JP2002126627A - Method for forming multilayered coating film, multilayered coating film and water-based intermediate coating material composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a coating film by which a coating film having excellent appearance without changing into yellow can be formed in a three-wet coating system aiming at the reduction in the coating process, cost and load on the environment. SOLUTION: The method for forming a multilayered coating film includes a process (1) to apply a water-based intermediate coating material to form an unhardened intermediate coating film on the objective body to be coated where an electrodeposition coating film is preliminarily formed, a process (2) to apply a water-based base coating material on the intermediate coating film to form an unhardened base coating film, a process (3) to apply a clear coating material thereon to form an unhardened clear coating film, and a process (4) to heat and harden the intermediate coating film, base coating film and clear coating film at a time to obtain a multilayered coating film. The water-based intermediate coating material and/or the water-based base coating material contains a pigment dispersion paste compounded therein which is prepared by preliminarily dispersing a pigment and a pigment dispersant substantially containing no volatile basic substance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of applying an aqueous intermediate coating, an aqueous base coating, and a clear coating on a workpiece on which an electrodeposition coating film has been formed by wet-on-wet, and performing heat curing at a time. More particularly, the present invention relates to a method for forming a multilayer coating film using a three-wet coating system, and more particularly, to a method for forming a multilayer coating film capable of obtaining a multilayer coating film having an excellent finished appearance, and a multilayer coating method obtained thereby. Membrane, and
The present invention relates to an aqueous intermediate coating composition which can be suitably used in the method for forming a multilayer coating film.

[0002]

2. Description of the Related Art In recent years, in the field of paints, especially in the field of automotive coatings, water-based paints have attracted attention in order to solve the problem of reducing environmental load (VOC, etc.). Aqueous paints are generally used to coat a water-based film-forming resin having a hydrophilic functional group and a curing agent, and a pigment-dispersed paste obtained from a pigment and a pigment-dispersed resin with low pollution such as water and alcohol. In a hydrophilic medium.

[0003] As the pigment-dispersed resin, it is common to use a part of the above-mentioned water-based film-forming resin blended in a water-based paint. As the film-forming resin, a resin in which an anionic functional group such as a carboxyl group or a sulfonic acid group is introduced, and a combination of a basic substance such as an amine as a counter ion are often used.

On the other hand, in the conventional automobile coating finishing procedure, a three-coat three-bake coating method in which an electrodeposition coating film, an intermediate coating film, and a top coating film are baked after each coating has been performed. After baking the electrodeposition coating film after electrodeposition coating, three coating processes of intermediate coating, base coating and clear coating are performed on the coating by wet-on-wet, and these three wet coatings are baked collectively. Attempts have also been made to reduce the number of baking steps with a coating system to save resources and save energy.

However, in a three-wet coating system in which intermediate coating, base coating and clear coating are performed on a wet-on-wet basis, a pigment-dispersed paste containing a pigment-dispersed resin in which a basic substance such as an amine is combined as a counter ion is blended. When the water-based paint is used, there is a problem that the obtained coating film turns yellow.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to form a coating film having an excellent appearance without yellowing in a three-wet coating system aiming at shortening the coating process, reducing costs and reducing environmental load. To provide a method for forming a coating film.

[0007]

According to the present invention, there is provided a step (1) of forming an uncured intermediate coating film by applying an aqueous intermediate coating material on an object on which an electrodeposition coating film is formed. Step (2) of applying an aqueous base paint on the intermediate coating film to form an uncured base coating film, and further applying a clear paint thereon to form an uncured clear coating film (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, The aqueous intermediate coating composition and / or the aqueous base coating composition contains a pigment dispersion paste obtained by previously dispersing a pigment and a pigment dispersant substantially free of a volatile basic substance. A method for forming a multilayer coating film characterized by the following.

[0008] The present invention is also a multilayer coating film formed by the above-mentioned method for forming a multilayer coating film. The present invention further relates to an aqueous intermediate coating composition comprising a pigment and a pigment dispersion paste obtained by previously dispersing a pigment and a pigment dispersant, wherein the pigment dispersant substantially comprises a volatile basic substance. It is also an aqueous intermediate coating composition characterized in that it is not included in the composition. Hereinafter, the present invention will be described in more detail.

In the method for forming a multilayer coating film of the present invention, a step (1) of applying an aqueous intermediate coating material on an object on which an electrodeposition coating film is formed to form an uncured intermediate coating film; Step (2) of applying an aqueous base paint on the intermediate coating film to form an uncured base coating film, and further applying a clear paint thereon to form an uncured clear coating film (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.

[0010]Process (1)  In the method for forming a multilayer coating film of the present invention, the step (1)
Is an aqueous intermediate coating on the object on which the electrodeposition coating is formed.
Paint to form an uncured intermediate coating film.
You. In the method for forming a multilayer coating film of the present invention,
Water-based intermediate coating used in (1), and
Less of the water-based paint used in step (2)
One is substantially free of pigments and volatile basic substances.
Pigment dispersion paper obtained by previously dispersing a pigment dispersant
The strike is blended.

[0011]Waterborne intermediate coating  The above-mentioned waterborne intermediate coating composition contains a pigment and a volatile basic substance.
Face obtained by pre-dispersing a pigment dispersant substantially free of
It is preferable that it contains a filler dispersion paste
No. In this specification, volatile basic substances are substantially
Does not include volatile bases in the solids of the pigment dispersant
No or no more than 3% by weight
Is meant.

In the present invention, the volatile dispersant is substantially free of a volatile basic substance, so that the volatile base in the intermediate coating film formed from the aqueous intermediate coating composition is reduced. The amount of the active substance is reduced, and yellowing of the obtained multilayer coating film can be suppressed. Therefore, when the content exceeds 3% by weight, the obtained multilayer coating film turns yellow and the finished appearance is inferior. Preferably, no volatile basic substance is contained at all, that is, no amine-neutralized pigment-dispersing resin which is conventionally generally used is used.

The volatile basic substance has a boiling point of 30.
It means a basic substance at 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, for example, ammonia, and examples of the organic basic substance include:
C1-C20 linear or branched alkyl group-containing primary to tertiary amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine and dimethyldodecylamine; monoethanolamine; C1-20 linear or branched hydroxyalkyl group-containing primary or tertiary amines such as diethanolamine and 2-amino-2-methylpropanol; C1-20 linear or such as dimethylethanolamine and diethylethanolamine; Primary to tertiary amines containing a branched alkyl group and a C1-20 linear or branched hydroxyalkyl group; C1-C1 such as diethylenetriamine and triethylenetetramine;
20 substituted or unsubstituted linear polyamines; morpholine, N
C1 such as -methylmorpholine, N-ethylmorpholine, etc.
20 substituted or unsubstituted cyclic monoamines; piperazine, N
-Methylpiperazine, N-ethylpiperazine, N, N-
Examples thereof include amines such as C1-20 substituted or unsubstituted cyclic polyamines such as 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 affinity portion and the hydrophilic portion include, for example, nonionic, cationic and anionic functional groups,
One molecule of the pigment dispersant may have two or more of the above functional groups. Examples of the nonionic functional group include a hydroxyl group, an amide group, and a polyoxyalkylene group. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. In addition, examples of the anionic functional group include a carboxyl group, a sulfonic group, and a phosphate group. Such a pigment dispersant can be manufactured by a method well known to those skilled in the art.

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

As the pigment dispersant, commercially available pigment dispersants can be used. Examples of the commercially available products include Disperbyk 190 and Disperby.
k 182, Disperbyk 184 (both by Big Chemie), EFKAPOLYMER 4550
(Manufactured by EFKA), Solsperse 27000, Solsperse 41000, Solsperse 53095 (all manufactured by Abisia), and the like.

The number average molecular weight of the pigment dispersant is 100
It is preferably from 0 to 100,000. If it is less than 1000, the dispersion stability is not sufficient, and if it exceeds 100,000, the viscosity is too high and handling becomes difficult. More preferably,
2000 to 50,000, more preferably 4000 to 5
It is ten thousand.

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

The pigment is not particularly limited as long as it is a pigment used in ordinary intermediate coatings, but is preferably a colored pigment from the viewpoint of improving weather resistance and securing concealing properties. In particular, titanium dioxide is more preferable since it is excellent in white color hiding properties and is inexpensive.

Examples of the pigments other than the above titanium dioxide include, for example, azochelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments , Isoindolinone pigments, organic complex pigments such as metal complex pigments; graphite, yellow iron oxide, red iron oxide,
And inorganic color pigments such as carbon black. As the above pigment, further, calcium carbonate, barium sulfate,
An extender pigment such as clay or talc may be used.

As the above-mentioned pigment, a standard gray intermediate paint containing carbon black and titanium dioxide as main pigments can be used, or a combination of a top paint and set gray or various colored pigments having the same lightness or hue can be used. A so-called color intermediate coating can also be used.

The pigment preferably has a weight ratio of the pigment (PWC) to the total weight of the pigment and the resin solids (PWC) in the aqueous intermediate coating composition of 10 to 60% by weight. If it is less than 10% by weight, the concealing property may be reduced due to insufficient pigment. If the amount exceeds 60% by weight, the viscosity at the time of curing is increased due to an excessive amount of the pigment, whereby the flowability is reduced and the appearance of the coating film may be reduced.

The above-mentioned aqueous intermediate coating composition can be prepared by mixing the above-mentioned pigment-dispersed paste, a film-forming resin and a curing agent. The content of the pigment dispersant in the aqueous intermediate coating composition is 0.5 to 1 based on the solid content when a pigment dispersant containing substantially no basic substance is used.
It is preferably 0% by weight. If the amount is less than 0.5% by weight, the dispersion stability of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 10% by weight, the properties of the coating film may be poor. Preferably, it is 1 to 5% by weight.

The film-forming resin is not particularly restricted but includes, for example, acrylic resins, polyester resins, alkyd resins, epoxy resins, urethane resins and the like.
From the viewpoint of pigment dispersibility and workability, a combination of an acrylic resin and / or a polyester resin and an amino resin and / or a blocked isocyanate resin as a curing agent is preferable. The film-forming resin and the curing agent are each 1
Although only the species may be used, two or more species may be used to balance the coating film performance.

As the above-mentioned film-forming resin, a water-soluble resin is used, or the resin is emulsified and dispersed by applying a dispersing agent such as a dispersing resin or a surfactant, thereby stably forming the resin in the aqueous intermediate coating composition. It can exist. The aqueous intermediate coating composition may further contain additives such as an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, and an anti-bake agent.

[0026]Intermediate coating film formation method  The above-mentioned aqueous intermediate paint is a coating material on which the above-mentioned electrodeposition coating film is formed.
A coating is applied on the garment to form an uncured intermediate coat.
As the electrodeposition paint for forming the electrodeposition coating film, cationic
And anionic types can be used. Corrosion protection
From the point that a coating film excellent in
Are preferred.

The material for electrodeposition coating is not particularly limited, and examples thereof include iron, copper, aluminum, tin, zinc and the like; 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 are subjected to a chemical conversion treatment with a phosphate, a chromate or the like before the electrodeposition coating is performed.

The method for applying the aqueous intermediate coating composition is not particularly limited, and includes, for example, an air electrostatic spray so-called “react gun”;
It can be carried out by using a rotary atomizing type electrostatic coating machine called “μ) bell”, “micro (μ) bell”, “metabell” or the like. Preferably, a method using a rotary atomization type electrostatic coating machine or the like is used.

The dry film thickness of the intermediate coating film varies depending on the application, but is preferably 5 to 50 μm. If the amount exceeds the upper limit, the sharpness may be reduced, or sagging may occur during coating or baking may occur. If the amount is below the lower limit, the appearance may be reduced.

In the present invention, to form an uncured coating film with respect to the aqueous intermediate coating, the aqueous base coating, and the clear coating means that the aqueous intermediate coating, the aqueous base coating, and the clear coating are wet-on-wet in this order. Means painting. In the present specification, the term “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 application. For the purpose of obtaining a good finished appearance, it is preferable to perform preheating after applying the aqueous intermediate coating and after applying the aqueous base coating.

[0031]Step (2)  The above step (2) is performed in the uncured state formed as described above.
A water-based base paint is applied on the intermediate coating film and is not cured
To form a base coating film.

[0032]Water-based paint  In the present invention, pigments and pigments are also used as the aqueous base paint.
And a pigment dispersant substantially free of volatile basic substances
Contains a pigment dispersion paste obtained by pre-dispersion
Things can be used. The volatile basic substance
Quality, and as the pigment dispersant, the aqueous intermediate coating
Examples of the materials can be given.

As the pigments contained in the aqueous base paint, the above-mentioned coloring pigments and extender pigments can be used. In addition, a glittering pigment can be blended and used as a metallic base paint, or a glittering pigment can be blended. Instead, a color pigment such as red, blue or black and / or an extender may be blended and used as a solid base paint.

The glitter pigment is not particularly limited.
For example, a non-colored or colored metallic glitter such as a metal or an alloy, and a mixture thereof, interference mica powder, colored mica powder, white mica powder, graphite, and a colorless colored flat pigment can be used. A non-colored or colored metallic brilliant material such as a metal or an alloy, and a mixture thereof are preferable since a coating film having excellent dispersibility and high transparency can be formed. Specific examples of the metal include aluminum, aluminum oxide, copper, zinc, iron, nickel, and tin.

The shape of the glitter pigment is not particularly limited.
Further, it may be colored, but for example, has an average particle size (D ₅₀ ) of 2 to ₅₀ μm and a thickness of 0.1 to 5 μm
Is preferable. Average particle size 10-35μ
Those having a range of m are excellent in glitter and are more preferable.

One or more of the above-mentioned pigments can be used, and one or more of a combination of a color pigment and an extender pigment, and if necessary, a flat pigment and a brilliant pigment are used. Can be used.

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

The aqueous base paint can be prepared by mixing the pigment dispersion paste, a film-forming resin and a curing agent. The pigment concentration (PWC) in the aqueous base paint, including the above-mentioned brilliant pigment and all other pigments, is as follows:
It is generally 0.1 to 50% by weight, preferably 0.1 to 50% by weight.
5 to 40% by weight, more preferably 1 to 30% by weight
It is. If it exceeds 50% by weight, the appearance of the coating film will be reduced.

The content of the pigment dispersant in the aqueous base paint may be 1 to 20% by weight based on the solid content when a pigment dispersant containing substantially no basic substance is used.
It is preferable that If the amount is less than 1% by weight, the dispersion stability of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 20% by weight, the physical properties of the resulting coating film may be reduced.

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

[0041]Base coating film formation method  The base paint is the uncured material formed as described above.
Apply to form an uncured base coating on top of the coating
You. As the above coating method, apply an aqueous intermediate coating
The method exemplified at that time can be mentioned. Base coating
When applying paint to automobile bodies, etc.,
Multistage coating with air electrostatic spray
Coating, preferably in two stages, or air
Combine the electrostatic spray with the above-mentioned rotary atomizing type electrostatic coating machine.
It is preferable to carry out by a combined coating method.

The dry film thickness of the base coating film varies depending on the application, but is preferably from 5 to 35 μm. Exceeding the upper limit may result in poor clarity and / or inconveniences such as unevenness and flow during coating. Below the lower limit, color unevenness may occur.

[0043]Step (3)  In the step (3), the uncured base formed as described above is used.
Apply a clear paint on the uncoated
To form a coating film.

[0044]Clear paint  The clear coating film contains a glitter pigment as a base paint.
Due to glitter pigments when using tallic base paint
Smoothes the unevenness and flicker of the base coating film.
It is also formed to protect the base coating.
You. There is no particular limitation on the clear paint, for example,
For example, from film-forming resins, curing agents and other additives.
Can be mentioned.

The film-forming resin is not particularly restricted but includes, for example, acrylic resins, polyester resins, epoxy resins, urethane resins, etc., which are combined with a curing agent such as an amino resin and / or a blocked isocyanate resin. Used. From the viewpoint of transparency or acid etching resistance, etc., a combination of an acrylic resin and / or a polyester resin and an amino resin, or a carboxylic acid.
It is preferable to use an acrylic resin and / or a polyester resin having an epoxy curing system.

As the clear paint, a viscosity control agent is added as an additive in order to apply the above-mentioned base paint in an uncured state after the application, and to prevent break-in, inversion or sagging between layers. Is preferred. The amount of the viscosity control agent to be added is 0.01 to 10 parts by weight, preferably 0.1 to 100 parts by weight, based on 100 parts by weight of the resin solid content of the clear paint.
The amount is from 02 to 8 parts by weight, more preferably from 0.03 to 6 parts by weight. If the amount is more than 10 parts by weight, the appearance deteriorates. If the amount is less than 0.1 part by weight, the effect of controlling the viscosity cannot be obtained, which causes problems such as sagging.

The form of the clear coating composition may be any of an organic solvent type, an aqueous type (water-soluble, water-dispersible, emulsion), a non-aqueous dispersion type, and a powder type. An adjusting agent or the like can be used.

[0048]Clear coating film forming method  The preparation method and coating method of the above clear paint are as follows.
It can be done according to the following method. Above clear coating
The dry film thickness varies depending on the application, but is 10 to 70 μm
It is. When the dry film thickness exceeds the upper limit, the sharpness deteriorates.
Troubles such as unevenness and flow during painting
Yes, if it is below the lower limit, the appearance may be reduced.

[0049]Step (4)  In the step (4), the intermediate coating film, the base
Heat-curing simultaneously the clear coating film and the clear coating film,
Obtain a multilayer coating. The temperature for heat curing is 1
Perform at 10 to 180 ° C, preferably 120 to 160 ° C
By doing so, it is possible to obtain a cured coating film with a high degree of crosslinking.
You. When the temperature exceeds 180 ° C., the coating film becomes hard and brittle,
If the temperature is lower than ℃, curing is not sufficient. Curing time depends on curing temperature
It changes more, but it takes 10-60 minutes at 120-160 ° C
Appropriate.

The thickness of the multilayer coating film obtained by the coating film forming method of the present invention is usually 30 to 300 μm, preferably 5 to 300 μm.
0 to 250 μm. If it exceeds 300 μm, the physical properties of the film such as a thermal cycle decrease, and if it is less than 30 μm, the strength of the film itself decreases.

The multilayer coating film obtained by the method for forming a multilayer coating film of the present invention is obtained by using a pigment obtained by using a water-based intermediate coating material and / or an aqueous base coating material with a pigment dispersant substantially containing no volatile basic substance. Since a dispersing paste is blended, the coating film can have an excellent finished appearance without yellowing.

Further, according to the three-wet one-bake coating method of the present invention, the baking step of the intermediate coating paint can be omitted as compared with the conventional three-coat two-bake method, so that the process can be shortened and the cost can be reduced. A new coating system aiming at reduction of energy consumption and environmental load can be constructed.

[0053]

EXAMPLES The present invention will be described in detail with reference to specific examples, but the present invention is not limited to the following examples. Parts and% mean parts by weight and% by weight, respectively.

Production Example 1 Water-soluble polyester resin A-1
Production of 200.0 parts of isophthalic acid and 1 part of phthalic anhydride
79.0 parts, adipic acid 176.0 parts, trimethylolpropane 150.0 parts, neopentyl glycol 29
After 5.0 parts and 2 parts of dibutyltin oxide were charged and heated in a nitrogen stream to melt the raw materials, the temperature was gradually raised to 170 ° C. with mixing and stirring. Thereafter, the mixture was further subjected to dehydration transesterification while raising the temperature to 220 ° C. over 3 hours. When the acid value reached 10, the mixture was cooled to 150 ° C. Further, 114.0 parts of hexahydrophthalic acid was added and reacted for 1 hour to complete the reaction. Further, after cooling to 100 ° C., 112.0 parts of butyl cellosolve was added to obtain a polyester resin A. The obtained polyester resin A
Had an acid value of 50, a hydroxyl value of 65, and a weight average molecular weight of 10,000 obtained by GPC (gel permeation chromatography). The polyester resin was cooled to 60 ° C.
0 parts and ion-exchanged water were added to obtain a polyester resin A-1 having a nonvolatile content of 50%.

Production Example 2 Production of Polystyrene Resin Emulsion A-2 In 94 parts of ion-exchanged water, 33.4 parts of Newcol 1120 (manufactured by Nippon Emulsifier Co.) and 6 parts of ethylene glycol mono-n-hexyl cellosolve were dispersed. While stirring 111.1 parts of the polyester resin A, the polyester resin emulsion A was coated with a nonionic dispersant having a non-volatile content of 47.5% by dropping the dispersion into the dispersion.
-2 was obtained.

Production Example 3 Production of Colored Pigment Paste C-1 9.4 parts of a commercially available pigment dispersant Disperbyk 190 (manufactured by BYK Chemie; solid content: 40% by weight), 36.8 parts of ion-exchanged water, rutile titanium dioxide 34 After premixing .5 parts, 34.4 parts of barium sulfate, 6 parts of talc, and 0.1 part of carbon black, a glass bead medium was added in a paint conditioner, and mixed and dispersed at room temperature until the particle size became 5 μm or less. Thus, a colored pigment paste C-1 was obtained. In the production of the pigment paste C-1, the time required until the particle size became 5 μm or less was 15 minutes.

Production Example 4 Production of Color Pigment Paste C-2 50.0 parts of water-soluble polyester resin A-1, 17.9 parts of ion-exchanged water, 34.5 parts of rutile type titanium dioxide, 34.4 parts of barium sulfate Colored pigment paste C-2 having a particle size of 5 μm or less was obtained in the same manner as in Production Example 3, except that talc was changed to 6 parts and carbon black was changed to 0.1 part. In the production of the pigment paste C-2, the time required until the particle size became 5 μm or less was 60 minutes.

Production Example 5 Preparation of aqueous base paint (1) Production of water-soluble acrylic resin Dipropylene glycol methyl ether 2 was placed in a reaction vessel.
3.9 parts and propylene glycol methyl ether 1
6.1 parts were added. 120 while mixing and stirring in a nitrogen stream
The temperature was raised to ° C. Then, 54.5 parts of ethyl acrylate.
A mixed solution of 12.5 parts of methyl methacrylate, 14.7 parts of 2-hydroxyethyl acrylate, 10.0 parts of styrene, 8.5 parts of methacrylic acid and 10.0 parts of dipropylene glycol methyl ether, t-butyl peroxy -2-ethylhexanoate, 2.0 parts of an initiator solution
It was dropped into the reaction vessel in parallel over time. After completion of the dropping, aging was performed at the same temperature for 0.5 hour. Further, an initiator solution consisting of 5.0 parts of dipropylene glycol methyl ether and 0.3 parts of t-butylperoxy-2-ethylhexanoate was dropped into the reaction vessel over 0.5 hour. After completion of dropping, aging was performed at the same temperature for 1 hour. Then, the solvent is removed under reduced pressure (70 Torr) 110
After distilling off 16.1 parts of the solvent at a temperature of 45 ° C., 45 mg of dimethylethanolamine was added to 1 g of the solid content of the resin, and ion-exchanged water was added. An acrylic resin was obtained.

(2) Production of Colored Pigment Paste 67.0 parts of the above water-soluble acrylic resin, ion-exchanged water 2
After premixing 3.2 parts and 67.0 parts of rutile type titanium dioxide, a glass bead medium was added in a paint conditioner, and the mixture was mixed and dispersed at room temperature for 1 hour to obtain a color pigment paste having a particle size of 5 μm or less.

(3) Preparation of Aqueous Base Coating Using the above water-soluble acrylic resin, Cymel 204 (manufactured by Mitsui Cytec) and the above-mentioned colored paste, the solid content was mixed so as to be as follows. Rutile type titanium dioxide 20.0 parts Water-soluble acrylic resin 48.0 parts Cymel 204 32.0 parts

Example 1 (1) Preparation of aqueous intermediate coating material Colored paste C-1 obtained in Production Example 3, water-soluble polyester resin A-1 obtained in Production Example 1, polyester resin emulsion obtained in Production Example 2 A-2 and a melamine resin were blended so as to have a solid content as shown in Table 1. Cymel 235 (manufactured by Mitsui Cytec) was used as the melamine resin.

[0062]

[Table 1]

(2) Method for forming a coating film A power top U-50 was applied to a dull steel plate treated with zinc phosphate.
(Nippon Paint cationic electrodeposition paint)
On a plate which was electrodeposited so as to have a thickness of 0 μm and baked at 160 ° C. for 30 minutes, a pre-diluted Olga P-2 (a melamine-curable polyester resin-based white intermediate coating from Nippon Paint) was coated with a 35 μm dry coating film. Thus, two-stage coating was performed by air spray, baked at 140 ° C. for 30 minutes, and then cooled to obtain a substrate. The obtained substrate was coated with the above-mentioned aqueous intermediate coating material by air spray coating at a thickness of 25 μm.
After heating at 5 ° C. for 5 minutes, the base paint of Production Example 5 was applied by air spray coating at 20 μm, and heated at 80 ° C. for 5 minutes.
Further, Macflow O-1801W clear (manufactured by Nippon Paint) is applied to the coated plate by air spray coating to a thickness of 50 μm.
After painting, baking was performed under each condition.

(3) Evaluation of coating film (3-1) Evaluation of yellowing: Baking at 140 ° C. for 30 minutes;
The b value of the baked coating film was measured using an SM color computer SM-4 (manufactured by Suga Test Instruments Co., Ltd.) before and after baking for minutes. The results are shown in Table 1.

(3-2) Visual Evaluation of Appearance The appearance of the coating film when baked at 140 ° C. for 30 minutes was visually evaluated. ：: no yellowing is observed Δ: slight yellowing is observed ×: yellowing is observed

Example 2, Comparative Examples 1 to 3 A coated plate having a multilayer coating film was prepared and evaluated in the same manner as in Example 1 except that the aqueous intermediate coating composition was prepared according to the formulation shown in Table 1. Was done. The results are shown in Table 1.

From the Examples and Comparative Examples, the aqueous intermediate coating composition produced in this Example contains a pigment dispersion paste substantially free of volatile basic substances, and thus the resulting multilayer coating is obtained. The coating was excellent in appearance without yellowing.

[0068]

According to the method for forming a multilayer coating film of the present invention, a pigment dispersion paste containing a pigment dispersant substantially containing no volatile basic substance is mixed with an aqueous intermediate coating composition and / or an aqueous base coating composition. Therefore, an excellent finished appearance can be obtained without yellowing of the coating film.

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) C09D 5/00 C09D 5/00 Z 5/02 5/02 17/00 17/00 201/00 201/00 (72) Inventor Chisato Yabushita 19-17 Ikedanakamachi, Neyagawa-shi, Osaka F-term in Nippon Paint Co., Ltd. DB001 DD001 DD121 DD231 DG001 KA08 KA09 MA08 NA01 NA23 PA12 PA19 PB07

Claims (6)

[Claims] 1. A step (1) of applying an aqueous intermediate coating composition on an object to be coated with an electrodeposition coating film to form an uncured intermediate coating film, A step (2) of applying an aqueous base paint to form an uncured base coating, and a step (3) of applying a clear coating thereon to form an uncured clear coating; and A method for forming a multilayer coating film comprising 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, wherein the aqueous intermediate coating material and / or The water-based coating composition is characterized in that a pigment dispersion paste obtained by previously dispersing a pigment and a pigment dispersant substantially free of a volatile basic substance is blended, and a multilayer coating film is formed. Method. 2. The method according to claim 1, wherein the content of the pigment dispersant in the aqueous intermediate coating composition is 0.5 to 10% by weight based on the solid content. 3. The content of the pigment dispersant in the aqueous base paint is 1 to 20% by weight based on the solid content.
The method for forming a multilayer coating film according to the above. 4. A multilayer coating film formed by the method for forming a multilayer coating film according to claim 1, 2 or 3. 5. An aqueous intermediate coating composition comprising a pigment and a pigment dispersing paste obtained by previously dispersing a pigment and a pigment dispersing agent, wherein the pigment dispersing agent substantially contains a volatile basic substance. A water-based intermediate coating composition characterized in that it is not contained in a water-based intermediate coating composition. 6. The aqueous intermediate coating composition according to claim 5, wherein the content of the pigment dispersant in the aqueous intermediate coating composition is 0.5 to 10% by weight based on the solid content.