JP2011131133A - Method for forming multilayer coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming multilayer coating film applying aqueous base coating and clear coating by a wet-on-wet process and curing in a two-coat one-bake coating method so as to achieve a coating film having excellent in flip-flop properties (FF) even when the coating contains high solid content. <P>SOLUTION: There is provided a method for forming multilayer coating film by heating and curing a base coating film and a clear coating film, wherein an organic solvent-base clear coating film is applied to form the clear coating film after an aqueous base coating is applied to form the base coating film on an intermediate coating film formed on a steel sheet used in automotive application. 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, and (c) a melamine resin which is 20-40 mass% in terms of solid content. <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 Patent Document 3 exhibits excellent performance and can be sufficiently used for automobile painting. On the other hand, recently, in the case of water-based paints, it has been increasing from the viewpoint of resource saving to reduce the use of an aqueous solvent to achieve high solid differentiation. However, in the technique of Patent Document 3 described above, Since the property (so-called FF property) is not always good, further improvement is desired.

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

  This invention improves the technique of the said patent document 3, and provides the multilayer coating-film formation method with favorable flip-flop property (FF property) even with high solid content.

That is, in the present invention, a water-based base coating is formed on an intermediate coating film formed on an automotive steel plate to form a base coating, and then an organic solvent-type clear coating is applied to form a clear coating. And a method of forming a multilayer coating film in which the base coating film and the clear coating film are heated and cured,
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,
Provided is a method for forming a multilayer coating film characterized by containing.

  The crosslinkable monomer is preferably allyl methacrylate, ethylene glycol dimethacrylate or divinylbenzene.

  The crosslinkable monomer 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, when forming a multilayer coating film by a two-coat one-bake method in which a water-based base paint and an organic solvent-type clear paint are applied by wet-on-wet and both coating films are baked and cured simultaneously, It has been found that by blending a predetermined amount of a crosslinkable monomer into the monomer mixture forming the acrylic resin emulsion in which the paint is blended, high solid differentiation of the aqueous base paint and good flip-flop properties (FF properties) can be achieved.

  High solidification and FF properties are considered to be basically contradictory performances. The FF property requires that the pigment, particularly the glitter pigment, maintain the orientation in the coated film. When the solid content is low, the pigment has time to take the orientation, but when the solid content is high, It is considered that the solid content rises before the pigment takes sufficient orientation, disturbing the orientation of the pigment, and the FF property becomes insufficient.

  In the present invention, a crosslinkable monomer is introduced during the formation of the acrylic resin emulsion of the aqueous base paint, and three-dimensional crosslinks are formed on the particles of the acrylic resin emulsion to give some restriction to the particle state. Even so, it is considered that the time for which the pigment can be oriented is given.

In the method for forming a composite coating film of the present invention, a water-based base coating is formed on an intermediate coating film formed on a steel plate for automobiles to form a base coating, and then an organic solvent type clear coating is applied to the clear coating. In the method for forming a multilayer coating film in which a coating film is formed and the base coating film and the clear coating film are heated and cured, the water-based base coating 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,
It is characterized by containing.

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 the water-soluble acrylic resin in solids, and (c) 20 to 40% by mass of the melamine resin in solids.

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 0.2% when the mass of the monomer mixture is 100%. Contains 20% 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, high solid differentiation and good FF properties cannot be obtained. When it exceeds 60 mass%, the solid content at the time of coating will become low.

  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 volume average particle diameter 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 solid content tends to decrease too much. 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.

Other Components The aqueous base paint of the present invention contains 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.

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 can be advantageously used for various substrates such as metals, plastics, foams, etc., particularly metal surfaces, and castings. Can be used particularly preferably.

  Examples of the metal products include iron, copper, aluminum, tin, zinc and the like and alloys containing these metals. Specific examples include automobile bodies and parts such as passenger cars, trucks, motorcycles, and buses. These metals 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 product include polypropylene resin, polycarbonate resin, urethane resin, polyester resin, polystyrene resin, ABS resin, vinyl chloride resin, polyamide resin and the like. 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.

  If necessary, an intermediate coating film may be formed on the substrate. An intermediate coating is used to form the intermediate coating film. The intermediate coating contains a film-forming resin, a curing agent, various organic and inorganic color pigments, extender pigments, and the like.

  The film-forming resin contained in the intermediate coating is not particularly limited, and film-forming resins such as acrylic resin, polyester resin, alkyd resin, epoxy resin, and urethane resin can be used. It is used in combination with the curing agent mentioned in the previous aqueous base coat composition. An amino resin and / or an isocyanate resin is generally used in view of various performances and costs of the obtained coating film.

  As the color pigment contained in the intermediate coating composition, those described in the description of the aqueous base coat composition can be used in the same manner. As a standard, it is preferable to use a so-called color intermediate coating that combines carbon black and titanium dioxide as main pigments with a gray intermediate coating or a set gray that combines the hue of the top coating and various colored pigments. . Furthermore, flat pigments such as aluminum powder and mica powder may be added. In these intermediate coating materials, in addition to the above components, additives usually added to the coating material, for example, surface conditioners, antioxidants, antifoaming agents and the like may be blended.

Multilayer coating film forming method In the multilayer coating film forming method of the present invention, a base coating film and an organic solvent-type clear coating composition using an aqueous base coating material are coated on an object on which an electrodeposition coating film and an intermediate coating film are formed if necessary. The clear coating film can be formed sequentially.

  In the present invention, when an aqueous base paint is applied to an automobile body, in order to enhance the appearance, it is applied in multiple stages by air electrostatic spray painting, preferably in two stages, or air electrostatic spray painting. , A method of forming a coating film by a coating method combined with a rotary atomizing electrostatic coating machine called “μμ (micro micro) bell”, “μ (micro) bell” or “metabell” Can be used.

  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 is preferably formed by so-called two-coat one-bake, which is baked together with an uncured base coating film as described above. By setting the baking temperature to 80 to 180 ° C, preferably 120 to 160 ° C, a cured coating film having a high degree of crosslinking can be obtained. 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, 29.61 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 1.00 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 1 having an acid value of 20 mgKOH / g and a hydroxyl value of 40 mgKOH / g was obtained.

Production Example 2 Production of Acrylic Resin Emulsions 2 to 7 and i, ii In the production method of acrylic resin emulsion 1 of Production Example 1 above, the same procedure as in Production Example 1 except that the monomer mixture is changed to the composition described in Table 1. Resin emulsions 2 to 7 were obtained. In producing the acrylic resin emulsion 7, 0.5 part of 4,4′-azobis (4-cyanovaleric acid) was used instead of 0.3 part of ammonium persulfate. The average particle diameter, non-volatile content, solid content acid value and hydroxyl value of the obtained acrylic resin emulsion are the same as those of the acrylic resin emulsion 1 of Production Example 1.

Production Example 3
126.5 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. Next, the first monomer mixture 100 of methyl acrylate 29.61 parts, ethyl acrylate 37.97 parts, styrene 4.00 parts, 2-hydroxyethyl methacrylate 7.42 parts and allyl methacrylate 1.00 parts Parts, Aqualon HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate ester, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), Adekaria soap NE-20 (α- [1-[(allyloxy) methyl] -2- (Nonylphenoxy) ethyl] -ω-hydroxyoxyethylene (manufactured by Asahi Denka)) from a monomer emulsion consisting of 0.5 parts and 80 parts of deionized water, 0.24 parts of ammonium persulfate, and 10 parts of deionized water The resulting initiator solution was added dropwise to the reaction vessel in parallel over 2 hours. After completion of the dropping, aging was performed at the same temperature for 1 hour.

  Further, at 80 ° C., 100 parts of a second-stage monomer mixture of 15.7 parts of ethyl acrylate, 1.86 parts of 2-hydroxyethyl methacrylate and 3.07 parts of methacrylic acid, and 0.2 part of Aqualon HS-10 A monomer emulsion consisting of 50 parts of deionized water and an initiator solution consisting of 0.06 part of ammonium persulfate and 10 parts of deionized water were run together over 0.5 hour and dropped into the reaction vessel. 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, 20 parts of deionized water and 0.32 part of dimethylaminoethanol were added to adjust the pH to 6.5, the average particle diameter was 150 nm, the non-volatile content was 25%, the solid content An acrylic resin emulsion 8 having an acid value of 20 mgKOH / g and a hydroxyl value of 40 mgKOH / g was obtained.

Production Example 4 Production of Acrylic Resin Emulsions 9 to 11 and iii Acrylic Resin Emulsion as in Production Example 3 except that the monomer mixture is changed to the composition described in Table 2 in the production method of the acrylic resin emulsion 8 in Production Example 3 above. 9 to 11 were obtained. The average particle diameter, non-volatile content, solid content acid value and hydroxyl value of the obtained acrylic resin emulsion are the same as those of the acrylic resin emulsion 8 of Production Example 3.

Production Example 5 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.).

Production Example 6 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 the ester exchange 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 volume average particle size, only ion-exchanged water is put into a dedicated cell, and only one drop of this dispersion is added and mixed, and the resin solid content concentration is adjusted to 0.1% by mass to adjust the dynamic light scattering particle size. It was 35 nm when measured at 20 degreeC with the measuring apparatus LB-500 (made by Horiba Ltd.).

Production Example 7 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%.

Examples 1-15 and Comparative Examples 1-3
<Manufacture of water-based metallic base paint>
160 parts of acrylic resin emulsions 1 to 11 and i to iii obtained in the previous production examples 1 to 4 (resin solid content 25%) or mixtures thereof shown in Examples 12 to 15 in Table 3 (the ratios are all) 160 parts of 10% by mass dimethylaminoethanol, 33 parts of water-soluble acrylic resin of Production Example 5 (30% resin solids), and polyester resin 33 of Production Example 6 Parts (resin solid content 30%), 38 parts Cymel 204 (mixed alkylated melamine resin, solid content 80%, water tolerance 3.6 ml, manufactured by Mitsui Cytec Co., Ltd.) as a melamine resin, Primepole PX-1000 (Sanyo Chemical Industries) 10 parts of Bifunctional Polyether Polyol), 21 parts of Alpaste MH8801 (Alumina Pigment made by Asahi Kasei) as a bright pigment (Solid content 65%, PWC 12%), phosphoric acid group-containing acrylic resin 5 parts, lauryl acid phosphate 0.3 part, and 2-ethylhexanol 30 parts, Adecanol UH-814N 3.3 parts (manufactured by ADEKA) An aqueous metallic base paint was obtained by uniformly dispersing a thickener (solid content 30%).

<Composite coating film forming method>
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. The solid content of the paint was measured at 105 ° C. for 3 hours. Thereafter, two-stage coating was performed with a “cartridge bell” so as to obtain a dry film thickness of 15 μm under conditions of a room temperature of 23 ° C. and a 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 Macflow-O-1810 (solvent type clear paint manufactured by Nippon Paint Co., Ltd.) was applied as a clear paint in 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.

FF property was evaluated about the obtained coating film. The evaluation method was as follows.
<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).

  The results are shown in Table 3. Table 3 also lists the paint solids.

  As is clear from the above results, in the case of an example using an acrylic resin emulsion containing a predetermined amount of a crosslinkable monomer, the FF property is 2.15 or more at a high solid content with a solid content of 23% or more, It can be seen that the FF property and the high solid content are compatible. In that case, even when the acrylic resin emulsion was formed in two stages as in Examples 8 to 11, good results were obtained as long as a crosslinkable monomer was contained in the core portion. Moreover, as can be seen from Examples 12 to 15, the acrylic resin emulsion may be a mixture of a plurality of acrylic resin emulsions.

  Comparative Example 1 uses a single-layer acrylic resin emulsion that does not contain a crosslinkable monomer. The solid content is as low as 21.4%, and the FF property is not 2.15 or higher. Comparative Example 2 is a single-layer acrylic resin emulsion containing a crosslinkable monomer. The amount of the crosslinkable monomer is as large as 30% by mass, and a high solid content of 27.5% can be achieved. The FF property shows a low value of 2.05. Comparative Example 3 is an example in which an acrylic resin emulsion is formed in two stages and does not contain a crosslinkable monomer. Although the FF property has a good value of 2.18, the solid content is as low as 20.5%.

  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)

The base coating film is formed by coating a water-based base paint on the intermediate coating film formed on the steel sheet for automobiles to form a base coating film, and then applying an organic solvent type clear coating composition to form the clear coating film. And a clear coating film is heated and cured to form 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,
A method for forming a multilayer coating film, comprising:   The method for forming a multilayer coating film according to claim 1, wherein the crosslinkable monomer is allyl methacrylate, ethylene glycol dimethacrylate, or divinylbenzene.   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.