US3444063A - Method for improving operational stability of electrocoating bath - Google Patents

Description

G. E. F. BREWER ET AL 3,444,063 METHOD FOR IMPROVING OPERATIONAL STABILITY May 13, 1969 OF ELECTROCOATING BATH Filed Dec. 6, 1366 4 r foe/v5 VS Sheet 2 of 2 G. E. F. BREWER ET AL METHOD FOR IMPROVING OPERATIONAL STABILITY OF ELECTROCOATING BATH 650/965 5/? BRA/V6? 6/185??? 4 BZ/AA S/fiE I N VENTORS BY Q4 25?" 04;, 5% 47'7'OR/VEVS May 13, 1969 Filed Dec. 6, 1966 A M.Nv\\\ M NM Ni Qwi T N3 82 Q@ United States Patent 3,444,063 METHOD FOR IMPROVING OPERATIONAL STA- BILIZITY OF ELECTROCOATING BATH GeorgeEfF. Brewer, Novi, and Gilbert L. Burnside, Oak

Park, Mich., assignors to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Dec. 6, 1966, Ser. No. 599,438

Int. Cl. C23b 13/00 US. Cl. 204-181 10 Claims This invention relates to the art of coating an electrically-conductive workpiece in an aqueous bath by electrically induced deposition of an organic coating material having functional groups in its molecular structure which are ionizable in aqueous medium. More particularly, this invention is concerned with a method for maintaining bath stability in a continuous or intermittently continuous electrocoating process of the type herein described which comprises isolating the bath material in time or space from the coating operation and maintaining a difference of electrical potential between electrodes in contact with the aqueous bath material that is below the threshold voltage for the resin employed and sufiicient to effect removal of ions that accumulate in the bath during coating operations.

The threshold voltage, i.e. the difference of potential at which electrically irreversible and/or bath insoluble deposition of the resin is initiated, will vary somewhat with the chemical and physical properties of the resin to be de posited. Ordinarily, this will be above about 5 volts and less than about 20 volts, more commonly below about volts. At voltages above the threshold voltage, an electrically resistant, resin-comprising film forms upon the receiving electrode, the anode in anodic deposition, the current falls rapidly and coating is effectively terminated as the current flow drops to insignificance.

The instant process is useful with either an anodic or a cathodic deposition process. Resins suitable for anodic electrocating or electropainting are polycarboxylic acid resins which are dispersed in the aqueous coating bath with the aid of a water soluble amino compound, such term herein including water soluble amines and ammonia. A resin suitable for cathodic deposition has in its molecular structure ionizable groups which upon ionization leave ionically positive sites. Examples of such groups are amine and substituted amine groups such as quaternary ammonium groups. Dispersion of such resins is effected by the addition of water-ionizable acidic dispersal assistants, e.g. water soluble carboxylic acids and suitably buffered forms of certain inorganic acids. Formic acid and acetic acid are exemplary of the former and buffered phosphoric acid is exemplary of the latter.

To date anodic deposition has gained primary acceptance in this field and is exemplified by US. Patent 3,23 0,- 162 to A. E. Gilchrist which patent is herein incorporated by reference. This embodiment of the electrocoating process is used herein for the detailed description of the invention, it being understood that the polarity of the electrodes shown in the drawings are reversible for cathodic deposition embodiments.

During the coating process, the coating material is first deposited upon the more easily coated areas, i.e. those areas of the workpiece about which the electric field is strongest. As these areas are coated with an electrically resistant film, the coating material is then deposited upon the uncovered areas and the areas having thinner coatings. Hence, this coating process tends to be self-leveling.

In industrial electrocoating processes, the coating tank may contain in excess of 10,000 gallons of the aqueous bath and maintenance of bath quality through many turnovers, i.e. complete replacement of coating solids, is essential to economic operation. The initial conductivity of a freshly prepared coating bath can be controlled within limits to provide optimum coating conditions in the given system. However, in anodic deposition, the quantitative use of water soluble amines used to solubilize the acidic resins affect such conductivity and hence the pH of the resultant bath. During continued use of the bath, an increase of electrical conductivity is frequently observed. In many instances, this conductivity increase is accompanied by increased gas evolution at the anode, higher current consumption, lowered throwing power and formation of unsightly paint films upon curing. This increase in conductivity also results from the accumulation of extraneous materials which ionize in the bath. Such ions are introduced into the coating bath through addition of water, leaching of pigment, entrainment of salts with incoming workpieces, absorption of carbon dioxide from the air, etc. By these methods phosphate ions, sulfate ions, chloride ions and various metal ions are introduced into the bath.

It now has been discovered that bath stability, i.e. constancy of bath composition, is improved by placing the bath between electrodes of opposite polarity and maintaining a difference of electrical potential between such electrodes that is sufiicient to effect neutralization of at least a portion of the offending ions at the receiving elec' trode with resultant deposition or evolution of the reaction product. The term receiving electrode refers to electrode of opposite polarity with respect to the charge of a given ion. Such difference of potential must be maintained below the threshold voltage to prevent deposition of the resinous coating material upon the paint-receiving electrode, the anode in anodic deposition, with resultant loss of coating material and termination of effective current flow. At the same time, positive ions are deposited at the cathode.

The process can be carried out within the coating tank employing the electrodes used in the coating process when the coating process is not in use or, preferably, a portion of the bath is continuously removed and passed through a separate cell which can be operated continuously. A potential difference of about 5 volts will be suitable with most baths. Depending upon the resin employed, a potential above about 1 and below 8 volts, more commonly between about 2 and about 6 volts, will be found advantageous. It will be understood that the bath in use can be analyzed to determine the extraneous ions that have accumulated therein since initiation of coating with the bath involved. A review of standard deposition tables will provide information as to the deposition potentials for the materials involved. Since the receiving electrode may be of inexpensive sheet stock, it is advisable to replace this from time to time. Likewise, the entire cell can be disconnected and cleansed periodically.

The process to which the instant invention is directed together with the method of this invention will be more fully understood from the following detailed description when read in conjunction with the accompanying schematic drawings, wherein:

FIGURE 1 is a schematic side view of apparatus which may be used for carrying out the instant invention; and

FIGURE 2 is a schematic and partial top view of an alternative design for a deionizing cell employed in conjunction with a conventional electrocoating tank.

Referring first to FIGURE 1, an electrically conductive tank 11 serves as the cathode of the coating cell when the coating process carried on therein is one of anodic deposition. Tank 11 contains an aqueous coating bath 13 in which a polycarboxylic acid resin and a water soluble amine are dispersed. Tank 11 is in electrical connection with ground and serves as the negative electrode in the coating process being connected to a negative lead of a DC. power source 17 via conductor 15. Article 19 which is to be electrocoated is placed upon conductor hangers 21 and 23 which in turn are suspended from and transported through bath 13 by a grounded conveyor 35. Conveyor 35 is a conventional, electrically powered, chain conveyor. Hangers 21 and 23 are electrically insulated from conveyor 35 by insulators 25 and 27 to isolate article 19 from the grounded conveyor. Contact plates or brushes 29 and 31 ride against and are shown in electrical connection with bus bar 33. Bus bar 33 is electrically connected to a positive lead of power source 17 through conductor 37.

An article 19 moves from right to left with respect to the drawing and enters bath 13, a difference of electrical potential is provided between article 19 and tank 11 of sufficient magnitude to initiate electrodeposition of the dispersed resin in bath 13 upon article 19. This difference of potential may be as low as the threshold voltage for the particular resin under the conditions of temperature, bath conductivity, etc., then existing or it may be as high as about 500 volts or more if the film of coating material being laid down has sufficient strength to avoid rupture at such potential. Ordinarily, in industrial production, the voltage employed will be above about 50 volts, commonly in the range of about 100 to about 250 volts.

A conduit 39 provides means for withdrawing coating bath 13 from coating tank 11 and introducing the same into a second and smaller tank 41. Conduit 43 and pump 45 provide means for returning aqueous bath material 13-1 from tank 41 into coating tank 11. Coating tank 41 is in electrical connection with power source 17 via conductor 47 and serves as the cathode of a second and separate cell which is used to cleanse the bath. A conductor 49 and an anode 51 positioned Within tank 41. Anode 51 here establishes electrical connection between power source 17 shown plaited has a large surface area relative to the quantity of bath material 131 in which it is immersed. The ratio of anode surface area to bath volume in tank 41 is advantageously at least times greater, preferably at least 100 times greater, than the corresponding ratio of the surface area of anode 19 to volume of bath 13 in tank 11. Preferably, the distance between electrodes is as small as is feasible and the unit may take the form of two parallel sheet electrodes separated by a thin stream of coating bath electrolyte. Conventional control means, not shown, in or connected with power source 17 and/ or condoctors 47 and 49 provide a difference of electrical potential between tank cathode 41 and anode 51 that is independent of the potential difference between tank cathode 11 and workpiece anode 19.

In practice, the bath material 13-1 in tank 43 is sufficiently isolated from the flow of current between the electrodes 11 and 19 to avoid electrodeposition of the coating material upon anode 51 as a result of such current. This may be effected by separating the tanks by a distance sufficient to reduce to insignificance the effect of the electrodes of the main bath on the coating material in tank 41, the conduits 39 and 43 may be constructed to provide effective electrical shielding between baths, etc. Those skilled in this art will recognize electrical shielding as that phenomenon which occurs even within the coating bath and renders more difiicult the coating of relatively inaccessible internal surface areas of hollow workpieces. In the coating process, this difficulty is overcome by use of secondary electrodes inserted into such hollows and insulated from the workpiece anode, by utilizing coating materials and/ or coating conditions which increase throwing power of the system, i.e. the ability to reach and coat such areas with an electrically irreversible coating, etc.

Referring now to FIGURE 2, there is shown a portion of an electrocoating tank 111 containing an aqueous coating bath 113 each of which may be the same as or similar to tank 11 and bath 13 of FIGURE 1. Positioned alongside coating tank 111 is a deionizing unit 140 including a tank 142, inlet conduits 144 and 150, outlet conduits 146 and 152 and loop conduits 148 and 154. Tank 142 comprises two electrode sides 142-1 and 1422 which, as here shown, function as cathodes, and a U- shaped non-conductive member 142-3 of which only ends 142-31 and 14232 are visible in this view. Member l423 also forms the bottom of tank 140 and is united with electrode sides 1421 and 142*2 in water-tight seal completing the tank. Positioned within tank is a centrally positioned electrode 156 which, as here shown, functions as an anode. Positioned between electrode 156 and electrode 142-1 is a separator 158. Between electrode 156 and electrode 1421 is a separator 160. The construction and function of separators 158 and 160 are hereinafter described. In accordance with this embodiment, electrocoating bath is removed from coating bath 113 in tank 111 via conduit 144, enters tank 142, passes between electrode 142-1 and separator 158, exits and reenters via conduit 154, passes between electrode 156 and separator 158, and is returned to tank 111 via conduit 146. In similar fashion, coating bath 113 is removed from tank 111 via conduit 150, enters tank 142, passes between electrode 142-2 and separator 160, exits and reenters via conduit 148, passes between anode 156 and separator 160, and is returned to tank 111 via conduit 150. It should be understood that one or more of the conduits of this unit may be operatively connected with conventional pumping means, not shown, where necessary to effect passage of the liquid bath as aforedescribed.

Separators 158 and 160 should be sufiicient obstacles to liquid flow to substantially reduce mechanical mixing of the liquid on opposite sides thereof and sufiiciently open or porous that ionic conductivity between center electrode 156 and each of the outer electrodes 142-1 and 142-2 is not significantly limited thereby. Separators 158 and 160 may be textile sheets, e.g. glass fiber cloth, porous plastic sheets, e.g. polypropylene, etc. The counterfiow principle here employed provides contact of the bath stream with the anode on return to coating tank and separation to the degree feasible from the cathode at which amine and/ or amine ion concentration is highest.

The coating material in the bath is periodically or continuously replaced as it is used up with replacement of the components thereof, i.e. binder resin, pigments, etc., being made in accordance with the ratio of their deposit. This ratio may vary considerably from the ratio of such components in the coating bath. Water is also added from time to time to maintain the paint solids level relatively constant, e.g. 5 percent.

In this application, painting by electrodeposition is meant to include the deposition of finely ground pigment and/or filler in the ionizable resin herein referred to as the binder, the deposition of binder without pigment and/ or filler or having very little of same, but which can be tinted if desired, and the deposition of other water reducible surface coating compositions containing the binder which might be considered to be broadly analogous to enamel, varnish, or lacquer bases, and the coating material for such deposition is termed a paint. Thus, the binder, which is converted to a water-resistant film by the electrodeposition and ultimately converted to a durable film resistant to conventional service conditions by final curing, can be all or virtually all that is to be deposited to form the film, or it can be a vehicle for pigmentary and/ or mineral filler material or even other resins on which it exerts the desired action for depositing the film. Suitable resins include but are not limited to those specifically listed in US. Patent 3,230,162 to A. E. Gilchrist. The preferred resins for anodic deposition have an acid number between about 30 and about 300 and an electrical equivalent weight between about 1,000 and about 20,000. The term electrical equivalent weight is employed herein to mean that amount of resin or resin mixture that will deposit per Faraday of electrical energy input. The conditions, procedures, and calculations which can be employed to determine electrical equivalent weight are set forth in detail in the aforementioned US. Patent 3,230,162.

EXAMPLE 1 A five-gallon bath of eleetrodepositable automobile primer paint was removed from an industrial production coating bath after the bath had undergone twenty turnovers. The aqueous bath had intimately dispersed therein a polycarboxylic acid resin having an electrical equivalent weight of about 1,040 and an acid number of about 65, pigments and other inorganic additives conventional to electrocoating paints. This material was placed in a coating tank and test panels were coated by anodic deposition as hereinbefore described with a potential difference between anode and cathode of 160 volts. The panels were cured by conventional bake oven procedure and upon examination were found to have a relatively rough surface.

An anode having exposed surfaces measuring 72 square inches was suspended in the bath with a cotton filter bag serving as a membrane around the anode. A potential difference of 5 volts and a constant current flow of about 0.125 ampere was maintained between anode and cathode for 16 hours. A test panel was then inserted into the bath and coated at 160 volts. The film was cured as before and a decrease in roughness of the coated surfaces was observed when compared with the panels coated prior to the 5-volt treatment.

EXAMPLE 2 A bath purification cell is prepared in a tank cathode 6 feet long, 6 inches deep and 0.75 inch in width. An anode in the form of a metal panel measuring 5.5 feet in length, 8 inches in height, and 0.040 inch in thickness is supported so as to extend into the tank to a depth of about 5 inches and positioned an even distance from the corresponding sides of the tank. The tank is filled to a depth of 5 inches with an aqueous electrocoating bath. A first conduit introduces additional quantities of the bath continuously from a larger electrocoating tank spaced apart from the aforedescribed cell and in which steel workpieces are being painted by electrodeposition at an impressed voltage of 180 volts. A second conduit provides means for continuously removing bath material from the first mentioned tank and returning it to the larger tank wherein the coating operation is in progress. In the smaller unit, a constant difference of potential of about 5 volts is maintained betweeen the tank cathode and the anode suspended therein.

EXAMPLE 3 The procedure of Example 2 is repeated employing a variety of commercially available, industrial electrocoating paints. Tests are run with a potential difference of 2, 4, 6 and 8 volts between the anode and cathode in the bath purification cell. With some electrocoating paints, coating of the receiving electrode (anode) is found to reduce the current flow with resulting decrease in the efficiency of the cell. In such instances, the potential difference is reduced to about 5 volts. With other formulations comprising polycarboxylic acid resin and pigment, operation at 8 volts does not result in significant deposition of resin upon the receiving electrode and continuous operation at such voltage can be maintained.

EXAMPLE 4 This invention is practiced within an industrial electrocoating tank without removal of the bath therefrom to a separate cell by maintaining a difference of potential of about 6 volts between the cathode tank and a metal sheet anode suspended therein during periods in which the bath is not in use.

The foregoing examples are solely for purposes of illustration and should not be considered as limitations upon the true scope of the invention as set forth in the appended claims.

We claim:

1. In a method for coating an electrically conductive object in an aqueous bath with an organic resin having ionized sites thereon and intimately dispersed within said bath, 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 sec- 0nd 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 sufiicient 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 resin upon said object, and removing the resultant coated object from said bath and wherein said bath accumulates an increasing concentration of ionized inorganic materials with continued use, the improvement which comprises placing at least a portion of said bath in contact with and between a cathode and an anode and maintaining a difference of electrical potential between said cathode and said anode which is below the potential required to effect electrodeposition of said resin upon either said anode or said cathode and above that difference of electrical potential required to effect removal from said bath of ionized inorganic material.

2. The method of claim 1 wherein said difference of electrical potential between said anode and said cathode is above about 1 volt and below the potential at which electrodeposition of said resin is essentially electrically irreversible.

3. The method of claim 1 wherein said difference of electrical potential between said anode and said cathode is in the range of about 2 to about 8 volts.

4. The method of claim 1 wherein said difference of electrical potential between said anode and said cathode is sufficient to cause evolution of gas from said bath and insufficient to effect electrodeposition of said resin.

5. The method of claim 1 wherein a portion of said bath is continuously removed from said electrical circuit, passed between said anode and said cathode while an electrically conductive object is being coated within said electrical circuit, and returned to said electrical circuit.

6. In a method for coating an electrically conductive object in an aqueous bath situated within a coating tank and having a polycarboxylic acid resin and a water soluble amine intimately dispersed therein which comprises immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as the first and positive electrode of an electrical circuit comprising said bath, said first electrode, and a second electrode in contact with said bath, spaced apart from said first electrode, and negative relative to said first electrode providing a difference of electrical potential between said first electrode and said second electrode, a direct current of electrical energy through said bath and between said first electrode and said second electrode, and electrodepositing a coating of said resin upon said first electrode and wherein said bath accumulates an increasing concentration of ionized inorganic materials with continued use, the improvement which comprises removing a portion of said bath from said coating tank, placing said portion of said bath between a cathode and an anode externally positioned with reference to said coating tank, maintaining a difference of electrical potential between said cathode and said anode which is below that required to effect electrodeposition of an electrically resistant film of said resin upon said anode and above that required to effect removal from said portion of said bath of ionized inorganic material, and returning said portion of said bath to said coating tank.

7. A method in accordance with claim 6 wherein said resin has an acid number in the range of about 30 to about 300 and an electrical equivalent weight in the range of about 1,000 to about 20,000.

8. A method in accordance with claim 6 wherein said difference of electrical potential between said anode and said cathode is between about 2 and about 6 volts.

9. A method in accordance with claim 5 wherein the difference of electrical potential between said first electrode and said second electrode is in the range of about to about 300 volts.

10. In a method for coating an electrically conductive object in an aqueous bath situated within a coating tank and having a polycarboxylic acid resin and a water soluble amine intimately dispersed therein which comprises immersing said object within said bath, utilizing said bath as the aqueous electrolyte and said object as the first and positive electrode of an electrical circuit comprising said bath, said first electrode and a second electrode in contact with said bath, spaced apart from said first electrode, and negative to said first electrode, providing a difference of electrical potential between said first electrode and said second electrode in the range of about 50 to about 500 volts with resultant direct current of electrical energy through said bath and between said first electrode and said second electrode, electrodepositing a coating of said resin upon said first electrode, and removing the resultant coated object from said bath and wherein said bath is maintained at a pH at which said bath absorbs carbon dioxide from the atmosphere, the improvement which comprises removing a portion of said bath from said References Cited UNITED STATES PATENTS 3,355,373 11/1967 Brewer et al 20418l 3,355,374 11/1967 Brewer et al. 204-181 JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner.

US. Cl. X.R. 204-431

Claims (1)

1. IN A METHOD FOR COATING AN ELECTRICALLY CONDUCTIVE OBJECT IN AN AQUEOUS BATH WITH AN ORGANIC RESIN HAVING IONIZED SITES THEREON AND INTIMATELY DISPERSED WITHIN SAID BATH, 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 RESIN UPON SAID OBJECT, AND REMOVING THE RESULTANT COATED OBJECT FROM SAID BATH AND WHEREIN SAID BATH ACCUMULATES AN INCREASING CONCENTRATION OF IONIZED INORGANIC MATERIALS WITH CONTINUED USE, THE IMPORVEMENT WHICH COMPRISES PLACING AT LEAST A PORTION OF SAID BATHIN CONTACT WITH AND BETWEEN A CATHODE AND AN ANODE AND MAINTAINING A DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAID CATHODE AND SAID ANODE WHICH IS BELOW THE POTENTIAL REQUIRED TO EFFECT ELECTRODEPOSITION OF SAID RESIN UPON EITHER SAID ANODE OR SAID CATHODE AND ABOVE THAT DIFFERENCE OF ELECTRICAL POTENTIAL REQUIRED TO EFFECT REMOVAL FROM SAID BATH OF IONIZED INORGANIC MATERIAL.

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