JP3991923B2 - Painting method - Google Patents

Description

[0001]
【Technical field】
The present invention relates to a coating method that is preferably applied to an automobile body or the like, and more particularly to a coating method that can prevent secondary sagging of an undercoat paint.
[0002]
[Background]
There are 3 types of paint systems for automobile bodies: undercoating with electrodeposition coatings based on epoxy resins, intermediate coatings based on polyester resins, and top coatings with polyester based coatings. After applying the base coat with various types of paint, baking it, applying the intermediate coat on the cured base coat, baking it, and applying the top coat on the cured intermediate coat A so-called 3-coat 3-bake coating method, which is completed by baking, is employed.
[0003]
In this type of coating system, secondary sagging of electrodeposition paint has been regarded as a problem. In other words, the electrodeposition coating is performed by immersing the automobile body in the electrodeposition paint, and this causes the electrodeposition paint to penetrate into narrow areas such as the mating surfaces of the various panels constituting the body to form a coating film. .
[0004]
However, the electrodeposition paint that has entered the narrow part such as the mating surface of the panel is not sufficiently removed only by the electrodeposition water washing step, and it flows out to the outer plate part in the electrodeposition drying furnace and hardens. Since the secondary sagging generated in this way is an abnormal coating compared to the surrounding electrodeposition coating, correction work or the like is required before intermediate coating as a coating failure.
[0005]
By the way, in order to shorten the coating process and save energy, a so-called two-coat one-bake coating system in which electrodeposition coating and intermediate coating are applied wet-on-wet and these are simultaneously baked and cured has been studied. When this 2-coat 1-bake coating system is employed, the secondary sacrificial electrodeposition appears after the intermediate coating drying furnace. As a result, when the correction for removing the secondary sagging is performed, it is necessary to sharpen the intermediate coating film, which is not preferable in terms of rust prevention quality and appearance quality.
[0006]
DISCLOSURE OF THE INVENTION
An object of this invention is to provide the coating method which can prevent secondary sagging of undercoat.
[0007]
In order to achieve the above object, according to the present invention, in the coating method of applying at least three paints of a cationic undercoat, an intermediate coat, and a top coat to an object to be coated, There is provided a coating method characterized by treating the object to be coated with an ammonium hydroxide solution before the intermediate coating is applied and when the coated undercoat is in an uncured state.
[0008]
The inventors of the present invention have examined the appearance defect, which is a problem of secondary sagging, and as a result, completed the present invention. That is, the viscosity of the undercoat paint that flows out as a secondary sagging is high, and the paint that generates this secondary sagging causes problems in appearance by re-dissolving the undercoat paint film formed on the general surface. It is in. As a result of studying these phenomena, the nature of the secondary sauce is that the base coat that is concentrated by evaporation of the solvent during the baking process of the base coat still shows strong acidity, and that the high-viscosity base coat gradually flows out. It was found that this is a typical problem.
[0009]
That is, the cationic electrodeposition paint, which is a typical undercoat paint, is weakly acidic, and the deposited uncured coating has the property of re-dissolving when it comes into contact with an acidic solution, but remains in a narrow part of the vehicle body panel. It is presumed that the solvent is evaporated as the temperature of the vehicle body rises in the process of baking in the drying furnace, and the remaining paint is presumed to become concentrated and show strong acidity. It becomes a high-viscosity liquid, the surrounding deposited coating film is dissolved, and it is estimated that a certain amount of liquid is flowing out.
[0010]
Based on these findings, the present invention treats the object to be coated with an ammonium hydroxide solution when the undercoat film is uncured after the undercoat paint is applied and before the intermediate paint is applied. To do. As a result, it is possible to insolubilize the undercoat paint remaining in the narrow part of the vehicle body in the solvent, and as a result, even if it flows out in the baking process, the surrounding deposited coating is not re-dissolved. Occurrence is suppressed.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a process diagram showing a coating method according to an embodiment of the present invention.
[0012]
The coating method according to this embodiment is a three-coating system of undercoating, intermediate coating, and topcoating. Undercoating and intermediate coating are applied wet-on-wet, and these are baked at the same time, followed by topcoating. Is a so-called 3-coat 2-bake coating system. Speaking of undercoating and intermediate coating, it is a 2-coat 1-bake coating system.
[0013]
In other words, the car body, which is the object to be cleaned and pretreated, is immersed in the electrodeposition tank, and then it is applied to the intermediate coating booth without baking and curing. After the intermediate coating is applied onto the film, these uncured electrodeposition coating and intermediate coating are simultaneously baked in an intermediate drying oven.
[0014]
Here, the electrodeposition coating used in this embodiment is a water-based coating, and the electrodeposition coating and the intermediate coating have substantially the same curing temperature and curing temperature holding time. Conventional curing conditions for electrodeposition coatings are generally maintained at 170 ° C. for 20 minutes, and curing conditions for intermediate coatings are generally maintained at 140 ° C. for 20 minutes, but in this embodiment, electrodeposition coatings are used. In addition, the curing conditions for the intermediate coating are both maintained at 160 ° C. for 20 minutes.
[0015]
In other words, the electrodeposition paint of this example lowers the curing temperature of the existing electrodeposition paint and intermediate coating, and the intermediate coating of this example has a higher curing temperature. It is.
[0016]
As a method for lowering the curing temperature of the electrodeposition paint, for example, when the electrodeposition paint is a material containing an epoxy resin as a base resin and a block polyisocyanate as a crosslinking agent, a block polyisocyanate is used. An alcohol-based blocking agent having an alkyl group shorter than that of the existing alcohol-based blocking agent is employed as the blocking alcohol-based blocking agent. By using a short alkyl group alcohol-based blocking agent, the dissociation temperature of the blocking agent is lowered, and thereby the reaction temperature of the electrodeposition paint can be lowered.
[0017]
Further, as a method of increasing the curing temperature of the intermediate coating, for example, when the intermediate coating is a material containing a base resin having a hydroxyl group and a carboxyl group and a block polyisocyanate as a crosslinking agent, An alcohol-based blocking agent having an alkyl group longer than that of the existing alcohol-based blocking agent is employed as the alcohol-based blocking agent for blocking the block polyisocyanate. By using a long alkyl group alcohol-based blocking agent, the dissociation temperature of the blocking agent increases, and thereby the reaction temperature of the intermediate coating can be increased.
[0018]
As another method for increasing 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, An amine having a boiling point higher than that of the existing amine is adopted for the amine neutralized in the paint. As a result, the binding 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 can be increased.
[0019]
The coating method of the present invention is not limited to the curing conditions of 160 ° C. × 20 minutes held as described above, and both the electrodeposition coating material and the intermediate coating material are cured at T ° C. × t minutes ( T and t may be arbitrary numbers). In this case, the conventional curing conditions are changed using the above-described method for both the electrodeposition coating and the intermediate coating.
[0020]
When the electrodeposition coating is completed, the excess electrodeposition paint adhering to the body is then washed using industrial water or the like. In this example, the body water is sprayed on the body in the first-stage electrodeposition washing process, and the body is immersed in a washing tank filled with the construction water in the next-stage electrodeposition washing process. However, the structure of this electrodeposition water washing step does not limit the present invention, and either spray water washing or immersion water washing may be employed, or these may be provided in multiple stages.
[0021]
Excess electrodeposition paint is washed on the body of the body after the electrodeposition washing process, especially the outer plate and flat parts, but the spray should not reach the narrow part such as the meeting surface of the panel. Therefore, electrodeposition paint is often left behind. For this reason, the body is immersed in a water rinsing tank filled with an alkaline aqueous solution in the next alkaline water rinsing step, and the alkaline aqueous solution is sprayed onto a narrow portion where the electrodeposition paint is likely to remain.
[0022]
The alkaline aqueous solution used in the alkaline water washing step is preferably an aqueous solution that does not contain miscellaneous ions so that miscellaneous ions do not remain on the surface of the coating film after baking and curing. For example, ammonium hydroxide or ethanolamine such as aminomethylpropanol In addition to aqueous solutions of organic bases such as representative amino alcohols, alkaline electrolyzed water and the like can be exemplified. In particular, since alkaline electrolyzed water obtained by electrolyzing water contains almost no cations, when this is used as an alkaline aqueous solution, the next electrodeposition pure water washing step can be omitted or the step can be shortened. Also, in the case of providing an alkaline water washing process by diverting the conventional electrodeposition water washing process, the coating method of this embodiment can be realized simply by replacing the conventional electrodeposition pure water washing process with the alkaline water washing process. The cost of remodeling equipment is also low.
[0023]
When the alkaline water washing is finished, the miscellaneous ions adhering to the body are washed with pure water. In this example, pure water industrial water is sprayed on the body in the first electrodeposition pure water washing step, and the body is immersed in a washing tank filled with pure water in the next electrodeposition pure water washing step. However, the structure of this electrodeposition pure water washing process does not limit this invention, Either spray water washing or immersion water washing may be employ | adopted and these may be provided in multistage.
[0024]
In addition, the narrow part such as the panel meeting surface according to the present embodiment is a typical example. The meeting surface of the sill outer panel and each pillar lower part, door hemming part, back door hemming part, trunk lid hemming part , The surface of the roof panel and weather strip reinforcement, etc., especially the part where secondary sagging occurs on the outer plate surface (part that can be seen as a finished car) due to the electrodeposition paint remaining in the narrow part It is desirable to process it automatically.
[0025]
Next, an example of a coating line to which the coating method of this embodiment is applied will be described. FIG. 2 is a diagram showing a coating line to which the coating method according to the embodiment of the present invention is applied, and is a line layout diagram corresponding to the process diagram of FIG.
[0026]
The automobile body assembled as a white body is brought into the painting factory from the body assembly process, and oil and dust adhering to the body are removed at the first pretreatment booth 1 and the steel plate surface constituting the body is protected. A rust conversion coating is formed.
[0027]
The body on which the clean and chemical conversion film is formed by passing through the pretreatment booth 1 is transported to the electrodeposition coating tank 2 filled with the electrodeposition paint and immersed in the electrodeposition paint. Here, the electrodeposition coating tank 2 is filled with a water-based electrodeposition paint whose curing condition is maintained at 160 ° C. for 20 minutes. In the electrodeposition coating tank 2, the electrodeposition paint is electrophoresed by applying a high voltage to the electrodeposition paint, whereby an electrodeposition coating film is formed on the body.
[0028]
The body leaving the electrodeposition coating tank 2 is carried into the electrodeposition water washing booth 3, where excess electrodeposition paint adhering to the body is washed away. Next, the body is brought into the alkaline water washing booth 3, and the electrodeposition paint remaining on the body is removed by spraying an alkaline aqueous solution on the body on which the electrodeposition coating film is formed or by immersing the body in the alkaline aqueous solution. To sum up.
[0029]
The body that has passed through the alkaline water washing booth 4 is carried into the electrodeposition pure water washing booth 5, where pure water is sprayed on the body or the body is immersed in pure water in order to remove miscellaneous ions attached to the body. As described above, when alkaline electrolyzed water is used in the alkaline water washing booth 4, the electrodeposition pure water washing booth 5 can be omitted or shortened.
[0030]
In the conventional coating method, the body after the electrodeposition pure water washing is carried into an electrodeposition drying furnace, and the electrodeposition coating film is baked and cured, but in this embodiment, the electrodeposition coating film is not provided. Is transported to the intermediate coating booth 6 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.
[0031]
In the intermediate coating booth 6, an intermediate coating is applied to the body with the curing conditions maintained at 160 ° C. for 20 minutes. The painting method is not particularly limited, and painting is performed using a bell type painting gun or a spray type painting gun. In addition, the material of the intermediate coating is not particularly limited, and either a water-based coating or an organic solvent-based coating can be used. However, if the electrodeposition coating is a water-based coating, the intermediate coating should also be a water-based coating. Is more preferable.
[0032]
The body in which the intermediate coating is applied onto the uncured electrodeposition coating is carried into the intermediate coating drying furnace 7, where the electrodeposition coating and the intermediate coating are simultaneously baked and cured.
[0033]
In this example, since the electrodeposition paint and the intermediate coating composition having the same curing conditions are used, as shown in FIG. 3, when the body temperature reaches around 160 ° C., the electrodeposition coating film and the intermediate coating film are At the same time, the curing reaction starts, and the viscosity of both the electrodeposition coating film and the intermediate coating film decreases once the glass transition point of each of the electrodeposition coating material and the intermediate coating material is exceeded. Both membranes become smooth. When the body temperature further rises, the viscosities of the electrodeposition coating film and the intermediate coating film increase again, and finally cure.
[0034]
Returning to FIG. 2, the body on which the cured electrodeposition coating film and the intermediate coating film are formed after passing through the intermediate coating drying oven 7 is a sealing process, a stone guard coating process, an undercoat coating process, and an insulator setting process (whichever Are also not shown).
[0035]
The sealing process is a process for applying a vinyl chloride resin sealing material to the panel mating surface or hemming portion of the automobile body to ensure watertightness, airtightness, or rust prevention.
[0036]
The stone guard coating process is to apply stone guard coat (chipping-resistant paint) to prevent damage to the coating film due to flying stones while driving on the outer body of the car body, the lower part of the door, the lower part of the front fender, the lower part of the rear fender, etc. It is a process.
[0037]
The undercoat application process is a process for applying a vinyl chloride resin undercoat material to the underside of a car body, a tire house, or the like to prevent coating damage caused by a stepping stone during traveling.
[0038]
The insulator setting step is a step of setting an insulator (soundproof / vibration-proof material) at an appropriate location on the dash panel or the floor surface.
[0039]
Since materials used in these steps, that is, a sealing material, a stone guard coat, an undercoat material, and an insulator all require a heating step, a drying furnace 9 is provided in the next step. As the automobile body passes through the drying furnace 9, the sealing material, the stone guard coat, and the undercoat material are cured, and the insulator is softened temporarily, so that it becomes familiar with the panel surface.
[0040]
Next, the automobile body is carried into the top coat booth 8. In the top coat booth 8, the top coat is applied by a conventionally known method, and the top coat film is baked and cured in the next top coat drying furnace 9.
[0041]
As described above, according to the coating method of the present embodiment, a conventionally required electrodeposition drying furnace is not necessary, and the process space of the coating line is significantly reduced. Moreover, since the heat energy used in the electrodeposition drying furnace is not required, the production cost can be reduced.
[0042]
In addition, according to the coating method of the present embodiment, when the electrodeposition coating film is in an uncured state, the body is treated with an alkaline aqueous solution, so that the electrodeposition paint remaining on the narrow part of the body is removed as a solvent. As a result, even if it flows out in the intermediate coating drying furnace 7, the surrounding deposited coating film is not dissolved again, and the occurrence of secondary sagging is suppressed.
[0043]
The embodiment described above is described for facilitating the understanding of the present invention, and is 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.
[0044]
For example, in the above-described embodiment, the present invention is applied to a system in which electrodeposition coating and intermediate coating are applied by wet-on-wet. However, the coating method of the present invention burns this after performing electrodeposition coating, Of course, the present invention can also be applied to a coating system in which an intermediate coating is performed on top of the coating.
[Brief description of the drawings]
FIG. 1 is a process diagram showing a coating method according to an embodiment of the present invention.
FIG. 2 is a diagram showing a painting line to which a painting 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.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Pretreatment booth 2 ... Electrodeposition coating tank 3 ... Electrodeposition washing booth 4 ... Alkaline water washing booth 5 ... Electrodeposition pure water washing booth 6 ... Intermediate coating painting booth 7 ... Intermediate coating drying furnace 8 ... Top coating painting booth 9 ... Top coating drying Furnace

Claims (5)

In a coating method in which at least three paints of a cationic undercoat, an intermediate coat and a top coat are applied to an object,
After the undercoat paint is applied and before the intermediate coat paint is applied, and when the applied undercoat film is in an uncured state, the article to be coated is treated with an ammonium hydroxide solution. How to paint. The coating method according to claim 1 , wherein the object to be coated is treated with an ammonium hydroxide solution and then washed with industrial water or pure water. The treatment with ammonium hydroxide solution, according to claim 1 or 2, characterized in that is carried out by the either spraying ammonium hydroxide solution to the coating object, or immersing the object to be coated in ammonium hydroxide solution The coating method as described in 4. The method of coating according to any one of claims 1 to 3, characterized in that spraying the ammonium hydroxide solution in the narrow portion of the object to be coated. The coating method according to any one of claims 1 to 4 , wherein the undercoat paint is applied, the intermediate paint is applied wet-on-wet, and these are baked and cured at the same time, and the topcoat paint is applied. .

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