US3728242A - Continuous electrodeposition process - Google Patents

Abstract

IN A CONTINUOUS ELECTRODEPOSITION PROCESS WHERE THE ARTICLE ENTERS THE ELECTRODEPOSITION BATH IN A POWER-OFF CONFIGURATION, IT HAS BEEN FOUND THAT SURFACE IRREGULARITIES OCCUR ON AN ARTICLE WHEN THAT ARTICLE IS INTRODUCED POWEROFF INTO THE ELECTRODEPOSITABLE COMPOSITION WHILE THE PRE-

CEDING ARTICLE IS STILL BEING COATED AND IS IN A POWER-ON CONDITION. THIS PROBLEM IS SOLVED BY APPLYING A SMALL VOLTAGE TO THE ARTICLE BEING INTRODUCED UNTIL SUCH TIME AS THE COATING VOLTAGE IS APPLIED.

Description

United States Patent 3,728,242 CONTINUOUS ELECTRODEPOSITION PROCESS Frederick M. Loop, North Olmsted, and Boyd J. Smith, Brecksville, Ohio, assiguors to PPG Industries, Inc., Pittsburgh, Pa.

Filed Apr. 28, 1971, Ser. No. 138,260 Int. Cl. B01k 5/02; C23h 13/00 US. Cl. 204-181 9 Claims ABSTRACT OF THE DISCLOSURE In a continuous electrodeposition process where the article enters the electrodeposition bath in a power-off configuration, it has been found that surface irregularities occur on an article when that article is introduced poweroff into the electrodepositable composition while the preceding article is still being coated and is in a power-on condition. This problem is solved by applying a small voltage to the article being introduced until such time as the coating voltage is applied.

STATE OF THE ART Electrodeposition of organic resinous coatings on a conductive substrate from an aqueous bath comprising an ionically-solubilized synthetic organic coating resin has rapidly achieved commercial success in the United States due too material cost advantages, labor savings, performance and etficiency of coating as well as environmental control advantages.

In the electrodeposition process, articles to be electrocoated are contacted with an aqueous dispersion of a solubilized, ionized film-forming material such as synthetic organic vehicle resins. An electric current is passed between the article to be coated, serving as an electrode, and a counter-electrode to cause deposition of a coating of the vehicle resin on the article. The article is then withdrawn from contact with the electrodepositable composition and usually rinsed and the coating either air-dried or baked in the manner of a conventional finish.

The article may be introduced into the electrodepositable composition either in a power-on or power-off mode. In power-on configurations, the article has an impressed current, the circuit being completed upon contact with the electrodepositable composition and coating commences immediately as the advancing portion of the article is introduced into the electrodepositable composition. Those skilled in the art have found numerous technical and safety cousideratioons which have lead them to use'a power-off introduction of the article into the electrodepositable composition.

In the power-off configuration, the article is first introduced into the electrodepositable composition and then the electrical circuit is completed, power flowing through the article and the article coated.

In an automated production line utilizing power-off introduction, it has been frequently found that the articles are placed on a line in such close proximity that when a first article is in a power-on configuration being coated in the electrodepositable composition, a second immediately-following article in the power-off state is impinging beneath the surface of the electrodeposition bath. It would be through that this article, being insulated and in a non-coating state, would not present any problem ice in approaching its electrodeposition position and impinging upon the electrodeposition bath prior to the comple tion of the coating of the article immediately ahead of it. It has been found, however, that since current is flowing in the bath between the article being coated and the counter-electrode, i.e., the tank containing the electrodepositable composition, the second article being introduced in a non-conductive state functions as a dipole, current flowing through this article to the counter-electrode. In this dipole state, a portion of the article has a more positive charge and a portion of the article has a more negative charge. In those portions of the article where the relatively negative charge is created, it has been discovered that there is a tendency, as with any metal in an anodic state, for the metal to function as an anode and cause dissolution of the metal and or metal pretreatment contained on the metal. This phenomena is then translated into discernible irregularities in the film subsequently electrodeposited on the article. These irregularities are frequently sutficiently severe to affect the appearance of a topcoat even when the material deposited is a primer, such as on automobile bodies. This, again, is especially critical where the article must have a uniform, high-quality appearance, such as an automobile body.

Description of the Invention It has now been found that surface irregularities caused by the introduction of an article into an electro-depositable composition in a power-off state While the electrodepositable composition itself is in a conductive state due to the fact that there is a second preceding article in the bath in a power-on condition being electrocoated can be eliminated or greatly reduced by the application of a small current to the article being entered into the electrodepositable composition in a power-off state. The voltage applied is a voltage below that which will cause substantial electro-deposition but will prevent the occurrence of surface irregularities due to othe article serving as a dipole. Voltages above about 10 volts are sufiicient to show substantial effect, while voltages above about volts are unnecessary. Typically, the voltage applied should be between about 25 and about 50 volts.

For better understanding of the invention, one may refer to the accompanying schematic drawing depicting one embodiment of the apparatus. This schematic drawing is related to an anionic resin composition which coats upon an anode. The invention is equally applicable to a cationic coating which coats upon the cathode, the power reversal being obvious. In the drawing there is shown a steel tank 1 which contains an electrodepositable composition 2 which serves as a cathode in the coating process. Tank 1 is electrically connected to a power supply unit 20 through connection 22. The articles to be coated (for example, automobile bodies) 18 and 19 are shown suspended from a conveyor 9 by hangers 10, 11, 12 and 13. The conveyor 9 can be a conventional electrically-powered, chain-driven conveyor constructed and arranged for the transportation of articles to be coated through the bath. The hangers include insulators 14, 15, 16 and 17, which insulate the article from the grounded conveyor. Busbar 8 may be segmented and contain segments that have no connection to the principal power supply unit 20 and arranged to allow electrical contact through the hangers with the article being coated for predetermined intervals so that the article becomes conductive only when immersed in the electrodepositable composition. For example, portion of the busbar 8 in the schematic drawing between points 6 and 7 may be considered conductive and in electrical contact with the article 18, completing the electrical circuit and allowing for coating of the article 18. Thus, article 18 serves as the positive electrode or anode of an electrodeposition cell as the article is passed through the bath being coated.

Article 19, being entered into the electrodeposition bath is not in electrical contact with the coating power supply source 20.

Article 19 is, however, in contact with the secondary power supply source 3 through busbar 8 between points 6 and 6' operating at low voltage. This portion of busbar 8 is insulated from the portion of busbar 8 defined between the points 6' and 7, which carries the primary coating voltage.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable composition utilized in the practice of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polyacid or polybasic resinous material can be electrocoated and, if film-forming, provides coatings which may be suitable for certain purposes. Any of such electrodepositable compositions is included among those which may be employed in the present invention, even though the coating obtained may not be entirely satisfactory for certain specialized uses. Electrodepositable compositions, while referred to as solubilized, in fact are considered a complex solution, dispersion or suspension or combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution. numerous such resins are described in U.S. Pats. Nos. 3,230,162, 3,441,489, 3,422,044, 3,403,088, 3,369,983, 3,366,563, 3,382,165 and British Pat. 1,132,267, as well as other patents to be found in Class 204, sub-class 181 of the U.S. Patent Ofiice. Since these materials are wellknown, art-recognized class or materials, it is deemed unnecessary to set forth a description of those resins in detail. The resin and electrodepositable composition descriptions of the above-mentioned patents are hereby incorporated by reference. For a general review of electrodeposition paint formulation, reference may be had to R. L. Yeates, Electropainting, Robert Draper, Ltd., Teddington, England (1966).

Presently the most widely used electrodeposition vehicle resins are synthetic polycarboxylic acid resinous materials; however, polyacids other than polycarboxylic acids are known in the art as electrodepositable resins. Likewise, polybasic resins may be employed.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline metals such as amine or alkaline metal hydroxides and when subjected to electric current they migrate to the anode. Polybasic resins solubilized by acids are cationic in nature and when these resins are water-dispersed or solubilized with an acid, the material is deposited on the cathode under an electric current. Although most electrodepositable compositions are a complex mixture, most commercially-utilized electrodepositable compositions are a complex mixture of either the anionic or cationic resins described above formulated with adjuvants such as pigments, solvents and surfactants, crosslinking resins and the like.

The invention is further described in conjunction with the following example, which is to be considered illustrative rather than limiting. All parts and percentages in the example and throughout the specification are by weight unless otherwise stated.

4 EXAMPLE The vehicle resin in this example is a maleinized tall oil fatty acid-"adipic acid ester of a styrene-allyl alcohol of 1100 molecular weight and 5 hydroxyl functionality (Schell X-450) comprising 39.7 percent X450, 52.9 percent tall oil fatty acids, 1.3 percent adipic acid and 6.1 percent maleic anhydride as a percent solids solution in 4-methoxy-4-methylpentanone-Z, having a viscosity of 36,700 centipoises and an acid Value of 38.2. The electrodeposition primer had the composition:

Percent Lead silicate Manganese dioxide Norn.-0rganic solvent-=4 methoxy+methylpentanone-2 in 20/80 ratio to vehicle resin (above). Amine=ll4 diethyl/ti'iethylamine.

The composition was diluted to 11 percent solids with deionized water.

Automobile bodies were continuously electrocoated in an electrodeposition tank of the general nature of the drawing but without the secondary power source. Each automobile body was immersed power-off and then processed through the tank, being coated at between 350 and 425 volts for two minutes. The bodies were in such proximity that a following body was entering into the electrodepositable composition at the rear of the electrodeposition tank while the preceding body was being coated. Surface defects were noticable in the finished coating on the forward portion of bodies thus coated.

When a low voltage, i.e., 30 volts, 120 amps, was applied to the automobile bodies as they entered the electrodeposition bath in the same manner as above, the surface irregularities previously noted were eliminated.

Other electrodepositable compositions, materials and conditions such as those herein described or within the skill of the art may be substituted for those exemplified.

According to the provisions of the Patent Statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. In a continuous electrodeposition process wherein a first metal article is being electrocoated at a first coating voltage, sufficient to cause electrodeposition of a coating, while serving as an electrode in a first circuit comprising said first article and a tank containing an electrodepositable composition comprising an ionically solubilized synthetic vehicle resin, said tank serving as a counter-electrode in said circuit, while a second metal article is entering said electrodepositable composition, not subjected to said coating voltage in said first circuit, the improvement comprising applying to said second metal aiticle in a second circuit comprising said second article and said tank containing said resin, a second voltage suificient to prevent substantial opposed polarity to develop in respective portions of said second article, but insufficient to cause substantial deposition of coating until said second article is subjected to a coating voltage in said first circuit.

2. A process as in claim 1 wherein the voltage applied to said second metal article in said second circuit is between. about 10 volts and about volts.

3. A process as in claim 2 wherein the voltage applied in said second circuit is between about 25 and about 100 volts.

4. A process as in claim 3 wherein the voltage applied in said second circuit is between about 25 volts and about 5 50 volts.

5. A process as in claim 1 wherein the vehicle resin is a base-solubilized synthetic polycarboxylic acid resin.

6. A process as in claim 5 wherein the voltage applied in said second metal article in said second circuit is between about 10 volts and about 100 volts.

7. A process as in claim 7 wherein the voltage applied in said second circuit is between about 25 and about 100 volts.

8. A process as in claim 7 wherein the voltage applied in said second circuit is between about 25 volts and about 50 volts.

9. A process as in claim 1 wherein the article is an automobile body.

6 References Cited UNITED STATES PATENTS 3,305,467 2/1967 Igras et al 204-181 3,471,389 10/1969 Swanson 204181 3,492,213 1/1970 Johnson 204-181 3,496,082 2/1970 Orem et a1. 204-181 OTHER REFERENCES Burnside et 211., Ford Electrocoating Process, Journal of Paint Technology, vol. 38, No. 493, February 1966, p. 96.

JOHN H. MACK, Primary Examiner 15 A. C. PRESCOTT, Assistant Examiner US. Cl. X.R.. 204300

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