JP2009261997A - Method of forming composite-layered coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite-layered coating film having more excellent coating film appearance by regulating the reaction curing speed of an aqueous intercoating material by a three-coat one-bake method using an aqueous intercoating material and aqueous base coating. <P>SOLUTION: The method includes applying an aqueous intercoating material to the surface of an electrodeposition coating film to form the intercoating film, applying an aqueous base coating to the surface of the intercoating film to form a base coating film, further applying a clear coating to the base coating film to form a clear coating film and baking the intercoating film, the base coating film and the clear coating film and curing at the same time to form the composite-layered coating film. The aqueous intercoating material contains at least an emulsion of a hydroxyl group-containing acrylic resin and a melamine resin containing an alkyl-etherified melamine resin (wherein the number average of imino groups per triazine nucleus is below 1.0 and the number average molecular weight is below 1,000), and the content of alkyl-etherified melamine resin contained in the aqueous intercoating material is 10-35 pts.mass per 100 pts.mass of the resin solid. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for forming a multilayer coating film, and more particularly to a method for forming a multilayer coating film comprising an intermediate coating film, a base coating film and a clear coating film on an automobile body using a three-coat one-bake method.

  Car body painting is basically performed by sequentially laminating an electrodeposition coating, an intermediate coating, and a top coating consisting of a base coating and a clear coating on the steel sheet that is the object to be coated. . Conventionally, these coating films have been formed by applying a coating composition whose composition is adjusted according to the function of each coating film and baking and curing each coating film. When multiple coatings are applied, unless the underlying layer is completely formed and smoothed, the adjacent coating layers interfere with each other, and the unevenness of the underlying layer is reflected in the upper layer. This is because the appearance of the film is deteriorated.

  However, in order to increase work efficiency and achieve energy savings, which have been particularly demanded in recent years, even in the automobile body coating industry, multiple coatings are formed in such a way that multiple paints are applied in an uncured state and then cured simultaneously. Methods such as a 3-coat 1-bake method including a step of simultaneously baking and curing an intermediate coating film, a base coating film and a clear coating film have been gradually adopted.

  Japanese Patent Application Laid-Open No. 2003-105257 (Patent Document 1) discloses an aqueous intermediate coating composition having excellent base concealability and smoothness, including a carboxyl group-containing aqueous polyester resin and a melamine resin. However, in Patent Document 1, an aqueous intermediate coating composition is applied, baked and cured to form an intermediate coating film, and then a top coating is applied thereon, followed by heat curing to form a multilayer coating film. The application of the aqueous intermediate coating composition to the 3-coat 1-bake method is not disclosed at all. Specifically, in Patent Document 1, although the base concealing property and smoothness of the water-based intermediate coating are studied in detail, three coats for further applying a top coating on the formed uncured intermediate coating film The coating film appearance of the multilayer coating film obtained by the 1-bake method has not been studied at all.

  Japanese Patent Application Laid-Open No. 2003-251264 (Patent Document 2) describes the viscosity of an intermediate coating film in a 3-coat 1-bake method including a step of simultaneously heating and curing an intermediate coating film, a base coating film, and a clear coating film. Disclosed is a multilayer coating film forming method capable of preventing a mixed layer between an intermediate coating film and a base coating film by controlling and forming a coating film having an excellent appearance without yellowing. ing. Moreover, in patent document 2, the aqueous | water-based intermediate coating which consists of acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste, and a thickener is used as the intermediate coating which forms an intermediate coating film, and a thickener is used. Is added to the intermediate coating to increase the viscosity, thereby improving the appearance of the resulting multilayer coating film.

  Japanese Patent Application Laid-Open No. 2003-251275 (Patent Document 3) discloses a water-based intermediate coating having an increased viscosity in a three-coat one-bake method including a step of simultaneously heating and curing an intermediate coating film, a base coating film, and a clear coating film. A multilayer coating film forming method capable of forming a coating film having an excellent appearance by using a paint is disclosed. In Patent Document 3, as an intermediate coating material for forming an intermediate coating film, an aqueous intermediate coating material consisting of an acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste and a thickener is used. Preferably, by using a urethane associative thickener to increase the viscosity of the paint, it suppresses sagging during coating, improves the thixotropy of the paint, and forms a coating film having an excellent appearance. Yes.

  Japanese Patent Application Laid-Open No. 2003-251276 (Patent Document 4) is a three-coat one-bake method including a step of simultaneously heating and curing an intermediate coating film, a base coating film, and a clear coating film. By controlling the stress-strain characteristics (Young's modulus and elongation at -20 ° C) of the intermediate coating film, it is possible to form a coating film that has excellent chipping resistance and impact resistance and has an excellent appearance. A layer coating formation method is disclosed. In Patent Document 4, an aqueous intermediate coating comprising an acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste and a thickener is used as the aqueous intermediate coating, and the thickener is used as the aqueous intermediate coating. By adding it to increase the viscosity of the paint, a multilayer coating film having an excellent finished appearance is formed.

  As described above, in the so-called three-coat one-bake method, attempts have been made to improve the appearance of the resulting multilayer coating film by adjusting the viscosity of the intermediate coating. However, in the multilayer coating film obtained by the 3-coat 1-bake method using the water-based intermediate coating material and the water-based base coating material, further improvement in coating film appearance (finishing property) is required. Therefore, it has become necessary to examine methods other than the viscosity adjustment of the water-based intermediate coating.

  JP-A-2002-294148 (Patent Document 5) discloses an aqueous intermediate coating composition containing a carboxyl group-containing aqueous polyester resin and a melamine resin, wherein the degree of polymerization of the melamine resin is 2.5 or less, and The mononuclear ratio is 50% or more, the reactive group of the melamine resin is a complete alkyl type, and the weight average molecular weight of the carboxyl group-containing aqueous polyester resin is 5,000 to 15000, A waterborne intermediate coating composition is described. However, in the aqueous intermediate coating composition described in Patent Document 5, the type of the base resin is different from that of the present invention, and the influence on the appearance due to curing is presumed to be completely different.

JP 2003-105257 A JP 2003-251264 A JP 2003-251275 A JP 2003-251276 A JP 2002-294148 A

  The present invention solves the above-described conventional problems, and is further improved by adjusting the reaction curing rate of the aqueous intermediate coating film in the 3-coat 1-bake method using the aqueous intermediate coating material and the aqueous base coating material. An object of the present invention is to provide a multilayer coating film having an excellent coating film appearance.

  As a result of diligent research, the present inventors have found that an aqueous intermediate coating used for forming an intermediate coating film in a three-coat one-bake method including a step of simultaneously baking and curing an intermediate coating film, a base coating film, and a clear coating film. By using a melamine resin containing an alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000 in the coating, It has been found that the reaction curing rate can be adjusted and the coating film appearance of the resulting multilayer coating film can be improved. Accordingly, the present invention provides the following.

The present invention
An aqueous intermediate coating is applied on the electrodeposition coating to form an intermediate coating, and then an aqueous base coating is applied on the intermediate coating to form a base coating. A method of forming a multilayer coating film by coating a clear coating on a film to form a clear coating film, and then simultaneously baking and curing the intermediate coating film, base coating film and clear coating film,
This water-based intermediate coating is
A melamine resin comprising a hydroxyl group-containing acrylic resin emulsion, and an alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000;
And the content of the alkyl etherified melamine resin contained in the aqueous intermediate coating is 10 to 35 parts by mass with respect to 100 parts by mass of the resin solid content.
A method for forming a multilayer coating film is provided, whereby the above object is achieved.

The aqueous intermediate coating further contains a hydroxyl group-containing polyester resin, and the resin solid content mass ratio (with respect to the resin solid content) of each component contained in the aqueous intermediate coating is:
5-50% by mass of a hydroxyl group-containing acrylic resin emulsion,
5 to 80% by mass of a hydroxyl group-containing polyester resin, and 10 to 50% by mass of a melamine resin,
Is more preferable.

The hydroxyl group-containing acrylic resin emulsion contained in the aqueous intermediate coating material has an acid value of 3 to 50 mgKOH / g and a hydroxyl value of 10 to 150 mgKOH / g, and the hydroxyl group-containing polyester resin contained in the aqueous intermediate coating material is The acid value is more preferably 5 to 50 mgKOH / g and the hydroxyl value is 5 to 150 mgKOH / g.

  The present invention further provides an aqueous intermediate coating material used in the method for forming a multilayer coating film.

  This invention also provides the multilayer coating film obtained by the formation method of the said multilayer coating film.

  According to the present invention, in the 3-coat 1-bake method using an aqueous intermediate coating material and an aqueous base coating material, the reaction curing rate of the aqueous intermediate coating film can be adjusted. Thereby, the multilayer coating film which has the outstanding coating-film external appearance can be provided, maintaining the coating-film physical property of a multilayer coating film. Since the coating film forming method of the present invention is also a method relating to a three-coat one-bake method using an aqueous intermediate coating material and an aqueous base coating material, it shortens the painting process, reduces the baking process and costs, and reduces the burden on the environment. It is possible.

  The present invention is described in detail below. First, the water-based intermediate coating, water-based base coating, and clear coating used in the present invention will be described, respectively, and then the method for forming a multilayer coating film will be described in detail.

Aqueous intermediate coating The aqueous intermediate coating used in the method of the present invention contains at least a hydroxyl group-containing acrylic resin emulsion and a melamine resin in a state of being dispersed or dissolved in an aqueous medium. The aqueous intermediate coating material may further contain a hydroxyl group-containing polyester resin and a pigment, and may further contain additives usually contained in an aqueous intermediate coating material for automobile bodies such as a viscosity agent or a filler.

Hydroxyl group-containing acrylic resin emulsion The hydroxyl group-containing acrylic resin emulsion contained in the aqueous intermediate coating used in the method of the present invention comprises (meth) acrylic acid alkyl ester (a), acid group-containing ethylenically unsaturated monomer (b), and hydroxyl group. The monomer mixture containing the ethylenically unsaturated monomer (c) can be obtained by emulsion polymerization. In addition, you may use the compound illustrated below as a component of a monomer mixture, combining 1 type (s) or 2 or more types as appropriate.

  The waterborne intermediate coating used in the method of the present invention has a hydroxyl group-containing acrylic resin emulsion and the number of imino groups per triazine nucleus described in detail below in an average of less than 1.0 and a number average molecular weight of less than 1000. By including a certain alkyl etherified melamine resin, it is possible to form a multilayer coating film having a good finished appearance while ensuring good adhesion between the intermediate coating film and the top coating film.

  The (meth) acrylic acid alkyl ester (a) is used to constitute the main skeleton of the hydroxyl group-containing acrylic resin emulsion. Specific examples of the (meth) acrylic acid alkyl ester (a) include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, and (meth) acrylic. Isobutyl acid, t-butyl (meth) acrylate, hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, ( Examples thereof include dodecyl (meth) acrylate and stearyl (meth) acrylate.

  The acid group-containing ethylenically unsaturated monomer (b) improves the stability of the resulting hydroxyl group-containing acrylic resin emulsion, such as storage stability, mechanical stability, stability against freezing, etc. Used to accelerate the curing reaction with curing agents such as resins. The acid group is preferably selected from a carboxyl group, a sulfonic acid group, a phosphoric acid group, and the like. A particularly preferred acid group is a carboxyl group from the viewpoints of the above-mentioned various stability improvements and curing reaction promoting functions.

  Examples of the carboxyl group-containing ethylenically unsaturated monomer include acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride, and fumaric acid. . Examples of the sulfonic acid group-containing ethylenically unsaturated monomer include p-vinylbenzenesulfonic acid, p-acrylamidepropanesulfonic acid, t-butylacrylamidesulfonic acid, and the like. Examples of the phosphoric acid group-containing ethylenically unsaturated monomer include light ester PM (manufactured by Kyoeisha Chemical Co., Ltd.) such as 2-hydroxyethyl acrylate phosphate monoester and 2-hydroxypropyl methacrylate phosphate monoester.

  The hydroxyl group-containing ethylenically unsaturated monomer (c) imparts hydrophilicity based on a hydroxyl group to a hydroxyl group-containing acrylic resin emulsion, increases the workability and stability against freezing when this is used as a paint, melamine resin, etc. It is used to impart curing reactivity with the curing agent.

  Examples of the hydroxyl group-containing ethylenically unsaturated monomer (c) include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, and ε-caprolactone-modified acrylic monomer. Is mentioned.

  Specific examples of the ε-caprolactone-modified acrylic monomer include “Placcel FA-1”, “Placcel FA-2”, “Placcel FA-3”, “Placcel FA-4”, and “Placcel FA-4” manufactured by Daicel Chemical Industries, Ltd. Plaxel FA-5 "," Placcel FM-1 "," Placcel FM-2 "," Placcel FM-3 "," Placcel FM-4 "," Placcel FM-5 "and the like.

  The monomer mixture may contain at least one monomer selected from the group consisting of a styrene monomer and (meth) acrylonitrile as the other monomer component. Examples of the styrenic monomer include α-methylstyrene in addition to styrene.

  The monomer mixture may contain a crosslinkable monomer such as a carbonyl group-containing ethylenically unsaturated monomer, a hydrolyzable polymerizable silyl group-containing monomer, and various polyfunctional vinyl monomers. In that case, the resulting hydroxyl group-containing acrylic resin emulsion is self-crosslinkable.

  Emulsion polymerization in the preparation of a hydroxyl group-containing acrylic resin emulsion can be carried out by heating the monomer mixture in an aqueous liquid with stirring in the presence of a radical polymerization initiator and an emulsifier. The reaction temperature is, for example, about 30 to 100 ° C., and the reaction time is preferably about 1 to 10 hours. The reaction temperature is adjusted by batch addition or temporary dropping of the monomer mixture or monomer pre-emulsified liquid in a reaction vessel charged with water and an emulsifier. It is good to do.

  As said radical polymerization initiator, the well-known initiator normally used by emulsion polymerization of an acrylic resin can be used. Specifically, for example, persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate can be used in the form of an aqueous solution as a water-soluble free radical polymerization initiator. In addition, so-called redox initiators in which an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate or hydrogen peroxide and a reducing agent such as sodium bisulfite, sodium thiosulfate, Rongalite or ascorbic acid are combined are aqueous solutions. Can be used in the form of

  As the emulsifier, an anion selected from a micelle compound having a hydrocarbon group having 6 or more carbon atoms and a hydrophilic moiety such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule. System or nonionic emulsifiers are used. Among these, as anionic emulsifiers, alkali metal salts or ammonium salts of alkylphenols or higher alcohol sulfuric acid half esters; alkali metal salts or ammonium salts of alkyl or allyl sulfonates; polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl ethers Or the alkali metal salt or ammonium salt of the sulfuric acid half ester of polyoxyethylene allyl ether is mentioned. Examples of nonionic emulsifiers include polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, and polyoxyethylene allyl ether. In addition to these general-purpose anionic and nonionic emulsifiers, there are radical polymerizable unsaturated double bonds in the molecule, that is, groups such as acrylic, methacrylic, propenyl, allyl, allyl ether, etc. Various anionic and nonionic reactive emulsifiers and the like are also used alone or in combination of two or more.

  In the emulsion polymerization, the use of an auxiliary agent (chain transfer agent) for adjusting the molecular weight such as a mercaptan compound or a lower alcohol promotes the smooth and uniform formation of the coating film from the viewpoint of promoting the emulsion polymerization. From the viewpoint of improving the adhesion to the material, there are many cases where it is preferable, and it is carried out depending on the situation as appropriate.

  Emulsion polymerization can be performed by any one of the usual one-step continuous monomer uniform dropping method, core-shell polymerization method which is a multi-stage monomer feed method, and power feed polymerization method which continuously changes the monomer composition fed during the polymerization. Can be legal.

  In this way, the hydroxyl group-containing acrylic resin emulsion used in the present invention is prepared. Although the mass average molecular weight of the obtained acrylic resin is not particularly limited, it is generally about 50,000 to 1,000,000, for example, about 100,000 to 800,000.

  The acrylic resin has a glass transition temperature (Tg) of -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 within this range, when an aqueous intermediate coating containing a hydroxyl group-containing acrylic resin emulsion is used in a wet-on-wet system, the affinity and adhesion with the undercoat and topcoat are improved, and the wet Familiarity at the interface with the upper coating of the state 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 may be weakened. On the other hand, if the Tg of the resin exceeds 20 ° C., the coating film becomes hard and brittle, so that the impact resistance may be lacking and the chipping resistance may be weakened. The type and amount of each monomer component are selected so that the Tg of the resin falls within the above range.

  The solid content acid value of the acrylic resin in the hydroxyl group-containing acrylic resin emulsion is preferably 3 to 50 mgKOH / g, and more preferably 5 to 40 mgKOH / g. By adjusting the acid value to the above range, various stability such as storage stability, mechanical stability, stability against freezing of resin emulsion and water-based intermediate coating using the same is improved. Curing reaction with a curing agent such as melamine resin occurs sufficiently at the time, and various strengths, chipping resistance, and water resistance of the coating film are improved. When the solid content acid value is less than 3 mgKOH / g, the above-mentioned various stability may be inferior, and the curing reaction with a curing agent such as a melamine resin is not sufficiently performed. Chipping resistance and water resistance may be inferior. On the other hand, if the solid content acid value exceeds 50 mgKOH / g, the polymerization stability of the resin may be deteriorated, the above-mentioned various stability may be deteriorated, or the water resistance of the obtained coating film may be deteriorated.

  Adjustment of the solid content acid value of the acrylic resin in the hydroxyl group-containing acrylic resin emulsion is performed by selecting the type and blending amount of each monomer component used for the preparation of the hydroxyl group-containing acrylic resin emulsion so that the acid value of the resin falls within the above range. Can be adjusted. In this adjustment, it is preferable to use a carboxyl group-containing monomer in the acid group-containing ethylenically unsaturated monomer (b), and among the acid group-containing ethylenically unsaturated monomer (b), the carboxyl group-containing monomer is preferably 50 masses. %, More preferably 80% by mass or more is more preferable.

  The hydroxyl value (solid content) of the acrylic resin in the hydroxyl group-containing acrylic resin emulsion is preferably 10 to 150 mgKOH / g, and more preferably 20 to 100 mgKOH / g. By adjusting the hydroxyl value within the above 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 a curing agent such as a melamine resin. And the curing reactivity is improved. When the hydroxyl value is less than 10 mgKOH / g, the curing reaction with the curing agent becomes insufficient, the mechanical properties of the coating film are weak, chipping resistance is poor, and water resistance and solvent resistance are also poor. There is a risk. On the other hand, when the hydroxyl value exceeds 150 mgKOH / g, the water resistance of the resulting coating film is lowered, the compatibility with the curing agent is poor, the coating film is distorted, and the curing reaction occurs unevenly. The various strengths of the coating film, particularly chipping resistance, solvent resistance and water resistance, may be inferior. The hydroxyl value of the acrylic resin in the hydroxyl group-containing acrylic resin emulsion is adjusted by selecting the type and amount of each monomer component used in the preparation of the hydroxyl group-containing acrylic resin emulsion so that the hydroxyl value of the resin falls within the above range. Can be adjusted.

  In order to maintain the stability of the hydroxyl group-containing acrylic resin emulsion by neutralizing a part or all of the carboxylic acid to the obtained hydroxyl group-containing acrylic resin emulsion, a basic compound may be added. As such a basic compound, ammonia, various amines, alkali metals and the like are generally used, and can be appropriately used in the present invention.

  The content of the hydroxyl group-containing acrylic resin emulsion contained in the aqueous intermediate coating is preferably 5 to 50% by mass, more preferably 7 to 40% by mass, based on the solid content of the coating resin. When the content of the hydroxyl group-containing acrylic resin emulsion is less than 5% by mass, the coating strength such as chipping resistance may be deteriorated. On the other hand, when the content of the hydroxyl group-containing acrylic resin emulsion exceeds 50% by mass, the water resistance of the resulting coating film may be lowered.

Melamine resin The aqueous intermediate coating composition of the present invention comprises an alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000. 10-35 mass% is included with respect to solid content. The content of the alkyl etherified melamine resin is more preferably 13 to 32% by mass. In addition, when content of the said alkyl ether melamine resin is less than 10 mass%, the external appearance improvement at the time of forming a coating film is not enough. Moreover, when it exceeds 35 mass%, when a coating film is formed, adhesiveness will fall.

  In the aqueous intermediate coating material in the present invention, by including the alkyl etherified melamine resin, the reaction curing rate of the intermediate coating film closer to the object to be coated can be suppressed in the 3-coat 1-bake method. As a result, the reaction curing rates of both coating films at the time of curing the intermediate coating film and the base coating film are approximated, and the appearance of the resulting multilayer coating film is improved.

Alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000 The melamine resin used in the aqueous intermediate coating composition of the present invention is triazine nucleus 1 An alkyl etherified melamine resin having an average number of imino groups of less than 1.0 and a number average molecular weight of less than 1000 is included. This alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000 is melamine (2,4,6-triamino-1, A part of the amino group of 3,5-triazine) can be prepared by reacting formaldehyde with methylol and then alkyl etherifying part of the resulting methylol group with alcohol.

Melamine has three amino groups (—NH ₂ ) bonded to the carbon atoms of the triazine nucleus constituting melamine. Since formaldehyde can be added to the two hydrogen atoms constituting this amino group, theoretically, 6 mol of formaldehyde can be added to 1 mol of melamine, and one triazine nucleus can be added. Six methylol groups can be introduced. The methylol group thus introduced into melamine is alkyletherified by reacting with alcohol.

In the present invention, in the methylolation using formaldehyde, not all the hydrogen atoms of the amino group of melamine are reacted to form a methylol, but an imino group ( —NH— CH ₂ OR; where R is H or an alkyl group, Is averaged to less than 1.0, preferably 0.01 to 0.5 per triazine nucleus. If the number of imino groups in the alkyl etherified melamine resin is 1.0 or more per triazine nucleus, the storage stability of the intermediate coating itself may be lowered. The content of imino groups is preferably 0.01 to 0.5 per triazine nucleus from the viewpoint of improving the appearance of the multilayer coating film.

  In alkyl etherification, a monohydric alcohol having 1 to 4 carbon atoms is used as an alcohol to be reacted with a methylol group introduced into melamine. Examples of such alcohol include methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, n-butyl alcohol, and isobutyl alcohol. The alcohol used for the alkyl etherification reaction may be one kind or a combination of two or more kinds. For example, alkyl etherification may be performed using two kinds of alcohols such as methyl alcohol and butyl alcohol. The methylolation reaction and the alkyl etherification reaction can be performed by known methods. Moreover, it is preferable from the point of the coating-film external appearance at the time of forming a coating film that a methyl alcohol is used for alkyl etherification, or it is the system which used methyl alcohol and butyl alcohol together.

  The alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000 has a ratio of methyl groups / butyl groups in the alkyl etherified moiety, The molar ratio is preferably 50/50 to 100/0. When the ratio of methyl group / butyl group is less than 50/50, the appearance may be deteriorated when a coating film is formed. The ratio of this methyl group / butyl group is more preferably 55/45 to 100/0, still more preferably 60/40 to 100/0.

  The thus prepared alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 has a number average molecular weight of less than 1000. When the number average molecular weight is 1000 or more, the smoothness when a coating film is formed is lowered. The number average molecular weight is preferably 300 to 900, and more preferably 400 to 700.

  While the use of the alkyl etherified melamine resin has various advantages, the alkyl etherified melamine resin generally has a film-forming resin containing a hydroxyl group (hydroxyl group-containing acrylic resin emulsion and There is a problem that it is difficult to cause a sufficient curing reaction with a hydroxyl group-containing polyester resin as required. Therefore, in the water-based intermediate coating, it is more preferable to use other melamine resins described in detail below together with the alkyl etherified melamine resin. When the alkyl etherified melamine resin and the other melamine resin are used in combination, the content ratio of the alkyl etherified melamine resin and the other melamine resin is particularly preferably 10/90 to 45/55.

Other Melamine Resins In the aqueous intermediate coating composition of the present invention, the number of imino groups per triazine nucleus is less than 1.0 on average and the number average molecular weight is less than 1000. A melamine resin may further be included. Examples of other melamine resins include melamine resins having an average number of imino groups of 1.0 or more per triazine nucleus and a number average molecular weight of 500 to 2500.

  In other melamine resins, if the number of imino groups is less than 1.0 per triazine nucleus, the curability of the intermediate coating may be lowered. The content of the imino group is preferably 1.2 to 2.5 per triazine nucleus from the viewpoint of physical properties of the coating film when a coating film is formed. When the number average molecular weight is less than 500, the curability when a coating film is formed may be lowered, and when it exceeds 2500, the appearance of the coating film may be lowered. The number average molecular weight is more preferably 550 to 1200, and even more preferably 600 to 1100. Thus, by using the above alkyl etherified melamine resin in combination with other melamine resins, it is possible to ensure good reaction curability of the intermediate coating film, and good coating film properties of the formed multilayer coating film. Can be secured.

  The total content of the melamine resin contained in the aqueous intermediate coating is preferably 10 to 50% by mass, more preferably 15 to 40% by mass, based on the solid content of the coating resin. When content of a melamine resin is less than 10 mass%, there exists a possibility that the water resistance of the coating film obtained may fall. Moreover, when content of a melamine resin exceeds 50 mass%, there exists a possibility that the chipping property of the coating film obtained may fall. In the present specification, the number average molecular weight is measured by GPC (gel permeation chromatogram), and a value converted into a polystyrene polymer molecular weight is used.

Other resins

  The aqueous intermediate coating material may further contain a resin component other than the resin component. Although it does not specifically limit as a resin component which may be included, For example, a polyester resin, a water-soluble acrylic resin, a polyether resin, an epoxy resin etc. can be mentioned. Among these, a hydroxyl group-containing polyester resin is more preferably used.

Hydroxyl-containing polyester resin Hydroxyl-containing polyester resin includes an oil-free polyester resin obtained by condensing a polyhydric alcohol component and a polybasic acid component, or a castor oil and a dehydrated castor oil in addition to the polyhydric alcohol component and the polybasic acid component. , Paulownia oil, safflower oil, soybean oil, linseed oil, tall oil, coconut oil, and the like, and an oil component that is one or a mixture of two or more of these fatty acids is added to the acid component and the alcohol component. And oil-modified polyester resins obtained by reacting the three. Moreover, the polyester resin which grafted acrylic resin and vinyl resin can also be used. Furthermore, a urethane-modified polyester resin obtained by reacting a polyisocyanate compound with a polyester resin obtained by reacting a polyhydric alcohol component and a polybasic acid component can also be used.

  Examples of the polyhydric alcohol component that can be used in the hydroxyl group-containing polyester resin include, for example, ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, , 6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentylglycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentylglycol ester, 2-butyl-2 Diols such as ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol, hydrogenated bisphenol A, and trimethylolpropane, trimethylolethane, glycerin , Penta Trivalent or higher polyol components such as lithitol, and 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, 2, Mention may be made of hydroxycarboxylic acid components such as 2-dimethyloloctanoic acid.

  Examples of polybasic acids that can be used in the hydroxyl group-containing polyester resin include, for example, phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, pyromellitic anhydride, etc. Aromatic polyhydric carboxylic acids and anhydrides; alicyclic polyhydric carboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, 1,4- and 1,3-cyclohexanedicarboxylic acid; maleic anhydride And polybasic acid components such as aliphatic polycarboxylic acids and anhydrides such as fumaric acid, succinic anhydride, adipic acid, sebacic acid and azelaic acid, and anhydrides thereof. A monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination as necessary.

  When preparing a hydroxyl group-containing polyester resin, as reaction components, mono-epoxide compounds such as monohydric alcohol, Cardura E (trade name: manufactured by Ciel Chemical), and lactones (β-propiolactone, dimethylpropio) Lactone, butyrolactone, γ-valerolactone, ε-caprolactone, γ-caprolactone, γ-caprolactone, crotolactone, δ-valerolactone, δ-caprolactone, etc.) may be used in combination. In particular, lactones are used for ring-opening addition to a polyester chain of a polyvalent carboxylic acid and a polyhydric alcohol to form a polyester chain by itself, and further to improve the chipping resistance of an aqueous intermediate coating composition. These may be contained in 3 to 30%, preferably 5 to 20%, in particular 7 to 15% of the total mass of all reaction components.

  The hydroxyl group-containing polyester resin can be easily made aqueous by adjusting its acid value and neutralizing the carboxyl group with a basic substance (for example, 50% or more). Examples of basic substances used here include ammonia, methylamine, ethylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, dimethylethanolamine, diethanolamine, and triethanolamine. Among these, diethanolamine, dimethylethanolamine, and triethanolamine are included. Ethanolamine and the like are preferred. Moreover, the neutralization rate in the case of the said neutralization is not specifically limited, For example, it is 80 to 120%.

  The number average molecular weight (Mn) of the hydroxyl group-containing polyester resin is preferably from 800 to 10,000, more preferably from 1,000 to 8,000. If the number average molecular weight is less than 800, the stability when the polyester resin is dispersed in water may be lowered. On the other hand, if the number average molecular weight exceeds 10,000, the viscosity of the resin increases, so that the solid content concentration in the case of coating is lowered, and the coating workability may be lowered.

  The hydroxyl value (solid content) of the hydroxyl group-containing polyester resin is preferably 5 to 150 mgKOH / g, and more preferably 30 to 130 mgKOH / g. When the hydroxyl value is less than 5 mgKOH / g, the curability of the resulting coating film may be reduced. On the other hand, if the hydroxyl value exceeds 150 mgKOH / g, the chipping resistance of the coating film may be lowered.

  The hydroxyl group-containing polyester resin preferably has an acid value (solid content) of 5 to 50 mgKOH / g, and more preferably 10 to 45 mgKOH / g. If the acid value is less than 5 mgKOH / g, the water dispersion stability of the hydroxyl group-containing polyester resin may be lowered. Moreover, when it exceeds 50 mgKOH / g, there exists a possibility that the water resistance of the coating film formed may fall.

  Moreover, it is preferable that the glass transition temperature of the said hydroxyl-containing polyester resin is -40-50 degreeC. When the said glass transition temperature is less than -40 degreeC, there exists a possibility that the hardness of the coating film obtained may fall, and when it exceeds 50 degreeC, there exists a possibility that base concealment property may fall. More preferably, it is -40-10 degreeC. The glass transition temperature can be measured by a differential scanning calorimeter (DSC) or the like.

  The content when the aqueous intermediate coating material contains the hydroxyl group-containing polyester resin is preferably 5 to 80% by mass with respect to the resin solid content mass contained in the aqueous intermediate coating material, and is 20 to 75% by mass. Is more preferable. When content of a hydroxyl-containing polyester resin exceeds 80 mass%, there exists a possibility that the coating-film physical property of the coating film obtained may fall. Moreover, when content of a hydroxyl-containing polyester resin is less than 5 mass%, there exists a possibility that the storage stability of intermediate coating may fall.

Other Components The aqueous intermediate coating composition used in the present invention may further contain a pigment dispersion paste, a viscosity agent, other additive components, and the like.

Pigment dispersion paste A pigment dispersion paste can be prepared by previously dispersing a pigment and a pigment dispersant in a small amount of an aqueous medium. The solid content of the pigment dispersant does not contain any volatile basic substance or contains 3% by mass or less. In the aqueous intermediate coating composition used in the present invention, by using such a pigment dispersant, the amount of volatile basic substances in the coating film formed from the aqueous intermediate coating composition is reduced, and the resulting multilayer coating is obtained. Yellowing of the film can be suppressed. Therefore, if the volatile basic substance is contained in the solid content of the pigment dispersant in an amount of more than 3% by mass, the resulting multilayer coating film tends to turn yellow and the finished appearance tends to deteriorate. Absent.

  The volatile basic substance means a basic substance having a boiling point of 300 ° C. or less, and examples thereof include inorganic and organic nitrogen-containing basic substances. As an inorganic basic substance, ammonia etc. are mentioned, for example. Examples of the organic basic substance include linear or branched alkyl having 1 to 20 carbon atoms such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, and dimethyldodecylamine. Group-containing primary to tertiary amines; linear or branched hydroxyalkyl group-containing primary to tertiary amines having 1 to 20 carbon atoms such as monoethanolamine, diethanolamine, 2-amino-2-methylpropanol; dimethylethanolamine A primary or tertiary amine containing a linear or branched alkyl group having 1 to 20 carbon atoms and a linear or branched hydroxyalkyl group having 1 to 20 carbon atoms, such as diethylethanolamine; diethylenetriamine , Carbon such as triethylenetetramine 1-20 substituted or unsubstituted linear polyamines; substituted or unsubstituted cyclic monoamines having 1 to 20 carbon atoms such as morpholine, N-methylmorpholine, N-ethylmorpholine; piperazine, N-methylpiperazine, N-ethylpiperazine, Mention may be made of amines such as substituted or unsubstituted cyclic polyamines having 1 to 20 carbon atoms such as N, N-dimethylpiperazine.

The aqueous intermediate coating composition used in the present invention may contain a volatile basic substance in components other than the pigment dispersant. Therefore, it is more preferable that the amount of the volatile basic substance contained in the pigment dispersant is suppressed. That is, it is preferable to disperse using a pigment dispersant that does not substantially contain a volatile basic substance. Further, it is more preferable not to use an amine-neutralized pigment dispersion resin generally used conventionally. And it is preferable to use a pigment dispersant so that the volatile basic substance per unit area 1 mm < ² > may be 7 * 10 < ^-6 > mmol or less at the time of multilayer coating film formation.

  The pigment dispersant is a resin having a structure including a pigment affinity part and a hydrophilic part. Examples of the pigment affinity portion and the hydrophilic portion include nonionic, cationic and anionic functional groups. The pigment dispersant may have two or more of the above functional groups in one molecule.

  Examples of the nonionic functional group include a hydroxyl group, an amide group, and a polyoxyalkylene group. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. Examples of the anionic functional group include a carboxyl group, a sulfonic acid group, and a phosphoric acid group. Such pigment dispersants can be produced by methods well known to those skilled in the art.

  The pigment dispersant is not particularly limited as long as it does not contain a volatile basic substance in the solid content or has a content of 3% by mass or less. What can disperse | distribute a pigment is preferable. For example, commercially available products (hereinafter, trade names) can also be used. Specifically, Disperbyk 190, Disperbyk 181, Disperbyk 182 (polymer co-polymers), which are anionic and nonionic dispersants manufactured by Big Chemie, Inc., can be used. Polymer), Disperbyk 184 (polymer copolymer), EFKAPOLYMER 4550, an anionic / nonionic dispersant manufactured by EFKA, Solsperse 27000, a nonionic dispersant manufactured by Abyssia, Solsperse 41000, an anionic dispersant, And Solsperse 53095.

  The number average molecular weight of the pigment dispersant is preferably a lower limit of 1000 and an upper limit of 100,000. If it is less than 1000, the dispersion stability may not be sufficient, and if it exceeds 100,000, the viscosity may be too high and handling may be difficult. More preferably, the lower limit is 2000 and the upper limit is 50,000, and even more preferably, the lower limit is 4000 and the upper limit is 50,000.

  The pigment dispersion paste is obtained by mixing and dispersing a pigment dispersant and a pigment according to a known method. It is preferable that the ratio of the pigment dispersant at the time of manufacturing the pigment dispersion paste is a lower limit of 1% by mass and an upper limit of 20% by mass with respect to the solid content of the pigment dispersion paste. If it is less than 1% by mass, it is difficult to disperse the pigment stably, and if it exceeds 20% by mass, the physical properties of the coating film may be inferior. Preferably, the lower limit is 5 mass% and the upper limit is 15 mass%.

  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.

  The pigment preferably has a pigment weight content (PWC) 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.

Associative viscosity agent The water-based intermediate coating composition used in the present invention may further contain a viscosity agent. Although it does not specifically limit as a viscosity agent, For example, a cellulose type thing, an alkali thickening type thing, and an association type thing can be mentioned. Examples of the association type include polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter, trade names) such as Adecanol UH-420, Adecanol UH-462, Adecanol UH-472, UH-540, Adecanol UH. -814N (manufactured by Asahi Denka Kogyo Co., Ltd.), Primal RH-1020 (manufactured by Rohm & Haas), Kuraray Poval (manufactured by Kuraray Co., Ltd.) and the like. These may be used alone or in combination of two or more.

  By containing the viscosity agent, 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, the coating workability during coating is improved, and the finished appearance of the resulting multilayer coating film can be made excellent.

  The content in the case of using a viscosity agent is 0.01 parts by mass lower limit and 20 parts by upper limit with respect to 100 parts by mass of resin solid content of the above-mentioned aqueous intermediate coating (solid content of all resins contained in the aqueous intermediate coating). The lower limit is 0.1 parts by mass, and the upper limit is more preferably 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.

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

Preparation of aqueous intermediate coating The aqueous intermediate coating used in the present invention has an average number of imino groups per hydroxyl group-containing acrylic resin emulsion and triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000. It is prepared by mixing a melamine resin containing an alkyl etherified melamine resin and a hydroxyl group-containing polyester resin as required. In addition, the preferable ratio of each resin component is as above-mentioned.

  When the hydroxyl group-containing polyester resin is included, the ratio of hydroxyl group-containing acrylic resin emulsion / hydroxyl group-containing polyester resin (solid content mass ratio) is preferably 1/6 to 2/1. By setting the ratio of the hydroxyl group-containing acrylic resin emulsion / hydroxyl group-containing polyester resin within the above range, the coating film viscosity and the smoothness, water absorption rate, and elution rate of the intermediate coating film can be made preferable ranges.

  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 mass%, it is difficult to increase the solid content concentration in the coating material, which is not preferable.

  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.

Aqueous base paint In the method for forming a multilayer coating film of the present invention, an aqueous base paint is used for forming the aqueous base paint film. This aqueous base paint may contain a film-forming resin, a curing agent, various organic or inorganic color pigments, extender pigments and, if necessary, glitter pigments.

  The film-forming resin contained in the aqueous base coating material used in the coating film forming method of the present invention preferably has a number average molecular weight of 5000 to 30000, more preferably 7000 to 25000. If it is less than 5000, workability and curability may be insufficient. On the other hand, if it exceeds 30000, the non-volatile content at the time of coating becomes too low, and the workability may be deteriorated. In this specification, the molecular weight is determined by the GPC method using styrene polymer as a standard.

  The coating film-forming resin preferably has a hydroxyl value (solid content) of 20 to 180, preferably 30 to 160. If the upper limit is exceeded, the water resistance of the coating film may be reduced, and if it is less than the lower limit, the curability of the coating film may be reduced. The coating film-forming resin also preferably has an acid value (solid content) of 10 to 80 mgKOH / g, more preferably 15 to 70 mgKOH / g. If the upper limit is exceeded, the water resistance of the coating film may be reduced, and if it is less than the lower limit, the curability of the coating film may be reduced.

  Preferred examples of the coating-forming resin include film-forming resins such as acrylic resins, polyester resins, alkyd resins, polyether resins, polyolefin resins, and urethane resins. The form of these resins may be water-soluble, water-dispersible or emulsion, and can be used alone or in combination of two or more. It is preferable to use an acrylic resin and / or a polyester resin as the coating-forming resin from the viewpoint of coating film performance such as weather resistance and water resistance.

  Preferred examples of the curing agent include amino resins, blocked isocyanate resins, epoxy compounds, aziridine compounds, carbodiimide compounds, and oxazoline compounds. A butylated and / or butylated melamine resin and / or a blocked isocyanate resin are generally used in view of various performances and costs of the obtained coating film.

  Content of the said hardening | curing agent is 20-100 mass% with respect to the total solid with a film-forming resin. If the content is less than 20% by mass, the curability may be insufficient, and if it exceeds 100% by mass, the cured film may be too hard and brittle.

  Examples of the colored pigment include organic azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, and perylene pigments. Pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, metal complex pigments, and the like. Examples of inorganic pigments include yellow lead, yellow iron oxide, bengara, carbon black, and titanium dioxide. As extender pigments, calcium carbonate, barium sulfate, clay, talc and the like are used. Further, flat pigments such as aluminum powder and mica powder may be added.

The shape of the glitter pigment is not particularly limited, and may be further colored. For example, the average particle diameter (D ₅₀ ) is 2 to ₅₀ μm and the thickness is 0.1 to 5 μm. Those are preferred. Those having an average particle diameter in the range of 10 to 35 μm are excellent in glitter and are more preferably used.

  The pigment concentration (PWC) of the glitter pigment in the paint is generally 20.0% by mass or less. When the upper limit is exceeded, the appearance of the coating film is lowered. Preferably, it is 0.01-18.0 mass%, More preferably, it is 0.1-15.0 mass%. When the content of the brightening agent exceeds 20.0% by mass, the coating film appearance may be deteriorated.

  Examples of the glitter pigment include non-colored or colored metal glitter such as metals or alloys such as aluminum, copper, zinc, iron, nickel, tin, and aluminum oxide, and mixtures thereof. Further, interference mica pigments, white mica pigments, graphite pigments and other colored or colored flat pigments may be used in combination.

  The total pigment concentration (PWC) in the paint including the glitter pigment and all other pigments is 0.1 to 50% by mass, preferably 0.5 to 40% by mass, more preferably Is 1.0 to 30% by mass. If the upper limit is exceeded, the coating film appearance may be reduced.

  In addition, a viscosity control agent can be added to the water-based base coating material in order to prevent familiarity with the clear coating film and ensure coating workability. As the viscosity control agent, those generally showing thixotropy can be used, for example, a fatty acid amide swelling dispersion, an amide fatty acid, a polyamide-based one such as a long-chain polyaminoamide phosphate, a colloidal swelling dispersion of polyethylene oxide Polyethylene-based materials, organic acid smectite clays, organic bentonite-based materials such as montmorillonite, inorganic pigments such as aluminum silicate and barium sulfate, flat pigments that develop viscosity depending on the shape of the pigment, cross-linked or non-cross-linked Resin particles and the like can be mentioned as viscosity control agents.

  In the aqueous base paint used in the present invention, additives usually added to the paint in addition to the above components, for example, a surface conditioner, an antioxidant, an antifoaming agent and the like may be blended. These compounding amounts are within the range known to those skilled in the art.

Preparation of aqueous base paint The method for producing the aqueous base paint used in the method for forming a multilayer coating film of the present invention is not particularly limited, and a compound such as each resin and pigment is kneaded and dispersed using a kneader or a roll. Any method known to those skilled in the art can be used.

  Moreover, what is marketed as a water-based base coating material for motor vehicle bodies can be used as the water-based base coating material used in the present invention. For example, AQUAREX AR-2000 Silver Metallic (trade name) (Nippon Paint Co., Ltd. water-based metallic base paint), NWB-230 (trade name) (Nippon Paint Co., Ltd. water-based metallic base paint), and the like can be mentioned.

Clear paint The clear paint used in the method of the present invention may be a paint composition usually used as a clear paint for automobile bodies. For example, what contains a film-forming resin, a curing agent and other additives as necessary, in a state of being dispersed or dissolved in a medium can be mentioned. Examples of the film-forming resin include acrylic resin, polyester resin, epoxy resin, and urethane resin. These can be used in combination with a curing agent such as an amino resin and / or an isocyanate resin. From the viewpoint of transparency or acid etching resistance, it is preferable to use a combination of an acrylic resin and / or a polyester resin and an amino resin, or an acrylic resin and / or a polyester resin having a carboxylic acid / epoxy curing system.

  The coating form of the clear paint may be any of an organic solvent type, an aqueous type (water-soluble, water-dispersible, emulsion), a non-water-dispersed type, and a powder type. It may be included.

Multilayer Coating Film Forming Method In the multilayer coating film forming method of the present invention, first, an article to be coated on which an electrodeposition coating film is formed is provided. The electrodeposition coating film is formed by applying an electrodeposition paint to an object to be coated and baking and curing it. The object to be coated is not particularly limited as long as it is a metal product capable of cationic electrodeposition coating. For example, iron, copper, aluminum, tin, zinc, and an alloy containing these metals can be given.

  The electrodeposition paint is not particularly limited, and a known cationic electrodeposition paint or anion electrodeposition paint can be used. Electrodeposition coating and baking may be performed by a method and conditions usually used for electrodeposition coating of automobile bodies.

  Next, an aqueous intermediate coating is applied on the electrodeposition coating to form an intermediate coating. Intermediate coatings are, for example, air electrostatic sprays known as “react guns”, commonly known as “micro-microbells (μμbells)”, “microbells (μbells)”, “metallic bells (metabells)”, etc. It can be applied by spraying using a rotary atomizing electrostatic coating machine.

  The coating amount is adjusted so that the film thickness of the cured coating film is 10 to 40 μm, preferably 15 to 30 μm. If the film thickness is less than 10 μm, the appearance and chipping resistance of the resulting coating film may be reduced, and if it exceeds 40 μm, problems such as sagging during coating and pinholes during baking hardening may occur.

  The intermediate coating film is preferably pre-dried (preheated) by heating or blowing before applying the aqueous base paint. The reason is that when the drying is insufficient, the water remaining in the coating film causes bumping in the step of baking the multilayer coating film, and it is easy to generate a crack. Further, when the base is coated on the intermediate coating, it is easy to mix with the base and the appearance may be deteriorated. The preliminary drying temperature may be a temperature lower than the temperature at which the coating film is completely cured, and is, for example, room temperature to 100 ° C, preferably 60 to 100 ° C, more preferably 70 to 90 ° C. The preliminary drying time is not particularly limited, and is, for example, 2 to 20 minutes, preferably 3 to 10 minutes.

  Next, the base coating film and the clear coating film are formed by sequentially applying a water-based base coating and a clear coating on the intermediate coating without being cured, and wet-on-wet. Here, the wet-on-wet coating means that the plurality of coating films are applied without being cured, after being subjected to the preliminary drying or the like as necessary.

  The coating amount of the aqueous base paint is usually adjusted so that the film thickness after curing of the coating film is 10 to 30 μm. If the film thickness after curing is less than 10 μm, there is a risk that the underlaying will be insufficiently concealed and color unevenness may occur. If it exceeds 30 μm, sagging will occur during painting and pinholes will occur during heat curing. There is a risk of doing so.

  The coating amount of the clear paint is usually adjusted so that the film thickness after drying and curing of the coating film is 10 to 70 μm. When the film thickness after curing is less than 10 μm, the appearance such as glossiness of the multilayer coating film is deteriorated, and when it exceeds 70 μm, the sharpness is deteriorated, and defects such as unevenness and flow occur during coating. If necessary, the preliminary drying may be performed after the aqueous base paint is applied and before the clear paint is applied.

  Next, the intermediate coating film, the base coating film, and the clear coating film are simultaneously baked and cured. Baking is usually performed by heating to a temperature of 110 to 180 ° C, preferably 120 to 160 ° C. Thereby, a cured coating film having a high degree of crosslinking can be obtained. If the heating temperature is less than 110 ° C, curing tends to be insufficient, and if it exceeds 180 ° C, the resulting coating film may be hard and brittle. Although the time to heat can be suitably set according to the said temperature, for example, when temperature is 120-160 degreeC, it is 10 to 60 minutes.

  The following examples further illustrate the present invention, but the present invention is not limited thereto. In the examples, “parts” and “%” are based on mass unless otherwise specified.

Production Example 1 Preparation of Pigment Dispersion Paste Disperbyk 190 (Nonion / Anionic Dispersant, Product Name) manufactured by BYK Chemie Co., Ltd. 4.5 parts, BYK-011 (Antifoam Agent manufactured by BYK Chemie Company Co., Ltd., product name) 0.5 parts, ion exchange After 22.9 parts of water and 72.1 parts of rutile titanium dioxide were premixed, 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.

Production Example 2 Preparation of Hydroxyl-Containing Acrylic Resin Emulsion A reaction vessel for producing a typical acrylic resin emulsion equipped with a stirrer, a thermometer, a dropping funnel, a reflux condenser, a nitrogen introduction tube, and the like, was charged with 445 parts of water and Newcol 293 ( 5 parts of an emulsifier manufactured by Nippon Emulsifier Co., Ltd., trade name) was charged and heated to 75 ° C. while stirring. A mixture of the monomer mixture of Table 1, 240 parts of water and 30 parts of New Coal 293 was emulsified using a homogenizer, and the monomer pre-emulsified liquid was dropped into the reaction vessel over 3 hours with stirring. Along with the dropping of the monomer pre-emulsified solution, an aqueous solution prepared by dissolving 1 part of APS (ammonium persulfate) as a polymerization initiator in 50 parts of water is uniformly dropped into the reaction vessel until the end of the dropping of the monomer pre-emulsified solution. did. After completion of the dropwise addition of the monomer pre-emulsion, the reaction was further continued at 80 ° C. for 1 hour, and then cooled. After cooling, an aqueous solution in which 2 parts of dimethylaminoethanol was dissolved in 20 parts of water was added to obtain a hydroxyl group-containing acrylic resin emulsion having a nonvolatile content of 40.6% by mass (solid content hydroxyl value 80 mgKOH / g; solid content acid value 10 .4 mg KOH / g). The obtained hydroxyl group-containing acrylic resin emulsion was adjusted to pH 7.2 using a 30% aqueous dimethylaminoethanol solution.

Production Example 3 Preparation of Melamine Resin A To a reaction vessel containing 128.2 g (4.0 mol) of methanol and 74.1 g (1.0 mol) of n-butanol, an aqueous sodium hydroxide solution was added to adjust the pH to 11.8. Was adjusted to 169.7 g (5.2 mol) of paraformaldehyde (92% CH ₂ O). After heating at 60 ° C. for 20 minutes to dissolve paraformaldehyde in methanol, 126.1 g (1.0 mol) of melamine was added, and the pH was adjusted to 13.0 with an aqueous sodium hydroxide solution. The reaction was carried out for 1 hour while distilling methanol out of the system at the reflux temperature, and further concentrated at atmospheric pressure until the internal temperature reached 110 ° C. Next, 320.4 g (10.0 mol) of methanol and 741.2 g (10.0 mol) of n-butanol were added, the pH was adjusted to 2.0 with sulfuric acid, and the reaction was performed at 30 ° C. for 3.5 hours. I let you. Thereafter, the pH was adjusted to 9.2 with an aqueous sodium hydroxide solution. The neutralized salt was removed by filtration, and the filtrate was concentrated under reduced pressure to obtain melamine resin A.

Production Example 4 Preparation of Melamine Resin B 973 g (12.0 mol) of 37% formalin and 1.0 mL of 50% aqueous sodium hydroxide solution were added to a reaction vessel and kept at 37 ° C., and then 126 g (1.0 mol) of melamine ) Was added. After the mixed solution was reacted at 90 ° C. for 25 minutes, 2000 g of room temperature water was added over 40 minutes to cool the mixed solution to 40 ° C. The slurry precipitate was filtered off and dried at room temperature. After adding 180 g of this dried precipitate and 350 g of methanol and heating to 29 ° C., 8.0 mL of a 70% nitric acid aqueous solution was added and the reaction temperature was kept at about 35 ° C. for 25 minutes to carry out an etherification reaction. After stopping the reaction by adding 50% aqueous sodium hydroxide solution to pH 9.8-10.5, 37% formalin 192 g and sodium hydroxide were added to adjust the pH to 9. Adjusted to 8-10.5. After removing 517 g of the fraction at 80 ° C. under reduced pressure, 350 g of methanol and 4.4 mL of 70% nitric acid were added and reacted at about 30 ° C. for 15 minutes. After adjusting the pH to 9.8-10.5 by adding 192 g of 50% sodium hydroxide and 37% formalin, 510 g was distilled off at 80 ° C. under reduced pressure. Similarly, 350 g of methanol and 4.4 mL of 70% nitric acid were added and reacted at about 30 ° C. for 15 minutes, and then 50% sodium hydroxide was added to adjust the pH to 9.8-10.5. Volatile components were distilled off at 120 ° C. under reduced pressure, and filtration was performed to obtain a melamine resin B which is a colorless, transparent and viscous liquid.

Production Example 5 Preparation of Melamine Resin C 973 g (12.0 mol) of 37% formalin and 1.0 mL of 50% aqueous sodium hydroxide solution were added to a reaction vessel and kept at 37 ° C., and then 126 g (1.0 mol) of melamine ) Was added. After the mixed solution was reacted at 90 ° C. for 25 minutes, 2000 g of room temperature water was added over 40 minutes to cool the mixed solution to 40 ° C. The slurry precipitate was filtered off and dried at room temperature. After adding 180 g of this dried precipitate and 350 g of methanol and heating to 29 ° C., 8.0 mL of a 70% nitric acid aqueous solution was added and the reaction temperature was kept at about 35 ° C. for 25 minutes to carry out an etherification reaction. After stopping the reaction by adding 50% aqueous sodium hydroxide solution to pH 9.8-10.5, 37% formalin 192 g and sodium hydroxide were added to adjust the pH to 9. Adjusted to 8-10.5. After removing 517 g of the fraction at 80 ° C. under reduced pressure, 350 g of methanol and 4.4 mL of 70% nitric acid were added and reacted at about 30 ° C. for 15 minutes. After adjusting the pH to 9.8-10.5 by adding 192 g of 50% sodium hydroxide and 37% formalin, 510 g was distilled off at 80 ° C. under reduced pressure. Similarly, 350 g of butanol and 4.4 mL of 70% nitric acid were added and reacted at about 30 ° C. for 15 minutes, and then 50% sodium hydroxide was added to adjust the pH to 9.8-10.5. Volatile components were distilled off at 120 ° C. under reduced pressure, followed by filtration to obtain a melamine resin C which was a colorless, transparent and viscous liquid.

Production Example 6 Preparation of Melamine Resin D In a reaction vessel containing 370.6 g (5.0 mol) of n-butanol, 169.7 g (5.2 mol) of paraformaldehyde (92% CH ₂ O) and 126. 1 g (1.0 mol) was added, the temperature was raised to 100 ° C., and the mixture was reacted for 20 minutes. Next, 0.33 g of 88% formic acid was added and further reacted at the same temperature for 20 minutes. Alkyl etherification was carried out by adding 500 g of xylene, maintaining the reflux temperature for 3 hours, and removing water from the reaction system using a decanter. Further, after distilling off xylene under reduced pressure, a predetermined amount of 2-ethylhexyl alcohol and 0.3 g of triethylamine were added to obtain a butylated melamine resin solution (melamine resin D) having a nonvolatile content of 65%.

Production Example 7 Preparation of Melamine Resin E To a reaction vessel containing 160.2 g (5.0 mol) of methanol, an aqueous sodium hydroxide solution was added to adjust the pH to 11.8, and then paraformaldehyde (92% CH ₂ O ) 169.7 g (5.2 mol) was added. After heating at 60 ° C. for 20 minutes to dissolve paraformaldehyde in methanol, 126.1 g (1.0 mol) of melamine was added, and the pH was adjusted to 13.0 with an aqueous sodium hydroxide solution. The reaction was carried out for 1 hour while distilling methanol out of the system at the reflux temperature, and further concentrated at atmospheric pressure until the internal temperature reached 110 ° C. Next, 640.8 g (20.0 mol) of methanol was added, the pH was adjusted to 2.0 with sulfuric acid, and the mixture was reacted at 30 ° C. for 3.5 hours. Thereafter, the pH was adjusted to 9.2 with an aqueous sodium hydroxide solution. The neutralized salt was filtered off, and the filtrate was concentrated under reduced pressure to obtain melamine resin E.

In the resulting melamine resin A~E than the molecular weight, triazine nucleus per imino group (- NH _-CH 2 OR;., Where R is H or an alkyl group) number of triazine nuclei per piece The number of methylol groups (—NH— CH ₂ OH ) and the ratio of alkyl groups in the alkyl etherified portion (methylation rate and butylation rate) are shown. The number of these groups and the ratio of alkyl groups are both average values. The proportion of the alkyl group can be determined from the peak area ratio by nuclear magnetic resonance spectroscopy (NMR method).

Production Example 8 Preparation of hydroxyl group-containing polyester resin In a reactor, 25.6 parts of isophthalic acid, 22.8 parts of phthalic anhydride, 5.6 parts of adipic acid, 19.3 parts of trimethylolpropane, 26.7 parts of neopentyl glycol, 17.5 parts of ε-caprolactone and 0.1 parts of dibutyltin oxide were added, and the temperature was raised to 170 ° C. with mixing and stirring. Thereafter, while the temperature was raised to 220 ° C. over 3 hours, water produced by the condensation reaction was removed until the acid value reached 8. Subsequently, 7.9 parts of trimellitic anhydride was added and reacted at 150 ° C. for 1 hour to obtain a polyester resin having an acid value of 40. Further, after cooling to 100 ° C., 11.2 parts of butyl cellosolve was added and stirred until uniform, and after cooling to 60 ° C., 98.8 parts of ion-exchanged water and 5.9 parts of dimethylethanolamine were added, and the solid content was 50% by mass. Thus, a hydroxyl group-containing aqueous polyester resin having a solid content acid value of 40, a hydroxyl value of 110, a number average molecular weight of 2870, and a glass transition temperature of −3 ° C. was obtained.

Example 1
Preparation of aqueous intermediate coating paint 139 parts of pigment dispersion paste obtained from Production Example 1, 24.6 parts of hydroxyl group-containing acrylic resin emulsion obtained from Production Example 2, 109.8 hydroxyl group-containing polyester resin obtained from Production Example 8 Parts, 15 parts of melamine resin B obtained from the production example, 20 parts of melamine resin A, Adecanol UH-814N (urethane associative thickener, active ingredient 30%, manufactured by Asahi Denka Kogyo Co., Ltd., trade name) 3.33 parts were mixed and stirred to obtain an aqueous intermediate coating.

Formation of multi-layer coating film A zinc phosphate-treated dull steel plate is electrodeposited with Powernics 110 (Cation Electrodeposition Paint, trade name, manufactured by Nippon Paint Co., Ltd.) so that the dry coating film has a thickness of 20 μm. A steel plate substrate was prepared by cooling after heat curing for 30 minutes. The obtained substrate was coated with the water-based intermediate coating material prepared in the above examples or production examples to a film thickness of 20 μm using the following robot coating machine. Next, preheating was performed at 80 ° C. for 5 minutes, and Aqualex AR-2000 Silver Metallic (trade name) (aqueous metallic base paint manufactured by Nippon Paint Co., Ltd.) was applied with a film thickness of 10 μm using the following robot coating machine. Minute preheating was performed. Furthermore, as a clear paint on the coated plate, Mac Flow O-1800W-2 Clear (Nippon Paint Co., Ltd. acid epoxy curable clear paint, product name) was applied by air spray coating to 35 μm, and then heated at 140 ° C. for 30 minutes. Curing was performed to obtain a test piece.
(Robot painting machine)
Model: ABB cartridge bell Linear speed: 600mm / s
Rotation speed: 25000rpm
Shaving air: 1.5 kg / cm ³
Applied voltage: -90kV

  The aqueous intermediate coating, aqueous base coating and clear coating were diluted under the following conditions and used for coating.

(Water-based intermediate coating)
Thinner: ion-exchanged water 40 seconds / NO. 4 Ford Cup / 20 ° C
The solid content of the paint was 54% by mass.

(Water-based paint)
Thinner: ion exchange water 45 seconds / NO. 4 Ford Cup / 20 ° C

(Clear paint)
Thinner: EEP (Ethoxyethyl propionate) / S-150 (Exxon aromatic hydrocarbon solvent, trade name) = 1/1 (mass ratio) mixed solvent 30 seconds / NO. 4 Ford Cup / 20 ° C

Examples 2-3 and Comparative Examples 1-4
An aqueous intermediate coating was prepared in the same manner as in Example 1 except that the type and amount of the melamine resin were changed to the types and amounts described in Tables 3 and 4. A multilayer coating film was formed in the same manner as in Example 1 using the obtained aqueous intermediate coating material and the aqueous base coating material and clear coating material used in Example 1.

Example 4
139 parts of pigment dispersion paste obtained from Production Example 1, 24.6 parts of hydroxyl group-containing acrylic resin emulsion obtained from Production Example 2, 140 parts of hydroxyl group-containing polyester resin obtained from Production Example 8, obtained from Production Example After 10 parts of melamine resin B and 10 parts of melamine resin A were mixed, 3.33 parts of Adecanol UH-814N (urethane associative thickener, active ingredient 30%, manufactured by Asahi Denka Kogyo Co., Ltd., trade name) were mixed and stirred. A waterborne intermediate coating was obtained.
A multilayer coating film was formed in the same manner as in Example 1 using the obtained aqueous intermediate coating material and the aqueous base coating material and clear coating material used in Example 1.

Comparative Example 5
139 parts of pigment dispersion paste obtained from Production Example 1, 24.6 parts of hydroxyl group-containing acrylic resin emulsion obtained from Production Example 2, 60 parts of hydroxyl group-containing polyester resin obtained from Production Example 8, obtained from Production Example After mixing 40 parts of melamine resin B and 20 parts of melamine resin A, 3.33 parts of Adecanol UH-814N (urethane associative thickener, active ingredient 30%, manufactured by Asahi Denka Kogyo Co., Ltd., trade name) are mixed and stirred. A waterborne intermediate coating was obtained.
A multilayer coating film was formed in the same manner as in Example 1 using the obtained aqueous intermediate coating material and the aqueous base coating material and clear coating material used in Example 1.

  The following evaluation was performed about the multilayer coating film obtained by the said Example and comparative example.

Finished Appearance of Multilayer Coating Film The finished appearance of the multilayer coating films obtained from the above Examples and Comparative Examples was evaluated with Wave Scan (surface roughness measuring instrument manufactured by BYK Gardner, trade name). The W2 value is a long-wavelength test and corresponds to coating film skin evaluation, and the W1 value is a short-wavelength test and corresponds to evaluation of a feeling of a coating film. These values indicate that the smaller the value, the better the appearance of the coating film. The results are shown in Tables 3 and 4. In addition, the quantity of each resin component in the following table | surface is resin solid content mass of each component contained in a water-based intermediate coating material.

Primary adhesion evaluation method A small-sized cutter knife is vertically applied to a coated specimen having a multilayer coating film obtained from the above Examples and Comparative Examples, and 11 parallel lines at equal intervals reaching the base are provided at intervals of 2 mm. Pulling 11 parallel lines that intersect perpendicularly to these parallel lines at 2mm intervals to engrave 100 squares of 2mm square surrounded by 4 straight lines, then adhesive tape (24mm wide) Was pressure-bonded without containing bubbles in the cut portion of the test coating film, and then rapidly pulled, and the adhesion between the intermediate coating film and the top coating film was evaluated from the degree of peeling of the top coating film. The number of grids peeled off is described.

  As shown in Table 3 above, the multilayer coating films obtained by the examples are all excellent in finished appearance, and adhesion (primary adhesion) between the intermediate coating film and the top coating film. Was also good. On the other hand, all of Comparative Examples 1 to 3 which do not contain an alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000 are It was confirmed that the finished appearance was inferior. Further, in Comparative Example 4 in which the amount of the alkyl etherified melamine resin was 5% by mass, an improvement in the finished appearance of the multilayer coating film was confirmed, but it was inferior to the present invention. In Comparative Example 5 in which the amount of the alkyl etherified melamine resin is 40% by mass, the finished appearance of the multilayer coating film is improved, while the adhesion between the intermediate coating film and the top coating film is greatly inferior. became.

  In the present invention, by adjusting the reaction curing rate of the aqueous intermediate coating film in the 3-coat 1-bake method using the aqueous intermediate coating composition and the aqueous base coating composition, the coating film properties of the multilayer coating film are maintained and excellent. A multilayer coating film having a coating film appearance can be provided. Since the coating film forming method of the present invention is also a method relating to a three-coat one-bake method using an aqueous intermediate coating material and an aqueous base coating material, it shortens the painting process, reduces the baking process and costs, and reduces the environmental burden Is possible.

Claims (5)

An aqueous intermediate coating is formed on the electrodeposition coating to form an intermediate coating, and then an aqueous base coating is applied on the intermediate coating to form a base coating. A method for forming a multilayer coating film by coating a clear coating on a film to form a clear coating film, and then simultaneously baking and curing the intermediate coating film, base coating film and clear coating film,
The aqueous intermediate coating is
A melamine resin comprising a hydroxyl group-containing acrylic resin emulsion, and an alkyl etherified melamine resin having an average number of imino groups per triazine nucleus of less than 1.0 and a number average molecular weight of less than 1000;
And the content of the alkyl etherified melamine resin contained in the aqueous intermediate coating is 10 to 35 parts by mass with respect to 100 parts by mass of the resin solid content.
A method for forming a multilayer coating film. The aqueous intermediate coating further contains a hydroxyl group-containing polyester resin, and the resin solid content mass ratio of each component contained in the aqueous intermediate coating, with respect to the resin solid content,
5-50% by mass of a hydroxyl group-containing acrylic resin emulsion,
5 to 80% by mass of a hydroxyl group-containing polyester resin, and 10 to 50% by mass of a melamine resin,
Is,
The method for forming a multilayer coating film according to claim 1. The hydroxyl group-containing acrylic resin emulsion contained in the aqueous intermediate coating material has an acid value of 3 to 50 mgKOH / g and a hydroxyl value of 10 to 150 mgKOH / g, and the hydroxyl group-containing polyester resin contained in the aqueous intermediate coating material is an acid. A value of 5 to 50 mg KOH / g and a hydroxyl value of 5 to 150 mg KOH / g.
The formation method of the multilayer coating film of Claim 1 or 2.   The water-based intermediate coating used in the formation method of the multilayer coating film in any one of Claims 1-3.   The multilayer coating film obtained by the formation method of the multilayer coating film in any one of Claims 1-3.