JP2004313943A - Coating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating method for carrying out wet-on-wet coating system comprising an undercoating and a middle coating while preventing coating failures such as insufficient side coating and defective skin. <P>SOLUTION: In the coating method for applying at least three coating materials; an undercoating material, a middle coating material, and a top coating material; an undercoating material and a middle coating material having practically same hardening temperature and retention time at the hardening temperature each other are used and the undercoating material is applied and then the middle coating material is applied in wet-on-wet manner and both coating materials are simultaneously baked and hardened and a top coating material is applied. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
【Technical field】
The present invention relates to a coating method which is preferably applied to an automobile body and the like, and more particularly to a two-coat one-bake coating method in which an undercoat and an intermediate paint are applied wet-on-wet and baked simultaneously.
[0002]
[Background Art]
The coating system of the automobile body is divided into three types: an undercoating coating to which an electrodeposition coating mainly containing an epoxy resin is applied, an intermediate coating coating mainly to a polyester resin, and a top coating coating mainly to a polyester coating. After applying an undercoat, using a kind of paint, baking it, applying an intermediate coat on the cured undercoat, then baking it and applying a topcoat on the cured intermediate coat A so-called three-coat three-bake coating method, which is completed by baking this, is adopted.
[0003]
However, in such a three-coat, three-bake coating system, a drying furnace is required for each of the undercoating step, the intermediate coating step, and the topcoating step, so that a large process space for installing the drying furnace is required. The energy consumed in the drying oven reflects on the production cost of the car.
[0004]
Therefore, in order to solve the above problem by reducing the number of drying ovens provided in these three processes to two or less, it is considered to apply undercoating and intermediate coating or intermediate coating and topcoating on a wet-on-wet basis. Have been.
[0005]
However, a coating system in which the undercoating and the intermediate coating are applied wet-on-wet and then baked simultaneously has the following problems.
[0006]
That is, while the baking temperature of the undercoat is maintained at 170 ° C. for 20 minutes, the baking temperature of the intermediate coating is maintained at 140 ° C. for 20 minutes. After the intermediate coating film applied to the lower layer starts to harden at around 140 ° C., the undercoat film applied to the lower layer starts to harden at around 170 ° C.
[0007]
For this reason, when the undercoat film of the lower layer is cured, the solvent is pierced and evaporates by breaking through the already cured intermediate coat film, or the undercoat film is cured after the intermediate coat film is cured. There is a problem that the intermediate coating film becomes defective due to curing and shrinkage.
[0008]
DISCLOSURE OF THE INVENTION
SUMMARY OF THE INVENTION An object of the present invention is to provide a coating method capable of realizing a wet-on-wet coating system of undercoating and intermediate coating while preventing coating defects such as armpit defects and skin defects.
[0009]
In order to achieve the above object, according to the present invention, in a coating method for applying at least three paints of an undercoat, an intermediate paint, and an overcoat,
Using an undercoat and an intermediate paint having substantially the same curing temperature and curing temperature holding time,
A coating method is provided in which the undercoat is applied, the intermediate coat is applied wet-on-wet, and these are simultaneously baked and cured to apply the topcoat.
[0010]
In the present invention, the undercoat paint and the intermediate coat paint having substantially the same curing temperature and curing temperature holding time are used, and the undercoat paint and the intermediate paint are applied in a wet-on-wet manner and then baked and cured at the same time. At the time of curing, the undercoat paint applied to the lower layer and the intermediate paint applied to the upper layer simultaneously start curing, and cure through the same change in viscosity. Accordingly, the solvent of the lower layer undercoat can be appropriately evaporated without breaking through the upper layer intermediate coat which is hardened to the same extent, and the occurrence of armpit defects can be prevented. In addition, since the lower undercoat film and the upper middle coat film are cured through the same change in viscosity, it is possible to prevent skin defects of the middle coat film.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a process chart showing a coating method according to an embodiment of the present invention.
[0012]
In the coating method according to the present embodiment, in a three-coat coating system of undercoating, intermediate coating and topcoating, undercoating and intermediate coating are applied wet-on-wet, and these are simultaneously baked and then topcoated. , A so-called three-coat two-bake coating system. Speaking of undercoating and intermediate coating, it is a coating system of two coats and one bake.
[0013]
That is, the automotive body, which has been washed and pre-treated, is immersed in an electrodeposition bath to perform electrodeposition coating, and is conveyed to an intermediate coating booth without being baked and cured. After applying the intermediate coating on the film, the uncured electrodeposition coating film and the intermediate coating film are simultaneously baked in an intermediate coating drying oven.
[0014]
Here, the electrodeposition paint and the intermediate paint used in the present embodiment have substantially the same curing temperature and curing temperature holding time. The curing condition of the conventional electrodeposition paint is generally 170 ° C. × 20 minutes, and the curing condition of the intermediate coating material is generally 140 ° C. × 20 minutes. In this embodiment, (1) The curing conditions of the electrodeposition paint and the intermediate coating are both kept at 140 ° C. × 20 minutes, or (2) the curing conditions of both the electrodeposition paint and the intermediate coating are kept at 170 ° C. × 20 minutes.
[0015]
In the case of the above (1), the curing condition of the electrodeposition paint is lowered to match the curing condition of the existing intermediate coating paint. In the case of a material containing a resin and a blocked polyisocyanate as a cross-linking agent, an alcohol-based blocking agent having an alkyl group shorter than the existing alcohol-based blocking agent is used as the alcohol-based blocking agent that blocks the blocked polyisocyanate. . The use of the alcohol-based blocking agent having a short alkyl group lowers the dissociation temperature of the blocking agent, thereby lowering the reaction temperature of the electrodeposition paint.
[0016]
In the case of the above (2), the curing condition of the intermediate paint is raised to a high temperature in order to match the curing condition of the existing electrodeposition paint. And a material containing a blocked polyisocyanate as a cross-linking agent, the alcohol-based blocking agent that blocks the blocked polyisocyanate has a longer alkyl-based alcohol-based block than the existing alcohol-based blocking agent. Adopt the agent. By using an alcohol-based blocking agent having a long alkyl group, the dissociation temperature of the blocking agent is increased, whereby the reaction temperature of the intermediate coating material can be increased.
[0017]
Further, as another method of raising the curing temperature of the intermediate coating material, when the intermediate coating material is a material containing a base resin having a hydroxyl group and a carboxyl group and a block polyisocyanate as a crosslinking agent, The amine having a higher boiling point than the existing amine is used for the amine neutralized in the coating composition. As a result, the bonding force between the amine and water is reduced, and the carboxyl group can be blocked for as long as possible, so that the reaction temperature of the intermediate coating material can be increased.
[0018]
In addition, in the coating method of the present invention, in addition to the above (1) and (2), the curing conditions of the electrodeposition paint and the intermediate paint are both maintained at T ° C. × t (T and t are arbitrary numbers). You can also. In this case, the conventional curing conditions for both the electrodeposition paint and the intermediate paint are changed using the above-described method.
[0019]
Next, an example of a coating line to which the coating method of the present embodiment is applied will be described. FIG. 2 is a diagram showing a coating line to which a coating method according to an embodiment of the present invention is applied, and FIG. 3 is a diagram showing a relationship between time and temperature, and a relationship between time and viscosity in an intermediate coating drying furnace in a coating method according to an embodiment of the present invention. 6 is a graph showing the relationship of FIG.
[0020]
The car body assembled as a white body is transported to the paint shop from the car body assembly process, where the first pre-treatment booth 1 removes oil and dust attached to the body and protects the body from the surface of the steel plate constituting the body. A rust conversion coating is formed.
[0021]
The body on which the clean and conversion coating is formed by passing through the pretreatment booth 1 is transported to the electrodeposition coating tank 2 filled with the electrodeposition paint, and is immersed in the electrodeposition paint. In the electrodeposition coating tank 2, when a high voltage is applied to the electrodeposition paint, the electrodeposition paint electrophoreses, whereby an electrodeposition coating film is formed on the body. The body leaving the electrodeposition coating tank 2 is carried into the electrodeposition washing booth 3, where excess electrodeposition paint adhered to the body is washed away. Here, in the case of the above (1), the electrodeposition paint tank 2 is filled with the electrodeposition paint with the curing condition kept at 140 ° C. × 20 minutes. In the case of the above (2), the electrodeposition coating tank 2 is filled with an electrodeposition coating material in which the curing condition is maintained at 170 ° C. × 20 minutes.
[0022]
In the conventional coating method, the body after the electrodeposition washing is carried into an electrodeposition drying furnace, and the electrodeposition coating film is baked and cured, but in the present embodiment, the electrodeposition coating film is provided without the electrodeposition drying furnace. The body is transported to the intermediate coating booth 4 in an uncured state. When the existing coating line is used as it is, the operation of the electrodeposition drying furnace is stopped and the body on which the uncured electrodeposition coating film is formed is passed as it is.
[0023]
In the middle coat booth 4, the middle coat paint in which the curing condition is kept at 140 ° C. × 20 minutes (in the case of the above (1)) or the middle coat paint in which the curing condition is kept 170 ° C. × 20 minutes (in the case of the above (2)) ) Is painted on the body. The coating method is not particularly limited, and the coating is performed using a bell-type coating gun, a spray-type coating gun, or the like.
[0024]
The body in which the intermediate coating is applied on the uncured electrodeposition coating is carried into the intermediate coating drying furnace 5, where the electrodeposition coating and the intermediate coating are simultaneously baked and cured. FIG. 3 is a graph showing the temperature rise curve of the body and the viscosity curve of the coating film in the intermediate coating drying furnace 5. In this embodiment, the electrodeposition paint and the intermediate paint having the same curing conditions are used. When the body temperature reaches around 140 ° C. (in the case of (1)) or around 170 ° C. (in the case of (2)), the electrodeposition coating film and the intermediate coating film simultaneously undergo a curing reaction as shown in FIG. Start. Further, as shown in the viscosity curve at the bottom of the figure, when transported to the intermediate coating drying furnace 5, initially, the solvent components contained in both the electrodeposition coating film and the intermediate coating film increase with the temperature rise. Evaporation causes an increase in viscosity, but when the body temperature exceeds the glass transition point of the electrodeposition paint and the intermediate coating, the viscosity of both coatings decreases once, and the coating surface of both coatings is smooth. become. In the present embodiment, since the point at which the coating surface becomes smooth is set to the same point for both the electrodeposition coating film and the intermediate coating film, the coating skin quality of the cured intermediate coating film is improved. When the body temperature further rises, both the electrodeposition coating film and the intermediate coating film start a curing reaction, whereby the viscosity of both coating films increases again.
[0025]
A cured electrodeposition coating film and an intermediate coating film are formed on the body that has passed through the intermediate coating drying furnace 5, and are then carried into the top coating booth 6. In the top coat booth 6, a top coat is applied by a conventionally known method, and the top coat is baked and cured in the next top coat drying furnace 7.
[0026]
As described above, according to the coating method of the present embodiment, the conventionally required electrodeposition drying furnace is not required, so that the process space of the coating line is significantly reduced. Further, thermal energy used in the electrodeposition drying furnace is not required, so that the production cost can be reduced.
[0027]
In addition, since the electrodeposition coating film and the intermediate coating film are baked and hardened under the same hardening conditions, coating problems such as wrinkles and skin defects can be prevented.
[0028]
The embodiments described above are described for facilitating the understanding of the present invention, and are not described for limiting the present invention. Therefore, each element disclosed in the above embodiment is intended to include all design changes and equivalents belonging to the technical scope of the present invention.
[Brief description of the drawings]
FIG. 1 is a process chart showing a coating method according to an embodiment of the present invention.
FIG. 2 is a view showing a coating line to which a coating method according to an embodiment of the present invention is applied.
FIG. 3 is a graph showing a relationship between time and temperature and a relationship between time and viscosity in an intermediate coating drying furnace according to an embodiment of the present invention.
FIG. 4 is a graph showing a relationship between time and temperature and a relationship between time and viscosity in an intermediate coating drying furnace of a conventional coating method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Pre-processing booth 2 ... Electrodeposition coating tank 3 ... Electrodeposition rinsing booth 4 ... Intermediate coating booth 5 ... Intermediate coating drying furnace 6 ... Top coating booth 7 ... Top coating drying furnace

Claims (4)

In a coating method of applying at least three paints of undercoat, intermediate paint and topcoat,
Using an undercoat and an intermediate paint having substantially the same curing temperature and curing temperature holding time,
A coating method, comprising applying the undercoat paint, applying the intermediate paint in a wet-on-wet manner, baking and curing these simultaneously, and applying the overcoat paint. 2. The coating method according to claim 1, wherein the undercoat contains an epoxy resin as a base resin and a blocked polyisocyanate as a crosslinking agent, and the blocked polyisocyanate is blocked with an alcohol-based blocking agent having a short alkyl group. 3. . The said intermediate coating material contains a base resin having a hydroxyl group and a carboxyl group, and a blocked polyisocyanate as a cross-linking agent, wherein the blocked polyisocyanate is blocked with an alcohol-based blocking agent having a long alkyl group. The painting method described. The coating method according to claim 1, wherein the intermediate coating composition includes a base resin having a hydroxyl group and a carboxyl group, a blocked polyisocyanate as a crosslinking agent, and a high-boiling amine.

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