US3816289A

US3816289A - Method of repair electrocoating a metal substrate

Info

Publication number: US3816289A
Application number: US00209305A
Authority: US
Inventors: L Landauer; R Zukowski
Original assignee: American Can Co
Current assignee: Rexam Beverage Can Co
Priority date: 1971-12-17
Filing date: 1971-12-17
Publication date: 1974-06-11
Anticipated expiration: 1991-06-11
Also published as: CA971508A

Abstract

An improved method of electrodepositing a repair coat of material upon discontinuities such as scratches in the base coat of a metal substrate, which comprises including as a step in the conventional method, the step of contacting the electrocoating medium utilized with a weak electrolyte having a dissociation constant no greater than 1 X 10 4 at 25*C in an amount and at a rate such that more of the coating material from the medium repair coats and more effectively seals the discontinuities in the base coat in less time than if the weak electrolyte were not employed. The weak electrolyte in an anodic electrocoating medium is carbonic acid. In a cathodic electrocoating medium, the weak electrolyte is a weak base such as triethylamine.

Description

United States Patent [191 Landauer et al.

[ June 11, 1974 l METHOD OF REPAIR ELECTROCOATING A METAL SUBSTRATE [73] Assignee: American Can Company,

Greenwich. Conn.

1 Filed: Dec. 17. 1971 21 Appl. No.: 209,305

3,681,224 8/l972 Stromberg 204/181 Primary ExaminerHoward S. Williams Attorney, Agent. or FirmRobert P. Auber; Paul R. Audet; Harries A. Mumma. Jr.

[ 5 7 ABSTRACT An improved method of electrodepositing a repair coat of material upon discontinuities such as scratches in the base coat of a metal substrate. which comprises including as a step in the conventional method. the step of contacting the electrocoating medium utilized with a weak electrolyte having a dissociation constant no greater than 1 X 10" at 25C in an amount and at a rate such that more of the coating material from the medium repair coats and more effectively seals the discontinuities in the base coat in less time than if the weak electrolyte were not employed. The weak electrolyte in an anodic electrocoating medium is carbonic acid. In a cathodic electrocoating medium, the weak electrolyte is a weak base such as triethylamine.

13 Claims, No Drawings METHOD OF REPAIR ELECTROCOATING A METAL SUBSTRATE BACKGROUND OF THE INVENTION This invention relates broadly to a method of coating a metal substrate. More particularly, it relates to the art of electrocoating metal substrates to protect the substrates from, for example, attack by corrosive environmental influences. More particularly, the invention is directed to an improved method of electrodepositing a repair coat on raw edges, breaks, scratches or other discontinuities in the base coat of the substrates, especially metal cans or can components.

Basically, electrocoating of a metal substrate is the electrodeposition thereon of organic resinous coating materials, from polyelectrolytic electrocoating material mediums which can be anodic or cathodic aqueous or non-aqueous base bath solutions, suspensions or dispersions. The electrocoating mediums ultimately contain polyelectrolytic particles, which, in the case of anodic mediums, carry a negative charge in the bath and when a voltage is applied and current is induced to flow through the medium, these particles migrate to and discharge onto any positively charged metal substrate, i.e., the anode, which may be immersed into the medium. Conversely, in the case of cathodic mediums, the polyelectrolytic particles carry a positive charge in the medium, and, upon application of a voltage, migrate to and discharge onto any negatively charged metal substrate, i.e., the cathode, which may be immersed into the medium. A layer of particulate coating material is electrodeposited adjacent the electrically charged metal substrate as the direct current flows between it and an oppositely electrically charged electrode wire or rod, immersed in the coating bath. The process is driven by an electrical potential which can be in the range of from 1 up to 500 volts, but more typically is from about 50 to 500 volts. The electrodeposition of the coating material takes place only at electricallyconductive surface areas of the metal object because only at such areas is there an electrical circuit and the electrical action which allows the flow of direct current needed to cause the polyelectrolytic particles to be electrodeposited adjacent the electricallyconductive surface.

The thickness of the layer of particulate material electrodeposited is automatically regulated by the characteristically low electrical conductivity of the particular mediums used. Once a certain layer thickness of coating material has attached to the electricallyconductive surface area of the metal substrate, the electrodeposited coating material, in having a low elec trical conductivity characteristic, increasingly tends to insulate the surface area from the coating bath in which it is immersed, transforming it into a non-conductive surface, whereby direct current flow therein greatly diminishes and eventually ceases, with the resulting inhibition of further electrodeposition of coating material.

One particular field where it has been found desir able to coat metal substrates is in the manufacturing of metal cans, where it is necessary that all exposed, uncoated surface areas on the metal can be coated to protect the metal from corrosion.

Heretofore, metal substrates, especially can ends and/or bodies, such as used in the packaging of beer or carbonated beverage, have been protected from environmental product attack by a double coat system which includes a base coat, usually roller coated onto the metal stock while it is in the flat and usually scratched during the can-making operation, and by a top coat applied to the fabricated can or component by, for example, a spraying-on process, to seal any such scratches or discontinuities which appear in the base coat. The top coat usually is an overall coat since the location of the discontinuities cannot precisely and reliably be ascertained.

Recently, the top coat has not been applied on an overall basis but only as a repair or spot coat applied only to the localized raw edges, breaks, scratches or other discontinuities, or other electrically porous or conductive areas of the base coat. This repair or spot coating can be effected because of the nature of the electrodeposition process wherein the ionized organic particles in the medium are automatically selectively deposited adjacent only thelocalized electrically porous or conductive areas where there is a flow of electric current through the medium, the discontinuity and the exposed conductive metal substrate.

It has been found, however, that present electrodeposition repair coating processes are too slow for use with high speed can electrocoating machines, practical use of which requires that deposition be effected in 2 seconds or less. The processes take too long to deposit and do not deposit enough particulate electrocoating material adjacent the site of the base coat discontinuity to obtain a satisfactory seal. The amount of material deposited merely fills the gap within and abuts the generally vertical walls of the cavitious area of the discontinuity and does not sufficiently lap over and seal the surrounding non-cavitious relatively continuous surface of the base coat. An insufficient amount of material deposited at the repair site is unsatisfactory because the adhesion at the abutment or connection between the deposit and the surrounding walls of the cavity involves only a small area. The adhesion therefore is weak and inflexible and allows the product to corrode through the connection, undercut the repair and base coats and again allow corrosive current flow to the metal substrate.

Efforts to increase the amount of deposit in less time by increasing the voltage used to drive the electrodeposition process fail because, the higher the voltage, the greater the corrosive attack of the repair site and metal substrate, as measured for example by iron-pickup in products contained in metal cans, and, the higher the voltage, the greater the evolution of gases adjacent the site, the gases often being trapped in the deposited material which thereby increases the likelihood of having porous seals and breakdowns in the repair deposit.

Present electrodepositing processes are also unsatisfactory because they often cannot be run at a constant driving voltage. Voltage must be adjusted to suit the particular electrodeposition properties and stability characteristics of each of the variety of batches of electrocoating materials used, so that a set voltage will cause the same amount of resinous material to deposit on repair sites of each substrate run through the batches.

When electrocoating materials are formulated and produced, whether in concentrate or bath form, they must necessarily be manufactured so that they are stable and do not, for example, precipitate out during handling, shipment and storage. However, this handling and storage stability characteristic of the materials is often in conflict with the instability characteristic required to allow rapid and heavy electrocoating deposition adjacent the repair site of the substrate. The balance obtained between storage and shipping stability and rapid and heavy deposition instability has resulted in inconsistent stability characteristics between various batches of the same electrocoating material because batches are manufactured and transported at different times and under slightly different conditions. In view of the above, under electrocoating operating conditions, it has heretofore been difficult and often not possible to attain a steady, controlled instability characteristic common to different batches of electrocoating materials used. For these reasons, at point of use, it has been difficult and often not possible to run the electrodeposition processes at a constant driving voltage.

Another disadvantage of present electrocoating processes is that electrocoating material mediums used therein do not possess sufficient throwing power. That is, they do not have the power or ability to throw or apply coating material to all areas of the substrate electrode, so that, in terms of relative distance from the other electrode, areas like cracks, crevices and seams that may be remote from the other electrode, coat as well as areas of the substrate closer to the electrode.

Still another disadvantage of present electrocoating processes is that too many of the ionic groups of resinous coating particles of the electrocoating medium that should be neutralized by hydrogen cations, are neutralized by ferric or ferrous cations formed by dissolution of the anodic metal can component.

The method of repair coating electrically conductive areas of metal substrates by contacting the electrocoating material with a weak electrolyte according to the improved method of this invention, is advantageous because it overcomes all of the aforementioned disadvantages by depositing a heavy enough amount of resinous coating particulate material adjacent a repair site to lap over and seal the discontinuity in a manner that does not allow corrosion through the seal or product contact with or current flow to the underlying metal substrate surface. The heavy deposit is effected at a rate greater than present electrocoating processes and fast enough for high speed can manufacturing operations. The heavy deposit can be effected with less voltage and gassing than present processes and voltage can be dept constant despite the use of a variety of electrocoating material batches initially having varying degrees of instability. Lastly, the improved method greatly increases the throwing power of electrocoating mediums and assures that more ionic groups of the carboxylic acid resin, for example, will be neutralized by hydrogen ions than under the present electrocoating processes, less will be available for neutralization by metallic ions, say cations from anodic dissolution, and more resinous coating material will be available for deposition adjacent repair sites.

BRIEF SUMMARY OF THE INVENTION This invention is an improved method of repair coating a metal substrate, which can be an open-ended can or a can component and which has a base coat of material thereon, by passing an electric current through a circuit which includes an organic polyelectrolytic electrocoating material medium containing organic polyelectrolytic particles, the medium being in contact with an electrically-conductive metal anode and an electrically-conductive cathode, and the current being sufficient to cause the particles to deposit upon, repair coat and seal any electrically-conductive raw edges breaks, scratches or other discontinuities in the base coat of the metal substrate. The repair coating method can also include, after the passing of current through the medium, the steps of removing any residual droplets of the medium from the repaircoated metal substrate, and heating the substrate to cure the repair coatmg.

The improvement comprises contacting the medium with a weak electrolyte before passing the current through the medium. The weak electrolyte has a dissociation constant not greater than about 1 X 10" measured at 25C, and is carbonic acid for treating an anodic electrocoating medium, or a weak base for treating a cathodic electrocoating medium, the eelctrolyte being provided in an amount and at a rate sufficient to non-precipitatingly electrically partially neutralize some of the unneutralized ionically charged sites.

of the organic polyelectrolytic particles and thereby increase the conductivity of the medium and/or increase the size of the particles in the medium such that when the current is passed through the medium, more of the organic particles deposit upon, repair coat and more effectively seal the raw edges and discontinuities of the base coat of the metal substrate in less time than if the weak electrolyte were not applied to the electrocoating medium.

The weak acid which can be employed for treating anodic electrocoating mediums is carbonic, preferably obtained by contacting the medium with carbon dioxide gas, although it can also be obtained by contacting the medium with dry ice or an aqueous solution of carbon dioxide. Examples of weak bases which can be employed for treating cathodic electrocoating mediums are ammonia, ammonium hydroxide, triethylamine, and trimethylamine. The preferred weak base is triethylamine.

The metal substrate to be coated can be anodic or cathodic. The organic polyelectrolytic electrocoating material medium can be a concentrate containing from about 20 to 50 percent solids and the concentrate can be reduced to below about 16 percent solids, based on the medium, in a manner that does not change the charges of the ionic groups of the organic polyelectrolytic particles or substantially directly affect the polyelectrolytic behavior'of the particles. The reducing can be effected by contacting the medium concentrate with a solvent consisting of deionized water. The anodic organic polyelectrolytic electrocoating material medium preferably is a polycarboxylic acid resin and the anodic medium preferably is a polyamide.

DETAILED DESCRIPTION OF THE INVENTION electrically conductive metal anode and an electrically conductive cathode. Electrocoating mediums can be either anodic or cathodic. When the electrocoating medium is anodic, the metal substrate is the anode. The metal substrate or anode can be an open-ended metal can or can component. When current is induced to flow through the circuit by an electromotive force or voltage, some of the organic particles in the medium deposit upon and repair coat any electrically conductive raw edges, breaks, scratches or other discontinuities in the base coat of the anodic metal substrate. When the repair coat is heavy enough, the discontinuities are fully and properly sealed.

The improvement of this invention comprises contacting the organic electrocoating polyelectrolytic material medium with a weak electrolyte before passing current through the medium.

The weak electrolyte which can be used to contact the medium according to the improved method of this invention has dissociation constant not greater than about 1 X 10" measured at 25C. When the medium is anodic, the weak electrolyte utilized is carbonic acid.

When the medium is cathodic, the weak electrolyte utilized is usually a weak base such as ammonia, ammonium hydroxide, trimethylamine or triethylamine. The preferred weak base is triethylamine. Less preferably utilizable as weak electrolytes according to the improved method of this invention are those salts which are only partially ionized in an aqueous solution.

The manner in which the weak electrolyte is brought into contact with the organic polyelectrolytic electrocoating material medium can be by any suitable means. Most of the aforementioned weak acids and weak bases can, when possible, be added or introduced directly into the medium in their gaseous, liquid, solid or glacial states. When the weak electrolyte is an acid or base which, due to its properties, cannot per se be added to the medium, the acid or base can be brought into contact with the medium indirectly as a reaction product obtained by first contacting the medium with a substance which when reacted with the medium and/or its constituents will produce the desired weak acid or base. For example, carbonic acid is a weak electrolyte utilizable according to the method of this invention but it is unstable and per se has not been isolated as such. Therefore, it cannot simply be added per se to the medium. Rather, it must be obtained by first contacting the medium with a substance such as carbon dioxide which for example, when it reacts with say any water in the medium, will produce the desired carbonic acid. Thus, the phrase weak electrolyte presently especially meaning weak acid as utilized in relation to the improved method of this invention, is defined as including substances such as for example carbon dioxide which will produce in the medium weak acids such as carbonic acid that are unstable and not per se otherwise isolatable. According to the improved method of this invention, the method of contacting carbonic acid with the medium is to first contact the medium with carbon dioxide gas, with an aqueous solution of carbon dioxide or with dry ice or some other suitable substance which will produce carbonic acid in the medium. The preferred method is to inject or bubble carbon dioxide gas into the electrocoating medium.

The method of contacting ammonia with the medium preferably is to dissolve the gas in water and to bring the aqueous solution into contact with the medium.

The weak electrolytes, when not injected or bubbled into the medium in their gaseous state, are added slowly during rapid agitation of the medium.

Generally, speaking, when the weak electrolyte brought into contact with an aqueous organic electrocoating medium is a weak acid, one theory of what occurs is that the weak acid ionizes into hydronium or hydrogen cations and anions. When the electrolyte is a weak base, it ionizes into say for example amine cations and hydroxide anions. in the case of carbonic acid in contact with the medium, hydrogen cations from the acid are believed to ultimately become attached to some of the ionically oppositely charged carboxyl groupings on the carboxylic resin particle or to other ionized groupings on other polyelectrolytic organic particles which exist in the organic polyelectrolytic electrocoating material medium. When this occurs, it is thought that the particles to which the hydrogen cations attach become less negatively charged than the more negatively charged particles to which less or none of the hydrogen cations become attached. The effect of the above in most electrocoating mediums is that the less negatively charged particles tend to repel one another less. Reduction of these mutually repelling tendencies results in a tendency for these particles to agglomerate or pre-agglomerate in the electrocoating medium. When this pre-agglomeration occurs, there is an increase in size of some particles in the medium.

Contacting a weak electrolyte with an electrocoating medium according to the improved method of this invention increases the conductivity of the medium. Conductivity increases in the case of anodic baths treated with weak electrolyte due to the formation of salts in the medium. The increased conductivity helps achieve rapid deposition. Contacting a medium with a weak electrolyte can also cause pre-agglomeration of and increase in size of the polyelectrolytic coating particles of the medium. By way of example, the increase in particle size is shown in Table l which is a particle size distribution profile of the organic polyelectrolytic particles present in each 2 percent solids organic polyelectrolytic electrocoating medium bath tested.

treated with carbon dioxide gas The electrocoating medium or bath designated A is a Clear Electrocoat Starting Material made by SCM Corporation, Glidden Coatings and Resins, purchased under the designation EXM 691 15. Baths B and C are resinous polymeric electrocoating mediums made by Pittsburg Plate Glass Industries, Inc. purchased under the respective designations C5568 and C5561, both mediums being aqueous and believed to be comprised of butyl acrylate-styrene-methacrylic acidhydroxyethyl methacrylate copolymers. All baths were diluted with 100 parts pI-I-adjusted deionized water to every one part medium. Table I indicates that when an untreated 2 percent solids bath of each of the aforementioned electrocoating mediums was treated by injecting carbon dioxide gas thereinto until conductivity of the particular bath reached the shown desired value, many immeasurable (beyond the range of the instrument used) small particles below the 2 micron size agglomerated into larger sized particles and caused a significant increase in the number of larger particles, especially those in the 2-4 micron range. The machine used to count and categorize the particles by size was a HlAC Automatic Particle Counter, Model PC-203- S5. The sample baths were pumped through the instrument at 15 ml/min.

Once the weak electrolyte, say carbonic acid, is brought into contact with an anodic electrocoating medium, and pre-agglomeration has occured, current is passed through the medium. The pre-agglomerated particles now possessing less charges that have to be neutralized in order to precipitate from the medium and still possessing an overall negative charge, migrate toward electricallyconductive areas of the anode or metal substrate where the particles pick up positive charges, become substantially neutralized and deposit upon and repair coat the electrically-conductive areas of the anodic metal substrate in less time and with less voltage than when conventional non-pre-agglomerating electrocoating systems are employed. Because the particles that are deposited are large, they do not stack tightly adjacent the repair site and therefore current flow gradually diminishes rather than immediately stops when the site is sealed. This gradual diminishment of current allows more particles to accumulate adjacent the sites and to stack high, over a wide area adjacent the site. This obtains an effective seal because the build-up reflows over a wide area surrounding the discontinuity when the repair coated metal substrate is later cured at conventional curing temperatures. The high film build-up is deposited in little time because the bath is more conductive and the large preagglomerated particles have fewer ionic sites left and therefore require relatively less current to neutralize and deposit them adjacent a repair site. Since less current is needed, and current is more easily carried in the more conductive bath, ample deposits are obtained even for only slight discontinuities in the base coat which allow very little current flow therethrough.

When the metal substrate is baked to cure the repair coat deposit, anions of the coating particles, in the case of anodic mediums treated with a weak acid, and cations of the particles, in the case of cathodic mediums treated with a weak base, are released and do not significantly affect the taste, odor, flexibility and water sensitivity characteristics of the repair coat.

That the improved method of this invention obtains better sealing repair coats and less metal corrosion with less operating voltages and/or times is shown in Table 11 wherein data is compiled for test packs made on tinfree steel can cylinders having lap seams cemented with a polymeric material and having a tin free steel end doubleseamed to the can cylinder. The interior of the open-ended can was roller-coated with single layer of a conventional epoxy, phenolic, or butadiene type of base coat and then discontinuities in the base coat were electrocoat repaired by pouring a carbon dioxidetreated electrocoat bath into each can. The two baths B and C were of the type used for Table I, each bath used in Table 11 having a different ratio of resin components. A v inch steel bar electrode was inserted into the can bath and a direct current potential (DC voltage) was applied for a certain period of time. The cans were emptied, rinsed with de-ionized water, dried and their repair coats baked for (5) minutes in a oven at 325F. One set of cans was packed with beer and the other with lemon-lime carbonated beverage test products, and carbon dioxide gas flow doubleseal closures were effected on the cans. The beer cans were stored at F and the lemon-lime cans at 98F for the times indicated, after which the products were analyzed for iron pickup, i.e., the amount or parts of ferrous or ferric ions present per million parts product. lron pickup results from corrosion attack by the product on the anodic metal substrate. As a result of anodic dissolution effects, thiscorrosion of the substrate at the required sites is increased by the use of higher voltages and/or longer electrodeposition times used in the repair coating process. The data in Table 11 illustrates this effect and shows the corresponding benefits of reducing voltage and time for the repair deposition.

TABLE I1 Volts/Time Average lron Analysis (ppm) (sec.) Beer (3 mo.) Lemon (1 mo.)

Electrocoat B 300 1.0 18 1.01 B 300 2.5 .29 2.11 B 300 e 3.5 .33 2.29 C 150 1.0 19 1.56 C 300 1.0 23 1.61 C 300 1.5 .26 2.69

The amount of voltage employed to electrodeposit a repair coat from an organic polyelectrolytic electrocoating medium treated with a weak electrolyte according to the improved method of this invention preferably is the least amount which will give a satisfactory repair coat in the time desired for the particular metal substrate, electrocoating medium and weak'electrolyte employed. Typical voltages utilized in electrocoating processes are in the range of from about 50 to 500 volts, more commonly from about 100 to 350 volts and still more commonly from about 100 to 200 volts. Table II and Examples I through IV (hereinafter) indicates typical voltages which can be used to satisfactorily repair coat particular base coated metal can substrates with particles from electrocoating mediums treated with weak electrolytes. The Examples also indicate that less voltage is required to satisfactorily repair coat a metal can substrate with a treated medium than with an untreated one.

The amount of weak electrolyte brought into contact with the polyelectrolytic organic electrocoating medium to obtain a rapid, satisfactorily sealing, high film build repair coat will vary depending among other things upon the metal substrate being coated, on the type of weak electrolyte introduced and on the coating and stability characteristics of the medium contacted. The amount or rate of a particular weak electrolyte required for a given medium can be determined empirically by ascertaining the conductivity or resistivity, the pH, or both of the particular electrocoating medium that is used when a certain added amountof weak electrolyte obtains a satisfactory repair coat. Once this amount is determined, it can be established for use on a commercial production basis and pH or resistivity of the medium can be monitored periodically or continuously by any suitable means such as a pH meter or more preferably with a conductivity bridge to assure that the right amount of weak electrolyte is always being employed. For example, for aqueous butyl acrylate-styrene-methacrylic acid-hydroxyethyl methacrylate copolymer electrocoating mediums having an untreated resistivity of from about 8,000 to 12,000 ohm/cm and containing no more than weight percent of total polymer units derived from methacrylic acid and containing no more than 7 weight percent of total polymer units derived from hydroxyethyl methacrylate, the amount of carbon dioxide needed to be injected into the medium to obtain acceptable quick tests and iron pickup is that amount requisite to obtain a resistivity preferably between from about 3,000 to 5,000 ohm/cm. Thus, the amount of weak electrolyte utilized in a medium is that amount of required to obtain a rapid, high build-up repair deposit that provides an effective seal.

The adequacy of the film build-up and effectiveness of the repair coat seal can be determined by a conventional quick test and by measuring the-iron pickup of the product contained in a metal substrate can. A typical quick test is to pour an aqueous 1 percent sodium chloride solution into an open-ended tin-free steel cylindrical can having its lap seam cemented with a polymeric material, its closed end sealed with a steel end closure and its interior roller coated with a single layer of a conventional lacquer. An electrode is placed in the solution and a 6 volt potential is induced in the circuit. The current flowing through the circuit, measured in milliamperes, determines the effectiveness of the repair coat. A perfect repair coat coverage is obtained when there is no current flow. As explained previously, an iron pickup test determines over a period of time the number of parts of ferrous or ferric metal ions dissolved into or picked up by a million parts of product contained in a metal substrate container that has been repair coated and that is still allowing some minute current flow to and corrosion of the metal substrate.

When carbon dioxide is used to bring carbonic acid into contact with a medium, it is preferable and advantageous to use an excess thereof to assure maximum preagglomeration of the organic polyelectrocoating particles in the electrocoating medium. An excess assures that the repair deposit will be the most rapid and the heaviest possible for a given voltage. Use of an excess of carbon dioxide is error-proof because there is no danger that the particles will pre-agglomerate to the point where they will precipitate out of the medium. Any excess carbon dioxide evolves from the medium as a gas.

Weak electrolytes other than carbonic acid are not error-proof because excess amounts can cause preagglomeration to the point where polyelectrolytic organic particles undesirably precipitate from the medium. For weak electroytes other than carbonic acid, it is desirable that the amount employed be less than that amount which causes precipitation of the particles. As a rule, the amount of weak electrolyte introduced can be up to that amount which so neutralizes the organic electrolytic particles of the medium that the medium no longer acts as a polyelectrolyte.

Polyelectrolytic electrocoating material mediums which can be treated with a weak electrolyte according to the method of this invention can be any of the organic resin-containing materials utilizable as electro coating concentrates or baths in metal electrocoating systems. The medium can be non-aqueous but preferably are aqueous and they can be modified, extended, and stabilized with solubilizers or other materials. Examples of aqueous mediums which can be employed are those disclosed in US. Pat. No. 3,230,162 issued to A. E. Gilchrist on Jan. 18, 1966. Disclosed therein are numerous concentrate compositions generally comprising about 50 to percent by weight polycarboxylic acid resin, about 1 to 10 percent water soluble amino compound and the balance water. The polycarboxylic acid resins are film-forming at electrodeposition bath temperatures, and are curable to a tack-free film. The resins may include coupled siccative oils such as glyceride drying oil treated with an olefinic coupling agent such as maleic anhydride or fumaric acid. The resins may also include acidic hydrocarbon drying oil polymers such as those made from maleinized copolymers of butadiene and diisobutylene; diphenolic acid and like polymer resins; and acrylic and vinyl polymers and copolymers having carboxylic acid groups such as butyl acrylate-methyl methacrylate-methacrylic acid copolymers, vinyl acetate-acrylic acid copolymers, acrylic acid and lower alkyl (C substituted acrylic acid-containing polymers.

Gilchrist discloses that when the polycarboxylic resin dispersion concentration in a bath is between from about l/2 to 1 percent to a practical maximum concentration of about 35 to 40 percent by weight, best film deposition results are obtained. Conventional oxygen sequestering agents can be added to the vehicle in the bath. Scavenger polybasic acids have a molecular weight approaching 1,000, usually in the region of about 500 to 800, can also be added to the bath to prevent it from accumulating an excess of amino. An example is the dimer of linoleic acid. Such polybasic acids apparently form soaps which are codeposited with the resins in the resulting film.

The amino compounds which Gilchrist states can be employed with anodic baths include ethanolamine, triethanolamine, diethylene triarnine and diethylamine.

Other aqueous polyelectrolytic electrocoating material mediums which can be treated with a weak electrolyte according to the improved method of this invention are disclosed in US. Pat. No. 3,366,563 issued to Hart on Jan. 30, 1968. Generally, Hart discloses an aqueous electrocoating bath containing a solubilized vehicle resin which comprises the reaction product of a drying oil fatty acid ester or a semi-drying oil fatty acid ester with an alpha, beta-ethylenically unsaturated dicarboxylic acid or an anhydride of such an acid.

Harts esters of fatty acids are those which can be derived from drying or semi-drying oils such as linseed oil, tall oil esters and alkyd resins perpared by using drying oils. Examples of the unsaturated dicarboxylic acid reagent of anhydrides thereof are maleic acid, fumaric acid and maleic anhydride. The bath may also include any ethylenically unsaturated monomer such as for example styrene, vinyl acetate, methyl acrylate, ethyl methacrylate and acrylonitrile.

Amines can be employed to neutralize the acidity of l-larts reaction product. Examples of such amines are: ethylamine, ethanol amine, aniline and pyrrolidine. In addition, Hart coating compositions can include pigments in ratio to the vehicle of not higher than 1.5 to l, and can include antioxidants, wetting agents, driers, foaming agents, suspending agents and bactericides.

Examples of non-aqueous polyelectrolytic electrocoating material mediums which can be employed according to the improved method of this invention are disclosed in US. Pat. No. 3,463,714 issued to W. D. Suomi and A. R. Ravve on Aug. 26, 1969. Disclosed therein are electrocoating baths prepared generally by dissolving a carboxyl-containing polymer and a basic nitrogen-containing compound in an organic solvent and adding a sufficient amount of a polar organic nonsolvent having a solubility parameter greater than 12 and a hydrogen bond index greater than 7.5 to convert the solution into a suspension. Suitable carboxylcontaining polymers are polymers or copolymers of a, B-ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, maleic anhydride, crotonic acid, citraconic acid and its anhydride, itaconic acid and its anhydride, chloromaleic acid and the like. Other examples are copolymers of the aforementioned a, B-ethylenically unsaturated carboxylic acids and vinyl monomers such as vinyl chloride, ethylene, styrene, vinyl acetate, methyl and ethyl acrylate, and methacrylates and mixtures thereof.

The basic nitrogen-containing compound disclosed in Suomi et al include: hydrazine, alkyl amines such as methyl amine, ethyl amine, diethyl amine, ethanol amine, heterocyclic amines such as piperidine, morpholine, N-methyl morpholine, polyamines such as melamine and triazine and polymers thereof such as melamine-formaldehyde resins and triazineformaldehyde resins. The organic solvents disclosed in Suomi et al include: methyl ethyl ketones, methyl isobutyl ketone, dimethyl acetamide, acetonitrile, and ethyl acetate. Examples of polar organic solvents disclosed by Suomi et al as utilizable include benzyl alcohol and diethylene glycol.

Methods of making the aforementioned and other disclosed electrocoating mediums are described in the respective aforementioned US. Patents. The polyelectrolytic electrocoating material mediums disclosed therein are merely examples of some of the many aqueous and non-aqueous mediums which are utilizable according to the improved method of this invention.

For example, although the aforementioned disclosed electrocoating mediums are formulated from polycarboxylic acids, it is to be understood that also utilizable as electrocoating mediums are non-polycarboxylic acid resins such as rubber lattice suspended resins adsorbed by hydroxyl ions, and resins formulated for example 12 from phenolics, polyvinyl ethers, cellulosic resins, polyimides and silicones.

Electrocoating mediums utilizable according to the improved method of this invention are usually manufactured and shipped as a concentrate having from about 20 percent to 50 percent solids. Although the mediums employed in the improved method of this invention can be in such a concentrated condition when contacted with the weak electrolyte, it is preferred that the concentrate be reduced so that the medium is a bath having from a measurable percent to about 20 percent solids. Preferably, the medium is from about 1 to 6 percent solids, more preferably from about I to 3 percent solids, especially when the mediums are employed in conjunction with high speed can electrodepositing machines. At high electrodeposition speeds such as 550 cans/min, low percentages of solids mean less drag out or loss of solids as the can is dragged out of the bath of electrocoating medium.

, Reduction of the electrocoating mediums can be effected by any known and suitable means which does not change the charges of the ionic groups of the organic polyelectrolytic particles and does not substantially directly affect the polyelectrolytic behavior of the particles. For example, reduction can be, and for aqueous mediums preferably is effected by adding during agitation, a solvent such as deionized water to the medium, the deionized water being added until the desired percentage of solids is attained. Reduction of the medium is effected before contacting the medium with the weak electrolyte and before current is passed through the medium.

The effect of contacting the aforementioned amino compound-solubilized electrocoating mediums with a weak electrolyte as by first injecting carbon dioxide into the medium is believed to be that nitrogencontaining cations, which were attached to and were neutralizing and stabilizing some of the formerly charged ionic sites of say the carboxylic acid groups of the polycarboxylic acid resin are separated from these acid groups and are themselves neutralized by the carbonate anions. This appears to cause preagglomeration of particles in the medium.

Below are several specific examples which illustrate the manner of carrying out the improved method of this invention. It is to be understood, however, that the examples are for the purpose of illustration, and the invention is not to be regarded as limited to any of the specific materials, techniques or conditions recited therein.

EXAMPLE I An anodic aqueous electrocoating bath was prepared by charging a glass vessel with 76 gms of ethylene acrylic acid copolymer resin containing from about 18 to 20 percent acrylic acid, 350 gms of deionized water and about 8.5 gms monoethanol amine. The resin is manufactured by Union Carbide Corporation and is sold under the designation EAA 9500. The charged vessel was covered to reduce evaporation of the mixture and brought to a bath temperature of about to 200F while providing constant agitation to the bath. Complete dispersion required about 4 hours at about F. The resin as prepared provided a 20 percent solids dispersion with approximately 65 percent neutralization of the resin.

The dispersion was agitated while it was cooled to 100F, and then it was filtered to reduce itto a percent solids bath. The resulting bath had a specific resistivity of 1,000 ohm cm and a pH of 8.5. After injecting carbon dioxide into the bath, it had a specific resistivity of 550 ohm cm and a pH of 8.3.

Throwing power tests were run on untreated and carbon dioxide-treated electrocoating baths made from this resin. A glass tube with an inside diameter of 5 1 inch was immersed a depth of 3 inches into a bath having a depth of approximately 4 inches. A 34 inch wide aluminum foil anode was placed inside the tube, the bottom edge of the foil being even with the tube bottom. A stainless steel cathode was placed parallel to and 2 inches from the anode. Current was supplied at 300 volts for 2 secs. The foil anodes were rinsed, dried at 325F for 5 minutes and immersed in a suitable dye solution to detect the presence of electrodeposited film. The untreated bath coated to a height of 1-54 inches whereas the treated bath coated to a height of 2-l/l6 inches. The maximum coating thickness was 1 mil with the untreated bath whereas with the treated bath it was 2 mils.

EXAMPLE II An anodic aqueous electrocoating bath was prepared by diluting a concentrate designated X1222 to a 2 percent solids solution with deionized water. The X1222 that was diluted was a 22 percent solids concentrate solution of butyl acrylate-styrene-methacrylic acidhydroxyethyl methacrylate and containing no more than 20 weight percent of total polymer units derived from methacrylic and containing no more than 7 weight percent of total polymer units derived from hydroxyethyl methacrylate. The X1222 concentrate is manufactured by Pittsburgh Plate Glass. The resulting bath had a specific resistivity of 9,000 ohm cm and had a pH of 8.7. When the bath was treated with carbon dioxide, the resulting carbon dioxide-treated bath had a specific resistivity of 5,600 ohm cm and had a pH of 8.4. When a weak electrolyte comprising a 0.25 percent solution of formic acid was added very slowly to a portion of the untreated bath during rapid agitation, the formic acid treated bath had a specific resistivity of 5700 ohm cm and a pH of 8.5. Although both treated baths provide satisfactory rapid repair deposition characteristics, the carbon dioxide-treated bath is preferred because, after 24 hours, there was some slight settling of precipitate in the'formic acid-treated bath.

EXAMPLE Ill A cathodic aqueous electrocoating bath was pre pared as follows: 60 gms of an oil-soluble non-heatreactive phenolic resin manufactured by Union Carbide Corporation and sold under the designation CKR 2400, was dissolved with agitation in 20 gms methyl isobutyl ketone, and the mixture was heated to about 190F. Upon dissolution of the phenolic resin, the source of heat was removed and 40 gms of a polyamide resin manufactured by General Mills, Inc. and sold as Versamid 100 and 4.8 gms of glacial acetic acid were added. The mixture was stirred approximately 5 minutes until it was homogeneous. Then, 167 gms of deionized water was added with good agitation. The dispersion was allowed to cool to about 90F and was filtered. Solids in the final dispersion were about 30 to 33 percent.

For electrocoat repairing of cans, the concentrated dispersion was further diluted with deionized water to 2-l/ 2 percent solids. Resistivity of the bath at F was 9,000 ohm cm and its pH was 4.0. The bath was then treated by adding very slowly and with good agitation, 2.75 gms of a 0.8 percent solution of ammonium hydroxide. Resistivity of the treated bath was measured at 5,900 ohm cm and its pH was 4.8.

Whereas before treatment, the bath was incapable of depositing any significant bead of electrocoat repair adjacent a base coat scratch, after treatment, a scratch was readily covered and overlapped with a one second deposition at 200 volts DC.

EXAMPLE IV A non-aqueous electrocoating bath for repair coating cans was prepared by dissolving in 864 gms of butyl acetate, 72 gms of a vinyl chloride-vinyl acetate carboxylcontaining resin manufactured by Union Carbide Corporation and sold as Solution Vinyl VMCH. Upon dissolution of the resin, 12 gms of an activator, monoethanol amine, was added with agitation until the solution was homogeneous and clear. Then, 600 gms of methanol was slowly added, producing a cloudy dispersion. When the dispersed bath contained 4.75 percent solids, its resistivity at 75F was measured at 11,000 ohm cm and its pH was 9.4. The bath was then separated into two parts. Carbon dioxide was bubbled into one part until its resistivity was lowered to 5,500 ohm cm and its pH lowered to 9.05. Stability checks showed that after six months had passed no precipitate had formed.

Both the treated and untreated baths were separately used to repair coat tin-free steel beverage cans of the type previously described as used in the aforementioned Quick Test. Conductivity Quick Tests to evaluate the effectiveness of repair coating coverages of the discontinuities in the enamel base coat on the inside of the cans were conducted by filling the cans with a 1 percent solution of sodium chloride and measuring conductivity of the solutions when a standard 6 volt potential was applied between the can bodies and a steel electrode immersed in the centre of each of the solutions. Quick Test results were that whereas 300 volts were required to repair coat the cans with the untreated bath so that the enamel coverage only allowed a current flow therethrough of about 0.70 ma, only 200 volts were required to repair coat the cans with the carbon dioxide-treated bath to obtain a coverage which allowed a current flow of only 0.22 ma. These results show that the bath treatment with carbon dioxide produced greatly improved repair coverage at a reduced voltage. a

We claim:

1. In a method of repair coating a metal substrate having a base coat of material thereon, which includes passing an electric current through a circuit including a anodic organic polyelectrolytic resin-containing electrocoating material medium in contact with an electrically-conductive metal substrate anode and an electrically-conductive cathode, said medium containing organic polyelectrolytic particles and said current being sufficient to cause said particles to deposit upon, repair coat and seal any electrically conductive raw edges, breaks, scratches or other discontinuities in said base coat of said metal substrate, the improvement which comprises positively admixing said medium with carbonic acid before passing said current through said medium, said carbonic acid having a dissociation constant not greater than about l X 10 at 25C, and supplying said acid in an amount and at a rate sufficient to non-precipitatingly electrically partially neutralize some of the unneutralized ionically charged sites of said organic polyelectrolytic particles and to thereby increase the conductivity of said medium and/or increase the size of said organic polyelectrolytic particles in said medium such that when said current is passed through said medium'more of said organic polyelectrolytic particles deposit upon, repair coat and more effectively seal said raw edges and discontinuities in said base coat of said substrate in less time than if said carbonic acid were not applied to said electrocoating medium.

2. The improved method of claim 1 wherein said metal substrate anode is an open-ended can or can component, and said method of repair coating includes, after said passing of said current through said medium, the steps of removing any residual droplets of said medium from said base coat of said metal substrate and heating said substrate to cure said repair coat.

3. The improved method of claim 2 wherein said medium of polycarboxylic resin-containing and said carbonic acid is obtained by injecting carbon dioxide into said medium.

4. The improved method of claim 1 wherein said metal substrate anode is an open-ended can or can component, said medium is a concentrate containing from about to 50 percent solids and said method of repair coating said metal substrate includes, before positively admixing said medium with said carbonic acid, the step of reducing said medium concentrate to below about 16 percent solids based upon said medium, said reducing being effected in a manner that does not change the charges of the ionic groups of said 16. organic polyelectrolytic particles or substantially directly affect the polyelectrolytic behavior of said particles.

5. The improved method of claim 4 wherein said positive admixing of said carbonic acid is effected by adding dry ice to said medium. 6. The improved method of claim 4 wherein said positive admixing of said carbonic acid is effected by adding an aqueous solution of carbon dioxide to said medium.

7. The improved method of claim 4 wherein said pos- 'itive admixing carbonic acid is effected by injecting carbon dioxide into said medium.

8. The improved method of claim 7 wherein said resin in said medium is polycarboxylic and said reducing of said medium is effected by contacting said medium with a solvent consisting of deionized water.

9. The improved method of claim 1 wherein said positive admixing of said carbonic acid is effected by injecting carbon dioxide into said medium.

10. The improved method of claim 1 wherein said positive admixing of said carbonic acid is effected by adding dry ice to said medium.

11. The improved method of claim 1 wherein said,

positive admixing of said carbonic acid is effected by adding an aqueous solution of carbon dioxide to said medium.

12. The improved method of claim 1 wherein said resin in said medium is polycarboxylic.

13. The improved method of claim 1 wherein said method of repair coating includes, after said passing of said current through said medium, the steps of removing any residual droplets of said medium from said base coat of said metal substrate and heating said substrate to cure said repair coat.

Claims

2. The improved method of claim 1 wherein said metal substrate anode is an open-ended can or can component, and said method of repair coating includes, after said passing of said current through said medium, the steps of removing any residual droplets of said medium from said base coat of said metal substrate and heating said substrate to cure said repair coat.
3. The improved method of claim 2 wherein said medium of polycarboxylic resin-containing and said carbonic acid is obtained by injecting carbon dioxide into said medium.
4. The improved method of claim 1 wherein said metal substrate anode is an open-ended can or can component, said medium is a concentrate containing from about 20 to 50 percent solids and said method of repair coating said metal substrate includes, before positively admixing said medium with said carbonic acid, the step of reducing said medium concentrate to below about 16 percent solids based upon said medium, said reducing being effected in a manner that does not change the charges of the ionic groups of said organic polyelectrolytic particles or substantially directly affect the polyelectrolytic behavior of said particles.
5. The improved method of claim 4 wherein said positive admixing of said carbonic acid is effected by adding dry ice to said medium.
6. The improved method of claim 4 wherein said positive admixing of said carbonic acid is effected by adding an aqueous solution of carbon dioxide to said medium.
7. The improved method of claim 4 wherein said positive admixing carbonic acid is effected by injecting carbon dioxide into said medium.
8. The improved method of cLaim 7 wherein said resin in said medium is polycarboxylic and said reducing of said medium is effected by contacting said medium with a solvent consisting of deionized water.
9. The improved method of claim 1 wherein said positive admixing of said carbonic acid is effected by injecting carbon dioxide into said medium.
10. The improved method of claim 1 wherein said positive admixing of said carbonic acid is effected by adding dry ice to said medium.
11. The improved method of claim 1 wherein said positive admixing of said carbonic acid is effected by adding an aqueous solution of carbon dioxide to said medium.
12. The improved method of claim 1 wherein said resin in said medium is polycarboxylic.
13. The improved method of claim 1 wherein said method of repair coating includes, after said passing of said current through said medium, the steps of removing any residual droplets of said medium from said base coat of said metal substrate and heating said substrate to cure said repair coat.