US3591478A - Electrocoating process - Google Patents

Abstract

A METHOD FOR IMPROVING AND MAINTAINING THE QUALITY OF THE ELECTROCATING DEPOSITED ON A METALLIC ANODE WHICH COMPRISES REMOVING INORGANIC ACID ANIONS FROM THE ELECTROCOATING BATH.

Description

Unimd tates Patent 3,591,478 ELECTROCOATING PROCESS James R. Erickson, Parma, Ohio, assignor to SCM Corporation, Cleveland, Dhio No Drawing. Filed May 6, 1968, Ser. No. 727,009 Int. Cl. Btllk 5/02; (323i) 13/00 US. Cl. 204181 7 Claims ABSTRACT OF THE DICLOSURE A method for improving and maintaining the quality of the electrocoating deposited on a metallic anode which comprises removing inorganic acid anions from the electrocoating bath.

This invention relates to an improved method of continuously electrocoating metallic substrates wherein the quality of the electrically deposited film is maintained through the removal of interfering inorganic, acid anions from the electrocoating bath.

In one aspect, this invention relates to an improved method of electroplating a series of aluminum articles, in a single bath, with an ionized, multicomponent paint.

Electrocoating is the electrodeposition of a resinous material (with or without pigments) on surfaces in an electrical circuit, from ionic, Water dispersions of these resins by means of an electrical current. In the electrocoating process, the object being coated is the anode, and the cathode is usually the tank containing the aqueous dispersion of electropaint. The anode and cathode are electrically connected to an external power source to complete the circuit.

In the electrocoating process a coating bath is formed by dispersing, in an aqueous medium, a synthetic organic resin having a plurality of water ionizable, anionic, functional groups within its molecular structure with a water ionizable dispersal assistant for such resin. The article to be coated is immersed in this coating bath while the article is connected as the anode in an electrical circuit. The tank containing the bath serves as the cathode in the circuit (alternatively, immersion cathodes can be used) and a direct current potential (or its electrical equivalent) is impressed across these electrodes. As the electrical current flows between the electrodes, the ionized, anionic dispersed resin is electrically deposited on the anode, and is converted into an essentially water insoluble coating thereon. The coating on the anode is then thermally cured if required.

This electrocoating is done with direct (unidirectional) current. In most cases, such current ordinarily is rectified AC current having about a 5 to 15% ripple factor. However, the current can have a greater or lesser ripple factor, or even can be half wave rectified alternating current, and so on, providing the net effect is unidirectional and thus, the current is direct current.

Useful voltages across the bath can be as low as 15 or even lower, and should not be so high as to burn through the deposited coating. Practical maxi mum deposition voltages are 350-500 volts for many resinous systems, although higher voltages can be used with selected systems, particularly if the duration of the higher voltage period is very short.

The electropaint is, of course, electrolytic in nature and completes the circuit between the anode and the cathode. This electropaint dispersion comprises a film forming paint binder containing a polycarboxylic acid resin at least partially neutralized with a water soluble amino compound. The polycarboxylic acid resin usually has an electrical equivalent weight between about 500 and about 20,000, and an acid number between about 20 and about 300. The polycarboxylic resin exhibits ani- 3,591,478 Patented July 6, 1971 Too onic polyelectrolyte behavior as indicated by the deposition on the anode substantially directly proportional With the electric current being passed through the bath.

The film-forming material employed in an electrocoating process of the type herein contemplated can constitute the sole coating material within the bath or it may include or be employed with pigments, metallic particles, dyes, drying oils, extenders, etc., and may be dispersed as a colloid, emulsion, emulsoid or apparent solution. The primary or backbone resin or resins employed in preparing the film-forming binder may include, but not by way of limitation, alkyd resins, acrylate resins, epoxy resins, phenol-formaldehyde resins, hydrocarbon resins, and other organic resins or mixtures of one or more of the foregoing resins with another of the resins heretofore mentioned or with other film-forming organic materials including binding agents and extenders conventionally employed with paints. Such materials may be reacted with or accompanied by other organic monomers and/or polymers including, but not by way of limitation, hydrocarbons and oxygen substituted hydrocarbons such as ethylene glycol, glycerol, monohydric alcohols, carboxylic acids, ethers, aldehydes and ketones.

Since the binder material is to be deposited anodically the resin will have free or Water dissociable carboxyl groups or their equivalent within the molecular structure of the resin. These can be the result of the original formulation of the resin or subsequently introduced by chemically reacting a suitable resin with monomers and/ or polymers which introduce such groups into the binder to be used in coating. Film-forming materials that have been found to be particularly suitable for anodic deposition include, but not by way of limitation, coupled siccative oils, e.g. coupled glyceride drying or semidrying oils such as linseed, sunflower, safilower, perilla, hempseed, walnut seed, dehydrated castor oil, rapeseed, tomato seed, menhaden, corn, tung, soya, oiticica, or the like, the olefinic double bonds in the oil being conjugated or nonconjugated or a mixture, the coupling agent being an acrylic olefinic acid or anhydride, preferably maleic anhydride, but also crotonic acid, citraconic acid, or anhydride, fumaric acid, or an acrylic olefinic aldehyde or ester of an acyclic olefinic ester such as acrolein, vinyl acetate, methyl maleate, etc., or even a polybasic acid such as phthalic or succinic, particularly coupled glyceride oils that are further reacted with about 225% of a polymerizable vinyl monomer; maleinized unsaturated fatty acids; maleinized rosin acids, alkyd resins, e.g. the esterification products of a polyol with a polybasic acid, particularly glyceride drying oil-extended alkyd resins; 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, i.e. those having carboxyl groups contributed by alpha, beta unsaturated carboxylic acids or residues of these acids, etc. Dispersion of these polycarboxylic acid resins in water is assisted by the addition of a suitable basic material such as ammonia, water soluble amines, mixtures of monomeric and polymeric amines, KOH etc. The pH of the bath is, of course, dependent upon the relative concentrations of acidic and basic materials therein.

The acid number of resins without appreciable content of anhydride groups can be determined with KOH by the ASTM standard method 55554. If appreciable anhydride groups are present, the acid number can be determined by refluxing a 1.5-2 gram sample of the portion of the resin for one hour with 50 ml. of 0.5 N aqueous KOH and 25 ml. of pyridine, then back titrating with 0.5 N HCl to a phenolphthalein end point.

The electrical equivalent weight of a given resin or resin mixture is herein defined as that amount of resin or resin mixture that will deposit per Faraday of electrical energy input under the conditions of operation set forth in detail in the succeeding paragraph. For this purpose the value of one Faraday in coulombs is herein taken to be 107.88 (atomic weight of silver) +0.001118 (grams of silver deposited by one coulomb from silver nitrate solution) or 96,493. Thus, if 0.015 gram of coating, the binder polycarboxylic acid resin moiety of which is 90% by weight and the balance of which is amino compound used to disperse it in the bath is transferred and coated on the anode per coulomb input to the process, the electrical equivalent weight of the resin is about 1303 or 0.0l l07.88+0.001118.

By way of further illustration, the electrical equivalent weight (in the nature of a gram equivalent weight in accordance with Faradays laws) of a particular polycarboxylic acid resin or resin mixture is simply and conveniently found for typical process conditions standardized on as follows: a polycarboxylic acid resin concentrate is made up at 65.56 C. (150 F.) by thoroughly mixing 50 grams of the polycarboxylic acid resin, 8 grams of distilled water and diisopropanolamine in an amount sulficient to yield resin dispersion pH of 7.8 or slightly lower after the concentrate has been reduced to 5% by weight resin concentration with additional distilled water. The concentrate is then diluted to one liter with additional distilled water to give 5% resin concentration in the resulting dispersion. (If a slight insufiiclency of the amine has been used, and the dispersion pH is below 7.8, pH is brought up to 7.8 with additional diisopropanolamine). The dispersion is poured into a metal tank, the broadest walls of which are substantially parallel with and 2.54 cm. out from the faces of a thin metal panel anode. The tank is wired as a direct current cathode, and the direct current anode is a gauge, 10.17 cm. (4 inches) wide, tared steel panel immersed in the bath 7. 62 cm. (3.5 inches) deep. At 26.67 C. (80 F.) bath temperature direct current is impressed from anode to cathode at 100 volts for one minute from an external power source, the current measured by use of a coulometer, and the current turned off. The anode panel is removed immediately, rinsed with distilled water, baked for 20 minutes at 176.67 C. (350 F.) and weighed. All volatile material such as water and amine is presumed to be removed from the film for practical purposes by the baking operation.

The difference between tared weight of the fresh panel and final weight of the baked panel divided by the coulombs of current used, times 107.88, divided by 0.001118 gives the electrical equivalent weight of the resin for purposes of this invention.

To avoid duplication, the method of this invention is explained in further detail using for purposes of illustration that embodiment of electropainting which has proven most practical to date in the light of the present state of this techniology, i.e. electropainting wherein at least a substantial portion of the film-forming resinous material is a synthetic polycarboxylic acid resin and is employed with a dispersal assistant comprising a water soluble amino compound.

The especially suitable water soluble amino compounds are soluble in water at 20 C. to the extent of at least about 1% basis weight of solution and include hydroxy amines, polyamines and monoamines such as: monoethanolamine, diethanolamine, triethanolamine, N-rnethyl ethanolamine, N-aminoethyl ethanolamine, N-methyl diethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, polyglycol amines such as HO(C H O) C H NH hydroxylamine, butanolamine, hexanolamine, methyldiethanolamine, octanolamine, and alkylene oxide reaction products of monoand polyamines such as the reaction product of ethylene diamine with ethylene oxide or propylene oxide, laurylamine with ethylene oxide, etc.; ethylene diamine, diethylene triamine, triethylene tetramine, hexamethylene tetramine, tetraethylene pentamine, propylene diamine, 1,3-diaminopropane, imino-bispropyl amine, and the like; and mono-, di-, and trilower alkyl (C amines such as mono-, di-, and triethyl amine.

The best films are deposited when about 30-60% of the total amino equivalents present in the bath, both combined and free, are contributed by water soluble polyamine, and thus it is preferred to operate that way. Preferably, it is diethylene triamine for efficiency and economy. The polyamine can be added to the bath along with supplemental binder concentrate composition dosing or separately.

The hydroxy amines, particularly those that are allphatic in nature at points of hydroxyl attachment, such as the alkanol amines, are also very useful for treating the polycarboxylic acid resin for dispersion and appear to have some desirable resin solubilizing effect in water over and above their neutralizing action. As a practical matter, the water soluble amino compound present in the bath over and above that amount necessary to impart anionic polyelectrolyte behavior to the particular polycarboxylic acid resin in the binder can be considered excess and is desirable, providing that the bath pH does not get so high that the bath absorbs CO from atmosphere, or the high amine concentration lowers the bath resistance to below about 500 ohm-centimeters. Broadly, the proportion of amine used can be between about 2 and about 7 times, and preferably between about 3.5 and about 5.5 times, the minimum amount necessary for imparting anionic polyelectrolyte behavior to the particular binder resin or resin mixture in the bath. Specific resistance of the bath as made up is advantageously between about 700 and about 1000 ohm-centimeters to deposit coatings about 25 microns thick as a priming coat. Higher bath resistance gives a thinner film and vice-versa.

Ammonia alone can be used but is less advantageous in my process for partially neutralizing the acid resin or resin mixture because it is so highly volatile at operating temperatures, small additions of it can cause comparatively large changes in pH of the bath, and baths using it tend to pick up CO from the air easily and thus are susceptible to uncontrolled change in electrical characteristics. Accordingly, ammonia is used only to assist in dispersing the resin in the bath along with other water soluble amino compounds, and not to the exclusion of other water soluble amino compounds.

When the polycarboxylic acid resin binder in the bath is substantially below about 1%, the film deposition is not as good as at higher concentrations. At even lower resin concentrations in the bath the evenness, smoothness, adhesion and thickness of the film deteriorates extremely rapidly. When the resin dispersion concentration is substantially above about -40% by weight, the bath viscosity can become so high that there is paint dragging when the coated body is withdrawn from the bath, that is, paint adheres and flows off non-uniformly to give an uneven deposit. The upper practical limiting concentration, it should be understood, will be to some extent a function of the particular resin in the bath at operating temperature (e.g. about 1550 C. generally) correlative to its ease of dispersion of dissolution in water, its electrical equivalent weight, and its specific rate of change of viscosity with dispersion concentration. The 3540% represents a practical maximum.

Also, the bath viscosity is especially important in large scale operations where electrical energy converted to bath heat has a relatively small area per unit volume of bath container to dissipate from. Accordingly, as viscosity goes up, the efficiency of heat transfer with cooling devices internal or external to the bath and from the tank walls themselves decreases substantially. Handling of the fluid in the bath and its drainage from the coated articles as they are withdrawn also are distinctly inferior when the viscosity of the bath rises greatly above that of water, i.e. more than about 200 times that of water. Heat control in the bath is important within a temperature range of roughly 15 to 50 C. to prevent the generation of undesirable volatile. materials and even the destabilizing or undue additional polymerization of the paint dispersions in some cases. With a bath viscosity not above about 30 times that of water the heat control can be very simple since the efficiency of heat transfer is quite high.

The proportion of amino compound, particularly hydroxy amines in the bath, can be used to manipulate bath viscosity, the higher proportions generally promoting apparent solubilization of the resin and some reduction in viscosity.

In the exemplary paint baths described hereafter, the resin in the bath dispersions shows anionic polyelectrolyte behavior because deposition of the resin on the anode is essentially directly proportional with the direct current passing through the bath. The quotient of coulombs of electricity per gram of a particular resin binder deposited is virtually independent of voltage in the operating range (less than about 5-10% variation), when allowance is made for the additional current used to drive the varying concentrations of amino compound to the cathode, even when the maximum voltage is doubled or trebled in the operating range of 100-500 volts. It further appears that when the polyelectrolyte resin binder coats tenaciously on a pigment or other particle in the bath, such particle assumes the migration properties to the anode similar to the polycarboxylic acid resin itself.

The polycarboxylic acid resins in the bath appear to exhibit the electrical migration property of anionic solutes, the resin ion present capable of being thought of as [R(COO)n] having n negative charges (where R represents the resin nucleus and COO represents a carboxyl radical). For illustration the amino ions resulting from neutralizing the resin in the bath (where the water soluble amine is used, is for example, a primary monoamine) can be thought of as [R'NH Where R' represents the amino compound nucleus.

Direct current is passed through the bath at a potential gradient of about 50 to 1000 volts to deposit a coating film of the anionic carboxylic resin on the anode workpiece.

As the coating builds up on the anode, the electrical resistance of the circuit increases, thus concentrating the electrical energy on covering pinholes, and other inacces sible areas until a coating of uniform resistance (and uniform thickness) is obtained. The coated workpiece is then removed from the bath and the coating is cured at elevated temperatures according to conventional practices.

One of the primary advantages of this process is that a large number of workpieces can be coated in a short period of time using a single tank of electropaint dispersion.

Accordingly, once the tank has been filled with the resin dispersion, the coating operation can be continually conducted over prolonged periods (i.e., weeks or months) without dumping, cleaning and refilling the tank. It is, of course, necessary to replenish the tank with the anionic, polycarboxylic resin, as this resin is removed from the bath in the form of workpiece coating.

While extended periods of commercially acceptable productions have been attained by this method, considerable difficulty is often encountered when a series of aluminum workpieces are successively coated. It has been observed that the quality of the coating deposited on the individual workpieces suffers as the series progresses. For instance, the quality of the coating deposited after one month of operation are often not as smooth, uniform and glossy as those coatings deposited initially. Additionally, it has been observed that the coating color is often off specification after prolonged operation. This is particularly true in the case of white and other light colored coatings.

I have found that this deterioration in coating quality is due primarily to an accumulation of inorganic, anionic contaminants in the coating bath. These anionic contaminants are primarily inorganic mineral acid anions such as sulfates acid sulfates (HSOg), chlorides c1 fluorides (F), bromides (BI nitrates (NO nitrites (N05) carbonates (CO acid carbonates (HCO phosphates (PO phosphites (PO chromates (CrOE) and sulfites (80 principally halides and oxygenated anions of elements having atomic numbers between 6 and 25 inclusive.

In spite of efforts to prevent these contaminants from entering the bath, they are often introduced in the form of improperly deionized water, or by improperly cleaned workpieces. This is particularly true in commercial operations where thorough deionization of large workpieces is not practical.

The inorganic anion concentration in the bath that causes a detrimental effect is quite small as compared to the concentration of the polycarboxylic resin, in that rough, heavy, non-uniform electrocoatings are deposited on aluminum substrates when the total concentration of the above and other inorganic acid anions exceeds about 30 parts per million (p.p.m.). According to the present invention, the total inorganic anion concentration is maintained below 10 p.p.m. and preferably below 5 p.p.m. through an ion exchange process.

The detrimental effects of the individual inorganic anions are additive in approximate proportion to their respective concentrations in p.p.m. For example, a concentration of 15 p.p.m. of sulfate (S05) and 20 p.p.m. nitrate (NO can produce an unsatisfactory coating, while coatings deposited in a bath containing either anion alone would be satisfactory.

Since anions will exert electrolytic influence in proportion to the number of ionic equivalents that are present, there is no theoretical basis for using the concentration in p.p.m. of chemically different anions as a common basis of comparison. This is just a convenient, empirical relationship of practical significance.

In practicing the present invention, the total inorganic anion concentration is maintained at a below 30 p.p.m. (usually less than 10 p.p.m.) by contacting the electrocoat bath with an anion exchange medium capable of exchanging an innocuous ion for inorganic anionic contaminants in the bath, without substantially removing the anionically dispersed polycarboxylic coating resin from the bath. These requirements can 'be fulfilled by synthetic anion exchange resins in the hydroxyl form. An anion exchange resin is in the hydroxyl form when it has hydroxyl ions (OH) available for external exchange.

Ion exchange is a phenomenon whereby a highly active ion associated with a relatively inactive nucleus (ion exchange resin matrix) can be exchanged for another active but different ion in solution. Ion exchange resins are well known in the art and can be visualized as having an elastic, three dimensional hydrocarbon network containing ion active groups. The network is prepared from a copolymer of styrene and divinylbenzene, the latter serving as a cross-linking agent and also to form a three dimensional structure. Additional information on ion exchange resins is given in US. Pats. 2,366,007; 2,341,907; 2,591,574; 2,591,573 and 2,614,099; which are incorporated herein by reference.

Synthetic anion exchange resins are cross-linked polyelectrolytes comprising a polymeric matrix containing a large number of ion active groups. The polymeric matrix is dimensionally stable, porous, and inert. Cationic active groups are firmly attached in this polymeric matrix, and are immobile. The electrical charge of these immobile cationic groups is balanced by an equivalent number of anionic groups (e.g. hydroxyl) which are mobile and can exchange with other ions of similar charge from an external source. When the anionic exchange resin has hydroxyl groups (OH-) as the mobile exchangeable group, the anionic exchange resin is said to be in the hydroxyl form.

According to the present ion exchange process, harmless hydroxyl ion enter the bath while the detrimental inorganic acid anions are removed.

In selecting particular anion exchange resins, pore size of the polymeric matrix is selected to be large enough to admit the inorganic anionic contaminants from the bath, but small enough to exclude the larger anionic polycarboxylic coating resin. Suitable commercially available anion exchange resins include Dowex l, Dowex 2, Dowex 11 and Dowex 21K resins in the hydroxyl form. These resins are strong base anion exchange resins incorporating quaternary ammonium functionality and are sold by the Dow Chemical Company. Other suitable resins include the hydroxyl form of the IRA series of the Macroreticular Amberlite Ion Exchange Resins sold by the Rohm and Haas Company.

The anion exchange resin can be contacted with the contaminated electrocoat bath by any conventional technique. For instance, a quantity of the anion exchange resin can be mixed in the bath and then removed after the inorganic acid anion concentration is measured to be below the acceptable level. Alternatively, the ion exchange can be accomplished by circulating (either periodically or continuously) the bath through an external bed of the ion exchange resin. External circulation through a resin bed is preferred in the interest of commercial practicality.

The reasons that these anions interfere with the coat ing quality is not presently understood, although it is strongly suspected that these anions, being quite mobile electrically, compete with the polycarboxylic resin anions in collecting at the anode. These inorganic acid anions then deposit on, and chemically react with, the aluminum workpiece together with the coating film. As the coating builds up, these anions become entrapped on the aluminum surface and encapsulated within the coating film. This results in the deposition of a heavy, rough coating of nonuniform character. The coating also tends to be darkened by the presence of these anions. This is particularly undesirable when white or other light colored coatings are being deposited.

Besides having a detrimental effect on the quality of the coating as deposited, the anions entrapped in the coating film also provide latent chemical reaction sites which can be activated during the service life of the coated workpiece. This is particularly true in the case of aluminum in that bubbles and blisters often develop in electrocoated aluminum articles. This is probably caused by the high chemical reactivity of aluminum. For instance, an aluminum workpiece that has been coated in an electrocoating bath containing excessive mineral acid anion contaminants will be more sensitive to ordinary solvents and alkaline cleaning solutions than an aluminum workpiece that has been coated in a similar electrocoating bath that is free from such contaminants.

The following exampes show how the invention can be practiced, but should not be construed as limiting the invention. All parts are parts by weight and all percentages are weight percentages unless otherwise indicated.

EXAMPLE 1 An acrylic resin is made by slowly adding a mixture of 60 parts of butyl acrylate, 25 parts of styrene, 15 parts of methacrylic acid, and 2 parts of dicumyl peroxide into 23 parts of 2-butoxy ethanol. This reaction mixture is maintained at about 158 to 170 C. for a four hour period in an agitated reactor equipped with a reflux condenser. The resulting reaction product is cooled to about 100 C., and 18 parts of hexakis (methoxy methyl) melamine (Cymel 300, sold by American Cyanamid Chemical Company) is added over a 15-minute period. The re- 8 sulting acrylic resin dispersion is then allowed to cool to room temperature. This acrylic resin dispersion apparently soluble in the electrocoating bath hereinafter described.

A white paint concentrate, containing about 35% NVM (non-volatile materials) is prepared by blending 260 parts of the acrylic resin dispersion prepared above, 17 parts of mineral spirits, 44 parts of diisopropanolamine, 442 parts of deionized water, and 122 parts of a pigment grind. The pigment grind is prepared by mixing 92 parts of the acrylic resin dispersion prepared above, 18 parts of diisopropanolamine and 453 parts of deionized water with 387 parts of kaolin clay and 580 parts of pigment grade TiO powder in a pebble mill.

The electrocoating bath is then prepared by diluting the above described white paint concentrate with deionized water to 8% NVM with deionized water. The deionized water has a specific resistance greater than 50,000 ohm-cm. at F.

The term NVM has been used above. This term refers to non-volatile material and is determined according to the ASTM test for Non-volatile Content of Varnishes, test designation D-l644-59.

The anodes used are 10.16 cm. wide by 8.89 cm. deposit length sheet aluminum panels, and the painting operation is conducted in a metal tank equipped with an agitator. The tank is wired as the cathode. The tank contains about 1100 mls. of the electrocoating bath described above, at a temperature of about 30 C. A constant D.C. potential of about volts is impressed across the tank cathode and the aluminum panel anode, from an external circuit. The anode is then slowly immersed in the bath over a 15-second period, and electrical current flows between the anode and cathode, while the D.C. potential is maintained constant at about 150 volts. After one minute of electropainting under these conditions (including the 15-second immersion period), a coating is deposited on the aluminum anode. This electrodeposited paint film is white in color, water resistant, slightly tacky, and tenaciously adherent. After oven drying at about 176 C. for about 15-30 minutes, the coating is about 1 mil in thickness, and is tough, uniform and quite glossy.

EXAMPLE 2 The electrocoating bath of Example 1 is contaminated with about 75 parts per million (ppm) of sulfate ion by adding sulfuric acid to the bath. This condition is designed to simulate a commercial coating operation wherein mineral acid anionic contaminants enter the bath with commercial aluminum parts and/or improperly deionized water.

The coating operation is conducted as in Example 1. The aluminum panels, after coating, but before baking, have a very rough, uneven texture and are off-white in appearance. After baking, the coatings have a slight yellowish cast and are very rough and non-uniform in appearance and in texture. Additionally, the coatings are very low in gloss.

To further demonstrate the detrimental effects of inorganic acid anionic contaminants in the electrocoating bath, several additional experiments are performed using the method of Example 1. In these experiments, the voltage is held constant at the level indicated in each run, and the coating time is constant in each run at one minute. As in Example 1, the anode panel is immersed in the electrocoating bath at a uniform rate requiring about 15 seconds for the panel to be immersed to the 8.89 cm. deposit length. This entrance time is included in the one minute coating period.

The following table presents the thickness of the coating deposited in mils, as a function of the constant D.C. voltage impressed across the electrodes in an anionically contaminated electrocoating bath.

The gloss of the coating deposited (after oven drying) is evaluated by a reflectance gloss meter test method wherein light beams strike the surface at a given angle and the light beams reflected from the surface are detected and measured by a photocell. The test method used is the ASTM test D-523-62-T, entitled Specular Gloss. In this test method, the higher test values indicate higher gloss. The angle of the incident beam is 60. The test results are set forth below:

Thickness 60 of coating gloss Constant voltage between deposited to anode and cathode (mils) value EXAMPLE 3 The sulfate contaminated electrocoating bath of Example 2 is circulated through a bed of ion exchange resin until the sulfate ion level is reduced to about 5-10 p.p.m. The bed is a /2 inch diameter by 36 inch glass tube containing about 120 mls. of anion exchange resin. The resin used is Dowex 2X8 ion exchange resin in the hydroxyl form. The flow rate of the electrocoating bath through the bed is about 10 mls. per minute.

Aluminum panels are coated by the method of Example 2 in the ion exchanged electrocoating bath of this example. The coated panels are then evaluated by the method of Example 2. The results are set forth below:

Thickness 60 of coating gloss Constant voltage between deposited test anode and cathode (mils) value EXAMPLE 4 The bath of Example 1 is contaminated with about 50 p.p.m. of nitrate ion by adding nitric acid to the bath. This example demonstrates the detrimental effect of the presence of the nitrate ion. The nitrate ion is a common contaminant in commercial processes. Aluminum panels are coated by the method of Example 2 and thick, uneven, rough yellowish coatings are obtained. The physical characteristics of these coatings are set forth below.

Thickness of coating deposited (mils) Constant voltage between anode and cathode Appearance Smooth.

0 Rough.

3 Very rough. g D

0. Extremely rough.

EXAMPLE 5 The coating bath of Example 4 is circuited through an ion exchange resin bed similar to the bed described Thickness of coating deposited (mils) Constant voltage between anode and cathode Appearance Very smooth. 0.

These panels are very white, very glossy and have a smooth, uniform surface texture.

These coated panels are superior to the panels prepared in Example 4 as is evident by comparing the physical data.

While the invention has been described in exemplary detail with respect to sulfate and nitrate contaminants, the present method will improve the performance of electrocoating baths that contain any inorganic acid anionic contaminants.

By aluminum anode or workpiece I mean to include those having surfaces of aluminum, anodized aluminum, and alloys which are preponderantly aluminum, eg about to 99+% aluminum.

Having thus described the invention, what is claimed is:

1. In the method for electrically depositing a coating on a series of aluminum articles wherein each of said articles successively serves as an anode in an electrical circuit while in contact with an electrocoating bath, said bath comprising an aqueous anionic dispersion of a polycarboxylic, film-forming resin, said bath further containing inorganic acid anions as impurities; and an electric current flows between said anode and a cathode in electriacl communication with said bath under sufficient electrical potential to deposit a coating of said film-forming resin upon said anode from said bath, the improvement which comprises:

maintaining the inorganic acid anion concentration of said electrocoating bath below about 30 p.p.m. by contacting said bath with an anion exchange resin in the hydroxyl form, whereby the deposited coating properties of uniformity and glossiness are enhanced.

2. The method of claim 1 wherein said bath is passed through a bed of said anion exchange resin.

3. The method of claim 1 wherein said ion exchange resin is mixed in the electrocoating bath and then removed from said bath when said inorganic acid anion concentration is below about 30 p.p.m.

4. The method of claim 1 wherein the inorganic acid anion concentration is maintained below about 10 p.p.m.

5. In a method for electrocoating a series of aluminum articles wherein each of said articles is successively contacted With an aqueous coating bath having paint dispersed therein and a cathode in electrical communication therewith, said aluminum article serving as an anode, wherein mineral acid anions are present as impurities in said bath during the electrocoating process, said paint containing a predetermined fraction of the film-forming paint binder a synthetic polycarboxylic acid resin at least partially neutralized with a suflicient quantity of a water soluble amino compound to maintain said polycarboxylic acid resin as a dispersion of anionic polyelectrolyte in said bath, said acid resin having electrical equivalent weight between about 500 and about 20,000, acid number between about 20 and about 300 and said resin exhibiting anionic polyelectrolyte behavior in said bath as indicated by its depositing as a coating on said anode substantially directly proportionally to electric current flow through said bath under the influence of an electric potential between said 1 1 1 2 cathode and said anode, the improvement which com- References Cited prises: NT

contacting said bath with an ion exchange resin in the UNITED STATES FATE 5 hydroxyl form to maintain the mineral acid anion 3,355,373 11/1967 Brewer a1 204 181 concentration of said bath below about 30 p.p.rn. 5 3,444,064 5/1969 Johnson wheregy the deposited colating groperties of uniform- FOREIGN PA 1ty an g ossmess are en ance 6. The method of claim 5 wherein said bath is passed ggiggg i zfi g through a bed of said anion exchange resin.

7. The method of claim 6 wherein said mineral acid 10 1504209 10/1967 France 204 181 anion concentration is maintained below about 10 ppm. HOWARD S, WILLIAMS, Primary Examiner