US3471389A - Electrocoating process wherein the initial amperage surge is controlled - Google Patents

Description

R. G. swANsoN Oct. 7, 1969 ELECTROCOATING PROCESS WHEREIN THE INITIAL 3,471,389

AMPERAGE sums 1s CONTROLLED Filed Aug. 26, 1966 FIG?) INVENTOR RALPH e. SWANSON BY #M/M AGENT United States Patent Office 3,471,389 ELECTROCOATING PROCESS WHEREIN THE INITIAL AMPERAGE SURGE IS CONTROLLED Ralph G. Swanson, Flint, Mich., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Aug. 26, 1966, Ser. No. 575,296 Int. Cl. C231) 13/00; Blllk 5/02 US. Cl. 204-181 8 Claims ABSTRACT OF THE DISCLOSURE This invention concerns an improved process for the application of aqueous coating compositions to metal substrates and more particularly to an improved electrocoating process for depositing such compositions to metal substrates.

Corrosion of automobile bodies and other metal articles that are exposed to 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, difiiculty has also been encounterel in adequately coating all surfaces of these articles with a primer and a paint in order to provide the necessary corrosion protection. It is well known that incomplete coverage of metal structures with protective coatings provides sites for initial corrosion which accelerates deterioration of the coating.

Electrocoating protective coating compositions on metal substrates overcomes many of the deficiencies present in conventional coating procedures such as spraying, dipping, and the like. By using an electrocoating process, the recessed areas, for example, of automobile frames and bodies and the interiors of tubular structures, are readily coated with a protective finish since the electric current can reach these areas. The electrocoating process for applying protective coatings to metal substrates has attracted the attention of manufactures interested in prolonging the useful life of their metal products.

One major problem which has plagued the prior art electrocoating process is an initial high amperage surge which occurs when the metal article which is to be coated enters the electrocoating bath. Generally, the metal article forms the anode of the cell and the protective coating is electro-deposited thereon. This initial amperage surge occurs when the cell is first energized after the article has entered and is submerged in the bath and usually lasts for less than 30 seconds. The surge is often 20, and up to 50, times higher than amperage required for the subsequent electrocoating process. This high amperage surge requires the use of electrical rectifiers that have 20-50 times more capacity than required for the subsequent electrocoating process and greatly increases the initial capital expense of process equipment. For example, a car body having a surface coating area of 1000 square feet, exhibits an initial amperage surge of about 25 amperes/ft while the amperage required for electrocoating is about 1 ampere/fe This electrocoating system requires an initial current of 25,000 amperes when the cell is first energized while the subsequent electrocoating process requires only about 1000 amperes. This par- 3,471,389 Patented Oct. 7, 1969 ticular electrocoating system for car bodies requires an electrical rectifier that has at least 25 times more capacity than is necessary for the final electrocoating process because of the high initial amperage surge.

The improved electrocoating process of this invention overcomes this problem of the prior art electrocoating process of the initial high amperage surge after the article has entered and is submerged in the electrocoating bath and results in savings to the manufacturer of lower equipment costs and provides for the even and uniform deposition of the coating composition since objectionable gas formation at the anode which is caused by the high current surge is substantially reduced.

In the standard continuous electrocoating process in which a metal article is coated with a protective coating, the metal article is submerged in and passed through an energized bath of an electrocoating cell in which the article forms the anode of the cell. The bath of the electrocoating cell contains a film-forming polymeric dispersion which is deposited as a polymeric coating on the article. The process of this invention is an improvement over the aforementioned standard electrocoating process since the initial amperage surge is reduced and held to a minimum when the article to be coated is immersed in the bath of the electrocoating cell. The improvement of this invention comprises the following steps:

(1) Electrocoating the article by initially having the cathode/anode area ratio at about 0.01 at the start of said process; and

(2) Increasing the cathode/anode area ratio from 0.01 to at least 0.25, preferably to about 0.5, and more preferably to about 1.0, in about 1030% of the total residence time the article is in the electrocoating bath.

The term cathode/anode area ratio is a numerical value resulting from the division of the area of the cathode by the area of the anode. The cathode of an electrocoating cell is generally the length of the cell but the only area of the cathode involved in determining the above ratio is that area of the cathode which is directly opposite the anode, i.e. the article being coated.

The metal piece being coated can be the anode or the cathode in this invention, depending upon the type of coating composition utilized but preferably it is the anode and for convenience, the system described particularly herein will refer to the metal piece being coated as the anode. Also, the process of this invention can be used in an electrocoating process in which the article to be coated is energized before it enters the electrocoating bath. High immersion rates, i.e., passing the article to be coated through the bath at a rate of about 8 feet per minute and above, even when the article is preenergized, cause an amperage surge which can be minimized by the process of this invention.

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

FIGURE 1 is a schematic side view of a preferred embodiment of the electrocoating apparatus used in the process of this invention showing the flow of the coating composition.

FIGURE 2 is a schematic top view of the apparatus of FIGURE 1 also showing the flow of the coating composition.

FIGURE 3 is a schematic side view of the preferred cathode illustrating a method for shielding the cathode.

Referring to FIGURES 1 and 2, the apparatus used in the process of this invention comprises a tank 1 having a plurality of cathodes 7 positioned along its walls. The inner surface of the tank 1 is electrically insulated from its liquid contents by an insulating layer 2. The coating composition can be withdrawn from the tank 1 through an outlet 9, positioned in the side of the tank and can be continuously passed into the tank through an inlet 8. Preferably, the coating composition is continuously passed through the tank 1 through a conduit 3 which is connected to a mixing tank 6 or an in-line mixer 6 where the electrocoating composition is uniformly mixed with makeup coating compositions from tank to increase the con centration of the coating composition which is conducted back to tank 1 by the conduit 3 and enters the tank at inlet 8.

The flow of the coating composition can be actuated by gravity or by a pump 4 depending on the rate of fiow desired and on thelocation of the inlet 8 and outlet 9. The inlet and the outlet are located to provide the greatest uniformity of the coating composition in the bath as possible.

The make-up coating composition is preferably added continuously to tank 6 (or the in-line mixer 6) from tank 5 and has a pH in solids composition adjusted so that when the make-up coating composition is mixed with the coating composition being recycled from tank 1 and transmitted to tank 1, the pH and solids composition of the coating composition in tank 1 will remain substantially at the original level when the process started. The electrocoating process operates continuously in this way at the same high level of efiiciency and effectiveness as when the process was started and without wide variations in solids content and pH.

In FIGURE 3, the cathode 7, as shown in FIGURES l and 2, is shown in detail and represents one preferred method of shielding the cathode used in the process of this invention to reduce the cathode/anode area ratio. The conductive metal cathode 10 holds a non-conductive sheet material 11 which is slidably mounted on the cathode and is held in place by a tab 12. The exposed cathode area can be changed by moving the sheet material in the holder or by cutting a section from the sheet material as shown in FIGURE 3.

The important step in this invention is to have the cathode/anode area ratio at the beginning of the process at about 0.01 to reduce the initial high amperage surge when the article to be coated is immersed in the bath. Several procedures can be used to reduce the cathode/anode area ratio in this invention; for example:

1) Painting or coating the cathode to reduce the available cathode area;

(2) Immersing only a portion of the cathode in the electrocoating bath to effectively reduce available cathode area;

(3) Using a shaped cathode which increases in size as the piece advances through the bath, e.g., starting with a small V-shaped cathode which gradually increases in size until the desired cathode/anode area ratio is reached;

(4) Positioning a non-conductive plastic sheet material in front of the cathode as shown in FIGURE 3.

For the process of this invention to be economical, the cathode/anode area ratio should be increased from the initial 0.01 to at least 0.25 and preferably to about 0.5 and more preferably to about 1 in about 1030% of the residence time, i.e., the time the article is in the coating bath. Under preferred operating conditions of the process of this invention, the cathode/anode area ratio is increased from 0.01 to 1 in about 15-25% of the residence time the article is in the electrocating bath.

Conventional materials of construction are used to form the apparatus used in the process of this invention. The tank 1 can be fabricated from iron, steel, plastic or any other strong durable material. The tank lining 2 can be any nonconducting known material suitable for use in a conventional electrocoating process. A mixture of epoxy resins and tar with a catalyst applied to the interior surface of the tank at thicknesses of about A; inch and then air-dried is very satisfactory but other durable insulating materials can be used. As is readily apparent to anyone skilled in the art, the cathode elements, transmisslon lines, pumps, mixing tanks, etc., should be of the appropriate materials necessary to handle this type of process. The slidable plastic sheet material as shown in FIGURE 3 or any other materials used to shield the cathode are preferably plastic materials, such as polyvinyl chloride, polymethyl methacrylate, polyethylene and the like.

Preferably, the electrocoating conditions used in the process of this invention are about 75 to 500 volts with a current of about 1 to 3 amperes per square foot of article being coated. The process of this invention should reduce the initial amperage surge when the article enters the coating bath to less than 4 amperes per square foot and preferably below 3 amperes per square foot. One preferred use for the process of this invention is in electrocoating of primer compositions onto steel car bodies. In electrocoating car bodies, an electrocoating voltage of about volts is used along with a current of 1 ampere per square foot and the initial amperage surge at the start of the process is reduced to less than 3 amperes per square foot when the car body is initially submerged in the bath.

A variety of electrocoating compositions can be used in the process of this invention. Belgian Patent 624,488, which is incorporated herein by reference, discloses a wide variety of film-forming polymeric materials that can be used in electrocoating. Preferably, an electrocoating composition that is used in the process of this invention has a solids content of about 3-30% and a pH of about 7 to 10 and comprises a uniformly dispersed film-forming polymer of a carboxylic acid polymer which has an acid number from about 6 to 300 and which is neutralized with a base such as ammonia, primary amines, secondary amines, tertiary amines, polyamines, hydroxy amines, potassium hydroxide, sodium hydroxide and the like. A variety of carboxylic polymers can be used with the only requirement being that the polymer is dispersible in water and forms an aqueous coating composition from which a polymeric coating can be electrodeposited.

Typical polymers useful in this invention are alkyd resins, epoxy resins, acrylic resins, the reaction product of dicarboxylic anhydride and the drying oil, and the like. It is preferred to use a thermosetting nitrogen containing resin with the aforementioned carboxylic polymer in the amounts of S to 50% by weight based on the weight of the film-forming polymer. Nitrogen containing resins that are useful are, for example, condensates of formaldehyde with melamine, urea, benzoguanamine or melamine/toluene sulfamamides. In most instances, the coating composition is used as a primer and preferably pigment particles are dispersed therein. Iron oxide is one preferred pigment. Barium chromate, because of its rust inhibitive properties is another useful primer particularly for automobile bodies. Other pigments that can be used in the process of this invention are titanium dioxide, basic lead chromate, chromium phosphate, lead chromate and the like.

The following examples illustrate the process of this invention.

Example 1 A laboratory size electrocoating tank is used which is constructed of steel and lined with plastic and measures about 6 inches wide by 72 inches long and is about 10 inches deep and is filled with an electrocoating composition which is described hereinafter. An electrically driven pulley and guidewiring arrangement is positioned over the electrocoating tank so that metal articles which are to be coated can be attached thereto and passed through the tank in about 60 seconds. The metal samples that are to be coated are electrically connected to the positive pole of a DC rectifier and form the anode of the electrocoating cell. An ammeter is connected to the cell and the amperage output of the rectifier is measured and recorded at the beginning and at the end of electrocoating process.

The electrocoating composition is a pigmented aqueous dispersion of an alkyd polymer and a heat reactive resin. The alkyd polymer is formed by charging the following ingredients into a polymerization vessel:

Parts by weight Trimellitic anhydride 238 Dehydrated castor oil fatty acid 229 1,5-pentanediol 149 Hydrogenated Bisphenol A (4,4'-iso-propylidene dicyclohexanol) 344 Methylisobutyl ketone 40 Total 1000 The ingredients are blanketed with nitrogen and the batch is heated to 140 C. with constant agitation. The batch is then heated to about 180-185 C. and maintained at this temperature for about 8-9 hours until an acid number of about 21 is reached. The batch is then cooled to room temperature.

The resulting alkyd polymer has a molecular weight of about 1300-1400 and an acid number of about 21.

The viscosity of the polymer is measured by diluting the polymer with butyl Cellosolve to 60% solids solution; this polymer solution has a Gardner-Holdt viscosity of W.

The following ingredients are used to form a pigment dispersion for an electrocoating composition:

Parts by weight Alkyd polymer solution prepared above (96% solids) 100 Diacetone alcohol 63.7 Hexamethoxymethylmelamine 9.5 Iron oxide pigment 20 Total 193.2

(64% solids) 186 Deionized water 999.5 N,N-dimethylethanolamine 3 Total 1188.5

N,N-dimethylethanolamine is slowly added with constant agitation to the pigment dispersion over about a 15 minute period. The water is preheated to about 70- 80 C. and is slowly added to the mixture with constant agitation; after all the water is added, the mixture is agitated for about 1 hour. The pH of the composition is then adjusted to about 7.8-8 with N,N-dimethylethanol amine.

Steel plates measuring about 4" x 8" x are treated on both sides with Bonderite 100 which is believed to be essentially an aqueous solution of dilute phosphoric acid and zinc phosphate. About 225 milligram of zinc phosphate per square foot of steel plate are deposited.

Two electrocoating processes are utilized and the initial ampere surge is determined for each; Process 1 is the improved electrocoating process according to this invention and Process 2 is the conventional electrocoating process. Both the initial amperage surge at the start of each process when the metal article is submerged in the bath and the final electrocoating amperage for each process are recorded in Table I.

In Process 1, the Bonderite coated steel plates are electrocoated with the initial cathode/anode area ratio being 0.01 at the start of the process when the metal plate enters and is submerged in the electrocoating bath. In about 20% of the total residence time the metal plate is in the bath, the cathode/ anode area ratio is increased to 1 and held at this ratio until the electrocoating process is completed. To obtain the desired cathode/anode area ratios, the cathodes are shielded as shown in FIGURE 3 with a plastic non-conductive material. The coated steel plates are electrocoated at three voltage, 50, and volts respectively. The initial amperage surge is measured at the start of the process when the metal article is submerged in the electrocoating bath and again the amperage is measured just before the article is removed from the coating bath, i.e., about 60 seconds after the start of the process. These values are recorded in Table I. The initial amperage surge at 50 volts is about 8 times that of the final electrocoating amperage while at 100 volts and 150 volts, the amperage surge is about 11 times the final amperage necessary for electrocoating.

In Process 2, the cathode/anode area ratio is maintained at 1 and the cathodes are not shielded as in Process 1. The Bonderite coated steel plates are coated at 50, 100 and 150 volts respectively, and the initial amperage and final amperage data are recorded in Table I. As can and final amperage data are recorded in Table I. As can be times of the final amperage necessary to electrocoat the plates.

The data of Table I indicate that Process 2, which is an example of a conventional electrocoating process, has a much higher initial amperage surge when the metal article enters the bath than does the improved process of this invention as exemplified by Process 1.

TABLE I Cathode/Anode Area Ratio C/A Ratio 0.01 CIA Ratio 1 Process 1 Start Finish Cathode/Anode Area Ratio Constan t 1 1. In a continuous electrocoating process for coating a metal article by electrodeposition in an electrocoating cell wherein the article being coated is the anode of said cell, by immersing the article in an aqueous electrocoating bath containing a dispersion of polymeric film-former, electrically energizing said bath and passing said article through said energized bath the improvement in combination therewith for controlling initial amperage surge when said article is first immersed in said bath comprising (1) electrocoating said article by having the cathode/ anode area ratio at about 0.01 at the start of said process;

(2) increasing the cathode/ anode area ratio from about 0.01 to at least 0.25 in about 10-30% of the total residence time said article is in the electrocoating bath.

2. The process of claim 1 in which the cathode/anode area ratio is increased to about 1 after about 20% of the total residence time said article is in the electrocoating bath.

3. The process of claim 1 in which the cathode/ anode area ratio is controlled by placing an electrically nonconductive shielding material between the cathode and the anode and said shielding material being positioned such that the cathode/anode area ratio progresses from about an initial 0.01 to about 0.5.

4. The process of claim 1 in which the cathode/anode area ratio is controlled by using multiple cathodes each being immersed a different degree in said electrocoating bath to provide an initial cathode/anode area ratio of about 0.01 at the start of said process and increasing said ratio to about 0.5

5. The process of claim 1 in which the cathode/anode area ratio is controlled in the process by using an elongated V shaped cathode which increases in size as the article advances through the bath starting with a small V shape which gradually increases in size and is positioned in said bath to provide an initial cathode/ anode area ratio of about 0.01 at the start of said process and increasing to about 0.5.

6. The process of claim 1 in which the cathode/anode area ratio is controlled by using multiple cathodes in which each cathode has a non-conductive panel slidably mounted on said cathode, the panels being adjusted to provide sufficient cathode exposure to provide an initial cathode/ anode ratio of about 0.01 at the start of said process and increasing to about 0.5.

7. The process of claim 1 in which the voltage used in said electrocoating cell is about 75-500 volts and the amperage is 1-3 amperes per square foot of article being coated and said initial amperage surge is less than 4 amperes per square foot of article being coated.

8. The process of claim 7 in which the article being electrocoated is a steel car body.

References Cited UNITED STATES PATENTS 3,355,374 11/1967 Brewer et a1. 2'0418l JOHN H. MACK, Primary Examiner E. ZAGARELLA, JR., Assistant Examiner U.S. Cl. X.R. 204300

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