US3498897A - Method for manufacturing multilayered product - Google Patents

Description

United States Patent 3,498,897 METHOD FOR MANUFACTURING MULTILAYERED PRODUCT Harold N. Bogart, Detroit, George E. F. Brewer, Novi, and Gilbert L. Burnside, Oak Park, Mich., assignors to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed July 25, 1967, Ser. No. 655,912 Int. Cl. C23b 13/00, 5/78; B01k 5/02 U.S. Cl. 204-181 8 Claims ABSTRACT OF THE DISCLOSURE Background of the invention The art of electrophoretic deposition of organic materials such as rubber, wax, polytetrauoroethylene, natural resins, and certain nonionic synthetic polymers from colloidal dispersions of the same is widely described in the literature. More recently, industrial electrodeposition of paint has become a reality through a method of electrodeposition utilizing ionized macromolecules to produce from conductive substrates resinous paint films which exhibit high electrical resistance. The resins most suitable for this purpose are polycarboxylic acid resins, i.e. organic resins having dissociable carboxyl groups in their molecular structure. These are dispersed in an aqueous bath with water soluble amines or similar water soluble amino compounds.

In this method of painting, the workpiece serves as one electrode of an electrodeposition cell and the aqueous dispersion of paint extends between the workpiece and another electrode while a direct current of electrical energy is passed through this dispersion with resultant electrodeposition of a resin film upon the workpiece. This film rapidly achieves high electrical resistance.

In the electrodeposition of a polycarboxylic acid resin, the workpiece serves as the anode of the electrodeposition cell and the noncoating electrode serves as the cathode. A number of such resins are described by Gilchrist in U.S. Patent 3,230,162 and by Huggard in U.S. Patent 3,297,- 557 which patents are incorporated herein by reference.

Ordinarily, electrodeposition of coating materials as hereinbefore and hereinafter described is effected by immersing the workpiece in an aqueous coating bath. Such deposition can also be effected by passing a continuous stream of the dispersion in contact with an electrode and simultaneously in contact with the workpiece while providing a sufficient difference of electrical potential between the electrode and the workpiece to effect electrodeposition of a water-insoluble film of such coating material from the stream onto the workpiece.

The depth of such film can be varied by varying the effective potential applied with resultant variance of current density at the workpiece surface, the residence coating time, the formulation of the coating bath, etc. The upper depth limit of such deposit is dependent upon the composition of the paint binder employed since electrical rupture characteristics of the binder will limit the difference of electrical potential that can be advantageously employed between the electrodes in the given bath arrangement. The high electrical resistance developed by the 3,498,897 Patented Mar. 3, 1970 electrodeposited film promotes coating of shielded areas and areas less easily coated as a result of location, etc., such deposition tending to be self-leveling. This film resistance also limits coating depth at a given constant voltage under any given electrode arrangement.

lChemical plating of surfaces with a thin coating of metal, e.g. nickel, copper, silver, gold, tin, cobalt, etc., is an established art. The term chemical plating as employed in this art and in this specification refers to the chemical reduction of ions of the plating element to elemental form upon the catalytic surface to be plated. This method is most Widely used and is best known with respect to nickel plating. Primarily, it is this embodiment that will be used for illustration in -this specification. The chemical plating of nickel upon a catalytic surface from an aqueous bath is based upon the catalytic reduction of nickel cations to metallic nickel and the corresponding oxidation of hypophosphite anions to phosphite anions with the evolution of hydrogen gas at the catalytic surface. This reaction is lcatalytic and autocatalytic, its initiation usually being accomplished by way of the catalytic effect of the article being plated and its continuance being catalyzed by the nickel which is deposited. The literature discloses iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum as catalytic for these reactions. The following U.S. patents are illustrative -of this plating process: 2,501,737; 2,532,283; 2,532,284;

3,268,353; and 3,282,723.

If the surface to be plated is not catalytic, it can be rendered catalytic. Thus, this method of plating has been extended to the plating of nonconductive and noncatalytic substrates, e.g. plastics, hard rubber, ceramics, etc., by implanting catalytic growth nuclei upon the surface to be plated. These nuclei catalyze the initial reduction of nickel from the plating bath and the nickel thus deposited catalyzes further reduction of nickel cations to complete the plating of the surface. The following U.S. patents are illustrative of chemical plating of nonconductive and noncatalytic substrates: 2,690,401; 2,690,402; 2,690,403; 2,848,359; 2,996,408; 3,015,014; 3,075,855; 3,075,856; 3,095,309; 3,179,575; and 3,186,863.

In the plating of nonconductive and noncatalytic substrates, the usual procedure is to first freshen the surface to be plated. The literature discloses a variety of methods for this including bufiing, sanding, chemical etching, etc. This is followed by planting the catalytic nuclei, as for example, by reducing palladium ions from a dilute aqueous solution of a palladium Salt, e.g. the chloride, nitrate, bromide, or sulfate, to palladium metal upon the surface to be plated. This is effected by immersing the surface to be plated in an aqueous solution of the salt of the catalytic metal, rinsing, and subsequently irnmersing the surface in a reducing solution. Reducing solutions disclosed in the literature include aqueous solutions of hypophosphorous acid, hydrazine hydrochloride, hydroquinone, etc. IIn a variation of this method, the surface is first immersed in an aqueous solution of stannous chloride, rinsed and subsequently immersed in the seeding solution. Other methods include forming the nonconductive and noncatalytic body with particles of the catalyst imbedded therein so as to project at the surface or to mechanically impregnate the surface with catalyst after forming.

The plating of metallic surfaces with nickel and other metals by electrodeposition is also well known to the art. The electrolytic process is reversible :and the amount of nickel, silver, copper or other metal removed from the plate metal-source electrode can be readily calculated when the quantitative ow of electrical energy between such electrode and the electrode to be plated is known.

Summary of the invention The present invention springs from the surprising discovery that anodic deposition of paint can be made upon a metal lm covering a rst layer of paint upon a metal substrate Without removing such lm or interrupting its continuity through lo'ss of metal from the film via cathodic deposition. This is particularly surprising when one considers that the number of coulombs necessary to effect such anodic deposition of paint is in excess of, normally 2 to 4 times as great as, the number of coulombs calculable to be necessary to cathodically deposit a quantity of the Vmetal of equal weight to such layer. The phenomena permitting such deposition is not fully understood.

Thus, in :accordance kwith the method of this invention, a lmetal substrate is iirst painted, the paint iilm applied to the workpiece is cured thereon, e.g. polymerized by baking or other conventional curing means, a thin layer of metal is chemically plated upon the exposed surface of the paint, and a second coat of paint is anodically deposited upon said layer of metal from an aqueous dispersion of paint which serves as the electrolyte of an electrodeposition cell in which such layer of metal serves as an anode.

The paint :applied immediately to the substrate may be applied by conventional means such as spraying, ow coating, roller coating, etc., when the workpiece is of simple form with no recessed or otherwise shielded areas which cannot be reached by conventional painting methods. When the workpiece is of complex conliguration having recessed and/or otherwise shielded areas, this paint is electrodeposited from an aqueous bath in the same manner as the paint applied to the aforementioned layer of metal, the unique characteristics of the electrodeposition process being singularly :adapted to afford complete coverage of all surfaces. In the preferred embodiment, the predominant fraction of the hlm-forming paint binder of each of such paints is a synthetic polycarboxylic acid resin having an acid number of at least about 10, preferably above about 30. In the coating bath, this synthetic polycarboxylic acid resin is at least partially neutralized with a sutcient quantity of water soluble amino compound to maintain said polycarboxylic acid resin as a dispersion of fanionic polyelectrolyte in said bath.

In accordance with the method of this invention, the coating of paint applied immediately to the substrate and the coating of paint anodically deposited over the layer of metal applied by chemical plating each have an average thickness in excess of about 0.3 ml (0.0003 inch), advantageously in the range of about 0.5 to about 1.5 mils, and preferably in the range of about 0.7 to about 1.25 mils. The electrical resistance of such films after polymerization is above about 1 million ohm-cm., preferably above about 10 million ohm-cm.

In the preferred embodiment, the metal chemically plated over the painted workpiece is nickel. However, it may be silver, copper or other chemically platable metal. The metal lm chemically plated upon the painted workpiece need only be continuous and of sufficient thickness to provide good electrical conductivity. Ordinarily, this layer of metal will be deposited to an average thickness in the range of about 0.001 to about `0.01 mil, preferably in the range of about 0.001 to about 0.005 mil. For the purposes of this invention, such layer advantageously does not substantially exceed about .O3 mil.

Brief description of the drawing FIGURE 1 is a block type ilow diagram illustrating the method steps carried out in a preferred embodiment of this invention; and

FIGURE 2 is a partially sectional, fragmentary view Description of the preferred embodiment Referring now to FIGURE 1, at the irst electro-coating station, a metal object such as an automobile body is immersed in an aqueous dispersion of paint which also comprises the electrolyte of an electrodeposition cell. The metal workpiece is electrically connected with the coating circuit of such cell while immersed in said dispersion so as to serve .as the anode of such cell. The aqueous coating dispersion comprising the coating bath is prepared in the follow manner:

An extended coupled glyceride drying oil paint binder is made by reacting in an agitator `tank 8,467 parts of alkali-refined linseed oil and 2,025 parts of maleic anhydride (heated together at 232.2 C. for about three hours until an acid value of -90 results), then cooling this intermediate to 157.2 C., adding 1,789 parts of vinyl toluene containing 48 parts of ditertiary butyl peroxide and reacting at 218.3 C. for about an hour. The resulting vinyl toluenated material is then cooled to 157.2 C. and 5,294 parts of non-heat reactive, thermoplastic, oil-soluble phenolic resin is added, the temperature raised to 232.2 C. and the mixture held one hour. The phenolic resin is a solid lump resin having a softening point of 1Z0-150 C., specific gravity of 1.03-1.05 at 20 C., and has been stripped to get out excess phenol and low molecular weight materials. It is a condensation product of about equimolar quantities of para tertiary butyl phenol and formaldehyde. The electrical equivalent weight of the resulting acid resin as extended is about 1,640, and it has acid number of 65. A method for determining electrical equivalent weight is described in detail in the aforementioned Gilchrist patent.

The material then is cooled to 93.3 C., and 1,140 parts are taken for forming a paint dispersion. To these 1,140 parts, parts of water are added, then 13.6 parts of triethylamine, the mixture agitated for a few minutes, then 74 more parts of water and 92.5 parts diisopropanol amine added. This mixture is further reduced with 1,825 parts water and 32.5 parts diethylene triamine while agitation is continued.

To this paint dispersion there is added 50 parts of a treating mixture of Imineral spirits, a light hydrocarbon liquid having A.P.I. gravity of 45-49.5, specific gravity at 15.6 C. of 078-080, flash point (Cleveland Open Cup) between 37.8-46 C., a negative doctor ytest and no acidity, 12 parts of a wetting agent (the oleic ester of sarcosine, having a maximum of 2% free fatty acid, a specic gravity of 0.948, color on the Gardner scale of 6, and a molecular weight of 340-350). This material is compatible with the paint dispersion; no distinct hydrocarbon phase results either at this time, even though a substantial amount of hydrocarbon (predominantly aliphatic) has been used, nor after further addition of the pigment grind and addition of extra Water to -make the initial painting bath.

A pigment grind is ymade from 123 parts of vinyl toluenated, maleic-coupled linseed oil made :in the same manner as the resin hereinbefore shown (except that the resulting polycarboxylic acid resin is not extended with the phenolic resin), 8.4 parts of diisopropanol amine, 0.7 part of an antifoam agent (a ditertiary acetylenic glycol with methyl and isopropyl substitution on the tertiary carbon atoms), 233 parts of line kaolin clay, parts of pigmentary titanium dioxide, 7.8 parts of ne lead chromate, 15.5 parts of ne red iron oxide, 16.9 parts of carbon black, and 201 parts of water. The resulting pigment grind is then blended with the foregoing paint dispersion and treating mixture to make a concentrated paint. The resulting paint is reduced further with water in the ratio of one part of the resulting paint per 5 parts of water to make an initial painting bath for electropainting operations. The resulting bath has resin solids (non-volatile matter) concentration of 7.24%. The total of amine equivalents used in making up the initial bath is about 4.5 times the minimum amount necessary to keep this polycarboxylie acid resin, once dispersed, in anionic polyelectrolyte condition in the bath and about 1.25 Atimes full neutralization of the acid resin with respect to its acid number. The number of coulombs of direct current used to electrodeposit a gra-m of this resin on an anode at minimum amine concentration in the bath to develop requisite polyelectrolyte characteristics for this coating process is virtually constant at 24. Specific resistance of the initial bath is about 900 ohm-centimeters.

The replace-ment paint solids are made by dispersing 1,140 parts of the same kind of extended polycarboxylic acid resin with 100 parts of water and 13.6 parts of triethylamine. To this is added the mineral spirits, the wetting agent, and the foregoing pigment grind, all of the same compositions and in proportions as are used to make lup the Original paint dispersion for the bath.

The tank containing the coating bath serves as Vthe cathode of this cell. A difference of electrical potential in the range of about 50 to about 1,000, preferably about 100 to about 300, volts is maintained between the automobile body-anode and the cathode and a direct current of elec- -trical energy is thereby initiated through the bath between anode and cathode with resulting anodic deposition of paint from the bath upon all surfaces of the automobile body. The automobile body-anode is immersed in the bath until a paint film having an average thickness of about 1 mil is deposited thereon. It is then removed, rinsed with water `which may include a detergent.

The painted body is then passed to the first curing station where this coating is polymerized in air maintained at a temperature of about 400 F. for about 25 minutes.

The painted automobile body is next immersed in an acid etching bath for about 2 minutes. This bath is maintained at a temperature of about 135 F. and comprises water, potassium dichromate and sulfuric acid. The quantitative ratio of the components used in making up this bath are about 37.5 grams of potassium dichromate and about 2500 cc. of concentrated sulfuric acid (66 Bau-m) to 5,000 cc. water. The etched body is then removed from the etching bath and rinsed thoroughly with water. It is then immersed in an aqueous bath comprising a 5% sodium hydroxide solution for about seconds at about 75 F. to neutralize any remaining etchant upon the surface of the workpiece. The body is again rinsed with water.

The etched body is next passed to the seeding station. Here, it is first immersed in a reducing bath comprising an aqueous solution of stannous chloride (20 grams per liter). This bath is maintained at about 75 F. and the body is in residence for about 2 minutes. The workpiece is removed from this bath and again rinsed with water. The workpiece is next immersed in a seeding bath. This bath comprises water, hydrochloric acid and palladium chloride. Palladium ions from Vthis solution are reduced to palladium at a plurality of sites upon the etched paint surface which retains minute amounts of stannous chloride ions. The quantitative ratio of the components used in making up this bath are about 10 cc. concentrated hydrochloric acid (B7-38%) and 0.75 gram of palladium chloride to about 3775 cc. of water. This bath is maintained at room temperature. The workpiece is again thoroughly rinsed with water.

The palladium-seeded body is then moved to the chemical plating tank and immersed therein for about 5 minutes. This bath is maintained at about 190 F. and comprises an aqueous solution of nickel sulfate, sodium citrate, sodium acetate, sodium hypophosphite, and magnesium sulfate. The quantitative ratio of the components used in making up this bath are about 35 grams/liter nickel sulfate, 10 grams/liter sodium citrate, 10 grams/ liter sodium acetate, grams/liter sodium hypophosphite, and grams/liter magnesium sulfate. About 10` cc. of a wetting agent, sodium lauryl sulfate, is advantageously 6 added to this bath. The body is then removed from the chemical plating bath with a continuous nickel film covering the continuous coating of paint which forms the immediate covering of the workpiece. It is then rinsed with water.

The nickel coated workpiece is then moved to the second electrocoating station where a polycarboxylic acid comprising paint is electrodeposited thereon in the same manner hereinbefore described for anodically depositing the first coating upon the workpiece. The coating dispersion used for this coating step is prepared in the following manner:

Thirty-one pounds of red iron oxide and fifty-one pounds of lead silico chromate are dispersed in a pebble mill with a mixture of 1% gallons 0f water and 4 gallons of a water reducible Vehicle prepared in the following manner:

A mixture of 878 parts tung oil and 203 parts fumarie acid are heated to 420 F. and held at this temperature for ten minutes and then cooled to 250 F. The adduct product is clear. A cut in xylene at 65% non-volatile resin solids has a viscosity of U-V (Gardner-Holdt) and an acid number of 158.4. The resin is reduced to 40% non-volatile resin solids in a blend of 79% (by weight) water, 15% ammonium hydroxide (28%), and 6% ethanol. The viscosity is X-Y (Gardner-Holdt).

The resulting paste is then reduced with an additional 331/3 gallons of the above described water reducible vehicle. To the resulting pigment-resin mixture is added, under agitation, an oil soluble partially esterified styrenated-allyl alcohol copolymer resin solution which consists of 40.3% styrene-allyl alcohol copolymer, 14.2% nonheat-hardening phenol f-omaldehyde, 11.4% rosin, 25.6% tall oil fatty acid (4% rosin acid), and 8.5% oiticica oil processed to an acid value of 44 to 46 and reduced to 72.5% non-volatile resin solids in a mixture of aromatic hydrocarbon solvents having a boiling range of about 375-410 F.

After the 72.5% solids solution, described above, has been added to the pigment dispersion-resin mixture, 33 gallons of water are added under agitation. The resulting emulsion is approximately 42 percent solids and has a viscosity of 14-20 seconds measured in a #4 Ford cup at 80 F.

This emulsion is further reduced with the addition of 300-600 percent of its volume of water.

ln the electrodeposition of the above described coating composition over the chemically plated layer of nickel, electrode contact with the nickel film is effected by clamping a steel wool pad between the nickel film and the anodic lead.

The last deposited coating is then moved to the second curing station and baked at an air temperature of about 400 F. for about 25 minutes.

In this embodiment, the body next moves to a conventional sanding station where the exposed surface of paint is checked for blemishes and sanded where necessary prior to application of a finishing coat. The workpiece then proceeds to one or more coating stations where a finish coat, e.g. enamel or lacquer, is applied by spraying or other conventional paint application means before passing to the final curing station.

FIGURE 2 is illustrative of a metal substrate coated in accordance with the embodiment above described. Upon the substrate 1 there is shown the first electrodeposited paint 2. Immediately below paint 2 is a layer of chemically plated nickel 3. The depth of the nickel layer is here shown enlarged with respect to the adjoining paint layers to facilitate its identification and location on the drawing. Immediately below the nickel layer, there is shown a second electrodeposited paint 4 which in turn is followed by a nish paint 5.

A further study of the method of this invention was made upon zinc phosphate treated steel panels with commercially available, polycarboxylic acid resin comprising 7 paints and commercially available chemical plating solutions.

In one such study, a first coating of a polycarboxylic acid resin comprising automobile primer was electrodeposited from an aqueous bath upon the test panel. The difference of electrical potential between anode and cathode was maintained at about 180 volts. The curing, etching and seeding steps hereinbefore described were employed to prepare the surface of the paint for chemical plating. A layer of silver was chemically plated over the seeded surface by conventional chemical plating techniques. A polycarboxylic acid comprising black enamel was then electrodeposited over the layer of silver and cured by conventional baking.

In another such example, a first coating of another polycarboxylic acid resin comprising automobile primer was electrodeposited from an aqueous bath upon the test panel. The conditions of electrodeposition Were immersion for 2 minutes at 75 F., the impressed potential was 180 volts, the maximum current was about 4.65 amps/ft2, and current at termination was about 0.135 amp/ ft2. The coating obtained measured 0.7 mil in thickness. The coating Was cured, acid etched, and seeded with palladium as in the previous examples. The seeded coating was then chemically plated with nickel and immersed in an electrodeposition bath containing an aqueous dispersion of another polycarboxylic acid resin comprising paint. The immersion time for this coating step was 2 minutes at 75 F. The coating potential Was about 200 volts. Current density ranged from about 2.25 amps/ft.2 at coating initiation to about 0.195 amp/ft.2 at coating termination.

In this application, painting by electro-deposition 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 miner-al filler material or even other resins on Which it exerts the desired action for depositing the film.

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. The method of coating an electrically-conductive, automobile lbody which comprises immersing said body in a first aqueous coating bath having dispersed therein a paint in which the predominant fraction of the film-forming paint binder thereof is a synthetic polycarboxylic acid resin at least partially neutralized with a sufficient quantity of Water soluble amino compound to maintain said resin as a dispersion lof anionic polyelectrolyte in said bath, anodically depositing upon said body from said bath a continuous, essentially Water-insoluble, film of said paint having an average depth in excess of about 0.5 mil, polymerizing said film of paint upon said body until said film has an average electrical resistance in excess of about 1 million ohm-cm, immersing the body in a chemical plating bath containing metal cations which are chemically reduced upon the `painted surface of said body until a continuous, adherent, electrically-conductive layer of metal is formed thereon to an average thickness in the range of 4about 0.001 to about 0.01 mil, immersing said body in a second aqueous coating bath having dispersed therein a paint in which the predominant fraction of the film-forming paint binder is a synthetic polycarboxylic acid resin at least partially neutralized With a sufficient quantity of Water soluble amino compound to maintain same as a dispersion of anionic polyelectrolyte in said second aqueous coating bath, and anodically depositing upon said layer of metal from said second aqueous coating bath a continuous, essentially Water-insoluble film of paint having an average depth in excess of about 0.7 mil.

2. The method of claim 1 wherein said layer of metal is nickel.

3. The method of claim 1 wherein said layer of metal is silver.

4. The method of coating an automobile body which comprises applying a rst, continuous coating of paint having an average thickness in excess of about 0.3 mil upon the surfaces of said body, polymerizin-g said first -coating of paint upon said body, applying upon the polymerized coating by chemical plating a continuous, adherent, electrically-conductive layer of metal having an average thickness not substantially in excess of about 0.03 mil, and anodically depositing upon said layer of metal a continuous coating of paint having an average thickness in excess of about 0.3 mil.

5. The method of coating an electrically-conductive substrate which comprises applying a first, continuous, coating of paint having an average thickness in excess of about 0.3 mil upon the surfaces of said substrate, polymerizing said first coating of paint upon said ibody, applying upon the polymerized coating by chemical plating a continuous, adherent, electrically-conductive layer of metal having an average thickness not substantially in excess of about 0.03 mil, and anodically depositing upon said layer of metal a continuous coating of paint to an average thickness in excess of about 0.3 mil.

6. The method of coating a metal substrate which comprises anodically depositing upon said substrate a coating of a polycarboxylic acid resin comprising paint having an average thickness in excess of about 0.3 mil, polymerizing said coating of paint upon said substrate, applying upon the polymerized coating a continuous, adherent, electrically-conductive layer of metal having an average thickness not substantially in excess of about 0.03 mil, and anodically depositing upon said layer of metal a continuous coating of paint having an average thickness in excess of about 0.5 mil thereby forming an article of manufacture comprising a metal substrate covered by a continuous coating of paint which is covered by a continuous layer of metal and in turn is covered with a continuous coating of paint.

. 7. An automobile body comprising a metal substrate,

upon said substrate a first, continuous, coating of paint which has been electrodeposited thereon to an average thickness in excess of about 0.3 mil from an aqueous dispersion of paint in which the predominant fraction of the film-forming binder thereof is a synthetic polycarboxylic acid resin at least partially neutralized with a sufficient quantity of Water soluble amine compound to maintain said polycarboxylic acid resin as a dispersion of anionic polyelectrolyte and subsequently polymerized to an average electrical resistance in excess of about 1 million ohm-cm., upon said first coating of paint a continuous adherent, electrically-conductive layer of nickel which has been chemically reduced from nickel ions to an average depth in the range of about 0.001 to about 0.01 mil, and upon said layer of metal a second, continuous coating of paint which has been electrodeposited from an aqueous dispersion thereof to an average depth in excess of about 0.3 mil.

8. A multi-layered article of manufacture comprising a metal substrate, upon the surface of said substrate a first, continuous, coating of paint which has an average depth in excess of about 0.3 mil and has been polymerized thereon, upon said `first coating of paint a continuous, adherent, electrically-conductive layer of metal having an average thickness not substantially in excess of about 9 0.03 mil, and u-pon said layer of metal, a second, continuous coating of paint which has been electrodeposited from an aqueous dispersion thereof to an average thicknes's in excess of about 0.3 mil.

References Cited UNITED STATES PATENTS 10 3,230,162 1/1966 Gilchrist 204--181 3,449,229 6/ 1969 Freeman et al. 204-181 3,232,856 2/1966 Klach et al. 204-181 5 HOWARD S. WILLIAMS, Primary Examiner

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