US3399128A - Electrodeposition process and apparatus having a movable conduit electrode - Google Patents

Description

g- 27, 1968 G. E. F. BREWER ETAL ,1 8

ELECTRODEPOSITION PROCESS AND APPARATUS HAVING A MOVABLE CONDUIT ELECTRODE Filed Nov. 2, 1964 3 Sheets-Sheet 1 M u m Q 3 amwzfi amen [k G/ZBf/W'LBUAMf/Df INVENTORY BY A M @Zg 6T ATTORNEYS Aug. 27, 1968 G. E. F. BREWER ETAL 3, ,1

ELECTRODEPOSITION PROCESS AND APPARATUS HAVING A MOVABLE CONDUIT ELECTRODE 3 Sheets-Sheet 2 Filed Nov. 2, 1964 g. 27, 1968 G. E. F. BREWER ETAL 3,

ELECTRODEPOSITION PROCESS AND APPARATUS HAVING A MOVABLE CONDUIT ELECTRODE Filed Nov. 2, 1964 3 Sheets-Sheet I5 I 11 IN. WWW SYLY L 1 ..l||l. ..o.- o 0 @E .l

fi jlj d 3b |l.|l .l|ll|l. I m$N lllxlllllrllllll Mllllll x MMN \MN INVENTORY 4 Wu ATTORNEYS United States Patent 3,399,128 ELECTRODEPOSITION PROCESS AND AP- PARATUS HAVING A MOVABLE CON- DUIT ELECTRODE George E. F. Brewer, Novi, and Gilbert L. Burnside, Oak

Park, Mich., assignors to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Nov. 2, 1964, Ser. No. 408,016 8 Claims. (Cl. 204181) ABSTRACT OF THE DISCLOSURE A method for electrodepositing film-forming, organic, coating material upon electrically-conductive objects which comprises immersing said objects in a coating bath in contact with a first electrode and comprising an aqueous dispersion of film-forming, organic, coating material, passing said objects through said bath and removing said objects from said bath, each of said objects while passing through said bath serving as a second electrode, and providing a direct current of electrical energy through said bath and between said first electrode and said second electrode, said method being further characterized by moving a conduit electrode through said bath with said second electrode and spaced apart therefrom, providing a direct current of electrical energy through said bath and between said conduit electrode and said second electrode, and forcing a stream of liquid coating material through said conduit electrode, into said bath, and against said second electrode; and means for carrying out said method.

This invention relates to the coating of an electrically conductive object by electrically induced deposition of an organic coating material from a liquid bath. In particular, this invention is concerned with continuous electrocoating processes wherein metallic objects are passed through and coated within an aqueous bath by an organic coating material dispersed therein. More particularly, this invention is concerned with an improved method and means for coating cavitous and/or compartmentalized objects in a continuous process.

In continuous electrocoating each workpiece becomes a part of an electrical circuit and is passed through a coating bath that is in electrical communication with at least one other electrode of the circuit which is at a different electrical potential with respect to the workpiece. For purposes of simplicity this invention will be described with reference to an embodiment of electrocoating wherein the workpiece is electrically connected to the power source in a manner such that in passing through the coating bath it is positively charged and serves as the anode of the deposition cell. In this embodiment the chemical composition of the material to be deposited is anodically attracted, e.g., comprises an organic compound having one or more water dissociable carboxylic acid groups within its molecular structure. It is to be understood however that the method and apparatus of this invention may be utilized where the polarities of the electrodes are reversed and a cathodically attracted coating maaterial is employed.

In each embodiment chemical composition of the dis persed coating material is such that it is attracted to the workpiece and deposited thereon when the unidirectional electric current employed for deposition is passed between the workpiece and one or more opposing electrodes. Commonly, the coating bath retainer serves as the primary opposing or driving electrode. Under the conditions maintained in the bath the coating established is essentially electrically irreversible.

In attempting to provide complex metal structures with an evenly distributed coating, difficulties are encountered in coating areas which are shielded from the driving electrode or electrodes by other portions of the workpiece. Thus, for example, in electrocoating assembled or semiassembled automobile bodies the longitudinally extending compartments formed by rocker panels and floor members have proven to be among the most difficult areas to coat. Auxiliary electrodes in the form of rods or bars have been inserted into shielded areas to aid in the coating process.

It has now been discovered that the coating of shielded surfaces can be markedly improved by introducing coat ing material to such surfaces through a conduit-comprising electrode via an orifice therein which is submersed in the coating bath. The term shielded surface as employed herein refers to a surface upon a workpiece submersed within a liquid coating bath to which liquid communication is restricted and/or to which the most direct conductance path from the nearest electrode of opposite p0- larity is intercepted by a solid body.

It is one object of this invention to provide an improved method for electrocoating compartmentalized metal objects.

It is another object of this invention to provide in an electrocoating process localized control over the composition of the coating bath in combination with localized control over electrical inducement to coating.

It is another object of this invention to provide novel apparatus for use in continuous electrocoating of compartmentalized objects.

With the foregoing and other objects in view as will hereinafter become apparent, this invention comprises the methods, combinations, construction and arrangement of parts hereinafter described and/or illustrated in the accompanying drawings, wherein:

FIGURE 1 is a schematic side view of apparatus suitable for use in a continuous electrocoating operation;

FIGURE 2 is a schematic side view of apparatus for carrying out one embodiment of this invention;

FIGURE 3 is an end view of one embodiment of a hollow electrode suitable for use with this invention;

FIGURE 4 is a view taken along line 4-4 of FIG- URE 3;

FIGURE 5 is an end view of another embodiment of a hollow electrode suitable for use with this invention; and

FIGURE 6 is a view taken along line 66 of FIG- URE 5.

Referring now to FIGURE 1, an electrically conductive coating tank 11 is in electrical communication with a negative terminal of power supply unit 15 via conductor 13 and serves as the principal cathode of the coating cell. Coating tank 11 is also in electrical communication with ground. A positive terminal of power supply unit 15 is in electrical communication with a bus bar 19 via conductor 17. A metal part 21 is shown approaching the coating tank 11 supported from conveyor rail 23 by a hanger 25. Conveyor rail 23 is representative of a conventional electrically powered conveyor system wherein an endless chain, not shown, is moved along rail 23 to impel the workpieces through the coating bath.

The lower portion of hanger 25 is electrically insulated from the grounded conveyor by an insulator 27. This portion of hanger 25 carries a contact brush or plate 29 by which hanger 25 and workpiece 21 are maintained in electrical connection with bus bar 19.

Power supply unit 15 is constructed and arranged to provide between the aforementioned electrodes and through the coating bath a direct current flow of electrical energy that is commensurate with the size of the electrocoating operation contemplated. Ordinarily, such current is provided by rectification of an alternating current power source or by a direct current generator. Potentials in the range of about to about 500, usually to 300 volts is suitable with most coating compositions.

In FIGURE 2 there is shown a sectional view of a coating tank 111 which contains a coating bath 112 comprising an aqueous dispersion of a carboxylic acid resin.

Organic coating materials which may be employed in coating zone 112 include, but not by way of limitation, alkyd resins, acrylate resins, epoxy resins, phenol-formaldehyde resins, hydrocarbon resins, and other organlc resins or mixtures of one or more of the foregoing resins with each other or other film-forming organic materials including binding agents and extenders conventionally employed with paints. Such materials may include or be employed with other organic monomers and/ or polymers including, but not by way of limitation, hydrocarbons and oxygen substituted hydrocarbons such as ethylene glycol, propylene glycol, glycerol, monohydric alcohols, carboxylic acids, ethers, aldehydes and ketones. The film-forming material may include or be employed with pigments, metallic particles, dyes, drying oils, etc., and may be dispersed as a colloid, emulsion or emulsoid. Coating materials adapted for anodic deposition may include one or more of the aforementioned resins having free carboxyl groups or their equivalent in their polymeric structure. Dispersion of these resins in water can be effected by the addition of a suitably basic material such as ammonia, water soluble amines, mixtures of monomeric and polymeric amines, etc. Coating materials adapted for cathodic deposition may include one or more of the aforementioned resins having amine or substituted amine groups, e.g., quaternary ammonium groups, in their resin structure. Dispersion of the latter resins can be effected by the addition of suitably acidic materials such as water soluble carboxylic acids, e.g., formic acid, acetic acid, propionic acid, and suitably modified or buffered forms of certain inorganic acids, e.g., phosphoric.

Tank 111 is in electrical connection with a negative terminal of a direct current power source, not shown, via conductor 113. A positive terminal of such power source is electrically connected via conductor 117 to bus bar 119 of which only a fragment is shown in this view. An automobile body 121 is carried by a sled 122 which in turn is suspended from an overhead conveyor 123, also shown in fragment view, by a bifurcated support member 124 and cables 126 and 128. Body 121 is in electrical connection with bus bar 119 via conductor and brush 129. In this embodiment sled 122 is of wood. Support member 124 provides support for a conduit-support arm 130 and associated brace member 132. Secured to and supported by arm 130 is a tubular member 133. Tubular member 133 is constructed of electrically conductive material, e.g., copper, and is in electrical connection with a second bus bar 139 via conductor 135 and brush 137. Bus bar 139 in turn is in electrical connection with a negative terminal of the aforementioned power source via conductor 141. Conduit 133 is in fluid and electrical communication with elbow conduit 143 which is in turn in fluid and electrical communication with a nozzle electrode 145. Nozzle electrode constructed of suitably conductive metal, e.g., stainless steel, is positioned within compartment 147, one of the rocker panel cavities of body 121, and is provided with a plurality of orifices distributed about its perimeter and along a substantial portion of its length. Elbow conduit 143 is further supported by and secured to a bracket 144 which in turn is fixedly mounted on sled 122.

In this embodiment a supply of coating material is fed into compartment 147 via electrically charged conduits 133, 143 and 145. The nozzle electrode 145 is of different polarity from that of the body 121, the difference in potential existing between the same being sufficient to cause electrically induced deposition of coatin material upon the walls of compartment 147. This difference of potential may be the same or different from that existing between tank 111 and the workpiece. The coating material introduced into compartment 147 via nozzle electrode 145 may be in the form of an aqueous dispersion identical to that of bath 112 or the concentration and composition of its components may be different.

In one embodiment of the apparatus illustrated in FIG- URE 2, coating material is introduced intorigid conduit 133 at a predetermined rate via flexible tubing 131 which in turn is connected to a reservoir means, not shown. This material is transferred from such reservoir means through tubing 131, conduits 133, 143 and 145 and into compartment 147 through a plurality of orifices in nozzle electrode 145 at a predetermined rate either by gravity feed or via pumping means, not shown.

To simplify the illustration, only one hollow electrode is shown in FIGURE 2. It should be understood, however, that in the practice of this embodiment, two or more fiowthrough electrodes may be employed simultaneously in the same manner as described for the electrode shown in FIGURE 2.

In one embodiment, the flow-through electrodes have internal diameters that decrease in the direction of the liquid flow therethrough. Such an electrode is illustrated in FIGURES 3 and 4. The embodiment shown in FIG- URES 3 and 4 has a tapered end portion 247 which is provided with a single orifice 249. End portion 247 is in fluid communication with a central tubular section 251 and a second end section 253 which is of reduced external diameter to facilitate sliding connection with conduit 143 of FIGURE 2. Central section 251 and end section 253 are of even internal diameters. In the flow-through electrode of FIGURES 5 and 6, the internal diameter is constant and a plurality of orifices 261 are evenly spaced about its perimeter. In another embodiment, not shown, these designs are combined to provide a tapered nozzle having a plurality of orifices.

The following examples illustrate some of the advantages provided by operating an electrocoating process in accordance with the invention hereinbefore and hereinafter described.

EXAMPLE I An electrocoating bath is formed by dispersing one volume of a commercially available (black) automobile primer paint containing about 40 weight percent solids within four volumes of water with the aid of an emulsifying amount of a water soluble amine. The primary constituents of the primer are a conventional organic binder, i.e., an organic resin having free or water dissociable carboxylic acid groups in its molecular structure, and a black. pigment.

A grounded copper tube is immersed in the bath and served as the cathode of the cell. The mild steel workpiece is positively charged in relation to the copper tubing.

In the first coating operation a difference of potential of 100 volts is maintained between the workpiece and the copper tube until a smooth coating of about 0.5 mil thickness is obtained upon the surfaces of the workpiece directly exposed to the cathode. The difference of potential is increased to 175 volts. The film obtained with this paint under these conditions with the same electrode surface area and spacing ruptures leaves a plurality of blisters in the coating surface. Employing identical conditions a portion of the bath is pumped through the copper cathode and directed against a surface of the workpiece. A flow rate of about 4.5 gallons per minute from the interior of the cathode is employed. Under these conditions smooth films are obtained upon workpieces at potentials of 100, 150 and 250 volts.

EXAMPLE II An electrocoating paint is prepared in the following IIIZIDIICII Step 1. 1893 pounds of tall oil fatty acids (a mixture containing 98.9 percent tall oil acids and 0.5 percent resin acids, acid number 199, saponification number 200, viscosity Gardner sec. 0.9, unsaponifiables 0.6) are charged to a vessel and heated to 150 F.

(2) To the heated acids are added one pound sodium benzoate.

(3) To the resulting mixture are added 1509 pounds of an epoxy resin (glycidyl ether of bisphenol A, visc. cps. at 25 C. 7000-9000, epoxy equivalent 185-195, i.e., grams of resin containing one gram-equivalent of epoxide).

(4) The resulting material is heated to 500 F. and for a time sutficient to provide an acid number of less than about 0.2.

(5) The resulting material is cooled to 330 F. and 527 pounds of the anhydride of 1,2,4-benzene tricarboxylic acid are added and the temperature maintained at 330 F. until the resulting resin has an acid number of 62.

(6) To this resin at 330 F. are added 974 pounds of methyl ethyl ketone and the mixture is allowed to cool.

(7) 44 pounds of the solution of resin-methyl ethyl ketone from the preceding step and 3.2 pounds of black iron oxide are admixed and ground for 16 hours after which 30 additional pounds of the resin-methyl ethyl ketone material of the preceding step are added and the resulting mixture is ground for an additional hour.

(8) To a sonic type homogenizer containing 47.6 pounds deionized water, 0.3 pound triisopropanolamine, and 1.1 pounds diethylamine, are added 25 pounds of the material prepared in the preceding step and 5 pounds methyl ethyl ketone. This material is agitated until fully emulsified and the solvent is removed. Specifically, the temperature of the emulsion is raised to about 150 F. to vaporize the methyl ethyl ketone. The vaporized solvent is then removed by reducing the pressure above the oil in-water type emulsion to about 50 mm. Hg with a conventional vacuum pump.

An electrocoating bath having a pH of 8.25 and containing 5.2 weight per-cent paint solids is prepared by diluting this emulsion with deionized water.

Electrically induced deposition of the paint thus prepared is carried out within a coating tank which is grounded and serves as the principal cathode of the electrodeposition cell. An automobile body is immersed in the bath and is positively charged in relation to cathode. Tubular electrodes having a plurality of orifices are connected to flexible tubing and inserted within the rocker panel compartments of the automobile body. The tubular electrodes are independently suspended and move through the bath in conjunction with the body into which they are inserted. These electrodes are grounded and negatively charged in relation to the body.

In a first coating operation a portion of the coating bath is removed from the aforesaid tank and pumped through the tubular electrodes and into rocker panel compartments with these electrodes maintained at the same potential as the major cathode, i.e., the bath retaining tank. During this test the difference in potential between the major cathode and the automobile is maintained at about 225 volts.

In a second coating operation the difference in potential between the primary cathode and the workpiece is maintained at about 225 volts while the difference in potential between the tubular electrodes and the workpiece is increased to about 250 volts.

In a third coating operation the difference in potential between the primary cathode and the workpiece is main tained at about 225 volts while the difference in potential between the tubular electrodes and the workpiece is decreased to about 200 volts.

In a fourth coating operation the same paint is used but the supply of coating material pumped through the tubular electrodes into the rocker panel compartments is from an independent source and not from the main coating bath. The water content of this feed stream is reduced 25 percent in relation to the composition of the main coating bath. Deionized water is continuously added to the main coating bath and water is continuously removed from the main coating bath by dialysis to maintain a constant water content and to maintain the bath at a constant pH.

In a fifth coating operation the same paint is used but the supply of coating material pumped through the tubular electrodes into the rocker panel compartments is from an independent source and not from the main coating bath. The water content of this feed is increased 25 percent in relation to the main bath. Water is continuously withdrawn from the coating tank by dialysis.

It will be understood by those skilled in the art that modifications can be made in the details of the foregoing specific examples of this invention without departing from the spirit and scope of this invention as set forth in the appended claims.

We claim:

1. In a method for electrodepositing film-forming organic, coating material upon electrically-conductive objects which comprises immersing said objects in a coating bath in contact with a first electrode and comprising an aqueous dispersion of film-forming, organic, coating material, passing said objects through said bath and removing said objects from said bath, each of said objects while passing through said bath serving as a second electrode, and providing a direct current of electrical energy through said bath and between said first electrode and said second electrode while the latter is passing through said bath thereby electrically inducing deposition of filmforming, organic, coating material upon said second electrode, the improvement which comprises moving a conduit electrode through said bath with said second electrode and spaced apart therefrom, providing a direct current flow of electrical energy through said bath and between said conduit electrode and said second electrode, and forcing a stream of liquid coating material through said conduit electrode, into said bath, and against said second electrode.

2. In a method for electrodepositing film-forming organic, coating material which comprises passing compartmentalized, electrically-conductive objects through an aqueous coating bath having film-forming, organic, coating material dispersed therein and a first electrode in contact therewith, each of said objects while passing through said bath serving as a second electrode, providing a direct current of electrical energy through said bath and between said first electrode and said second electrode while the latter is passing through said bath thereby electrically inducing deposition of film-forming, organic coating material upon said second electrode, the improvement which comprises positioning a tubular conduit electrode within a compartment of said second electrode and spaced apart therefrom, providing a direct current of electrical energy through said bath and between said tubular conduit electrode and said second electrode, and forcing a stream of liquid coating material through said conduit electrode, into said bath, and against said second electrode.

3. A method in accordance with claim 1 wherein said liquid coating bath contains an aqueous dispersion of an organic resin having a plurality of water dissociable carboxylic acid groups within its molecular structure.

4. A method in accordance with claim 2 wherein the difference in electrical potential between said first electrode and said second electrode is greater than the difference in electrical potential between said second electrode and said conduit electrode.

5. A method in accordance with claim 2 wherein the difference in electrical potential between said first electrode and said second electrode is less than the difference in electrical potential between said second electrode and said conduit electrode.

6. A method in accordance with claim 2 wherein the stream of liquid coating material passing through said conduit electrode is of the same composition as that of said coating bath.

7. A method in accordance with claim 2 wherein the stream of liquid coating material passing through said conduit electrode is of difierent composition from that of said coating bath.

8. Apparatus for use in electrically induced deposition of an organic coating material upon compartmentalized, electrically conductive objects comprising in combination a coating vessel adapted to retain an aqueous dispersion of an organic coating material, conveyor means for propelling an electrically conductive object through said dispersion spaced apart from said vessel, electrical means for passing a direct current flow of electrical energy through said dispersion and between said vessel and an electrically conductive object within said dispersion, an electrode comprising an electrically conductive conduit having inlet means and outlet means and adapted to admit of the flow of an aqueous dispersion of organic coating material therethrough, movable carrier and support means constructed and arranged to move said electrode through said dispersion with said electrode inserted within a compartment of a compartmentalized object propelled by said conveyor means and spaced apart therefrom, electrical means for passing a direct current flow of electrical energy between said electrode and the object into which it is inserted, and pumping means associated with said electrode and adapted to force liquid coating material through said electrode and into said compartment.

References Cited UNITED STATES PATENTS 950,973 3/1910 Wallace 204-237 2,431,629 11/1947 Wind et al. 204181 3,325,390 6/1967 Burnside et al. 204181 3,355,374 1l/1967 Brewer et a1 204-181 FOREIGN PATENTS 10,317 6/ 1899 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner.

E. ZAGARELLA, Assistant Examiner.

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