US3682814A - Cathodic electrocoating process - Google Patents

Abstract

AN IMPROVEMENT IN PROCESS IS SHOWN FOR ELECTROCOATING CATHODIC SUBSTRATES WITH PAINT FROM AN AQUEOUS BATH REPLENISHED PERIODICALLY OR CONTINUOUSLY WITH CATIONIC RESINOUS PAINT BINDER, SAID BINDER DISPERSED IN THE BATH WITH IONIZING ACID. THE ACID USED IS ONE READILY DECOMPOSABLE DURING THE OPERATING, E.G., FORMIC OR CITRIC; THE ANODE ZONE CONDITIONS ARE ESTABLISHED AND MAINTAINED FOR DECOMPOSITION OF AT LEAST ABOUT A QUATER OF SUCH ACID SUBSTANTIALLY AS FAST AS IT IS DEIONIZED AT THE ANODE; A DECOMPOSITION RESIDUE IS SEPARATED FROM THE BATH: AND ADDITIONL SUCH ACID IS ADDED TO THE BATH AT THE RATE THE ACID IS BEING DECOMPOSED. ADVANTAGEOUSLY FOR SUCH DECOMPOSITION THE ANODE ZONE CONTAINS AN OXIDATION PROMOTER, IS AT HIGHER TEMPERATURE THAN THE CATHODE ZONE, AND THERE IS HIGHER ANODE THE CATHODE ELECTRIC CURRENT DENSITY.

Description

US. Cl. 204-181 9 Claims ABSTRACT OF THE DISCLOSURE An improvement in process is shown for electrocoating cathodic substrates with paint from an aqueous bath replenished periodically or continuously with cationic resinous paint binder, said binder being dispersed in the bath with ionizing acid. The acid used is one readily decomposable during the operation, e.g., formic or citric; the anode zone conditions are established and maintained for decomposition of at least about a quarter of such acid substantially as fast as it is deionized at the anode; a decomposition residue is separated from the bath; and additional such acid is added to the bath at the rate the acid is being decomposed. Advantageously for such decomposition the anode zone contains an oxidation promoter, is at higher temperature than the cathode zone, and there is higher anode than cathode electric current density.

This invention relates to an improvement in process for electrodeposition of paints from an aqueous bath onto a cathode substrate wherein painting components, e.g., paint binder, are added to the bath continuously or periodically for replenishment of the bath. Advantages of the invention over prior suggestions include simplified maintenance of the compositional and electrical properties of the bath.

'Electrodeposition of paints containing synthetic filmforming resinous cationic paint binders onto cathodically charged substrates from an aqueous bath dispersion of the same has been shown, for example, in the following patents and applications: U.S. Pats. 2,345,543; 3,446,- 723; 3,454,482; 3,455,806; and copending US. Ser. No. 662,866. As metal and oxygen pickup of the depositing Lfilm is nil, such deposition is attractive. The teachings of this art are incorporated herein by reference, particularly those of US. Ser. No. 662,866 and U8. Pat. 3,454; 482, which suggests formic acid and strong mineral acids as being suitable ionizing acids for forming stable aqueous dispersions of cationic resin for electrocoating. However, maintenance of the desired narrow range of ionizing acid concentration of such bath has been difficult be cause of strong acid buildup as operations continue and the necessity for feed adjustments and/or bath treatment (e.g., ion exchange) responsive thereto.

The instant improvement minimizes or eliminates this deficiency and permits the user to make up cationic resin and ionizing acid in a great latitude of proportions, separately or premixed, with ease of miscibility of these components for extended operations with little or no appreciable strong acid buildup and effective dispersion of bath materials. The improvement in such electrocoating process comprises:

Employing as at least the prepouderant fraction of the acidic ionizing agent an acid which is readily decomposable into innocuous residues, at least one of which is readily separable from said bath dispersion during the electrocoating process;

Establishing and maintaining anode zone conditions for the decomposition of at least about a quarter and preferably substantially all of said acid into said residues United States Patent ice substantially as rapidly as said acid is deionized in said anode zone;

Separating said readily separable residue from said bath dispersion;

And feeding said bath with additional of said acid substantially at the rate it is being decomposed.

The most suitable ionizing acids for this use are those decomposable into water and carbon oxides; they in clude the acids formic, citric, malic and carbonic, and are suplied as such, in water solution, or as the appropriate anhydride. The first two are preferred for efficiency and economy.

At least the preponderance and preferably all of the acid equivalents in the electrocoating bath are of such readily decomposable acid or acids to minimize and preferably to eliminate acid buildup in the bath for prolonged operating periods. Small proportions of oxidation-resistant acids such as hydrochloric, acetic, sulfuric, maleic, and/or phosphoric can be used to lower bath pH, assist in solubilizing resin, and to carry current, but these can accumulate in very prolonged periods unless they are especially carefully proportioned in the bath relative to their attrition from the bath. Generally the acid equivalents used are slightly in excess (about 5- 25%) of that amount necessary to establish and maintain stable aqueous dispersion of the resin in the bath homogeneously with little or no agitation at cathode zone temperature. Restriction of such acid equivalents is one way for establishing and maintaining anode zone conditions for the decomposition of the easily decomposable acid substantially fairly rapidly. When more such acid equivalents are present, it usually is necessary to accelerate the decomposition as described hereinafter.

If necessary or desired, water formed from the decomposition of the acid above that tolerable in the bath can be removed periodically or continuously, e.g., by vacuum evaporation of bath liquid, electrodialysis of acid and water from the bath through an anionic dialyzing membrane, ultrafiltration or reverse osmosis of the bath dispersion, or the like.

The cationic resins for aqueous dispersion are those with sufficient ionizable amino functionality 'in their structure to form stable aqueous dispersions in the electrocoating bath containing such decomposable ionizing acid or mixture of acids. Tertiary amino functionality is preferred to achieve the marked transformation of the cathodic resin at the charged cathode from the stably dispersed state to a manifestly electrically resistant and 'water-resistant state (e.g., virtually water-insoluble for practical purposes) and to resist aminolysis of ester linkages in the resin. At least about 0.05-0.15 gram equivalent of such amino functionality per grams of cationic resin is needed to get good stable dispersion in water. When such minimum amino functionality is present, the acid equivalents used in the bath should be at least approximately equal to such amine equivalents. When the amine equivalents are substantially higher, the acid equivalents present can be only a fraction of them for stability of dispersion.

Typical cationic resins include the tertiary amino alkanol ester resins more fully described in commonlyassigned copending US. patent application S.N. 662,866, and the amino-containing copolymers described in U.S. Pats. 3,454,482; 3,455,806; and 3,446,723. These are nitrogen basic cationic polymers, the especially preferred type being polymeric tertiary amino alkanol esters of polycarboxylic acid. If hydroxylated substances are incorporated into the cationic resin, e.g., hydroxy alkyl acrylic or methacrylic acid units, etc. or into the bath, e.g., 2-alkoxy ethanol-1 such as 2-butoxy ethanol-1, glycols, monoalkyl ethers of diethylene glycols, etc. stable aqueous dispersions can be maintained at slightly lower levels of ionizable amino nitrogen in the resin.

Paint for the purpose of this application is meant to include simply the cationic resin, any extender resins codispersed for deposition with the same, and any conventional driers, U.V. sensitizers or absorbers, curing catalysts, film plasticizers, solvents, antifoam agents, finely ground pigments and/or fillers dispersed for codeposition with such resin or resin mixture. The deposit, and thus the paint also can contain tints or stains, if desired, and may be considered broadly analogous to enamel, varnish, lacquers, or paint.

Preferably the pigments and other solid particles to be codeposited with the cationic resin from the bath are extremely fine, e.g., for example having average particle size not substantially in excess of 2 microns for solid particles having density of 3.5 or more, and average particle size not over about 5 microns when such pigmentation and/or fillers are less dense. If as much as about 5% and preferably not more than about 2% by weight of the solid pigment, fillers, and codepositing solid resins are above microns particle size, this can be tolerated without substantial sacrifice in performance, but virtually nothing should be coarser than about 44 microns for good suspension in the bath and good film appearance.

Where the cationic resins are blended and/or reacted with other resins or plasticizing material for codeposition and resulting hardness or other desirable film properties, such extending resins usually include amino aldehyde resins such as melamine or urea formaldehyde resins, phenolic resins, and materials which blend well with or are soluble in the cationic resin by itself or in the presence of solvents such as alkoxy alkanols. Also appropriate for codeposition are: butadienestyrene latices, vinyl chloride and vinylidine chloride homopolymer and copolymer latices, polyolefin resins, fluorocarbon resins, bis-phenol glycidyl ether resins, dicyclodieopxy carboxylate resins, diolefins petroleum resin dispersions, resinous polyols, and resinous polyols esterified with carboxylic acids such as monocarboxylic acids, film plasticizers such as dioctylphthalate and the like. Such resins, if used as a simple blending resin, generally are used in smaller proportion in the bath than the cationic ones and never so high as to mask the good cathodic deposition of the cationic resin blend and cause manifest unevenness of film.

Those resins that are normally solid at deposition tem perature but readily fusible, including dicyclodiepoxy carboxylate resins, polyethylene resins, polyvinyl chloride resins, certain phenol formaldehyde resins such as those modified with rosin, powdered chlorinated hydrocarbon waxes, epichlorohydrin bisphenol type epoxy resins advantageously having molecular weight about 1000 or higher to resist hydrolysis in the bath, chlorinated rubber, vinyl acetate-vinyl chloride copolymers, and mixtures of same, can be melted and coalesced with the cationic resin at a force curing temperature of 300 to 400 F. Some extenders, particularly hexamethylol melamine and the like, can be considered as assisting in curing in some cases for pro ducing harder or tougher films.

The electrodeposition is done with direct (net unidirectional) current. In most cases such current ordinarily is rectified AC current having about a 5 to ripple factor. However, the current can be half wave rectified alternating current, pulsed current and so on, providing the net effect is unidirectional and thus the current broadly a direct current. Useful voltages across the bath can be as low as 15 or even lower, and should not be so high as to cause melting and deterioration of the deposited coating with attendant film disruption. Practical maximum 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 (0.01 to 0.5 second).

Where the ostensibly electrolytic deposition of the cathodic resin or resinous mixture containing cationic resin is slower than the, collateral electrodeposition of pigment (including mineral fillers and solid particulates of structurally nonionic or substantially non-ionized resinunder the conditions of deposition), the cationic resin-to-pigment weight ratio in the resulting cathodic paint deposit can be significantly lower than the corresponding ratio of the bath suspension. To take care of this and prevent the deposition of chalky, resin-starved appearing films, I prefer to establish the weight ratio of the cationic resinto-pigment at least about 2:1 in the bath, and to make up the bath with an aqueous replenishment composition having cationic resin-to-pigment weight ratio of at least about 1.5: 1, said weight ratio being less than the weight ratio of cationic resin to pigment in thebath and substantially the same as that in the film being deposited. Where a structurally nonionic or substantially nonionized resin that is film-forming on or before curing with the cationic resin is to be codeposited, I can lower the cationic resin-to-pigment weight ratio in the bath somewhat, e.g., about 1.5:1 or even lower, say about 1:1 and still get satisfactory films because of the extra pigment binding action obtained from said non-ionized resin after the coated substrate has been cured.

Replenishment of the bath also involves feeding incrementally or continuously ionizing acid, separately or together with the binder resin, substantially as rapidly as it is depleted from the bath. Generally, pH of the bath is maintained between about 1.5 and 5.5 and generally 2-3.5.

As previously mentioned, useful voltages in the present operation can be from about 15 volts up to 350 and higher in some cases, provided, however, that the film is not ruptured because of heat generated with large amperage flows. The bath for ordinary service is a tank of paint dispersion in which the cathode is immersed for electrocoating then removed. However, with special equipment designs the electrocoating can be done by spraying the aqueous bath dispersion onto the cathodicallycharged part, provided that the spray makes a continuous stream which carries the necessary electric current between the electrodes. Dipping of the cathode in a bath tank is preferred for reaching remote surfaces most simply, the techniques being essentially the electrical converse of those which have been proposed previously for dipping anodes for electrocoating. The cathodes can be rinsed, before or after coating, with water, electrolyte solutions, or aqueous bath dispersion, dipped with power ofl? or on, coated at substantially constant amperage or at substantially constant voltage or at increasing voltage, and removed from the bath with power off or on.

Ordinarily the total binder resin solids (cathodic resin plus any extender resin and plasticizers, N .V.M.) is maintained in the bath between about /2% and about 20%, and advantageously between about 5% and about 15%. Replenishment composition generally can have about 30- such binder resin solids (N.V.M.) and preferably 60% to 100%. Customarily the freshly electrocoated part is rinsed with water and blown with air to perfect the film, then the part is air-cured or force-cured, e.g., with heat or other known means, to yield a tack-free film. Typical baking cure is 350 F. for 20 minutes in air, and this also is used conveniently as a test for non-volatile material (N.V.M.) of a paint deposit.

For most efficient and greatest latitude of operation, I prefer to maintain decomposition of the readily decomposable ionizing acid roughly as rapidly as said acid is deionized at the anode. When less than about a quarter of such acid is being decomposed as it is attracted to the anode, and thus redisperses in the bath for further neutralization reactions, so little make-up acid is being added with the makeup resin to the bath that such resin is quite difficult to disperse rapidly and thoroughly in the bath. When, however, at least about 25-40% and advantageously even more of such acid is being decomposed, the dispersion of makeup resin is greatly facilitated,

particularly where the makeup acid is well mixed with the makeup resin before adding same to the bath. The makeup mixture preferably also contains all the other ingredients necessary to keep the bath in substantial balance as to composition.

As the operating voltage causes electrolysis of water, oxygen is generated at the anode. This in itself tends to oxidize the readily decomposable ionizing acid, which is attracted to the anode as an anion and converted there into the corresponding molecular species for decomposition. Maintenance of the anode warm (above the temperature of cathode and, for example, at least at 90- 130 F.) assists in the decomposition of the acid. The anode and zone around it can be maintained thus by external heat input into the anode and/or by the waste heat generated in the electrocoating process.

A fairly simple way to accomplish the latter anode zone heating and get intensive action is to make the wetted anode surface area substantially smaller than the wetted cathode area to be coated, e.g., by 4-200 times and preferably -100 times, thus getting higher current density at the anode than the cathode. Heat retention about the anode can be assisted by separating the anode (or anodes if more than one anode is used) from the cathode (or cathodes if more than one cathode is being coated) by a porous diaphragm or membrane permeable at least to the anions present and Water, which membrane is a poor conductor of heat. Suitable membranes include textile cloth, Woven polyolefin fiber screens, uncoated regenerated cellulose, paper or the like.

Advantageously, the cathode zone bath liquid is maintained at a moderate temperature (e.g., 60-120" F.) with cooling if necessary, while the anode zone is maintained 10-100 higher than this to accelerate the molecular acid decomposition.

Other techniques for accelerating such acid oxidation include adding an oxidation promoter to the bath, most suitably to the anode zone when the anode is separated from the cathode by a porous diaphragm or membrane. The type and concentration of such promoter should be regulated so as to suppress uncontrolled decomposition of the acid except in the anode zone. Thus chromic acid must be used with care, only in minute concentration and this in an isolated anode area. Cobalt, lead, manganese, vanadium, copper and other heavy metal salts of very weak acids (e.g., metal driers such as naphthenates, octoates, acetates, etc.) preferably sparingly soluble in Water if actually soluble at all, are useful in concentrations (measured as metal) of 0.0010.1 weight percent of the bath dispersion. Other suitable promoters in very small concentration (0.001-.1%) include peroxides and the like, for example, methyl ethyl ketone peroxide, hydrogen peroxide, sodium peroxide, benzoyl peroxide, ozone, etc.

Additionally the anode surface can be coated with oxidation catalyst sites, for example, platinum as in the form of platinum black, other precious metals, chromates, manganates, vanadates, molybdates, cobalt, nickel, chromium, and various oxides of these metals or other heavy metals, particularly if they do not materially suppress the passage of electric current.

When carbonic acid is used as all or part of the ionizing acid, it usually is necessary to use superatmospheric pressure in the cathode zone to maintain adequate amino resin ionization for stable dispersion, for example, 10-60 p.s.i.g. When the anode zone is maintained at a substantially lower pressure than this, for example, atmospheric pressure, decomposition of the acid in the anode zone can be accelerated.

The materials of apparatus construction are conventional and corrosion-resistant where necessary or desirable, for example, austenitic stainless steel, carbon, plastic, etc., and where necessary are electrically conductive. Advantageously for efficiency and economy the current used is DC made by rectifying AC current.

The following examples show ways in which this invention can be and has been practiced, but should not be construed as limiting this invention. Unless otherwise indicated, all temperatures herein are in degrees Fahrenheit, all pressures in pounds per square inch gauge, all parts are parts by weight, and all percentages weight percentages.

EXAMPLE 1 There is reacted in an agitated tank 10,364 parts of alkali-refined linseed oil and'2,479 parts of maleic anhydride (heated together at 232 C. for about 3 hours or until Acid No. of -100 results), then cooling this intermediate to 157 (3., adding 2,084 parts of vinyl toluene containing 42 parts of di-tertiary butyl peroxide and reacting at about 218 C. for about one hour. The resulting vinyl toluenated material then is cooled to 157 and 2,643 parts of thermoplastic coumarone-indene, oil-soluble extender resin is added, the temperature raised to 195 and the mixture held one hour. The coumarone-indene resin is a solid lump resin and it is considered heat reactive.

One thousand five hundred and twenty-five parts of the extended resin intermediate is reacted with 135 parts of N,N-dimethyl ethanolamine to yield an almost completely esterified resin having 0.084 equivalent of tertiary amine per grams of resulting resin. One hundred parts of extended resin product then is neutralized with 4.2 parts of formic acid (in the form of a 50% aqueous solution of the acid) to form a concentrate composition. A portion of this neutralization product is further reduced with water to give a 5% (N.V.M.) resin solids electrocoating bath dispersion, having the following characteristics: pH 3.5; specific resistivity at 80 of 1,100 ohm-centimeters; dispersion colored amber and somewhat cloudy; 7 /2% acid equivalents above that necessary for the amine neutralization.

This dispersion is held at 80 in an insulated agitated tank having immersed therein a cathode cold-rolled, smooth steel plate wired to a like anode steel plate through an external circuit. Voltage between the anode and cathode is held at 50 volts DC (rectified alternating current) for 60 seconds, and this yields a deposit of about 0.75 mil of resinous film on the cathode plate, the coulombs per gram of film deposited being 61. There is 0.091 acid equivalents and 0.841 amino equivalents per 100 grams of the esterified resin in the bath at the start of electrodeposition. About 40% of the formic acid decomposes into carbon dioxide and water in the anode area substantially as rapidly as it comes into contact with the anode, this being estimated by titration of a sample of the bath at the start, middle, and at the end of the coating series to determine acid content of the bath. The resulting cathodic deposit is substantially insoluble in water and withstands rinsing with water.

At the end of the coating period the coated cathode plate is disconnected, withdrawn from the tank, and force cured in air at 350 F. for 20 minutes to yield a hard, protective paint film.

A series of 11 more like steel cathodes are coated and cured this Way in succession using the same bath. After the 6th and 12th panel is coated, additional of the amino resin and formic acid are added in the form of an aqueous replenishment concentrate composition containing 1.68 parts of formic acid (as 50% aqueous solution) per 100 parts of extended resin to bring the solids content of the bath to about 5% EXAMPLE 2 A copolymer is made by reacting 25 parts styrene, 50 parts butyl acrylate, 10 parts hydroxypropyl methacrylic acid and 15 parts of dimethylaminoethyl methacrylate in the presence of 20 parts of 2-butoxy-ethanol-l and 2.5 parts of dicumyl peroxide catalyst. The resulting product, with the solvent unseparated, is blended with 25 parts of hexamethylol melamine and 2.9 parts formic acid in the form of 50% aqueous solution of the acid to form a bath replenishment concentrate composition. A portion of this composition is further reduced with water to give a 5% (N.V.M.) resin solids (excluding butoxyethanol solvent) electrocoating bath dispersion having the following characteristics: percent acid equivalents above that necessary for stable bath dispersion about 18%; pH about 3.6; solution color amber and clear; bath resistivity 1,640 ohmcentimeters.

This solution is poured into an insulated tank having immersed therein a cathode steel plate wired to a roughened, nickel-plated copper anode plate through an external circuit. The anode area immersed is about 1 that of the cathode area immersed. The anode is near the wall of the tank; interposed between the anode and the cathode, to separate the anode zone from the cathode zone, is a very fine polypropylene fabric screen standing out about 1 inch from the surface of the anode that faces the cathode. The cathode zone is agitated with a mixer and the temperature thereof is about 80, maintained thereat with a cooling water coil as necessary. Voltage between the anode and the cathode is held at 75 volts DC for 60 seconds. This yields a deposit of about 1 mil thick of resin film on the cathode plate, the coulombs per gram of film deposited at 75 volts being 44. There is about 0.051 acid equivalents and 0.051 amino equivalents per 100 grams of the esterified resin the bath at the beginning of the electrodeposition. The formic acid decomposes into carbon dioxide in water in the anode area substantially as rapidly asit comes into contact with the anode, this being estimated by titrations of electrodeposition bath and the coulombs of electricity flowing. The resulting cathodic deposited film is substantially insoluble in water and withstands rinsing with water. At the end of the coating period, the coated cathode plate is disconnected, withdrawn from the tank, and force-cured in air at 350 F. for 20 minutes to yield a hard, protective paint film.

Successive coating runs are conducted with a series of like fresh steel cathodes in the same Way using the same bath without noticeable buildup in ionizing acid. After each third panel of a dozen is coated, additional of the amino resin and formic acid are added in the form of the aqueous replenishment concentrate composition to bring solids content of the bath to about 5%. After a few panels are run, the temperature in the anode zone is about 110. To further promote the decomposition of the formic acid in the anode zone, a small amount of aqueous hydrogen peroxide can be added thereto continuously or incrementally.

Having thus described the invention, what is claimed 1. In a process for electrocating with paint a cathodically-charged substrate immersed in a coating bath containing an aqueous dispersion of said paint, said bath having a cathode zone containing said substrate and an anode zone containing a charged anode, said charged electrodes being maintained in electrical contact with each other by means of said bath, wherein a resinous cationic paint binder is added to the bath periodically or continually and stably dispersed in said bath with the aid of an acidic ionizing agent, the improvement which comprises:

employing as at least the preponderant fraction of the acidic ionizing agent an acid which is readily decomposable into innocuous residues, at least one of which is readily separable from said bath dispersio during the electrocoating process;

establishing and maintaining anode zone conditions for the decomposition of at least about a quarter of said acid into said residues substantially as rapidly as said acid is deionized in said anode zone;

separating said readily separable residue from said bath dispersion; and,

feeding said bath with additional of said acid substantially at the rate it is being decomposed.

2. The process of claim 1 wherein acid is formic acid.

3 The process of claim 1 wherein said acid is citric acid.

4. The process of claim 1 wherein said acid is malic acid.

5. The process of claim 1 wherein said acid is carbonic acid.

6. The process of claim 1 wherein said acid is decomposed into said residues substantially as rapidly as said acid is deionized in said anode zone.

7. The process of claim 1 wherein said anode zone contains an oxidation promoter.

8. The process of claim 1 wherein said anode zone and said cathode zone are maintained at different temperatures, the anode zone being the higher.

9. The process of claim 1 wherein the current density on said anode is maintained substantially higher than the current density on said cathode.

References Cited UNITED STATES PATENTS HOWARD S. WILLIAMS, Primary Examiner