JP2003251276A - Method of forming multilayered coating film and water based intermediate coating material composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of forming a multilayered coating film by which a coating film exhibiting excellent chipping resistance, impact resistance and appearance is formed in a 3-wet coating system aiming the reduction of coating process, cost saving and the decrease of environmental load. <P>SOLUTION: In the multilayered coating film forming method composed of a process (1) of applying a water based intermediate coating material to form an uncured intermediate coating film on a material to be coated on which an electrodeposition coating film is formed, a process (2) of applying a water based base coating material to form an uncured base coating film, a process (3) of applying a clear coating material to form an uncured clear coating lime and a process (4) of heating and curing the intermediate coating film, the base coating film and the clear coating film simultaneously to form the multilayered coating film, a coating film obtained by applying the water based intermediate coating material on a polypropylene-made test plate and baking in a state of a single film has the stress-strain characteristic of ≤27,000 kg/cm<SP>2</SP>Young's modulus and ≥4.2% elongation measured at 20°C. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for forming a multilayer coating film and an aqueous intermediate coating composition.

[0002]

2. Description of the Related Art In recent years, in the field of paints, especially in the field of automobile coatings, water-based paints have attracted attention in order to solve the problem of reducing environmental load (VOC, etc.). Water-based paints are generally prepared by incorporating a hydrophilic functional group such as an anionic functional group or a cationic functional group into a resin and combining it with a counterion to form a salt-forming film-forming resin in water or alcohol. It is water-soluble, water-dispersed or emulsified in a low-pollution hydrophilic medium such as.

Of these, when an anionic functional group such as a carboxyl group, a sulfonic acid group, a sulfuric acid ester group or a phosphoric acid ester group is introduced into the coating film forming resin, a basic ion such as amine is used as a counter ion. The substance is being used.

On the other hand, conventionally, in the coating process for automobile coating, an uncured base coating film is formed by coating a base coating composition on an intermediate coating film obtained by applying an intermediate coating composition and heating and curing. After that, a clear coating is applied, and the coating is heated and cured at the same time by a three-coat, two-bake coating method. In this coating method, the Young's modulus and the elongation rate of the intermediate coating film in the resulting coating film are obtained. It is possible to impart good chipping resistance to the resulting multilayer coating film.

However, in the three-coat, two-bake coating method, after the intermediate coating material is applied and cured by heating to obtain the intermediate coating film, the intermediate coating film is repaired before the base coating material is applied. Therefore, it is necessary to design a relatively hard coating film, and it has been practically difficult to design an intermediate coating film having a low Young's modulus and a high elongation.

On the other hand, in recent years, after the electrodeposition coating is baked after the electrodeposition coating, three coating steps of an intermediate coating, a base coating and a clear coating are applied on it by wet-on-wet. Attempts have been made to save resources and energy by reducing the number of baking steps by a three-wet coating system that performs batch baking.

[0007]

SUMMARY OF THE INVENTION In view of the above situation, the present invention provides a three-wet coating system aiming at shortening of coating process, cost reduction and environmental load reduction, which is excellent in chipping resistance and impact resistance and has an excellent appearance. It is an object of the present invention to provide a method for forming a multilayer coating film capable of forming a coating film having the same.

[0008]

The present invention comprises a step (1) of forming an uncured intermediate coating film by coating an aqueous intermediate coating composition on an object to be coated on which an electrodeposition coating film has been formed. Step (2) of forming an uncured base coating film by applying an aqueous base coating on the above intermediate coating film, and further coating a clear coating on it to form an uncured clear coating film. The method for forming a multilayer coating film comprises the step (3) of: and the 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. The above-mentioned water-based intermediate coating is a stress-strain characteristic of a coating film obtained by baking on a polypropylene test plate and then baking as a single film.
Young's modulus of 27,000 Kg / at measurement temperature of -20 ° C
The method for forming a multilayer coating film is characterized in that it has a cm ² or less and an elongation rate of 4.2% or more.

The water-based intermediate coating composition is an acrylic emulsion.
Resin, melamine resin, dispersant pigment dispersion paste and
It is composed of a thickener, and includes step (1), step (2) and
And the coating film formed in step (3) is subjected to step (4).
Unit area 1mm before ^TwoVolatile bases in
The total amount of organic substances is 7 × 10^-6must be below mmol
And are preferred.

The present invention is also an aqueous intermediate coating composition used in a method for forming a multilayer coating film, wherein the aqueous intermediate coating composition is an acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste and a thickening agent. Consists of agents
The weight average molecular weight of the acrylic emulsion resin is 1
It is a water-based intermediate coating composition characterized by being 00000-200000. Hereinafter, the present invention will be described in detail.

In the method for forming a multilayer coating film of the present invention, a step (1) of forming an uncured intermediate coating film by coating an aqueous intermediate coating composition on an object to be coated on which an electrodeposition coating film has been formed. A step (2) of forming an uncured base coating film by applying an aqueous base paint on the intermediate coating film, and further applying a clear paint thereon to form an uncured clear coating film. It comprises a forming step (3) and a 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.

[0012]Process (1) In the method for forming a multilayer coating film of the present invention, the above step (1)
Is a water-based intermediate coating on the object on which the electrodeposition coating is formed.
In the process of coating the material to form an uncured intermediate coating film
It

[0013]Water-based intermediate coating The above water-based intermediate coating hides the base,
Ensures surface smoothness (improves appearance), impact resistance, chipping resistance
It is applied to impart coating properties such as
is there.

The above-mentioned waterborne intermediate coating composition is obtained by coating a polypropylene test plate and baking it as a single film so that the stress-strain characteristic of the coating film is Young's modulus 27 at a measurement temperature of -20 ° C.
It is not particularly limited as long as it is 000 Kg / cm ² or less and the elongation rate is 4.2% or more. That is, after coating the water-based intermediate coating composition on a polypropylene test plate,
The stress-strain characteristics of a coating film obtained by baking a single film have a Young's modulus of 27,000 Kg / cm ² or less and an elongation rate of 4.2% or more at a measurement temperature of −20 ° C.

A coating film obtained by applying the above water-based intermediate coating composition to a polypropylene test plate and baking it as a single film is
The above water-based intermediate coating composition was air-sprayed onto a polypropylene test plate of 10 × 30 × 2 mm to give a dry film thickness of 40 μm.
And a coating film obtained by baking at 140 ° C. for 30 minutes.

The above-mentioned stress-strain characteristic means that the coating film obtained after baking is peeled from the test plate, and the peeled coating film is used as a test piece to measure temperature -20 ° C., measurement length 50 mm, pulling speed 10 mm / min. It means Young's modulus and elongation measured by conducting a tensile test under the conditions of, for example, "Autograph AGS-G type" and "analog meter" (both manufactured by Shimadzu Corporation). Can be measured.

The Young's modulus can be obtained from the slope of the chart of the relationship between stress and strain immediately after the start of the tensile test. The elongation is obtained from the length of the test piece before starting the tensile test and the length until the test piece breaks in the tensile test, for example, in the tensile test, the axis of the test piece before the test. When the length in the direction is L ₀ (mm) and the length in the axial direction until the test piece breaks L (mm), the elongation percentage is (L−L ₀ ) × 100 / L. _It means ₀ and can be obtained from the chart in the above tensile test.

In the case of 3 coat 2 bake, the base paint is applied after the intermediate paint is applied, baked and hardened. Normally, the repair process of the intermediate paint film formed before applying the base paint is performed. become. In order to be able to carry out this step, it is necessary to harden the intermediate coating film formed to a certain extent. Usually, the Young's modulus of the intermediate coating film is usually about 30,000 to 35,000 Kg / cm ² , and the elongation is usually 2 The paint design of the intermediate coating paint will be carried out so as to be about 3%. On the other hand, since the method for forming a multilayer coating film of the present invention is a 3-wet coating system, there is no need to perform the above-mentioned repair process and it is not necessary to harden the coating film formed by the intermediate coating film to some extent. The aqueous intermediate coating composition can be designed so that the Young's modulus of the coating film is 27,000 Kg / cm ² or less and the elongation rate is 4.2% or more.

Since the Young's modulus is 27,000 Kg / cm ² or less and the elongation is 4.2% or more, the multilayer coating film obtained by the above multilayer coating film forming method has excellent chipping resistance and impact resistance. It will be excellent.

The Young's modulus is 27,000 Kg / cm.^Two
It is the following. 27,000 Kg / cm^TwoIf you exceed
May be inferior to ping level. Upper limit 27,000 Kg
/ Cm ^Two, Lower limit 17,000 Kg / cm^TwoLike to be
Good The elongation rate is 4.2% or more. 4.2%
If it is less than 1, the chipping level may be poor.
The lower limit is preferably 4.2% and the upper limit is 23%.

An acrylic emulsion resin is preferably used as the coating film-forming resin in the above-mentioned aqueous intermediate coating composition. By using the acrylic emulsion resin, it is possible to reduce the amount of amine in the water-based intermediate coating composition, it is possible to prevent yellowing of the resulting multilayer coating film, and the Young's modulus of the coating film. 27,000 Kg /
cm ² or less, the elongation rate can be 4.2% or more,
The obtained multilayer coating film can have excellent chipping resistance and impact resistance.

The weight average molecular weight of the acrylic emulsion resin obtained by gel permeation chromatography (GPC) has a lower limit of 100,000 and an upper limit of 200.
It is preferably 000. This makes it possible to increase the viscosity of the uncured intermediate coating film formed in the above step (1), prevent a layer mixture between the intermediate coating film and the base coating film, and finish the coating film. The appearance can be made excellent. If it is less than 100,000, physical properties with good chipping resistance cannot be obtained, and if it exceeds 200,000, the undercoat hiding property as an intermediate coating is lowered, and it may be difficult to obtain an excellent finished appearance.

As a method for measuring the weight average molecular weight of the acrylic emulsion resin by gel permeation chromatography (GPC), a conventionally known method can be used.

The glass transition temperature (Tg) of the acrylic emulsion resin preferably has a lower limit of -50 ° C and an upper limit of 20 ° C. If it is less than -50 ° C, the hardness of the multilayer coating film becomes too low, and if it exceeds 20 ° C, the Young's modulus is 2700.
It may not be possible to set the elongation rate to 0 Kg / cm ² or less and the elongation rate to 4.2% or more.

The acrylic emulsion resin has an acid value of 1 (mg KOH / g resin) as the lower limit and 100 (mg as the upper limit).
KOH / g resin) is preferable. When it is less than 1, the coating film adhesion may be poor and curing may be poor, and when it exceeds 100, the hydrophilicity may be too high and the water resistance of the coating film may be deteriorated. More preferably, the lower limit is 3 and the upper limit is 70.

When the acrylic emulsion resin has a hydroxyl group, the hydroxyl value has a lower limit of 30,
The upper limit is preferably 150. If it is less than 30,
There is a case where curing failure occurs, and when it exceeds 150, excess hydroxyl groups remain in the coating film after curing, resulting in poor water resistance. More preferably, the lower limit is 40 and the upper limit is 100.

The method for producing the acrylic emulsion resin is not particularly limited and can be appropriately selected according to the kind and properties of the desired acrylic emulsion resin, for example, melt polymerization method, transesterification method, interface. A conventionally known production method such as a polymerization method or a solution polymerization method can be used.

The above-mentioned water-based intermediate coating composition preferably contains a thickening agent. Thereby, the viscosity before performing the above step (2) becomes higher, the mixed layer between the intermediate coating film and the base coating film can be further suppressed, and the finish appearance of the obtained coating film is excellent. Can be one.

The thickener is not particularly limited, and examples thereof include swollen dispersion of fatty acid amide, amide fatty acid,
Polyamide-based thickeners such as long-chain polyaminoamide phosphates; polyethylene-based thickeners such as colloidal swelling dispersions of oxidized polyethylene; organic acid smectite clays
Examples thereof include organic bentonite-based thickeners such as montmorillonite; inorganic pigments such as aluminum silicate and barium sulfate; and flattened pigments that exhibit viscosity depending on the shape of the pigment. Among them, the associative thickener is preferable from the viewpoint of preventing mixed layers.

The addition amount of the above-mentioned thickening agent is 0.01 parts by mass as the lower limit and 10 parts by mass as the upper limit, preferably 0.02 as the lower limit, with respect to 100 parts by mass of the resin solid content of the aqueous intermediate coating composition.
Parts by mass, upper limit of 8 parts by mass, more preferably lower limit of 0.03
Parts by mass, the upper limit is 6 parts by mass. If it exceeds 10 parts by mass,
If the amount is less than 0.1 part by mass, the thickening effect cannot be obtained, and there is a possibility that a mixed layer may occur between the intermediate coating film and the base coating film.

The above aqueous intermediate coating composition preferably contains a melamine resin as a curing agent. By using an acrylic emulsion resin and a melamine resin in combination in the above aqueous intermediate coating composition, it is easier to achieve a Young's modulus of the coating film of 27,000 Kg / cm ² or less and an elongation rate of 4.2% or more. can do.

In the above melamine resin, the reactive group is N
- _{(CH 2 OR)} 2 in which complete alkyl type, N-(CH
₂ OR) methylol type is _{CH 2} OH, N- _(CH 2
OR) H, an imino type, N- (CH ₂ OR) H and N-
There can be exemplified four types of imino / methylol type in which (CH ₂ OR) CH ₂ OH is mixed, but a complete alkyl type is preferable from the viewpoint of the underlayer hiding property.

Examples of commercially available melamine resins include Cymel 235, Cymel 236, Cymel 267, Cymel 327, Cymel 350, Cymel 370, Cymel 325 (all manufactured by Mitsui Cytec). it can. These melamine resins may be used alone or in combination of two or more.
Among them, Cymel 327, Cymel 370, and Cymel 325, which can obtain sufficient curability in a 3 wet coating system, are preferable. These may be used alone or in combination of two or more.

The solid content mass ratio of the coating film-forming resin and the curing agent in the aqueous intermediate coating composition is 85/15 to 50.
It is preferably / 50. If the ratio exceeds 85/15, the curability may decrease, and if it is less than 50/50, various properties of the obtained coating film may decrease.

The above-mentioned aqueous intermediate coating composition preferably contains a dispersant pigment dispersion paste obtained by previously dispersing a pigment and a pigment dispersant substantially free of a volatile basic substance. In the present specification, the phrase "substantially free of a volatile basic substance" means that the solid content of the pigment dispersant does not contain a volatile basic substance at all or contains 3% by mass or less. Is meant.

In the present invention, the volatile base in the intermediate coating film formed from the aqueous intermediate coating composition is prepared by making the above pigment dispersant substantially free of a volatile basic substance. The amount of the functional substance is reduced, and yellowing of the obtained multilayer coating film can be suppressed. Therefore, when the content of the volatile basic substance exceeds 3% by mass, the obtained multilayer coating film turns yellow and the finished appearance is inferior. Preferably, it does not contain a volatile basic substance at all, that is, it does not use an amine-neutralized pigment-dispersing resin that is generally used in the past.

The volatile basic substance has a boiling point of 30.
It means a basic substance of 0 ° C. or lower. For example,
Mention may be made of inorganic and organic nitrogen-containing basic substances. Examples of the inorganic basic substance include ammonia and the like, and examples of the organic basic substance include:
C1-C20 linear or branched alkyl group-containing primary or tertiary amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, dimethyldodecylamine; monoethanol Amine,
C1-C20 linear or branched hydroxyalkyl group-containing primary or tertiary amines such as diethanolamine and 2-amino-2-methylpropanol; dimethylethanolamine, diethylethanolamine and other C1-C20 1 to 1 containing a straight-chain or branched alkyl group and a straight-chain or branched hydroxyalkyl group having 1 to 20 carbon atoms
Tertiary amines; substituted or unsubstituted chain polyamines having 1 to 20 carbon atoms such as diethylenetriamine and triethylenetetramine; substituted or unsubstituted cyclic monoamines having 1 to 20 carbon atoms such as morpholine, N-methylmorpholine and N-ethylmorpholine. An amine such as a substituted or unsubstituted cyclic polyamine having 1 to 20 carbon atoms such as piperazine, N-methylpiperazine, N-ethylpiperazine, and N, N-dimethylpiperazine.

The pigment dispersant is a resin having a structure containing a pigment-affinity portion and a hydrophilic portion, and does not substantially contain the volatile basic substance. Examples of the pigment-affinitive portion and the hydrophilic portion include nonionic, cationic and anionic functional groups,
Two or more kinds of the above functional groups may be contained in one molecule of the pigment dispersant.

Examples of the nonionic functional group include, for example,
Examples thereof include a hydroxyl group, an amide group, and a polyoxyalkylene group, and examples of the cationic functional group include:
Examples thereof include an amino group, an imino group and a hydrazino group. Further, examples of the anionic functional group include a carboxyl group, a sulfonic acid group, and a phosphoric acid group. Such a pigment dispersant can be manufactured by a method well known to those skilled in the art.

The above-mentioned pigment dispersant is not particularly limited as long as it does not substantially contain a volatile basic substance, but it is preferable that the pigment dispersant can efficiently disperse the pigment with a small amount of the pigment dispersant. .

As the pigment dispersant, a commercially available one may be used. Examples of the commercially available product include Disperbyk 190 and Disperby.
k 182, Disperbyk 184 (all manufactured by Big Chemie), EFKA POLYMER 4550
(Manufactured by EFKA), Solsperse 27000, Solsperse 41000, Solsperse 53095 (all manufactured by Avicia), and the like.

The number average molecular weight of the above pigment dispersant has a lower limit of 1
000, and the upper limit is preferably 100,000. If it is less than 1000, the dispersion stability may not be sufficient, and 1
If it exceeds 0,000, the viscosity may be too high and handling may be difficult. The lower limit is 2,000 and the upper limit is 50,000, more preferably the lower limit is 4,000 and the upper limit is 50,000.

The above pigment dispersant is mixed and dispersed with a pigment according to a known method to obtain a dispersant pigment dispersion paste. The mixing ratio of the pigment dispersant in the dispersant pigment dispersion paste is preferably 1% by mass as the lower limit and 20% by mass as the upper limit with respect to the solid content of the dispersant pigment dispersion paste. If it is less than 1% by mass, the pigment cannot be dispersed stably, and if it exceeds 20% by mass, the physical properties of the coating film may be poor. Preferably, the lower limit is 5% by mass and the upper limit is 15% by mass.
Is.

The above-mentioned pigment is not particularly limited as long as it is a pigment used in ordinary intermediate coating paints, but from the viewpoint of improving weather resistance and ensuring hiding power, it is preferably a color pigment. In particular, titanium dioxide is more preferable because it has excellent white color hiding power 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. , Organic color pigments such as isoindolinone pigments and metal complex pigments; yellow lead, yellow iron oxide, red iron oxide,
Inorganic color pigments such as carbon black can be used. As the pigment, further, calcium carbonate, barium sulfate,
An extender pigment such as clay or talc may be used.

As the above-mentioned pigment, it is possible to use a standard gray-based intermediate coating composition containing carbon black and titanium dioxide as main pigments, or a combination of a top-coat coating composition and a set gray in which lightness or hue is matched or various coloring pigments. It is also possible to use a so-called color intermediate coating material.

The above pigment preferably has a pigment mass ratio (PWC) to the total mass of the pigment and resin solids (PWC) of 10 to 60 mass% in the aqueous intermediate coating composition. If it is less than 10% by mass, the hiding property may be deteriorated due to lack of pigment. If it exceeds 60% by mass, the excess pigment may cause an increase in viscosity at the time of curing, resulting in poor flowability and poor coating film appearance.

The content of the pigment dispersant in the above waterborne intermediate coating composition is, on the solid content basis, a lower limit of 0.5% by mass when a pigment dispersant containing substantially no basic substance is used. The upper limit is preferably 10% by mass. When it is less than 0.5% by mass, the dispersion stability of the pigment may be deteriorated due to the small amount of the pigment dispersant compounded. If it exceeds 10% by mass, the physical properties of the coating film may be poor. Preferably lower limit 1
The upper limit is 5% by mass and the upper limit is 5% by mass. The above water-based intermediate coating,
Further, additive components such as an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, an anti-armpitting agent and the like can be added.

It is more preferable that the above-mentioned waterborne intermediate coating composition comprises an acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste and a thickener. As a result, the stress-strain characteristic of the coating film obtained by baking after coating as a single film has a Young's modulus of 270 at a measurement temperature of -20 ° C.
It will be 00 Kg / cm ² or less and an elongation rate of 4.2% or more. Also, since the amount of amine in the paint is reduced,
It is possible to prevent yellowing of the obtained multilayer coating film and further prevent a mixed layer between the intermediate coating film and the base coating film, thereby obtaining a multilayer coating film having an excellent appearance. You can

In the step (1), from the viewpoints of preventing mixed layers, preventing yellowing and improving the appearance, preheating is performed after the aqueous intermediate coating composition is applied and before the step (2) is performed. preferable. When preheating is performed in the step (1), the viscosity before performing the step (2) in the method for forming a multilayer coating film of the present invention means the viscosity after preheating.

[0051]Intermediate coating film formation method The above-mentioned water-based intermediate coating is a coating on which the above electrodeposition coating is formed.
It is applied on the article to form an uncured intermediate coating film.
As the electrodeposition paint for forming the above electrodeposition coating film, cation
Type and anionic type can be used. Anticorrosion
From the point that a coating film excellent in
Those are preferable.

The object to be coated (material for electrodeposition coating) is not particularly limited, and examples thereof include iron, copper, aluminum,
Tin, zinc, etc .; and alloys and castings containing these metals. Specific examples include automobile bodies and parts such as passenger cars, trucks, motorcycles and buses. It is particularly preferable that these metals have been previously subjected to chemical conversion treatment with a phosphate, chromate or the like before the electrodeposition coating is performed.

The method of applying the above-mentioned water-based intermediate coating composition is not particularly limited. For example, an air electrostatic spray commonly called "react gun"; commonly called "micro / micro (μ
This can be carried out by using a rotary atomization type electrostatic coating machine called “μ) bell”, “micro (μ) bell”, “metabell” or the like. A method using a rotary atomization type electrostatic coating machine or the like is preferable.

The dry film thickness of the intermediate coating film varies depending on the use, but is preferably 5 to 50 μm. If it exceeds the upper limit, sharpness may be deteriorated, and defects such as sagging at the time of coating and bake at the time of baking may occur. If it is less than the lower limit, the appearance may be deteriorated.

In the present invention, regarding the water-based intermediate coating composition, the water-based coating composition and the clear coating composition, uncuring the coating film means that the water-based intermediate coating composition, the water-based coating composition and the clear coating composition are wet-on-wet in this order. It means to be painted on. In the present specification, uncured is a concept including, for example, a state after preheating. The preheating is, for example, a step of leaving or heating at room temperature to less than 100 ° C. for 1 to 10 minutes after coating. For the purpose of obtaining a good finished appearance, it is preferable to perform preheating after applying the water-based intermediate coating composition and after coating the water-based base coating composition.

[0056]Process (2) The step (2) is an uncured film formed as described above.
Water-based base paint is applied on top of the intermediate coating film and uncured
Is a step of forming a base coating film.

[0057]Water-based paint The above-mentioned water-based base paint is mainly used for the color and luster of the coating film.
Painted to maintain and maintain aesthetics and design
It is something. In the present invention, the above water-based base coating
However, it is substantially free of pigments and volatile basic substances.
Dispersant obtained by previously dispersing a pigment dispersant
It is possible to use the one which is blended with the first. Up
As a volatile basic substance and as the above pigment dispersant
Examples of the water-based intermediate coating composition
be able to.

As the pigment contained in the above-mentioned water-based base paint, in addition to the above-mentioned coloring pigments and extender pigments, it is also possible to blend a glittering pigment and use it as a metallic base paint, or to blend a glittering pigment. Instead, it is also possible to mix a color pigment such as red, blue or black and / or an extender pigment to use as a solid type base paint.

The bright pigment is not particularly limited,
Examples thereof include uncolored or colored metallic luster pigments such as metals or alloys and mixtures thereof, interference mica powder, colored mica powder, white mica powder, graphite or colorless colored flat pigments. A non-colored or colored metallic glitter material such as a metal or an alloy and a mixture thereof are preferable because they can form a coating film having excellent dispersibility and high transparency. Specific examples of the metal include aluminum, aluminum oxide, copper, zinc, iron, nickel and tin.

The shape of the bright pigment is not particularly limited,
Further, although it may be colored, for example, the average particle size (D5
0) has a lower limit of 2 μm and an upper limit of 50 μm, and a scale-like substance having a lower limit of 0.1 μm and an upper limit of 5 μm is preferable. The average particle size is more preferably the lower limit of 10 μm and the upper limit of 35 μm, because it is excellent in brilliance.

The above-mentioned pigments may be used alone or in combination of two or more, and one or more of them may be combined from coloring pigments and extender pigments and, if necessary, flat pigments and luster pigments. Can be used.

In the above aqueous base paint, the blending ratio of the pigment dispersant in the dispersant pigment dispersion paste is preferably 3 to 50% by mass based on the solid content of the dispersant pigment dispersion paste. If it is less than 3% by mass, the pigment cannot be dispersed stably, and if it exceeds 50% by mass, the physical properties of the resulting coating film may be deteriorated.

The aqueous base paint can be prepared by mixing the dispersant-pigment-dispersed paste with the film-forming resin and the curing agent. Pigment concentration (PW) in water-based paints, including the above-mentioned luster pigments and all other pigments
C) is generally a lower limit of 0.1% by mass and an upper limit of 50% by mass, preferably a lower limit of 0.5% by mass and an upper limit of 40% by mass, more preferably a lower limit of 1% by mass and an upper limit. Thirty
It is% by mass. If it exceeds 50% by mass, the appearance of the coating film deteriorates.

The content of the pigment dispersant in the water-based base coating composition is, when a pigment dispersant containing substantially no basic substance is used, a lower limit of 1% by mass based on solid content,
The upper limit is preferably 20% by mass. When the amount is less than 1% by mass, the dispersion stability of the pigment may be deteriorated due to the small amount of the pigment dispersant compounded. If it exceeds 20% by mass, the physical properties of the resulting coating film may deteriorate.

The coating film-forming resin, curing agent and other additives used in the above water-based base coating material are not particularly limited, and those mentioned above can be mentioned. From the viewpoint of pigment dispersibility and workability, a combination of an acrylic resin and / or a polyester resin and a melamine resin is preferable. Also,
Regarding the method for preparing the water-based base coating material, the methods exemplified for the above-mentioned water-based intermediate coating material can be mentioned.

[0066]Base coating film formation method The base paint is an uncured medium formed as described above.
Apply uncured base coating on top of coating
It As the above coating method, a water-based intermediate coating is applied.
The method exemplified at that time can be mentioned. Above base coating
When designing the paint on the car body, etc.
Multistage coating with air electrostatic spray to enhance
Coating, preferably in two stages or by air
-A combination of electrostatic spray and the above rotary atomizing type electrostatic coating machine
It is preferable to use a combined coating method.

The dry film thickness of the base coating film varies depending on the use, but is preferably 5 to 35 μm. If it exceeds the upper limit, sharpness may be deteriorated, and defects such as unevenness and flow during coating may occur. If it is less than the lower limit, color unevenness may occur.

[0068]Process (3) The above step (3) is performed by the uncured resin formed as described above.
Apply a clear paint on the base coating and apply an uncured clear coating.
This is a step of forming a coating film.

[0069]Clear paint The clear coating is a base coating containing a bright pigment.
Due to the luster pigment when using a taric base paint
Smooth the unevenness and flicker of the base coating.
Also, it is formed to protect the base coating film.
It The clear paint is not particularly limited, for example,
A film-forming resin, curing agent and other additives.
I can name one.

The coating film-forming resin is not particularly limited, and examples thereof include acrylic resin, polyester resin, epoxy resin and urethane resin, which are combined with a curing agent such as amino resin and / or blocked isocyanate resin. Used. From the viewpoint of transparency or acid etching resistance, a combination of an acrylic resin and / or polyester resin with an amino resin, or a carboxylic acid
It is preferable to use an acrylic resin and / or polyester resin having an epoxy curing system.

As the above clear paint, after the above base paint is applied, it is applied in an uncured state, so that it contains a viscosity control agent as an additive in order to prevent familiarity between layers, reversal, or sagging. It is preferable. The addition amount of the viscosity control agent is 0.01 parts by mass as the lower limit and 10 parts by mass as the upper limit, and preferably 0.02 parts by mass as the lower limit and 8 parts by mass as the upper limit, relative to 100 parts by mass of the resin solid content of the clear paint. ,
More preferably, the lower limit is 0.03 parts by mass and the upper limit is 6 parts by mass. 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 of any of an organic solvent type, an aqueous type (water-soluble, water-dispersible, emulsion), a non-aqueous dispersion type and a powder type, and if necessary, a curing catalyst and a surface. A regulator or the like can be used.

[0073]Clear coating film formation method As a method for preparing the clear paint and a coating method,
It can be done according to traditional methods. Clear coating above
The dry film thickness of 10-70 μm varies depending on the application.
Is. If this dry film thickness exceeds the upper limit, the image clarity will decrease.
Or unevenness or flow may occur during painting.
If it is below the lower limit, the appearance may be deteriorated.

[0074]Process (4) The step (4) includes the intermediate coating film, the base coating film, and
And the above clear coating film are cured by heating at the same time to obtain a multilayer coating film.
Is a step of obtaining. As the temperature for heating and curing,
Lower limit 110 ° C, upper limit 180 ° C, preferably lower limit 120
℃, the upper limit of 160 ℃, high crosslinking degree
A cured coating film can be obtained. If it exceeds 180 ℃,
The film becomes hard and brittle, and curing is not sufficient below 110 ° C.
Yes. The curing time varies depending on the curing temperature, but it is 120-1
A temperature of 60 ° C. for 10 to 60 minutes is suitable.

In the method for forming a multilayer coating film of the present invention, the coating film formed in the above step (1), step (2) and step (3) has a unit area before the step (4). The total amount of volatile basic substances in 1 mm ² is preferably 7 × 10 ⁻⁶ mmol or less.

Volatile basic substances contained in the water-based intermediate coating composition and the water-based coating composition remain in the coating film formed in the steps (1), (2) and (3), and the The total amount of basic substances is 1 mm ² per unit area of the coating film.
In the case of more than 7 × 10 ⁻⁶ mmol, when the uncured coating film is heated to cure, the volatile basic substance partially remains, causing a chemical change to cause yellowing. In addition, basic substances that volatilize from the uncured intermediate coating film and base coating film are trapped inside the clear coating film, and the clear coating film also turns yellow, resulting in poor color reproducibility and design. To do. Preferably, 6.5 × 10 ⁻⁶ mmol
It is the following.

In the present specification, the unit area is 1 mm ²
Total amount of the above volatile basic substances in (mmol)
Means that a section (S) having an area of 1 mm ² on the surface of the coating film formed in the steps (1), (2) and (3) is projected vertically onto the coated surface of the coating object. Assuming the obtained section (S '), the content (mm) of the volatile basic substance in the section (V) sandwiched by the section (S) and the section (S')
ol) is meant.

The total amount (mmol) of the volatile basic substance in the unit area of 1 mm ² is equal to the portion (V).
The total amount of the volatile basic substances (mmol) contained in each of the intermediate coating film, the base coating film and the clear coating film in 1. That is, the amount of the volatile basic substance contained in the intermediate coating film in the portion (V) [Ammo
l], the amount [Bmmol] of the volatile basic substance contained in the base coating film in the part (V), and the volatile basic substance contained in the clear coating film in the part (V). Amount [Cmmol] and total amount [(A + B +
C) mmol] is used herein as a coating film unit area of 1 m
It is the total amount of the above volatile basic substances in m ² .

The total amount (mmol) of the volatile basic substance in the unit area of 1 mm ² is obtained by sampling the coating film formed in the steps (1), (2) and (3). It can be calculated by assuming that 1 g is 1 cm ³ from the content of the volatile basic substance quantified by gas chromatography of the sample and the dry film thickness.

The film thickness of the multilayer coating film obtained by the coating film forming method of the present invention is usually a lower limit of 30 μm, an upper limit of 300 μm,
The lower limit is preferably 50 μm and the upper limit is 250 μm. Three
If it exceeds 00 μm, the physical properties of the film such as cooling and heating cycles may deteriorate, and if it is less than 30 μm, the strength of the film itself may decrease.

The aqueous intermediate coating composition of the present invention is an aqueous intermediate coating composition used in the method for forming a multilayer coating film, wherein the aqueous intermediate coating composition is an acrylic emulsion resin, a melamine resin or a dispersion. The pigment-dispersed paste and the thickener, and the acrylic emulsion resin has a weight average molecular weight of 100,000 to 200,000. The water-based intermediate coating composition is used as a water-based intermediate coating in the method for forming a multilayer coating film, and by using it, a coating excellent in chipping resistance, impact resistance and appearance is also used. A membrane can be obtained.

The weight average molecular weight of the acrylic emulsion resin obtained by gel permeation chromatography (GPC) has a lower limit of 100,000 and an upper limit of 200.
It is preferably 000. By forming an uncured intermediate coating film using an aqueous coating composition containing an acrylic emulsion resin in the above range, it is possible to prevent a mixed layer between the formed intermediate coating film and the base coating film, The finished appearance of the film can be made excellent.

In the method for forming a multilayer coating film of the present invention, the aqueous intermediate coating composition used in the above step (1) is obtained by baking an uncured intermediate coating film formed by coating this as a single film. The stress-strain characteristics of the resulting coating film have a Young's modulus of 27,000 Kg / cm ² or less and an elongation of 4. at a measurement temperature of −20 ° C.
Since it is 2% or more, the multilayer coating film obtained by the above-mentioned multilayer coating film forming method is excellent in chipping resistance and impact resistance. Further, in the coating film forming method, the water-based intermediate coating composition comprises an acrylic emulsion resin, a melamine resin, a dispersant-pigment-dispersed paste, and a thickener, and the step (1), the step (2) and the step ( The coating film formed in 3) is obtained when the total amount of volatile basic substances in a unit area of 1 mm ² is 7 × 10 ⁻⁶ mmol or less before performing step (4). Since yellowing of the coating film can be prevented, it is possible to obtain a coating film which is excellent not only in chipping resistance and impact resistance but also in finished appearance.

Further, according to the 3-wet 1-bake coating method of the present invention, the baking step of the intermediate coating material can be omitted as compared with the conventional 3-coat 2-bake method, so that the steps can be shortened and the cost can be reduced. It is possible to build a new coating system aiming to reduce energy consumption and environmental load.

[0085]

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples. In the examples, "parts" means "parts by mass" unless otherwise specified.

Production Example 1 Production of Colored Pigment Paste 9.4 parts of a commercially available dispersant "Disperbyk 190" (manufactured by Big Chemie), 36.8 parts of ion-exchanged water, 34.5 parts of rutile titanium dioxide, barium sulfate 34. After premixing 4 parts and 6 parts talc, add glass bead media in a paint conditioner and add a particle size of 5 at room temperature.
The pigment was mixed and dispersed until it became less than or equal to μm to obtain a color pigment paste.

Production Example 2 Acrylic Emulsion Resin A-
35.75 parts of deionized water was added to the production reaction vessel of Example 1, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then α, β-
Styrene 10 as the ethylenically unsaturated monomer mixture
Parts, methyl methacrylate 11.16 parts, butyl acrylate 5.48 parts, ethyl acrylate 56.42 parts, acrylate-4 hydroxybutyl 15.40 parts, methacrylic acid 1.53 parts, Aqualon HS-10 (poly Oxyethylene alkylpropenyl phenyl ether sulfate,
Daiichi Kogyo Seiyaku Co., Ltd.) 3.0 parts, ADEKA rear soap NE-
20 (manufactured by Asahi Denka Co., Ltd.) and a monomer emulsion consisting of 3.75 parts and 50 parts of deionized water, and ammonium superfluate 0.30
Parts and an initiator solution consisting of 15.0 parts deionized water 2
It was added dropwise to the reaction vessel in parallel over time. After completion of dropping, aging was carried out at the same temperature for 2 hours. Then, cool to 40 ° C. and filter with a 400 mesh filter to give a particle size of 20.
An acrylic emulsion resin A-1 having 0 nm, a non-volatile content of 50%, an acid value of 10, a hydroxyl value of 60, a glass transition temperature of -30 ° C, and a weight average molecular weight of 100,000 was obtained.

Production Example 3 Acrylic Emulsion Resin A-
35.75 parts of deionized water was added to the production reaction container of Example 3, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then α, β-
Styrene 20 as the ethylenically unsaturated monomer mixture
Parts, methyl methacrylate 27.76 parts, ethylhexyl acrylate 9.26 parts, ethyl acrylate 33.16
Parts, hydroxyethyl acrylate 8.28 parts, methacrylic acid 1.53 parts, thiocalcol 20 (thiol, manufactured by Kao Corporation) 2.0 parts, Aqualon HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate ester, Daiichi Kogyo) Pharmaceutical Co., Ltd.) 3.0 parts, ADEKA REASOAP N
E-20 (manufactured by Asahi Denka Co., Ltd.) 0.5 parts and 50 parts of deionized water as a monomer emulsion, and ammonium persulfate 0.3.
An initiator solution consisting of 0 parts and 15.0 parts of deionized water was added dropwise to the reaction vessel in parallel over 2 hours. After completion of dropping, aging was carried out at the same temperature for 2 hours. Then, cool to 40 ° C. and filter with a 400 mesh filter to give a particle size of 20.
An acrylic emulsion resin A-3 having 0 nm, a non-volatile content of 50%, an acid value of 10, a hydroxyl value of 60, a glass transition temperature of 20 ° C., and a weight average molecular weight of 50,000 was obtained.

Production Example 4 Acrylic Emulsion Resin A-
35.75 parts of deionized water was added to the production reactor of Production No. 5, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then α, β-
Styrene 20 as the ethylenically unsaturated monomer mixture
Parts, 9.5 parts of methyl methacrylate, butyl acrylate 1
5.27 parts, ethyl acrylate 45.42 parts, hydroxyethyl acrylate 8.28 parts, methacrylic acid 1.53
Part, Thiocalcol 20 (Thiol, manufactured by Kao Corporation) 1.5
Parts, AQUALON HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) 3.0 parts, Adecaria Soap NE-20 (manufactured by Asahi Denka Co., Ltd.) 0.5 parts and deionized water 50 Parts of the monomer emulsion and an initiator solution consisting of 0.30 parts of ammonium persulfate and 15.0 parts of deionized water were added dropwise to the reaction vessel in parallel over a period of 2 hours. After completion of dropping, aging was carried out at the same temperature for 2 hours. Then cool to 40 ° C., 40
After filtering through a 0 mesh filter, an acrylic emulsion resin A-4 having a particle size of 200 nm, a nonvolatile content of 50%, an acid value of 10, a hydroxyl value of 60, a glass transition temperature of 0 ° C. and a weight average molecular weight of 80,000 was obtained.

Production Example 5 Acrylic Emulsion Resin A-
35.75 parts of deionized water was added to the production reactor of Production No. 5, and the temperature was raised to 80 ° C. while mixing and stirring in a nitrogen stream. Then α, β-
Styrene 20 as the ethylenically unsaturated monomer mixture
Parts, methyl methacrylate 0.18 parts, butyl acrylate 9.08 parts, ethyl acrylate 58.94 parts, acrylate-4 hydroxybutyl 10.27 parts, methacrylic acid 1.53 parts, thiocalcol 20 (thiol, Kao 1.5 parts, Aqualon HS-10 (polyoxyethylene alkylpropenyl phenyl ether sulfate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) 3.0 parts, ADEKA REASOAP N
E-20 (manufactured by Asahi Denka Co., Ltd.) 0.5 parts and 50 parts of deionized water as a monomer emulsion, and ammonium persulfate 0.3.
An initiator solution consisting of 0 parts and 15.0 parts of deionized water was added dropwise to the reaction vessel in parallel over 2 hours. After completion of dropping, aging was carried out at the same temperature for 2 hours. Then, cool to 40 ° C. and filter with a 400 mesh filter to give a particle size of 20.
An acrylic emulsion resin A-5 having 0 nm, a non-volatile content of 50%, an acid value of 10, a hydroxyl value of 60, a glass transition temperature of -10 ° C, and a weight average molecular weight of 80,000 was obtained.

Example 1 (1) Preparation of aqueous intermediate coating composition 30 parts of the color pigment paste obtained in Production Example 1 and 42 parts of the acrylic emulsion resin A-1 obtained in Production Example 2 were prepared.
Part, Cymel 327 (imino type melamine resin manufactured by Mitsui Cytec Co., Ltd.), 28 parts, and Adecanol UH-814N (thickener manufactured by Asahi Denka Co., Ltd., 2.33 parts) were added and mixed with stirring to prepare an aqueous intermediate coating composition B. -1 was obtained.

(2) Method for forming coating film Power Top U-50 was applied to dull steel sheet treated with zinc phosphate.
(Cationic electrodeposition paint made by Nippon Paint)
A pre-diluted Olga P-2 (Nippon Paint's melamine-curable polyester resin-based white intermediate coating) was electrodeposited to a thickness of 0 μm and baked at 160 ° C. for 30 minutes. 2 stage coating was performed by air spraying so that it was baked at 140 ° C. for 30 minutes and then cooled to obtain a substrate. 20 μm of the above water-based intermediate coating composition B-1 was applied onto the obtained substrate by air spray coating.
After painting and preheating at 80 ° C. for 5 minutes, 10 μm of AQUALEX AR-2000 base (water-based paint by Nippon Paint Co., Ltd.) was applied by air spray painting.
Preheating was performed at 0 ° C. for 3 minutes. Further, Macflow O-1800W-2 clear (clear paint manufactured by Nippon Paint Co., Ltd.) was applied to the coated plate by air spray coating for 35 μm, and then baked at 140 ° C. for 30 minutes to obtain a test piece.
An aqueous intermediate coating composition (aqueous intermediate coating composition B-
1), water-based paint (Aqualex AR-2000
Base), clear paint (Macflow O-1800
W-2 clear) was diluted and coated under the following conditions. (Water-based intermediate coating) Thinner: Ion-exchanged water 40 seconds / NO. 4 Ford cup / 20 ° C (water-based paint) Thinner: Ion-exchanged water 45 seconds / NO. 4 Ford cups / 20 ° C (clear paint) Thinner: EEP (ethoxyethyl propionate) /
S-150 (Aromatic hydrocarbon solvent manufactured by Exxon) = 1
/ 30 seconds / NO. 4 Ford Cup / 20 ℃

(3) Evaluation of coating film [Method for measuring stress-strain characteristic] The obtained waterborne intermediate coating composition B-1 was applied onto a polypropylene test plate by air spraying so that the dry film thickness was 40 μm. , 140 ° C for 3
The coating film obtained by baking for 0 minutes was peeled off from the test plate,
A test piece was cut into a size of 10 × 70 mm. An autograph AGS-G type and an analog meter (manufactured by Shimadzu Corporation) are used for measuring stress-strain characteristics, and the measurement length is 50 m.
The measurement was performed at m, temperature of -20 ° C, and tensile speed of 10 mm / min. Young's modulus was obtained from the slope of the chart immediately after the tensile test. The elongation percentage is the Δ length of the chart [when the length of the test piece before the test in the axial direction is 70 (mm) and the length of the test piece in the axial direction until breakage is L (mm), (%) = (L-70) × 100/70].

[Chipping resistance evaluation method] Stepping stone tester
Cool to -30 ° C in a test holder (made by Suga Test Instruments Co., Ltd.)
Mounted the test piece so that the approach angle of the stone is 90 ° C.
3 kg / cm of 100g crushed stone ^TwoJet with air pressure
A crushed stone is made to collide with a test piece. The peeling scratch at that time
Visually check the degree (number, size, location of destruction) in 5 steps
evaluated. The results are shown in Table 1. 1: There are peeling scratches on the entire surface and peeling from the substrate 2: No peeling scratches on the entire surface and no peeling from the substrate 3: Some peeling scratches and peeling from the substrate 4: Some peeling scratches, no peeling from the substrate 5: Almost no peeling

[Multilayer Coating Hardness] Pencil Hardness A test was conducted using Mitsubishi Pencil “UNI” based on JIS K 5400, 8.4, and Table 1 shows the hardness limit at which the coating is not scratched. .

[Method of Measuring Amount of Residual Basic Substance] Samples of the intermediate coating film and the base coating film were sampled after the aqueous intermediate coating composition and the aqueous base coating composition were applied and preheated in the above coating method, respectively. 0.3 g of the collected sample was weighed, 0.6 ml of a standard solution (100 ml of methanol was mixed with 1.0 g of isobutanol) was added, and the mixture was sufficiently stirred by a touch mixer and ultrasonic vibration.
The supernatant was collected and the amount of basic substance was quantified by gas chromatography. From the obtained amount of basic substance and dry film thickness, assuming that 1 g is 1 cm ³ , the total amount of basic substance in a unit area of 1 mm ² of the coating film was calculated. The results are shown in Table 1.

Example 2 A test piece was obtained in the same manner as in Example 1 except that an aqueous intermediate coating composition was prepared by the method described below. Moreover, the same evaluation was performed and the results are shown in Table 1. (1) Preparation of water-based intermediate coating composition 30 parts of the coloring pigment paste obtained in Production Example 1 and Rack Star 3622A (manufactured by Dainippon Ink and Chemicals, Inc., nonvolatile content 52.
5%, designed glass transition temperature -30 ° C) 42 parts, Cymel 327 (Mitsui Cytec imino type melamine resin)
Was added, and 2.33 parts of Adecanol UH-814N (a thickener manufactured by Asahi Denka Co., Ltd.) was added, and the mixture was stirred with stirring to obtain an aqueous intermediate coating composition B-2.

Comparative Example 1 A test piece was obtained in the same manner as in Example 1 except that an aqueous intermediate coating composition was prepared by the method shown below. Moreover, the same evaluation was performed and the results are shown in Table 1. (1) Preparation of aqueous intermediate coating composition 30 parts of the color pigment paste obtained in Production Example 1 and 42 parts of the acrylic emulsion resin A-3 obtained in Production Example 3
Part, Cymel 327 (imino type melamine resin manufactured by Mitsui Cytec Co., Ltd.), 28 parts, and Adecanol UH-814N (thickener manufactured by Asahi Denka Co., Ltd., 2.33 parts) were added and mixed with stirring to prepare an aqueous intermediate coating composition B. -3 was obtained.

Comparative Example 2 A test piece was obtained in the same manner as in Example 1 except that an aqueous intermediate coating composition was prepared by the method shown below. Moreover, the same evaluation was performed and the results are shown in Table 1. (1) Preparation of aqueous intermediate coating composition 30 parts of the color pigment paste obtained in Production Example 1 and 42 parts of the acrylic emulsion resin A-4 obtained in Production Example 4
Part, Cymel 327 (imino type melamine resin manufactured by Mitsui Cytec Co., Ltd.), 28 parts, and Adecanol UH-814N (thickener manufactured by Asahi Denka Co., Ltd., 2.33 parts) were added and mixed with stirring to prepare an aqueous intermediate coating composition B. -4 was obtained.

Comparative Example 3 A test piece was obtained in the same manner as in Example 1 except that an aqueous intermediate coating composition was prepared by the method shown below. Moreover, the same evaluation was performed and the results are shown in Table 1. (1) Preparation of aqueous intermediate coating composition 30 parts of the color pigment paste obtained in Production Example 1 and 42 parts of the acrylic emulsion resin A-5 obtained in Production Example 5
Part, Cymel 327 (imino type melamine resin manufactured by Mitsui Cytec Co., Ltd.), 28 parts, and Adecanol UH-814N (thickener manufactured by Asahi Denka Co., Ltd., 2.33 parts) were added and mixed with stirring to prepare an aqueous intermediate coating composition B. -5 was obtained.

[0101]

[Table 1]

From the examples and comparative examples, the test pieces obtained in the examples are the stress of the coating film obtained by baking the uncured intermediate coating film formed by coating the aqueous intermediate coating composition with a single film. The strain characteristic is a Young's modulus of 2700 at a measurement temperature of -20 ° C.
Since it was 0 Kg / cm ² or less and the elongation rate was 4.2% or more, it was excellent in chipping resistance as compared with the test piece obtained in the comparative example. Further, the obtained coating film was excellent in appearance without yellowing.

[0103]

Since the method for forming a multilayer coating film of the present invention has the above-mentioned constitution, it comprises a step of heating and curing an uncured coating film formed from an aqueous intermediate coating composition, an aqueous base coating composition and a clear coating composition. In the multi-layer coating film forming method, particularly in the 3 wet 1 bake coating method, a coating film excellent in chipping resistance and impact resistance can be obtained, and the obtained coating film is excellent without yellowing. It has an appearance. Therefore, the method for forming a multilayer coating film of the present invention can be suitably used particularly for vehicle painting such as automobile bodies.

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) C09D 7/12 C09D 7/12 133/00 133/00 161/28 161/28 201/00 201/00 ( 72) Inventor Teruzo Higashi 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan Paint Co., Ltd. (72) Inventor Chisatoshi Yabushita 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan (72) Invention Kyoko Kobayashi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor, Kazuya Kitagawa No location, Taura Port Town, Yokosuka City, Kanagawa Kanto Automobile Industry Co., Ltd. F term (reference) 4D075 AE12 BB26Z BB89X BB91Z CA02 CA04 CA13 CA33 CA38 CA44 CA47 DA06 DB02 DB05 DB06 DB07 DC12 DC13 EA06 EA13 EA43 EB20 EB22 EB32 EB33 EB35 EB38 EB45 EB52 EC11 EC33 4J038 CG141 DA162 DF022 DH022 HA026 HA216 HA376 HA456 HA556 JB13 KA03 KA07 KA08 KA09 MA08 MA10 MA14 NA01 NA11 PA04 PA19

Claims (3)

[Claims] 1. A step (1) of forming an uncured intermediate coating film by coating an aqueous intermediate coating composition on an object to be coated on which an electrodeposition coating film has been formed, on the intermediate coating film. A step (2) of applying an aqueous base paint to form an uncured base coating film, and a step (3) of applying a clear paint thereon to form an uncured clear coating film, and A method for forming a multilayer coating film comprising the 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, wherein the aqueous intermediate coating composition comprises: The coating film obtained by baking on a polypropylene test plate and then baking it as a single film has a Young's modulus of 27,000 Kg / cm ² or less and an elongation rate of 4.2% or more at a measurement temperature of -20 ° C. And a method for forming a multilayer coating film. 2. The water-based intermediate coating composition comprises an acrylic emulsion resin, a melamine resin, a dispersant pigment dispersion paste and a thickener, and is formed by the step (1), the step (2) and the step (3). The multilayer coating film forming method according to claim 1, wherein the coating film has a total amount of volatile basic substances in a unit area of 1 mm ² of 7 × 10 ⁻⁶ mmol or less before performing the step (4). . 3. An aqueous intermediate coating composition for use in the method for forming a multilayer coating film according to claim 1, wherein the aqueous intermediate coating composition is an acrylic emulsion resin, a melamine resin,
Dispersant An aqueous intermediate coating composition comprising a pigment-dispersed paste and a thickener, wherein the acrylic emulsion resin has a weight average molecular weight of 100,000 to 200,000.