JP3271857B2 - How to paint a car body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for painting an automobile body. More particularly, the present invention relates to a method for coating an automobile body having significantly improved chipping resistance even at low film thickness.

[0002]

2. Description of the Related Art An outer plate of a vehicle body, for example, a lower portion, a lower portion, and an underbody portion of the vehicle body outer plate always hit by a stepping stone during running, so that so-called "chipping peeling" occurs, and the aesthetic appearance is reduced. The body steel plate at the peeling part is exposed and rusts, and corrosion proceeds. For this reason, a great deal of attention has been paid to countermeasures against "chipping peeling", and research and development of paints having excellent chipping resistance have been carried out, for example, as a countermeasure against chipping peeling at the lower part of the body outer panel. As a primer or intermediate coating composition for automobiles, a specific pigment is used to prevent damage to the substrate even when the specific pigment is used (JP-A-53-45813), and a coating composition using an isocyanate compound (JP-A-54). JP-A-73836, JP-A-57-68176
Publication).

However, in the method using a composition containing 20 to 50% by weight of talc having a particle diameter of 10 to 20 μm (Japanese Patent Application Laid-Open No. 53-45813), scratches due to chipping hardly reach the base steel sheet. Although the rust of the outer steel plate of the vehicle itself is reduced, there is a disadvantage that the peeling area of the top coat is increased and the appearance is impaired.

The method using isocyanate (Japanese Patent Application Laid-Open No. 54-73836) has a disadvantage that the chipping resistance is not sufficient, and a compound obtained by reacting a polybutadiene having a hydroxyl group with an isocyanate group is used. In the case of the conventional method (Japanese Patent Laid-Open No. 57-68176), the chipping resistance is good, but there is a drawback that the finished appearance after the top coat is inferior.

[0005] Also, various so-called chipping-resistant paint compositions for floors have been proposed for the purpose of preventing rust below a vehicle body. 174570
However, in a method using a chipping-resistant paint using this composition, if good chipping resistance is to be obtained, it is necessary to apply a film having a thickness of 300 μm or more. When the thickness is 300 μm or less, there is a disadvantage that the chipping resistance is rapidly reduced. Further, the appearance of the coating film is inferior, so that it cannot withstand use as an outer steel plate of a vehicle body.

On the other hand, there has been proposed a method of coating a so-called "interlayer chipping-resistant paint" on an outer steel plate of a vehicle body for the purpose of improving the chipping resistance between the electrodeposition paint and the intermediate paint. For example, a method for applying a coating composition comprising a soft polyester resin and a melamine resin, a method for applying a coating composition comprising a polyhydroxy compound and a blocked isocyanate compound, and a method for applying a coating composition comprising a polyolefin resin grafted with maleic acid Methods are known.

[0007]

According to the method using such an interlayer chipping-resistant paint, the chipping resistance can be improved as compared with the case of using the above-mentioned intermediate coating for automobiles or a primer composition. It is necessary to apply the paint to a high film thickness of 200 μm or 300 μm. Further, the higher the thickness of the paint, the lower the finish appearance of the top coat becomes. In this case, the thickness must be in the range of 25 to 150 μm, but if the thickness is 25 to 150 μm, the chipping resistance is poor. In addition, coating with a high film thickness increases the weight of the vehicle body, lowers the fuel efficiency of the vehicle, and contradicts the need for energy saving. Under such circumstances, there has been a long-awaited desire to develop a method for coating an external steel plate having a low film thickness and good chipping resistance.

[0008]

In view of this situation, the present inventors have made intensive studies to improve the above-mentioned problems,
The present inventors have found a coating method that exhibits good chipping resistance at a low film thickness that does not deteriorate the appearance of the topcoat paint, that is, a film thickness of 25 to 150 μm, and has finally completed the present invention.

That is, according to the present invention, an outer plate of an automobile body is provided with:
In a coating method in which an electrodeposition coating, an anti-chipping coating, an intermediate coating, and a top coating are repeatedly applied, (1) the anti-chipping coating comprises: (A) a resin solid content of an epoxy resin having a hydroxyl group of 10;
A hydroxyl group-containing, number-average molecular weight of 5 obtained by reacting 10 to 70 parts by weight of a cyclic ester with 0 part by weight
(B) a hydroxyl group-containing epoxy resin having a number average molecular weight of 400 to 100
0 of the polyester polyol resin and (C) a polyisocyanate compound in which the isocyanate group is blocked, with respect to 1 equivalent of the hydroxyl group of the component (A) and the equivalent of the hydroxyl group of the component (B), the isocyanate group of the component (C) is 0.5 to 0.5%. A coating composition blended at a ratio of 2.0 equivalents, wherein the glass transition temperature of a cured coating film of the composition is −
20 ° C. to 0 ° C., and the elongation is 1 at a temperature of 20 ° C.
0 to 100%, 1 to 50% at a temperature of −20 ° C., and a tensile strength of 400 to 700 kgf at a temperature of 20 ° C.
/ Cm ² , wherein the chipping-resistant coating is applied in a dry film thickness of 25 to 150 μm, and then dried so that the coating film gel fraction of the chipping-resistant coating is in a range of 70 to 100%. And then applying an intermediate coating.

The chipping-resistant coating composition used in the present invention will be described. Hereinafter, when referred to as the chipping-resistant paint, it means a paint applied between the electrodeposition paint and the intermediate paint in the coating of the outer steel plate of the vehicle body.

A hydroxyl group-containing epoxy resin obtained by reacting 10 to 70 parts by weight of a cyclic ester with 100 parts by weight of a resin solid content of an epoxy resin having a hydroxyl group contained in a coating composition used in the present invention. And a number average molecular weight of 500
The epoxy resin of ~ 5000 is a bisphenol A type,
One or two or more hydroxyl group-containing epoxy resins having a number average molecular weight of 500 to 5,000 formed by reacting an epoxy resin such as bisphenol F and a cyclic ester and / or a ring-opening polymer of the cyclic ester. It is a mixture. The grounds for using such a resin are described below.

According to the study of the inventors, 25 to 150 μm
If good chipping resistance is to be obtained with a low film thickness of m,
The glass transition temperature of the cured coating film is -20C to 0C,
The elongation percentage is 10 to 100% at a temperature of 20 ° C, and the temperature is -2.
It should be 1 to 50% at 0 ° C, and the tensile strength should be in the range of 400 to 700 kgf / cm ^{2 at} a temperature of 20 ° C.

The tensile strength in the range of 400 to 700 Kgf / cm ² is effective for preventing the coating film from being damaged or broken by a stone splash, and is 10 to 100% at a temperature of 20 ° C. and 1 to 100 at a temperature of −20 ° C. By having an elongation of 50%, sufficient adhesion of the chipping-resistant paint to the electrodeposition coating film can be maintained, and chipping peeling can be prevented.

The environment in which the automobile body is used is generally -20.degree. C. to 40.degree. C., and a glass transition point of -20.degree.
Must be within the range.

In order to realize the physical properties of the cured coating film, it is preferable to use a urethane resin. For example, by combining a polyester polyol and a blocked isocyanate, the glass transition temperature of the cured coating film is from -20.degree. ° C and an elongation percentage of 10 to 100% at a temperature of 20 ° C and 1 to 50% at a temperature of -20 ° C, but the tensile strength in this case is at most 300 Kgf / cm. ² , and the composition intended for the present invention cannot be obtained.

Therefore, by further using a bisphenol A epoxy resin such as Epicoat 1004 (manufactured by Yuka Shell Epoxy Co., Ltd.) in combination with the polyester polyol and the blocked isocyanate, the tensile strength is improved to 400 kgf / cm ^{2 to} 700 kgf / cm ² . However, conversely, the elongation at 20 ° C. drops to 1 to 5%, and the desired physical properties cannot be obtained.

On the other hand, when a hydroxyl group-containing epoxy resin obtained by reacting an epoxy resin with a cyclic ester is used in combination with a combination of a polyester polyol and a blocked isocyanate, it is possible to obtain the cured coating film properties. The inventors have found and used the anti-chipping coating composition of the present invention.

The reason that the desired cured coating film properties can be obtained by using a hydroxyl group-containing epoxy resin obtained by reacting a cyclic ester with an epoxy resin is that the epoxy resin has a linear molecular structure obtained by reacting the cyclic ester. It is considered that it is good to simultaneously contain both the resulting flexible structure and the rigid structure derived from the bisphenol skeleton originally contained in the epoxy resin main chain.

As an example of such a resin, for example, a reaction product of 100 parts by weight of a bisphenol A-based epoxy resin having an epoxy equivalent of 450 g / 1 equivalent and 25 parts by weight of ε-caprolactone, an epoxy equivalent of 700 g / 1 equivalent. 100 parts by weight of bisphenol F epoxy resin and ε
A reaction product of 16 parts by weight of caprolactone, a reaction product of 100 parts by weight of a bisphenol A epoxy resin having an epoxy equivalent of 450 g / 1 equivalent and 40 parts by weight of γ-valerolactone are known.

If the weight ratio of the above-mentioned cyclic ester to 100 parts by weight of the epoxy resin is less than 10 parts by weight, the finished appearance is reduced, and if it is more than 70 parts by weight, the chipping resistance is undesirably reduced.

The molecular weight of the epoxy resin obtained by reacting the cyclic ester is 500 to 5000, preferably 100
0 to 4000. If the molecular weight is lower than 500, the chipping resistance decreases, and if it exceeds 5,000, the coating workability deteriorates and the finished appearance is unfavorably deteriorated.

It also contains a hydroxyl group and has a number average molecular weight of 40
The polyester polyol resin of 0 to 1000 is one or a mixture of two or more of reaction products of a polyhydric alcohol and a polyhydric carboxylic acid and / or an anhydride thereof in an amount smaller than the stoichiometric amount. . Number average molecular weight 400
If it is lower, the chipping resistance is reduced, and 100
When it exceeds 0, similarly, chipping resistance, especially at high temperatures,
The chipping resistance after exposure to a degraded environment such as a high humidity environment is lowered, which is not preferable.

Examples of polyhydric alcohols include, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, isomeric pentanediols, isomeric hexane Diols or octanediols such as 2-ethyl-1,3-hexanediol, 1,2-, 1,3- and 1,4-bis (hydroxymethyl) -cyclohexanone, trimethylolpropane, glycerin and the like. it can.

Examples of polycarboxylic acids and polycarboxylic anhydrides include oxalic acid, succinic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, maleic acid and itaconic acid. Polycarboxylic acids,
Trivalent carboxylic acids such as trimellitic acid, phthalic anhydride,
Examples include polycarboxylic anhydrides such as tetrahydrophthalic anhydride, maleic anhydride, and trimellitic anhydride, and dimer or trimer fatty acids, for example, trimer of castor oil fatty acid.

In the coating composition used in the present invention, a mixture of the above-mentioned epoxy resin and polyester polyol resin is used. The mixing ratio of the epoxy resin and the polyester polyol resin is not particularly limited, but is preferably 50 to 90 parts by weight of the polyester polyol resin per 100 parts by weight of the epoxy resin.

Next, the isocyanate group-blocked polyisocyanate compound contained in the coating composition used in the present invention refers to one of the compounds in which the isocyanate group of the polyisocyanate compound is blocked by a commonly used blocking agent. Or a mixture of two or more thereof, and examples of the polyisocyanate compound include 2,4- and / or
Or 2,6-diisocyanate toluene, 2,4-diisocyanate-dicyclohexylmethane, 4,4'-
Diisocyanate-dicyclohexylmethane, hexamethylene diisocyanate, 1-isocyanate-3,
Diisocyanates such as 3,5-trimethyl-5-isocyanatomethylcyclohexane, these diisocyanates and less than equivalent polyhydric alcohols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, Adducts formed by the reaction with 1,4-butanediol, trimethylolpropane, glycerin, etc., biuret trimers of the above-mentioned diisocyanates, trimers of isocyanurates of the above-described diisocyanates, and the like are equivalent to these polyisocyanate compounds. Or an equivalent or more of a blocking agent.

Examples of such a blocking agent include phenol, p-substituted phenol, alcohols, ε-caprolactam, ketoximes, and acetone oximes, and various selections are possible.

The mixing ratio of the above-mentioned mixture of the epoxy resin and the polyester polyol to the blocked isocyanate is such that the isocyanate group of the blocked isocyanate component is 0.5 to 2.0 with respect to 1 equivalent of the hydroxyl group of the mixture of the epoxy resin and the polyester polyol. It is a mixture of both components at an equivalent ratio, and has an isocyanate group content of 0.1.
If the amount is less than 5 equivalents or more than 2.0 equivalents, the chipping resistance is reduced.

The coating composition used in the present invention may further comprise, if desired, a resin component comprising the above-mentioned epoxy resin, polyester polyol resin and blocked isocyanate.
Coloring agents such as carbon black, pigments such as titanium dioxide, tertiary amines, curing catalysts such as organotin compounds, fluidity modifiers, various additives such as defoamers and hydrocarbons such as toluene, xylene, and solvent naphtha, Organic solvents such as esters such as ethyl acetate, butyl acetate and amyl acetate, and ketones such as methyl ethyl ketone, methyl butyl ketone, holone and isophorone can be contained. The content of these pigments, additives, solvents, and the like is not particularly limited, but the content of pigments is set to 150% by weight or less based on the total weight of the resin components in order to obtain the above-described cured coating film properties. It is desirable. If it exceeds 150% by weight, the elongation at 20 ° C. and at −20 ° C. may both decrease.

In the present invention, the above-mentioned coating composition is applied to the electrodeposition coating film on the outer steel plate of the vehicle body by a usual coating method, for example, by means of air spray, airless spray, electrostatic coating, dip coating or the like. , Coating film thickness is 25 ~ 150
μm, preferably 25 to 75 μm. 25-150μ
m, good chipping resistance can be obtained.
If the thickness is less than 5 μm, the chipping resistance is reduced. Also one
If the thickness is more than 50 μm, the chipping resistance is further improved, but the appearance is reduced, and it is not preferable from the viewpoint of energy saving and cost.

The coating composition is heated and dried after being applied by the above-mentioned means. The heating and drying conditions are such that the coating film gel fraction of the coating composition is in the range of 70 to 100%, preferably 75 to 90%. If it is less than 70%, not only defects in coating workability such as pinholes, armpits, or reduction in finished gloss at the time of intermediate coating, but also tensile strength may be reduced among various properties of the cured coating film. Not preferred.
When the gel fraction is less than 70%, the reason for the decrease in tensile strength is that the solvent contained in the intermediate coating that is subsequently applied to the chipping-resistant coating dissolves the chipping-resistant coating layer, resulting in incomplete curing. Conceivable. As an example of the drying conditions for obtaining such a coating film gel fraction, 100 to 140
C. for 3 to 30 minutes.

In the coating method of the present invention, there are no particular restrictions on the electrodeposition coating, the intermediate coating, and the top coating, excluding the composition used for the chipping-resistant coating, and conventionally used commercial products can be used. .

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto. In the description, parts and% are by weight unless otherwise specified.

[0034]

【Example】

Manufacture of anti-chipping coating composition As epoxy resin, Praxel G-702 (a reaction product of 100 parts of bisphenol A type epoxy resin having an epoxy equivalent of 1700 g / 1 equivalent and 25 parts of ε-caprolactone, number average molecular weight 4000) Xylene solution (manufactured by Daicel Chemical Industries, Ltd.) (resin solid content 60%) 1
2.5 parts, as a polyester polyol resin, dismophen 670 (number average molecular weight 900, hydroxyl value 140
7.5 parts of mgKOH equivalent, solid content 100%, manufactured by Sumitomo Bayer Urethane Co., Ltd., and as a blocked isocyanate, Vernock DB-980K (theoretical isocyanate content 10.5%, solid content 75%, Dainippon Ink and Chemicals, Inc.) 18.1 parts, 36.7 parts of xylene, 25 parts of titanium dioxide, and 0.2 part of carbon black were kneaded using a sand mill to produce a chipping-resistant coating.

Production of anti-chipping coating composition [0036] The anti-chipping coating composition was produced in the same manner as the anti-chipping coating composition with the composition shown in Table 1 below.

Example 1 A chipping-resistant paint was spray-coated on a steel sheet of a car body subjected to a chemical conversion treatment and subjected to cationic electrodeposition coating so as to have a dry film thickness of 30 μm, and dried at 100 ° C. × 5 minutes. Next, a polyester-melamine resin-based intermediate coating was spray-coated so as to have a dry film thickness of 30 μm, and baked at 140 ° C. for 20 minutes.
Further, a polyester-melamine resin top coat is applied by spray coating so as to have a dry film thickness of 30 μm.
Bake for 20 minutes.

When the obtained test plates were subjected to various coating film performance tests, the results were good as described in Table 2 below.

Examples 2 to 6 A test plate was prepared in the same manner as in Example 1, and a coating film performance test was performed. The results were all good as described in Table 2 below.

Comparative Example 1 As a comparative example, the following chipping-resistant paint was produced using a composition comprising a polyester polyol, a blocked isocyanate and an amino resin. A chipping-resistant paint was applied in the same manner as in Example 1 to prepare a test plate, and a chipping-resistance test was carried out, but it was not practical.

The production of the chipping-resistant paint was carried out as follows. 633 parts of adipic acid, 109 parts of trimethylolpropane, 206 parts of neopentyl glycol,
210 parts of diethylene glycol and 0.5 part of dibutyltin oxide are subjected to a dehydration reaction at a reaction temperature of 180 ° C. to 240 ° C. When the acid value reaches 1.5, the reaction is stopped, 125 parts of xylene, 125 parts of cellosolve acetate To obtain a polyester polyol. Next, 100 parts of this polyester polyol was mixed with Olester P49-75.
Block isocyanate produced by dropping 260 parts of methyl ethyl ketoxime into 1000 parts of S (prepolymer obtained by reaction of trimethylolpropane and toluene diisocyanate, nonvolatile content of 75%, manufactured by Mitsui Toatsu Chemicals, Inc.) NCO 11.9%) 83.9
Parts, 24.5 parts of Uban 2020 (butylated melamine resin, nonvolatile content 80%, manufactured by Mitsui Toatsu Chemicals, Inc.), 222 parts of calcium carbonate, and 2 parts of carbon black.

Comparative Examples 2 to 4 In Comparative Examples 2 to 4 in Table 3 below, test plates were prepared by a coating method outside the scope of the present invention. All of them had a problem in chipping resistance and were not practical.

Comparative Examples 5 to 9 Comparative Examples 5 to 9 in Table 3 below are examples in which a test plate was prepared using a composition outside the scope of the present invention. Table 1 shows the composition of the chipping-resistant coating film. All of them had a problem in chipping resistance and were not practical.

Various tests were carried out under the following test conditions.

(1) Appearance The coated film after overcoating is visually observed to check for defects such as sagging, armpits, pinholes, and uneven gloss.

(2) Chipping resistance A stepping stone tester (Suga Test Machine Co., Ltd., model JA-400) is used. A painted test plate is attached to the holder of the test machine at an angle of 20 ° from the horizontal, and 500 g of basalt No. 7 crushed stone is jetted at an air pressure of 4 kg / cm ² at the pressure of the test machine to collide. Thereafter, the test plate was washed with water and dried, and the coating film lifted up by chipping was removed with an adhesive tape, and then the degree of peeling damage was evaluated (◎ best; ○ good; △ poor; × poor).

(3) Water resistance The test plate was immersed in warm ion-exchanged water at 40 ° C. for 240 hours.
Immediately after the removal, the water droplets are wiped off, the appearance is checked for abnormalities, and 100 cross hatches having a width of 2 mm are cut with a cutter knife, and a peeling test is performed using an adhesive tape. A case where there was no peeling and all 100 grids remained remained as a pass.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

[0051]

According to the present invention, 25 which has been conventionally difficult.
It is possible to obtain a coating of a steel sheet on the outside of a vehicle body having a low film thickness of μm to 50 μm and having good chipping resistance.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI B05D 7/24 302 B05D 7/24 302U 302V C09D 175/04 C09D 175/04 175/06 175/06 (72) Inventor Yasuo Sumitomo 6-10-73 Minamitsukaguchi-cho, Amagasaki City, Hyogo Prefecture, Japan Shinto Paint Co., Ltd. (72) Inventor Fumitaka Nakayama 6-10-73 Minamitsukaguchi-cho, Amagasaki City, Hyogo Prefecture, Japan Shinto Paint Co., Ltd. (56) References JP-A-59-160572 (JP, A) JP-A-62-65773 (JP, A) JP-A-62-45381 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B05D 1/36 C09D 175/04 C09D 175/06

Claims (1)

(57) [Claims] 1. A coating method in which an electrodeposition coating, an anti-chipping coating, an intermediate coating, and a top coating are repeatedly applied to an outer panel of an automobile body. (1) The anti-chipping coating comprises: (A) an epoxy resin having a hydroxyl group Resin solid content of 10
A hydroxyl group-containing, number-average molecular weight of 5 obtained by reacting 10 to 70 parts by weight of a cyclic ester with 0 part by weight
(B) a hydroxyl group-containing epoxy resin having a number average molecular weight of 400 to 100
0 of the polyester polyol resin and (C) the polyisocyanate compound in which the isocyanate group is blocked, with respect to 1 equivalent of the hydroxyl group of the component (A) and the equivalent of the hydroxyl group of the component (B), the isocyanate group of the component (C) is from 0.5 to A coating composition blended at a ratio of 2.0 equivalents, wherein the glass transition temperature of a cured coating film of the composition is −
20 ° C. to 0 ° C., and the elongation is 1 at a temperature of 20 ° C.
0 to 100%, 1 to 50% at a temperature of −20 ° C., and a tensile strength of 400 to 700 kgf at a temperature of 20 ° C.
/ Cm ² , wherein the chipping-resistant coating is applied in a dry film thickness of 25 to 150 μm, and then dried so that the coating film gel fraction of the chipping-resistant coating is in a range of 70 to 100%. And then applying an intermediate coating.