Electrophoretic Coating Process

Abstract

An improved process for the electrophoretic deposition of a coating on a metal article in which the bath contains an electrically charged film-forming material and a porous partition which separates the article being coated (e.g., the anode) from electrode (e.g., the cathode) and in which a portion of the bath which is deficient in film-forming material is removed and replenished with film-forming material and recycled into the bath in the proximity of the article being coated. An apparatus having the required novel features necessary for the improved electrophoretic process is also disclosed.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of my copending application Ser. No. 457,551 filed May 21, 1965, now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention concerns application of aqueous coating compositions to metal substrates and more particularly to an improved process for electrophoretic deposition of such compositions and apparatus useful therefor.

Corrosion of automobile bodies and other metal articles ordinarily exposed to exterior atmospheric conditions has been a bothersome problem for many decades. Apart from the task of developing suitable primers and paints for protecting these structures from corrosion, difficulty has also been encountered in adequately coating all surfaces of these articles with such primers and paints in order to provide the protection which such compositions can afford. It is well known that incomplete coverage of metal structures with protective coatings provides sites for initial corrosion which then accelerates deterioration of the coatings in other portions of the structure.

Electrophoretic deposition of coating compositions on metal substrates overcomes many of the deficiencies attending conventional coating procedures such as spraying, dipping, and the like. By means of electrophoretic deposition, recessed areas of automobile frames and bodies and the interiors of tubular structures are readily coated with a protective finish where electric current can reach these areas. Understandably, therefore, the process of applying protective coatings to metal articles by electrophoretic deposition has attracted the attention of manufacturers interested in prolonging the useful life of their metal products.

Two major problems have beset the process of electrophoretic deposition of water borne paints as heretofore practiced and because of these problems continuous surveillance and control has been required to render the process useful in commercial installations operating at high work load. Thus prior art processes of this type have been plagued by a progressively decreasing throwing power and concentration of paint solids in the electrophoretic deposition tank. Also, undesirable variations in paint thickness, film smoothness and corrosion resistance on the coated products have resulted from changes in pH of the bath composition in the vicinity of the anode. In operating prior art processes this pH level has had to be carefully watched and adjusted periodically in order to insure effective deposition of paint. Nevertheless undesirable wide variations in pH do occur and these affect the coatings adversely. Each electrophoretic paint composition has its own optimum pH level which must be closely maintained.

In view of the above shortcomings of prior art electrophoretic deposition processes it is not surprising that coatings produced by such a process have had quality defects and the process has not achieved general use. Obviously the most uniform coatings are achieved if the concentration of paint solids and the critical pH level surrounding the work piece being coated can be maintained close to optimum values. The process and apparatus of the instant invention achieve this objective.

SUMMARY OF THE INVENTION

The process of this invention involves coating a metal article by electrophoretic deposition by passing an electric current between an electrode and said article through an electrolyte bath consisting essentially of an aqueous coating composition containing an electrically charged film-former, said electrode (e.g., cathode) and said metal article (e.g., anode) being at least partially immersed in the coating composition bath and electrically communicative through the electrolyte, but partially isolated from each other by a porous partition, removing at the cathode a portion of the coating bath (e.g., catholyte) including pigments, and simultaneously adding to the portion of the bath in the vicinity of the anode, for uniform distribution therein, aqueous coating composition of a pH and composition suitable for maintaining the pH and solids concentration of the coating bath at predetermined levels, usually those optimum for electrophoretic deposition of the coating bath being utilized.

DESCRIPTION OF THE INVENTION

The metal workpiece being coated can be the anode or the cathode in operating this invention, depending upon the type of electrolyte utilized but preferably it is the anode and for convenience the system described particularly herein will refer to the workpiece as the anode, the electrolyte in the vicinity of the work piece as anolyte; the pole opposite in polarity to the workpiece as the cathode and the electrolyte in the immediate vicinity of the cathode as catholyte.

The process and apparatus of this invention are not only more effective than prior art procedures of this type but also permits continuous coating without substantial change of composition or pH of the coating bath. Also, coatings applied to metal articles according to this invention are more uniform and adhesive than heretofore.

Another advantage of this invention is the greater ease of dispersion of the acidic "makeup" primer into the bath contents. This advantage is obtained through the concentration of alkalinity in the confined catholyte which more effectively reacts with the "makeup" primer than would analyte of lower pH.

In accordance with a preferred embodiment catholyte withdrawn from the tank is admixed with a make up coating composition having a pH and solids composition adjusted to provide a mixture suitable for addition to the coating bath as aforesaid to maintain its pH and composition practically constant.

The present invention can be readily understood by reference to the drawings:

FIG. I is a schematic side view of a preferred embodiment of the apparatus of this invention showing the flow of coating bath.

FIG. II is a schematic top view of the apparatus of FIG. I with the flow of coating bath also shown.

FIG. III is a schematic cross-sectional view of the apparatus showing the location of the cathodes and the porous partitions positioned around the cathodes and separating them from the main body of the coating bath. Catholyte flow is also indicated by arrows.

FIG. IV is a schematic cross-sectional view of another embodiment of the apparatus showing a different arrangement at the cathodes and a different catholyte flow pattern.

Referring to FIGS. I and II, the apparatus comprises a tank 1 having a plurality of compartments 2 along its walls and enclosed by porous partitions 3. The inner surface of tank 1 is electrically insulated from its liquid contents by insulating layer 4. Provision is made to withdraw catholyte from each compartment at outlets 5, preferably continuously, pass it through a common conduit 6 to either a mixing tank 7 or in-line mixer 7 where it is uniformly mixed with make up coating composition from tank 8 to form a concentrated anolyte which is conducted back to tank 1 by conduit 9 entering the tank at inlet 10.

The flow of the catholyte can be actuated by gravity or by pump 11 depending upon the rate of flow desired and the location of outlets 5 and inlets 9. The latter are located to provide the greatest uniformity of coating bath composition possible.

Make up composition is preferably added continuously to tank 7 (or in-line mixer 7) from tank 8 and has a pH and solids composition such that when mixed with catholyte entering tank 7 via conduit 6, and transmitted back to tank 1 the pH and solids composition of tank 1 will be maintained substantially at their original level. In this way the electrophoretic deposition process continues to operate continuously at the same high level of efficiency and effectiveness as at the start up and without the wide variations in solids content and pH which has characterized prior art processes.

In FIG. III the cathode 12 is shown in close proximity to tank wall 4 and with porous or semipermeable membrane partitions 3 partially isolating the cathode and separating it from the coating bath, thereby permitting isolation and removal of catholyte. Each partition is topped by a weir 17 over which coating bath passes when the coating bath level in the tank becomes high enough. Coating bath including solids dispersed therein, can also pass through porous partition 3 and it is important for purposes of this invention that the pore or mesh size of the porous partition be no larger than essential to permit passage of the coating bath and its dispersed solids, including pigment, through the partitions to the cathode. Failure of the partition to so function will detract substantially from the effectiveness of this process by permitting excessive back diffusion of alkalinity into the anolyte. Back diffusion should be as low as possible. Back diffusion can be held to a minimum by using a semipermeable membrane as the partition.

It will be noted from FIG. III that catholyte leaving the tank at outlet has a higher pH (indicated to be about 10 for exemplary purposes) than catholyte near the weir. The flow cycle prevents catholyte having a high pH from mixing with the main body of the coating bath so that the latter is maintained at its optimum pH, while catholyte with a pH higher than this optimum value is continuously removed and replaced with the new coating bath to maintain the optimum pH. Coating bath solids are simultaneously replenished by additions to mixing tank 7 (or in-line mixer 7).

The arrangement of porous partitions 3 in FIG. IV is slightly different from those in FIG. III in that they are topped by an impervious member which extends from slightly below to slightly above (for example, one inch below to one inch above) the surface of the coating bath. The latter can pass through the partitions, if porous, as in FIG. III but can pass even more readily under the partitions at 14 to reach the cathode. In this arrangement outlet 5 is fitted with standpipe 15 the top 16 of which is just below the surface of the coating bath so that coating flows upward along the cathode in order to enter the standpipe for withdrawal through outlet 5. Naturally the pH of catholyte near the bottom of the cathode approximates that of the main coating bath and pH of the catholyte increases as catholyte approaches the top of the standpipe. The porous partition is selected so that it provides sufficient impedance to flow of coating bath to prevent channeling and short-circuiting of the bath and favors sweeping much of the catholyte past the full length of the cathode before entering the standpipes.

The materials of construction of the apparatus of this invention need not be other than conventional. The tank 1 can be fabricated from iron, steel, plastic or any other strong and durable material. The tank lining 4 can be any known nonconducting material suitable for use in conventional electroplating processes. A mixture of epoxy resin and tar with catalyst, applied to the interior surfaces of the tank at a thickness of about one-eighth inch and then air dried, has been found to be satisfactory but other durable insulation materials can be used. It will be readily apparent to anyone skilled in the art that the cathode elements, transmission lines, pumps, mixing tanks, etc. should be appropriate to the materials being handled. The walls 3 of cathode compartments 2 can be any porous fabric or semiporous sheeting which will permit electrolytic conduction of current between cathode and anode and which has pores sufficiently small to prevent substantial intermixing of catholyte (bath in the immediate vicinity of the cathodes) with anolyte (bath outside of the cathode compartments 2). The sidewalls 17 of compartments 2 can be the same material as the front walls 3 but need not be porous and are preferably a plastic sheet material of polyvinyl chloride or polymethyl methacrylate or the like which is strong and rigid enough to impart sufficient structural rigidity to the compartment to prevent collapse.

The following examples illustrate the operation of the apparatus.

EXAMPLE 1

This describes a laboratory size dip tank, isolated cathode compartment in the tank, catholyte flow rate, electrical conditions for deposition, the anode area, and the electrophoretic paint compositions used.

A metal dip tank measuring inside 12 inches long, 4 inches wide and 14 inches tall is electrically insulated from ground. Inside the tank a single cathode compartment is installed on the end wall. The compartment measures overall 4 inches wide, one-half inch deep and 14 inches tall with one face exposed and the top and bottom ends open. The cathode compartment is positioned with its bottom open end about one-half inch from the bottom of the tank so that dispersion can enter the compartment at a rate at least as great as its withdrawal rate but not substantially greater, thus intermixing of catholyte and anolyte is avoided. It is lined overall with an electrically insulating paint coating except for a bare inside area of 2 inches .times. 10 inches which serves as the cathode. A porous fabric or semiporous plastic membrane is used to separate the cathode surface from the main section of the tank and form a cathode compartment. A syphon tube of 1 mm. inside diameter leads from the top of the cathode compartment to a separate vessel. A screw clamp on the syphon controlled the catholyte flow rate which is adjusted to yield 10 cc. per minute. The dip tank is charged with 11,100 grams water diluted material consisting of 1,110 grams electrophoretic primer solids, neutralized "charging primer," and the balance is deionized water to yield a 10 percent solids bath. The material is maintained in uniform dispersion by means of constant agitation with a propeller agitator.

Another primer used, called "makeup primer," has the same composition of solids as the neutralized "charging primer" except that it does not contain water and is acidic. This "makeup" primer is added to the catholyte, as it is removed, at a controlled flow rate and the mixture added to the main or anolyte section of the tank so as to maintain the anolyte pH and solids at optimum value. The composition and preparation of the primers follow: ---------------------------------------------------------------------------

Alkyd Resin __________________________________________________________________________ Tall Oil Fatty Acid 825 grams Neopentylglycol 918 Trimellitic Anhydride 818 Methylisobutylketone 129 2,690

Less water of esterification 160 Total 2,530 grams __________________________________________________________________________

The above ingredients are charged to a reactor and cooked at reflux temperature which rises to 404.degree. F. while removing 160 grams of water of esterification. Methylethylketone is added to produce an acid number of 45.2 which represents 45.2 milligrams of KOH per gram of alkyd solids. The viscosity is Z2 plus one-half bubble Gardner Holt viscosity. --------------------------------------------------------------------------- Final temperature is 228.degree. F.

acidic Vehicle A __________________________________________________________________________ Alkyd Resin 2,530 grams Hexamethoxymethylmelamine 329 Methylethylketone 351 Total 3,210 __________________________________________________________________________

The alkyd resin temperature is raised to 148.degree. F. and then the amine and ketone are admixed while cooling to 114.degree. F. ---------------------------------------------------------------------------

neutralized Vehicle B __________________________________________________________________________ Acidic Vehicle A 3,210 grams Triethylamine 146 Deionized water 3,466 Total 6,822 __________________________________________________________________________

Acid Vehicle A is heated to 114.degree. F. and then the amine and water are mixed until clear. Final viscosity is T plus one-half.

The metal panels to be electrocoated are 4 inches .times. 12 inches size and are electrically connected to the positive pole of a DC rectifier capable of yielding 150 DV volts. The cathode face of the cathode compartment is electrically connected to the negative pole of the rectifier. To electrocoat the panel, the system is energized at 150 DC volts for 90 seconds during which time the amperage falls from an initial high value to a final low value. The initial and final amperes are characteristic with the type of metal used and its prior surface conditioning. Clean ferrous and zinc metals show an initial value of about 6 amperes and a final value of about 0.3 amperes. Clean aluminum foil used in the following examples showed an initial value of about 3 amperes and a final value of about 0.6 amperes during a 90 second deposition time. --------------------------------------------------------------------------- Neutralized Charging Primer

Neutralized Vehicle B 7.72 % Iron Oxide No. 5.55 Bentone 34 0.62 Methylethylketone 1.61 __________________________________________________________________________

Mill above to 0.25 mil pigment fineness, then add the following mix to yield uniform dispersion.

Neutralized Vehicle B 76.9 Creosole 0.3 Deionized Water 7.3 Total 100.0 %

Acidic Makeup Primer __________________________________________________________________________ Acidic Vehicle A 6.38 % Iron Oxide No. 9.75 Bentone 34 1.08 Methylethylketone 18.63

Mill above to 0.25 mil pigment fineness, then add the following and mix to yield uniform dispersion.

Acidic Vehicle A 63.63 Creosole 0.53 Total 100.00 %

EXAMPLE 2

This example illustrates the objectional pH rise that takes place in a conventional tank operating at the same deposition rate per unit volume of primer in the tank as in the following examples. The equipment in example 1 is used except that the cathode compartment is excluded. Instead a separate bare steel cathode of the same dimension is used. Thirty aluminum foil panels are coated in the course of 60 minutes. The pH of the tank contents is measured initially and after 10, 20 and 30 panels. The pH is measured as 7.25, 7.35, 7.40 and 7.45, respectively, illustrating the rapid pH rise for conventional coating processes. In another trial 90 phosphated steel panels are coated. The initial and final pH values were respectively 7.2 and 8.3 and the corresponding film thicknesses were 0.85 mils and 0.6 mils showing a rapid deterioration of coating efficiency due to the rapid pH rise for this short period. By doubling the number of panels, coating efficiency and throwing power progressively deteriorate and film thickness quickly approaches zero.

EXAMPLE 3

This example illustrates the negligible pH rise of the anolyte when the catholyte is substantially isolated from the anolyte and withdrawn from the cathode compartment. The equipment of example 1 is used including the cathode compartment. A porous fabric separating the cathode from the anode consists of loosely woven fiber glass of 60 thread-count per inch; two layers of this fabric are used. As panels are coated, the catholyte is syphoned into a collecting vessel at the rate of 10 cc. per panel per 2 minutes. The pH of the initial anolyte and catholyte is identical at 7.25. After the tenth, twentieth and thirtieth panel the pH of the anolyte and catholyte are: ---------------------------------------------------------------------------

Panel Anolyte Catholyte __________________________________________________________________________ Initial 7.25 7.25 Tenth 7.25 8.05 Twentieth 7.30 8.25 Thirtieth 7.35 8.35 __________________________________________________________________________

In this example the small rise in pH of the anolyte is due to some back diffusion of catholyte alkalinity through the porous fabric of the cathode compartment. This back diffusion of alkalinity decreases as the porosity of the fabric decreases. The coating on all panels is smooth and of uniform thickness (0.7 mils).

EXAMPLE 4

This example illustrates that resistance to back diffusion is increased with a major decrease in porosity of the cathode wall construction by using a semipermeable membrane. In this instance the procedure is the same as that in example 3 except that the fiber glass wall of the cathode compartment is replaced with a semipermeable membrane coded 61 AZL-066 as supplied by Ionics, Inc. of Cambridge, Massachusetts. This membrane contains a cationic exchange resin. Thirty panels are coated to yield a total of 48 grams of dry film. Determinations of pH produced the following values: ---------------------------------------------------------------------------

Anolyte Catholyte __________________________________________________________________________ Initial 7.20 7.20 After 30 panels 7.30 9.00 __________________________________________________________________________

The addition of 48 grams of makeup primer solids to the catholyte and addition of this mixture to the anolyte restored the anolyte to its initial pH value of 7.2. The panels' coatings are smooth and are uniformly 0.7 mils thick.

EXAMPLE 5

This example illustrates the complete controlled catholyte process. The equipment covered in example 3 is used. The catholyte flow rate is adjusted to 20 cc. per 4 inches .times. 12 inches panel per 2 minutes. The panels are coated at the rate of 1.6 grams of dry film per panel. The catholyte is added to a collecting vessel which contains an agitator and admixed with acidic makeup primer of the composition given in example 1. This mixture is admixed with the anolyte of the dip tank. After completing depositions on 30 panels over a period of one hour, 600 cc. of catholyte has flowed and been simultaneously mixed with 48 grams of "makeup primer" solids. The pH of the neutralized catholyte is 7.1, just 0.15 less than the anolyte. In this instance the neutralized catholyte offsets the slight rise in pH of the anolyte caused by back diffusion of alkalinity as mentioned in example 3. All panels have a smooth uniform coating of film former 0.7 mils thick.

Claims

What is claimed is:

1. In a method for coating an electrically conductive object in an aqueous bath with a paint comprising an organic resin having ionized sites thereon and intimately dispersed within said bath with an ionized dispersal assistant, said method comprising immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as a first electrode of an electrical circuit comprising said bath, said first electrode, and a second electrode in contact with said bath and spaced apart from said object, providing a difference of electrical potential between said first electrode and said second electrode sufficient to cause a direct current of electrical energy through said bath and between said first electrode and said second electrode having direction and sufficiency to effect electrodeposition of a coating of said paint upon said object from said bath, the improvement which comprises interposing a diffusion barrier admitting of liquid flow therethrough between said first electrode and said second electrode forming a bath-withdrawal zone, withdrawing from said zone aqueous electrolyte of reduced paint concentration and increased concentration of dispersal assistant relative to the corresponding concentrations of said bath where said bath is in contact with said first electrode, admixing said electrolyte with replenishment paint for said bath and returning the resultant replenishment feed to said bath.

2. The method of claim 1 wherein said electrically conductive objects are continuously introduced into said bath and said replenishment feed is introduced into said bath at a rate sufficient to maintain the concentration of paint and dispersal assistant in said bath at substantially constant levels.

3. In a method for coating an electrically conductive object in an aqueous bath with a paint comprising a polycarboxylic acid resin having ionized sites thereon and intimately dispersed within said bath with ionized amine, said method comprising immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as an anode of an electrical circuit comprising said bath, said anode and a cathode in contact with said bath and spaced apart from said anode, providing a difference of electrical potential between said anode and said cathode sufficient to cause a direct current of electrical energy through said bath and between said anode and said cathode having direction and sufficiency to effect electrodeposition of a coating of said paint upon said anode from said bath, the improvement which comprises interposing a diffusion barrier admitting of liquid flow therethrough between said anode and said cathode forming a cathode zone otherwise separated from the bath in an anode zone wherein said anode is in contact with said bath, withdrawing from said cathode zone aqueous catholyte of reduced paint concentration and increased concentration of said amine relative to the corresponding concentrations of said paint and said amine in said bath in said anode zone, admixing said catholyte with replenishment paint for said bath and returning the resultant replenishment feed to said bath in said anode zone.

4. The method of claim 3 wherein said replenishment feed contains a lower concentration of said amine than said bath in said anode zone.