JPH0987891A - Electrodeposition coating method and electrodeposition coating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the concentrated flow of a large current to part of materials to be coated and the formation of stepped coating films when the continuously transported materials to be coated enter an electrodeposition coating vessel in electrodeposition coating of an electrical entrance system of a low equipment cost and maintenance cost without the occurrence of a bipolar phenomenon. SOLUTION: The electric resistance value of the polar liquid in a diaphragm chamber M is set highest in the value of the entrance section diaphragm chamber M1 of the materials 2 to be coated and is set successively lower toward the progression direction of the materials 2 to be coated. Pure water is supplied to the diaphragm chamber M1 at the foremost end and is sent backward in a cascade system for this purpose. Further, the thickness w1 of the diaphragm chamber M at the foremost end is set max. and is successively decreased toward the progression direction of the materials 2 to be coated. As a result, the degradation in the throwing power by the voltage drop in the exit section is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electrodeposition coating, and more particularly to electrodeposition coating using a diaphragm chamber in an energization bath type.

[0002]

[Prior Art and Problems] Electrodeposition coating has been used for a long time on products such as automobiles processed with metal plates. In recent years, cationic electrodeposition coating has been widely used in Japan and abroad because it provides a highly corrosion-resistant coating. Has been done. Cationic electrodeposition coating is performed by bathing a water-soluble paint neutralized with an organic acid and having a specific resistance value of 300 to 1000 Ω-cm in an electrodeposition (coating) tank to form a counter electrode. Then, a DC voltage is applied between the main electrodes, which are usually arranged in 5 to 20 diaphragm chambers. This causes the cationic resin paint to deposit on the surface of the negative counter electrode, while the organic acid concentrates in the diaphragm chamber, so pure water is supplied to remove the acid and maintain the neutrality of the paint. ing. Further, the polar liquid concentration in the diaphragm chamber, that is, the acid concentration, is controlled by measuring the specific resistance value or the specific electric conductivity.

By the way, the object to be coated is suspended on an overhead traveling type conveyor and continuously immersed in an electrocoating tank formed in a boat shape, and the electrodeposition coating proceeds in tandem with the progress of the object to be coated. However, the method of applying a DC voltage to the object to be coated is a so-called energization bath method in which the object to be coated is energized and a voltage is applied after the object is completely immersed in the electrodeposition tank. There is a so-called totally submerged energization method. In either case, when energized in a bare state where no coating film is formed, the surface resistance is small and a large current flows, which causes various problems. That is, in the case of the energization tank system, the liquid surface is shaken due to agitation and the impact at the time of entering the object to be coated, so that the electrodeposition coating cannot be performed uniformly and a so-called step phenomenon occurs. Therefore, the coating quality deteriorates, polishing is required, and the cost is increased. In order to solve this, a method of separating the position of the main electrode from the entry point of the object to be coated has been adopted, but a sufficient effect was not obtained. In addition, in the case of the totally submerged energization method, a step phenomenon does not occur, but a large current flows at the same time as energization, so that a large power source and a current collector are required. In addition, in this method, when the article to be coated is continuously conveyed, it is necessary to completely insulate between the article to be coated and the conveying device, so that the structure of the suspension hanger or the current collector becomes complicated, The cost is significantly higher than that of the energized bath system. Also, when the continuously conveyed coating object enters the tank in a non-energized state, a current flows from the preceding energized coating object, a so-called bipolar phenomenon occurs in which the deposited coating film is dissolved, and the coating film It caused a defect. Therefore, a method has been adopted in which the energization zone is divided into a plurality of zones, the voltage in the zone near the bath section is lowered, and the voltage is sequentially increased toward the bath section. Not only that, but also when moving from the low voltage zone to the high voltage zone, sparks on the sliding part of the current collector were unavoidable and required a great deal of cost for maintenance.

On the other hand, the electrode is arranged in the diaphragm chamber, and a plurality of diaphragm chambers are usually used in parallel with the traveling direction of the object to be coated. Further, as the electrodeposition coating progresses, the acid concentration of the polar liquid in the diaphragm chamber gradually increases.
Pure water having a specific resistance of kΩ-cm is supplied to and maintained in the diaphragm chamber, but both the inlet-side diaphragm chamber and the outlet-side diaphragm chamber of the object to be coated have a specific resistance of 1000 to 3000Ω-. It was controlled so that it would be cm. Therefore, when the electrodeposition coating is carried out by the energization bath method, a large current flows due to the small surface resistance of the coating object in the bath section, despite the small electrodeposition area of the coating object, against slight shaking. However, the current density was changed, resulting in a step phenomenon. On the other hand, in the tank portion, the high resistance of the polar liquid in the diaphragm causes a decrease in the application to the object to be coated, resulting in a decrease in throwing power. Therefore, if a large number of bare electrodes are provided in addition to the diaphragm inner electrodes, the throwing power does not deteriorate, but the acid concentrated on the bare electrodes is not removed, and the electrodeposition coating solution is often discarded, or the filtrate is filtered with an ultrafilter. Had to be discarded to remove excess acid, resulting in complicated work and increased cost. Therefore, an object of the present invention is to provide a method and a device for stabilizing electrodeposition coating without causing a step phenomenon in the continuous electrodeposition coating of the energization bath method and without reducing the throwing power. Is to provide.

[0005]

[Means for Solving the Problems] In order to solve the above problems,
Means provided by the present invention is an article to be continuously conveyed to an electrodeposition coating tank having a main electrode and a plurality of diaphragm chambers arranged in parallel with the traveling direction of the article to be coated. However, in the electrodeposition coating that is carried out with the tank energized, the electric resistance of the polar liquid in the diaphragm chamber closest to the entry point of the object to be coated is set to the maximum value. It is characterized in that it is adjusted so as to gradually decrease, and the electrical resistance value of the polar fluid in the diaphragm chamber is adjusted by supplying pure water to the diaphragm chamber closest to the entry point of the object to be coated, To the highest value,
The following is performed by sequentially supplying the polar liquid in each septum chamber in the traveling direction of the article to be coated in a cascade manner, and adjusting the electric resistance value of the polar fluid in the membrane chamber is most effective at the entry point of the article to be coated. This includes maximizing the thickness of the adjacent diaphragm chamber and successively decreasing the thickness in the advancing direction of the article to be coated.

[0006]

According to the above means, the electric resistance of the polar liquid in the diaphragm chamber closest to the entry point of the article to be coated is maximized,
Since the electrical resistance value of the polar liquid in each diaphragm chamber is successively decreased from the tank part to the tank part, the diaphragm surface potential of the tank part is the lowest, and then sequentially increases. Therefore, the current does not suddenly concentrate and flow in the tank portion of the object to be coated, and no step is generated. In addition, since the voltage drop is small and the diaphragm surface potential is high in the bath section, the throwing power does not decrease and the current efficiency does not decrease. Further, if pure water is supplied to the leading diaphragm chambers and then sequentially supplied to each diaphragm chamber in the advancing direction of the object to be coated, the resistance value is gradually reduced from the tank portion, and thus the diaphragm surface potential is gradually increased. Can be automatically adjusted to. Furthermore, since the concentration of the acid to be discarded becomes the highest, the amount of pure water used decreases. Further, if the thickness of the leading diaphragm chamber is maximized and then the thickness is successively decreased, the electrical resistance of the diaphragm chamber will automatically and sequentially decrease from the bath section, and the diaphragm surface potential will successively increase.

[0007]

Example 1 An embodiment of the present invention will be shown by way of example. FIG. 1 is a vertical sectional view of an electrodeposition coating apparatus showing the present invention, in which six diaphragm chambers M _{1 to} M ₆ are arranged. FIG. 2 is a diagram showing a part of the horizontal cross section of this embodiment.

In FIG. 1, the article to be coated 2 moves from the left side to the right in the figure, enters the point P, and the electrodeposition coating proceeds as the article to be coated advances, and leaves the point Q. In the meantime, the article to be coated 2 is normally grounded by the current collector plate 5 through a current collector (not shown) to maintain the negative potential. The diaphragm chamber M is
As shown in FIG. 2, the shortest distance 1 is about 30 cm along the wall of the electrodeposition coating tank 4 and the electrodes are arranged in parallel with the traveling direction of the article to be coated 2. As shown in FIG. 1, the most advanced diaphragm chamber M ₁ is arranged at a position 3 to 5 m away from the point P, and each diaphragm chamber is arranged with an appropriate electrode density. The number of diaphragm chambers M can be sufficient even if it is a number that is normally used, but in the present invention, the larger the number, the greater the effect. Therefore, the number is preferably 3 or more, and more preferably 5 or more. More preferable. Main electrodes D ₁ -in the diaphragm chambers M ₁ -M ₆
A positive DC voltage is constantly applied to D ₆ and bare electrodes D ₇ and D ₈ by a power source (not shown). That is, the object to be coated is energized and deenergized.

The pure water or the polar liquid is supplied to the diaphragm chamber M by
As shown in FIG. 1, it is performed by a cascade method. That is, Iriso section diaphragm chamber M ₁ is pumped by pump 12 from the pure water storage tank 10, from the diaphragm chamber M ₁ is supplied to the diaphragm chamber M ₂ overflows by downcomers 13. The following M ₂ to M ₃
To the final diaphragm chamber M ₆ in a cascade manner from M ₃ to M ₄ by overflow, and is discharged to the drain tank Z. When the number of diaphragm chambers M is large, as shown in FIG. 1, a cushion tank 14 may be arranged in the middle and pumped by a pump 16. As for the polar liquid concentration in the diaphragm chamber M, the specific electric conductivity meters or the specific resistance meters 20 and 21 are provided to measure the specific electric conductivity or the specific resistance of the leading and final diaphragm chambers to adjust the pure water supply amount. Is preferred. The pure water or the polar solution may be supplied to each diaphragm chamber individually, but if a cascade type is used as shown in FIG. 1, the electric resistance of each diaphragm chamber is automatically adjusted in the traveling direction of the object to be coated. It is preferable that the diaphragm surface potential automatically and sequentially increases. Also, since the polar liquid with a high acid concentration is discharged,
Compared to the individual supply method, the pure water supply amount can be significantly reduced.

As the thickness w of the diaphragm chamber M, that is, the distance from the main electrode plate D to the diaphragm B stretched on the side of the object to be coated, the larger the electric resistance, the more effective it is. As shown in FIG. It is preferable to increase the thickness w ₁ of the partial diaphragm chamber M ₁ and successively decrease the thickness in the advancing direction of the article to be coated, because the electric resistance value is gradually decreased in the traveling direction of the article to be coated and the diaphragm surface potential is sequentially increased. Therefore, it is preferable to increase the thickness w ₁ at the leading edge at least. The practical thickness is usually preferably 3 cm or more, more preferably 5 cm or more. But thicker,
Since the electrodeposition tank becomes larger by that much, it should be 15 cm or less. When used in combination with cascade type pure water supply, a synergistic effect can be exhibited.

The polar liquid concentration in the most advanced diaphragm chamber M ₁ is preferably 10 times or more the specific resistance of the electrodeposition coating, and in practice the specific resistance of the electrodeposition coating is 300 to 1000 Ω-cm. The specific resistance of the most advanced diaphragm chamber M ₁ is preferably 3000 Ω-cm or more, more preferably 10000 Ω-cm or more, and further preferably 20000 Ω-cm or more. Further, the closer the polar liquid concentration in the rearmost diaphragm chamber is to the specific resistance value or the specific electric conductivity value of the electrodeposition coating itself, the more preferable the throwing power is.
The pure water supply amount may be adjusted by. Further, after the rearmost diaphragm chamber, in order to further improve the throwing power, one or two pipe-shaped bare electrodes may be arranged facing the bottom portion of the article to be coated, as shown in FIG.

[0012]

According to the present invention, since the electric resistance of the diaphragm chamber is high in the tank portion and is gradually decreased in the advancing direction of the object to be coated, the diaphragm surface potential is low in the tank portion and the object to be coated is lowered. It becomes higher in the direction of travel. Therefore, when a material to be coated enters the electrodeposition tank, a rapid current does not flow, a step is not generated, a uniform coating film is formed, and the appearance quality is improved. Further, the voltage drop in the tank portion is small, the throwing power does not deteriorate, and the current efficiency improves. Therefore, the corrosion resistance is improved and the electricity bill is reduced. Further, the position where the bare electrode is disposed can be limited to the tail end where the surface resistance of the object to be coated is large and thus the amount of current is small, so that the acid can be reliably removed. Therefore, it is not necessary to dispose of the electrodeposition paint specially, and the paint cost is reduced. In addition, if pure water is supplied to the most advanced diaphragm chamber and then the polar solution is sent sequentially in a cascade manner, the concentration of the polar solution in each diaphragm chamber, and hence the electrical resistance, will be high at the leading edge and It is automatically adjusted so that it gradually decreases, and stable electrodeposition coating is possible. Further, since the highly concentrated polar liquid is discarded, the amount of pure water used is reduced and the cost is reduced. In addition, if the thickness of the most advanced diaphragm chamber is maximized and the thickness is successively reduced, the electrical resistance of each diaphragm chamber can be automatically adjusted so as to decrease in the traveling direction of the object to be coated. Can be painted.

Although the present invention has been described with respect to a cationic type coating, the present invention is not limited to this, and an anionic type in which a coating film is deposited on the anode and the amine as the neutralizing agent is concentrated in the diaphragm chamber containing the cathode. It goes without saying that the present invention can also be used in the case of electrodeposition coating using a resin coating material, which is an electric bath.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an embodiment of the present invention.

FIG. 2 is a diagram showing a part of a horizontal cross section of an embodiment of the present invention.

[Explanation of symbols]

 2, the object to be coated 4, the electrodeposition coating tank M, the diaphragm chamber D, the main electrode 10, the pure water storage tanks 20 and 21 are a resistivity meter or a conductivity meter.

Claims (4)

[Claims] 1. An object to be coated, which is continuously transported to an electrodeposition coating tank having a plurality of diaphragm chambers, which incorporates a main electrode and is arranged in parallel with a traveling direction of the object to be coated, is energized. In the electrodeposition coating carried out in the tank in the state, the electric resistance value of the polar liquid in the diaphragm chamber closest to the entry point of the object to be coated becomes the maximum value,
Hereinafter, the electrodeposition coating method is characterized in that it is adjusted so as to gradually decrease in the traveling direction of the object to be coated. 2. The adjustment of the electric resistance value of the polar liquid in the diaphragm chamber is performed by supplying pure water to the diaphragm chamber closest to the entry point of the object to be coated so that the specific resistance value becomes the maximum value. The electrodeposition coating method according to claim 1, wherein the electrodeposition coating is performed by sequentially supplying the polar liquid in the chamber in a traveling direction of the article to be coated. 3. The adjustment of the electrical resistance of the polar liquid in the diaphragm chamber is such that the thickness of the diaphragm chamber closest to the entry point of the article to be coated is maximized, and the thickness is successively reduced in the advancing direction of the article to be coated. The electrodeposition coating method according to claim 1, wherein 4. A means for immersing an object to be coated in an electrodeposition coating tank in an energized state, a plurality of diaphragm chambers arranged in parallel in the traveling direction of the object to be coated, and a means for supplying pure water to the leading diaphragm chamber, And an electrodeposition coating apparatus having means for sequentially sending the polar liquid in each diaphragm chamber backward in a cascade manner, and means for measuring at least the specific resistance position or the electric conductivity of the leading diaphragm chamber.