JP2011131135A - Method for forming multilayer coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming multilayer coating film applying intermediate coating, aqueous base coating and clear coating by a wet-on-wet process so as to achieve a coating film with uniform appearance by applying a steel sheet and a plastic substrate. <P>SOLUTION: There is provided a method for forming multilayer coating film by heating and curing three layers of an intermediate coating film, a base coating film and a clear coating film, wherein an organic solvent-based clear coating film is applied to form the clear coating film after aqueous intermediate coating is applied to form a coating film on a substrate and an aqueous base coating is applied to form the base coating film. In the resin solid content of 100 mass%, the aqueous base coating is characterized by containing (a) an acrylic resin emulsion which is 10-60 mass% in terms of solid content obtained by emulsion polymerizing a monomer mixture containing 0.2-20 mass% of a crosslinking monomer, (b) an aqueous acrylic resin which is 5-40 mass% in terms of solid content, (c) a melamine resin which is 20-40 mass% in terms of solid content, and (d) propylene glycol monoalkyl ether which is 10-40 mass% in the coating resin solid content of 100 mass%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for forming a multilayer coating film formed on an automobile body and the like, and a composite coating film obtained by the method.

  In recent years, considering the influence on the environment, in order to reduce the amount of the organic solvent contained in the paint, water-based paints have been studied. In automotive coatings, it is common to form a base coating and then form a clear coating thereon, but it has been practiced to make the base coating water-based in this system.

  As an aqueous base paint adapted to such a system, for example, JP-A-63-193968 (Patent Document 1) discloses a water-dispersible film-forming acrylic polymer that is an acrylic emulsion, a hydrophobic melamine resin, and a pigment. An aqueous coating composition is disclosed. Japanese Patent Application Laid-Open No. 63-265974 (Patent Document 2) discloses an aqueous coating material containing an emulsion resin having a core portion of a monomer having an alkyl group having 4 to 12 carbon atoms and a shell portion containing acrylamide or methacrylic acid. Each is disclosed.

  The present inventors have already proposed improvement of coating film appearance and workability in a coating method using an aqueous base paint in Japanese Patent Application Laid-Open No. 2001-240791 (Patent Document 3). In this technique, in a water-based base coat paint, the use of a specific monomer for the synthesis of an acrylic emulsion resin improves the familiarity with water, improves the appearance of the coating film, and adjusts the acid value to improve the paint workability. The technique of this Patent Document 3 exhibits excellent performance, and is an automobile coating, particularly a so-called two-coat / one-bake method (a method of baking and curing both coating films simultaneously after forming an aqueous base coating film and a clear coating film). It is enough to use. Recently, however, the above two-coat one-bake process has been advanced one step further, so-called three-coat one-bake coating, in which the intermediate coating film formed before the base coating film is not cured and is baked and cured simultaneously with the base coating film and the clear coating film. A method is being adopted, and the object to be coated is applied not only to a steel plate but also to a material having both a steel plate and a plastic substrate. When the three-coating / one-bake coating method is performed on a base material provided with both such a steel plate and a plastic base material, the technique disclosed in Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-240791) uses all coating methods. It is difficult to make the appearance uniform on the film surface (particularly, the coating surface of both the steel plate and the plastic substrate).

JP-A-63-193968 JP-A 63-265974 JP 2001-240791 A

  The present invention improves the technique of Patent Document 3 above, and a multilayer coating film forming method in which the appearance is uniform even when coated on a substrate having both a steel plate and a plastic substrate by a three-coat / one-bake coating method I will provide a.

That is, in the present invention, an aqueous intermediate coating is formed on a substrate having both a steel plate and a plastic substrate to form an intermediate coating, and the aqueous base coating is applied on the formed intermediate coating. After forming the base coating, an organic solvent-type clear coating is applied to form a clear coating, and the above three layers of intermediate coating, base coating and clear coating are heated and cured. A method for forming a film, comprising:
The water-based paint is in a resin solid content of 100% by mass,
(A) An acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing 0.2 to 20% by mass of a crosslinkable monomer has a solid content of 10 to 60% by mass,
(B) Water-soluble acrylic resin is 5 to 40% by mass in solid content, and (c) Melamine resin is 20 to 40% by mass in solid content,
(D) Provided is a method for forming a multilayer coating film, characterized by containing 10 to 40 parts by mass of propylene glycol monoalkyl ether with respect to 100 parts by mass of a coating resin solid content.

  The propylene glycol monoalkyl ether (d) is preferably propylene glycol monopropyl ether.

  The propylene glycol monoalkyl ether is preferably blended in an amount of 20 to 30 parts by mass with respect to 100 parts by mass of the coating resin solid content.

  The crosslinkable monomer used for the synthesis of the acrylic resin emulsion is preferably contained in an amount of 0.5 to 20% by mass in the monomer mixture for synthesizing the acrylic resin emulsion.

  The monomer mixture for synthesizing the acrylic resin emulsion (a) is preferably a crosslinkable monomer, an α, β-ethylenically unsaturated monomer having an acid group, an α, β-ethylenically unsaturated monomer having a hydroxyl group, and others. Of α, β-ethylenically unsaturated monomers.

  The acrylic resin emulsion (a) preferably has an acid value of 3 to 50 mgKOH / g and a hydroxyl value of 10 to 150 mgKOH / g.

  It is preferable that the water-based paint further contains a glitter pigment.

  It is preferable that the aqueous base paint further contains a polyester resin.

  In the present invention, on a substrate having both a steel plate and a plastic substrate, three layers of a water-based intermediate coating material, a water-based base coating material, and an organic solvent-type clear coating material were applied by wet-on-wet and obtained. When a multilayer coating is formed by the three-coat / one-bake method, in which three-layer coatings are baked and cured simultaneously, by using a predetermined amount of propylene glycol monoalkyl ether as part of the solvent in the aqueous base coating, It was found that the appearance was uniform on both coating surfaces of the substrate.

  Although not bound by a specific theory, when propylene glycol monoalkyl ether is blended, a water-based base paint is spray-painted and the paint is applied to the surface of the object, and usually preheating is performed. It is thought that the increase in viscosity is moderated between application and preheating, and there is a room for aligning the orientation of pigments, particularly glittering pigments, so that the appearance becomes uniform.

In the composite coating film forming method of the present invention, an aqueous intermediate coating is formed on a substrate having both a steel plate and a plastic substrate to form an intermediate coating, and the aqueous coating is formed on the formed intermediate coating. After applying the base paint to form the base paint film, apply an organic solvent type clear paint to form a clear paint film, and heat and cure the above three layers of the intermediate paint film, the base paint film and the clear paint film. A method for forming a multilayer coating film,
The water-based paint is in a resin solid content of 100% by mass,
(A) An acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing 0.2 to 20% by mass of a crosslinkable monomer has a solid content of 10 to 60% by mass,
(B) Water-soluble acrylic resin is 5 to 40% by mass in solid content, and (c) Melamine resin is 20 to 40% by mass in solid content,
(D) The propylene glycol monoalkyl ether is contained in an amount of 10 to 40 parts by mass with respect to 100 parts by mass of the resin solid content.

Aqueous base paint The aqueous base paint used in the method for forming a multilayer coating film of the present invention is a solid (a) acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing 0.2 to 20% by mass of a crosslinkable monomer. 10 to 60% by mass, (b) 5 to 40% by mass of water-soluble acrylic resin, (c) 20 to 40% by mass of melamine resin and (d) propylene glycol monoalkyl ether 10-40 mass parts is contained with respect to 100 mass parts of solid content.

Ingredient (a)
The acrylic resin emulsion of component (a) is a monomer mixture of a crosslinkable monomer and another α, β-ethylenically unsaturated monomer, and the crosslinkable monomer is 0.2 to 20 when the mass of the monomer mixture is 100%. Contains by mass%. Therefore, the other α, β-ethylenically unsaturated monomer is contained in an amount of 80 to 99.8% by mass.

  The acrylic resin emulsion of component (a) is blended in the aqueous base paint in an amount of 10 to 60% by mass, preferably 15 to 50% by mass. If it is less than 10% by mass, good flip-flop properties (FF properties) cannot be obtained. When it exceeds 60 mass%, the viscosity increase at the time of application will become quick, and skin appearance will worsen.

  The monomer mixture forming the acrylic resin emulsion of component (a) contains 0.2 to 20% by mass, preferably 0.5 to 20% by mass of the crosslinkable monomer. When the crosslinkable monomer is less than 0.2% by mass, the solid content at the time of coating is lowered and sagging occurs. Further, the orientation of the glitter pigment is disturbed and the FF property is lowered. If the crosslinkable monomer exceeds 20% by mass, the resin gels, making it difficult to synthesize the emulsion.

  The crosslinkable monomer is a compound having two or more radically polymerizable ethylenically unsaturated groups in the molecule. For example, divinylbenzene, ethylene glycol di (meth) acrylate, hexanediol di (meth) acrylate, polyethylene glycol Di (meth) acrylate, allyl (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexane di (meth) acrylate, neopentyl glycol di (meth) acrylate, pentaerythritol di (meth) acrylate And diaryl compounds such as triallyl cyanurate, pentaerythritol tri (meth) acrylate, trimethylolpropane tri (meth) acrylate, and dipentaerythritol hexa (meth) acrylate. The crosslinkable monomer may be a combination of the above monomers. Preferred crosslinkable monomers are allyl (meth) acrylate, ethylene glycol di (meth) acrylate and divinylbenzene. This is because these crosslinkable monomers are relatively easy to handle. In this specification, “(meth) acryl” means both acrylic and methacrylic.

  The acrylic resin emulsion (a) preferably has an acid value or a hydroxyl value as required. The acid value or hydroxyl value of the acrylic resin is substantially the same as the acid value or hydroxyl value of the monomer mixture in which it is synthesized. The monomer mixture used in the present invention has an acid value of preferably 3 to 50 mgKOH / g, more preferably 7 to 40 mgKOH / g. If the acid value is less than 3 mgKOH / g, the workability cannot be improved, and if it exceeds 50 mgKOH / g, the water resistance of the coating film decreases. On the other hand, when the aqueous base coating material needs to have curability, the monomer mixture needs to have a hydroxyl value of 10 to 150 mgKOH / g, preferably 20 to 100 mgKOH / g. When the hydroxyl value is less than 10 mgKOH / g, sufficient curability cannot be obtained, and when it exceeds 150 mgKOH / g, the water resistance of the coating film is lowered. Moreover, it is preferable from the point of the mechanical physical property of the coating film obtained that the glass transition temperature of the polymer obtained by copolymerizing the said monomer mixture is between -20-80 degreeC.

  The said monomer mixture can have the said acid value and hydroxyl value by including the alpha, beta-ethylenically unsaturated monomer which has an acid group or a hydroxyl group in it.

  Examples of the α, β-ethylenically unsaturated monomer having an acid group include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethylphthalic acid, 2-acryloyloxyethyl succinic acid, ω- Carboxy-polycaprolactone mono (meth) acrylate, isocrotonic acid, α-hydro-ω-((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-acryloyloxyethyl acid phosphate, 2-acrylamido-2-methylpropanesulfonic acid, and the like. Among these, acrylic acid, methacrylic acid, and acrylic acid dimer are preferable.

  On the other hand, as the α, β-ethylenically unsaturated monomer having a hydroxyl group, hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, allyl alcohol, methacryl alcohol, (meth) Mention may be made of adducts of hydroxyethyl acrylate and ε-caprolactone. Among these, preferred are hydroxyethyl (meth) acrylate, hydroxybutyl (meth) acrylate, and an adduct of hydroxyethyl (meth) acrylate and ε-caprolactone.

  Furthermore, the monomer mixture may further contain other α, β-ethylenically unsaturated monomers. Examples of the other α, β-ethylenically unsaturated monomers include (meth) acrylate esters (for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, (meth) acrylic). N-butyl acid, isobutyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth) acrylate, cyclohexyl methacrylate, ( (Meth) acrylic acid t-butylcyclohexyl, (meth) acrylic acid dicyclopentadienyl, (meth) acrylic acid dihydrodicyclopentadienyl, etc.), polymerizable amide compounds (for example, (meth) acrylamide, N-methylol ( (Meth) acrylamide, N-butoxymethyl (meth) acrylic N, N-dimethyl (meth) acrylamide, N, N-dibutyl (meth) acrylamide, N, N-dioctyl (meth) acrylamide, N-monobutyl (meth) acrylamide, N-monooctyl (meth) acrylamide 2, 4-dihydroxy-4′-vinylbenzophenone, N- (2-hydroxyethyl) acrylamide, N- (2-hydroxyethyl) methacrylamide, etc.), polymerizable aromatic compounds (for example, styrene, α-methylstyrene, vinyl ketone, t-butylstyrene, parachlorostyrene, vinylnaphthalene, etc.), polymerizable nitriles (eg, acrylonitrile, methacrylonitrile, etc.), α-olefins (eg, ethylene, propylene, etc.), vinyl esters (eg, vinyl acetate, propionic acid) Vinyl), dienes ( Examples thereof include butadiene and isoprene), polymerizable aromatic compounds, polymerizable nitriles, α-olefins, vinyl esters, and dienes. These can be selected depending on the purpose, but (meth) acrylamide is preferably used when hydrophilicity is easily imparted.

  The acrylic resin emulsion contained in the aqueous base paint of the present invention is obtained by emulsion polymerization of the monomer mixture. The emulsion polymerization carried out here can be carried out using a generally well-known method. Specifically, it can be carried out by dissolving the emulsifier in water or an aqueous medium containing an organic solvent such as alcohol as required, and dropping the monomer mixture and the polymerization initiator under heating and stirring. . A monomer mixture emulsified in advance using an emulsifier and water may be similarly dropped.

  Suitable polymerization initiators include azo oily compounds (for example, azobisisobutyronitrile, 2,2′-azobis (2-methylbutyronitrile) and 2,2′-azobis (2,4 -Dimethylvaleronitrile) and the like, and aqueous compounds (for example, anionic 4,4'-azobis (4-cyanovaleric acid), 2,2-azobis (N- (2-carboxyethyl) -2-methylpropion) Amidine and cationic 2,2′-azobis (2-methylpropionamidine)); and redox oily peroxides (eg, benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide and t-butyl perbenzoate) Etc.), and aqueous peroxides such as potassium persulfate and ammonium persulfate It is below.

  As the emulsifier, those commonly used by those skilled in the art can be used. However, reactive emulsifiers such as Antox MS-60 (manufactured by Nippon Emulsifier Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries Ltd.) Adekaria soap NE-20 (Asahi Denka Co., Ltd.) and Aqualon HS-10 (Daiichi Kogyo Seiyaku Co., Ltd.) are particularly preferred.

  In order to adjust the molecular weight, a mercaptan such as lauryl mercaptan and a chain transfer agent such as α-methylstyrene dimer may be used as necessary.

  The reaction temperature is determined by the initiator, and for example, it is preferably 60 to 90 ° C. for an azo initiator and 30 to 70 ° C. for a redox system. In general, the reaction time is 1 to 8 hours. The amount of the initiator relative to the total amount of the monomer mixture is generally 0.1 to 5% by mass, preferably 0.2 to 2% by mass.

  The emulsion polymerization can be performed in multiple stages, for example, in two stages. That is, first, a part (monomer mixture 1) of the α, β-ethylenically unsaturated monomer mixture is subjected to emulsion polymerization, and the remainder of the α, β-ethylenically unsaturated monomer mixture (monomer mixture 2) is added thereto. In addition, emulsion polymerization is performed.

  The monomer mixture 1 preferably contains an α, β-ethylenically unsaturated monomer having an amide group from the viewpoint of preventing the compatibility with the clear coating film. At this time, it is more preferable that the monomer mixture 2 does not contain an α, β-ethylenically unsaturated monomer having an amide group. Since the mixture of monomer mixtures 1 and 2 is the monomer mixture, the requirement for the α, β-ethylenically unsaturated monomer mixture described above is the combination of monomer mixtures 1 and 2. Will meet.

  The volume average particle size of the acrylic resin emulsion is preferably in the range of 0.01 to 1.0 μm. When the volume average particle diameter is less than 0.01 μm, the effect of improving workability is small, and when it exceeds 1.0 μm, the appearance of the obtained coating film may be deteriorated. The particle size can be adjusted, for example, by adjusting the monomer composition and emulsion polymerization conditions.

  The said acrylic resin emulsion can be used by pH 5-10 by neutralizing with a base as needed. This is because the stability in this pH region is high. This neutralization is preferably carried out by adding a tertiary amine such as dimethylethanolamine or triethylamine to the system before or after emulsion polymerization.

Ingredient (b)
The aqueous base paint of the present invention contains a water-soluble acrylic resin (b). The water-soluble acrylic resin is contained in the aqueous base paint in an amount of 5 to 40% by mass, preferably 10 to 30% by mass. When the amount is less than 5% by mass, the increase in viscosity at the time of coating is increased and the skin appearance is deteriorated. When the amount exceeds 40% by mass, good FF properties are not obtained.

  The water-soluble acrylic resin has a number average molecular weight of 3,000 to 50,000, preferably 6,000 to 30,000. If it is less than 3000, workability and curability are not sufficient, and if it exceeds 50000, the non-volatile content at the time of coating becomes too low, and workability is rather deteriorated.

  The water-soluble acrylic resin preferably has an acid value of 10 to 100 mg KOH / g, more preferably 20 to 80 mg KOH / g. When the upper limit is exceeded, the water resistance of the coating film is lowered, and when the lower limit is exceeded, the water dispersibility of the resin is reduced. Decreases. Further, it preferably has a hydroxyl value of 20 to 180 mgKOH / g, more preferably 30 to 160 mgKOH / g. When the upper limit is exceeded, the water resistance of the coating film decreases, and when the lower limit is exceeded, the curability of the coating film decreases.

  This water-soluble acrylic resin has an α, β-ethylenically unsaturated monomer having an acid group described in the previous monomer mixture as an essential component, and performs solution polymerization together with other α, β-ethylenically unsaturated monomers. Can be obtained.

  The water-soluble acrylic resin is usually neutralized with a basic compound, for example, an organic amine such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine, and dimethylethanolamine, and added to water. Although it dissolves and uses, this neutralization may be performed with respect to water-soluble acrylic resin itself, or may be performed at the time of manufacture of the water-based base coating material mentioned later.

Ingredient (c)
The aqueous base paint of the present invention contains melamine resin (c) as a curing agent. The melamine resin may be water-soluble or water-insoluble. Among the melamine resins, it is preferable in terms of stability to use a water tolerance of 3.0 or more. The water tolerance used here is for evaluating the degree of hydrophilicity, and the higher the value, the higher the hydrophilicity. The water tolerance value was measured by mixing 0.5 g of a sample in 10 ml of acetone and dispersing in a 100 ml beaker at 25 ° C., and gradually adding ion-exchanged water using a burette until the mixture became cloudy. The amount (ml) of ion exchange water required is measured, and this amount (ml) of ion exchange water is used as a water tolerance value.

  The content of the melamine resin is preferably 20 to 40% by mass with respect to the resin solid content in the aqueous base coat composition. If it is less than 20% by mass, the curability is lowered, and if it exceeds 40% by mass, the stability of the paint may be lowered.

Ingredient (d)
In the present invention, propylene glycol monoalkyl ether is blended in the water-based base coating material as a part of the solvent. The propylene glycol monoalkyl ether is blended in an amount of 10 to 40 parts by mass, preferably 10 to 30 parts by mass, with respect to 100 parts by mass of the resin solid content of the paint. When the amount is less than 10 parts by mass, the increase in viscosity at the time of applying the paint is accelerated, and the appearance of the skin of the coating film is deteriorated. When the amount is more than 40 parts by mass, the FF property is lowered.

  As the component (d), a more preferable alkyl group of propylene glycol monoalkyl ether has 2 to 4 carbon atoms, and more preferably 3 carbon atoms. Specific examples include propylene glycol monoethyl ether, propylene glycol monopropyl ether, and propylene glycol monobutyl ether.

Other Components The aqueous base paint of the present invention can contain a pigment. Examples of pigments include glitter pigments and colored pigments. The shape of the glitter pigment is not particularly limited and may be colored. For example, the pigment having an average particle diameter (D ₅₀ ) of 2 to ₅₀ μm and a thickness of 0.1 to 5 μm Is preferred. Those having an average particle diameter in the range of 10 to 35 μm are excellent in glitter and are more preferably used. Specific examples include non-colored or colored metallic brightening agents such as aluminum, copper, zinc, iron, nickel, tin, and aluminum oxide, and alloys, and mixtures thereof. In addition, interference mica pigments, white mica pigments, graphite pigments, and the like are included in this.

  On the other hand, as the colored pigment, for example, organic azo chelate pigment, insoluble azo pigment, condensed azo pigment, phthalocyanine pigment, indigo pigment, perinone pigment, perylene pigment, dioxane pigment, quinacridone pigment, isoindo Examples thereof include linone pigments and metal complex pigments. Examples of inorganic pigments include chrome lead, yellow iron oxide, bengara, carbon black, and titanium dioxide.

  The total pigment concentration (PWC) in the aqueous base paint of the present invention is preferably 0.1 to 50% by mass. More preferably, it is 0.5-40 mass%, Most preferably, it is 1.0-30 mass%. When the upper limit is exceeded, the appearance of the coating film deteriorates. When a bright pigment is included, the pigment concentration (PWC) is generally preferably 18.0% by mass or less. When the upper limit is exceeded, the appearance of the coating film deteriorates. More preferably, it is 0.01-15.0 mass%, Most preferably, it is 0.01-13.0 mass%.

  The aqueous base paint may contain a polyether polyol. By including this polyether polyol, the flip-flop property of the composite coating film obtained from this can be further improved, so that the coating film appearance can be further improved.

  The polyether polyol has at least one primary hydroxyl group in one molecule, has a number average molecular weight of 300 to 3000, a hydroxyl value of 30 to 700 mgKOH / g, and a water tolerance of 2.0 or more. It is preferable to use a certain one. If the above conditions are not satisfied, the water resistance may not be lowered or the target appearance may not be improved.

  As such a polyether polyol, a compound obtained by adding an alkylene oxide such as ethylene oxide or propylene oxide to an active hydrogen-containing compound such as a polyhydric alcohol, a polyhydric phenol, or a polyvalent carboxylic acid can be used. Specific examples include commercial products such as Prime Pole PX-1000, Sanix SP-750 (all of which are manufactured by Sanyo Chemical Industries), and PTMG-650 (manufactured by Mitsubishi Chemical). The polyether polyol is preferably contained in an amount of 1 to 40% by mass, more preferably 3 to 30% by mass in the solid content of the coating resin.

  Furthermore, other viscosity control agents can be added to the water-based base paint of the present invention in order to prevent familiarity with the top coat film and to ensure coating workability. As the viscosity control agent, those generally showing thixotropy can be used, for example, polyamide-based ones such as crosslinked or non-crosslinked resin particles, swelling dispersions of fatty acid amides, amide fatty acids, and long-chain polyaminoamide phosphates. Viscosity develops depending on the shape of polyethylene, such as colloidal swelling dispersion of polyethylene oxide, organic bentonite such as organic acid smectite clay, montmorillonite, inorganic pigments such as aluminum silicate and barium sulfate, etc. A flat pigment etc. can be mentioned.

  In the aqueous metallic base paint used in the present invention, in addition to the above-mentioned components, additives usually added to the paint, such as surface conditioners, thickeners, antioxidants, UV inhibitors, antifoaming agents, etc. You may mix | blend. These compounding amounts are within the range known to those skilled in the art.

  In addition to the melamine resin curing agent of component (c), the aqueous base paint may contain other curing agents. As such a hardening | curing agent, what is generally used for the coating material can be used. Examples of such compounds include blocked isocyanates, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds, and metal ions. A blocked isocyanate resin is generally used from the viewpoint of various performances and costs of the obtained coating film.

  The above-mentioned blocked isocyanate can be obtained by adding a blocking agent having active hydrogen to a polyisocyanate such as trimethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, isophorone diisocyanate, etc. Examples include those that dissociate and generate isocyanate groups.

  If necessary, the water-based base coating material of the present invention may further contain a polyester resin. The polyester resin is obtained by condensation polymerization of an acid component and an alcohol component. The acid component is not particularly limited, and examples thereof include polyvalent carboxylic acid compounds such as adipic acid, sebacic acid, isophthalic acid, and phthalic anhydride, and anhydrides thereof. Furthermore, as an acid component, a compound having a carboxylic acid group and a hydroxyl group in one molecule such as dimethylolpropionic acid can be used. Moreover, it does not specifically limit as said alcohol component, Polyhydric alcohol compounds, such as ethylene glycol, a trimethylol propane, neopentyl glycol, can be mentioned. The polyester resin preferably has a resin solid content acid value of 10 to 80 mgKOH / g and a number average molecular weight of 1000 to 15000. The polyester resin is added to suppress color unevenness of the obtained coating film. The compounding quantity in the coating material of a polyester resin is 5-30 mass%, Preferably it is 10-20 mass%. If it is less than 5% by mass, the effect of suppressing color unevenness is small, and if it is more than 30% by mass, the flip-flop property may be lowered.

  The method for producing the paint used in the present invention is not particularly limited, including those described later, and all methods well known to those skilled in the art such as kneading and dispersing a compound such as a pigment using a kneader or a roll. Can be used.

Water-based intermediate coating The water-based intermediate coating used in the method of the present invention conceals the base, ensures a surface skin appearance after top coating (improves appearance), and imparts coating properties such as impact resistance. To be applied. The aqueous intermediate coating material contains a resin, a curing agent, and the like that are acrylic resin emulsions in a state of being dispersed or dissolved in an aqueous medium.

  The acrylic resin emulsion functions as a resin in the water-based intermediate coating material, and is an (meth) acrylic acid alkyl ester (i), an α, β-ethylenically unsaturated monomer (ii) having an acid group, and an α, β- having a hydroxyl group. It can be obtained by emulsion polymerization of a monomer mixture containing an ethylenically unsaturated monomer (iii). Specific examples of these monomers include cross-linkable monomers exemplified in the component (a) of the above-mentioned aqueous base paint, α, β-ethylenically unsaturated monomers having acid groups, and α, β-ethylene having hydroxyl groups. And unsaturated α-, β-ethylenically unsaturated monomers. In addition, you may use these monomers as a component of a monomer mixture, combining 1 type (s) or 2 or more types as appropriate. Specific examples of the emulsion polymerization and the polymerization initiator and emulsifier used in the emulsion polymerization include those exemplified for the component (a) of the aqueous base paint described above.

  The glass transition temperature (Tg) of the acrylic resin is −50 ° C. to 20 ° C., preferably −40 ° C. to 10 ° C., more preferably −30 ° C. to 0 ° C. By setting the Tg of the resin in this range, when an aqueous intermediate coating containing an acrylic resin emulsion is used in a wet-on-wet system, the affinity and adhesion with the undercoat and the topcoat are improved, and the wet state Familiarity at the interface with the upper coating is good and no reversal occurs. Moreover, the moderate softness | flexibility of the coating film finally obtained is acquired, and chipping resistance is improved. As a result, a multilayer coating film having a very high appearance can be formed. If the Tg of the resin is less than −50 ° C., the mechanical strength of the coating film is insufficient and the chipping resistance is weak. On the other hand, if the Tg of the resin exceeds 20 ° C., the coating film becomes hard and brittle, so that it lacks impact resistance and weakens chipping resistance. The type and amount of each monomer component are selected so that the Tg of the resin falls within the above range.

  The acid value of the acrylic resin is 2 to 60 mgKOH / g, preferably 5 to 50 mgKOH / g. By setting the acid value of the resin within this range, various stability such as storage stability, mechanical stability and stability against freezing of the resin emulsion and the water-based intermediate coating material using the resin emulsion are improved. A curing reaction with a curing agent such as a melamine resin at the time of formation sufficiently occurs, and various strengths, chipping resistance, and water resistance of the coating film are improved. When the acid value of the resin is less than 2 mg KOH / g, the above-mentioned various stability is inferior, and the curing reaction with a curing agent such as a melamine resin is not sufficiently performed, so that various strengths, chipping resistance and water resistance of the coating film are inferior. On the other hand, when the acid value of the resin exceeds 60 mgKOH / g, the polymerization stability of the resin is deteriorated, the above-mentioned various stability is deteriorated, or the water resistance of the obtained coating film is inferior. The type and amount of each monomer component are selected so that the acid value of the resin falls within the above range. As described above, it is important to use a carboxyl group-containing monomer among the acid group-containing ethylenically unsaturated monomers (b), and among the monomers (b), the carboxyl group-containing monomer is preferably 50% by mass or more, More preferably, it is contained by 80% by mass or more.

  The hydroxyl value of the acrylic resin is 10 to 120 mgKOH / g, preferably 20 to 100 mgKOH / g. By setting the hydroxyl value of the resin within this range, the resin has moderate hydrophilicity, and when used as a coating composition containing a resin emulsion, the workability and stability against freezing are increased, and the melamine resin or isocyanate type is used. The curing reactivity with the curing agent is also sufficient. When the hydroxyl value is less than 10 mg KOH / g, the curing reaction with the curing agent is insufficient, the mechanical properties of the coating film are weak, chipping resistance is poor, and water resistance and solvent resistance are also poor. On the other hand, when the hydroxyl value exceeds 120 mgKOH / g, the water resistance of the obtained coating film is decreased, the compatibility with the curing agent is poor, the coating film is distorted, and the curing reaction is uneven. As a result, the various strengths of the coating film, particularly chipping resistance, solvent resistance and water resistance are inferior. The type and amount of each monomer component are selected so that the hydroxyl value of the resin falls within the above range.

  A basic compound is added to the obtained acrylic resin emulsion in order to neutralize part or all of the carboxylic acid and maintain the stability of the acrylic resin emulsion. As these basic compounds, ammonia, various amines, alkali metals and the like are usually used, and they are also used appropriately in the present invention.

  The curing agent is not particularly limited as long as it generates a curing reaction with an acrylic resin or urethane resin contained as an emulsion and can be blended in an aqueous intermediate coating, for example, a melamine resin, an isocyanate resin, an oxazoline compound. And carbodiimide compounds. These 1 type (s) or 2 or more types are used in combination as appropriate. It is preferable to contain a melamine resin or a carbodiimide compound in terms of coating film performance.

  It does not specifically limit as a melamine resin, What is normally used as a hardening | curing agent can be used. However, hydrophobic melamine resins are preferred. This is because the appearance of the obtained multilayer coating film is improved.

  As the melamine resin, those usually used as a curing agent can be used. For example, an alkyl etherified melamine resin obtained by alkyl etherification is preferable, and a melamine resin substituted with a methoxy group and / or a butoxy group is more preferable. As such melamine resins, those having a methoxy group alone, Cymel 325, Cymel 327, Cymel 370, Mycoat 723; those having both a methoxy group and a butoxy group, Cymel 202, Cymel 204, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 254, Cymel 266, Cymel 267 (all trade names, manufactured by Mitsui Cytec); My Coat 506 (trade name, Mitsui Cytech, Inc.) having a butoxy group alone Product), Uban 20N60, Uban 20SE (both trade names, manufactured by Mitsui Chemicals) and the like. These may be used alone or in combination of two or more. Of these, Cymel 325, Cymel 327, and My Coat 723 are more preferable.

  The isocyanate resin is obtained by blocking a diisocyanate compound with an appropriate blocking agent. The diisocyanate compound is not particularly limited as long as it has two or more isocyanate groups in one molecule. For example, aliphatic diisocyanates such as hexamethylene diisocyanate (HMDI) and trimethylhexamethylene diisocyanate (TMDI); isophorone Alicyclic diisocyanates such as diisocyanate (IPDI); aromatic-aliphatic diisocyanates such as xylylene diisocyanate (XDI); aromatic diisocyanates such as tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI) Hydrogenated diisocyanates such as dimer acid diisocyanate (DDI), hydrogenated TDI (HTDI), hydrogenated XDI (H6XDI), hydrogenated MDI (H12MDI), Adducts of beauty more diisocyanates and isocyanurates thereof, and the like. Furthermore, these 1 type (s) or 2 or more types can be used in combination as appropriate.

  The blocking agent for blocking the diisocyanate compound is not particularly limited. For example, oximes such as methyl ethyl ketoxime, acetoxime, and cyclohexanone oxime; phenols such as m-cresol and xylenol; butanol, 2-ethylhexanol, cyclohexanol, ethylene Alcohols such as glycol monoethyl ether; lactams such as ε-caprolactam; diketones such as diethyl malonate and acetoacetate; mercaptans such as thiophenol; ureas such as thiouric acid; imidazoles; Can be mentioned. Of these, oximes, phenols, alcohols, lactams and diketones are preferred.

  The oxazoline-based compound is preferably a compound having two or more 2-oxazoline groups, and examples thereof include the following oxazolines and oxazoline group-containing polymers. These 1 type (s) or 2 or more types can be used in combination. The oxazoline compound is obtained by using a method of dehydrating cyclization by heating amide alcohol in the presence of a catalyst, a method of synthesizing from alkanolamine and nitrile, or a method of synthesizing from alkanolamine and carboxylic acid. .

  Examples of the oxazolines include 2,2′-bis- (2-oxazoline), 2,2′-methylene-bis- (2-oxazoline), 2,2′-ethylene-bis- (2-oxazoline), 2,2'-trimethylene-bis- (2-oxazoline), 2,2'-tetramethylene-bis- (2-oxazoline), 2,2'-hexamethylene-bis- (2-oxazoline) and the like. . These 1 type (s) or 2 or more types can be used in combination as appropriate.

  The oxazoline group-containing polymer is obtained by polymerizing an addition polymerizable oxazoline and at least one other polymerizable monomer as required. Examples of the addition polymerizable oxazoline include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline and the like. Can be mentioned. These 1 type (s) or 2 or more types are used in combination as appropriate. Of these, 2-isopropenyl-2-oxazoline is preferred because it is easily available industrially.

  The amount of addition polymerizable oxazoline used is not particularly limited, but is preferably 1% by mass or more in the oxazoline group-containing polymer. If the amount is less than 1% by mass, the degree of curing tends to be insufficient, and durability, water resistance and the like tend to be impaired.

  The other polymerizable monomer is not particularly limited as long as it is a monomer that can be copolymerized with an addition-polymerizable oxazoline and does not react with the oxazoline group. For example, methyl (meth) acrylate, (meth) (Meth) acrylic acid esters such as butyl acrylate and 2-ethylhexyl (meth) acrylate; Unsaturated nitriles such as (meth) acrylonitrile; Unsaturated amides such as (meth) acrylamide and N-methylol (meth) acrylamide Vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogenated α and β such as vinyl chloride, vinylidene chloride and vinyl fluoride -Unsaturated monomers: α, β-unsaturated aromatics such as styrene and α-methylstyrene Mer, etc. can be mentioned. These 1 type (s) or 2 or more types can be used in combination as appropriate.

  The oxazoline group-containing polymer can be produced by adding an addition-polymerizable oxazoline and, if necessary, at least one other polymerizable monomer by a conventionally known polymerization method such as suspension polymerization, solution polymerization, emulsion polymerization and the like. Examples of the supply form of the oxazoline group-containing compound include organic solvent solutions, aqueous solutions, non-aqueous dispersions, and emulsions, but are not particularly limited to these forms.

  As the carbodiimide-based compound, those produced by various methods can be used. Basically, those obtained by synthesizing isocyanate-terminated polycarbodiimide by a condensation reaction involving decarbonization of organic diisocyanate are listed. be able to. More specifically, in the production of a polycarbodiimide-based compound, (a) a polycarbodiimide compound containing at least two isocyanate groups in one molecule, and (b) a polyol having a hydroxyl group at the molecular end are combined with the above (a ) A step in which the molar amount of the isocyanate group of the polycarbodiimide compound is reacted in a ratio exceeding the molar amount of the hydroxyl group of the polyol (b), and the reaction product obtained in the above step has a hydrophilic property having active hydrogen and a hydrophilic portion. A hydrophilized modified carbodiimide compound obtained by the step of reacting the agent (c) can be mentioned as a preferable one.

  Although it does not specifically limit as a carbodiimide compound (a) which contains at least 2 isocyanate group in 1 molecule, From a reactive viewpoint, it is preferable that it is a carbodiimide compound which has an isocyanate group in both ends. A method for producing a carbodiimide compound having isocyanate groups at both ends is well known to those skilled in the art. For example, a condensation reaction involving decarbonization of an organic diisocyanate can be used.

  The hydrophilized modified carbodiimide compound contained in the water-based intermediate coating used in the present invention has a carbodiimide unit and a polyol unit alternately and continuously present via urethane bonds. A monoalkoxypolyalkylene oxide unit is bonded to a carbodiimide unit via

The carbodiimide unit is obtained by removing an isocyanate group from a polycarbodiimide compound (a) containing at least two isocyanate groups in one molecule, and — (— N═C═N—R ₇ —) _n — ( R ₇ is a hydrocarbon group which may be saturated or unsaturated, and may contain a nitrogen atom and / or an oxygen atom, n is the degree of polymerization, and is represented by a natural number of 2 to 20) Unit.

  The polycarbodiimide compound (a) is the same as that described in the method for producing the hydrophilic modified carbodiimide compound.

  The polyol unit refers to a unit obtained by removing active hydrogen from a polyol (b) containing at least two hydroxyl groups in one molecule.

  The polyol (b) is the same as described in the method for producing the hydrophilic modified carbodiimide compound.

  The carbodiimide unit and the polyol unit in the hydrophilized modified carbodiimide compound are alternately and continuously present via a urethane bond represented by -NHCO-. The number of repetitions is not particularly limited, but is preferably 1 to 10 from the viewpoint of reaction efficiency.

  The hydrophilized modified carbodiimide compound of the present invention is a monoalkoxypolyalkylene oxide unit at both molecular ends, and the monoalkoxypolyalkylene oxide unit is bonded to the carbodiimide unit via the urethane bond.

The monoalkoxypolyalkylene oxide unit is one obtained by removing active hydrogen from the monoalkoxypolyalkylene glycol, and R ₅ —O— (CH ₂ —CHR ₆ —O—) _m — (wherein R ₅ , R ₆ , and m are the same as above.

  The monoalkoxypolyalkylene glycol is specifically the same as that described in the method for producing the hydrophilic modified carbodiimide compound.

  The aqueous intermediate coating material used in the present invention can further contain the following components. For example, additional resin components, pigment dispersion pastes, thickeners, other additive components, and the like.

  Although it does not specifically limit as said additional resin component, For example, a urethane resin emulsion, a polyester resin, an acrylic resin, a carbonate resin, an epoxy resin etc. can be mentioned.

  Although it will not specifically limit if it is a pigment used for a normal water-based paint, A colored pigment is preferable from the point which improves a weather resistance and ensures concealment property. In particular, titanium dioxide is more preferable because it is excellent in color concealment and is inexpensive.

  Examples of pigments other than titanium dioxide include azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, and isoindolinone. Organic pigments such as pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments and metal complex pigments; inorganic pigments such as yellow lead, yellow iron oxide, Bengala and carbon black. These pigments may be used in combination with extender pigments such as calcium carbonate, barium sulfate, clay and talc.

  As the pigment, a standard gray paint having carbon black and titanium dioxide as main pigments can also be used. In addition, it is also possible to use a paint in which the top coat paint is combined with lightness or hue or a combination of various color pigments.

  Although it does not specifically limit as a thickener, Cellulose type | system | groups, such as viscose, methylcellulose, ethylcellulose, hydroxyethylcellulose, and what is marketed, such as Tyroze MH and Tyroze H (all are the Hoechst company make, brand name), etc. Things; Sodium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, and commercially available products (all are trade names below) include Primal ASE-60, Primal TT-615, Primal RM-5 (all manufactured by Rohm & Haas) ), Alkali thickening type such as Euker Polyphobe (manufactured by Union Carbide); polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter referred to as trade names) include Adecanol UH-420 and Adecanol UH- 4 2, Adecanol UH-472, UH-540, Adecanol UH-814N (Asahi Denka Kogyo Co., Ltd.), Primal RH-1020 (Rohm & Haas), Kuraray Poval (Kuraray Co., Ltd.) be able to. These may be used alone or in combination of two or more.

  By containing the thickener, the viscosity of the aqueous intermediate coating can be increased, and the occurrence of dripping can be suppressed when the aqueous intermediate coating is applied. Moreover, the mixed layer between an intermediate coating film and a base coating film can be suppressed more. As a result, compared to the case where no thickener is included, the coating workability during coating is improved, and the finished appearance of the resulting coating film can be made excellent.

  Examples of other additives include additives usually added in addition to the above-mentioned components, such as ultraviolet absorbers; antioxidants; antifoaming agents; surface conditioners; pinhole inhibitors. These compounding amounts are within the range known to those skilled in the art.

  The aqueous intermediate coating used in the present invention is prepared by mixing the above-mentioned acrylic resin emulsion, a curing agent, and other components as required.

  The curing agent has a lower limit of 2% by mass and an upper limit of 50% by mass, preferably a lower limit of 4% by mass and an upper limit of 40% by mass, more preferably a lower limit of 5% by mass and an upper limit of 30% by mass with respect to the solid content of the curing agent and the acrylic resin emulsion. Use to be. When it is less than 2% by mass, the water resistance of the resulting coating film tends to decrease. Moreover, when it exceeds 50 mass%, there exists a tendency for the peeling property of the coating film obtained to fall.

  Appropriate amounts of the additional resin component, pigment dispersion paste and other additives may be mixed. However, the additional resin component is preferably blended at a ratio of 50% by mass or less based on the solid content of all the resins contained in the aqueous intermediate coating composition. When it exceeds 50% by mass, it is difficult to increase the solid content concentration in the paint, which is not preferable.

  The pigment preferably has a pigment concentration (PWC; pigment weight content) of 10 to 60% by mass with respect to the solid content of all the resins contained in the aqueous intermediate coating and the total mass of the pigment. If it is less than 10% by mass, the concealability may be lowered. When it exceeds 60 mass%, viscosity increase at the time of hardening will be caused, flow property may fall and a coating-film external appearance may fall.

  The content of the thickener is 0.01 parts by mass at the lower limit and 20 parts by mass at the upper limit with respect to 100 parts by mass of the resin solid content (solid content of all resins contained in the water-based intermediate paint) of the aqueous intermediate coating. It is preferable that the lower limit is 0.1 parts by mass, and the upper limit is 10 parts by mass. If it is less than 0.01 parts by mass, the thickening effect cannot be obtained, and sagging may occur during coating. If it exceeds 20 parts by mass, the appearance and various performances of the resulting coating film may be reduced. is there.

  The order of adding these components may be before or after adding the curing agent to the emulsion. The form of the aqueous intermediate coating is not particularly limited as long as it is aqueous, and may be in the form of, for example, water-soluble, water-dispersed, or aqueous emulsion.

Organic solvent type clear paint The organic solvent type clear paint is not particularly limited, and a clear paint containing a film-forming resin and a curing agent can be used. Furthermore, a color pigment can also be contained as long as it does not interfere with the design of the base.

  Preferred examples of the organic solvent-type clear coating include a combination of an acrylic resin and / or a polyester resin with an amino resin and / or an isocyanate, or a carboxylic acid / epoxy curing system in terms of transparency or acid etching resistance. And acrylic resin and / or polyester resin system.

  Furthermore, it is preferable that a viscosity control agent is added to the organic solvent-type clear coating material in order to ensure coating workability. As the viscosity control agent, those generally showing thixotropy can be used. As such a thing, what was described in the above-mentioned aqueous | water-based basecoat composition can be used, for example. Moreover, a curing catalyst, a surface conditioner, etc. can be included if necessary.

The coating film forming method of the present invention is used for a substrate having both a steel plate and a plastic. Examples of the steel sheet include iron, copper, aluminum, tin, zinc, and the like and alloys containing these metals. These steel plates are particularly preferably those subjected to chemical conversion treatment with phosphate, chromate or the like in advance.

  An electrodeposition coating film may be formed on the chemical conversion treated steel sheet. As this electrodeposition coating, a cation type and an anion type can be used, but the cationic type electrodeposition coating composition is excellent in corrosion resistance. This is preferable because it gives a multilayer coating film.

  Examples of the plastic include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, and polyamide resin. Specific examples include automobile parts such as spoilers, bumpers, mirror covers, grills, and door knobs. These plastic products are preferably those washed with steam or neutral detergent with trichloroethane, and may further be provided with a primer coating to enable electrostatic coating.

Multi-layer coating film forming method In the multi-layer coating film forming method of the present invention, an aqueous intermediate coating is applied on an object to be coated having both a steel plate and a plastic (an electrodeposition coating film may be formed on the steel plate). An intermediate coating film using a paint, a base coating film using an aqueous base coating, and a clear coating film using an organic solvent-type clear coating can be sequentially formed.

  In the present invention, when the water-based intermediate coating and the water-based base coating are applied to the automobile body, in order to enhance the appearance, the multi-stage coating by air electrostatic spray coating, preferably two-stage coating, or air Coating film by a combination of electrostatic spray coating and a rotary atomizing electrostatic coating machine called “μμ (micro micro) bell”, “μ (micro) bell” or “metabell” The method of forming can be used.

  In the present invention, the thickness of the coating film applied with the aqueous intermediate coating varies depending on the desired application, but in many cases, 15 to 30 μm is useful. If the upper limit is exceeded, sagging or peeling occurs during painting, and if the lower limit is not reached, the skin appearance may deteriorate.

  In the present invention, the film thickness of the coating film when applied with a water-based base paint varies depending on the desired application, but in many cases, 10 to 30 μm is useful. If the upper limit is exceeded, sharpness may deteriorate, or defects such as unevenness or flow may occur during coating. If the lower limit is exceeded, the substrate cannot be concealed and film breakage occurs.

  In the multilayer coating film forming method of the present invention, a clear coating is further applied on the uncured base coating to form a clear coating (wet-on-wet coating), omitting the baking and drying oven. It is preferable from the viewpoint of economy and environment. In order to obtain a good finished coating film, it is desirable to heat the uncured base coating film at 40 to 100 ° C. for 2 to 10 minutes before applying the clear coating.

  In the coating film forming method of the present invention, the clear coating film that is applied after forming the base coating film is formed to smooth and protect the unevenness, flickering, and the like caused by the base coating film. Specifically, as a coating method, it is preferable to form the coating film by a rotary atomizing electrostatic coating machine such as the μμ bell or μbell described above.

  The dry film thickness of the clear coating film formed from the clear coating is generally preferably about 10 to 80 μm, more preferably about 20 to 60 μm. If the upper limit is exceeded, workability problems such as peeling or sagging may occur during painting, and if the lower limit is not reached, the underlying irregularities cannot be concealed.

  The clear coating film obtained as described above can obtain a cured coating film having a high degree of crosslinking by setting the baking temperature to 80 to 180 ° C, preferably 120 to 160 ° C. If the upper limit is exceeded, the coating film becomes hard and brittle, and if it is less than the lower limit, curing is not sufficient. The curing time varies depending on the curing temperature, but 10 to 30 minutes at 120 to 160 ° C. is appropriate.

  In many cases, the film thickness of the multilayer coating film formed in the present invention is 30 to 300 μm, preferably 50 to 250 μm. When the upper limit is exceeded, film physical properties such as a cooling cycle decrease, and when the lower limit is exceeded, the strength of the film itself decreases.

  EXAMPLES Hereinafter, although a specific Example is given and this invention is demonstrated in detail, this invention is not limited by a following example. Hereinafter, “part” means “part by mass”.

Production Example 1 Production of acrylic resin emulsion 1 126.5 parts of deionized water was added to a reaction vessel, and the mixture was heated to 80 ° C. while being mixed and stirred in a nitrogen stream. Next, 27.61 parts of methyl acrylate, 53.04 parts of ethyl acrylate, 4.00 parts of styrene, 9.28 parts of 2-hydroxyethyl methacrylate, 3.07 parts of methacrylic acid and 3.00 parts of allyl methacrylate Monomer mixture 100 parts, Aqualon HS-10 (polyoxyethylene alkyl propenyl phenyl ether sulfate ester, Daiichi Kogyo Seiyaku Co., Ltd.) 0.7 parts, Adeka Soap NE-20 (α- [1-[(allyloxy) methyl]) 2- (nonylphenoxy) ethyl] -ω-hydroxyoxyethylene (manufactured by Asahi Denka Co., Ltd.), 0.5 part, and a monomer emulsion comprising 80 parts of deionized water, 0.3 part of ammonium persulfate, and deionized water An initiator solution consisting of 10 parts was dropped into the reaction vessel in parallel over 2 hours. After completion of the dropping, aging was performed at the same temperature for 2 hours. Next, after cooling to 40 ° C. and filtration through a 400 mesh filter, 70 parts of deionized water and 0.32 part of dimethylaminoethanol were added to adjust the pH to 6.5, the average particle size was 150 nm, the non-volatile content was 25%, the solid content was A single-layer acrylic resin emulsion having an acid value of 20 mgKOH / g and a hydroxyl value of 40 mgKOH / g was obtained.

Production Example 2 Production of water-soluble acrylic resin 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to a reaction vessel, and the mixture was heated to 105 ° C. with mixing and stirring in a nitrogen stream. Next, an initiator solution consisting of 100 parts of the following monomer mixture, 10.0 parts of tripropylene glycol methyl ether and 1 part of tertiary butyl peroxy 2-ethylhexanoate was dropped into the reaction vessel in parallel over 3 hours. After completion of the dropping, aging was performed at the same temperature for 0.5 hours.

  Further, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 part of tertiary butyl peroxy 2-ethylhexanoate was dropped into the reaction vessel over 0.5 hours. After completion of the dropping, aging was performed at the same temperature for 2 hours.

  After removing 16.1 parts of the solvent at 110 ° C. under reduced pressure (70 torr) using a solvent removal apparatus, 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added to obtain a water-soluble acrylic resin solution. The obtained water-soluble acrylic resin solution had a nonvolatile content of 30%, a solid content acid value of 40 mg KOH / g, a hydroxyl value of 50 mg KOH / g, and a viscosity of 140 poise (E-type viscometer 1 rpm / 25 ° C.).

  Monomer mixture Methyl methacrylate 13.1 parts Ethyl acrylate 68.4 parts 2-Hydroxyethyl methacrylate 11.6 parts Methacrylic acid 6.9 parts

Production Example 3 Production of Polyester Resin In a reaction vessel equipped with a stirrer, condenser and thermometer, 372 parts of dimethyl terephthalic acid, 380 parts of dimethyl isophthalic acid, 576 parts of 2-methyl-1,3-propanediol, 1,5-pentanediol 222 parts and 0.41 part of tetrabutyl titanate were charged, and a transesterification reaction was carried out over 4 hours while heating from 160 ° C to 230 ° C. Next, the pressure inside the system was gradually reduced, the pressure was reduced to 5 mmHg over 20 minutes, and the pressure was further reduced to a vacuum of 0.3 mmHg or less, and a polycondensation reaction was performed at 260 ° C. for 40 minutes. The mixture was cooled to 220 ° C. in a nitrogen atmosphere, 23 parts of trimellitic anhydride was added, and a reaction was performed at 220 ° C. for 30 minutes to obtain a polyester resin. As a result of NMR compositional analysis, the obtained polyester resin had a carboxylic acid component in a molar ratio of terephthalic acid / isophthalic acid / trimellitic acid: 48/49/3, and a polyol component in a molar ratio of 2-methyl-1 , 3-propanediol / 1,5-pentanediol: 65/35. That is, when the total amount of each of the polycarboxylic acid component and the polyol component is 100 mol%, the aromatic dicarboxylic acid is 97 mol%, the isophthalic acid is 49 mol%, the total amount of the predetermined diol is 65 mol%, ethylene glycol Was 0 mol%.

About the obtained resin, the characteristic value was measured as follows.
(1) Number average molecular weight: It was 12000 when measured by gel permeation chromatography (GPC) using polystyrene standard sample standards.
(2) Acid value: 0.2 g of a sample was precisely weighed and dissolved in 20 ml of chloroform, and then titrated with 0.01 N potassium hydroxide (ethanol solution) to find 16.1 mg KOH / g.

  To 100 parts of this polyester resin, 40 parts of butyl cellosolve and 2.7 parts of triethylamine were added, followed by stirring at 80 ° C. for 1 hour for dissolution. Next, 193 parts of ion-exchanged water was gently added to obtain a polyester aqueous dispersion 1 containing a polyester resin having a nonvolatile content of 30%. In order to measure the average particle size, only ion-exchanged water is put into a dedicated cell, and only 1 drop of this dispersion is mixed, and the resin solid content concentration is adjusted to 0.1% by mass to measure the dynamic light scattering particle size. It was 35 nm when measured at 20 degreeC with apparatus LB-500 (made by Horiba, Ltd.).

Production Example 4 Synthesis of phosphoric acid group-containing acrylic resin 40 parts of ethoxypropanol was charged into a 1 liter reaction vessel equipped with a stirrer, a temperature controller, and a condenser, and 4 parts of styrene, 35.96 parts of n-butyl acrylate, Ethylhexyl methacrylate 18.45 parts, 2-hydroxyethyl methacrylate 13.92 parts, methacrylic acid 7.67 parts, ethoxypropanol 20 parts, Phosmer PP (Acid phosphooxyhexa (oxypropylene) monomethacrylate) 20 manufactured by Unichemical Co., Ltd. 40 parts of the dissolved solution and 121.7 parts of a monomer solution consisting of 1.7 parts of azobisisobutyronitrile were added dropwise at 120 ° C. for 3 hours, and stirring was further continued for 1 hour. The obtained resin was an acrylic acid varnish having an acid value of 105 mgKOH / g, of which an acid value of 55 mgKOH / g based on a phosphoric acid group, a hydroxyl value of 60 mgKOH / g and a number average molecular weight of 6000, and a nonvolatile content of 63%.

Example 1
<Manufacture of water-based metallic base paint>
160 parts of the acrylic resin emulsion obtained in the previous Production Example 1 (solid content 25%), 10 parts by mass of 10% by weight dimethylaminoethanol, 33 parts of the water-soluble acrylic resin of Production Example 2 (solid resin content 30%), production 33 parts of the polyester resin of Example 3 (resin solid content 30%), 38 parts of Cymel 204 (mixed alkylated melamine resin, solid content 80%, water tolerance 3.6 ml, manufactured by Mitsui Cytec Co., Ltd.) as a melamine resin, propylene glycol mono 15 parts of propyl ether (15 parts by mass with respect to 100 parts by mass of resin solid content of paint), 10 parts of Prime Pole PX-1000 (bifunctional polyether polyol manufactured by Sanyo Chemical Industries), Alpaste MH8801 (Asahi Kasei) as a bright pigment 21 parts (solid content 65%, PWC 12%), phosphoric acid group-containing acrylic 5 parts of fat and 0.3 part of lauryl acid phosphate are added, and further 30 parts of 2-ethylhexanol and 3.3 parts of Adecanol UH-814N (Adeka thickener, solid content 30%) are uniformly dispersed. Thus, a water-based metallic base paint was obtained.

<Composite coating film formation method (two-coat / one-bake)>
Cathodic electrodeposition paint "Power Top U-50" (manufactured by Nippon Paint Co., Ltd.) is electrodeposited onto a dull steel sheet with a thickness of 0.8mm, length 30cm, width 40cm treated with zinc phosphate so that the dry film thickness is 20 µm. Painted and baked at 160 ° C. for 30 minutes, pre-diluted gray intermediate coating “Olga OP-30” (Nippon Paint Co., Ltd.) in 25 seconds (measured at 20 ° C. using a No. 4 Ford cup) Polyester / melamine-based paint) was applied in two stages by air spraying to a dry film thickness of 35 μm, and baked at 140 ° C. for 30 minutes.

  After cooling, the aqueous metallic base paint of the previous production example was diluted to 6000 mPa · s (measured at 20 ° C. under the condition of 6 rpm using a B-type viscometer) using deionized water. Two-stage coating was performed with a “cartridge bell” so that the dry film thickness was 15 μm under conditions of room temperature of 23 ° C. and humidity of 68%. An interval setting of 1 minute 30 seconds was performed between the two coatings. After the second application, setting was performed with an interval of 1 minute and 30 seconds. Thereafter, preheating was performed at 80 ° C. for 5 minutes.

  After preheating, the coated plate was allowed to cool to room temperature, and -O-1810 (solvent type clear paint manufactured by Nippon Paint Co., Ltd.) was applied as a clear coating for one stage to a dry film thickness of 35 μm and set for 7 minutes. Next, the coated film was baked at 140 ° C. for 30 minutes with a dryer to obtain a composite coating film.

About the obtained coating film, FF property and the coating-film external appearance (skin external appearance) were evaluated. The evaluation method was as follows. The results are shown in Table 1.
<FF property evaluation>
Using a Minolta CM-512m3 commercially available from Minolta Co., Ltd., measurement is performed by irradiating a light source from angles of 25 °, 45 °, and 75 ° when the light receiving portion perpendicular to the coating film is 0 °. The L * value was measured, and the flip-flop value was calculated from the measured value.
FF value (flip-flop value) = expressed as a ratio of 25 ° L * value / 75 ° L * value. The FF property is 2.15 or more (L * value of 25 degrees is 98 or more).

<Skin appearance>
Appearance evaluation of laminated coating film About the surface of the obtained laminated coating film, the finished external appearance was evaluated by measuring W1 value and W4 value using "Wavescan-T" manufactured by BYK-Gardner GmbH. Here, the W1 value is an index for evaluating the skin swell (round) of the coating film, and the lower this value, the better the finished appearance. The W4 value is an index for evaluating the gloss and fine skin of the coating film, and the lower this value, the better the finished appearance.
The skin appearance pass is W1 <15, W2 <10, W3 <20 and W4 <15.

<Composite coating method (Three coats / one bake)>
Example of water-based intermediate coating (preparation of pigment dispersion paste)
72.1 parts of organic substance-treated titanium dioxide pigment (Ishihara Sangyo Co., Ltd. “Typaque CR-90-2”), commercially available dispersant (Big Chemie “Disperbyk 190”) 4.5 parts, and deionized water 23.4 parts are premixed. Thereafter, a glass bead medium was added in a paint conditioner and mixed and dispersed at room temperature until the particle size became 5 μm or less to obtain a pigment dispersion paste.

(Preparation of acrylic resin emulsion)
35.75 parts of deionized water was added to the reaction vessel, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then, as an α, β-ethylenically unsaturated monomer mixture, 10 parts of styrene, 24.02 parts of methyl methacrylate, 28.94 parts of butyl acrylate, 20.11 parts of ethyl acrylate, 4-hydroxybutyl acrylate-15. 40 parts, 1.53 parts of methacrylic acid, 3.0 parts of polyoxyethylene alkylpropenyl phenyl ether sulfate ("Aqualon HS-10" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), emulsifier ("Adekaria soap NE-" manufactured by Asahi Denka Co., Ltd.) 20 ") 0.5 part of a monomer emulsion consisting of 50 parts of deionized water and an initiator solution consisting of 0.30 part of ammonium persulfate and 15.0 parts of deionized water in parallel over 2 hours. Then, it was dropped into the reaction vessel. After completion of the dropping, aging was performed at the same temperature for 2 hours. Next, the mixture was cooled to 40 ° C. and filtered through a 400 mesh filter to obtain an acrylic resin emulsion having a particle size of 200 nm, a nonvolatile content of 50%, an acid value of 10 mgKOH / g, a hydroxyl value of 60 mgKOH / g, and a mass average molecular weight of 200,000.
The obtained acrylic resin emulsion was adjusted to pH 7.2 using a 30% aqueous dimethylaminoethanol solution.

(Preparation of water-based intermediate coating)
After mixing 55.5 parts of the pigment dispersion paste obtained as described above, 96 parts of an acrylic resin emulsion, and 13.3 parts of melamine resin (“Cymel 327” manufactured by Mitsui Cytec Co., Ltd.) as a curing agent, urethane association type 1.0 part of a thickener (Adecanol UH-814N manufactured by Asahi Denka Kogyo Co., Ltd., active ingredient 30%) was mixed and stirred to obtain an aqueous intermediate coating.

3C / 1B multi-layer coating method on steel plate Powernics 110 (Cation Electrodeposition Paint, trade name, manufactured by Nippon Paint Co., Ltd.) is electrodeposited on a zinc phosphate-treated dull steel plate so that the dry coating thickness is 20 μm. The steel sheet substrate was prepared by coating, cooling after heating and curing at 160 ° C. for 30 minutes.

  On the obtained substrate, the above-mentioned aqueous intermediate coating was applied by air spray coating so that the dry film thickness was 20 μm, preheated at 80 ° C. for 5 minutes, and then the aqueous metallic base coating obtained above was air-treated. 10 μm was applied by spray coating, and preheating was performed at 80 ° C. for 3 minutes. Furthermore, after coating Macflow O-1810 as a clear paint on the coated plate to a dry film thickness of 40 μm by air spray coating, heat curing at 140 ° C. for 30 minutes, and a test having a multilayer coating film I got a plate.

3C / 1B multilayer coating method for bumpers WB8200 (Nippon Bee Chemical's water-based primer coating for polypropylene materials, trade name) is dried by air spray coating on commercially available polypropylene material (70mm x 150mm x 3mm) The plastic member was produced by painting so that thickness might be set to 10 micrometers, and heat-drying for 10 minutes at 80 degreeC.

  The obtained plastic member was coated with a water-based metallic base paint by 10 μm by air spray coating, and preheated at 80 ° C. for 3 minutes. Further, Polyurexel O-1100 Clear (Nippon Paint Co., Ltd., isocyanate-cured clear paint, product name) was applied to the coated plate with an air spray to a dry film thickness of 40 μm and set for 7 minutes. . Furthermore, the obtained coated plate was heat-cured at 120 ° C. for 30 minutes in a hot air drying furnace to obtain a test plate having a multilayer coating film.

  The coating film obtained by the three-coat / one-bake coating method was also evaluated for FF properties and coating film appearance (skin appearance). The results are shown in Table 1.

Example 2 and Comparative Examples 1-3
Instead of 15 parts of propylene glycol monopropyl ether when producing the aqueous metallic base paint of Example 1, 30 parts by weight of propylene glycol monopropyl ether in Example 2 (30 parts by weight with respect to 100 parts by weight of resin solid content of the paint) In Comparative Example 1, 5 parts by mass of propylene glycol monopropyl ether (5 parts by mass with respect to 100 parts by mass of resin solid content of the paint), and in Comparative Example 2, 50 parts by mass of propylene glycol monopropyl ether (solid resin content of paint 100) 50 parts by weight), and in Comparative Example 3, an aqueous metallic base containing 15 parts by weight of butyl cellosolve instead of propylene glycol monopropyl ether (15 parts by weight with respect to 100 parts by weight of the resin solid content of the paint) A paint was produced. Using these paints, two-coat / one-bake coating and three-coat / one-bake coating were performed in the same manner as in Example 1 to obtain a laminated coating film. About the obtained laminated coating film, FF property and coating-film external appearance (skin external appearance) were measured similarly to Example 1, and a result is shown in Table 1.

  As is clear from the above results, when using an aqueous metallic base paint containing 15 parts by mass of propylene glycol monopropyl ether and a resin solid content of the paint and an aqueous metallic base paint containing 30 parts by mass, two-coat / one-bake coating is used. The FF property and the appearance (skin appearance) are excellent not only on the steel plate, but also on the three-coating / one-baked steel plate and the plastic bumper. On the other hand, in the comparative example, it can be seen that the performance deteriorates in any part of the FF property and the appearance (skin appearance) on the steel sheet of the two-coat / one-bake coating and the three-coat / one-bake coating.

  The multilayer coating film forming method of the present invention can be used when forming a high-appearance coating film such as an automobile body.

Claims (7)

An aqueous intermediate coating is applied to a substrate having both a steel plate and a plastic substrate to form an intermediate coating, and an aqueous base coating is applied to the formed intermediate coating to form a base coating. After forming, an organic solvent-type clear coating is applied to form a clear coating, and the three layers of the intermediate coating, base coating and clear coating are heated and cured to form a multilayer coating. And
The water-based paint is in a resin solid content of 100% by mass,
(A) An acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing 0.2 to 20% by mass of a crosslinkable monomer has a solid content of 10 to 60% by mass,
(B) Water-soluble acrylic resin is 5 to 40% by mass in solid content, and (c) Melamine resin is 20 to 40% by mass in solid content,
(D) A method for forming a multilayer coating film, comprising 10 to 40 parts by mass of propylene glycol monoalkyl ether with respect to 100 parts by mass of a coating resin solid content.   The method for forming a multilayer coating film according to claim 1, wherein the propylene glycol monoalkyl ether is propylene glycol monopropyl ether.   The method for forming a multilayer coating film according to claim 1, wherein the crosslinkable monomer is contained in the monomer mixture for synthesizing the acrylic resin emulsion in an amount of 0.5 to 20% by mass.   The monomer mixture for synthesizing the acrylic resin emulsion (a) is a crosslinkable monomer, an α, β-ethylenically unsaturated monomer having an acid group, an α, β-ethylenically unsaturated monomer having a hydroxyl group, and other α, β- The method for forming a multilayer coating film according to claim 1, comprising an ethylenically unsaturated monomer.   The method for forming a multilayer coating film according to claim 1, wherein the acrylic resin emulsion (a) has an acid value of 3 to 50 mgKOH / g and a hydroxyl value of 10 to 150 mgKOH / g.   The method for forming a multilayer coating film according to claim 1, wherein the aqueous base paint further contains a luster pigment.   The method for forming a multilayer coating film according to claim 1, wherein the aqueous base paint further contains a polyester resin.