US3304250A - Continuous electrocoating process utilizing electrodialysis to control the bath composition - Google Patents

Description

Feb. 14, 1967 A. E. GlLcHRlsT 3,304,250

CONTINUOUS ELECTROCOATTNC PROCESS UTTLTZTNG ELECTRODTALYSTS TO CONTROL THE BATH COMPOSITION Filed March 17, 1965 5 Sheets-Shea?l 1 F/G. Z

ZZMMQLL A. E. GlLcHRlsT 3,304,250 CONTINUOUS ELECTROCOATING PROCESS UTILIZING ELECTRODIALYSIS Feb. i4, 1967 To CONTROL THE BATH ColvlPosIT101-lA v 1 5 Sheets-Sheet 2 Filed March 1'7, 1965 n W L 5%4/6014, TTR/VV Feb. 14, 1967 A. E. GlLcHRlsr 3,304,250

CONTINUOUS ELECTROCOATING PROCESS UTILIZING ELECTRODIALYSIS TO CONTROL THE BATH COMPOSITION Filed March 17, 1965 5 lSheets-SheetI 3 7//// //////:/V/ lo n g s SI /f ""msV/////\/ l @y ya@ Z.

ATTQR/VEVS United States Patent O 3,304,250 CONTINUOUS ELECTROCOATING PROCESS UTI- LIZING ELECTRODIALYSIS TO CONTROL THE BATH COMPOSITION c Allan E. Gilchrist, Fairview Park, Ohio, asslgnor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Mar. 17, 1965, Ser. No. 440,583 11 Claims. (Cl. 204-181) This application is a continuation-in-part of my copending U.S. patent application S.N. 304,297 filed August 14, 1963 now U.S. Patent No. 3,230,162, which in turn is a continuation-in-part of U.S. patent application S.N. 249,812 tiled January 7, 1963, and now abandoned, which in turn is a continuation-in-part of U.S. patent applications S.N. 132,303 filed August 18, 1961; S.N. 183,024 iiled March 28, 1962; S.N. 186,496 tiled April 10, 1962; S.N. 202,691 filed June 15, 1962; and S.N. 218,575 tiled August 22, 1962, all of which are now abandoned, and copending application S.N. 186,320 filed April 10, 1962. The disclosures of each of the above recited U.S. patent applications beginning with S.N. 132,303 were incorporated by reference into the above recited U.S. patent application S.N. 249,812 while copending therewith, and the disclosures of said patent application S.N. 249,812 were likewise incorporated by reference into the above referred to copending U.S. patent application S.N. 304,- 2 97 while U.S. patent application S.N. 249,812 was copending therewith. The disclosures of my copending patent application S.N. 304,297 are incorporated herein by reference.

This invention relates to an improved method of electrocoating wherein relative concentrations of bath components are maintained through extended periods of continuous coating by electrodialysis. In particular, this invention relates to an improved method of electropainting with a multicomponent paint wherein a coating bath is formed by dispersing within an aqueous medium a synthetic organic resin having a plurality of Water ionizable functional groups within its molecular structure and a water ionizable dispersal assistant for such resin, the dispersed resin is converted into an essentially water-insoluble coating upon electrically conductive objects passing through such bath by electrically induced deposition and the relative concentrations of components of the bath are controlled by selectively removing a portion of such components by electrodialysis.

In the drawings:

FIGURE 1 is a cross sectional View of one embodiment of apparatus which can be used to demonstrate the instant method;

FIGURE 2 is a schematic side View of apparatus su1table for use in a continuous electrocoating operation; and

FIGURE 3 is a cross sectional view taken along line 3 3 of FIGURE 2.

The separate components of an electrocoating bath ordinarily'do not deplete at a rate corresponding to thelr relative concentrations in the coating bath. Quality coating on a continuous basis requires that the relative concentrations of the various bath components remain within limits dictated by the operational characteristics of the given coating bath. Maintenance of the desired concentrations of the various bath components would provide no problem if it were feasible in all instances to replace bath components in a continuous coating operation 1n the ratio at which such components deplete. A particular problem is provided by dispersal assistants which facilitate resin dispersion in the bath and hence are a necessary component of replacement feed. Ordinarily these materials deplete or paint out at a significantly lower rate than does the resin binder and their concentration 3,304,250l Patented Feb. I4, 1967 lCe within the bath continues to build as the painting process continues unless means for their selective removal from the bath are incorporated into the system.

The film-forming material employed in an electrocoating process -of the type hereincontemplated may 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.

Where 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 may be the result of the original formulation of the resin or subsequently introduced by chemicaly reacting a suitable resin with monomers and/ o1' 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, sunliower, saiiiower, perilla, hempseed, Walnut seed, dehydrated castor oil, rapeseed, tomato seed, menhaden, corn, tung, soya, oiticica, or the like, the olenic double bonds in the oil being conjugated or nonconjugated or a mixture, the coupling agent being an acyclic olenic acid or anhydride, preferably maleic anhydride, but also crotonic acid, citraconic acid, or anhydride, fumaric acid, or an acyclic oleiinic aldehyde or ester of an acyclic oleiinic ester such as acrolein, vinyl acetate, methyl maleate, etc., lor even a polybasic acid such as phthalic or succinic, particularly coupled glyceride oils that are further reacted With about 2-25% of a polymerizable vinyl monomer; maleinized unsaturated fatty acids; maleinized rosin acids, alkyd resins, e.g. the esteritication 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 (C1 4) 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, etc. The pH of the bath is, of course, dependent upon the relative concentrations of acidic and basic materials therein.

Where the binder material is to be deposited cathodically the primary resin may include one or more of the aforementioned resins having functional groups that ionize in the bath leaving the resin particle with a plurality of positive ion sites. Such groups may be amine or subp stituted amine groups, e.g. quaternary ammonium groups.

The conditions of operation are preferably such that there is a decided reduction in Water dispersibility when such resin contacts the cathode. Dispersion of the latter lilmforming materials is effected by the addition of suitably acidic materials such as water soluble carboxylic acids, e.g. formic acid, acetic acid, propionic acid, and suitably modified or buffered forms of certain inorganic acids, e.g. phosphoric.

In accordance with this invention an electrode in uid communication with the aqueous coating bath and of opposite polarity with respect to the electrode comprising the object being coated, is isolated from such object by a dialysis membrane and bath components are selectively removed from the coating bath.

The membranes thus employed rn'ay be of materials conventional to ordinary dialysis separations or they may be synthesized by methods known to the art to provide characteristics particularly desirable in the instant process. The membrane is preferably formed of a relatively nonconductive material having pores the diameters of which are limited by a predetermined maximum. These pores should be small enough to prevent the escape of significant amounts of the organic disperse phase, e.g. resin feedstock and particulate pigment where the latter is employed. The pores should be large enough to admit of the passage through such membrane of water and water soluble materials of lesser size or lower molecular weight than 'the resin particles provided for coating the workpiece. The membranes should be chemically resistant to the coating bath under the conditions employed in the painting process and of sufficient structural strength to withstand the agitational and other stressesl incidental to the coating operation in which they are employed. Such membranes include, but not by way of limitation, regenerated cellulose (from viscose), vinyl chloride polymers, styrene polymers, various cellulosic materials, and other synthetic resin sheets which are ion permeable. Ceramic plates may also be used.

-The pore size may vary in the range of about 20 A. to about 5,000 A. or even higher depending upon the size of the paint particles dispersed in the bath so long as no significant portion of paint desirable for deposition on theV workpiece migrates through the membrane. In those embodiments wherein the particles of the paint feedstock are of extreme iineness, e.g. the maximum resin particle size present fbeing estimated at not more than 'about 500 `rnillimicrons, an effective pore size between about 20 A.

and about 200 A. has been found suitable. Preferably, the'largest pores of the membrane have a diameter not substantially in excess of 5,000 A.

j Advantages of the instant process over prior practices include operating stability of the painting bath and paint binder compositions therein, even deposition of the paint film, simplicity of operating control, good paint coating into crevices and around corners, deposition in a selflimiting thickness, and resistance of the deposited film to condensed vapor washing prior to 'and during ultimate cure, as for example, by baking.

y.To avoid duplication, the method of this invention is hereinafter 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 technology, i.e. electropainting wherein at least a substantial portion of the film-forming resinous material isa synthetic polycarboxylic acid resin and is employed with a dispersal assistant comprising a Vwater soluble amino compound.

In one embodiment the polycarboxylic lacid resins have an acid number in the range of about 30 'to about 3,00 preferablyk between about 60 and about 200, and an electrical equivalent weight, hereinafter referred to as REW., between about 1,000 and about 20,000, preferably between about 1,000 and about 2,000, for ease of dispersion and efficiency of operation.

The acid number of resins Without appreciable content of anhydride groups can be determined with KOH by the ASTM standard method 555-54. If appreciable anhydride groups are present, the acid number can be determined by reiiuxing 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 HC1 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 Atransferred and coated on the anode per coulomb input to the process, the electrical equivalent weight of the resin is about 1303 or 0.015 O.9 l07.88+ 0.001118. By way of further illustration I tind electrical equivalent weight (in the nature of a gram equivalent Weight in accordance with Faradays laws) of -a particular polycarv boxylic acid resin or resin mixture simply and conveniently 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 50I grams of the polycarboxylic acid resin, 8 grams of distilled water and diisopropanolamine in an amount sufficient 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 dilu-ted to one liter with additional distilled Water to give 5% resin concentration in the resulting dispersion. (If a slight insufficiency of the amine has been used, and the dispersion pH is below 7.8, pH is brought up to 7.8 with additional dii-sopropanolamine.) 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 Vis Wired as la direct current cathode, and the direct current anode is a 20 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 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 iilm for practical purposes by the baking operation. The difference between tared Weight of the fresh panel and linal 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.

My preferred polycarboxylic acid resin for incorporating into the paint binder concentrate composition is a coupled siccative glyceride oil reacted with about 2-25%, basis the weight of the coupled oil, of a polymerization vinyl monomer such as vinyl toluene, styrene, alpha methyl styrene, acrylonitrile or the like. The reaction with the vinyl monomer appears to enhance the vdepositedl iilm by making it .convert especially rapidly from a fluent dispersed material into an immobile, adherent, nonwetta'ble (with water) and water resistant film on the sub-- strate anode when at least about 2% of the vinyl mono- Y mer is reacted into the coupled oil; the electrical resistancel of the bath is desirably raised when this resin is used also..V

The preferred vinyl i monomer for economy and etiiciency in the practice of my invention is vinyl toluene, preferably limited to maximum on the weight of the coupled oil. When this resin is extended with non-heat reactive phenolic resin by the high temperature process previously described, it is the most highly preferred one for metal priming because of its final hardness.

In general I have found that the more highly acidic polycarboxylic acid resins for my binder compositions, e.g. those having acid number substantially above about 100, will form good polyelectrolyte dispersions in my process at a fairly low bath pH. Thus, for example, I have operated at a bath pH as low as 5 with a polycarboxylic acid binder resin having acid number of 177. Using resins and blends thereof having substantially lower acid number, e.g. about 100 or below, generally requires a higher minimum pH, e.g. 7-7.3, to insure the consistent polyelectrolyte behavior in the bath when treated with the water soluble amino compound. Accordingly, the useful lower bath pH in my process will be a function of the kind and concentration of the particular paint binder to be dispersed; it will be broadly between about 5 and about 7.3, though it should be recognized that operation at a somewhat higher pH or pH range will give a greater margin of safety in maintaining consistent polyelectrolyte behavior. When adding supplemental resin having acid ,number below about 100, e.g. a coupled glyceride drying oil resin reacted with a polymerizable vinyl monomer and extended with, for example, phenolic resin, I have found it advantageous to control such supplemental addition to the bath to keep bath pH between about 7 and about 8.3.

The especially suitable water soluble amino compounds for the practice of my invention are soluble in water at C. to the extent of at least about 1% basis weight of solution and include hydroxy amines, polyamiues and monoamines such as: monoethanolamine, dieth'anolamine, triethanolamine, N-methyl ethanolamine, N-aminoethyl ethanolamine, N-methyl diethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, polyglycol amines such as HO(C2H4O)2C3H6NH2, hydroxylamine, butanolamine, hexanolamine, methyldiethanolarnine, 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-bis-propyl amine, and the like; and mono, di, and trilower alkyl (C1 8) amines such as mono, di-, and triethyl amine.

I have found that the best lms 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 I prefer 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 aliphatic 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 solubiiizing 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 CO2 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 lm 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 CO2 from the air easily 'and thus are susceptible to uncontrolled change in electrical characteristics. Accordingly, I prefer to use ammonia only to assist in dispersing the resin in the bath along with other water soluble amino compounds, and not to use it to the exclusion of other water soluble amino compounds.

Contrary to what would be expected from theoretical considerations, I have found that when the polycarboxylic acid resin binder in the b ath is substantially below about 1/2-1%, the lm 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 iilm deteriorates extremely rapidly. When the resin dispersion concentration is substantially above about 35- 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 15-50 C. generally) correlative to its ease of dispersion or dissolution in water, its electrical equivalent weight, and its speciic rate of change of viscosity with dispersion concentration. The 'S5-40% 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 Ithat 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 50-500 volts. It fur-ther 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 [RNH3] where R represents the amino compound nucleus.

In one embodiment, the electrodialyzing operation involves isolating the anode from the primary cathode to form anode and cathode zones separated by a dialysis membrane, charging the anode zone with the dilute, aqueous painting dispersion and the cathode zone with water. In another embodiment a dialysis membrane is interposed between the anode and one or more secondary cathodes which may or may not be within the bath retention vessel or coating tank.

In such electrodialysis operation only free amino Vcornpounds, low molecular weight resin breakdown products and amino electrolysis products, and stray inorganic cations will migrate through the membrane in any appreciable quantity while the paint deposits on the anode when the electrical circuit is made from anode to cathode through the compartmentai bath. When the titratable free amino compound concentration in the coating zone rises substantially -above about 2% by weight of the coating dispersion (bath) in that zone, the quality of the resulting coating and electrical characteristics of the bath are in- Vferior to those obtained in a fresh coating bath or a coating bath from which the accumulated excess of amine has been removed. Various coating inefficiencies can appear such as streakiness, chalkiness, etc. For most resins presently known to be suitable for this process the titratable free amino compound in the anode zone will constitute about 0.5 to about 1.5% by weight of the aqueous bath in said zone. If the amine concentration and hence the pH of the coating zone are reduced below a requisite minimum precipitation of the acid resin will ensue. It is preferred that the titratable free amino compound be maintained at a level sufficient to maintain the pH of the bath about one whole number above that pH at which precipitation of the bath resin is initiated. The pH at which precipitation is initiated will vary somewhat with the cornposition of the coating material in the bath. While the coating bath or a portion of it, can be removed periodically or continuously from around the anode and subjected to ordinary dialysis, the process is slow. yBy interposing a dialysis membrane between the anode and a cathode, I can accelerate the dialysis process to an advantageous degree. The purging operation can be operated continuously or periodically to maintain the free amino concentration of the anode coating zone or zones at the desired level. In the cathode Zone or zones shielded from the paint bath by the dialysis membrane or membranes water such as ordinary tap water preferably is used as the aqueous medium for eciency and economy, the water advantageously being added continuously and withdrawn continuously to the cathode zone or zones to maintain a fairly constant head almost equal to and only slightly below (e.g. up to about 3") the hydrostatic head of the paint dispersion in the anode zone. The removal of amino compound from the anode zone in this manner is facilitated by maintaining'the concentration of the same in the cathode zone or zones as low as possible.

The electrical equivalent weight of the exemplary polycarboxylic acid resins is between about 1,000 and about 2,000, and the acid numbers of these resins are between about 30 and 300.

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

The material then is cooled to 93.3 C., and 1,140 parts are taken for forming a paint dispersion. To these 1,140 parts, 100 parts of water are added, then 13.6 parts of triethylamine, the mixture agitated for a few minutes, then 74 more parts of water and 92.5 parts diisopropanolamine added. This mixture is further reduced with 1,825 parts water and 32.5 parts diethylene triamine while agitation is continued.

To this paint dispersion there is added 50 parts of a.

treating mixture of mineral spirits, a light hydrocarbon liquid having API gravity of'45-49.5, specific gravity at 15.6 C. of 0.78-0.80, ash point (Cleveland Open Cup) between 37.8-46 C., a negative Doctor test and no acidity, l2 parts of a wetting agent (the yoleic ester of sarcosine, having a maximum of 2% free fatty acid, a specific gravity of 0.948, color on the Gardner scale of 6, and a molecular weight of 340-350). This material is compatible with the paint dispersion; no distinct hydrocarbon phase results either at this time, even though a substantial amount of hydrocarbon (predominantly aliphatic) has been used, nor after further addition of the pigment grind and addition or extra water to make the initial painting bath.

A pigment gri-nd is made from 123 parts of vinyltoluenated, lmaleic-coupled linseed oil made in the same manner as the resin hereinabove shown in this example (except that the resulting polycarboxylic acid resin is n-ot extended with the phenolic resin), 8.4 parts of diisopropanolamine, 0.7 part of an antifoam agent (a ditertiary acetylenic glycol with methyl and isopropyl substitution on the tertiary carbon atoms), 233 parts of fine kaolin clay, parts -of pigmentary titanium dioxide, 7.8 parts of tine lead chromate, 15.5 parts of line red iron oxide, 16.9 parts of carbon black, and 201 parts of water` The resulting pigment grind is then blended with the foregoing paint dispersion and treatingmixture to make a concentrated paint. The resulting paint is reduced further with water to make an initial painting bath for electropainting operations. The resulting bath has resin solids (non-volatile matter) concentration of 5%. The total of amine equivalents used in making up the initial bath is about 4.5 times the minimum amount necessary to keep this polycarboxylic acid resin, once dispersed, in anionic polyelectrolyte condition in the bath and about 1.25 times full neutralization of the acid resin with respect to its acid number (determined by the pyridine method described hereinbefore). The number of coulombs of direct current used to electroplate a gram of this resin on an anode at minimum amine concentration in the bath to develop requisite polyelectrolyte characteristics for my coating process is virtually constant at 24. Specific resistance of the initial bath is about 900 ohm-centimeters.

Referring to the drawing, the anode zone has acrylic plastic walls 1l, leading down to bottom 12, which bottom is perforated to connect with and distribute upwards paint flowing in and from distributor box 13. Walls 11 are perforated as shown with a large number of large and small holes to give liquid access to dialysis membranes 17.

The interior width between walls 11 is 1.1901 centimeters; this anode zone is 22.85 centimeters broad and the paint depth therein 24.13 centimeters, as represented by paint level 16. The dialysis membranes 17 were a pair of regenerated viscous cellulose sheets, 0.01092 centimeter thick. Anode 14 is a 21.58 cm. X 30.5 cm. rectangular piece of phosphatized steel shim stock immersed to expose 929 square centimeters of surface to the anode zone paint bath. On each vertical edge 0.635 centimeter of the shim stock is bent toward a right angle to give it structural stability as an anode.

The cathode zones are built alike of acrylic plastic members 18, each forming a chamber whose interior dimensions are 21.58 centimeters broad, 20.33 centimeters high, and 0.953 centimeter wide; members 1S seal dialysis membranes 17 to the imperforate portions of anode zone walls 11. The seal between the two cathode chambers and the anode zone walls, in effect is gasketed with membrane 17; these joints additionally are sealed against water leakage with a heavy grease and the assembly is held tightly together with metal through bolts, not shown.

In each cathode zone there is a bronze screen cathode 19 covering the wall of the cathode chamber away from membranes 17. Fresh tap water is introduced into each cathode chamber by copper tubings 21 which discharge near the bottom of each cathode zone and also serve as electrical leads conducting to the bronze screen cathodes. Cathode efliuent water containing dialyzed amino compound is withdrawn from each cathode zone by copper tubings 23. Control of the water ow into and out of the cathode zones is maintained by means not shown. Items 24 of the drawing represent randomly positioned rubber tubing placed between the screen and the membrane to press the membrane against perforated panel 11.

Paint is pumped continuously by an external pump, not shown, into paint distributor box 13 and flows upwards through the anode zone, decanting at level 16 over a Weir, not shown, and owing to the pump suction for recirculation back to box 13. The cathode zones run virtually full at all times.

A power source, not shown, supplies direct current from anode 14 to cathodes 19 and is regulated by an external resistance, not shown, to maintain essentially constant current as a particular anode is being coated. The current is conducted to the cell by anode connector 15 connected to shim stock anode 14 and withdrawn from the cell by cathode connectors 22 attached to water inlet tubes 21. Except as otherwise mentioned herein, the body of the apparatus is made of clear, hard acrylic plastic.

During the coating of a particular anode the voltage rises to 150 volts across the combined electrocoating-electrodialysis cell. Initial operations are at room temperature with amperage for a particular anode being 15.84- 21.52. amperes per square meter at the start of the coating runs and approaching 37.36 `amperes per square meter as the temperature rises, the conductance of the cell rising as the temperature approaches 43.3 C. from room temperature in the series of runs.

A particular unpainted anode 14 is dipped in the anode zone, the voltage run up to 150, this anode removed from the anode zone, the excess adhering liquid blown oi the coated anode with air, and the coated anode baked for 10-15 minutes at 193.3 C. Before baking the electrophoretically-deposited paint iilm is slightly tacky and tenaciously adhering. After baking the lm is cured to a tough, exible, glossy ilm about 25.4 microns thick, free of tack and tightly adhering to the metal even under iiexure. The film on each anode so coated and cured is smooth, even and ostensibly awless.

When one anode is coated and withdrawn from the bath, another unpainted one is inserted and coated the same way. After each tive anodes are so coated, the bath is sampled to check for free amino substance (eg. diisopropanolamine) and Virtually no increase is found at Iany sampling, the increment all being dialyzed through membranes |17 into the cathode zones and washed out with the cathode effluents being withdrawn through tubes 23. After about every five panels additional dilute paint (5% resin solids) is added to make up the resin solids content in the anode bath dispersion to about 5%, there being about 1% drop in such solids for each ve panels coated. Ninety-seven successive panels are coated in the run series.

Example 2 Referring to FIGURES 2 and 3 of the drawings, in this embodiment a combination coating bath retainer and dialysis unit 111 has an electrically conductive unit 113 and an electrically nonconductive unit 115 which together with dialysis membrane 117 divide the interior of unit 11.1 into a coating zone 119 and a dialysis zone 121. Unit 113 serves as the primary cathode of the coating cell and is electrically connected with a direct current power supply via conductor 123 and with ground. Positioned Within dialysis zone 121 is a secondary cathode 127 which is electrically connected to power supply 125 via conductors 123 and 129 and with ground. Unit 113 is provided with conduit 131 for admitting fluid to coating zone 119 or for removal of the same. Unit 115 is provided with an inlet conduit 133 and an outlet conduit 135.

Power supply unit 125 is in electrical connection with bus bar 139 via conductor 137.

Conveyor means 141 may be any conventional conveyor system that is suitable for use in the operations hereinafter described, eg. an electrically powered, chain driven system adapted to continuously transport the intended workpiece through the coating bath within coating zone 119. In this embodiment conveyor means 141 is represented by conveyor rail 143 and hanger 145. A workpiece 147 is shown suspended from and in electrical contact with hanger 145. Hanger 145 is adapted to be engaged by chain drive means, not shown, and thus moved along rail 143. Rail 143 is constructed and arranged in a manner such that the workpiece 147 is lowered intothe coating bath as hanger 145 passes over coating zone 119 and raised from the bath as it approaches the downstream side thereof. Hanger 145 is insulated from the grounded conveyor rail 143 by lan insulator 149 and carries a contact plate or brush 151 by which hanger 145 and workpiece 147 are maintained in electrical connection with bus bar 139; In this embodiment the workpiece 147 serves as the anode for both the electrocoating and the electrodialysis operations and is representative of the continuous or sustained line of workpieces passing through the coating bath.

Coating zone 119 contains an aqueous dispersion of a polycarboxylic acid resin having a composition similar to the resin of the preceding example. A water soluble amine, e.g. diethylamine, is employed to assist in dispersing the resin within the bath. The anions formed by dissociation of the amine are attracted to the cathodes 113 and 127 and those 'attracted to the latter pass through membrane 117 into dialysis zone 121. Water is continuously circulated through dialysis zone i121 entering via conduit 133 and exiting via conduit 135 thereby continuously removing amino material from the coating zone |119. Power supply unit 125 provides a direct current ow of electrical energy between the electrodes via the liquid mediums within the coating zone 119 and the dialysis zone 121 that is commensurate with the size of the electrocoating operation contemplated. For constant current operation the power supply lunt 125 is preferably adjustable to provide a current flow of about 5 to 5,000 amperes. For constant voltage operation the power supply unit 125 is preferably adjustable to provide a potential in the range of about 50 to 500 volts or greater. The potential employed is somewhat dependent upon the characteristics of the resin being deposited and must be maintained a level below that -at which the given resin deposit begins to rupture.

In an alternative arrangement for anodic deposition the coating tank may be connected to the negative terminal of a direct current power source and insulated from ground while the bus bar and conveyor are positive in relation to the tank and grounded. In such an arrangement insulation of the bus bar from the conveyor can -be eliminated.

The term amino material las employed herein shall be understood to include ammonia and amines in compound and/ or ionic state.

The aforedescribed embodiments of the invention should be considered in all respects as illustrative and not restrictive, the scope of the invention Abeing indicated by the appended claims.

I claim:

1. In a continuous method of electrocoating wherein electrically conductive objects are passed through an aqueous coating bath comprising a dispersion of (a) a rst component consisting of a synthetic organic resin having within its molecular structure prior to dispersion functional groups which undergo chemical change in contact with water and the hereinafter recited second component creating ionic sites upon said resin and altering the pH of said bath, and

(b) a second component consisting of a water soluble and water ionizable resin dispersal assistant in an amount which at least partially neutralizes said resin, and having a rst electrode in electrical communication therewith, each of said objects while passing through said bath serving as a second electrode, a difference of electrical potential is provided between said rst electrode and said second electrode causing a direct current tlow of electrical energy through said bath between said first electrode and said second electrode which electrically induces deposition of said resin upon said second electrode While resin dispersal assistant is liberated in said bath, said resin is thereby removed from said dispersion at a substantially higher ratethan said resin dispersal assistant, 'and Water ionizable resin dispersal assistant is added to said bath with replenishment resin -at a rate greater than said dispersal assistant is removed by said method of electrocoating, the improvement wherein a dialysis membrane permeable to water and water soluble resin dispersal assistant material within Asaid bath and substantially impermeable to said resin is interposed between said iirst electrode and said second electrode forming a coating zone containing the aforesaid resin dispersion and a substantially resin free second Zone, ionized dispersal assistant is electrodialyzed through said membrane into said second zone, t-he concentration of titratable resin dispersal assistant in said coating zone is established and maintained at a level not substantially above about 2% by weight of said bath in said coating zone, and the aqueous material in said second zone is withdrawn from said zone.

2. The method of claim 1 wherein said dialysis membrane has pores the largest of which has a diameter not substantially in excess of about 5,000 A. and the limiting diameter of a majority thereof is in the nange of about 20 A. to about 5,000 A.

3. The method of claim 1 wherein the concentration of titratable resin dispersal assistant in said coating zone is maintained at a level in the range of about 0.5 to about 1.5 weight percent of said bath in said coating zone.

4. In a continuous method of electrocoating wherein electrically conductive objects are passed through an aqueous coating bath containing a dispersion of a synthetic polycarboxylic acid resin at least partially neutralized by a water soluble amino material and having a cathode in electrical communication therewith, each of said objects while passing through said bath serving as an anode, a difference of electrical potential is provided between said cathode and said anode causing a direct electxic current flow of electrical energy through said bath between said cathode and said anode which electrically induces deposition of said resin upon said anode, water soluble amino material is liberated in said bath, said resin is thereby removed from said dispersion at a substantially higher rate than said amino material, and water soluble amino material is added to said bath with replenishment resin at a rate greater than it is removed by said method of electrocoating, the improvement wherein a dialysis membrane permeable to water and water soluble amino material within said bath and substantially impermeable to said resin is interposed between said cathode and said anode forming an anode coating zone containing the aforesaid resin dispersion and a substantially resin free cathode zone, positively ionized amino material is electrodialyzed through said membrane into said cathode zone, the concentration of titratable amino material in said anode coating zone is established and maintained at a level not substantially above about 2% by weight of said bath in said anode coating zone, and the aqueous material in said cathode zone is withdrawn therefrom.

5. A method in accordance with claim 4 wherein the concentration of titratable water soluble amino material within said anode coating Zone is maintained at a level sufficient to maintain t-he pH of the bath at least about one whole number above that pH lat which precipitation of said resin from said dispersion is initiated.

6. A method in accordance with claim 4 wherein said resin is a synthetic polycarboxylic acid resin having an electrical equivalent weight between about 1,000 and about 20,000 andan acid number between about 30 and about 300.

7. The method of claim 4 wherein said dialysis Imembrane has pores t-he limiting diameters of which are in the range of about 20 A. to about 5,000 A.

8. The method of claim 4 wherein said dialysis membrane has pores the limiting diameters of which are in the range of about 20 A. to about 200 A.

9. The method of claim 4 wherein the concentration of said water soluble amino material in said coating zone is maintained at a level in the range of about 0.5 to 1.5 weight percent of said bath in said coating zone.

:10. In a continuous method of electrocoating wherein electrically conductive objects are passed through an aqueous coating bath having paint dispersed therein and a cathode in electrical communication therewith, each of said objects while passing through said bath serving as au anode, said paint containing'as the predominant fraction of the hlm-forming paint binder a synthetic polycarbOXylic acid resin at least partially neutralized with a suticient quantity of water soluble amino compound to maintain said polycarboxylic acid resin as a dispersion ofl anionic polyelectrolyte in said bath, said acid resin having electrical equivalent weight between about 1,000 and about 20,000, acid number between about 30 and about 300, and, in said bath, exhibiting anionic polyelectrolyte behavior as indicated by its depositing on said anode' substantially directly proportional to electric current iiow through said bath, a diference of electrical .potential is provided between said cathode and said anode causing a direct current flow of electrical energy through said bath between said cathode and said anode which electrically induces deposition of said resin upon said anode while water soluble amino material is liberated in said bath, said resin is thereby removed from said dispersion at a substantially higher rate than said amino material and water soluble amino materim is added to said bath with replenishment resin at a rate greater than it is removed by said method of electrocoating, the improvement wherein a dialysis membrane permeable to water and water soluble amino material within said bath and substantially impermeable to said resin is interposed between said cathode and said anode forming an anode coating zone containing the aforesaid resin dispersion and a substantially resin free cathode zone, positive ions of said amino material are electrodialyzed through saidy membrane into said cathode zone, the concentration of titratable amino material in said `anode coating zone is established and maintained at a level not substantially above about 2% by weight of said bath in said anode coating zone, and the aqueous material in said cathode zone is withdrawn therefrom.

11. In a continuous method of electrocoating wherein electrically conductive objects are passed through an aqueous coating bath comprising a dispersion of (a) a first component consisting of a synthetic organic iresin having within its molecular structure prior to dispersion functional groups which undergo chemical change in contact with water and the hereinafter recited second component creating ionic sites upon said resin, said sites being reconvertible into deionized condition upon electrodeposition of said resin upon an electrically conductive object with attendant alteration of the pH of said bath, and

(b) a 4second component consisting of a water soluble and water ionizable resin dispersal assistant in an amount which at least partially neutralizes said resin and having a rst electrode in electrical communication therewith, each of said objects while passing through said bath serving as a second electrode, a difference of electrical potential is provided between said rst electrode and said second electrode causing a direct current ow of electrical energy through said bath between said irst electrode and said second electrode which electrically induces deposition of said resin upon said second electrode -while resin dispersal assistant is liberated in said bath, said resin is thereby removed from said dispersion at a substantially higher rate than said resin dispersal assistant, and water ionizable resin dispersal assistant is added to said bath with replenishment resin at a rate greater than said dispersal assistant is removed by said method of electrocoating, the improvement wherein a'dialysis membrane permeable to water and Water soluble resin dispersal assistant material within said bath and substantially impermeable to said -resin is interposed between said iirst electrode and said second electrode forming a coating zone containing the aforesaid resin dispersion and a substantially resin free second zone, ionized dispersal assistant is electrodialyzed through said membrane into said second zone, the concentration of titratable resin dispersal aS- sistant in said coating zone is established and maintained at a level in the range of about 0.5 to about 1.5% by Weight of said bath in said Icoating zone, said level being sufiicient to maintain the pH of the bath at least one Whole number above that pH at which precipitation of said resin from said dispersion is initiated, and the aqueous material in said second zone is withdrawn from said zone.

References Cited by the Examiner UNITED STATES PATENTS 2,800,447 7/l957 Graham 204-181 FOREIGN PATENTS 164,699 8/ 1955 Australia.

JOHN H. MACK, Primary Examiner.

E. ZAGARELLA, Assistant Examiner.

Claims (1)

1. IN A CONTINUOUS METHOD OF ELECTROCOATING WHEREIN ELECTRICALLY CONDUCTIVE OBJECTS ARE PASSED THROUGH AN AQUEOUS COATING BATH COMPRISING A DISPERSION OF (A) A FIRST COMPONENT CONSISTING OF A SYNTHETIC ORGANIC RESIN HAVING WITHIN ITS MOLECULAR STRUCTURE PRIOR TO DISPERSION FUNCTIONAL GROUPS WHICH UNDERGO CHEMICAL CHANGE IN CONTACT WITH WATER AND THE HEREINAFTER SAID RESIN AND ALTERING THE PH OF SAID BATH, AND (B) A SECOND COMPONENT CONSISTING OF A WATER SOLUBLE AND WATER IONIZABLE RESIN DISPERSAL ASSISTANT IN AN AMOUNT WHICH AT LEAST PARTIALLY NEUTRALIZES SAID RESIN, AND HAVING A FIRST ELECTRODE IN ELECTRICAL COMMUNICATION THEREWITH, EACH OF SAID OBJECTS WHILE PASSING THROUGH SAID BATH SERVING AS A SECOND ELECTRODE, A DIFFERENCE OF ELECTRICAL POTENTIAL IS PROVIDED BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE CAUSING A DIRECT CURRENT FLOW OF ELECTRICAL ENERGY THROUGH SAID BATH BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE WHICH ELECTRICALLY INDUCES DEPOSITION OF SAID RESIN UPON SAID SECOND ELECTRODE WHILE RESIN DISPERSAL ASSISTANT IS LIBERATED IN SAID BATH, SAID RESIN IS THEREBY REMOVED FROM SAID DISPERSION AT A SUBSTANTIALLY HIGHER RATE THAN SAID RESIN DISPERSAL ASSISTANT, AND WATER IONIZABLE RESIN DISPERSAL ASSISTANT IS ADDED TO SAID BATH WITH REPLENISHMENT RESIN AT A RATE GREATER THAN SAID ASSISTANT IS REMOVED BY SAID METHOD OF ELECTROCOATING, THE IMPROVEMENT WHEREIN A DIALYSIS MEMBRANE PERMEABLE TO WATER AND WATER SOLUBLE RESIN DISPERSAL ASSISTANT MATERIAL WITHIN SAID BATH AND SUBSTANTIALLY IMPERMEABLE TO SAID RESIN IS INTERPOSED FORMING A COATING ZONE CONTAINING THE AFORESAID RESIN DISPERSION AND A SUBSTANTIALLY RESIN FREE SECOND ZONE, IONIZED DISPERSAL ASSISTANT IS ELECTRODIALYZED THROUGH SAID MEMBRANE INTO SAID SECOND ZONE, THE CONCENTRATION OF TITRATABLE RESIN DISPERSAL ASSISTANT IN SAID COATING ZONE IS ESTABLISHED AND MAINTAINED AT A LEVEL NOT SUBSTANTIALLY ABOVE ABOUT 2% BY WEIGHT OF SAID BATH IN SAID COATING ZONE, AND THE AQUEOUS MATERIAL IN SAID SECOND ZONE IS WITHDRAWN FROM SAID ZONE.

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