JPH1060688A - Cationic electrodeposition coating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the oxidation of lead ion, etc., and to efficiently remove lead, the metal ions of zinc, etc., eluted from a material to be coated and an increasing acid by using an anode with a metal low in oxygen overvoltage formed on a Ti substrate and electro-dialyzing a part of the electrodeposition coating bath soln. in a tank incorporated with an ion-exchange membrane. SOLUTION: An anode and a material to be coated as a cathode are arranged in an electrodeposition coating tank 10, and an anion-exchange membrane and a cation-exchange membrane are alternately arranged in an electrodialytic tank 11. An electrodeposition coating bath soln. 12 partially discharged from the coating tank 10 is sent to the electrodialytic tank 11 to remove acid and/or metal ion 13 therein, and then the electrodeposition coating bath soln. is returned to the coating tank 10. Since the anode with the metal or metal oxide low in oxygen overvoltage formed on a Ti substrate is used, the elution of the metal ion such as iron ion and the oxidation of the metal ion of lead, etc., close to the anode are prevented, iron oxide and lead oxide are not deposited on a coating film, and hence the quality is not lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cationic electrodeposition coating method, and more particularly, to the prevention of elution of metal ions from an anode, the prevention of oxidation of metal ions, particularly lead ions, eluted from paint components, and the electrodeposition coating bath. The present invention relates to a cationic electrodeposition coating method involving removal of acid and / or metal in a liquid.

[0002]

2. Description of the Related Art In a cationic electrodeposition coating, as shown in FIG. 3, an object to be coated (an automobile body, for example) 3 is immersed in an electrodeposition coating bath solution 2 in an electrodeposition coating tank 1. In this method, an object is electrically coated by passing a direct current between the anodes 4a and 4b and the cathode 3 arranged in the electrodeposition coating solution. In the cationic electrodeposition coating, a reaction of 2H ₂ O + 2e ⁻ → 2OH ⁻ + H ₂ occurs on the cathode side, and a reaction of 2H ₂ O → 4H ⁺ + O ₂ ⁺ + 4e ⁻ occurs on the anode side.

[0003] An electrodeposition coating solution (liquid containing an electrodeposition coating) is an aqueous coating obtained by dispersing or dissolving a pigment-processed resin (paint component) in water. The resin has, for example, an epoxy resin as a main skeleton. This is reacted with an amine or the like to introduce a cationic functional group, and neutralized with an acid as a neutralizing agent to disperse or dissolve in water.

When a direct current is passed, on the cathode (object) side, the positively charged paint component moves to the surface of the cathode (object) and solidifies to form a coating film on the cathode surface. As the coating film is formed, the acid in the electrodeposition bath solution is liberated and the acid concentration rises. However, if the acid concentration is too high, the solidification of the paint components is reduced and the coating is adversely affected. Therefore, the pH of the electrodeposition bath solution is within a predetermined range (for example, pH 5.5 to 6.
It is necessary to remove the acid so that it falls within 5). Conventionally, for this purpose, oxidizing radicals are collected on the anode side by partitioning the vicinity of the anode 4b in the electrodeposition coating tank with a diaphragm 5 made of an anion film, and the collected oxidizing radicals are removed as acid to the outside of the electrodeposition coating tank. Have been done.

[0005]

However, when the vicinity of the anode in the electrodeposition coating tank is divided by a diaphragm, extra space is required and the size of the electrodeposition coating tank becomes large. There is a disadvantage that the above-mentioned electrodeposition bath solution must be maintained. Further, since a large object to be coated enters and exits the electrodeposition coating tank, the diaphragm is liable to be damaged, and the shape and installation method of the diaphragm are considerably restricted in design.

Further, since the current efficiency of removing the acid by the diaphragm 5 is higher than the current efficiency of electrodeposition of the paint on the workpiece 3,
When all the anodes are partitioned by the diaphragm, the acid is removed too much. Therefore, as shown in FIG. 3, the anode (bare anode) 4a not partitioned by the anion membrane is also used in combination. Adjustment of the area ratio with the bare anode 4a and the current ratio is required, which is troublesome.

In addition, when electricity is supplied, the hydrogen ion concentration gradually increases near the anode and the anode potential rises, so that metal ions in the electrodeposition bath solution are oxidized, and especially lead eluted from paint components. Such metal ions are remarkably oxidized and become lead oxide and float in the electrodeposition bath solution. However, the lead oxide or the like formed in this way has a rough texture, and if it adheres to a coating film, its quality deteriorates.

Conventionally, stainless steel or ferrite has been used for the anode electrode. However, when a ferrite anode is used, the above-mentioned oxidation of metal ions such as lead is remarkable.
Further, iron ions are eluted from the stainless steel anode, and iron oxide formed by oxidizing the iron ions accumulates in the electrodeposition coating bath solution used for a long time and deposits on the object to be coated. Oxide is further often surface-treated by zinc phosphate or the like before the electrodeposition coating to be coated was eluted in electrodeposition coating baths solution such as zinc ions for the (Zn ^{+ +)} oxide It becomes zinc and electrodeposits on the coating film, deteriorating the quality of the coating film.

The present invention prevents oxidation of lead ions and the like present in the electrodeposition coating bath solution, and further increases the acid ions which are increased by electrodeposition, such as lead ions and zinc ions eluted from the object to be coated. It is an object of the present invention to provide a cationic electrodeposition coating method that efficiently removes odor and prevents its accumulation.

[0010]

According to the cationic electrodeposition coating method of the present invention, an object to be coated is used as a cathode, titanium is used as a base material, and a surface layer made of a metal or metal oxide having a low oxygen overvoltage is formed thereon. The anode is placed in an electrodeposition coating tank, and a direct current is applied to perform electrodeposition coating on the object to be coated, and a part of the electrodeposition coating bath solution is taken out of the electrodeposition coating tank and ionized. The method is characterized in that an acid and / or metal ions contained in the electrocoat bath are removed by introducing into an electrodialysis tank incorporating an exchange membrane and performing electrodialysis, and then returning to the electrocoat tank. And

Metal ions contained in the cationic electrodeposition coating bath solution include lead ions and the like, zinc ions eluted from the object to be coated, and acetic acid as a neutralizing agent for the electrodeposition paint. , Formic acid, lactic acid and the like.

The surface layer of the anode made of a metal or metal oxide having a low oxygen overvoltage is preferably made of at least one selected from iridium, tantalum, ruthenium, palladium, platinum and metal oxides thereof, or a combination thereof. . Among them, it is preferable to use iridium metal containing up to 30% by weight of ruthenium metal.

In the present invention, the oxygen overvoltage of the anode is preferably from 0.05 V to 0.3 V, and more preferably from 0.05 V to 0.1 V. If the oxygen overvoltage is 0.3 V or more, the amount of lead oxide generated is undesirably increased.

Further, the current density in the electrodeposition coating tank is adjusted so that the electrode potential at the anode is maintained at a potential equal to or lower than the electrode potential that generates lead oxide by oxidation of metal ions, particularly lead ions, at the anode. preferable. For example, the electrode potential at which lead ions are oxidized is E = 1.449-0.11.
Obtained from 82 × pH value ^{-0.02591 × log (Pb + +)} .

[0015] As applied current density in this order electrodeposition coating bath well that is ^{0.05A / dm 2 ~1A / dm 2} , more preferably 0.1A / dm ² ~0.2A /
dm ² is good. When the energizing current density is 1 A / dm ² or more, lead oxide is undesirably generated.

In the electrodialysis tank to which the electrodeposition bath solution is supplied and treated in the present invention, a cation exchange membrane and an anion exchange membrane are alternately arranged between an anode and a cathode and assembled by tightening the whole. A filter press type electrodialysis tank is preferably used. The electrodeposition coating bath solution taken out of the electrodeposition coating tank is supplied to the dilution chamber of the electrodialysis tank, and is preferably passed through the anion exchange membrane by applying a current of 0.01 to 5 A / dm ² . CH ₃ COO ^-
Metal ions such as zinc and lead are condensed in the concentrating chamber after passing through an acid root such as cation exchange membrane, and are discharged from the electrodialysis tank as a concentrated solution. CH ₃ COO ^- acid radical or, zinc, such as,
The diluent in the electrodeposition coating tank from which metal ions such as lead have been removed is circulated again to the electrodeposition coating tank.

The cation exchange membrane used in the electrodialysis tank is preferably a strongly acidic homogeneous membrane or a heterogeneous membrane, and the anion exchange membrane is preferably a strongly basic homogeneous or heterogeneous membrane.

As the cation exchange membrane of the electrodialysis tank, a permselective membrane that allows metal ions to pass through more selectively than paint components may be used. As such a permselective membrane, a membrane in which a polymer layer of ethylenediamine is formed on the surface of a cation exchange membrane, or an amphoteric ion exchange membrane in which a cation exchange membrane and an anion exchange membrane are bonded to each other are used.

Further, the electrodeposition coating bath liquid partially removed from the electrodeposition coating tank may be supplied to an electrodialysis tank after the resin or pigment of the coating component is separated by an ultrafiltration device (UF). When the resin and pigment in the paint are supplied to the electrodialysis tank in a state of being separated and removed in advance, the efficiency of removing the acid and / or metal ion in the electrodialysis tank is improved. Ultrafiltration equipment (U
The filter of F) preferably passes particles of 0.1 μm or less, preferably 0.01 μm or less in order to separate the resin and pigment in the paint. The resin and pigment in the paint separated by the ultrafiltration device (UF) are returned to the electrodeposition coating tank.

In the electrodialysis tank, the acid and metal ions contained in the electrocoat bath are removed as much as possible and then circulated to the electrocoat tank. However, in many cases, the metal ions contained in the electrodeposition bath solution are less than the acids,
In the electrodialysis tank, substantially all of the metal ions can be removed, while the acid can be removed only in an amount corresponding to the amount of electricity corresponding to the removal of the metal ions. In such a case, in order to crush the acid remaining in the electrodeposition bath solution, a corresponding amount of the paint component is replenished. In this case, the load on the electrodialysis tank can be reduced, and the acid-containing waste discharged from the enrichment chamber of the electrodialysis tank can be reduced as compared with the case where the entire amount of the acid is removed. Can manage the cation type electrodeposition bath solution.

Further, according to the present invention, metal ions such as iron, lead, nickel, and zinc contained in the cation-type electrodeposition coating bath are oxidized near the anode in the electrodeposition coating tank to form an insoluble metal oxide. , Not only makes it impossible to remove metal ions by electrodialysis, but also lowers the quality of a coating film on an object to be coated. In order to prevent this, in the present invention, the reducing agent is preferably present in the electrodeposition coating bath in the electrodeposition coating bath in an amount of 0.01 to 0.05 equivalent / liter. Excessive presence of the reducing agent is not preferable because it deteriorates the quality of the coating film on the object to be coated.

Various reducing agents can be used as the reducing agent. However, from the viewpoint of the effect on the coating film on the object to be coated and on the electrodialysis, it is preferable that when the reducing agent is oxidized, it itself becomes an acid, CO ₂ or H _2.
The use of aldehydes, ketones, acids, and alcohols that become ₂ O is preferred. Preferred examples of the reducing agent include formaldehyde, acetaldehyde and formic acid.

In the present invention, it is extremely effective to use formic acid as a neutralizing agent as an acid as the cationic electrodeposition coating. Formic acid also acts as a reducing agent for metal ions, so that oxidation of metal ions is prevented, and metal ions are effectively removed by electrodialysis, so that a good coating film on an object to be coated can be obtained.

Further, in order not to increase the hydrogen ion concentration near the surface of the anode, a propeller type stirrer (not shown) is provided near the anode, or a part of the electrodeposition coating bath is removed from the electrodeposition coating tank. It is preferable that the electrode surface is stirred by making a flow in the electrodeposition coating bath liquid near the anode by, for example, returning to the vicinity of the anode in the electrodeposition coating tank with a pump (not shown). It is also preferable to vibrate the anode.

Further, the anode is formed into a cylindrical body made of titanium metal as a base material, and at least the surface layer on the outer surface side is made of iridium,
Tantalum, ruthenium, palladium, formed from at least one or more selected from platinum and metal oxides thereof,
In addition, a plurality of holes are provided on the cylindrical side surface, and a part of the electrodeposition coating bath is circulated and returned to the cylindrical body, and is ejected from the hole to flow or stir the electrodeposition coating bath near the anode. A state may be created.

Thus, in the cationic electrodeposition coating solution of the present invention, the electrodeposition coating solution from which acid and metal ions have been removed by electrodialysis is circulated, and the pH in the electrodeposition coating solution is preferably 5 to 5. The metal ion concentration can be maintained at 6.5, more preferably 5.8 to 6.2, and preferably 1000 ppm or less, and more preferably 500 ppm or less.

[0027]

FIG. 1 is a view for explaining an electrodeposition coating method according to the present invention. An electrodeposition coating tank 10 is provided with an anode and an object to be coated as a cathode. On the other hand, in the electrodialysis tank 11, anion exchange membranes and cation exchange membranes are alternately arranged. Electrodeposition bath liquid partially removed from the electrodeposition bath
2 is sent to an electrodialysis tank 11 where the acid and / or metal ions 13 are removed, and then the electrocoat bath 14 is returned to the electrocoat tank 10.

In the electrodeposition coating method shown in FIG.
An ultrafiltration device (UF) 15 is disposed between the electrodialysis tank 11 and the electrocoating bath solution 12 partially removed from the electrodeposition coating tank and sent to the ultrafiltration device (UF) 15. Here, after the paint components are separated and removed in advance, they are sent to the electrodialysis tank 11, where the acid and / or metal ions 13 are removed, and the electrodeposition bath solution 14 is returned to the electrodeposition tank 10. The paint component 16 separated by the ultrafiltration device (UF) 15 is returned to the electrodeposition coating tank 10.

[0029]

【Example】

Example 1 An electrodeposition coating tank containing 100 liters of a cationic electrodeposition paint (PTU 1500 manufactured by Nippon Paint Co.) and having a titanium substrate anode (oxygen overvoltage 0.1 V) having iridium in the surface layer as an anode. , A current was applied at a current density of 0.2 A / dm ² to perform electrodeposition coating on steel. An electrodeposition paint having a composition of 200 g / liter, 0.05 eq / liter of acetic acid, 0.05 g / liter of lead acetate, 0.05 g / liter of zinc acetate, and 0.05 g / liter of iron acetate taken out of the electrodeposition coating tank. The coating bath was supplied to a dilution chamber tank of an electrodialysis tank (a Du-O type electrodialysis tank manufactured by Asahi Glass). The concentrated solution tank contains 0.
Acetic acid of 02 eq / liter was supplied, and 1 liter of a 0.5N sodium sulfate solution was supplied to the electrode solution tank.
The electrodialysis tank has an effective membrane area of 1.7 dm ² , 10 membrane pairs, a strong basic anion exchange membrane Selemion AMV (trade name of Asahi Glass Co.) as an anion exchange membrane, and a strongly acidic cation exchange membrane as a cation exchange membrane. Selemion CMV (trade name of Asahi Glass Co., Ltd.) was used.

In the electrodialysis tank, a direct current of 0.05 A was passed while circulating the diluent, the concentrate and the electrode solution. As a result of measuring the concentration of acetic acid in the diluted solution after 1 hour, 0.04 e
At q / liter, the current efficiency was found to be 65% and acetic acid was removed as desired. The concentration of zinc acetate remaining in the diluent was 0.01 g / liter, and the concentration of iron acetate was 0.1 g / liter.
It was 02 g / liter, and it was confirmed that metal ions had been removed. The diluent was circulated and supplied to the electrodeposition bath so that the pH of the bath was maintained at 6.0. As a result, the electrodeposition coating could be performed smoothly without any problem.

<Example 2> An electrodialysis tank was used in the same manner as in Example 1 except that a strongly acidic cation exchange membrane Selemion CMV was used as the cation exchange membrane and a membrane obtained by surface-treating the membrane with a polyamine of ethylenediamine was used. Carried out.

Although the current efficiency of removing acetic acid in the diluent in the electrodialysis tank was 75%, the concentration of zinc acetate remaining in the diluent was 0.025 g / liter, and the concentration of iron acetate was 0.03 g / liter.
At g / liter, the removal rate was slightly lower than in Example 1. However, the amount of amine, which is a coating component leaked into the concentrated solution, was 1 millieq / liter or less in the concentrated solution, and it was confirmed that the coating component was hard to leak. As in Example 1, the diluent in the electrodialysis tank could be circulated and used in the electrocoat tank without any problems.

Example 3 Example 3 was carried out in the same manner as in Example 1, except that an electrodialysis tank incorporating an amphoteric exchange membrane Selemion HSV (trade name of Asahi Glass Co.) was used as the cation exchange membrane.

The current efficiency in removing acetic acid from the diluent in the electrodialysis tank was 75%, but the concentration of zinc acetate remaining in the diluent was 0.04 g / liter, and the concentration of iron acetate was 0.04 g.
Per liter, the removal rate is lower than in Examples 1 and 2, but the amine, which is a paint component that has leaked into the concentrated solution, is 1 millieq / liter or less in the concentrated solution, and the paint component is less likely to leak. Was confirmed. The same diluent in the electrodialysis tank as in Example 1 could be circulated and used without any trouble in the electrocoat tank.

[0036]

According to the present invention, the elution of metal ions such as iron ions from the anode is achieved by forming a surface layer made of a metal or metal oxide having a low oxygen overvoltage on the anode made of titanium as a base material. In addition to this, it is possible to prevent oxidation of metal ions such as lead near the anode, prevent iron oxide or lead oxide from adhering to the coating film, and prevent deterioration in quality.

The current density and the pH of the electrodeposition bath solution
To maintain a potential equal to or lower than the electrode potential at which lead oxide is generated, add a reducing agent to the electrodeposition coating bath, stir the vicinity of the anode, make the anode into a cylindrical body, or vibrate the anode. By doing so, the increase and oxidation of lead ions near the anode can be prevented, preventing the adhesion of iron oxide and lead oxide to the coating film,
Quality deterioration can be prevented.

In the present invention, the electrodeposition coating bath is taken out of the electrodeposition coating bath, and the acid and metal ions are removed by the electrodialysis bath. Can be reduced. Further, since the diaphragm is not in the electrodeposition coating tank, the design of the electrodeposition coating tank is easy, and the diaphragm is not damaged. Further, it is not necessary to consider the difference between the current removal efficiency of the acid by the diaphragm and the current efficiency of the electrodeposition of the paint on the object to be coated, which facilitates the design of the electrodeposition coating tank.

In the electrodialysis tank, a paint component can be collected and returned to the electrodeposition coating tank by using a permselective membrane that selectively allows metal ions to pass therethrough as compared with the paint component.

Further, after the electrodeposition coating bath liquid taken out of the electrodeposition coating tank is filtered by an ultrafiltration device and supplied to the electrodialysis tank, the paint components can be recovered and returned to the electrodeposition coating tank. At the same time, by separating the paint components in advance, the load on the electrodialysis tank can be reduced, and the removal efficiency of acid and / or metal ions can be improved.

In the electrodialysis tank, only metal ions are separated without separating and removing the acid, and the paint components consumed in the electrodeposition coating on the object to be coated corresponding to the increased amount of the acid are applied to the electrodeposition tank. In this case, there is no need to treat or replenish the separated acid, and an efficient cationic electrodeposition coating can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a cationic electrodeposition coating method according to the present invention.

FIG. 2 is a diagram illustrating a cationic electrodeposition coating method according to the present invention.

FIG. 3 shows a conventional cationic electrodeposition coating apparatus.

[Explanation of symbols]

 10 Electrocoating tank 11 Electrodialysis tank 15 Ultrafiltration device

Claims (8)

[Claims] An object comprising a cathode as an object to be coated, a titanium substrate as a base material, and an anode having a surface layer made of a metal or a metal oxide having a low oxygen overvoltage formed thereon are arranged in an electrodeposition coating tank. Electrodeposition is applied to the object by applying a current, and a part of the electrodeposition coating solution is taken out of the electrodeposition coating tank and introduced into an electrodialysis tank with an ion exchange membrane. And removing the acid and / or metal ions contained in the electrodeposition bath solution, followed by returning to the electrodeposition bath. 2. The cationic electrodeposition coating method according to claim 1, wherein the cationic electrodeposition paint contained in the electrodeposition bath solution is a paint using formic acid as a neutralizing agent. 3. A surface layer comprising a metal or a metal oxide having a low oxygen overvoltage comprises at least one selected from iridium, tantalum, ruthenium, palladium, platinum and metal oxides thereof. Cationic electrodeposition coating method. 4. The cationic electrodeposition coating method according to claim 1, wherein at least one reducing agent selected from formic acid, formaldehyde and acetaldehyde is present in the electrodeposition coating solution. 5. An anode having a cylindrical body, having a plurality of holes on its cylindrical side surface, and circulating a part of the electrodeposition coating bath solution inside and outside the cylindrical body forming the anode through the holes. The cationic electrodeposition coating method according to any one of claims 1 to 4, wherein a flow of the electrodeposition coating solution is created near the anode. 6. The electrocoating bath solution taken out of the electrocoating tank is filtered by an ultrafiltration device to separate and remove paint components of 0.1 μm or less, and then supplied to an electrodialysis tank. 5. The method of cationic electrodeposition coating according to any one of the items 5. 7. The method according to claim 1, wherein the electrodialysis tank has a cation exchange membrane and an anion exchange membrane, and the cation exchange membrane is a permselective membrane through which metal ions pass selectively as compared with paint components. The cationic electrodeposition coating method according to any one of the above. 8. An electrodialysis tank, wherein substantially all of the metal ions in the electrodeposition coating bath are removed and removed, and an equivalent amount of paint components corresponding to the remaining acid radicals is supplied to the electrodeposition bath. 7
The cationic electrodeposition coating method according to any one of the above.