JP2822587B2 - Coating method - Google Patents

Description

The present invention relates to a novel method for forming a coating film.
More specifically, the present invention relates to a method for forming a coating film having excellent weather resistance, acid resistance, and finished appearance without causing a large amount of energy consumption and environmental pollution.

[Prior art] Conventionally, a coating containing a metallic pigment such as an aluminum pigment or a mica pigment or a coloring pigment (hereinafter referred to as a base coat) is applied to an outer panel of an automobile or the like from the viewpoint of aesthetics and weather resistance without baking. A method of applying a transparent paint (hereinafter referred to as a top coat) thereon and heat-curing to form a coating film has been adopted.

The base coat in these paints is a low solid paint containing a large amount of solvent to obtain aesthetics, and the top coat is a vinyl copolymer / melamine resin in order to secure weather resistance. It is.

However, in recent years, there has been a problem that the coating film is easily wetted by acid rain having a low pH due to air pollution. Also, since the base coat contains a large amount of solvent,
It is not preferable from the viewpoints of resource saving and environmental pollution. For this reason, if the solvent is to be reduced, the metallic pigment is difficult to disperse uniformly, and there is a problem that the aesthetic property of the obtained coating film is inevitably reduced.

[Problems to be Solved by the Invention] Under such circumstances, the present invention forms a coating film excellent in weather resistance, acid resistance, and finished appearance without causing a large amount of energy consumption and environmental pollution. It is intended to provide a method for doing so.

[Means for Solving the Problems] The present inventors have conducted intensive studies in order to achieve the above object, and as a result, after applying a water-based paint as a base coat on the surface of the object to be coated, the top surface of this painted surface was applied. A top coat containing a vinyl copolymer and a resin component composed of a specific ratio of a melamine resin and a blocked isocyanate compound is applied as a coat, and then heated and cured.
It has been found that the purpose can be achieved by the coat 1 bake treatment, and based on this finding, the present invention has been completed.

That is, according to the present invention, when a coating film is formed by a two-coat one-bake treatment, an aqueous paint containing a metallic pigment and / or a color pigment is applied to the surface of an object to be coated. Forming a coating film characterized by applying a top coat containing a polymer, a resin component comprising 5 to 25% by weight of a melamine resin and 5 to 40% by weight of a blocked isocyanate compound, and then heating and curing. It provides a method.

 Hereinafter, the present invention will be described in detail.

In the method of the present invention, a water-based paint is used as a base coat. This water-based paint mainly contains a water-soluble thermosetting resin or a water-dispersible thermosetting resin, a metallic pigment and / or a coloring pigment, a water-soluble organic solvent, and deionized water. It contains a pigment, a viscosity modifier, a coating surface modifier and the like.

Examples of the water-soluble or water-dispersible thermosetting resin include, for example, an alkyd resin, a polyester resin, an acrylic resin, and a water-soluble or water-dispersible base resin selected from cellulosic resins, and a crosslinking agent. For example, those containing a water-soluble or water-dispersible amino resin obtained by condensation or co-condensation of melamine, benzoguanamine, urea or the like with formaldehyde, or a water-dispersible blocked isocyanate resin are used. As these base resins and crosslinking agents, those already known per se can be used.

There is no particular limitation on the metallic pigment,
Conventionally used in water-based paints, for example, aluminum, copper, brass, bronze,
Stainless steel or the like, or mica-like iron oxide, flaky metallic powder, mica pieces coated with titanium oxide or iron oxide, or the like is used.

On the other hand, there is no particular limitation on the coloring pigment, and those usually used in paints, for example, inorganic pigments such as titanium dioxide, red iron oxide, yellow iron oxide, and carbon black, phthalocyanine blue, phthalocyanine green, quinacridone-based red pigment, and isoindone An organic pigment such as a linone-based yellow pigment can be used.

In the present invention, a top coat containing a resin component comprising a vinyl copolymer, a melamine resin, and a blocked isocyanate compound is used. The content of the melamine resin in the resin component must be in the range of 5 to 25% by weight, and the content of the blocked isocyanate compound must be in the range of 5 to 40% by weight. If the content of the melamine resin is less than 5% by weight, crosslinking may be insufficient and gasoline resistance may be deteriorated, and if it exceeds 25% by weight, ether bonds due to the melamine resin which is easily hydrolyzed in the coating film increase. There is a tendency for acid resistance to decrease because of too much. On the other hand, the content of the blocked isocyanate compound is 5% by weight.
If the amount is less than the above, crosslinking may be insufficient and the gasoline resistance may be poor. If the amount exceeds 40% by weight, the amount of the soft blocked isocyanate component becomes too large, and the hardness of the coating film tends to be insufficient.

As the melamine resin used for the top coat, formaldehyde is added to melamine, and then the melamine has 1 to 1 carbon atoms.
And those etherified with a monohydric alcohol. This melamine resin may be used alone or in combination of two or more. Examples of the blocked isocyanate compound include hexamethylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4 (or 2,6) -diisocyanate, 4,4'-methylenebis (cyclohexyl isocyanate), these diisocyanates, ethylene glycol and triglycol. Adducts with polyhydric alcohols such as methylolpropane, or burettes, which are adducts with water, and isocyanurate polymers of the above-mentioned diisocyanates, etc., can be used as monohydric alcohols, ε-caprolactam, oxime compounds, ethyl acetoacetate or malon. Examples include those blocked with a compound having active methylene such as ethyl acid. Among these, oxime compounds and compounds having active methylene such as ethyl acetoacetate and ethyl malonate are exemplified. Those blocked with compounds are particularly preferred in terms of low-temperature curability.

As the vinyl copolymer used in the overcoat,
Those having a hydroxyl value of 50 to 200 mgKOH / g, an acid value of 30 mgKOH / g or less, and a number average molecular weight in the range of 2,000 to 15,000 are preferred. When the hydroxyl value is less than 50 mgKOH / g, the crosslink density of the coating film is low, and the gasoline resistance may be poor. Also, the hydroxyl value may exceed 200 mgKOH / g,
If it exceeds 30 mgKOH / g, the polarity of the resin tends to be too high and the water resistance tends to decrease. On the other hand, the number average molecular weight is 2,
If it is less than 000, the crosslinking density of the coating film may be low and the gasoline resistance may be poor. If it exceeds 15,000, the fineness of the coating material may be insufficient and the smoothness of the coating film may be poor.

As the monomer component of the vinyl copolymer, for example, methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, dodecyl acrylate, octadodecyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate,
Acrylic monomer having no functional group other than unsaturated double bond such as octyl methacrylate, dodecyl methacrylate, octadodecyl methacrylate; acrylic monomer having a carboxy group such as acrylic acid and methacrylic acid; 2-hydroxy Acrylic monomer containing a hydroxyl group such as ethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate and a reaction product of these compounds with caprolactone; glycidyl acrylate;
An acrylic monomer having an epoxy group such as glycidyl methacrylate; an ester of maleic acid and a monohydric alcohol; and other vinyl monomers such as styrene, α-methylstyrene, and vinyl acetate can be preferably used.

Further, an organic solvent may be used in the top coat according to the present invention, if desired. Examples of the organic solvent include aromatic carbons such as toluene, xylene and Solvesso # 100 (trade name, manufactured by Exxon Chemical Co., Ltd.). Hydrogen solvents; ester solvents such as ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, and propylene glycol monomethyl ether acetate; methyl isobutyl ketone;
Ketone solvents such as methyl amyl ketone; alcohol solvents such as butyl alcohol and amyl alcohol; ether solvents such as ethylene glycol monobutyl ether can be used.

In the top coat of the present invention, additives or small amounts of inorganic pigments and organic pigments usually used for the top coat can be blended as a third component, if desired. Examples of the additive include a coating surface conditioner, an ultraviolet absorber, and a thermal deterioration inhibitor. These additives and a small amount of inorganic pigments and organic pigments may be used alone or in combination of two or more.

In the method of the present invention, first, the water-based paint is applied to the surface of the object to be coated. In this case, an atomization type coating machine, for example, an air spray coating machine, an airless spray coating machine, an air atomization type or a rotary mist is used. It is desirable to apply a dry film thickness of about 5 to 25 μm using a chemical electrostatic coating machine or the like.

After applying the water-based paint in this manner, the temperature is
After leaving for about 1 to 20 minutes at 0 ° C., the top surface is coated with a top coat paint, usually by an atomization type coating machine, for example, an air spray coating machine, an airless spray coating machine, an air atomization type or a rotary atomization type. Dry film thickness of 10 to 80 using an electric coating machine
The coating is applied so as to have a thickness of about μm, and then heat-cured at a temperature of 120 to 180 ° C. for 10 to 60 minutes in a hot-air oven, an infrared oven, a dielectric heating oven, or the like to form a desired coating film.

[Examples] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

 In addition, the coating film performance was determined as follows.

(1) Acid Resistance A test piece was spotted with 0.2 ml of 40% by weight sulfuric acid at 50 ° C. for 15 minutes to examine the coating film for abnormal appearance such as spots, blisters, and blemishes.

(2) Gasoline resistance The test piece was immersed in gasoline at 20 ° C. for 1 hour, and the presence or absence of abnormal appearance such as swelling of the coating film and dirt was examined.

(3) Water resistance The test piece was immersed in warm water of 40 ° C. for 240 hours, and the presence or absence of abnormal appearance such as swelling of the coating film and dirt was examined.

(4) Hardness of coating film Determined in accordance with JIS K-5400 6.14 pencil scratch test.

(5) Finish appearance of coating film The finish appearance of the coating film was evaluated using an image clarity measuring device (HA-ICM) manufactured by Suga Test Instruments Co., Ltd. The higher the value, the better the finished appearance of the coating film.

Production Example 1 Production of Aqueous Acrylic Resin Dispersion (a-1) for Water-based Paint In a flask equipped with a stirrer, reflux condenser and thermometer, 113 parts by weight of deionized water, 30% by weight Newco 1707F [anionic surfactant And 5 parts by weight of trade name, manufactured by Nippon Emulsifier Co., Ltd.] and 0.25 part by weight of ammonium persulfate. Next, the following monomer mixture was added dropwise over 3 hours, and then kept at 70 ° C. for further 3 hours, and a heating residue 44.5% by weight, pH 2.3 acrylic resin dispersion (a-1)
I got

Methyl methacrylate 40 parts by weight Styrene 10 parts by weight butyl acrylate 30 parts by weight 2-hydroxyethyl methacrylate 15 parts by weight Methacrylic acid 5 parts by weight Production Example 2 Production of aqueous acrylic resin solution (a-2) for water-based paint Stirrer, reflux condenser, 80 parts by weight of butyl cellosolve is added to a flask equipped with a thermometer, and the mixture is heated and stirred at 140 ° C. Next, the following monomer mixture was added dropwise over 3 hours.

Methyl methacrylate 45 parts by weight Butyl acrylate 25 parts by weight Styrene 10 parts by weight 2-hydroxyethyl methacrylate 10 parts by weight Acrylic acid 10 parts by weight t-butyl peroxybenzoate 1 part by weight Then, 0.5 parts by weight of t-butyl peroxybenzoate was added, and further The reaction was continued at 140 ° C. for 2 hours to obtain a resin solution having a heating residue of 55.2% by weight, an acid value of 43 and a number average molecular weight of 4,600. This is neutralized with an equivalent amount of dimethylaminoethanol, and further heated by adding deionized water.
An acrylic resin aqueous solution (a-2) of 50% by weight was obtained.

Production Example 3 Production of water-based paint Acrylic resin aqueous dispersion (a-1) obtained in Production Example 1
Dimethylaminoethanol was added thereto, and the mixture was neutralized and adjusted to pH 7.5. To 100 parts by weight, 50 parts by weight of the aqueous acrylic resin solution (a-2) obtained in Production Example 2 and aluminum paste AW-500B (manufactured by Asahi Kasei Metals Corporation, Trade name) A mixture of 6 parts by weight, 10 parts by weight of deionized water and 10 parts by weight of butyl cellosolve was added, and 25 parts by weight of melamine resin [Cymel 350: manufactured by Mitsui Cyanamid Co., Ltd.] was added and mixed well. did.
20 seconds / 2 in Ford Cup # 4 using deionized water
After dilution to 0 ° C., an aqueous paint was obtained.

Production Example 4 Production of Acrylic Resin for Top Coating Paint A flask equipped with a stirrer, a reflux condenser and a thermometer was charged with 80 parts by weight of xylene and heated to 140 ° C. Next, the following monomers and a polymerization initiator were added dropwise at 140 ° C. over 3 hours.

Styrene 20.0 parts by weight Butyl methacrylate 33.0 parts by weight Butyl acrylate 21.8 parts by weight 2-hydroxyethyl methacrylate 23.2 parts by weight Acrylic acid 2.0 parts by weight t-butyl peroxybenzoate 1.2 parts by weight Then, 0.5 parts by weight of t-butyl peroxybenzoate was added, and further The reaction was continued at 140 ° C for 2 hours, and the residue of the heating was 56.5 wt%, GPC method [Toyo Soda Kogyo H
A resin solution having a number average molecular weight of 4,300 based on polystyrene conversion using LC-802A (trade name) was obtained.

Example 1 73.0 parts by weight of the acrylic resin for a top coat obtained in Production Example 4,
Melamine resin [Uban 166-60, manufactured by Mitsui Toatsu Chemicals, Inc.
Product name, isobutylated melamine resin, heating residue 60wt%]
14.6 parts by weight, block isocyanate compound [Dismodur BL-3175, manufactured by Sumitomo Bayer Co., Ltd., trade name, oxime block of hexamethylene diisocyanate trimer, heating residue 75 wt%] 11.8 parts by weight, surface conditioner [Modaflow,
Monsanto, trade name] 0.1 part by weight, dibutyltin dilaralate 0.5 part by weight well mixed, Solvesso # 100
[Aromatic hydrocarbon, manufactured by Esso Corporation] / Ford cup # 4 for 30 seconds using a mixed solvent of n-butanol (weight ratio 80/20) /
After dilution at 20 ° C., a top coat was produced.

Next, zinc phosphate treatment [Nihon Parkerizing Co., Ltd.
Manufactured by Nippon Yushi Co., Ltd., trade name: Aqua # 410
0] was applied to a dry film thickness of 20 μm, baked at 175 ° C. for 30 minutes, and then coated with an intermediate coating [Nippon Yushi Co., Ltd. product name, Hiepico NO1] to a dry film thickness of 30 μm. A test plate was prepared by baking at 140 ° C. for 30 minutes.

The surface of the test plate was spray-coated with the water-based paint obtained in Production Example 3 so that the dry film thickness became 15 μm.
After drying at ℃ for 3 minutes, the above-mentioned top coat is dried
Electrostatic spray coating was performed so that the thickness became 30 μm, and baking was performed at 140 ° C. for 30 minutes to prepare a test piece.

 Table 1 shows the test results of the coating properties of the test pieces.

Examples 2 and 3 and Comparative Examples 1 to 3 Using the water-based paint obtained in Production Example 3 and the topcoat paint prepared in the same manner as in Example 1 based on the formulation shown in Table 1, the same as in Example 1 The test piece was prepared as described above, and the coating film performance was determined. The results are shown in Table 1.

Examples 4 and 5 Example 1 was obtained by using Aqua Base Silver (trade name, manufactured by ICI) as a water-based paint and using the composition shown in Table 1.
A test piece was prepared in the same manner as in Example 1 using the overcoat paint prepared in the same manner as in Example 1, and the coating film performance was evaluated. Table 1 shows the results.

As shown in Examples 1 to 5, according to the present invention, a coating film having excellent acid resistance, water resistance, gasoline resistance, coating film hardness, and finished appearance is formed. If the melamine resin and the blocked isocyanate compound are less than 5% by weight, the water resistance, gasoline resistance, coating film hardness, and finish appearance of the coating film are inferior. As shown in Comparative Example 2, the melamine resin contains 25% by weight.
When it exceeds, acid resistance and finished appearance are inferior. Further, as shown in Comparative Example 3, the blocked isocyanate compound
If it exceeds wt%, water resistance, gasoline resistance, coating film hardness and finished appearance of the coating film are inferior.

[Effects of the Invention] According to the present invention, a coating film excellent in weather resistance, acid resistance and finished appearance can be efficiently formed without causing a large amount of energy consumption and environmental pollution.

Claims (3)

(57) [Claims] In forming a coating film by a two-coat one-bake treatment, a water-based paint containing a metallic pigment and / or a color pigment is applied to the surface of an object to be coated. Forming a coating film characterized by applying a top coat containing a polymer, a resin component comprising 5 to 25% by weight of a melamine resin and 5 to 40% by weight of a blocked isocyanate compound, and then heating and curing. Method. 2. A vinyl copolymer having a hydroxyl value of 50 to 200 mgK.
OH / g, acid value 30 mgKOH / g or less and number average molecular weight 2,000 to 15,0
The method for forming a coating film according to claim 1, wherein 3. The method according to claim 1, wherein the blocked isocyanate compound comprises an aliphatic isocyanate compound.
The method for forming a coating film according to the above.