Chemistry: electrical and wave energy – Apparatus – Electrophoretic or electro-osmotic apparatus
Reexamination Certificate
2000-04-21
2002-08-20
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrophoretic or electro-osmotic apparatus
Reexamination Certificate
active
06436263
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrodeposition (hereafter referred to as ED) coating systems and methods, and more particularly to ED coating systems/methods utilizing a first electrode which is to be coated, and plurality of second electrodes provided in association with the first electrodes.
BACKGROUND OF THE INVENTION
ED coating generally may be broadly divided into two categories, including one using a coating material of an anion type and the other using a coating material of a cation type. Since, in either of these ED coatings, uniformity and adhesion of the coating on an article to be coated are excellent and the degree of pollution is generally low, these ED coating techniques have been widely applied recently to prime coating or one coat finishing of metal materials, such as automobile vehicle bodies.
As for the coating materials used in such ED coatings, as a coating material of an anion type, for example, a resin of molecular weight of 2000 often is used to which a carboxyl group is attached to make it water soluble; in the case of a coating material of a cation type, an amino group is attached to the resin to make it water soluble. Even with these water-soluble coating materials, however, the degree of ionization after being dissolved in water is very low. For this reason, at present, in the case of the coating material of an anion type, an alkaline neutralizing agent such as tai-ethylamine, for example, is mixed thereinto, while, in the case of the coating material of a cation type, an acidic neutralizing agent such as acetic acid is mixed thereinto. In both cases, neutralizing is effected, respectively, to thereby increase the degrees of ionization in the water.
As seen above, neutralizing agents are added and mixed to increase the degree of ionization in accordance with the properties of the resin components of the respective coating material. On the other hand, when the ED coating on the articles to be coated proceeds thereby decreasing the resin component in the solution, the coating material should be successively supplied from outside. Accordingly amine or acetic acid, as the neutralizing agent, accumulate in the solution, whereby a phenomenon such as redissolving of the coated film or pinholes occurs, so that the efficacy of the ED coating is impaired to a considerable extent.
For this reason, as described in Japanese Patent Kokoku (Post-Exam. Publn.) No. 22231/1970, for example, so-called pH control is performed to increase the efficiency. By such a method that a second electrode is separated from the article to be coated and from the aqueous solution by use of an ion-exchange membrane or the like, amine or acetic acid are osmotically extracted, to thereby prevent the accumulation of neutralizing agent in the aqueous solution.
ED coating of a cation type using a coating material of a cation type will be hereunder described. In ED coating of a cation type an anion exchange membrane has been used as a membrane. This anion exchange membrane normally has an efficiency of 8-10×10
−6
(mole/Coulomb) as an electric efficiency of removing the acid (coulombic acid removing rate). The acid (neutralizing agent) added to the aqueous solution (ED bath coating material) in the electrodeposition coating bath amounts to a value A contained in the coating material that is supplied to the ED bath.
On the other hand, the total amount of the acid taken out from the ED bath coating material to the outside equals a value B, which includes: (1) 10-20% of the value A taken out as acid contained in a UF filtrate which is used as a rinsing liquid after the ED coating; (2) 5-10% of the value A taken out as acid contained in the coated film; and (3) 70-80% of the value A, which is removed by the membrane electrodes. Although it is ideal that the value A is equal to the value B, it is difficult to adjust in order to obtain such an equality by conventional techniques. In general, B>A is adopted, whereby, if needed, a small amount of acid is added to the bath to keep a generally more exact acid balance.
For such reasons, when all of the electrodes provided in the electrodeposition bath happen to be the membrane electrodes for extracting acid, removal of the acid becomes highly excessive, whereby such disadvantages are presented that the acid as being the neutralizing agent lacks and the acid needs to be periodically supplied from the outside and so forth, so that the control of the neutralizing agent in the ED bath coating material becomes troublesome and the acid is uselessly consumed. For this reason, sometimes some of the electrodes are replaced with so-called bare electrodes having no membranes, or with membrane electrodes having extremely low acid removal rate, so that removal of the acid can be better balanced.
As described above, when the rate of removal is 8-10×10
−6
(mole/Coulomb), removal of the acid becomes excessive and when the rate of removal is 5-6×10
−6
(mole/Coulomb), removal of the acid becomes more nearly ideally balanced, so that a neutral membrane having the latter rate of acid removal may be used sometimes.
SUMMARY OF THE INVENTION
The above-mentioned conventional techniques require, in the event that the acid concentration in the ED bath becomes too low, to add acid directly from outside. There is, however, a disadvantage in this method as such work of addition of acid not only requires labor but also it is quite dangerous. Further, there is an additional problem with such techniques in that there is a sudden change in the acid concentration between before and after addition of acid, which tends to cause abrupt change in the paint characteristic.
The present invention aims to provide ED coating systems and methods which eliminate such problems of conventional techniques and provide a new technique, with interest paid to the function of acid removal of membrane electrodes, that enable adjustment without directly adding acid from outside when acid concentration in the bath tends to go too low.
To attain the goal mentioned above, an ED coating method is proposed which comprises a first electrode as an article to be coated provided in an ED bath and a plurality of second electrodes provided in association with the first electrode, wherein current is passed between the article to be coated and the second electrodes through an aqueous solution of a substance contained in the electrodeposition bath, to thereby electrodeposit the substance for forming a coating film onto the article to be coated, and the second electrodes comprise a number of membrane electrodes having a membrane portion which separates the electrode from the aqueous solution. Some of these second electrodes are a low acid removal type electrode, each of which is provided with corrosion resistant electrode material and first membrane portion having a function of precluding most of the flow of ionized neutralizing agent in the aqueous solution from being extracted, and the remaining second electrodes are high acid removal type membrane electrodes being each provided with a second membrane portion having a function of osmotically extracting the neutralizing agent, wherein these low acid removal type membrane electrodes and high acid removal type of membrane electrodes are placed along the bath paint tank wall.
Further each of the high acid removal type membrane electrodes is provided with a first electrolyte circulation system to run electrolyte from one end to the other end between its second type membrane and electrode pipe, likewise each of the low acid removal type membrane electrodes is provided with a second electrolyte circulation system functioning basically the same as the first electrolyte circulation system, independently from the first system. Both of the first and second electrolyte circulation systems are provided with correspondingly first and second conductivity control circuits/units which are activated if the conductivity exceeds a pre-set reference conductivity value in order to controllably introduce D.I. water to correspondin
Loudermilk & Associates
Mayekar Kishor
Polytechs Inc.
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