JPH1043674A - Method for forming composite coating film for automobile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method having good quality in smoothness, rust prevention, coating work, etc., in a method for forming a composite coating film for automobiles. SOLUTION: In a method for forming a composite coating film for automobiles in which a cationic electrodeposition coating film is formed on a steel plate for automobiles using a cationic electrodeposition coating, and a finishing coating film containing pigment is formed on the electrodeposition coating film, a cationic electrodeposition coating composition which contains a resin having cationic groups (base resin) A, block polyisocyanete B (curing resin) B, and a water-insoluble component C which is added as required and in which each component of A, B, C is exclusively composed of components the specific gravity of which is 2.0 or less is used as the electrodeposition coating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a composite coating film for automobiles.

[0002]

2. Description of the Related Art A composite coating film for automobiles is obtained by forming an electrodeposition coating film on a steel sheet for automobiles, and applying an intermediate coat, a top coat or a clear coat. Composite coatings for commercial vehicles and the like are often obtained by omitting the intermediate coating and applying an overcoat on the electrodeposition coating. The electrodeposition coating formed here has high coating resistance, mechanical strength, coloring, concealment,
Various fillers have been added for the purpose of ensuring rust prevention and the like.

[0003]

Typical fillers are pigments (color pigments, extender pigments, rust-preventive pigments). On the other hand, the addition of these pigments results in a smooth electrodeposition coating film to be formed, and The top coat was sharp and the workability of coating was such that the filter and the UF film were clogged with pigment, the finish of the horizontal portion was impaired, and continuous agitation was required to prevent sedimentation of the pigment. Conventionally, although an improvement effect has been seen by the pigment dispersion technique and the pigment / resin ratio, there has been a problem that a sufficient effect cannot be obtained from the viewpoint of the above-described improvement of the coating workability.

[0004]

In view of the above, the present inventors have found that the method of forming a two-coat automotive composite coating film is superior in smoothness, rust prevention, coating workability and the like. The purpose of this study was to develop a method with high quality. As a result, it has been found that the above-mentioned object is achieved only when an electrodeposition paint having the following characteristics is used.

That is, the present invention relates to a method for forming a composite coating film for automobiles, comprising forming a cationic electrodeposition coating film on a steel plate for automobiles using a cationic electrodeposition coating material, and further forming a pigmented top coating film thereon. The electrodeposition paint contains (A) a resin having a cationic group (base resin), (B) a blocked polyisocyanate (curing agent resin), and (C) a water-insoluble component added as needed. A),
Each of the components (B) and (C) is a method for forming a composite coating film for automobiles using a cationic electrodeposition coating composition comprising only components having a specific gravity of 2.0 or less.

In the method for forming a composite coating film of the present invention,
As the overcoating material for forming the overcoating film, conventionally known appropriate ones can be applied, but as the cationic electrodeposition coating material, one having only a component having a specific gravity of 2.0 or less must be used. When a component having a specific gravity of more than 2.0 is used, the smoothness of the electrodeposition coating film,
As a result, the overcoating clarity is impaired, which is not preferable.

As the resin having a cationic group used as the base resin (A) of the cationic electrodeposition coating composition of the present invention, an amino group is added to an epoxy resin, an epoxy group-containing acrylic resin, an epoxy group-free acrylic resin, a polyurethane resin and the like. To obtain the amino group by protonation with an acid. Particularly preferred acids include formic acid, acetic acid, lactic acid,
There are propionic acid, citric acid, malic acid, sulfamic acid and the like. Although the amount of the amino group is not particularly limited, the resin 1 is usually used.
0.5 to 3 equivalents per 000 g is suitable. Particularly preferred resins having a cationic group are epoxy resins and acrylic resins, and the specific gravity thereof is 2.0 or less.

[0008] The epoxy resin is preferably an epoxy resin having an average of two epoxy groups per molecule. To be specific, glycidyl ether of polyphenol having two phenolic hydroxyl groups in one molecule is preferable, and preferred polyphenols are resorcin, hydroquinone, 2,2-bis- (4
-Hydroxyphenyl) -propane (commonly known as bisphenol A), 4,4'-dihydroxybenzophenone,
1-bis- (4-hydroxyphenyl) -methane, 1,
1-bis- (4-hydroxyphenyl) -ethane, 4,
4'-dihydroxybiphenyl and the like can be mentioned. Others include glycidyl ethers of diols having two alcoholic hydroxyl groups in one molecule, and preferred diols include ethylene glycol,
Propylene glycol, 1,4-butanediol, 1,
6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, 1,4-
Examples thereof include low molecular weight diols such as cyclohexanediol, and oligomer diols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol, but are not limited thereto, and a mixture of the above epoxy resins is also possible.

The molecular weight of the epoxy resin is from 2000 to 100
00 is preferable, but from the viewpoint of coating quality represented by coating film appearance, rust prevention performance and the like, throwing power, pretreatment sensitivity, repelling resistance and the like, especially 3000
~ 8000 is preferred. To obtain a suitable molecular weight,
The epoxy resin is used as a linking agent to increase the molecular weight.
Preferred linking agents include the above-mentioned polyphenols or diols, and further dicarboxylic acids having two carboxyl groups in one molecule such as adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, Examples thereof include isophthalic acid, dimer acid, a butadiene polymer containing a carboxyl group, and a butadiene / acrylonitrile copolymer.
Examples of the amine for increasing the molecular weight include primary amines such as ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, monoethanolamine, dimethylaminopropylamine and diethylaminopropylamine, and hexamethylenediamine. And diamines in which each amino group has been secondaryized. It is also possible to extend the chain length with a polyisocyanate. Particularly preferable high molecular weight is a method in which the glycidyl ether of the above-mentioned polyphenol and the glycidyl ether of the above-mentioned diol are linked and reacted with the above-mentioned polyphenol, and the reaction temperature is suitably from 70 to 180 ° C.

Regarding the epoxy group-containing acrylic resin,
It is obtained by copolymerizing an epoxy group-containing unsaturated monomer, an acrylic monomer and, if necessary, an unsaturated monomer other than the acrylic monomer in the presence of a polymerization initiator. Examples of the epoxy group-containing unsaturated monomer include glycidyl acrylate, glycidyl methacrylate, and allyl glycidyl ether. The acrylic monomers include ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate,
-Hydroxyethyl acrylate, methyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl methacrylate, cyclohexyl methacrylate, acrylamide, methacrylamide, N-methylol acrylamide, N- (n-butoxymethyl) -acrylamide And unsaturated monomers other than acrylic monomers include styrene, α-methylstyrene, vinyltoluene, cyclohexylvinylether, acrylonitrile and the like.

As the aminating agent for aminating the epoxy group of the epoxy resin or the epoxy group-containing acrylic resin, methylamine, ethylamine, n-propylamine, isopropylamine, monoethanolamine, n-propanolamine, isopropanolamine , Diethylamine, diethanolamine, N-methylethanolamine, N-ethylethanolamine, ethylenediamine, diethylenetriamine, hydroxyethylaminoethylamine, ethylaminoethylamine, methylaminopropylamine, dimethylaminopropylamine and the like, and mixtures thereof.
Of these, alkanolamines having a hydroxyl group are particularly preferred. Alternatively, after the primary amino group is previously reacted with a ketone to form a block, the remaining active hydrogen may be reacted with an epoxy group. As a specific method of amination,
In the presence or absence of a solvent, usually 50 to 120
The reaction is carried out at a temperature of ° C.

[0012] The epoxy group-free acrylic resin includes an amino group-containing unsaturated monomer such as dimethylaminoethyl acrylate, diethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminopropyl acrylamide, diallylamine and the above-mentioned acrylic monomer. And an unsaturated monomer other than an acrylic monomer is copolymerized in the presence of a polymerization initiator.

As for the polyurethane resin, diisocyanate having two isocyanate groups per molecule and one isocyanate
A method in which a diol having two hydroxyl groups per molecule is reacted and an amino alcohol having a tertiary amino group is reacted with a terminal isocyanate group, or an epoxy group of a polyurethane resin obtained by reacting glycidol or the like with a terminal isocyanate group, It can be obtained by an amination method according to the method described above.

The (B) blocked polyisocyanate (curing agent) in the present invention is a reaction product of a polyisocyanate and a blocking agent. As the polyisocyanate, aromatic or aliphatic (including alicyclic) is used. A polyisocyanate, for example, 2,4- or 2,6
-Tolylene diisocyanate and mixtures thereof, p
-Phenylene diisocyanate, diphenylmethane-
4,4'-diisocyanate, polymethylene polyphenyl isocyanate, tetramethylene diisocyanate,
Hexamethylene diisocyanate, isophorone diisocyanate, 1,3 or 1,4-bis- (isocyanatomethyl) -cyclohexane, dicyclohexylmethane-4,4'-diisocyanate, bis- (isocyanatomethyl) -norbornane, 3 or 4-isocyanatomethyl- 1-methylcyclohexyl isocyanate, m- or p-xylylene diisocyanate, m
-Or p-tetramethylxylylene diisocyanate, or a burette-modified or isocyanurate-modified isocyanate, or a part of the isocyanate group of the isocyanate is converted to ethylene glycol, propylene glycol, 1,6-hexanediol, neopentyl Glycol, diethylene glycol,
Linked with low molecular weight diols such as triethylene glycol and 1,4-cyclohexanediol, oligomer diols such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol and polylactone diol, and polyols such as trimethylolethane, trimethylolpropane and pentaerythritol And mixtures thereof.

Examples of the blocking agent include aliphatic alcohol compounds such as methanol, ethanol, n-butanol and 2-ethylhexanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether and ethylene glycol monohexyl ether. Cellosolve compounds, diethylene glycol monoethyl ether, carbitol compounds such as diethylene glycol monobutyl ether, acetone oxime, methyl ethyl ketone oxime,
Oxime compounds such as cyclohexanone oxime; lactam compounds such as ε-caprolactam; phenolic compounds such as phenol, cresol and xylenol; and active methylene group-containing compounds such as ethyl acetoacetate and diethyl malonate.

The reaction between the polyisocyanate and the blocking agent can be carried out in a solvent or in a melt without a solvent. As the solvent used in the reaction, a solvent that does not react with the polyisocyanate, for example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone, and isophorone, and aromatics such as benzene, toluene, xylene, chlorobenzene, and nitrobenzene Examples thereof include hydrocarbons, cyclic ethers such as tetrahydrofuran and dioxane, and halogenated hydrocarbons such as chloroform and carbon tetrachloride. The reaction temperature is not particularly limited, but is preferably 30 to 1
50 ° C. The specific gravity of the blocked polyisocyanate thus obtained is 2.0 or less.

The water-insoluble component (C) optionally added to the cationic electrodeposition coating composition of the present invention includes a specific gravity of 2.
It is a powdery inorganic substance having a particle size of 0 or less, and a desired particle size can be obtained by a usual pigment dispersing method. Examples thereof include carbon black and the like. Also, a powdered organic substance having a specific gravity of 2.0 or less, an organic pigment, a dye or an inorganic pigment, a component colored or not colored by an organic pigment and a dye, and an inorganic substance, in order to adjust the fluidity regardless of the organic substance. Thickeners, anti-repellents, leveling agents, etc., which interact with the gelled substance or other components to be added, may be mentioned, all of which are limited in that they have a specific gravity of 2.0 or less.

(A) Cationic amine-modified epoxy resin (base resin) of the cationic electrodeposition coating composition of the present invention
And the content ratio of the (B) blocked polyisocyanate (curing agent) is from 90 to 40/10 to 60, preferably from 85 to 40/15 to 60, more preferably from 80 to 55/20 in terms of solids weight ratio. ~ 45. A part of the component (A) may be replaced with a resin obtained by cationizing a normal amine-modified epoxy resin other than the component (A), but the amount thereof is 30% by weight based on the component (A). The following is preferred. When rust prevention performance is emphasized, a water-soluble additive is used in a weight ratio of 0.1 to 10.0%, preferably 0.5 to 5.0% with respect to component (A) + component (B). P
Metal salts such as b or Zn can be used.

The cationic electrodeposition coating composition of the present invention can be applied to a desired base surface by a known cationic electrodeposition coating in a state of being generally dispersed in water. Specifically, the solid content concentration of the paint is preferably about 5 to 40% by weight, more preferably 10 to 20% by weight, the pH is adjusted to 5 to 8, the bath temperature is 15 to 35 ° C, and the load voltage is 100 to 450V. Under the above conditions, the object can be coated as a cathode. After the coated object is washed with water, it is heated at 100 to 200 ° C. in a baking oven.
For 10 to 30 minutes to obtain a cured coating film. The thickness of the coating film obtained from the cationic electrodeposition coating composition of the present invention is not particularly limited.
0 μm, preferably 10 to 40 μm is suitable. The top coat used in the present invention is an automotive top coat mainly composed of an acrylic resin, and after spray coating, can be baked in a baking oven at 120 to 140 ° C. for 15 to 25 minutes to obtain a cured coating film. The composite coating film of the present invention is formed by coating and baking the above-mentioned top coat paint to a film thickness of about 40 μm on the electrodeposition coating film which is paint-baked to a film thickness of about 20 μm by the above method.

[0020]

EXAMPLES The present invention will be described below with reference to production examples of (A) an amine-modified epoxy resin (base resin) and (B) a blocked isocyanate (curing agent), along with examples and comparative examples.

Production Example 1 (Production of Base Resin A1) Using the raw materials shown in Table 1, the amine-modified epoxy resin (A) of the present invention was produced by the following method.

[Table 1] Raw material (1) Glicier PP-300 manufactured by Sanyo Kasei Co., Ltd.
P Raw material (2) Etoto YD-128 manufactured by Toto Kasei Co., Ltd. Raw materials (1), (2), (3), and (4) are charged into a 5-liter 4-neck flask equipped with a stirrer, a thermometer, and a cooling tube. The mixture was stirred, heated and heated to 150 ° C. 150
After maintaining at 6 ° C. for 6 hours, the raw material (5) was gradually charged and
Cooled to ° C. Next, the raw material (6) was charged and the temperature was raised to 100 ° C. After keeping at 100 ° C. for 2 hours, it was cooled to 80 ° C. and taken out. The obtained amine-modified epoxy resin A1 had a molecular weight of 4,600, a specific gravity of 1.2, and a solid content of 65%.

Production Example 2 (Production of Acrylic Base Resin A2) Using the components shown in Table 2, an acrylic resin (A2) was produced by the method shown below.

[Table 2] The raw material (1) is charged into a 2 liter four-necked flask equipped with a stirrer, a thermometer, and a cooling tube, stirred, heated, heated, and refluxed, and the raw materials (2), (3), (4), (5),
The mixed solution of (6) and (7) was added dropwise over 2 hours. After the addition, the mixture was kept warm for 4 hours, cooled, and taken out. The obtained acrylic resin A2 had a molecular weight of 4,500, a specific gravity of 0.9, and a solid content of 65%.
Met.

Production Example 3 (Production of Base Resin A3 for Comparison) Using the raw materials shown in Table 3, a comparative amine-modified epoxy resin was produced by the following method.

[Table 3] Raw material (1) Epototo YD-128 manufactured by Toto Kasei Co., Ltd. Raw material (2) Epototo YD-011 manufactured by Toto Kasei Co., Ltd. Raw materials (1), (2) were placed in a 5-liter four-necked flask equipped with a stirrer, a thermometer, and a cooling tube. ), (3), stirring,
Heating was performed and the temperature was raised to 100 ° C. After holding at 100 ° C. for 1 hour, it was cooled to 80 ° C. Next, the raw material (4),
(5) was charged and the temperature was raised to 100 ° C. After keeping at 100 ° C. for 2 hours, it was cooled to 80 ° C. and taken out. The obtained amine-modified epoxy resin A3 had a molecular weight of 2,100, a specific gravity of 1.2, and a solid content of 70%.

Production Example 4 (Production of Curing Agent B) Using the raw materials shown in Table 4, a blocked polyisocyanate was produced by the following method.

[Table 4] Raw materials (1) Millionate MR manufactured by Nippon Polyurethanes
-400 Raw materials (2), (3) and (4) were charged into a 3 liter four-necked flask equipped with a stirrer, a thermometer and a condenser, and stirred.
Heating was performed, the temperature was raised to 45 ° C., and the temperature was maintained for 30 minutes. Then, while maintaining the temperature in the flask at 45 ° C, the raw material (1) was charged over 1 hour, and reacted at 40 to 50 ° C for 3 hours. Then, while maintaining the same temperature, the raw material (5) was added dropwise over 1 hour. After the addition, the temperature was raised to 100 ° C., and the temperature was maintained at 100 ° C. for 2 hours. The obtained blocked polyisocyanate B has a specific gravity of 1.1 and a solid content of 7
5%.

Examples 1, 2, 3, 4, 5 and Comparative Examples
1, 2 Electrodeposition paints were prepared according to the formulations shown in Table 5 and evaluated for quality. In the overcoat vividness test, a topcoat containing acrylic resin as a main component was formed by baking to a film thickness of about 40 μm on the electrodeposition coating film obtained by baking the coating to a film thickness of about 20 μm with each electrodeposition paint. A test was performed with the composite coating thus prepared.

Preparation of Resin Aqueous Dispersion A mixture of a base resin and a curing agent was charged into a mixture of propylene glycol monomethyl ether, formic acid, and deionized water with good stirring to obtain a resin aqueous dispersion. In Example 5, a mixture of a base resin, a curing agent, and a disazo-based dye (BK-1 manufactured by Taoka Chemical Co., Ltd.) was charged into a mixed solution of propylene glycol monomethyl ether, formic acid, and deionized water with good stirring. An aqueous dispersion was obtained. Preparation of Pigment Paste In Example 4, after sufficiently stirring a base resin, formic acid, deionized water, diethylene glycol monobutyl ether, and carbon black (specific gravity 1.8) with a dissolver, the particle size is reduced to 10 μm or less by a horizontal sand mill. It was dispersed to obtain a pigment paste. As Comparative Examples 1 and 2, base resin, formic acid, deionized water, diethylene glycol monobutyl ether, carbon black (specific gravity 1.8), titanium oxide (specific gravity 3.8), kaolin (specific gravity 2.5), dibutyltin oxide (Specific gravity 1.5), rust preventive pigment (specific gravity 6.
After 4) was sufficiently stirred with a dissolver, the mixture was dispersed by a horizontal sand mill until the particle diameter became 10 μm or less to obtain a pigment paste. Preparation of Electrodeposition Paint The above aqueous resin dispersion and pigment paste were blended in the amounts shown in Table 5 to obtain an electrodeposition coating liquid.

[0027]

[Table 5]

Using the electrodeposition coating liquid obtained above, a carbon electrode was used as an anode, and a zinc phosphate treated plate (manufactured by Japan Test Panel, Bt.
(3004, 0.8 × 70 × 150 mm) was used as a cathode, electrodeposition coating was performed under the condition that the film thickness after baking was 20 μm, and baking was performed at 175 ° C. for 25 minutes. Next, air spray coating was performed under the condition that the film thickness after baking of the automotive top coat was 40 μm, baking was performed at 140 ° C. for 20 minutes, and the results of the coating film performance evaluation are shown in Table 6.

[0029]

[Table 6] Evaluation method (1) Appearance of coating film Ra (μm) measurement with Surfcoder SE-30D manufactured by Kosaka Laboratory Ltd. (2) Top coat sharpness Brightness gloss meter PGd measurement (3) Salt water spray resistance JIS-Z-2731 It went according to. A flaw reaching the substrate is put on the electrodeposition coated surface with a cutter knife, and the rust width after 1000 hours is evaluated. (4) Hot salt water immersion test After immersing the coated test plate in a 5% saline solution at 50 ° C. for 1000 hours, the ratio of the coating film peeling area to the entire test plate is measured and expressed in%. (5) Sedimentation speed BYK Dinometer Measurement of amount of sediment (mg / hour) per unit time (6) Throwing power Pipe method Measurement of coating length (cm) (7) Finishing of horizontal part Panel bent into L-shape And evaluate the finish of the horizontal part. No difference in appearance from vertical part ○, small difference from vertical part △

[0030]

According to the present invention, it is possible to provide a method for forming a composite coating film for automobiles, which is excellent in smoothness and excellent in rust prevention, coating workability and the like.

Claims (4)

[Claims] 1. A method for forming a composite coating film for automobiles, comprising forming a cationic electrodeposition coating film on a steel sheet for automobiles using a cationic electrodeposition coating material, and further forming a pigmented top coating film thereon. It comprises (A) a resin having a cationic group (base resin), (B) a blocked polyisocyanate (curing agent resin), and (C) a water-insoluble component added as necessary.
A method for forming a composite coating film for automobiles using a cationic electrodeposition coating composition comprising components (A), (B) and (C) each having a specific gravity of 2.0 or less. 2. A cationic electrodeposition coating composition wherein the resin having a cationic group is a cationic amine-modified epoxy resin obtained by cationizing an amine-modified epoxy resin obtained by reacting an amine with an epoxy resin. Item 4. The method for forming a composite coating film for an automobile according to Item 1. 3. An amine-modified epoxy resin obtained by reacting an amine with an epoxy resin obtained by reacting a resin having a cationic group with glycidyl ether of polyphenol, glycidyl ether of diol and polyphenol, The method for forming a composite coating film for automobiles according to claim 2, wherein a cationic electrodeposition coating composition which is a cationic amine-modified epoxy resin is used. 4. The resin having a cationic group has a molecular weight of 30.
The method for forming a composite coating film for automobiles according to claim 3, wherein a cationic electrodeposition coating composition which is a cationic amine-modified epoxy resin having a size of 00 or more is used.