JPH11217696A - Production of aluminum product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an aluminum product improved in the defect of coating casued by the deposition of acidic substance to a coating film before baking. SOLUTION: This coating method for an aluminum product is one in which, in a method for producing an aluminum product including a coating stage for an aluminum base material composed of an anode oxidizing stage, an anionic electrodeposition coating stage and a coating stage, a washing stage and a baking stage, a basic compd. composed of alkali metal or alkaline-earth metal is added to the anionic electrodeposition coating soln. in the coating stage or to the wash water in the washing stage after the 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 producing an aluminum product by applying an anion electrodeposition coating to an aluminum substrate or an aluminum alloy substrate (hereinafter collectively referred to as an aluminum substrate). More specifically, the present invention relates to a method for producing an aluminum product, which comprises a step of coating an aluminum base material to prevent an abnormality of the surface of a coating film due to acidic mist often generated in anionic electrodeposition coating.

[0002]

2. Description of the Related Art Electrodeposition coating uses water as a solvent, so there is no danger of fire or explosion, continuous coating by automation is possible, and coating thickness control is easy. For these reasons, they are widely used in metal painting. Anionic electrodeposition coating using an anionic polymer is also used in industrial-scale coating of aluminum products. However, in the coating, there was a problem that white spots or white streaks appeared in some places of a normal clear coating film due to adhesion of the acidic substance to the coating film before baking. In anionic electrodeposition coating of an aluminum base material, an operation of oxidizing a surface to be coated before coating to form an aluminum oxide film on the surface (referred to as anodic oxidation) is performed. It has been known that an acidic substance such as sulfuric acid or oxalic acid used as one component of the electrolytic solution in the above-mentioned method contaminates the coating film. Therefore, in the actual coating line, a system is provided in which a water washing step and a washing step with hot water are provided after the anodization, and an acidic substance such as sulfuric acid adhered to the aluminum base material during the anodization is used. However, such water washing and hot water washing may not completely remove acidic substances, and contamination caused by sulfuric acid mist and the like generated from an anodizing tank and floating in the air cannot be ignored. Therefore, the problem has been completely solved. Absent.

As a solution to the above problem, in order to remove sulfuric acid and the like adhering to an aluminum base material and being carried into an electrodeposition coating solution due to insufficient washing with water after anodization, the electrodeposition coating solution is replaced with an ion exchange resin. Or ultrafiltration (JP-A-52-142744), or providing a barrier (curtain) made of a plastic sheet or film between a step before the coating bath and a step after the coating bath. A method of preventing sulfuric acid mist from adhering to the coating film has been proposed (Japanese Patent Application Laid-Open No. 1-279795). Further, in order to suppress the generation of sulfuric acid mist from the anodic oxidation tank. In addition, a method of adding a surfactant to the electrolyte has been studied. However, the above-mentioned proposed method has a problem that a large amount of cost is required for improving the equipment or the effect is insufficient, and a simple solution is required.

[0004]

SUMMARY OF THE INVENTION The present invention relates to an anion electrodeposition of an aluminum base material which can completely prevent an abnormality of a coating film caused by an acidic substance such as sulfuric acid without adding special equipment to a coating line. An object of the present invention is to provide a method of manufacturing an aluminum product including a painting process.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, found that an alkali metal or an alkali metal is contained in a coating liquid in an anion electrodeposition coating bath or a washing water in a washing step following the coating. By adding a trace amount of a basic compound consisting of an earth metal, without adversely affecting other properties of the coating film, it was found that abnormalities of the coating film caused by an acidic substance such as sulfuric acid can be prevented,
The present invention has been completed. That is, the present invention relates to a method for producing an aluminum product including a step of coating an aluminum substrate comprising an anodizing step, an anion electrodeposition coating step, a washing step, and a baking step, wherein the anion electrodeposition coating liquid or the anion electrodeposition coating liquid in the coating step is used. A method for coating an aluminum product, comprising adding a basic compound comprising an alkali metal or an alkaline earth metal to washing water in a washing step after coating. Hereinafter, the present invention will be described in more detail.

[0006] The main steps in anionic electrodeposition coating of aluminum substrates include: 1) acid cleaning of aluminum substrates;
2) washing with water, 3) washing with alkali, 4) washing with water, 5) anodic oxidation, 6) washing with water, 7) washing with hot water, 8) electrodeposition coating, 9) washing with water and 10) baking. In such anionic electrodeposition coating, there are two possible routes through which an acidic substance causing an abnormality of the coating film contaminates the coating film before baking. One of them is that the acidic substance in the anodizing electrolyte is brought into the electrodeposition coating solution while it is attached to the aluminum substrate due to insufficient cleaning after the anodization, and in the same manner, from the electrodeposition coating solution. As a result of being brought into the washing water of the next washing process,
This is the route in which the acidic substance in the solution adheres to the coating before baking, and the other is that the acidic substance in the mist generated in the anodic oxidation tank etc. comes into contact with the coating before baking while floating in the air. It is a route to do.

In the present invention, as described above, a basic compound comprising an alkali metal or an alkaline earth metal is added to the coating solution in the electrodeposition coating bath or the washing water used in the subsequent washing step. It is more preferable to add the compound to the washing water in the washing step after coating. Although the details of the design of the anion electrodeposition coating line for aluminum base materials are slightly different depending on the individual, the washing water in the washing step after painting is used when returning the active ingredient of the paint brought into the washing liquid to the coating bath. A system in which the active ingredient flows into a coating bath together with the active ingredient is generally employed. In such a case, the basic compound added to the washing water in the washing step also diffuses into the coating liquid after a certain period of time.

The basic compound used in the present invention includes hydroxides of alkali metals or alkaline earth metals such as sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide; sodium carbonate, potassium carbonate, Basic alkali metal salts such as sodium hydrogencarbonate and potassium hydrogencarbonate can be mentioned, and these can be used alone or in combination of two or more. Particularly preferred basic compounds are sodium hydroxide and potassium hydroxide. In addition, when an organic amine or ammonia was used instead of the above basic compound, the effect exhibited in the present invention was not obtained.

The basic compound in the present invention is presumed to be involved in acidic substances adhering to the coating film and neutralization. The preferred amount of the basic compound is difficult to determine uniformly because the amount of the acidic substance adhering to the coating film varies somewhat depending on the individual coating line or operating conditions. It is usually 20 ppm or more based on the weight of the washing water in the washing step after the liquid or coating, and the upper limit thereof is an amount such that the pH of the electrodeposition coating liquid or washing water does not exceed 9. PH of electrodeposition coating solution or washing water is 9
If the basic compound is used in an amount exceeding the above, the coating film will be dissolved on the liquid side. The basic compound may be added to a coating liquid or a cleaning liquid when the electrodeposition coating bath is built,
It may be added to the coating liquid or cleaning liquid in use.

As the electrodeposition paint in the present invention, a paint obtained by dissolving or emulsifying and dispersing an amine salt of a polymer having a carboxyl group and a crosslinking agent in an aqueous medium can be preferably used. The polymer having a carboxyl group is an acrylic polymer having an alkyl acrylate monomer unit or the like as a main component, and is preferably a polymer having an acid value of 5 to 150. Examples of the monomer that imparts a carboxyl group to the polymer include acrylic acid, methacrylic acid, α-chloroacrylic acid, itaconic acid, and maleic anhydride.
For the purpose of increasing the hydrophilicity of the above polymer, amine having a boiling point lower than the baking temperature (normally 140 ° C. to 220 ° C.) after forming the coating film should be used as the amine used for neutralizing the polymer. Is desirable. Ammonia may be used in place of the amine. As the cross-linking agent, an alkoxylated methylol melamine in which at least a part of the methylol group is alkoxylated with a lower alcohol having 4 or less carbon atoms, or the like can be used. From 5 to 60% by weight. When the polymer used in combination has self-crosslinking properties, the amount of the crosslinking agent used may be less than the above range. The anionic electrodeposition paint may contain an organic solvent up to 10% by weight of the aqueous medium.

As an example of the electrodeposition coating conditions that can be used in the present invention, the following conditions are given. Voltage: 50 to 250 V Electrodeposition time: 0.5 to 10 minutes In addition, the water washing after the electrodeposition coating is appropriately performed by immersing in water for 0.5 to 10 minutes once or twice. Can be usually employed at 140 to 220 ° C. for 5 to 40 minutes. Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

[0012]

EXAMPLES and COMPARATIVE EXAMPLES The electrodeposition paint and aluminum substrate used in the following specific examples are as follows. ○ Electrodeposition paint Toacalon ED4620 (a water-based paint composed of a polymer having a carboxyl group having an acid value of 40 and a mixed alkyl etherified methylolmelamine), which is a clear electrodeposition paint, has a solid content of 8% with pure water. The coating solution was diluted, and dimethylethanolamine was added thereto to adjust the pH to 8.1, and 2000 g of the coating solution was charged into a coating tank to form a coating bath. Further, as washing water for the coating film after the electrodeposition coating, 2,000 g of an aqueous solution was prepared by adding dimethylethanolamine to a solution obtained by diluting the above coating material to a solid concentration of 0.3% with pure water and adjusting the pH to 8.5. .

(A) Aluminum substrate After an acid cleaning treatment and an alkali cleaning treatment in the following order, an anodized aluminum alloy plate was prepared as a coating substrate. Immerse in 10% sulfuric acid aqueous solution for 5 minutes at room temperature Wash with running tap water for 5 minutes 5 Immerse in 5% sodium hydroxide aqueous solution at 40 ° C for 5 minutes Wash with running tap water for 5 minutes Immerse in 10% sulfuric acid aqueous solution for 5 minutes at room temperature Tap water running water Anodization (130 g / dm3 sulfuric acid aqueous solution at 22 ° C, 1 minute)
(Electrolysis under A / dm2 condition for 30 minutes) Washing with running tap water for 5 minutes Dipping in 80 ° C pure water for 3 minutes

[0014]

Comparative Example 1 1 g of the above aqueous solution for the washing step was added to 2000 g.
A solution obtained by adding 1 g of a solution (*) adjusted to pH = 8.0 by adding ammonia to a 10% aqueous sulfuric acid solution (sulfuric acid concentration;
5 ppm) was used as the washing water of this example. Using a test piece immersed in the electrodeposition paint bath as an anode and a stainless steel plate as a cathode, a DC current of 130 V was applied for 2 minutes to perform electrodeposition coating, followed by washing with the above-mentioned washing water.
Then, it baked for 30 minutes with the electric dryer set to 180 degreeC. As a result, some white stripes were observed on the obtained cured coating film. * Note: The reason for using an aqueous solution of ammonium sulfate instead of a dilute aqueous solution of sulfuric acid is that when a dilute aqueous solution of sulfuric acid is used, the coating liquid is acidified and the polymer in the coating liquid gels. This is because it is different from the coating film abnormality generated in the above.

[0015]

Example 1 An aqueous solution obtained by adding 100 ppm of sodium hydroxide to the liquid used as the washing water in Comparative Example 1 was used as the washing water of this example. Otherwise, the same operation as in Comparative Example 1 was performed, and the resulting coating film was cured by heating. As a result, no white streaks or spots were present on the obtained cured coating film, and there was no aesthetic problem. Further, the gloss of the coating film, the hardness of the coating film by a pencil scratch test, boiling water resistance, corrosion resistance, alkali resistance and the like were exactly the same as those of the coating film obtained in Comparative Example 1.

[0016]

Examples 2 to 3 The liquid used as the washing water in Comparative Example 1 was
Further, an aqueous solution to which 30 ppm of sodium hydroxide is added,
The washing water of Example 2 was used. Further, an aqueous solution obtained by further adding 500 ppm of sodium hydroxide to the liquid used as the washing water in Comparative Example 1 was used as the washing water of Example 3. In all other respects, the same operation as in Comparative Example 1 was performed, and the resulting coating film was cured by heating. As a result, Example 2 and Example 3
There were no white streaks or spots on the cured coating film obtained in any of the above, and there was no problem aesthetically. In addition, the other coating film properties were basically the same as Comparative Example 1, and were excellent.

[0017]

According to the present invention, a coating defect due to an acidic substance, which has been a long-standing problem in anionic electrodeposition coating of an aluminum base material, can be performed by an extremely simple operation without adding special equipment to the coating line. Can be prevented, and an aluminum product coated with a coating film having physical properties necessary for aesthetics and substrate protection can be obtained.

Claims (2)

[Claims] Anodizing step, anion electrodeposition coating step,
In a method for manufacturing an aluminum product including a coating step of an aluminum substrate comprising a washing step and a baking step, an alkali metal or an alkaline earth metal is contained in the anion electrodeposition coating solution in the coating step or in the washing water in the washing step after the coating. A method for coating an aluminum product, comprising adding a basic compound composed of a metal. 2. The method for coating an aluminum product according to claim 1, wherein the amount of the basic compound added is 20 ppm or more based on the weight of the coating liquid or the washing water.