US3531390A - Electrodeposition method - Google Patents

Description

United States Patent U.S. Cl. 204-181 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of improving the uniformity of the coating on an article applied by an electrodeposition process in which the article to be coated is entered into the electrodeposition bath under an applied voltage. The invention comprises prewetting the article to be electrocoated prior to its entry into the electrodeposition bath.

This invention relates to a method of electrodepositing a coating on a conductive base. More particularly, this invention relates to a method of improving the uniformity of the coating on an article applied by electrodeposition by prewetting the article to be electrocoated.

Electrodeposition is a relatively new coating technique which, although based on well-known principles, has only recently become technically feasible through the development of electrodepositable compositions which have the desired characteristics to meet the demands placed on a modern coating material. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition. The most commonly employed substrates include the base metals such as iron, steel, aluminum, copper, zinc, brass, tin, nickel and chromium, as well as other metals and pretreated metals, impregnated paper or other substrates rendered conductive under the conditions employed may also be coated.

While electrodeposition is in many respects advantageous compared to ordinary application methods, problems have arisen in the fact that under certain conditions the coating provided is not wholly uniform to the extent that visible hash marks, striations, craters, or variations in gloss are apparent either when the coating is observed or when the electrodeposited coating serves as a primer and the topcoat is observed. Often this lack of uniformity can only be noted from certain angles of observation, probably due to the angle of the incident light.

The above-described phenomena are observed only under certain conditions. Where the article, or part thereof, to be coated is immersed in the electrodeposition bath prior to the application of a voltage to the bath and then the voltage is applied, a more or less uniform coating is obtained since increased electrical resistance is caused in the areas of film buildup, therefore directing deposition to more thinly-coated areas and thereby providing uniform coatings. Also, when an article is preimmersed, the initial current density is relatively uniform over the entire article.

The phenomena described are observed, however, where the article or part thereof is introduced in the bath under an applied voltage so that coating of the article is progressively initiated as the article moves into the bath. As can readily be seen, this latter method is the method amenable to continuous or mass production of electrocoated articles, such as when the articles, such as automobile bodies, metal housings or parts, are being moved into the tank under an applied voltage by a conveyor belt, coated and then removed. Likewise, this method is amenable to coating of coiled sheet steel, where a section of Patented Sept. 29, 1970 the coil is progressively passed into a bath under voltage to produce a coated coil which is then fabricated.

The reasons for the production of less uniform coating Where the article is introduced into a bath under an applied voltage are not altogether clear and are no doubt complex. One theory which has been advanced, is that when the article is being moved into the bath, the current density at the leading edge of the paint film is inordinately high. Therefore, the wetting properties of the paint are reduced and the paint film builds at the leading edge rather than flowing as freely as normal. Since the operation of the conveyor or feed system is never totally smooth in practice, whenever the article hesitates on entry, an area of higher density coating is achieved, and since the current density at the leading edge is higher than when thearticle is completely immersed, the remaining surrounding area does not achieve as thick a film coating. In the extreme case, this localized effect might lead to localized rupture of the film, as evidenced by cratering. It has been found that electrodepositive coatings having excellent flow and Wetting properties do not present as severe a problem in respect to these film variations. It is likewise apparent that the higher the impressed voltage on the system, the greater the potential film variations and as the higher voltages are employed, the problem becomes more severe. It is also true that the problem is more noticeable where a relatively thin coating film is applied.

It has now been found that the incidence of film variation in a system where the article to be electrocoated is introduced into an electrodeposition bath under an applied voltage can be eliminated, or at least greatly reduced, by prewetting the surface of the article with an electrodeposition composition such as the electrodeposition composition contained in the bath or another electrodeposition composition differing in composition from that in the bath. Preferably, the liquid used to prewet the article is substantially the electrodeposition composition contained in the bath.

Prewetting, as used throughout the specification and claims, is defined as wetting the surface of the article to be coated at a time and in a manner that the surface is wet when introduced into the electrodeposition bath.

A number of electrodepositable compositions are known and can be employed to provide the electrodeposited coating applied according to the method of the instant invention. Virtually any water-soluble, water-dispersible, or water-emulsifiable resinous material can be electrodeposited and, if film-forming, provides a coating which may be suitable for certain purposes. Any such electrodepositable material is included among those that can be employed in the present invention, even though the coatings obtained may not be entirely satisfactory, except for certain specialized uses.

Several coating compositions are known which can be electrodeposited with especially good results to provide coatings of highly desirable properties for many widely used applications. A preferred class of coating compositions are those in which the vehicle is an at least partially-neutralized reaction product of a drying oil fatty acid ester with an alpha, beta-ethylenically unsaturated dicarboxylic acid or an anhydride of such an acid. The composition may also comprise the reaction product of the ester, acid or anhydride and one or more other ethylenically unsaturated monomers. The initial reaction products as above may also be partially reacted with an alcohol to esterify part of the carboxylic groups from the acid or anhydride prior to neutralization of all or part of the remaining acidic radicals with a base.

The fatty acid esters used to make the aforesaid vehicles are esters of fatty acids which are or can be derived from drying oils or from such sources as tall oil. (By drying oil is meant those oils having an iodine number of greater than 90, thus including so-called semi-drying oils.) Examples of such esters include linseed oil, saffiower oil, perilla oil, tung oil, oiticia oil, sunflower oil, tall oil esters, dehydrated castor oil, fish oils, and the like.

The fatty acid ester may also be an alkyd resin prepared utilizing semi-drying or drying oil; an ester of an epoxide with such fatty acids; a semi-drying or drying oil fatty acid ester of a polyol; or semi-drying or drying oil fatty acid ester of a resinous polyol. If desired, the ester may be modified with other acids, such as saturated, unsaturated or aromatic acids, as well as with such acid materials as rosin.

The alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride may be an anhydride such as maleic anhydride or itaconic anhydride, or an unsaturated dicarboxylic acid which forms an anhydride, for example, maleic acid or itaconic acid. The acids appear to function by first forming an anhydride. Fumaric acid, which does not form an anhydride, may also be employed, as well as mixtures of any of the above acids or anhydrides. Usu ally the anhydride or acid employed contains from 4 to 12 carbon atoms. The reaction between the acid or anhydride and the fatty acid ester takes place readily without the use of a catalyst at temperatures between about 100 C. and about 300 C., depending primarily upon the type of fatty acid ester used.

As indicated, the reaction product can also include one or more other ethylenically unsaturated monomers in polymerized form. Essentially, any ethylenically unsaturated monomer, e.g., those containing CH C groups, can be employed for this purpose, with the preferred compounds being styrene, substituted styrenes, alkyl acrylates, alkyl methacrylates, diolefins and acrylonitrile. When such an additional monomer is employed, the reaction is best carried out by first reacting the acid or anhydride with the fatty acid ester and then reacting this product with the monomer at somewhat lower temperatures.

Although the proportions of the components in the foregoing reaction products are not critical, it is preferred to utilize between about 10 percent and about 45 percent by weight of the unsaturated acid or acid anhydride with about 55 percent to about 90 percent by weight of fatty acid ester. If an ethylenically unsaturated monomer is incorporated in the reaction product, it is typically used in amounts between about 5 percent and about 35 percent by Weight based upon the total weight of acid or anhydride and ester.

Neutralization of these products is accomplished by reaction of all or part of the dicarboxylic anhydride groups with a base. Usually up to about half of such groups are neutralized in unesterified adducts; the partially esterified products are often neutralized to a greater extent, based on unesterified acid groups remaining. Inorganic bases, such as metal hydroxides or, more desirably, ammonia, can be used for this purpose, as can organic bases, particularly amines. Among the preferred class of neutralizing bases are ammonia and any basic amine, including alkyl amines, cycloalkyl amines, unsaturated amines, aromatic amines, aryl amines, aralkyl amines, cyclic amines, diamines and substituted amines, of the primary, secondary, tertiary or quaternary amine types, including hydroxylamines.

It is preferred in certain instances that the neutralization reaction be carried out in such a manner that amido groups are attached to part of the carbonyl carbon atoms derived from the dicarboxylic acid or anhydride. By amido groups are meant trivalent nitrogen atoms attached with one valence to the carbonyl carbon atom, with the other two walences being linked to hydrogen or carbon atoms in the same or different organic radicals. Amido groups are formed, for example, when the reaction with the neutralizing base is carried out with a water solution of ammonia, a primary amine or a secondary amine, or when the product is reacted with such an amine in the absence of water.

The electrodepositable coating compositions comprising the above reaction products described contain in most instances a pigment composition and, if desired, various additives such as anti-oxidants, surface active agents and the like. The pigment composition may be of any conventional type, comprising, for example, iron oxides, lead oxides, strontium chromate, carbon black, titanium dioxide, talc, barium sulfate, as well as color pigments such as cadmium yellow, cadmium red, chromic yellow and the like. Better results with pigmented compositions are attained if the weight ratio of pigment solids to vehicle solids is not higher than about 1.5 to 1, and preferably not higher than about 1 to 1. The coating compositions when employed for electrodeposition usually are made up of at least about percent water.

Compositions within this general class are described in co-pending applications, Ser. 'No. 222,674, filed Sept. 10, 1962, now US. Pat. No. 3,366,563, and Ser. No. 282,880, filed May 24, 1963, now US. Pat. No. 3,369,983.

Another type of electrodepositable coating composition which gives desirable results are the water-dispersible coating compositions comprising at least partially neutralized interpolymers of hydroxyalkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acids and at least one other ethylenically unsaturated monomer. These are employed in the composition along with an amine-aldehyde condensation product or a polyepoxide, or both, with the interpolymer usually making from about 50 percent to about percent by weight of the resinous composition.

The acid monomer of the interpolymer is usually acrylic acid or methacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids, such as ethacrylic acid, crotonic acid, maleic acid or other acids of up to about 6 carbon atoms can also be employed. The hydroxyalkyl ester is usually hydroxyethyl or hydroxypropyl acrylate or methacrylate, but also desirable are the various hydroxyalkyl esters of the above acids having, for example, up to about 5 carbon atoms in the hydroxyalkyl radical. Monoor diesters of the dicarboxylic acids mentioned are included. Ordinarily, the acid and ester each comprise between about 1 percent and about 20 percent by weight of the interpolymer, with the remainder being made up of one or more other copolymerizable ethylenically unsaturated monomers. The most often used are the alkyl acrylates, such as ethyl acrylate; the alkyl methacrylates, such as methyl methacrylate; and the vinyl aromatic hydrocarbons, such as styrene; but others can be utilized.

The above interpolymer is at least partially neutralized by reaction with a base as described above; at least about 10 percent, and preferably 50 percent or more of the acidic groups are neutralized, and this can be carried out either before or after the incorporation of the interpolymer inthe coating composition. The bases above can be used, with ammonia and amines being preferred; except when a polyepoxide is present, in which case there is preferably employed a hydroxide, such as sodium hydroxide, or if an amine, a tertiary amine.

The amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoguanamine, or urea with formaldehyde, although other amino-containing amines and amides, including triazines, diazines, triazoles, guanadines, guanamines and alkyl and aryl-substituted derivatives or such compounds can be employed, as can other aldehydes, such as acetaldehyde. The alkylol groups of the products can be etherified by reaction with an alcohol, and the products utilized can be water-soluble or organic solvent-soluble. Y

The electrodepositable compositions can also include a polyepoxide, which can be any epoxide compound or mixture with an epoxy functionality of greater than 1.0. Numerous such polyepoxides are known and are described in patents such as U.S. Pats. Nos. 2,467,171; 2,615,007; 2,716,123; 2,786,067; 3,030,336; 3,053,855; and 3,075,999. Included are polyglycidyl ethers of polyphenols, such as bisphenol A, or of aliphatic polyhydric alcohols, such as 1,4-butanediol; polyglicidyl esters of polycarboxylic acids, such as diglycidyl adipate; and polyepoxides from the epoxidation of unsaturated alicyclic compounds, such as 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy 6 methylcyclohexanecarboxylate.

Electrodepositable compositions comprising the above interpolymers and an amine-aldehyde resin or a polyepoxide, or both, are more fully described in copending application Ser. No. 368,394, filed May 18, 1964, now U.S. Pat. No. 3,403,088.

Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that is, a vehicle containing an alkyd resin and an amine-aldehyde resin. A number of these are known in the art and may be employed. Preferred are water-dispersible alkyds such as those in which a conventional alkyd (such as a glyceryl phthalate resin), which may be modified With drying oil fatty acids, is made with a high acid number (e.g., 50 to 70) and solubilized with ammonia or an amine, or those in which a surface active agent, such as a polyalkylene glycol (e.g. Carbowax), is incorporated. High acid number alkyds are also made by employing a tricarboxylic acid, such as trimellitic acid or anhydridc, along with a polyol in making the alkyd.

The above alkyds are combined with an amine-aldehyde resin, such as those described hereinabove. Preferred are water soluble condensation products of melamine or a similar triazine with formaldehyde with subsequent reaction with an alkanol. An example of such a product is hexakis(methoxymethyl) melamine.

The alkyd-amine compositions are dispersed in water and they ordinarily contain from about 10 percent to about 50 percent by weight of amine resin based on the total resinous components.

Examples of compositions of this class are described in U.S. Pats. Nos. 2,852,475; 2,852,476; and 2,853,459.

In electrodeposition processes employing the various coating compositions described above, an aqueous bath containing the coating composition is placed in contact with an electrically conductive anode and an electrically conductive cathode. The surface to be coated is employed as one of the electrodes. In the specific examples of compositions described above, the surface to be coated is employed as the anode. Upon the passage of electric current between the anode and the cathode, while in contact with the bath containing the coating composition, an adherent film of the coating composition is deposited. The conditions under which the electrodeposition step herein is carried out are those conventionally used in electrodeposition of coatings. The applied voltage may be varied greatly and can be, for example, as low as 1 volt or as high as several thousand volts, although typically between 50 volts and 500 volts are used. The current density is usually between about 0.1 ampere and amperes per square foot, and tends to decrease during the electrodeposition.

The concentration of the non-volatile components (i.e. vehicle and any pigments and the like) in the aqueous bath is not critical and relatively high levels can be em ployed. However, it is ordinarily desirable to use as low a concentration as gives satisfactory results, and in the cases of the above-described compositions, aqueous compositions containing as little as 1 percent by weight of non-volatile solids can be employed, with those containing between 5 percent and 35 percent by weight being preferred.

The essential step in the process of the instant inven tion is that thesurface be prewet by an -electredeposit75- able composition prior to entry into the electrodeposition bath. The prewetting of the article may be accomplished in various ways.

For example, the article may be prewet by a spray head, or a series of spray heads, over the electrodeposition bath just prior to entry into the tank. The entire article may be prewet, or in the case of large articles the spray may be directed on an area immediately prior to entry of the article. Where the prewetting is accomplished by the electrodeposition composition contained in the bath, a portion of the material in the tank may be pumped through the spray heads. Likewise, flow-coating may be employed in a similar manner.

Alternatively, the article to be coated may be sprayed or dipped in a separate operation prior to its positioning over and its entry into the electrodeposition bath. This technique would be preferred where the composition used to prewet was other than the electrodeposition composition in the bath or a composition intended to be added to the bath for various reasons.

Another method of prewetting the article prior to entry into the electrodeposition bath is the use of sponges or other wipers to prewet the article; this is especially amenable to wetting flat objects such as sheet or coil steel.

The invention is further described in conjunction with the following examples, which are to be considered illustrative rather than limiting. All parts and percentages in the examples and throughout this specification are by weight unless otherwise stated.

EXAMPLE I A coating composition was produced as follows:

A vehicle resin (Resin A) composition was produced by heating a 4-to-1 weight mixture of linseed oil and maleic anhydride to 250 C. over a two-hour period, and then holding this mixture at this temperature for 15 minutes. The product was mixed with deionized Water and diethylamine to give a solution with a pH of about 7.2 and a solids content of 35 percent.

A pigment paste (Paste B) was made by grinding the following in a steel ball mill for 16 hours; at the beginning of the grinding period suflicient diethylamine was added to make the pH about 9.

Parts by weight Resin A (above) 35% solids 230.0

Dispersing agent (combination oil-soluble sulfomate and non-ionic surfactant-Witco 912) 6.0 Red iron oxide 475.0 Carbon black (30% aqueous dispersion) 74.0 Strontium chromate 25.0

After 16 hours, the following was added to the mill and mixed 30 minutes:

Parts by weight Resin A (above) 115.0

Deionized water 68.0

The above paste was then let down as follows:

Resin A (above) 506.1 Paste B (above) 273.1

and were mixed, then deionized water was incrementally added to bring the solids content of the mixture to 8.0 percent.

The electrodeposition bath comprised a two-gallon stainless steel beaker which was filled with a sample of above final composition which had been aged under electrodeposition conditions for several months. The beaker served as the cathode in-these studies. The panel to be coated served as the anode.

Zinc phosphate treated steel panels (Bonderite 37) were lowered into the bath by a motor drive under a predetermined, preset, constant voltage. Unless otherwise indicated, the panels were lowered into the bath at a constantrate -of-20 seeondsfor-aseven-inchdip. The electroof soybean fatty acid, and 504 parts of 1,5-pentanediol were heated to a temperature of 260 F., and then there 7 depositable liquid used to prewet the panels was a portion of the electrodeposition composition contained in the bath.

TABLE I Bath Tempera- Current (amps) Panel ture (degrees Time, Designation Fahrenheit) Voltage Initial Final Minutes Remarks 71 100 1. 6 0. 2 3 Dry pane1severe hash marksfiln1 thickness 0.50 mil. 71 100 1. 9 0. 18 3 Panel prewet by spraying prior to entry-free of hash marksfilm thickness 0.50 mil. 71 100 2. 0.22 3 Control panel-preirn1nersed and then voltage applied-free of hash marks film thickness 0.50 mil. 1 84 100 2. 0 0. 22 3 Dry paneltrace hash marks-film thickness 0.6 mils. 84 100 2. 0 O. 22 3 Panel grfiewet by spraying prior to entryfree of hash marks-film thickness m1 84 100 1. 9 0. 22 3 Panel preimmersed in bath and then voltage applied-free of hash marks film thickness 0.6 mil. 84 75 1. 65 0.23 2. 5 Dry panelsevere hash marksfil.n1 thickness 0.4 mil. 84 75 1.65 0. 28 2. 5 Panel prewet by sprayingfree of hash marksfilm thickness 0.4 mil. 84 75 1. 65 0.23 2. 5 Panel prewet by dipping-very slight film irregularity.

EXAMPLE II In above experiments, the spraying was accom- Another example of a similar electrodeposition composition which may be employed in the process of the instant invention is as follows:

Two more vehicle resins (Resins C and D) were pro duced by reaction of a 4-to-1 weight ratio of linseed oil with maleic anhydride, as in Example I, and then reacting 300 parts of the product with 75 parts of styrene in the presence of ditertiary butyl peroxide to 150 C. to 200 C. The portion product obtained was partially esterified by mixing 250 parts thereof with 13.5 parts of methanol and heating to 90 C. for a total of 3 hours.

Both the styrenated maleinized oil and the partially esterified product were mixed with deionized water and ammonia to give solutions with a pH of about 7 and a solids content of 35.0 percent in the case of the styrenated maleinized oil and 31.0 percent in the case of the partially esterified product. The solutions were designated Resin C and Resin D respectively.

A pigment paste (Paste E) was made by grinding the following in a steel ball mill for 16 hours; at the beginning of the grinding period sutficient ammonia was added to make the pH about 9.

Parts by weight Resin C (above) 28.7

Red iron oxide 47.4 Strontium chromate 1.2 Dispersing agent (combination oil soluble sulfonate and nonionic surfactant-Witco 912) 0.5

After 16 hours, the following was added to the mill and mixed 30 minutes.

Parts by weight were mixed and then deionized water was incrementally added to bring the solids content of the mixture to 8 percent.

EXAMPLE HI A coating composition was produced as follows: 1155 parts of 2,2-bis(4-hydroxycyclohexyl) propane, 896 parts was added 896 parts of trimellitic anhydride and the mixture heated to 350 F.360 F. and reacted until an acid number of 5052 was reached. The reaction mixture was then cooled to 120 F. while 1071 parts of 4-methoxy-4- methylpentanone-Z were added. 19.4 parts of propylene imine were then added and the reaction mixture heated at 170 F. for 30 minutes. This reaction product was designated Resin S.

A vehicle was prepared employing parts of Resin S and 25 parts hexarnethoxymethylmelamine. This composition was solubilized with triethylamine to a pH of about 7.2 and reduced to 40 percent solids with deionized water. This vehicle was designated Vehicle T.

A pigment paste (Paste U) was made by grinding the following in an attritor to a Hegman grind gauge reading of 7; at the beginning of the grinding period sufficient diterylamine was added to make the pH about 9.

Parts by weight Deionized water 408.17

The above was mixed and the pH adjusted to about 8.0 with triethylamine and reduced to 10 percent solids with deionized water to form the electrodeposition composition employed in the following examples.

The electrodeposition bath comprised a two-gallon stainless steel beaker which was filled with a sample of the above final electrodeposition composition. The beaker served as the cathode in these studies. The panel to be coated served as the anode.

Five phosphate treated steel panels (Bonderite 37) were lowered into the bath by a motor drive under a predetermined, preset, constant voltage. Unless otherwise indicated, the panels were lowered into the bath at a constant rate of 20 seconds for a seven-inch dip. The electrodeposita'ble liquid used to prewet the panels was a portion of the electrodeposition composition contained in the bath.

TABLE II Bath Tempera- Time, Panel Desigture (degrew Volt- Amper- M Fllm Thicknation Fahrenheit) age age utes ness (Mils)1 Remarks 75 0. 9-0. 10 2 0. 45 Control panel lowered voltage onmany hash marks. 75 75 0.7-0. 10 2 0.38 Control panel lowered voltage on-slight hash marks. 75 100 0. SH). 10 2 0. 40 Prewet by sprayingsmooth panel produced. 75 75 0. 7-0. 10 2 0.38 Do.

Various other electrodepositable compositions, such as those hereinabove described, can be substituted for those of the examples.

While the description of the specification is primarily drawn to the use of various metals as substrates, it is also contemplated that the method of this invention is applicable to non-conducting substrates which have been rendered amenable to the electrodeposition process by first applying to the surface to be coated a layer of an electrically-conductive coating composition, and then employing the coated surface as an electrode in an electrodeposition process; for example, a process such as described in application Ser. No. 416,140, filed Dec. 4, 1964.

In the above and other tests, the general applicability of the method herein has been shown and it has been found that good results are attained using varying compositions, electrodeposition conditions and substrates.

According to the provisions of the patent statutes, there are descrioed above the invention and what are now considered to be its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

What is claimed is:

1. In a method of electrodepositing a coating upon a dry electrode surface, said method comprising introducing said surface into an aqueous electrodeposition bath, which is under an applied voltage, the improvement which comprises the step of prewetting said surface with an electrodepositable composition prior to its introduction into said bath.

2. A method as in claim 1 where said electrode surface is the anode.

3. In a method of electrodepositing a coating upon a dry anode surface, said method comprising introducing said surface into an aqueous electrodeposition bath, comprising a solubilized polycarboxylic acid resin, which is under an applied voltage, the improvement which comprises prewetting said surface with an aqueous electrodeposition composition comprising a solubilized polycarboxylic acid resin, prior to its introduction into said bath.

4. A method as in claim 3 where the electrodepositable composition used to prewet the electrode surface to be coated comprises substantially the same electrodeposition composition contained in the electrodeposition bath.

5. A method as in claim 4 where said electrode surface is the anode.

6. A method as in claim 5 when the prewetting is accomplished by spraying said electrode surface with the electrodeposition composition.

7. In a method of electrodepositing a coating upon a dry electrode surface, said method comprising introducing said surface under an applied voltage into an aqueous electrodeposition bath containing an electrodepositable composition comprising a vehicle which comprises a solubilized reaction product of a drying oil fatty acid ester with a member of the group consisting of alpha-ethylenically unsaturated dicarboxylic acids and an anhydride of said acids, the improvement which comprises the step of prewetting said electrode surface with said electrodepositable composition prior to its introduction into said bath.

8. In a method of electrodepositing a coating upon a dry electrode surface, said method comprising introducing said surface, under an applied voltage, into an aqueous electrodeposition bath containing an electrodepositable composition comprising a vehicle comprising a solubilized combination of an alkyd resin and an aminealdehyde condensation product, the improvement which comprises the step of prewetting said electrode surface with said electrodepositable composition prior to its introduction into said bath.

References Cited UNITED STATES PATENTS 1,589,327 6/1926 Eberlin et a1 204-181 3,200,057 8/1965 Burnside et al. 204181 3,304,250 2/1967 Gilchrist 204181 3,378,477 4/1968 Gentles et a1. 204-181 3,403,088 9/1968 Hart 204-181 3,420,762 1/1969 Shaw et al. 204-181 HOWARD S. WILLIAMS, Primary Examiner