JP2009114392A - Water-borne intermediate coating material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water-borne intermediate coating material with which a multi-layer coating film excellent in designability and functionality can be formed even when an intermediate coating film, a base coating film and a clear coating film are formed by a three-coat one-bake coating process. <P>SOLUTION: The water-borne intermediate coating material comprises (a) a water-dispersible vinyl-modified polyester resin, (b) a water-soluble epoxy-modified polyester resin and (c) a melamine resin. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a water-based intermediate coating, and more specifically, a multilayer coating excellent in design and functionality when an intermediate coating, a base coating, and a clear coating are formed by a 3-coat 1-bake method. The present invention relates to an aqueous intermediate coating material capable of forming a film.

  In recent years, in the paint field, especially in the automobile painting field, in order to solve the problems of resource saving, cost saving and environmental load (VOC and HAPs etc.) reduction, conversion from organic solvent-based paints to water-based paints and shortening of painting processes have been made. There is a strong demand. In terms of shortening the painting process, in the conventional automotive paint finishing procedure, after the electrodeposition coating was baked, an intermediate coating was formed and baked once, and then a base coating and a clear coating were formed. A 3-coat 2-bake coating method, which is baked later, has been used normally. However, in recent years, after the electrodeposition coating is baked, an intermediate coating, a base coating, and a clear coating are applied by wet-on-wet, and these layers are not layered. A three-coat one-bake coating method has been developed in which cured coatings are baked together.

  In general, the coating film has a role related to design properties such as appearance improvement and a function related to functionality such as protection of an object to be coated, and the intermediate coating film is particularly required to have excellent performance related to the design properties and functionality. The For example, for an automotive intermediate coating, it is necessary to form a coating film that satisfies the coating performance such as the concealment property of the electrodeposition roughness (underlying concealment property) and chipping resistance.

  In order to obtain the above-mentioned coating film performance, conventionally, a specific organic solvent-based intermediate coating is used, and after applying the intermediate coating, baking is performed once, and then a base coating and a clear coating are applied. It has been formed, and by this method, it has been possible to conceal minute waviness of the base such as a steel plate or an electrodeposition coating film and to improve chipping resistance. In recent years, a coating technique has been developed for making the intermediate coating water-based and adapting to the 3-coat 1-bake method. For example, Patent Document 1 discloses an aqueous intermediate coating composition containing a specific polyester resin, and an intermediate coating film excellent in adhesion to the top coating film is formed by the coating composition.

  However, even in the above-described coating technology, it has not yet been solved to reduce the appearance deterioration of the multilayer coating film formation due to the influence of the base roughness, which was a problem of the 3-coat 1-bake method, and has an excellent coating film appearance. In order to obtain it, it was necessary to manage the groundwork. Therefore, it has been desired to develop an intermediate coating material and a coating method that can form a multilayer coating film that has a sufficient base concealing effect even in the 3-coat 1-bake method and is excellent in design and functionality.

JP 2002-126737 A

  The object of the present invention is to form an intermediate coating film excellent in smoothness and pigment dispersibility even when an intermediate coating film, a base coating film, and a clear coating film are formed by a three-coat one-bake method. To provide a water-based intermediate coating material capable of forming a multilayer coating film having excellent functionality such as property and chipping resistance, a method for forming a multilayer coating film, and a coating film formed article on which the multilayer coating film is formed is there.

When the present inventors form a multilayer coating film by a three-coat one-bake method, that is, an uncoated intermediate coating film by applying an aqueous intermediate coating composition on an object to be coated on which an electrodeposition coating film has been formed. A step (1) of forming an uncured base coating film by applying an aqueous base coating on the uncured intermediate coating film (1), and an uncured base coating film on the uncured base coating film A multilayer coating comprising a step (3) of applying a clear coating to form an uncured clear coating, and a step (4) of simultaneously heating and curing the intermediate coating, the base coating, and the clear coating. When forming a multilayer coating film by the forming method, it has been found that a sufficient base concealing effect can be obtained by increasing the solid content concentration in the intermediate coating film before application of the base coating. However, according to this coating method, the skin on the surface of the water-based intermediate coating film is deteriorated (smoothness is lowered), so it is difficult to improve the appearance of the multilayer coating film by this method.
However, as a result of further diligent investigations, the present inventors have found that an intermediate coating film having excellent base concealing property and smoothness is formed by using a specific water-based intermediate coating material, and further has excellent water resistance and chipping resistance. It has been found that a multilayer coating film having properties is also formed.
That is, the present invention provides the following (1) to (13).
(1) (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester resin and (c) a water-based intermediate coating comprising melamine resin,
(2) (a) A water-dispersible vinyl-modified polyester resin is a polyester resin having an acid value of 20 to 100 mg KOH / g, a neutralization degree of 70 to 100%, and a weight average molecular weight of 10,000 to 150,000, and (b) a water-soluble epoxy. The aqueous intermediate coating composition according to (1), wherein the modified polyester resin is a polyester resin having an acid value of 25 to 50 mgKOH / g, a degree of neutralization of 70 to 100%, and a weight average molecular weight of 5000 to 30000,
(3) (c) The melamine resin has a reactive group represented by the formula (I)

A methylol type which is a group represented by formula (II)

And one or more melamine resins selected from a methylol / imino type in which groups represented by the formulas (I) and (II) are mixed. The water-based intermediate coating composition according to the above (1) or (2),
(4) (c) The melamine resin has a reactive group represented by the formula (I)

A methylol type which is a group represented by formula (II)

And one or more melamine resins selected from methylol / imino types in which the groups represented by formulas (I) and (II) are mixed, and the reactive group is ( III) Formula

(R ¹ and R ² each independently represents an alkyl group having 1 to 4 carbon atoms.)
(C) The content of the complete alkyl ether type melamine resin is 60% by mass or less based on the total amount of the melamine resin (1) ) Or (2) water-based intermediate coating,
(5) The aqueous intermediate coating composition according to any one of (1) to (4), further including (d) a water-soluble epoxy resin,
(6) The mass ratio of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin is 5:95 to 95: 5 in terms of resin solids, and in terms of resin solids, (C) Content of melamine resin is 25-100 mass% with respect to the total amount of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin, in terms of resin solid content. (D) The content of the water-soluble epoxy resin is 3 to 25% by mass with respect to the total amount of (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester resin and (c) melamine resin. The aqueous intermediate coating composition according to the above (5),
(7) (e) Further containing crosslinked resin particles, and in terms of resin solids, the content of (e) crosslinked resin particles is (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester. The aqueous intermediate coating composition according to (5) or (6) above, which is 3 to 40% by mass with respect to the total amount of resin, (c) melamine resin and (d) water-soluble epoxy resin,
(8) Any of (1) to (7) above, further comprising (f) a pigment, wherein the content of (f) pigment is 20 to 70% by mass in terms of pigment weight concentration (PWC) Water-based intermediate coating paint according to
(9) A step (1) of forming an uncured intermediate coating film by applying a water-based intermediate coating composition on an object on which an electrodeposition coating film has been formed, on the uncured intermediate coating film (2) forming an uncured base coating by applying an aqueous base coating, and (3) forming an uncured clear coating by applying a clear coating on the uncured base coating. ), An aqueous intermediate coating material used in a method for forming a multilayer coating film comprising the step (4) of simultaneously heating and curing the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film. A waterborne intermediate coating composition according to any one of the above (1) to (8),
(10) A step (1) of forming an uncured intermediate coating film by applying a water-based intermediate coating composition on an object on which an electrodeposition coating film has been formed, on the uncured intermediate coating film (2) forming an uncured base coating by applying an aqueous base coating, and (3) forming an uncured clear coating by applying a clear coating on the uncured base coating. ), An uncured intermediate coating film, the uncured base coating film, and a step (4) of simultaneously curing the uncured clear coating film by heating, wherein the aqueous film A multilayer coating film forming method comprising using the water-based intermediate coating material according to any one of (1) to (8) above as a coating material;
(11) The method for forming a multilayer coating film according to (10) above, wherein a base paint containing an acrylic emulsion is used as the water-based base paint.
(12) The multilayer coating film according to the above (10) or (11), wherein preheating is performed between the step (1) and the step (2) and / or between the step (2) and the step (3). Forming method,
(13) A multilayer coating film formed by the multilayer coating film forming method according to any one of (10) to (12) is provided.

  ADVANTAGE OF THE INVENTION According to this invention, when forming an intermediate coating film, a base coating film, and a clear coating film by a 3 coat 1 baking system, the aqueous intermediate coating material which can form the multilayer coating film which is excellent in design property and functionality Is provided.

  The aqueous intermediate coating material of the present invention contains (a) a water-dispersible vinyl-modified polyester resin and (b) a water-soluble epoxy-modified polyester resin as a film-forming resin. That is, the aqueous intermediate coating material of the present invention contains a hydrophilic polyester resin as a coating film-forming resin, and a water-dispersible polyester resin and a water-soluble polyester resin are used in combination as the hydrophilic polyester resin. Here, the water-soluble resin refers to a resin that is soluble in water, and the water-dispersible resin refers to a resin that is not water-soluble but is finely dispersed in water, such as an emulsion or suspension.

  Whether the resin used in the present invention (the polyester resin and (d) the water-soluble epoxy resin) is water-soluble or water-dispersible is determined by whether turbidity is observed when mixed with water, that is, by the turbidity method. be able to. In making the judgment, it is preferable that the solvent is only water, but a small amount (about 5% by weight with respect to the resin / water mixture) may contain a hydrophilic solvent such as butyl cellosolve together with water. .

  The (a) water-dispersible vinyl-modified polyester resin used in the present invention is a water-dispersible polyester resin to which a fatty acid chain having a vinyl polymer portion is bonded. Preferably, (a) 15-45% by mass of the water-dispersible vinyl-modified polyester resin is the vinyl polymer part, and 10-50% by mass of the vinyl polymer part has a carboxyl group. It is a structural unit derived from a saturated monomer.

  (A) The water-dispersible vinyl-modified polyester resin is, for example, in the presence of an unsaturated fatty acid, a carboxyl group-containing α, β-ethylenically unsaturated monomer, and other copolymerizable α, β-ethylenic monomers. Vinyl-modified fatty acid having a vinyl polymer portion obtained by radical polymerization of an unsaturated monomer [hereinafter referred to as (a-1) vinyl-modified fatty acid. ] And a polyester resin having a hydroxyl group to be described later (hereinafter referred to as (a-2) hydroxyl group-containing polyester resin). ] Can be obtained by a method of condensing.

  Examples of the unsaturated fatty acids are derived from various (semi) drying oils and non-drying oils such as paulownia oil, linseed oil, soybean oil, safflower oil, castor oil, dehydrated castor oil, rice bran oil, cottonseed oil, and palm oil. A fatty acid is mentioned, These can be used individually or in combination of 2 or more types.

  The amount of unsaturated fatty acid used is preferably in the range of 20 to 70% by weight, more preferably in the range of 30 to 60% by weight, based on the total raw materials used when (a-1) vinyl-modified fatty acid is produced. By using a vinyl-modified fatty acid obtained using an unsaturated fatty acid in such a range, (a) the dispersion stability of the water-dispersible vinyl-modified polyester resin, the storage stability of the aqueous paint of the present invention, and the aqueous of the present invention The coating film properties such as water resistance of the multilayer coating film obtained by using the intermediate coating can be improved.

  Examples of the carboxyl group-containing α, β-ethylenically unsaturated monomer include α, β-ethylenically unsaturated monocarboxylic acid such as acrylic acid, methacrylic acid, and crotonic acid, maleic acid, fumaric acid, and itaconic acid. Α, β-ethylenically unsaturated dicarboxylic acids such as maleic acid, itaconic acid and other acid anhydrides, and monoesterified products of these acid anhydrides, and at least one of them is preferably used. It is more preferable to use methacrylic acid in that the physical properties of the resulting coating film can be improved.

  The amount of the carboxyl group-containing α, β-ethylenically unsaturated monomer used is such that 10 to 50% by mass of the vinyl polymer portion in the obtained (a) water-dispersible vinyl-modified polyester resin is carboxyl group-containing α, It is preferable to set so as to be a structural unit derived from a β-ethylenically unsaturated monomer. Within this range, (a) the water dispersibility of the water-dispersible vinyl-modified polyester resin can be improved, and the storage stability of the aqueous intermediate coating composition of the present invention can be improved, and further after drying It is also possible to obtain a coating film that is difficult to whiten.

  (A-1) Among vinyl-modified fatty acids, vinyl-modified fatty acids having a vinyl polymer portion having a carboxyl group and an aryl group (hereinafter referred to as aryl group-containing vinyl-modified fatty acids). ] Is preferably used. By using a vinyl-modified polyester resin obtained by condensation of an aryl group-containing vinyl-modified fatty acid and (a-2) a hydroxyl group-containing polyester resin, an aqueous intermediate coating material having excellent storage stability can be obtained.

  The aryl group-containing vinyl-modified fatty acid is, for example, in the presence of an unsaturated fatty acid, a carboxyl group-containing α, β-ethylenically unsaturated monomer, an aryl group-containing α, β-ethylenically unsaturated monomer, and It can be obtained by radical polymerization of other copolymerizable α, β-ethylenically unsaturated monomers.

  Examples of the aryl group-containing α, β-ethylenically unsaturated monomer include styrene derivatives having a functional group such as an alkyl group at each position of the aromatic ring of styrene or styrene. Examples of the styrene derivative include tertiary butyl styrene, α-methyl styrene, vinyl toluene, and the like.

  The amount of the aryl group-containing α, β-ethylenically unsaturated monomer is not particularly limited, but the total α, β-ethylenically unsaturated monomer used for the polymerization of the vinyl polymer portion having a carboxyl group and an aryl group is not limited. Preferably it is 20 mass% or more with respect to the total amount of a saturated monomer, More preferably, it is 30-70 mass%. By making the use amount of the aryl group-containing α, β-ethylenically unsaturated monomer within the above range, (a) the water dispersibility of the water dispersible vinyl-modified polyester resin is improved, and the aqueous intermediate coating of the present invention is used. The storage stability of the paint is also improved.

  As described above, when the (a-1) vinyl-modified fatty acid is produced, the carboxyl group-containing α, β-ethylenically unsaturated monomer or aryl group-containing α, β-ethylenically unsaturated monomer In addition, other copolymerizable α, β-ethylenically unsaturated monomers can be used in combination. Other copolymerizable α, β-ethylenically unsaturated monomers include the aforementioned carboxyl group-containing α, β-ethylenically unsaturated monomers and aryl group-containing α, β-ethylenically unsaturated monomers. Α, β-ethylenically unsaturated monomer other than the body, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, i- (meth) acrylate Alkyl esters of (meth) acrylic acid such as butyl, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and cyclohexyl (meth) acrylate can be used.

  As other copolymerizable α, β-ethylenically unsaturated monomers, within the scope of achieving the object of the present invention, 2-hydroxyethyl (meth) acrylate, 2-methacrylate (meth) acrylate is used. Hydroxyalkyl esters of (meth) acrylic acid such as hydroxypropyl can be used. Further, α, β-ethylenically unsaturated monomers having nonionic surface active ability such as methoxypolyethylene glycol mono (meth) acrylate, polyethylene glycol mono (meth) acrylate, propylene glycol polyethylene glycol mono (meth) acrylate Can also be used.

  (A-1) The vinyl-modified fatty acid can be produced by a method such as a solution polymerization method or a bulk polymerization method.

  According to the solution polymerization method, for example, in the presence of a polymerization initiator in an organic solvent, in an inert gas atmosphere, the above-described carboxyl group-containing α, β-ethylenically unsaturated monomer α, β- (A-1) A vinyl modified fatty acid can be obtained by adding ethylenically unsaturated monomers and unsaturated fatty acid intermittently or continuously, or by adding them all at once and keeping at about 70 to 150 ° C. Further, the polymerization initiator may be added in advance to the organic solvent as described above, but is added simultaneously when various α, β-ethylenically unsaturated monomers and unsaturated fatty acids are dropped. Also good.

  Examples of the organic solvent that can be used in this solution polymerization method include aromatic solvents such as toluene and xylene, ketone solvents such as methyl ethyl ketone and methyl isobutyl ketone, and ester solvents such as ethyl acetate and butyl acetate. It is done. In addition, (a-1) alcoholic solvents such as isopropanol and n-butanol, ethyl cellosolve, butyl cellosolve and the like within a range that does not adversely affect the condensation reaction between the vinyl-modified fatty acid and (a-2) hydroxyl group-containing polyester resin. A coal ether solvent or the like can be used.

  Examples of the polymerization initiator that can be used in the solution polymerization method include t-butyl peroxybenzoate, di-t-butyl peroxide, t-butyl peroxy 2-ethylhexanoate, and benzoyl peroxide. Organic peroxides and azo compounds such as 2,2′-azobisisobutyronitrile and 2,2′-azobis-2-methylbutyronitrile, and the organic peroxides described above are preferably used. Is done.

  Moreover, when manufacturing (a-1) vinyl modified fatty acid by a solution polymerization method, a chain transfer agent can be used as needed, and as such chain transfer agent, t-dodecyl mercaptan, normal dodecyl mercaptan Alkyl mercaptans such as normal octyl mercaptan, α-methylstyrene dimer, and the like are preferable.

  Further, according to the bulk polymerization method, for example, without using an organic solvent, α, β-ethylenically unsaturated monomers such as the above-described carboxyl group-containing α, β-ethylenically unsaturated monomers, And (a-1) vinyl-modified fatty acid can be produced by heating and mixing while adding unsaturated fatty acids all together or intermittently or continuously.

  At this time, (a-2) water dispersion is carried out by bulk polymerizing various α, β-ethylenically unsaturated monomers and unsaturated fatty acids in the presence of (a-2) hydroxyl group-containing polyester resin described later. The vinyl-modified polyester resin can be directly produced.

  Next, (a-2) hydroxyl group-containing polyester resin will be described. The (a-2) hydroxyl group-containing polyester resin has a hydroxyl group among those obtained by condensation reaction using polybasic acid and polyhydric alcohol as main components. The (a-2) hydroxyl group-containing polyester resin may have either a linear structure or a branched structure, and may be urethane-modified or silicone-modified according to the purpose of use.

  (A-2) The polybasic acid used for producing the hydroxyl group-containing polyester resin preferably contains 2 to 4 carboxyl groups in one molecule, such as phthalic acid, isophthalic acid, terephthalic acid, and succinic acid. Acid, maleic acid, itaconic acid, fumaric acid, tetrahydrophthalic acid, hexahydrophthalic acid, methyltetrahydrophthalic acid, adipic acid, sebacic acid, azelaic acid, hymic acid, trimellitic acid, methylcyclohexeric acid, pyromellitic acid, etc. And anhydrides thereof.

  The (a-2) polyhydric alcohol used in producing the hydroxyl group-containing polyester resin is preferably one containing 2 to 6 hydroxyl groups in one molecule, such as ethylene glycol, propylene glycol, neopentyl. Examples include glycol, butanediol, pentanediol, 1.4-cyclohexanedimethanol, trimethylolethane, trimethylolpropane, glycerin, trisisocyanurate, pentaerythritol and the like.

  In addition, (a-2) when producing a hydroxyl group-containing polyester resin, in addition to the above-mentioned polybasic acid, animal oil, vegetable oil and fatty acids obtained by hydrolyzing them as required, (Registered trademark) E ”(a glycidyl ester of a branched aliphatic monocarboxylic acid manufactured by Shell) and the like can be used in combination as long as the object of the present invention is achieved.

  Examples of the animal oil, vegetable oil and fatty acids obtained by hydrolyzing them include, for example, coconut oil, hydrogenated coconut oil, rice bran oil, tall oil, soybean oil, castor oil, and dehydrated castor oil. And the fatty acid obtained. It is preferable from the viewpoint of the storage stability of the aqueous paint that the amount of the animal oil, vegetable oil and fatty acid obtained by hydrolyzing them does not exceed 50% by mass in the (a-2) hydroxyl group-containing polyester resin.

  (A-2) A hydroxyl group-containing polyester resin can be produced by a condensation reaction using a polybasic acid and a polyhydric alcohol as main components. For example, the polyhydric alcohol is present in excess of the polybasic acid. Depending on the conditions, a melting method and a solvent method can be applied.

  (A-2) As a hydroxyl group-containing polyester resin, after synthesizing a polyester resin having a hydroxyl group as described above, an adduct (TMP) of tolylene diisocyanate or methylene bisphenyl isocyanate, or, in some cases, hexamethylene diisocyanate trimethylolpropane. A polyisocyanate such as modified HDI) subjected to polyaddition reaction and urethane-modified can also be used.

  (A-2) The hydroxyl group-containing polyester resin preferably has a hydroxyl value of 50 to 300 mgKOH / g, more preferably 100 to 250 mgKOH / g. If the hydroxyl value is within this range, the condensation reaction between the (a-1) vinyl-modified fatty acid and the (a-2) hydroxyl group-containing polyester resin can be smoothly advanced, and the water resistance of the resulting coating film is improved. Property and durability can also be improved.

  The (a) water-dispersible vinyl-modified polyester resin used in the present invention is produced, for example, by mixing and heating (a-1) a vinyl-modified fatty acid and (a-2) a hydroxyl group-containing polyester resin and causing a condensation reaction. can do. Under the present circumstances, the polybasic acid illustrated in manufacture of (a-2) hydroxyl-containing polyester resin can also be added further.

  The temperature of the above condensation reaction is not particularly limited, and is preferably 170 to 210 ° C. From the viewpoint of the reaction rate, it is desirable to select an appropriate temperature depending on the kind of α, β-ethylenically unsaturated monomer having a carboxyl group.

  The condensation reaction takes place between (a-2) the hydroxyl group of the hydroxyl group-containing polyester resin and (a-1) the carboxyl group of the vinyl-modified fatty acid. (A-1) The vinyl-modified fatty acid has a carboxyl group derived from an unsaturated fatty acid and a carboxyl group derived from a vinyl polymer portion. Among them, a carboxyl group derived from an unsaturated fatty acid, (a- 2) A condensation reaction with a hydroxyl group of the hydroxyl group-containing polyester resin is preferable from the viewpoint that the water dispersibility of the obtained (a) water-dispersible vinyl-modified polyester resin can be further improved.

  In particular, when the carboxyl group derived from the vinyl polymer portion is a carboxyl group derived from methacrylic acid, the reactivity of the carboxyl group is lower than the reactivity of the carboxyl group derived from an unsaturated fatty acid. A carboxyl group derived from a saturated fatty acid is more preferred because it exclusively participates in the condensation reaction.

  In addition, (a) the water-dispersible vinyl-modified polyester resin may be prepared by adding a small amount of an organic solvent in the presence of (a-2) a hydroxyl group-containing polyester resin separately from the above-described method. It can also be produced by adding and mixing monomers such as a saturated fatty acid and an α, β-ethylenically unsaturated monomer having a carboxyl group, followed by a temperature rise and a condensation reaction.

  In addition, (a) water-dispersible vinyl-modified polyester resin is (a-2) an α, β-ethylenic acid having a carboxyl group described above in (a-2) a polyester resin obtained by condensation reaction of a hydroxyl group-containing polyester resin and an unsaturated fatty acid. It can also be obtained by a method in which monomers such as unsaturated monomers are added, mixed and polymerized.

  Furthermore, (a) water-dispersible vinyl-modified polyester resin is obtained by polymerizing a carboxyl group-containing vinyl polymer to a polyester resin obtained by condensation reaction of (a-2) a hydroxyl group-containing polyester resin and an unsaturated fatty acid. You can also

  (A) The acid value of the water-dispersible vinyl-modified polyester resin is preferably 20 to 100 mgKOH / g, and more preferably 25 to 60 mgKOH / g. When the acid value is less than 20 mgKOH / g, the water dispersibility is lowered and the appearance of the coating film is liable to deteriorate. When the acid value is more than 100 mgKOH / g, the water resistance and durability of the coating film are liable to be lowered. The hydroxyl value is preferably 20 to 150 mgKOH / g, more preferably 40 to 150 mgKOH / g. When the hydroxyl value is less than 20 mgKOH / g, the coating film hardness is lowered and the adhesiveness with the base tends to be insufficient, and when it exceeds 150 mgKOH / g, the water resistance tends to be lowered.

(a) The degree of neutralization of the water-dispersible vinyl-modified polyester resin may be a value obtained by neutralizing all of the carboxyl groups of the resin, or may be a value obtained by neutralizing a part thereof. It is preferable to use one that is ˜100%. More preferably, it is 80 to 100%. If it is less than a lower limit, hydrophilicity will fall and it will become difficult to obtain water dispersibility, and a coating-film external appearance will deteriorate easily.
Here, in the present specification, the degree of neutralization of the resin means the amount of neutralized carboxyl groups (unit: mol) out of the total amount (unit: mol) of carboxyl groups possessed by 1 g of the resin (solid content). Mol) is expressed as a percentage (unit: mol%). For example, when all of the carboxyl groups of the resin are neutralized, the neutralization degree is 100%, and when a part of the carboxyl groups of the resin is neutralized, the neutralization degree is neutralized. It is a value obtained by dividing the amount of carboxyl groups (unit: mol) by the total amount of carboxyl groups (unit: mol) of the resin and converting the percentage.
Here, as the neutralizing agent used for neutralizing the carboxyl group, for example, ammonia; ethylamine, propylamine, butylamine, benzylamine, monoethanolamine, neopentanolamine, 2-aminopropanol, 3-aminopropanol, etc. Secondary monoamines such as diethylamine, diethanolamine, di-n-propanolamine, di-iso-propanolamine, N-methylethanolamine, N-ethylethanolamine; dimethylethanolamine, trimethylamine, triethylamine, Tertiary monoamines such as triisopropylamine, methyldiethanolamine, dimethylaminoethanol; diethylenetriamine, hydroxyethylaminoethylamine, ethylaminoethylamine, Amino compounds such as Le aminopropyl amine, sodium hydroxide, potassium hydroxide, can be used alkali metal hydroxides such as lithium hydroxide or the like. From the viewpoint of workability and the like, a tertiary monoamine is preferable.

  Moreover, it is preferable that 15-45 mass% of the (a) water-dispersible vinyl-modified polyester resin used in the present invention is the vinyl polymer portion. If it is less than 15% by mass, the pigment dispersibility tends to be insufficient, and if it exceeds 45% by mass, the water dispersibility is lowered and the appearance of the coating film tends to be deteriorated.

  The weight average molecular weight of the (a) water-dispersible vinyl-modified polyester resin is preferably in the range of 10,000 to 150,000, and more preferably in the range of 30000 to 100,000. If it is less than 10,000, the coating film hardness is lowered and the adhesion to the substrate tends to be insufficient, and if it exceeds 150,000, the resin viscosity tends to be high, and the solids concentration of the paint is lowered more than necessary. There is a risk of getting lost. In addition, a weight average molecular weight is the value which converted the measured value by the gel permeation chromatography (GPC) method by the polystyrene standard.

  The aqueous intermediate coating material of the present invention has excellent pigment dispersibility derived from the vinyl polymer portion of (a) water-dispersible vinyl-modified polyester resin. Therefore, since the pigment is uniformly dispersed in the intermediate coating film, an intermediate coating film having high design properties is formed by the coating material. In addition, this excellent pigment dispersibility makes it easy to use colored pigments that are prominent in non-uniformity, and by combining with colored pigments in the base coating and clear coating, a multilayer coating with higher design is formed. can do. Further, (a) hydrophilicity of the surface of the intermediate coating film is improved by using a water-dispersible vinyl-modified polyester resin.

  The (b) water-soluble epoxy-modified polyester resin used in the present invention is a water-soluble polyester resin having an epoxy group. (B) The water-soluble epoxy-modified polyester resin is a polyester resin having two or more hydroxyl groups in one molecule (hereinafter referred to as (b-2) a hydroxyl group-containing polyester resin). ] Can be obtained by modifying.

  (B-1) As an epoxy resin, what is normally used in the coating-materials field can be used. Specifically, Epicoat 828, 834, 836, 1001, 1004, 1007, DX-225 (above, manufactured by Shell Chemical Co., Ltd.), Araldite GY-260, 6071, 6084 (manufactured by Ciba Geigy), DER-330, 331, Bisphenol-type epoxy resins such as 660, 661, 66 (manufactured by Dow Chemical), Epicron 800, 830, 850, 860, 1050, 4050 (manufactured by Dainippon Ink Industries, Ltd.), DEN-431, 438 (Dow Chemical) Phenol novolac type epoxy resins such as Araldite CT508 (Ciba Geigy), DER732, 736 (Dow Chemical Co.) and other polyglycol type epoxy resins, ester type epoxy resins, chain aliphatic epoxy resins, alicyclic An epoxy resin, a polyol type epoxy resin, or the like can be used. Moreover, in each said epoxy resin, you may use what contains a halogen.

  The (b-2) hydroxyl group-containing polyester resin is obtained, for example, by polycondensation (ester reaction) of a polyhydric alcohol and a polybasic acid or an anhydride thereof. It can be produced by the same method as the polyester resin. Examples of the polyhydric alcohol, polybasic acid or anhydride thereof may include those exemplified in the production of the above (a-2) hydroxyl group-containing polyester resin. For example, the polyhydric alcohol hydroxyl group and the polybasic acid or the same What is necessary is just to make it react so that it may become 1.2-1.8 by the molar ratio with the carboxyl group of an anhydride.

  The (b) water-soluble epoxy-modified polyester resin can be synthesized by a reaction between (b-1) an epoxy resin and (b-2) a polyester resin, and examples thereof include reactions such as addition, condensation, and grafting. Specifically, the reaction between the epoxy group of (b-1) the epoxy resin and the carboxyl group or hydroxyl group of (b-2) the polyester resin, or when the polyisocyanate compound is used and (b-1) Reaction with the hydroxyl group of an epoxy resin and (b-2) polyester resin is mentioned.

  (B) The water-soluble epoxy-modified polyester resin preferably has a weight average molecular weight of 5,000 to 30,000, more preferably 10,000 to 25,000. If the weight average molecular weight is less than 5,000, the coating film hardness tends to decrease and the adhesion to the substrate tends to be insufficient. If it exceeds 30,000, the resin viscosity tends to increase, and the solid content concentration of the paint is more than necessary. There is a risk that it will have to be lowered. The hydroxyl value is preferably 75 to 200 mgKOH / g, more preferably 80 to 150 mgKOH / g. When the hydroxyl value is less than 75 mgKOH / g, the coating film hardness tends to decrease and the adhesion to the base tends to be insufficient, and when it exceeds 200 mgKOH / g, the resin viscosity tends to increase, and the paint solids There is a possibility that the concentration must be lowered more than necessary. The acid value is preferably 25 to 50 mgKOH / g, and more preferably 27 to 40 mgKOH / g. When an acid value is less than 25 mgKOH / g, water dispersibility will fall and a coating-film external appearance will deteriorate easily. If it exceeds 50 mgKOH / g, the resin viscosity tends to increase, and the solids concentration of the paint may have to be lowered more than necessary, and the storage stability of the paint tends to be lowered. (B) The neutralization degree of the water-soluble epoxy-modified polyester resin may be one obtained by neutralizing all of the carboxyl groups of the resin, or may be obtained by neutralizing a part of the resin. It is preferable to use 100%. More preferably, it is 80 to 100%. If it is less than a lower limit, hydrophilicity will fall and it will become difficult to obtain water solubility, and a coating-film external appearance will deteriorate easily.

  (B) By using an aqueous intermediate coating material containing a water-soluble epoxy-modified polyester resin, a multilayer coating film having excellent chipping resistance and water resistance can be formed. Moreover, the hydrophilicity of the intermediate coating film surface improves by using (b) water-soluble epoxy-modified polyester resin.

  Conventionally, after applying a water-dispersible paint, pre-heating treatment tends to make the surface hydrophobic, but (b) a coating film made of an aqueous paint containing a water-soluble epoxy-modified polyester resin has a surface that remains after the pre-heating treatment. It can be kept hydrophilic. The aqueous intermediate coating composition of the present invention utilizes this effect. That is, by first increasing the solid content concentration in the intermediate coating film by preheating treatment or the like, an excellent base concealing effect can be obtained. In addition, even if the smoothness of the surface of the intermediate coating film may be temporarily deteriorated by this treatment, the hydrophilicity of the surface of the intermediate coating film is maintained, so that the coating is applied on the surface by a wet-on-wet method. The moisture in the water-based base paint thus transferred moves to the intermediate coating film, and finally the smoothness of the surface of the intermediate coating film (interface with the base coating film) is improved. In the present invention, (b) a water-soluble epoxy-modified polyester resin is used together with (a) a water-dispersible vinyl-modified polyester resin as a film-forming resin, thereby obtaining a hydrophilic effect on the surface of the intermediate coating film. It is.

  In the aqueous intermediate coating material of the present invention, the mass ratio of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin is 5:95 to 95: 5 in terms of resin solid content. Is preferred. (A) When the content of the water-dispersible vinyl-modified polyester resin is small, the pigment dispersibility is inferior, so that the base concealing property by the intermediate coating film is insufficient, and the design properties of the multilayer coating film tend to be lowered. On the other hand, if the content of the water-soluble epoxy-modified polyester resin (b) is small, the surface of the intermediate coating film before application of the base coating tends to become hydrophobic, and the smoothing effect of the interface between the intermediate coating film and the base coating due to water migration It is difficult to obtain and the surface smoothness of the base coating film tends to deteriorate. Furthermore, water resistance and chipping resistance are also reduced. More preferably, the mass ratio of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin is 10:90 to 90:10, more preferably 15 in terms of resin solid content. : 85-85: 15.

  In the present invention, (c) a melamine resin is used as a curing agent. (C) The melamine resin is obtained by polymerizing methylol melamine obtained by the reaction of melamine and formaldehyde, and further etherifying the methylol group with alcohol. In the present invention, a water-soluble melamine resin is preferable from the viewpoint of coating film performance and water-based coating, and the reactive group is particularly preferably represented by the formula (I)

A methylol type which is a group represented by formula (II)

And one or more melamine resins selected from methylol / imino types in which groups represented by formulas (I) and (II) are mixed are preferred. Since the melamine resin having the group represented by the formula (I) and / or the formula (II) has high curability, a multilayer coating film having high coating performance such as chipping resistance can be obtained.

  From the viewpoint of design, (c) the melamine resin, the methylol type, imino type, or methylol / imino type melamine resin, and the reactive group is represented by the formula (III)

It is preferable to use together with the complete alkyl ether type melamine resin which is group represented by these. Here, in formula (III), R ¹ and R ² each independently represents an alkyl group having 1 to 4 carbon atoms. The complete alkyl ether type melamine resin has a slower reaction rate than other melamine resins, so the combination time can adjust the curing time, and the smoothness of the interface between the intermediate coating and the base coating Can be formed. When using together, (c) 60 mass% or less is preferable, and, as for content of a complete alkyl ether type melamine resin with respect to the melamine resin whole quantity, 10-50 mass% is more preferable. When it exceeds 60% by mass, the coating film performance tends to deteriorate.

  (C) The degree of polymerization of the melamine resin is preferably 3.0 or less, more preferably 1.5 or less. The degree of polymerization indicates the degree of condensation of the melamine resin. When the degree of polymerization exceeds 3.0, the base concealing property tends to decrease. The mononuclear ratio of the melamine resin is preferably 50% or more, more preferably 60% or more. The mononuclear body ratio is a ratio of a mononuclear melamine resin having a degree of polymerization of 1.0 contained in the entire melamine resin. When the ratio is less than 50%, the base concealing property tends to decrease. is there.

  Examples of commercially available melamine resins include Cymel (registered trademark) 235, 267, 303, 325 and 350, My Coat (registered trademark) M506 (both manufactured by Mitsui Cytec), and the like. .

  (C) Content of melamine resin is 25-100 mass% with respect to the total amount of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin in terms of resin solid content. Is preferred. If the amount is less than 25% by mass, chipping resistance and water resistance may be lowered. If the amount is more than 100% by mass, the appearance may be poor. Further, from the above viewpoint, it is preferably 30 to 80% by mass.

  The aqueous intermediate coating material of the present invention can further contain (d) a water-soluble epoxy resin. (D) The water-soluble epoxy resin is a compound having at least one hydrophilic portion and two or more epoxy groups in one molecule. In particular, those having an ether bond or a hydroxyl group as the hydrophilic site are preferred.

  (D) Particularly preferable examples of the water-soluble epoxy resin include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, and polypropylene glycol diglycidyl ether. These may be used alone or in combination of two or more. (D) By containing the water-soluble epoxy resin, the chipping resistance of the coating film is improved.

  (D) Examples of commercially available water-soluble epoxy resins include "Denacol (registered trademark)" series EX-810, EX-811, EX-851, EX-821, EX-830, EX manufactured by Nagase ChemteX Corporation. -832, EX-841, EX-861, 40E, 100E, 200E, 400E of "Epolite" series manufactured by Kyoeisha Chemical Co., Ltd., ethylene glycol such as 212, 214 of "Epolide (registered trademark) NT" series manufactured by Daicel Chemical Co., Ltd. Polyglycol diglycidyl ether, Nagase ChemteX "Denacol (registered trademark)" series EX-911, EX-941, EX-920, EX-921, EX-931, Kyoeisha "Epolite" series of 70P, 200P, 400P “Daicel Chemical Co., Ltd.” ) Diglycidyl ether of propylene glycol or polypropylene glycol 228 such as the NT "series, and the like.

  The content of the (d) water-soluble epoxy resin is 3 in terms of solid content with respect to the total amount of (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester resin, and (c) melamine resin. It is preferably ˜25% by mass. When the content is less than 3% by mass, the chipping resistance is insufficient, and when the content exceeds 25% by mass, the coating stability is poor. From the above viewpoint, it is more preferably 5 to 20% by mass, and further preferably 7 to 15% by mass.

  The aqueous intermediate coating material of the present invention may further contain (e) crosslinked resin particles. The (e) crosslinked resin particles used in the present invention are fine particles made of a polymer crosslinked so as to be insoluble in an organic solvent for paint. In particular, it is preferable to have a property of being stably dispersed in an aqueous intermediate coating, that is, hydrophilic. (E) A particulate carboxyl group-containing acrylic resin is used as the crosslinked resin particles. The carboxyl group-containing acrylic resin particles are usually obtained as an aqueous dispersion.

  (E) In order to produce an aqueous dispersion of crosslinked resin particles, it is usually an ethylenically unsaturated monomer having at least one carboxyl group in the molecule (hereinafter referred to as a carboxyl group-containing ethylenically unsaturated monomer). Other ethylenically unsaturated monomers are emulsion polymerized in an aqueous medium.

  Examples of the carboxyl group-containing ethylenically unsaturated monomer that can be suitably used include styrene derivatives, (meth) acrylic acid derivatives, and unsaturated dibasic acids. Preferred are (meth) acrylic acid derivatives, and more preferred are acrylic acid, methacrylic acid, acrylic acid dimer and α-hydro-ω-((1-oxo-2-propenyl) oxy) poly (oxy ( 1-oxo-1,6-hexanediyl)).

  Other ethylenically unsaturated monomers that can be suitably used here are ethylenically unsaturated compounds that can be radically copolymerized with carboxyl group-containing ethylenically unsaturated monomers, for example, (meth) acrylates having no reactive functional group (For example, methyl acrylate, methyl methacrylate, ethyl acrylate, and ethyl methacrylate), polymerizable aromatic compounds (for example, styrene, α-methyl styrene, vinyl ketone, and vinyl naphthalene), hydroxyl-containing unsaturated compounds (for example, 2- Hydroxyethyl acrylate and 2-hydroxyethyl methacrylate), polymerizable amides (eg acrylamide, methacrylamide, N-methylol methacrylamide and N-methoxymethyl acrylamide), polymerizable nitriles (eg acrylonite) And methacrylonitrile), vinyl halides (such as vinyl chloride, vinyl bromide and vinyl fluoride), α-olefins (such as ethylene and propylene), vinyl esters (such as vinyl acetate and vinyl propionate), and Examples include dienes (such as butadiene and isoprene).

In addition, it is necessary to use two or more radically polymerizable ethylenically unsaturated compounds in the molecule as other ethylenically unsaturated monomers in order to provide sufficient crosslinking so that the crosslinked resin particles do not dissolve in the solvent. is there. Specifically, polymerizable unsaturated monocarboxylic acid ester of polyhydric alcohol (e.g., ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, etc.), polybasic acid Polymerizable unsaturated alcohol esters (such as diallyl terephthalate, diallyl phthalate and triallyl trimellitate), aromatic compounds substituted with two or more vinyl groups (such as divinylbenzene), and epoxy group-containing ethylenic compounds An adduct of an unsaturated monomer and a carboxyl group-containing ethylenically unsaturated group monomer (for example, a reaction product of glycidyl acrylate or glycidyl methacrylate with acrylic acid, methacrylic acid, crotonic acid, and maleic acid) It is below.

  Such other ethylenically unsaturated monomers may be used alone or in admixture of two or more.

  The blending ratio in the emulsion polymerization of the carboxyl group-containing ethylenically unsaturated monomer and the other ethylenically unsaturated monomer is based on the total amount of the ethylenically unsaturated monomer used to produce the (e) crosslinked resin particles, The carboxyl group-containing ethylenically unsaturated monomer is 1 to 50% by mass, preferably 10 to 40% by mass, and the other ethylenically unsaturated monomer is 99 to 50% by mass, preferably 90 to 60% by mass. Among the other ethylenically unsaturated monomers, the two or more radically polymerizable ethylenically unsaturated compounds in the molecule are preferably 1 to 10% by mass, preferably based on the total amount of the ethylenically unsaturated monomers. Is 1-7% by mass, more preferably 1-5% by mass.

  The emulsion polymerization reaction is carried out by dissolving an emulsifier in water or an aqueous medium containing an organic solvent such as alcohol if necessary, and dropping an ethylenically unsaturated monomer and a polymerization initiator under heating and stirring. . An ethylenically unsaturated monomer emulsified in advance using an emulsifier and water may be similarly added dropwise.

  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 such as anionic 4,4'-azobis (4-cyanovaleric acid) and cationic 2,2'-azobis (2-methylpropionamidine)); Redox oily peroxides (e.g. benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide and t-butyl perbenzoate), and aqueous peroxides (e.g. potassium persulfate and ammonium peroxide) Can be mentioned.

  As the emulsifier, those usually used by those skilled in the art can be used. However, reactive emulsifiers such as Antox MS-60 (manufactured by Nippon Emulsifier), Eleminol (registered trademark) JS-2 (Sanyo Chemical Industries) And Aqualon (registered trademark) HS-10 (Daiichi Kogyo Seiyaku) 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 initiator relative to the total amount of unsaturated compounds is generally from 0.1 to 5% by weight, preferably from 0.5 to 2% by weight.

  The crosslinked resin particles produced in an aqueous medium can be used as they are, but the fine resin particles are isolated by a method such as filtration, spray drying, freeze drying, etc., and the particle size is adjusted as it is or using a mill or the like. It can also be used after pulverization.

  (E) The particle diameter of the crosslinked resin particles is preferably in the range of 0.01 to 10 μm, and more preferably in the range of 0.01 to 1.0 μm. When the particle diameter is less than 0.01 μm, the effect of improving workability is small, and when it exceeds 10 μm, the appearance of the obtained coating film may be deteriorated. The particle size can be adjusted by methods well known to those skilled in the art. For example, it is possible by pulverization using the above-mentioned mill or the like or by changing the monomer composition.

  (E) The acid value of the aqueous dispersion of crosslinked resin particles is preferably 5 to 80 mgKOH / g, and more preferably 10 to 70 mgKOH / g. When the acid value is less than 5 mgKOH / g, the effect of improving workability is small, and when it exceeds 80 mgKOH / g, the water solubility increases and the particle property may be lost.

  (E) The aqueous dispersion of crosslinked resin particles can be neutralized with a base and used at pH 5-10. In this pH range, (e) the stability of the crosslinked resin particle aqueous dispersion is high. This neutralization is preferably carried out by adding a tertiary amine such as dimethylethanolamine and triethylamine before or after polymerization.

  The aqueous intermediate coating composition of the present invention contains (e) crosslinked resin particles, so that when the intermediate coating film and the base coating film are formed wet-on-wet, the appearance of the coating film is improved. Mixing of coating film components of the coating film and the base coating film can be prevented. Therefore, water migration from the base coating film can be achieved while preventing mixing of coating film components, and the design of the multilayer coating film can be improved.

  (E) The content of the crosslinked resin particles is (a) a water-dispersible vinyl-modified polyester resin, (b) a water-soluble epoxy-modified polyester resin, (c) a melamine resin, and (d) a water-soluble epoxy resin. 3-40 mass% is preferable with respect to the total amount. If it is less than 3% by mass, there is a possibility that a mixed layer of both coatings may be formed when the intermediate coating and base coating are formed wet-on-wet in combination with an aqueous base coating, exceeding 40% by mass. And there exists a possibility that the smoothness of a multilayer coating film may fall. From the above viewpoint, it is more preferably 5 to 30% by mass, still more preferably 8 to 25% by mass.

  The aqueous intermediate coating composition of the present invention may contain (f) a pigment. Examples of the pigment include coloring pigments and extender pigments. By adding the pigment, it is possible to conceal fine irregularities on the surface of the electrodeposition coating film, which is the base of the intermediate coating film, and to improve the smoothness of the coating film. Color pigments include azo lake pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perylene pigments, quinophthalone pigments, dioxazine pigments, quinacridone pigments, isoindolinone pigments Organic pigments such as pigments and metal complexes, inorganic pigments such as yellow lead, yellow iron oxide, bengara, titanium dioxide, carbon black and the like, as extender pigments, barium sulfate, barium carbonate, calcium carbonate, magnesium carbonate, Examples thereof include silica and talc. The pigment weight concentration (PWC) is preferably 20 to 70% by mass, more preferably 25 to 60% by mass.

  The aqueous intermediate coating composition of the present invention can contain other additives as long as the effects of the present invention are not hindered, such as an anti-settling agent such as polyamide wax and polyethylene wax, an ultraviolet absorber, an antioxidant, and a leveling agent. And surface conditioners such as silicone and organic polymers, anti-sagging agents, thickeners, antifoaming agents, and organic compounds such as cellulose alkyl ethers. The content of the additive is preferably 15% by mass or less and more preferably 10% by mass or less with respect to the entire coating material in terms of solid content.

  The method for obtaining the aqueous intermediate coating composition of the present invention is not particularly limited, and the above-mentioned resin and pigment blends are kneaded using a kneader, roll, etc., dispersed using a sand grind mill, disper, etc. All methods known to those skilled in the art can be used.

The water-based intermediate coating is combined with the water-based base coating to form a composite coating by wet-on-wetting with the water-based intermediate coating and the water-based base. Can migrate. Due to the transfer of moisture, the viscosity of the contact surface of the intermediate coating film with the base coating film is reduced, and the unevenness is relaxed. As a result, the effect of improving the smoothness of the surface of the base coating film is obtained. In particular,
A step (1) of forming an uncured intermediate coating film by applying the above-mentioned aqueous intermediate coating composition on an object on which an electrodeposition coating film is formed;
A step (2) of forming an uncured base coating by applying an aqueous base coating on the intermediate coating;
A step (3) of applying a clear coating onto the uncured base coating to form an uncured clear coating;
Step (4) of simultaneously heating and curing the intermediate coating film, the base coating film, and the clear coating film to obtain a multilayer coating film
It can be used as a water-based intermediate coating in a method for forming a multilayer coating film containing.
When the aqueous intermediate coating composition of the present invention is applied to the method for forming a multilayer coating film including the steps (1) to (4), the surface of the electrodeposition coating film is obtained due to the above-described effect of the combination of the aqueous intermediate coating composition and the aqueous base coating composition. These fine irregularities are canceled, and a multilayer coating film with good smoothness can be formed.

In particular, by performing preheating between the step (1) and the step (2) and reducing the amount of water while leaving the intermediate coating film uncured (increasing the solid content concentration), It is possible to prevent the base coating film from being mixed with the coating film and the water-based base coating, and to obtain a further excellent base concealing effect. In addition, since the surface of the intermediate coating film by the aqueous intermediate coating composition of the present invention can maintain hydrophilicity even after the preheating, a multilayer coating film having excellent smoothness is formed by moisture transfer from the base coating film. can do.
Moreover, by performing preheating between the said process (2) and the process (3), the water | moisture content in a base coating film can be transferred to an intermediate coating film during the said preheating, As a result, a multilayer coating film Can improve the smoothness. The preheating can also be performed between the steps (1) and (2) and between the steps (2) and (3). In that case, the preheating is performed in the steps (1) and ( The effect of performing preheating between 2) and the effect of performing preheating between the steps (2) and (3) can be achieved.

  In the said multilayer coating-film formation method, it does not specifically limit as a to-be-coated object, For example, iron, copper, aluminum, tin, zinc, etc .; Alloys and castings containing these metals are mentioned. Specific examples include automobile bodies and parts such as passenger cars, trucks, motorcycles, and buses. These metals are particularly preferably those that have been previously subjected to chemical conversion treatment with phosphate, chromate or the like before electrodeposition coating.

In the multilayer coating film forming method, a cationic electrodeposition coating is applied on an object to be coated to form a cationic electrodeposition coating. The cationic electrodeposition paint is not particularly limited, and a known cationic electrodeposition paint can be used. Examples of such cationic electrodeposition paints include paint compositions containing a cationic base resin and a curing agent.
Although it does not specifically limit as a cationic base resin, For example, it describes in the amine-modified epoxy resin type | system | group described in Japanese Patent Publication No.54-4978, Japanese Patent Publication No.56-34186, etc., Japanese Patent Publication No.55-115476, etc. Amine-modified polyurethane polyol resin system, amine-modified polybutadiene resin system described in JP-B-62-61077, JP-A-63-86766, etc., JP-A-63-139909, JP-B-1-60516 Examples include amine-modified acrylic resin systems described in JP-A No. 6-128351, sulfonium group-containing resin systems described in JP-A-6-128351, and the like. In addition to those described in the above references, phosphonium group-containing resin systems and the like can also be used. Among the above cationic base resins, it is particularly preferable to use an amine-modified epoxy resin system.
Examples of the curing agent include, but are not limited to, amino resins and block polyisocyanate compounds.

  In the multilayer coating film forming method, the electrodeposition coating film is heated and cured, and then the aqueous intermediate coating composition of the present invention is applied to form an intermediate coating film. The method of applying the intermediate coating is not particularly limited. For example, an air electrostatic spray called “react gun”; commonly called “micro / micro (μμ) bell”, “micro (μ) bell”, “metabell”, etc. This can be done by using a rotary atomizing electrostatic coating machine or the like. A method using a rotary atomizing electrostatic coating machine or the like is preferable.

  Although the dry film thickness of the said intermediate coating film changes with uses, it is preferable that it is 5-50 micrometers. If the upper limit is exceeded, defects such as sagging or bake-curing may occur during painting, and if the lower limit is exceeded, the appearance may be reduced.

In the multilayer coating film forming method, the base coating film is formed by applying an aqueous base coating material in an uncured state of the intermediate coating film (that is, by a wet-on-wet method). In this specification, uncured includes not only the case where preheating is not performed, but also the state after preheating.
The preheating is a step in which the applied intermediate coating is not cured after application of the intermediate coating, for example, it is allowed to stand or heat at room temperature to less than 100 ° C. for 1 to 10 minutes. In the method, it can carry out for adjustment of solid content. In particular, when the aqueous intermediate coating composition of the present invention is used, the solid content of the intermediate coating film is preferably 70% or more, more preferably 75% or more, in a state before applying the aqueous base coating composition. When the solid content of the intermediate coating film is 70% or more, an excellent base concealing effect can be obtained. In addition, when the solid content concentration is 70% or more, the smoothness of the surface of the intermediate coating film is lost, and the design of the multilayer coating film is deteriorated. Is applied by a wet-on-wet method, the smoothness of the interface between the intermediate coating film and the base coating film is improved, and a multilayer coating film having excellent design properties is obtained.

The base paint is not particularly limited as long as it is a water-based base paint, and examples thereof include water-based film-forming resins, curing agents, pigments, and other additives. The aqueous film-forming resin is not particularly limited, and examples thereof include acrylic resins, polyester resins, alkyd resins, epoxy resins, and urethane resins. The form of the aqueous film-forming resin is not particularly limited, and examples thereof include water dispersibility (emulsion type) and water solubility.
As the water-based base paint, it is preferable to use an acrylic emulsion and / or a water-soluble acrylic resin from the viewpoint of coating performance such as coating workability, weather resistance, water resistance, etc. Among them, an acrylic emulsion is more preferable.
The base paint can contain a curing agent such as a melamine resin and an epoxy resin, and among them, a melamine resin is preferable from the viewpoint of various performances and cost of the obtained coating film. From the viewpoint of improving curability at low temperatures, it is also preferable to add a block isocyanate resin, a carbodiimide compound, or an oxazoline compound in combination.
The base paint may contain the above-described viscosity control agent and other additives as appropriate in order to prevent familiarity with the clear coating film and to ensure coating workability.

  The water-based base paint can be used as a metallic base paint by blending with a glitter pigment, and without blending the glitter pigment, a color pigment such as red, blue or black, and if necessary, a body pigment. It can also be used as a solid base paint. The glitter pigment is not particularly limited, and examples thereof include non-colored or colored metallic glitter materials such as metals or alloys and mixtures thereof, interference mica powder, colored mica powder, white mica powder, graphite, or colorless colored flat pigment. Etc. Since it is excellent in dispersibility and can form a highly transparent coating film, non-colored or colored metallic glitter materials such as metals or alloys and mixtures thereof are preferred. Specific examples of the metal include aluminum, aluminum oxide, copper, zinc, iron, nickel, tin and the like.

The shape of the glitter pigment is not particularly limited, and may be further colored, for example, a scaly shape having an average particle diameter (D ₅₀ ) of 2 to ₅₀ μm and a thickness of 0.1 to 5 μm. Those are preferred. Those having an average particle size in the range of 10 to 35 μm are more preferable because of excellent glitter. The pigment concentration (PWC) in the base paint of the glitter pigment is generally 23% by mass or less. When it exceeds 23% by mass, the appearance of the coating film is deteriorated. Preferably, it is 0.01-20 mass%, More preferably, it is 0.01-18 mass%.

  As the pigment other than the glitter pigment, the color pigments and extender pigments described in the aqueous intermediate coating composition of the present invention can be used. As said pigment, it can be used 1 type or in combination of 2 or more types among a luster pigment, a coloring pigment, and an extender pigment. The pigment concentration (PWC) in the aqueous base paint including the glitter pigment and all other pigments is generally 0.1 to 50% by mass, preferably 0.5 to 40% by mass. More preferably, it is 1-30 mass%. When it exceeds 50% by mass, the coating film appearance is deteriorated. As for the water-based base paint, examples of other additives used and methods for preparing the base paint include those exemplified for the water-based intermediate paint.

  Examples of the coating method of the water-based base paint include the methods exemplified when applying the intermediate paint. In the case where the aqueous base paint is applied to an automobile body or the like, in order to enhance the design, multi-stage painting by air electrostatic spraying, preferably painting in two stages, or air electrostatic spraying and the above-mentioned It is preferable to carry out by a coating method in combination with a rotary atomizing type electrostatic coating machine.

  Although the dry film thickness of the said base coating film changes with uses, it is preferable that it is 5-35 micrometers. If the upper limit is exceeded, sharpness may deteriorate, or defects such as unevenness and flow may occur during painting, and if the lower limit is exceeded, color unevenness may occur.

  As described above, the intermediate coating film before application of the water-based base paint has a high solid content concentration of 70% or more when obtaining the base concealing effect. As a result, even if the surface of the intermediate coating film is temporarily inferior in smoothness, moisture in the aqueous base coating material is transferred to the surface layer of the intermediate coating film by applying the aqueous base coating material by the wet-on-wet method. Then, the surface of the intermediate coating film (interface between the intermediate coating film and the base coating film) is smoothed. The intermediate coating composition of the present invention is hydrophilic and prevents water from becoming hydrophobic even in the intermediate coating film after preheating. can do. In addition, the intermediate coating composition of the present invention suppresses the mixing of coating film components, although the above water transfer occurs. As a result, the glossiness does not deteriorate, and a multilayer coating film with high design properties can be formed. Furthermore, since it has excellent pigment dispersibility in addition to the above-mentioned mixing prevention effect, it is possible to use a color pigment in an intermediate coating film, and by combining a color pigment in an intermediate coating film and a color pigment in a base coating film The appearance can be improved, and a multilayer coating film with high design properties can be formed.

  In the method for forming a multilayer coating film, a clear coating film is formed by applying a clear coating material in a state where the base coating film is uncured. The clear coating is used to smooth the unevenness and flickering of the base coating caused by the bright pigment when the metallic base coating containing the bright pigment is used as the base coating, and to protect the base coating. Is formed. The clear coating is not particularly limited, and examples thereof include those composed of a film-forming resin, a curing agent, and other additives.

  The film-forming resin is not particularly limited, and examples thereof include acrylic resins, polyester resins, epoxy resins, urethane resins, and the like. These are used in combination with a curing agent such as an amino resin and / or a blocked 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.

  As the clear paint, since the above-mentioned aqueous base paint is applied in an uncured state, it may contain a viscosity control agent as an additive in order to prevent familiarity, reversal or sagging between layers. preferable. The addition amount of the viscosity control agent is 0.01 to 10 parts by mass, preferably 0.02 to 8 parts by mass, and more preferably 0.03 to 6 parts by mass with respect to 100 parts by mass of the resin solid content of the clear paint. Part. When it exceeds 10 parts by mass, the appearance is deteriorated, and when it is less than 0.1 parts by mass, the viscosity control effect cannot be obtained, which causes problems such as sagging.

  The clear coating composition may be any of organic solvent type, aqueous type (water-soluble, water-dispersible, emulsion), non-aqueous dispersion type, and powder type. If necessary, a curing catalyst, a surface conditioner, etc. Can be used.

  The clear coating material can be prepared and applied according to conventional methods. Although the dry film thickness of the said clear coating film changes with uses, it is 10-70 micrometers. If the dry film thickness exceeds the upper limit, the sharpness may be deteriorated, and defects such as unevenness and flow may occur during coating. If the dry film thickness is less than the lower limit, the appearance may be deteriorated.

  In the multilayer coating film forming method, the intermediate coating film, the base coating film, and the clear coating film are heated and cured simultaneously to obtain a multilayer coating film.

  The temperature for the heat curing is preferably 110 to 180 ° C, more preferably 120 to 160 ° C, whereby a cured coating film having a high degree of crosslinking can be obtained. If it exceeds 180 ° C., the coating film tends to be hard and brittle, and if it is less than 110 ° C., curing tends to be insufficient. The curing time varies depending on the curing temperature, but is suitably from 120 to 160 ° C. for 10 to 60 minutes. The film thickness of the multilayer coating film obtained by the coating film forming method of the present invention is usually 30 to 300 μm, preferably 50 to 250 μm. When it exceeds 300 μm, film physical properties such as a cooling cycle are lowered, and when it is less than 30 μm, the strength of the film itself is lowered.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by this example. In addition, unless otherwise indicated, a part represents a mass part and% represents mass%.

[Production Example 1] ((a) Production of water-dispersible vinyl-modified polyester resin)
(A-1) Production of vinyl-modified fatty acid A 3-liter four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen gas inlet tube was charged with the amount of fatty acid and xylene 650 parts shown in Table 1. While stirring, the temperature was raised to 130 ° C., and a mixture of vinyl polymerizable monomers and a polymerization initiator was added thereto over 3 hours. The mixture was stirred overnight at 130 ° C. and cooled to 80 ° C., and then 350 parts of methyl ethyl ketone was added to obtain a solution of (a-1) vinyl-modified fatty acid A. The non-volatile content of the solution is 50% by mass.

(A-2) Production of hydroxyl group-containing polyester resin Into a 3 liter four-necked flask equipped with a stirrer, a thermometer, a reflux condenser with a dehydration trap, and a nitrogen gas introduction tube, a raw material and dibutyltin having the composition shown in Table 2 0.5 parts by mass of oxide was charged, the temperature was raised to 220 ° C., and a dehydration condensation reaction was performed. At this time, a part of the resin solution was taken out, and the reaction was continued until the acid value of the solution when diluted with butyl cellosolve until the non-volatile content became 60% by mass was the value shown in Table 2. (a) Water dispersion A solid (a-2) hydroxyl group-containing polyester resin A, which is a raw material for the conductive vinyl-modified polyester resin, was obtained.

(A) Manufacture of water-dispersible vinyl-modified polyester resin Into a 3 liter four-necked flask equipped with a stirrer, a thermometer, a reflux condenser with a dehydration trap, and a nitrogen gas inlet tube, the raw materials having the composition shown in Table 3 were prepared. The mixture was gradually heated up to 180 ° C. to distill off xylene and methyl ethyl ketone, and a dehydration condensation reaction was performed. Here, the reaction was continued until the acid value of the solution when diluted with butyl cellosolve until the nonvolatile content was 50% by mass was 16.5 mgKOH / g. After completion of the reaction, the mixture was stirred at 150 ° C. for 1 hour, triethylamine was added at 90 ° C., and the mixture was stirred at the same temperature for 1 hour. Thereafter, ion-exchanged water was added so that the nonvolatile content was 40% by mass, and an aqueous dispersion of (a) water-dispersible vinyl-modified polyester resin A having the properties shown in Table 3 was obtained.

  As a result of judging the obtained (a) water-dispersible vinyl-modified polyester resin A by the following turbidity method, it was confirmed to be a water-dispersible resin.

Turbidity method (another method for judging water solubility and water dispersibility)
The said judgment was performed by following (i)-(iii).
(I) A resin to be measured is mixed in 100 g of a mixed solvent of water and butyl cellosolve, and at 20 ° C., the resin is 40% by weight in terms of resin solids, 55% by weight of water, and 5% by weight of butyl cellosolve. Adjust the mixture so that.
(Ii) A type 5 character is placed on the bottom of the glass beaker containing the mixed solution, and a person with a visual acuity of 1.5 tries to interpret the type 5 character from 0.5 m above.
(Iii) The resin is “water-soluble” when it is transparent enough to interpret the No. 5 type, and the resin is “water-dispersible” when not transparent to the extent that the No. 5 type can be read. It is judged that.

Table 3 shows property values of the water-dispersible vinyl-modified polyester resin A.

(B) Water-soluble epoxy-modified polyester resin Commercially available epoxy-modified polyester resins shown below were used.
Epoxy-modified polyester resin (manufactured by Dainippon Ink and Chemicals, trade name Watersol (registered trademark) S-370, hydroxyl value 100, acid value 40, weight average molecular weight 17000, uniform liquid type, solid content 55%)
When this resin was judged based on the turbidity method, it was confirmed to be water-soluble.

(C) Melamine resin The following commercially available melamine resins were used.
Melamine resin A (Mitsui Cytec, trade name: Cymel (registered trademark) 325, methyl group-containing imino type, number of nuclei 1)
Melamine resin B (Mitsui Cytec, trade name: Cymel (registered trademark) 303, methyl group-containing fully alkyl type, number of nuclei 1)

[(D) Water-soluble epoxy resin]
A commercially available epoxy resin (manufactured by Nagase Chemtech, trade name: Denacol (registered trademark) EX-861) was used.
When the epoxy resin was judged based on the turbidity method, it was confirmed to be water-soluble.

[Production Example 2] ((e) Production of crosslinked resin particles)
185 parts of deionized water was charged into a 500 ml reaction vessel equipped with a nitrogen introduction tube, a stirrer, a cooler, a temperature controller and a dropping funnel, and the temperature was raised to 83 ° C. 20 parts of Aronics (registered trademark) M-5300 (manufactured by Toagosei Co., Ltd.), 5.9 parts of dimethylethanolamine and 80 parts of styrene were charged into a dropping funnel to prepare a monomer solution. Then, the monomer solution was dropped into the reaction vessel over 2 hours. Further, 1 part of 4,4'-azobis-4-cyanovaleric acid was neutralized with 0.55 part of dimethylethanolamine, and a solution dissolved in 40 parts of deionized water was added dropwise as an initiator simultaneously with the monomer solution. . Further, stirring was continued at 83 ° C. for 1 hour, followed by cooling to obtain a milky white emulsion. The resulting emulsion of crosslinked resin particles had a solid content of 30% and a particle size of 100 nm (laser light scattering method).

[Examples 1 to 12, Comparative Examples 1 and 2] (Production of intermediate coating and multilayer coating)
Manufacture of intermediate coating paint A stainless steel container is charged with a predetermined amount of (a) water-dispersible vinyl-modified polyester resin, a predetermined amount of pigment, and the same amount of glass beads as shown in Table 4, and 2 hours in a paint shaker. Dispersed. Next, after a predetermined amount of (b) water-soluble epoxy-modified polyester resin, (c) melamine resin, (d) water-soluble epoxy resin, and (e) crosslinked resin particles were stirred for 5 minutes, glass beads were removed, and Ford cup no. The viscosity was adjusted so as to be 50 seconds (20 ° C.) at 4 to obtain an aqueous intermediate coating.
Preparation of multi-layer coating film Power deposition (registered trademark) U-50 (cation paint electrodeposition made by Nippon Paint Co., Ltd.) Electrodeposition coating with a dry film thickness of 30 μm on dull steel sheet treated with zinc phosphate And baked at 160 ° C. for 30 minutes. The above-mentioned aqueous intermediate coating was applied to the plate by rotary atomization coating to a film thickness of 20 μm and preheated at 80 ° C. for 3 minutes. The solid content concentration of the intermediate coating film after preheating was 85% by mass. In addition, the measurement of the solid content concentration of the coating film after the preheating of this aqueous intermediate coating was performed as follows.
A 150 × 100 mm aluminum foil whose mass was measured in advance was coated and pre-heated under the same conditions as the coating of the water-based intermediate coating on the dull steel plate to obtain an uncured water-based intermediate coating. The aluminum foil on which this uncured intermediate coating film was formed was quickly measured for the folding mass, and the solid content concentration was calculated.
After the preheating, Aqualex AR-2100 (Nippon Paint aqueous base paint) was applied by air spray coating to a film thickness of 12 μm and preheated at 80 ° C. for 3 minutes. Further, Macflow (registered trademark) O-1820 clear (clear paint manufactured by Nippon Paint Co., Ltd.) was applied to the coated plate by air spray coating to a film thickness of 35 μm, and then baked at 140 ° C. for 30 minutes. On the other hand, the base skin evaluation test material was prepared by baking at 140 ° C. for 30 minutes after applying the base paint.

  The following measurements and tests were performed on the obtained multilayer coating film. The results are shown in Table 5.

[Base skin appearance]
After applying the base paint, evaluation was performed according to the following criteria using a base skin evaluation test agent prepared by baking at 140 ° C. for 30 minutes. In addition, the said evaluation criteria are common with the following evaluation criteria of a multilayer coating-film external appearance.
◎: Unevenness is not confirmed on the surface and gloss is quite good. ○: Unevenness is not confirmed on the surface, and gloss is good. △: Unevenness is slightly confirmed on the surface, and the gloss is subtracted. ×: There is unevenness on the surface. , Glossy

[Multilayer coating appearance]
(Glossy)
The 20 ° gloss was measured using “PICOGLOSS Model 500MC” manufactured by ERICHSEN.
(PGD measurement)
The sharpness and glossiness (Gd) value of the multilayer coating film was measured using a portable sharpness gloss meter “PGD IV type” manufactured by Nippon Color Research Laboratory. The Gd value is an index of the sharpness of an image reflected on a surface. The larger the value, the higher the sharpness of the surface.
(Visually)
Evaluation was performed according to the same criteria as the above-mentioned evaluation of the base skin appearance.

[Coating performance]
(water resistant)
The coating film was immersed in warm water at 40 ° C. for 1 day, and changes in coating film appearance were visually evaluated according to the following criteria.
◎: No abnormality ○: Slightly whitening △: Whitening ×: Dissolution (chipping resistance)
Using a chipping resistance gravel tester (manufactured by Suga Test Instruments Co., Ltd.), the test was conducted under the following conditions and evaluated visually.
The size of the stone The amount of No. 7 crushed stone 50g
Distance 35cm
Air pressure 4.0 kg / cm ²
Angle 45 °
Test temperature -20 ° C
Evaluation criteria A: No peeling at all.
○: Slight peeling is observed.
Δ: Peeling of 1 mmφ or less is observed.
X: The peeling is conspicuous.

  ADVANTAGE OF THE INVENTION According to this invention, when forming an intermediate coating film, a base coating film, and a clear coating film by a 3 coat 1 baking system, the aqueous intermediate coating material which can form the multilayer coating film which is excellent in design property and functionality Is provided. With the aqueous intermediate coating composition of the present invention, the production of a multilayer coating film having excellent coating performance can be achieved together with the conversion from an organic solvent-based paint to a water-based paint and the shortening of the coating process.

Claims (13)

A water-based intermediate coating comprising (a) a water-dispersible vinyl-modified polyester resin, (b) a water-soluble epoxy-modified polyester resin, and (c) a melamine resin. (A) The water-dispersible vinyl-modified polyester resin is a polyester resin having an acid value of 20 to 100 mgKOH / g, a neutralization degree of 70 to 100%, and a weight average molecular weight of 10,000 to 150,000,
The water-based intermediate coating composition according to claim 1, wherein the water-soluble epoxy-modified polyester resin is a polyester resin having an acid value of 25 to 50 mgKOH / g, a neutralization degree of 70 to 100%, and a weight average molecular weight of 5000 to 30000. (C) The melamine resin has a reactive group represented by the formula (I)

A methylol type which is a group represented by formula (II)

And one or more melamine resins selected from a methylol / imino type in which groups represented by the formulas (I) and (II) are mixed. The aqueous intermediate coating material according to claim 1 or 2. (C) The melamine resin has a reactive group represented by the formula (I)

A methylol type which is a group represented by formula (II)

And one or more melamine resins selected from methylol / imino types in which groups represented by formulas (I) and (II) are mixed,
The reactive group has the formula (III)

(R ¹ and R ² each independently represents an alkyl group having 1 to 4 carbon atoms.)
A fully alkyl ether type melamine resin, which is a group represented by the formula (1), is used in combination, and (c) the content of the complete alkyl ether type melamine resin is 60% by mass or less based on the total amount of the melamine resin. Or the water-based intermediate coating material according to 2. (D) The waterborne intermediate coating composition according to any one of claims 1 to 4, further comprising a water-soluble epoxy resin. The mass ratio of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin is 5:95 to 95: 5 in terms of resin solids,
In terms of resin solid content, the content of (c) melamine resin is 25 to 100% by mass with respect to the total amount of (a) water-dispersible vinyl-modified polyester resin and (b) water-soluble epoxy-modified polyester resin. ,
In terms of resin solid content, the content of (d) water-soluble epoxy resin is based on the total amount of (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester resin and (c) melamine resin. The water-based intermediate coating composition according to claim 5, which is 3 to 25% by mass. (E) Further containing crosslinked resin particles, and in terms of resin solid content, the content of (e) crosslinked resin particles is (a) water-dispersible vinyl-modified polyester resin, (b) water-soluble epoxy-modified polyester resin, ( The aqueous intermediate coating composition according to claim 5 or 6, which is 3 to 40% by mass based on the total amount of c) melamine resin and (d) water-soluble epoxy resin. The aqueous solution according to any one of claims 1 to 7, further comprising (f) a pigment, wherein the content of the (f) pigment is 20 to 70% by mass in terms of pigment weight concentration (PWC). Paint. A step (1) of forming an uncured intermediate coating film by applying a water-based intermediate coating material on an object on which an electrodeposition coating film has been formed; an aqueous coating composition on the uncured intermediate coating film; A step (2) of applying a base coating to form an uncured base coating, a step (3) of applying a clear coating on the uncured base coating to form an uncured clear coating, The water-based intermediate coating used in the method for forming a multilayer coating comprising the step (4) of simultaneously heating and curing the uncured intermediate coating, the uncured base coating, and the uncured clear coating. The water-based intermediate coating material in any one of 1-8. A step (1) of forming an uncured intermediate coating film by applying a water-based intermediate coating material on an object on which an electrodeposition coating film has been formed; an aqueous coating composition on the uncured intermediate coating film; A step (2) of applying a base coating to form an uncured base coating, a step (3) of applying a clear coating on the uncured base coating to form an uncured clear coating, A multilayer coating film forming method comprising a step (4) of simultaneously heating and curing an uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film,
A method for forming a multilayer coating film, comprising using the aqueous intermediate coating composition according to claim 1 as the aqueous intermediate coating composition. The multilayer coating film forming method according to claim 10, wherein a base paint containing an acrylic emulsion is used as the water-based base paint. The multilayer coating film forming method according to claim 10 or 11, wherein preheating is performed between the step (1) and the step (2) and / or between the step (2) and the step (3). The multilayer coating-film formed by the multilayer coating-film formation method in any one of Claims 10-12.