JP2002212790A - Electrode deposition coating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrodeposition coating method which is capable of lessening the nonuniformity of electrodeposition coating films occurring in the uneven treatment of metallic blank surfaces even with an electrodeposition coating system of rinsing a material to be coated by circulating filtered water filtered by a UF filter. SOLUTION: This electrodeposition coating method includes a process step of rinsing the material to be coated by circulating the filtered water by the electrodeposition coating system having an electrodeposition tank for forming the electrodeposition coating film on the surface of the material to be coated; a rinsing means for rinsing the resulted material to be coated, a recovering and rinsing drum for recovering a waste rinsing liquid; a filter for separating the waste rinsing liquid of the recovering and rinsing drum to a concentrate containing coating material components and the filtered water; a means for introducing the concentrate to the recovering and rinsing drum; and a means for introducing the filtered water to the rinsing means for the material to be coated, in which the electrical conductivity in the bath of the electrodeposition tank is controlled to <=2,000 μS by substituting the filtered water with water containing no metal cation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrodeposition coating method and, more particularly, to an electrodeposition coating method excellent in concealing unevenness in treatment of a metal material.

[0002]

2. Description of the Related Art Electrodeposition coating is widely applied taking advantage of its easiness of management, economy and the like, and there are remarkable technological advances. Above all, more than 15 months have passed since the conversion from the anionic electrodeposition paint to the cationic electrodeposition paint was measured, and in the automobile field, almost 100% has become cationic electrodeposition paint today. Cationic electrodeposition paints have been introduced not only in the automobile field but also in a wide range of fields such as building materials, general metal products, electric products, and industrial machines.

[0003] The background of the development of cationic electrodeposition coating suitable for coating of automobiles and the like is as follows.
Development of cationic electrodeposition coatings with excellent F (ultrafiltration) properties,
There has been the development of UF films suitable for cationic electrodeposition coating and the development of UF systems that make use of these.

[0004] The electrodeposition coating system which has been developed as described above generally has a configuration shown in FIG. That is, an electrodeposition tank 2 for forming an electrodeposition coating film on the surface of the object 15;
A coating material washing means 3 for washing the obtained coating material 16 with water is provided. Performing advanced cleaning in electrodeposition coating
It is extremely important to improve the finish.

However, the amount of water required for washing is extremely large, and since the washing waste liquid contains the components of the paint, it is not preferable to directly discharge it to the outside. Therefore, the electrodeposition coating system is provided with a system 11 for collecting and reusing the washing water.

[0006] A system 11 for recovering and reusing the washing water comprises:
In general, as shown in FIG. 1, a recovery rinsing tank 4 for recovering the rinsing waste liquid; a filter 5 for separating the rinsing waste liquid in the recovery rinsing tank 4 into a concentrated liquid containing a paint component and filtered water; And a means 7 for introducing the filtered water into the coated product washing means.

That is, the system 11 for recovering and reusing the washing water comprises an electrodeposition tank 2, a washing tank 13 and a collecting water washing tank 4 in order to prevent wastewater containing paint components from being discharged to the outside.
A closed circuit is formed from the above. Therefore, when the coating is washed in the electrodeposition coating process, water circulates through the system, and the amount of water is kept constant in principle.

When a metal material is subjected to electrodeposition coating, the metal material before coating is subjected to a surface treatment in order to prevent corrosion by oxygen, sulfur oxides, rainwater, seawater and the like in the atmosphere. The surface treatment is usually performed by alkaline degreasing of the material surface using caustic soda or caustic potash, then zinc phosphate,
Film formation with a phosphate such as iron phosphate is performed.

However, if the film formation becomes non-uniform in this surface treatment, the electric conductivity of the surface of the metal material becomes non-uniform between the film-formed part and the part where the film is not formed. The amount of the components will also be non-uniform. Therefore, if there is unevenness in the surface of the metal material, unevenness also occurs in the appearance of the electrodeposition coating film.

On the other hand, the amount of paint solids deposited on the surface of the article to be coated also depends on the electrical conductivity of the electrodeposition paint. If this is large, the amount of deposition increases, and if it is small, the amount of deposition decreases. Therefore, when the electric conductivity of the electrodeposition paint is reduced, the non-uniformity of the electrodeposition coating film due to the uneven processing of the surface of the metal material is reduced and can be made almost inconspicuous.

However, the electrodeposition paint is essentially an electrolyte solution containing various miscellaneous ions. The metal cation (Na ⁺ , K ⁺ , Ca ²⁺ , Zn ²⁺ , Fe ^3+, etc.) used for the surface treatment is brought into the electrodeposition tank together with the metal material to be coated.

In such a case, in an electrodeposition coating system for rinsing a coated object by circulating filtered water filtered by a UF filter, the concentration of metal cations in the water increases with time, and the electrodeposition coating material in the electrodeposition tank is removed. Electric conductivity also increases with time. As a result, the appearance of the electrodeposition coating film tends to be non-uniform over time due to unevenness in the surface of the metal material.

[0013]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems.
Even in an electrodeposition coating system for circulating filtered water filtered by an F-filter to wash the coating, the electrodeposition coating method can reduce the non-uniformity of the electrodeposition coating film due to the unevenness of the metal material surface treatment. Is to provide.

[0014]

SUMMARY OF THE INVENTION The present invention is directed to an electrodeposition tank for forming an electrodeposition coating film on the surface of an object to be coated; a coating material washing means for washing the obtained coating; and a recovery washing for collecting a washing waste liquid. Tank; a filter for separating the washing waste liquid in the collecting washing tank into a concentrated liquid containing a paint component and filtered water; a means for introducing the concentrated liquid into the collected washing layer; and a means for introducing the filtered water to the coated article washing means;
An electrodeposition coating method comprising circulating filtered water with an electrodeposition coating system having a step of rinsing the coated object with water, wherein the filtered water is replaced with water substantially free of metal cations. The present invention provides an electrodeposition coating method for controlling the electric conductivity in a bath to 2000 μS or less, thereby achieving the above object.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to make the description easier to understand,
The method of the present invention will be described with reference to FIG.

In the cationic electrodeposition coating, an object 15 attached to a conveyor 14 is continuously flowed. First, for example, water is sprayed on the object 15 by the object watering means 1 such as a shower nozzle.
5 is put in the electrodeposition tank 2. Here, an electrodeposition coating film is formed on the surface of the article 15 to be coated, and the article 16 is obtained. The painted object 16 is carried to the washing tank 13 and washed with the painted object washing means 3 such as a shower nozzle.

The washing waste liquid flows in the countercurrent direction with the coating material and is collected in the recovery washing tank 4. The washing waste liquid is separated by the filter 5 into a concentrated liquid containing a paint component and filtered water.

It is preferable to use a semi-permeable membrane as the filter. As such a semipermeable membrane, RO (reverse osmosis)
There are membranes, UF membranes, and MF (microfiltration) membranes. When filtering waste liquid, a UF membrane is optimal. The RO film is excellent in the ability to remove paint components, but has a small processing capacity per unit time and is not economically preferable. Conversely, for example, an MF membrane having a pore size equal to or larger than UF has excellent membrane treatment ability, but a large amount of paint components enter the filtrate, and do not satisfy the drainage water quality standard for drainage. When the filtrate is reused as final washing water, The cleaning effect is poor and the paint hardens on the substrate,
There is a problem in that so-called bumps occur and stain the coating film.

The UF membrane has no problem in practical use of both the processing capacity and the components in the filtrate. The UF membrane may have a molecular weight cut off of about 3,000 to 1,000,000, and may be made of any material such as polyacrylonitrile, polysulfone, polyolefin, and chemically modified products thereof. The suitability of the material, the molecular weight cut-off, etc. differs depending on the liquid quality, so it is preferable to select the material by conducting a preliminary investigation.

The concentrated liquid that has come out of the filter is introduced into the recovered washing layer 4 by, for example, a line 6. The filtered water is introduced into the coated product washing means 3 by a line 7.

[0021] The UF filter does not have the ability to remove metal cations from the substrate or the like. Therefore, UF
In the electrodeposition coating system in which the filtered water is circulated to wash the coated object with water, the concentration of metal cations in the water increases with time, and the electrical conductivity of the electrodeposition coating in the electrodeposition tank 2 also increases. It will be.

In order to prevent this, in the electrodeposition coating method of the present invention, the electric conductivity of the electrodeposition paint in the bath of the electrodeposition tank 2 is measured by measuring the electric conductivity of the electrodeposition paint at all times or at predetermined time intervals. 200
It is controlled to 0 μS (Siemens) or less, preferably 1500 μS or less. The electric conductivity in the bath of the electrodeposition tank 2 is 2000
If it exceeds μS, if the surface of the metal material has unevenness in processing, the appearance of the electrodeposition coating film also becomes uneven.

As an electric conductivity control method, the filtered water filtered by the UF filter is discarded outside the coating system, and instead, water substantially free of metal cations is introduced into the coating system.

The amount by which the filtered water is replaced with water substantially free of metal cations is such that the electric conductivity in the bath of the electrodeposition tank is 200.
This is an amount sufficient to control the voltage to 0 μS or less. For example, while draining filtered water from the washing tank 13, water substantially free of metal cations may be supplied to the washing tank 13 until the electric conductivity in the bath of the electrodeposition tank 2 becomes 2000 μS or less.

"Substantially free of metal cations" means that the concentration of metal cations is 10 ppm or less, preferably 5 ppm or less. If the conductivity of water substantially free of metal cations exceeds 2,000, it becomes difficult to control the conductivity in the bath of the electrodeposition tank to 2000 μS or less.

The water substantially free of metal cations preferably contains a sufficient amount of acid to neutralize the cationized resin of the electrodeposition coating. This is because the stability of the electrodeposition paint is improved. Such acids include acetic acid, lactic acid, formic acid, sulfonic acid, nitric acid, hydrochloric acid, phosphoric acid and the like. Also,
The acid content is at least 20%, preferably 30-60
% To achieve a% neutralization rate.

This water may contain an appropriate amount of a component usually contained as a hydrophilic solvent for the electrodeposition paint. Examples of such components include butyl cellosolve (ethylene glycol monobutyl ether), hexyl cellosolve (ethylene glycol monohexyl ether), propylene glycol monobutyl ether, and the like, and a solvent containing ethylene oxide or propylene oxide.

The washed product 16 is further subjected to final washing as necessary. The painted object 16 is continuously conveyed to the second recovery rinsing tank 9, where the final rinsing is performed by pure water, working water, or another pure water different from the recovered water by the final rinsing means 8 such as a shower nozzle. Is done. After the final washing, the coating 16 is dried and baked to complete the electrodeposition coating process.

[0029]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 4 experiments were carried out using a pilot plant (capacity of the electrodeposition tank 2 of 200 kg) of the electrodeposition coating system shown in FIG.
Each experiment was distinguished by adding a symbol to AD. At this time, as the filter 5, "KCV-30" manufactured by Asahi Kasei Co., Ltd.
10 "was used.

In the electrodeposition tank 2, 24 kg of cationic electrodeposition paint “Power Top VF-1”, 76 kg of “Power Top VF-2” manufactured by Nippon Paint Co., and 100 kg of deionized water
g, the object to be coated was mounted on a conveyor, and the electrodeposition coating was started. The electrodeposition conditions were an effective voltage of 100 to 250 V and a film thickness of 20 to 30 μm.

Experiments A to D were operated for 8 hours a day for 2 months. During that time, replenishment of the electrodeposition paint is "Power Top VF
-1 ”for 26 kg / week,“ Power Top VF ”
"-2" was carried out at a rate of 72 kg / week. Also,
The filtration water was replaced under the following conditions for each experiment.

[0033]

[Table 1]

Further, the electric conductivity in the bath of the electrodeposition tank was measured every 0.5 months, and the finish of the painted surface was evaluated by electrodepositing a metal material having uneven surface treatment. The evaluation criteria were as shown in Table 2. Table 3 shows the results.

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

The present invention also provides an electrodeposition coating method which is excellent in concealing treatment unevenness of a metal material even in an electrodeposition coating system for circulating filtered water filtered by a UF filter to wash a coating.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an electrodeposition coating system.

[Explanation of Signs] 1 ... means for applying water to coated object, 2 ... electrodeposition tank, 3 ... means for washing coated object, 8 ... final washing means, 4, 9 ... recovered washing tank, 5 ... filter, 6, 7 ... line , 13 ... Washing tank.

Claims (1)

[Claims] 1. An electrodeposition tank for forming an electrodeposition coating film on the surface of an object to be coated; a coating material washing means for washing the obtained coating with water; a collecting washing tank for collecting washing waste liquid; For separating the concentrated liquid containing the paint component into filtrate and filtered water; means for introducing the concentrated liquid into the recovered washing layer; and means for introducing the filtered water to the coated article washing means. An electrodeposition coating method including a step of circulating water to wash a coated object, wherein the filtered water is replaced with water substantially free of metal cations to reduce the electric conductivity in the bath of the electrodeposition tank. An electrodeposition coating method characterized by controlling to 2000 μS or less.