US3585120A

US3585120A - Novel method of electrocoating hollow bodies

Info

Publication number: US3585120A
Application number: US757565A
Authority: US
Inventors: Felix Wehrmann; Franz Aigner
Original assignee: Stollack AG
Current assignee: Stollack AG
Priority date: 1967-09-08
Filing date: 1968-09-05
Publication date: 1971-06-15
Anticipated expiration: 1988-06-15
Also published as: DE1771953B2; DE1771953A1; CS152459B2; DE1771953C3; FR1582692A; AT294278B; GB1235127A

Abstract

A METHOD OF ELECTROCOATING THE INTERIOR OF HOLLOW OBJECTS REQUIRING A COATING OF A WATER-SOLUBLE OR WATER-DISPERSIBLE COATING AGENT WHICH OBJECTS HAVE AT LEAST AN ELECTRICALLY CONDUCTIVE SURFACE WHEREIN THE IMPROVEMENT RESIDES IN THE USE OF AT LEAST ONE BIPOLAR AUXILIARY ELECTRODE WHICH EXTENDS INTO THE INTERIOR OF THE HOLLOW OBJECT.

Description

June 15, 1971 F. WEHRMANN ETAL NOVEL METHOD OF ELECTROCOATING HOLLOW BODIES Filed Sept. 5, 1968 2 Sheets-Sheet 1 F I G 5 :3 F E G .4

la 4 5o 50 i; 1 I o o 80 INVENTORS FELIX WEHRMANN FRANZ AIGNER ELWJA. Azw/ ATTORNEYS Jllflfi F WEHR EIAL OVEL METHOD OF ELEGTROCOATING HOLLOW BODIES Filed Sept. 5, 1968 2 Sheets-sheet 2,

INVENTORS FELIX WEHRMANN FRANZ AIGNER ATTORNEYS United States Patent US. Cl. 204-181 12 Claims ABSTRACT OF THE DISCLOSURE A method of electrocoating the interior of hollow objects requiring a coating of a water-soluble or water-dispersible coating agent which objects have at least an electrically conductive surface wherein the improvement resides in the use of at least one bipolar auxiliary electrode which extends into the interior of the hollow object.

PRIOR ART Known electrocoating methods face great difficulties when attempting to provide the interior of hollow bodies with a coating. For the following discussions, it will be assumed that the coating agent is to be deposited on the anode which in this case will be the object to be coated and the counter-electrode is the electrode. The difiiculty in coating the interior of the hollow bodies resides in the fact that a minimum current density is required to form the coating. Since the electrolytic resistance between the cathode and the outer surface of the object to be coated is substantially lower than between the cathode and the inner surface of the object to be coated, the current densities appearing on the surfaces also vary considerably. The entire potential drop is practically between the cathode and the outer surface of the object to be coated. Only with increasing outside coating is the potential drop displaced partly into the interior of the object to be coated, so that only a partial and irregular coating of the inner surfaces is achieved. One attempt by the prior art to solve this problem required the use of an auxiliary cathode in the interior of the hollow body to be coated. However, this solution is expensive and has considerable technical disadvantages.

OBJECTS OF THE INVENTION It is an object of the invention to provide an economical and simple method of coating the interior of hollow bodies.

It is another object of the invention to provide a novel electrocoating apparatus wherein the interior of hollow bodies are coated with the aid of a bipolar electrode.

These and other objects and advantages of the invention will become obvious from the following detailed description.

THE INVENTION The method of the invention for electrocoating conductive surfaces of a hollow body with a coating material comprises immersing a hollow object having at least an electrically conductive surface which acts as an electrode in an electrically conductive coating bath containing an organic coating agent to be deposited on the hollow object and provided with a counter-electrode, passing an electric current thorugh the said coating bath and removing the coated hollow object from the coating bath, the coating of the interior of the hollow object being effected by at least one bipolar auxiliary electrode with one pole inside the hollow object and one pole outside the hollow 3,585,120 Patented June 15, 1971 'ice object. Bipolar electrodes are electrodes which act on one end or side as anodes and on the other end or side as cathodes and such electrodes are known such as in electrometallurgy for refining copper (Milazzo, Textbook of Electrochemistry) The method of the invention has great technical advantages compared to the conventional methods for coating hollow bodies. Since the bipolar auxiliary electrodes are not in metallic conductive contact with the counterelectrodes of the hollow body to be located, there is no danger of a short-circuit in case the bipolar auxiliary electrode comes in contact with the hollow body to be coated. Destruction of the hollow body, damage to the rectifier, impulse generator or any other current source are thus impossible. Since the bipolar auxiliary electrodes are smaller than conventional auxiliary electrodes connected in the direct circuit and are made of a less expensive material than the latter, considerable savings are possible. Moreover, the complicated assembly of the auxiliary electrodes immediately before the coating is eliminated. These bipolar auxiliary electrodes can rather be mounted at the most favorable time of the manufacturing process of the hollow body to be coated. For example, it is possible, to mount the auxiliary electrodes in the wall of a car body before the wall is assembled in the car body. With the method according to the invention it is also possible to coat cavities, for example, of car bodies, which could only be unsatisfactorily coated heretofore or not at all.

The following materials may be used for the production of water-soluble or water-dispersible coating agents in the electrically conductive baths for the method of the invention: organic and inorganic pigments, cutting agents, such as rutile, anatase, lithopone, blanc fix, kaolin, heavy spar, zinc sulfide, carbon black, iron oxide dye, zinc chromate, lead cyanamide, heliogen blue, as well as soluble dyes, such as varnish dye, in a ratio of pigment or dye to solid resin of 0 to 5.0:1.

The following materials may also be organic film forming agents: water-soluble or water-dispersible synthetic resins and natural resins as well as combinations thereof, water-soluble or water-dispersible vegetable or animal oils and fats and their transformation products, such as linseed oil and sardine oil, dehydrogenated castor oil, watersoluble or water-dispersible natural resins and their transformation products, such as shellac, resins, soaps and resin esters; water-soluble or water-dispersible synthetic resins modified with natural resins, water-soluble or waterdispersible maleinate resin; water-soluble or water-dispersible saturated and unsaturated polyesters; water-soluble or water-dispersible oil-free and styrenized alkyd resins, water soluble or water dispersible alkyd resins with synthetic fatty acids; water-soluble or water-dispersible alkyd resins with vegetable and animal fatty acids; watersoluble or water-dispersible acrylic alkyd resins; watersoluble or water dispersible silicone akyd resins; watersoluble or water dispersible phenol resins and novolaks; water-soluble or water-dispersible butanolized resol resins modified with fatty acids or by incorporation of polyesters; water-soluble or water-dispersible resols and coldhardening phenol resins; water-soluble or Water dispersible alkyl pheno resins; water-soluble or water-dispersible terpene-phenol resins; water-soluble or water-dispersible plasticized or unplasticized urea resins; water-soluble or water-dispersible melamine resins; water-soluble or water dispersible polyvinyl acetals; water-soluble or water-dispersible polyvinyl chlorides and polyvinyl chloride mixed polymerizates; water-soluble or water-dispersible polyvinyl acetates and other polyvinyl esters; water-soluble or water- 'dispersible polyvinyl ethers; Water-soluble or waterdispersible polyvinyl carbazoles; water-soluble or waterdispersible polyacrylic resins and resins of polyacrylic acid derivatives, such as water-soluble or water-dispersible polymethacrylic acids; water-soluble or water dispersible polystyrenes; water-soluble or water-dispersible polyethylcnes; water-soluble or water-dispersible polyisobutylenes; watersoluble or water-dispersible cumerone and indene resins; water-soluble or water-dispersible ketone and aldehyde resins; water-soluble or water dispersible aromatic derivatives of formaldehyde resins; water-soluble or waterdispersible anilin resin; water-soluble or water-dispersible carbamic acid resins; water-soluble or water-dispersible sulfonamide resins; water-soluble or water-dispersible chlorodiphenyl resins; water-soluble or water-dispersible polyamide resins, water-soluble or Water-dispersible polyaddition resins such as polyurethane; water-soluble or water-dispersible epoxide resins and their transformation products; water-soluble or water-dispersible nitrocellulose resins as well as other water-soluble or water-dispersible cellulose resins; water-soluble or water-dispersible natural and synthetic rubber resins and their dirivatives such as butadiene derivatives; water-soluble or water-dispersible silicone resins; water-soluble or water-dispersible resins of halogenated polyethylene; and water-soluble or waterdispersible polycarbonates.

Instead of all of the above indicated natural or synthetic resins, their water-soluble or water-dispersible soaps or other derivatives may also be used as binders.

The electric current used in the method of the invention may be direct current and/or alternating current and/or pulsating current of any form. The use of alternating current and pulsating currents is fully described in my copending US. patent applications Ser. No. 628,390 filed Apr. 4, 1967 and Ser. No. 635,977 filed May 2, 1967, the disclosures of which are hereby incorporated by reference thereto.

The bipolar electrodes may take any form. One embodiment is a copper wire with an iron mesh screen at one end as illustrated in FIG. 5. A particularly useful bipolar anode is a plastic pipe filled with steel wool and provided with openings along its length. The length of the flexible plastic pipe may be easily adjusted to conform to the form and size of the cavity to be coated. One part of the plastic pipe will protrude into the hollow body and the other end will protrude outside the cavity and the steel wool in the outer part will be coated during the coating process. In a subsequent coating process, the same electrode can be arranged so that the coated portion is in the cavity to be coated and uncoated portion is outside the cavity. During this coating process, the uncoated portion becomes coated and the coated portion is freed from the coating agent. This has the exceptional advantage of being able to use the bipolar electrode almost an infinite number of times.

The thickness of the coating on the interior of the hollow body can be regulated by varying the ratio of the surface area of the interior of the hollow body to the surface area of the coated portion of the bipolar auxiliary electrode.

Referring now to the drawings-- FIG. 1 illustrates a known apparatus for electrocoating with direct current and FIG. 2 is an equivalent circuit diagram of the apparatus of FIG. 1.

FIG. 3 illustrates a known apparatus for electrocoating interiors of hollow bodies with direct current and FIG. 4 is the equivalent current diagram of the apparatus of FIG. 3.

FIG. 5 illustrates one embodiment of the apparatus of the invention to coat the interior and exterior of a hollow object using direct current and FIG. 6 is the equivalent current diagram of the apparatus of FIG. 5.

FIG. 7 is another embodiment of the apparatus of the invention for coating hollow objects with direct current.

FIG. 8 is an embodiment of the apparatus of the invention for coating a hollow body with alternating current. FIG. 9 illustrates an embodiment of the invention in which two car bodies are coated simultaneously using a pulsating current.

FIG. 10 illustrates an embodiment of the invention in which a car body is coated using direct current.

In the prior art embodiment of FIGS. 1 and 2, the coating vessel 1 acts as a cathode with respect to the electrically conductive object 2 to be coated. The current supply for object 2 is derived from current-collecting rail 3. The object 2 is a can which has only an opening 4 in the bottom and 5 in the top surface. After this can 2 has been introduced into the coating bath, the current is turned on and the coating of the surface beings. FIG. 2 shows the equivalent circuit diagram of this arrangement wherein 1a denotes the cathode, 2a the contact point of the coating bath with the outside of the object to be coates, 3a the connecting point of the current source with the object to be coated, 4a the resistance of the coating bath outside the object to be coated, 5a the resistance of the film deposited on the outer surface of the object, 6a the resistance of the liquid column in the opening of the object (corresponding to opening 4 or 5 in FIG. 1); 7a the resistance of the coating bath inside the object, 8a the resistance of the film deposited on the inner surface of the object. It can be readily seen that with a very low external resistance almost all the entire current flows between the cathode and the outer surface of the object, since the points 2a and 3a have practically the same potential.

The resistance of the liquid column 6a is substantially higher due to the small cross section of the openings of the object than the resistance of the coating bath outside 4a and inside 7a of the object, and thus determines the current flowing on the inside. With increasing outside coating, the resistance 5a rises and with it, also according to Kirchhoff, the current flowing over the internal resistance, which is equal to the sum of the

resistances

6a, 7a and 8a. Between the points 2a and 3a is thus formed a constantly increasing potential difference. The higher the film resistance 5a, the better is the coating on the inside. Since the film resistance cannot be increased indefinitely, however, the current density inside the hollow body to be coated is limited to such low values that no sufiicient inside coating is possible.

FIGS. 3 and 4 illustrate the prior are method of overcoming the deficiencies of the method of FIG. 1 by introducing an auxiliary cathode into the interior of the hollow body to be coaaed. In FIG. 3, the hollow body 2 to be coated is connected by the current collecting rail 3 to the positive pole of the current source and the bath vessel 1 and auxiliary electrode 6 are connected the negative pole of the current source. FIG. 4 illustrates the equivalent circuit diagram for FIG. 3. FIG. 4 shows the equivalent circuit diagram of this arrangement, where 1a denotes the cathode, 9a the auxiliary cathode, 3a the connecting point of the current source with the object to be coated, 4a the resistance of the coating bath outside the object to be coated, 5a the resistance of the film deposited on the outside of the object, 7a the resistance of the coating bath inside the object and 8a the resistance of the film deposited on the inner surface of the object. The inside coating is elfected here independent of the outside coating. Points 1a and 9a have permanently the same cathode potential.

This apparently very elegant solution has not only considerable technical disadvantages, but is also rather expensive. Since the full operating voltage is constantly applied to these auxiliary electrodes, they must be so designed that short-circuits are positively avoided. In the case of a short-circuit, not only is the rectifier jeopardized but the auxiliary electrode and partly also the object to be coated are destroyed. The assembly, which must be carried out very carefully, requires additional personnel and causes additional cists, particularly for objects whose cavities are small and difiicult of access.

The present invention clearly avoids the disadvantages of the prior art and is more fully illustrated in FIGS.

to 10. In FIG. 5, a can 2 to be coated is immersed into coating vessel 1. The can 2is provided with anopening 4 in the top surface and an opening 5 in the bottom surfaceand is electrically connected by lead wire 3 to the positive pole of-adirect current source andthe vessel 1 is connected to the negative pole thereof. An auxiliary bipolar electrode consisting of copper wire 7 which passes through the side wall of can 2 at point 8 whichis insulated and outside the can the copper wire 6 is attached to an iron wire mesh 9. This auxiliary electrode is not'in galvanic contact with the current source. The outer part of the bipolar auxiliary electrode 9 is arranged in the proximity of the cathode and acts therefore as 'an anode, while the part 7 in the interior of the can 2 acts as a cathode with respect to the latter. The entire surface of the can 2 as well as the outer part 9 of the bipolar auxiliary electrode are coated.

After the coating, the coated can 2 is lifted out of the bath, and rinsed with water. The auxiliary electrode is removed from the can without damaging the film layer. The quality 'of the stoved lacquer coat inside the can 2 is of the same quality as the layer on the outer wall of the can. The coated wire mesh 9 is discarded.

FIG. 6 shows the equivalent circuit diagram of this arrangement where the points 1a to 8a have the same meaning as the points 1a-8a in FIG. 2. 10a denotes the resistance of the bipolar auxiliary electrode, which is substantially lower than that of the liquid column 6a. As it can be seen, there is always a relatively high cathode potential at point 11a, in contrast to the arrangement according to FIGS. 2 and 4, which attains with increasing outside coating practically the entire applied direct current voltage, without the bipolar auxiliary electrode being in any way in metallic contact with the cathode.

In the embodiment of FIG. 7, a can 2v to be coated is immersed in a coating vessel 1 provided with an electrode 8' connected to the negative pole of a direct current source. Can 2 by lead wire 3 is connected to the positive pole of the said current source and is provided with an opening 4 in the top surface and an opening 5 in the bottom surface. An auxiliary bipolar electrode 5b made of copper passes through the walls of can 2 at points 10 and 11 which are insulated. The inside and outside of the can are both then coated.

FIG. 8 illustrates the invention when alternating current is used for the coating process. The electrocoating bath vessel 1 is made of aluminum with an interior coating of aluminum oxide and is connected to the alternating current source. Vessel 1 in combination with the coating bath acts as a rectifier. The hollow body or can 2 to be coated is connected to the other pole of the alternating current source by lead wire 3 and is provided with opening 4 in the top surface and opening 5 in the bottom surface. The said object 2 acts as an anode. The auxiliary electrodes are comprised of a plate-shaped outer part 12 made of noncoatable material which is connected to inner part 13 which passes through the wall of object 2 at point 14 which is insulated. The inner part 13 of the auxiliary electrode acts as a cathode with respect to object 2 and effects the complete coating of the interior.

In the embodiment of FIG. 9, two

car bodies

12 and 13 are to be coated and are suspended one behind the other from a conveyor rail 14 in coating vessel 1. The coating vessel 1 acts as a cathode and the car bodies are always in the bath at the same time and move through the bath in the direction of the arrows. Car body 13 is galvanically connected by current collecting rail 15 to the current source and car body 12 is not in the direct circuit. The current source is an impulse generator which generates current in the form of rectangular impulses with a maximum voltage of 400 v., an impulse duration of 30 milliseconds and an impulse interval of 30 milliseconds.

Car bodies

12 and 13 are provided with auxiliary electrodes 16 and 17, respectively, of copper which is conducted insulated through the wall of the car body at point 18. Between point 18 and roller mechanism 19, the auxiliary electrode is coated with an insulating mate rial. The bipolar auxiliary electrode 17 is in galvanic contact with car body 12 by means of rail 20. In this position, the body 13 acts as an anode with respect to vessel 1. Body 12 represents the large-surfaced part of the bipolar auxiliary electrode 17. This part of the bipolar auxiliary electrode acts therefore as an anode with respect to vessel 1, and the coating of the outer surface of body 12 which is not in the direct current flow, starts.

The part of the auxiliary electrode 17 inside body 13 acts as a cathode with respect to body 13, so that the uniform coating of the interior is ensured. As soon as the coating of body 13 is completed, body 12 enters the direct circuit in its place. Auxiliary electrode 16 is in galvanic contact with the following body. Since body 12 is already somewhat coated, the current surge is not great when the current is turned on for this body 12. In previously known methods, however, the uncoated body had to be introduced under reduced voltage in order to avoid an excessive current surge at the start of the coating operation.

FIG. 10 illustrates a method of coating car bodies with direct current using bipolar auxiliary electrodes and an auxiliary cathode called a frame cathode. The coating is effected by means of direct current. Body 12 is introduced on the live rail 14 into the coating bath and acts as an anode. Vessel 1 acts as a cathode. Frame cathode 21 is inserted in body 12, insulated against the latter. The frame cathode is connected to the same pole of the DC source as the vessel and acts therefore as a cathode. Furthermore, two bipolar

auxiliary electrodes

22, 23 are provided. The bipolar auxiliary electrode 22 serves to coat a small cavity separated from the large cavity of the body by partition 24 and it is conducted insulated through partition 24. The large-surfaced part of the bipolar auxiliary electrode 25 is arranged close to the frame electrode and acts as an anode with respect to the latter, the other part 26 protrudes into the small cavity of the body and acts as a cathode with respect to the latter. The bipolar auxiliary electrode 23 protrudes partly into the coating bath outside the body in the proximity of the main cathode, partly into the interior of the body. This arrangement of the electrodes and auxiliary electrodes effects a uniform coating of the entire surface of the car body.

Various modifications of the method and apparatus of the invention may be made without departing from the spirit or scope thereof. For instance, the method is also suitable for water-soluble or water-dispersible coating agents which are deposited on the cathode with the corresponding reversal of the polarity of the current sources.

We claim:

1. A method for electrocoating conductive surfaces of a hollow body with a coating material which comprises immersing a hollow object having at least an electrically conductive surface which acts as an electrode in an electrically conductive coating bath containing an organic coating agent to be deposited on the hollow object and provided with a counter-electrode, passing an electric current through the said coating bath and removing the coated hollow object from the coating bath, the coating of the interior of the hollow object being effected by at least one bipolar auxiliary electrode with one pole inside the hollow object and one pole outside the hollow object, a pole of which is coated during the process.

2. The method of claim 1 wherein at least one additional electrode is also in the cavity of the hollow body and has the same potential as the counter-electrode.

3. The method of claim 1 wherein the portion of the bi-polar electrode which is coated during the process has a large surface area.

4. The method of claim 1 wherein the hollow body acts as an anode and counter-electrode is the cathode.

5. The method of claim 1 wherein at least a portion of the bipolar electrode is made of a non-coatable material.

6. The method of claim 1 wherein interior coating thickness is regulated by the ratio of the surface area of the cavity to the surface area of the coated part of the bipolar electrode.

7. The method of claim 1 wherein the bipolar electrode is in galvanic contact with the hollow body to be coated when dipped into the bath but not in galvanic contact with a current source.

8. An apparatus for the electrocoating of a hollow object having an electrically conductive surface at least with a coating agent comprising a hollow body adapted to act as an electrode and whose surface at least is electrically conductive, an electrically conductive coating bath adapted to an organic coating agent which is to be deposited on the hollow body, a counter-electrode adapted to be immersed in the said coating bath, an electrical source connected to the hollow body and the counter electrode and at least one bipolar auxiliary electrode adapted to extend partially into the interior of the hollow body and partially outside the hollow body and not in electrical contact with the hollow body.

i 9; The "apparatusxof claim 8 wherein the"por tion of the bipolar electrodes adapted to-extend outside the hollow body is many times greater in areatha'n the portion of the bipolar electrode adapted to extend into the hollow body. 10. The apparatus of-claim '8 wherein-the bipolar electrode is made of copper.

. 11.. The apparatus of claim 8wherein the bipolar elecw:

References Cited V 1 UN TED STATES PATENTS; 3,418,233

12/1968 Igvas et 51." 204 30d 3,4s3,097 12/1969 Bush et a1. 204 1s1x 3,476,667

11/1969 Gilchrist 20418l HOWARD. S. WILLIAMS, Primary Examiner