paint composition according to the present disclosure onto an object to be coated to form an uncured intermediate coat film, and applying an aqueous base paint composition onto the uncured intermediate coat film to form an uncured base coat film. There is no pre-drying step between the steps of forming the uncured intermediate coat film and forming the uncured base coat film. Omission of the pre-drying step reduces the environmental impact and production costs.
• pre-drying methods include, for example, natural drying and heat drying.
• natural drying the uncured intermediate coating film is left for 5 to 15 minutes at a temperature condition of, for example, 20°C to 25°C.
• Heat drying is performed under conditions where the curing reaction of the coating film-forming components does not proceed, or at least the curing reaction is not completed.
• heat drying the uncured intermediate coating film is heated for 30 seconds to 10 minutes at a temperature condition of, for example, 50°C to 80°C.
• Step of forming an uncured intermediate coating film The aqueous coating composition according to the present disclosure is applied to a substrate to form an uncured intermediate coating film.
• the intermediate coating film is interposed between the substrate and the base coating film.
• the intermediate coating film improves adhesion between the base coating film and the substrate.
• the intermediate coating film according to the present disclosure suppresses the formation of a mixed layer.
• the aqueous coating composition according to the present disclosure contains a polyester resin, the intermediate coating further smoothes the coating surface, making it easier to suppress unevenness in the base coating film.
• the thickness of the intermediate coating film after curing may be 6 ⁇ m or more and 35 ⁇ m or less, from the viewpoint of smoothness and chipping resistance of the coated article.
• the thickness of the intermediate coating film after curing may be 8 ⁇ m or more, and may be 10 ⁇ m or more.
• the thickness of the intermediate coating film after curing may be 25 ⁇ m or less, and may be 22 ⁇ m or less.
• the thickness of the coating can be measured using an electromagnetic coating thickness gauge (e.g., SANKO's SDM-miniR).
• the coating thickness is the average value of the coating thickness at any five points.
• Examples of coating methods include air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency.
• rotary atomization electrostatic coating machines commonly known as "micro-microbell ( ⁇ bell)", “microbell ( ⁇ bell)”, “metallicbell (metabell)", etc. are used.
• the material of the substrate include metal, resin, and glass. According to the aqueous coating composition of the present disclosure, a coating film having excellent chipping resistance can be obtained, and therefore the aqueous coating composition is particularly suitable for coating substrates containing metal.
• the shape of the object to be coated is not particularly limited.
• Specific examples of the object to be coated include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and parts for automobile bodies, as well as automobile parts such as spoilers, bumpers, mirror covers, grilles, and doorknobs.
• metals include nium, tin, zinc, and alloys thereof (e.g., steel).
• metallic substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrolytic galvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.
• the metallic substrate may be surface-treated. Examples of surface treatments include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. After surface treatment, the metallic substrate may be further coated with an electrocoating paint.
• the electrocoating paint may be of the cationic type or the anionic type.
• resins examples include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, polyvinyl chloride resin, and polyamide resin. It is preferable that the resin substrate has been degreased.
• the preliminary drying (also called preheating) process after applying the aqueous paint composition is omitted.
• mixing of the uncured undercoat film and the aqueous base paint composition applied on top of it is suppressed, and the formation of a mixed layer is suppressed. This improves the appearance of the resulting painted article.
• Step of forming an uncured base coating film The aqueous base coating composition is applied onto the uncured intermediate coating film to form an uncured base coating film.
• the base coating film may be a single layer or a laminated coating film of two or more layers.
• the multi-layer coating film is obtained by the so-called wet-on-wet coating method, in which coating is performed on an uncured coating film.
• Wet-on-wet coating makes it possible to omit the baking and drying oven, thereby reducing the environmental burden and production costs.
• the pre-drying step after coating the aqueous coating composition is also omitted, so the effect of reducing the environmental burden and production costs is even greater.
• the thickness of the base coating film after curing is, for example, 6 ⁇ m or more and 35 ⁇ m or less.
• the thickness of the base coating film after curing may be 8 ⁇ m or more, or 10 ⁇ m or more.
• the thickness of the base coating film after curing may be 25 ⁇ m or less, or 20 ⁇ m or less.
• the coating method can be, for example, the same method as that used for coating the undercoat paint composition.
• the above-mentioned preliminary drying process may be carried out.
• the aqueous base coating composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, a melamine resin), and a polyether polyol resin.
• the base coating film formed by such an aqueous base coating composition has excellent appearance.
• the aqueous base coating composition may further contain a pigment and various additives. Examples of the additives include an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, and a pinhole inhibitor.
• a clear coating composition may be further applied onto the uncured base coating film to form an uncured clear coating film.
• the thickness of the clear coating film after curing may be 20 ⁇ m or more and 50 ⁇ m or less from the viewpoint of scratch resistance and smoothness.
• the thickness of the clear coating film after curing may be 25 ⁇ m or more, or may be 30 ⁇ m or more.
• the thickness of the clear coating film after curing may be 45 ⁇ m or less, or may be 40 ⁇ m or less.
• the coating method is not particularly limited. Examples of coating methods include the same methods as those used for coating the undercoat coating composition. Among these, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. After coating the clear coating composition, preliminary drying may be performed in the same manner as described above.
• the clear coating composition may be solvent-based, water-based, or powder-type.
• the solvent-based clear coating composition may contain an acrylic resin and/or a polyester resin as a coating film-forming resin, and an amino resin and/or an isocyanate as a curing agent, from the viewpoint of transparency or acid etching resistance.
• the solvent-based clear coating composition may also contain an acrylic resin and/or a polyester resin having a carboxylic acid and/or an epoxy group.
• the clear coating composition may contain various pigments within a range that does not impair the transparency and the effect of the coating composition according to the present disclosure.
• the clear coating composition may contain the above-mentioned additives as necessary.
• the heating conditions are set appropriately depending on the composition of each paint composition and the material of the object to be coated.
• the heating temperature is, for example, 60°C or higher and 170°C or lower, and may be 90°C or higher and 150°C or lower.
• the heating time may be set appropriately depending on the heating temperature.
• the heating time may be, for example, 10 minutes or more and 60 minutes or less, or 15 minutes or more and 45 minutes or less.
• the heating time means the time during which the inside of the heating device reaches the target temperature and the coated object is maintained at the target temperature, and does not take into account the time until the target temperature is reached.
• Examples of heating devices include drying furnaces that use heat sources such as hot air, electricity, gas, and infrared rays.
• Examples 1 to 7, Comparative Examples 1 to 3 (1) Preparation of Aqueous Paint Compositions As shown in Table 1, aqueous paint compositions were prepared by varying the blending amounts of the aqueous polyester resin (A) and the aqueous polyurethane resin (B).
• the above water-based paint composition was applied onto the resulting electrocoated coating using a rotary atomizer electrostatic coating device so that the dry film thickness was 12 ⁇ m.
• Aqualex AR-3400 product name, water-based metallic base paint manufactured by Nippon Paint Automotive Coatings Co., Ltd.
• the coating was preheated at 80°C for 3 minutes.
• a clear coating composition PU Excel O-4300 (trade name, Nippon Paint Automotive Coatings, two-component clear coating) was applied using a rotary atomizing electrostatic coating device so that the dry film thickness was 35 ⁇ m. After that, the coating was heated and cured at 140°C for 30 minutes to obtain a coated article.
• the test panels after the stone chipping test were visually evaluated according to the following criteria.
• the evaluation was the average score of three test panels per level. If the following criteria score was 3.0 points or more, the panel was suitable for practical use. 5: Almost no peeling is observed.
• 2 The peeled area is large, and peeling is observed at the interface between the electrodeposition coating film and the undercoat coating film. 1: The peeled area is large and the electrodeposition coating is destroyed.
• the aqueous coating composition of the present invention can produce a multi-layer coating film with excellent appearance and chipping resistance even when pre-drying is omitted. Therefore, it is suitable for use in forming a variety of multi-layer coating films.

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