US 3640814 A
An electrocoating arrangement employing at least two tank baths for improved coating quality, each tank bath containing a resinous water-soluble paint, and a specific tank-article arrangement which provides a high safety factor in that the tank and the article have the same electrical polarity.
Claims available in
Description (OCR text may contain errors)
United States Patent Koch, H et al.
[ Feb. 8, 1972  ELECTROCOATING APPARATUS  Inventors: Robert L. Koch, II; Gilman Tredwell, both of Evansville, Ind.
 Assignee: Ashdee Corp., Evansville, Ind.
 Filed: Mar. 17, 1965  Appl. No.: 440,410
I  U.S.Cl ..204/300,204/l8l,204/301 5s 1 Field of Search ..204/299, 300, 18]
 References Cited UNITED STATES PATENTS 2,478,322 8/1949 Robinson etal ..204/181 2,898,279 8/1959 Metcalfe et al ..204/181 3,304,250 2/1967 Gilchrist ...204/ l 81 3,355,374 12/1967 Brewer et al ..204/ l 81 FOREIGN PATENTS OR APPLICATIONS 970,506 9/l964 Great Britain ..204/181 Primary Examiner-Howard S. Williams Attorney-Warren D. Flackbert  ABSTRACT An electrocoating arrangement employing at least two tank baths for improved coating quality, each tank bath containing a resinous water-soluble paint, and a specific tank-article arrangement which provides a high safety factor in that the tank and the article have the same electrical polarity.
6 Claims, 2 Drawing Figures PATENTEBFEB 8 I972 3.640.814
Fl E T IN VEN TOR6 ROJERT A. lac/1,1 y Gunny TAPIDIVEIL fforvzey ELECTROCOATING APPARATUS The present invention relates to electrocoating, and more particularly to a new and novel arrangement and structure for achieving a more effective and safe process of anodichydrocoating.
Although relatively in its infancy, electrocoating, sometimes termed anodic-hydrocoating, is a process of applying organic finishes through the use of electrical energy. In carrying out the basic process, an article to be coated is immersed in an electrolyte, defined as a specially formulated, water-soluble paint, where electrical current is then passed through the article and the electrolyte for a given period of time. Upon application of electrical energy, the resin and pigment forming part of the aforesaid electrolyte migrate to the article and a film is irreversibly deposited thereon. Thereafter, and in a typical procedure, the article may be rinsed, dried and baked, either directly, or after being overcoated wet on wet with a conventional compatible paint.
The process is highly advantageous in the control of film thickness on the article coated, even to the obtaining of thick films when using a paint which is formulated with a low solid content, and, additionally, results in rapid paint drain-off, good coverage of sharp edges without film recession, important penetration into welded seams and even into the interior surfaces of boxed sections, no solvent wash, and reduces any fire hazard by the use of water-soluble material.
In prior arrangements, the process has been characterized by the use of a single tank, where the implementation thereof was costly, whether operated as a batch process or a continuous process. In either the batch process or continuous process, of the article to be coated entered the bath rapidly, a costly power supply was required, considering the high-current surge at the beginning of the coating process. In the batch process, if the article was lowered slowly into the bath, much more time was involved to accomplish the coating. In a continuous process where the article was lowered slowly into the bath, not only way additional coating time required, but a much longer tank, necessitating the use of additional valuable floor space, as well as additional material in the tank.
Moreover, existing electrocoating arrangements have provided another drawback safetywise, in that the tank is of one electrical polarity, while the article being coated is of an opposite electrical polarity, reflecting possible danger to the operator, and necessitating complicated insulating hangers and commutators, and the use of bus bars, to make electrical contact with the article when it is in the paint bath.
By virtue of the instant invention, the applicants herein have provided a new and novel approach to the aforedescribed electrocoating process, teaching the use of a multitank electrocoating arrangement, and, as well, a specific tank-article arrangement which results in a high-safety factor and economical implementation, in that, in the preferred form of the invention, the tank and article have the same electrical polarity. Additionally, and as to be discussed further herebelow, the instant insulated tank readily lends itself to provision for electrodialysis for amine and pl-I-level control, without the necessity of using three different energy potentials as has typically been the case in the past.
With a multitank arrangement comprising, at least, a first tank having an approximate constant current of voltage characteristic and a second tank having an approximate constant voltage or current characteristic higher than that of the first tank, an effective smooth coated film is achievable, where the throw power of the paint component of the electrolyte and the flow of the paint, which is dependent upon supplied electrical energy, is more effectively controlled than has been possible heretofore. In other words, the instant invention permits a higher electrical energy level, and, hence, better throw power, in the second tank, not being limited to the lower nonbubbling" electrical energy level which is maintained in the known one-tank production arrangement to prevent any rupture of the finish. Moreover, inherent with the instant arrangement is the need for less article time in the first tank, as
long as a protective film is achieved which will then preclude any bubbling of the coating.
In addition to the aforesaid highly important safety factor resulting from having the article being coated and each of the tanks in any multitank arrangement at the same electrical polarity, the instant invention provides other advantages with the multitank arrangement, including shorter time for coating, a better control of the film makeup and color, the elimination of staining effect in pastel colors, i.e., rusting due, for example, to oxidation of ferrous ions upon baking, penetration into even deeper areas, the need for less area or floor space in any manufacturing activity, and the simplification, and resulting economic savings, of power supply equipment, coupled with better flexibility of the latter.
Accordingly, the principal object of the present invention is to provide new and novel apparatus and techniques in electrocoating process for applying organic finishes.
Another object of the present invention is to provide a multitank arrangement for effecting electrocoating which results in advantages not at all available through previous usage of a single tank operation.
A further object of the present invention is to provide a new and novel tank structure whereby the tank and the article being coated are maintained at the same electrical polarity.
A still further and more general object of the present invention is to provide, in an electrocoating process, a high-safety factor for the operator, an optimum use of equipment and floor space, and important finishing results not available heretofore.
Other objects and a better understanding of the present in vention will become more apparent from the following description, taken in conjunction with the accompanying drawing, wherein FIG. 1 is a view in elevation, partly fragmentary and partly in vertical section, showing a tank structure in accordance with the teachings of the instant invention; and
FIG. 2 is a plan view of the tank structure of FIG. 1.
For the purposes of promoting an understanding of the principles of the invention, specific language will be used to describe the embodiments illustrated in the drawing and described herein. However, it will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications thereof, and such further applications of the principles of the invention being contemplated as would normally occur to one skilled in the art to which the invention relates.
In accordance with the present invention, the technique of electrocoating is advanced through the provision of a multiple tank bath arrangement, instead of the previously utilized single tank bath. In order to avoid the high-current surge inherent with the use of a single tank, two tanks are utilized, where the electrolyte in the first tank is preferably maintained at an approximate constant current or voltage, requiring simplified power equipment than necessary heretofore with the single tank arrangement, and where the electrolyte in the second tank is preferably maintained at an approximate constant current or voltage, being higher in value than that which could be properly employed in a single tank arrangement,
representing importance to the arrangement by assuring less time for the overall operation and better results than those achievable heretofore.
While, depending upon the electrolyte in use, the electrical energy level in the first tank may be lower than that employed in a one-tank arrangement, such electrical energy level is probably equal to that in the one-tank arrangement to prevent bubbling and any rupturing of the finish being applied. In any event, standard equipment is typically employed to attain the desired electrical energy levels in the first tank and in the second tank.
As mentioned hereabove, the use of the instant multitank arrangement represents a simplification, both costwise, through equipment and space, and operationally, through control, over the prior known single tank procedure. In this latter regard, and by way of example, the operator can readily avoid a higher than desired voltage in the first tank, where the latter is not feasible because of the development of gas bubbles, as well as burned areas, in the organic finish. On the other hand, because of effective control, higher electrical energy can be supplied to the second tank than feasible in a single tank arrangement, whereby optimum coating results through higher power of the paint after initial coating in the first tank.
In a preferred practice of the invention, the electrolyte used is typically a water base soluble resin, the latter being, by way of example, of an acrylic, an epoxy or an alkyd type. Moreover, although commonly termed a paint, actually coating substance might be more appropriate to define the electrolyte used in the electrocoating process.
Although conventional tank structures may be employed in the instant above-described multitank arrangement for electrocoating, the applicants herein have provided a new and novel tank structure, shown in FIGS. 1 and 2, which permits, as mentioned hereabove, an important safety factor to the user. In this regard, and representing an important contribution to the art, the applicants tank structure permits the article being coated and the tank itself to be maintained at the same electrical polarity.
In order to accomplish the preceding, a conventional open tank 12 is provided having upstanding sidewalls and a bottom wall. The inner surface of the tank 12 is typically coated (not shown) with polyurethane, epoxy, polyesters, or the like, to prevent any irreversible depositing of coating material or paint thereon. Spaced inwardly around the sidewalls of the tank 12 are a series of electrodes 14, where nonconductive members 15 serve as mounting means therefor.
In that the presently practiced electrocoating process requires, because of the nature of available paint and in contrast to customary metal plating, the coating of a part which is maintained at a positive polarity, both the article being coated and the tank 12 are at the same electrical polarity, both being grounded, where the electrodes 14 are at an opposite electrical polarity. In other words, any brushing against the outside of the tank 12 does not endanger the operator because of the spaced apart relationship between the walls of the tank and the electrodes 14. In any event, the instant tank teaches an arrangement for maintaining the article being coated and the tank at the same electrical polarity, where any electrode is, of course, at the opposite electrical polarity.
The preceding is important because of the high voltages involved in electrocoating, in contrast, or example, to those employed in usual commonly known metal plating operations. Moreover, it should be evident that the instant tank arrangement does not require any specialized isolating of the article being coated, as the latter is at the same electrical polarity as the conveyor on which it moves during the coating process.
In this regard, the figures further illustrate a conveyor 18 in combination with the new and novel above-described tank arrangement. It might be again stressed that the use of the instant multitank technique affords an important advantage spacewise in a production area, and also permits almost optimum vertical immersing of the article being coated, in contrast to the gradual downwardly and upwardly inclined movement common to a single tank approach.
In order to attain optimum film thickness and coating, the electrodes 14 forming part of the applicants new and novel tank structure preferably have a combined area equal to or greater than the area of the article being coated, and, moreover, instead of utilizing a lined tank, the overall tank may be made from an electrically insulative material, such as polyethylene or fiberglass, for example, being electrically neutral in such instance.
Another feature of the instant insulated tank is its adaptability for utilizing the electrodialysis process for pH level and amine control, where previously a complicated arrangement was involved which entailed three potentials of electrical enerviz, a positive part, a negative electrode and a neutral tank. e control of the amine, and, hence, the pH level in the tank was accomplished through the use of a bag containing amine absorption material and made from a semipermeable membrane. With the invention at hand, an important advantage results in only a screen 16, defined as a semipermeable membrane for passage of the amine, but not the pigment or the like in the paint, need be employed, without any multielectrical energy problem, adjacent each row of negative electrodes for the desired results. In other words, the entire electrodialysis process has been simplified in accordance with the teachings of the instant invention where, at the outset, no specialized electrical equipment is required to carry out the process.
From the preceding, it should be understood that the applicants herein have provided new and novel contributions to the electrocoating art, in teaching the use of a multitank arrangement, and increased safety and utility in the form of a tank structure per se. As discussed, the multitank arrangement affords many advantages, including, by way of resume, the need for less floor space for the electrocoating operation, the shorter time required for coating with much more effective finished results than possible heretofore, a better control of film makeup and color, deeper penetration into recessed areas, a simplification of power supply equipment, coupled with the flexibility thereof, and the elimination of staining, a problem particularly prevalent with pastel colors.
It should be further understood that modifications may be made within the spirit of the invention. For example, the number of bath tanks may be increased over the discussed two, different materials used in each tank, and the electrode area increased for even faster coating. Thus, the preceding description should be considered illustrative and not as limiting the scope of the following claims.
1. A bath tank for electrocoating comprising an upstanding wall and a bottom wall, means for holding an article within said bath tank for coating, and at least one electrode mounted on said upstanding wall of said tank substantially parallel therewith and electrically insulated therefrom, and means for maintaining said article holding means and said tank at the same electrical polarity.
2. The bath tank of claim 1 where said means maintains said article holding means and said tank at an electrically grounded potential.
3. The bath tank of claim 1 where the inner surface thereof is coated with a nonconductive substance.
4. The bath tank of claim 1 where an electrodialysis screen is disposed within said tank in spaced-apart relationship with respect to said at least one electrode.
5. In electrocoating, a first bath tank and a second bath tank each adapted to contain a resinuous water-soluble paint, conveyor means for transferring an article to be coated through each of said tanks and serving as one electrode, electrical energy supply means for said first tank for applying a first coating to said article during a predetermined first time period, and electrical energy supply means for said second tank for applying a second coating to said article during a second predetermined time period of a different duration than said first predetermined time period after coating in said first tank, where said first tank, said second tank and said conveyor means are of the same electrical polarity.
6. The electrocoating arrangement of claim 5 where said second predetermined time period is the same as said first predetermined time period.