US3730866A

US3730866A - Catholyte rinse of electrocoat

Info

Publication number: US3730866A
Application number: US00887949A
Authority: US
Inventors: L Madejczyk
Original assignee: Ford Motor Co
Current assignee: EIDP Inc
Priority date: 1969-12-24
Filing date: 1969-12-24
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01
Also published as: DE2063160A1; FR2072044B1; NL7018810A; DK128943B; FR2072044A1; GB1278385A; DE2063160C3; JPS5016376B1; CA947233A; BE760778A; DE2063160B2

Abstract

AN IMPROVED METHOD FOR ELECTRODEPOSITING PAINT UPON AN ELECTRICALLY CONDUCTIVE SUBSTRATE FROM AN AQUEOUS DISPERSION OF SUCH PAINT IS PROVIDED WHEREIN CATHOLYTE LIQUID IS SEPARATED FROM THE COATING BATH AND DISCHARGED UPON AN ARTICLE WHICH HAS BEEN COATED IN AND REMOVED FROM SUCH BATH WITH SUCH ARTICLE POSITIONED IN A MANNER SUCH THAT THE LIQUID SO DISCHARGED RETURNS TO THE COATING BATH.

Description

l, 173 L. A. MADEJCZYK 3,730,866

CATHOLYTE RINSE OF' ELECTROCOT Filed Dec. 24. 1969 4 Sheets-Sheet l may l, 1973 Filed Deo. 24. 1969 4 Sheets-Sheet 3 WIW L w73 IW MAUEJCXYK IYIM l CATHOLYTE RINSE 0F ELECTROCOAT Filed Dec. 24. 1969 4 Sheets-Sheet .I5

. K I m. .WNW Wm. I I III I QI In wm Tw@ I. w \Im.d\ I I I W1 @d \..Im%\ ma I, IIIIIIII IIMIV I I I II M I TmNY vm@ N HTII MII I I w1 x I m I Fm@ M\ W u n M B II III I I I I I I Imxw @HGM Mwv @WNY I I I I I MI I I II, un I III \II, |II IIIIWIII wmmwmmm IIIIIIII II| IIIIIIIIIIIIIII l- IIMII IIIIIIIWIHIII I I II II II |I III! mx FI I|III.I md@ Il II IMI Im IIJ I I II I at w \w\ \-bm\\ I 7 II, II. W Iwwx wwwk \M\ \\I u /NWS .PQ MNKIMEO z /ImIm I I U HU/ N I N n/ N@ u@ I Q0 Im Iw\\ Tmw w W\M May l, 1973 l.. A. MADEJCZYK CATHOLYTE RINSE OF ELECTROCUAT 4 Sheets-Sheet 4 Filed Deo. 24.v 1969 rrokfvfy nitecl States Patent() CATHOLYTE RINSE F ELECTRGCOAT Leopold A. Madejczyk, Oak Park, Mich., assigner to Ford Motor Company, Dearborn, Mich. Filed Dec. 24, 1969, Ser. No. 887,949 Int. Cl. 01d 13/02; B01k 5/02; C23b 13/00 U.S. Cl. 204-181 4 Claims ABSTRACT oF THE DISCLOSURE An improved method for electrodepositing paint upon an electrically conductive substrate from an aqueous d ispersion of such paint is provided wherein catholyte liquid is separated from the coating bath and discharged upon an article which has been coated in and removed from such bath with such article positioned in a manner such that the liquid so discharged returns to the coating bath.

BACKGROUND OF THE TNVENTION The electrodeposition of paint from an aqueous dispersion thereof upon an electrically conductive workpiece is now well known in the art. See, for example, U.S. Pats. 3,230,162; 3,335,103; 3,378,477; and 3,403,- 088.

Conventionally, the coated object is withdrawn from the coating bath and takes with it a quantity of adhered or otherwise entrapped paint which is not a part of the electrodeposited paint iilm thereon. This material is conventionally removed by rinsing and/or by compressed' air streams. This material together with the rinse water conventionally drains into the plant waste system or is otherwise disposed of. The amount of paint lost by drag-out dilfers widely with the resin system employed in the coating tank and represents a substantial loss in coating efliciency as measured by the paint coverage obtained per unit volume of paint solids introduced into the coating bath.

If extraneous rinse water is returned to the bath in Order to return the paint solids therein, the water content of the bath is unduly increased. Such a method cannot be maintained unless there is devised some method for effecting water removal from the bath which is quantitatively essentially equal to the inflow of extraneous rinse water. The difficulties attendant to operating an electrodeposition bath with a constantly changing water supply are obvious to one skilled in the art.

THE INVENTION It has been discovered that coating bath liquid which has been freed or substantially freed of paint solids is particularly suitable for use as rinse liquid for an electrocoating process. It" this material is then returned to the bath with undeposited paint removed from a coated article through rinsing, it maintains the water balance of the bath and can be controlled so as not to introduce extraneous contaminants, always a potential danger if a new supply of water is being continuously added to the bath.

In the method of this invention liquid is separated from the bath which is free or substantially free of paint solids. This liquid is continuously or intermittently continuously pumped to a rinsing station constructed and arranged so that the rinse liquid and paint particles rinsed from the coated workpiece flow back into the coating bath and are redispersed.

The removal of liquid catholyte is eifected by placing in the bath a catholyte separation unit which comprises a water permeable wall and includes a cathode in electrical connection with the primary cathode of the coating cell. This auxiliary cathode assumes the polarity of 3,730,866 Patented May 1, 1973 ice the primary and may or may not be a part of the water permeable wall.

The vast majority of operations wherein paint is electrodeposited from an aqueous bath are systems wherein the paint has affinity for the anode and is anodically deposited, i.e., the object to be coated serves as the relatively positive electrode of the electrodeposition cell. Such systems conventionally employ a polycarboxylic acid resin paint binder and are dispersed within the bath with a water-soluble base. The preferred bases for this purpose are water-soluble amines. It is known in the art that other bases are operable for this purpose. These include ammonium, potassium hydroxide, lithium hydroxide, sodium hydroxide, etc.

Suitable binder resins for anodic deposition are synthetic polycarboxylic acid resins, i.e., organic resins having free or ionizable carboxyl groups in their molecular structure which are derived from a constitutent monomer. Acidic hlm-forming resins include, but not by way of limitation, any of the polycarboxylic acid resins used in the electrodeposition of paint from an aqueous bath. They include coupled oils such as sunower, safower, perilla, hempseed, walnut seed, dehydrated castor oil, rapeseed, tomato seed, menhaden, corn, tung, soya, oiticica, or the like, the oleiinic double bonds in the oil being conjugated or nonconjugated or a mixture, the coupling agent being an acyclic oleiinic acid or anhydride, preferably maleic anhydride, but also crotonic acid, citraconic acid, or anhydride, fumaric acid, or an acyclic -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 2 to about 25% of a polymerizable vinyl monomer; maleinized unsaturated fatty acids; maleinized rosin acids, alkyd resins, e.g., the esteriication products of a polyol with 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 vinyl polymers and copolymers having carboxylic acid groups such as butyl acrylate-methyl methacrylate-methaacrylic acid copolymers, acrylic acid and lower alkyl (C1 to C4) substituted acrylic acid-containing polymers, i.e those having carboxyl groups contributed by alphabeta unsaturated carboxylic acids or residues of these acids, etc. These and other suitable resins are described in detail in many patents of which the following are illustrative: -U.S. Pats. 3,230,162; 3,297,557 and 3,369,- 983. Such resins are electrodeposited with an impressed potential in the range of about 50 to about 500 volts or higher. The concentration of binder resin in the bath is advisedly in the range of about 5 to about 15 weight percent of the bath.

if a cathodically depositable paint is employed and the workpiece to be coated forms the cathode, relatively negative electrode of the electrodeposition cell, the auxiliary electrode is in electrical connection with and assumes the polarity of the anode. Cathodically depositable resins include those having ionizable amine groups in their molecular structure. They can be-dispersed in aqueous dispersion with the aid of a water soluble acid, e.g., acetic acid.

This invention will be more easily understood by referring to the accompanying drawings wherein:

FIG. 1 is a semidiagrammic sectional side view of apparatus used in carrying out one embodiment of this invention;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 4 is a semidiagramrnatic sectional side view ofl apparatus used in carrying out a second embodiment of this invention and taken along line 4-4 of FIG. 5

FIG. 5 is a view taken along line 5-5 of FIG. 4; and

FIG. 6 is a view taken along line 6-6 of FIG. 5 and is a partial end view of the apparatus shown in FIG. 4 and looking inthe direction of workpiece travel to and through the coating bath.

Referring now to FIGS. 1, 2 and 3, a steel coating tank 11 contains a coating bath 13, i.e., an aqueous dispersion of paint comprising pigment, a polycarboxylic acid resin and a water soluble base, and serves as a negative electrode (cathode) in the coating process. Tank 11 is electrically connected to D.C. power supply unit 17 via conductor 15. An article 19 to be coated, e.g., an automotive body, is shown suspended from a conveyor 35 by hanger 21. Conveyor 35 is a conventional, electrically powered, chain driven conveyor constructed and arranged for the transportation of articles to be coated through bath 13. Hanger 21 includes insulator 23 which electrically insulates article 19 from the grounded conveyor. Contact plate or brush 25 is attached to and in electrical connection with hanger 21. Article 19 is shown immersed in bath 13 and in electrical connection with bus bar 37 via contact plate 25 and in turn is in electrical connection with D.C. power supply unit 17 via conductor 39. Article 19, therefore serves as the positive electrode of an electrodeposition cell while the article is passing through bath 13.

It will be understood that bus bar 37 may be segmented and that certain of the various segments thereof may have no connection with power unit 17 or another power supply unit not shown so as to permit impression of a higher voltage than that impressed on other segments, or may be electrically connected with a power supply unit so as to admit of polarity reversal with respect to other segments of the bus bar.

Power supply unit 17 is constructed and arranged to convert (rectify) an alternating current power source to direct electric current or the equivalent thereof and to provide between the electrodes and through the coating bath a direct current ow of electrical energy that is commensurate with the size of the electrocoating operation contemplated. Design of the power supply unit should take into consideration the surface area of the workpieces that will be in residence within the coating bath at any given time, the workpiece surface area entering the bath per unit time, the deposition properties of the coating formulation, the conductivity of the coating bath, the thickness of the coating to be formed, etc.

At the left or exit side of tank 11 there is shown an article 27 that has been electrocoated in bath 13 and is now undergoing rinsing to wash adhered paint particles which are brought out of the bath with the coated article. Article 27 is suspended from conveyor 35 by a hanger 29 which is equipped with an insulator 31 and a contact plate or brush 33.

Positioned along one upper side of tank 11 is a catholyte separation unit 40. Catholyte separation unit 40 comprises a porous metal box, e.g., expanded metal or wire mesh, and a separation membrane 43 which admits of the passage of water and water soluble materials such as amines, KOH, LiOH, NaOH, etc., therethrough while preventing the passage of substantial amounts of paint solids. Thus, separator membrane 43 may be cloth, e.g., sailcloth, water permeable plastic membranes, and other water permeable synthetic or natural lter means. In another embodiment, the separation membrane is the porous or water permeable box itself and is structured so that the openings therein are suciently small to efrect the same degree of separation specified for the separate separation membranes. In still another embodiment, the walls of separation unit are formed of a porous ceramic and a conductor positicned inside such unit is in electrical connection with the primary cathode. The separation membrane 43 may be structured to provide an essentially complete barrier to the paint particles of bath 13 or it may admit of their limited passage to a degree such that the concentration of such particles in catholyte separation unit 40 is below about 50, preferably below l0 and more preferably below 5, percent of the corresponding concentration in coating bath 13. Otherwise exxpressed there exists through the barrier of differential ow with respect to water and solubilizer or resin dispersal assistant on the one hand and paint solids on the other.

Positioned within catholyte separation unit are pumping means 45 having intake means 47. Pumping means 45 is operatively connected to conduit 49 through which liquid from within catholyte separation unit 40 is passed by pumping means 45 to

conduits

51 and 53 and through the same to shower

heads

55 and 57 from whence the same is discharged upon article 27. In this embodiment, article 27 is thus rinsed while immediately over the coating bath and hence the drainage from such rinsing falls directly into coating bath 13. Conduit 49 is provided with valve 49-1 which can divert the flow from pump 45 through outlet conduit 49-2 for purging.

Porous metal box 41 is supported by and in electrical connection with the coating tank 11. Thus, it becomes a part of the cathode of the cell and attracts cations through separation membrane 43, primarily those of the water soluble base employed to disperse and/or solubilize the acidic paint binder resin. Thus, within catholyte separation unit 40, there exists an aqueous dispersion that is paint solids poor and base rich relative to bath 13. This fluid is therefore particularly suitable as a rinse uid for removing adhered paint particles from the coated workpiece.

FIGS. 4, 5 and 6 relate to a second embodiment of this invention, In this embodiment, there is shown a steel coating tank 111 which contains coating bath 113 and serves as the negative electrode in the coating process. Tank 111 is electrically connected to D.C. power supply unit 117 via conductor 115. An article to be coated 119 is shown suspended from a conveyor 135 by hanger 123. Conveyor 135 is a conventional, electrically powered, chain driven conveyor constructed and arranged for the transportation of articles to be coated through bath 13. Hanger 121 includes insulator 123 which electrically insulates article 119 from the grounded conveyor. Contact plate or brush 125 is attached to and in electrical connection with hanger 121. Article 119 is shownimmersed in bath 113 and in electrical connection with bus bar 137 via contact plate 125 and in turn is in electrical connection with D.C. power supply unit 117 via conductor 139. Article 119, therefore, serves as the positive electrode of an electrodeposition cell while the article is passing through bath 113.

Extending from coating tank 111 and in the direction of workpiece travel from coating tank 111 is a drainboard assembly 112 constructed and arranged so that rinse liquid falling thereon drains back into coating bath 113 in coating tank 111. Above drainboard assembly 112 there is shown an article 127 that has been electrocoated in bath 113 and is now undergoing rinsing to wash off adhered paint particles which are removed from bath 113 with the coated article. Article 127 is suspended from conveyor by a hanger 129 which is equipped with an insulator 131 and a contact plate or brush 133.

Positioned along the upper edge of coating tank 111 and positioned at the end of the tank where the articles to be coated enter the bath is a U-shaped catholyte separation unit 140. Catholyte separation unit comprises a metal box 141 having a porous bottom 141-1 which may be of expanded metal, wire mesh, etc., and a separation membrane 143 which admits of the passage of water and water soluble materials such as amine, ammonia, KOH, LiOH, etc., therethrough. The separator membrane 143 may be cloth, eg., saillQth., a water per'- meable plastic membrane, or other filter means of synthetic or natural materials.

The separation membrane 143 may be structured to provide an essentially complete barrier to the paint particles of bath 113 or it may admit of their limited passage to a degree such that the concentration of paint in catholyte separation unit 140 ranges up to 50, preferably below and more preferably below 1, percent of the corresponding concentration of the same in coating bath 113.

Catholyte separation unit 140 has outlet means on opposite sides of tank 111 in the form of conduits 160-1 and 160-2 through which liquid flows from the upper interior of catholyte separation unit 140 by gravity ow into collection units 161-1 and 161-2 which optionally are provided with lters 162. Collection units 161-1 and 161-2 are provided with drain conduits 163-1 and 163-2 which are operatively connected to pumps 145-1 and 145-2. Pumps 145-1 and 145-2 are operatively connected to conduits 149-1 and 149-2 through which liquid from catholyte separation unit 140 is passed by pumps 145-1 and 145-2 to shower heads 155 and 157 from whence the same is discharged upon article 127. In this embodiment, article 127 is thus rinsed while our drainboard assembly 112 and the rinse liquid drains back into coating bath 113.

Metal box 141 of catholyte separation unit 140 is in electrical connection with the tank 111. Thus, it becomes a part of the cathode of the cell and attracts cations through separation membrane 143, primarily those of the water soluble base employed to solubilize and/or disperse the acidic paint binder resin- Thus, within catholyte separation unit 140, there exists an aqueous dispersion that is paint solids poor and base rich relative to bath 113.

Catholyte separation units

40 and 140 are advantageously positioned near the point Where uncoated workpieces enter the coating bath since the greatest surge of current occurs before an electrically resistant lm builds upon the workpiece.

In this application, the term paint is meant to include pigment and/ or nely ground filler, the binder Without pigment and/or ller or having very little of the same, which can be tinted if desired. Thus, the binder which is ultimately converted to a durable film, can be all or virtually all that is used to form the film, or it can be vehicle for pigment and/or particulate iiller material.

It will be understood by those skilled in the art that modifications can be made within the design of the apparatus shown and described herein without departing from the scope of the invention as hereinafter claimed.

I claim:

1. In a process for electrodepositing paint wherein an aqueous dispersion comprising a synthetic polycarboxylic acid resin and water soluble base form a coating bath which is retained Within a bath container and is in contact with a relatively negative cathode, a relatively positive anode is immersed in said coating bath and a film of said paint is electrodeposited thereon, said anode is removed from said coating bath after electrodeposition of said film of paint and the resultant coated anode is sprayed with liquid to wash therefrom unattached paint particles thereon, the improvement which comprises separating from said bath within said bath container a liquid bath fraction which comprises water and water soluble base and a concentration of paint particles between 1 and 50% of the corresponding concentration in said bath by positioning within said bath a catholyte separation unit comprising an auxiliary cathode and a water permeable membrane through which a flow of paint particles is impeded thereby providing a differential flow through said membrane of paint particles on the one hand and water and water soluble base on the other and removing said liquid bath fraction from within said catholyte separation unit, discharging said liquid bath fraction upon said coated anode while said coated anode is outside said coating bath and returning said liquid bath fraction to said bath.

2. The method of claim 1 wherein said liquid bath fraction has a concentration of paint particles between 5 and 10% of the corresponding concentration within said coating bath.

3. The mehod of claim 1 wherein said liquid bath fraction is discharged upon said coated electrode while said coated electrode is directly above said coating bath and drains directly from said coated electrode by gravity ow into said coating bath.

4. The method of claim 1 wherein said liquid bath fraction is discharged upon said coated electrode after said coated electrode has been moved from a position over said coating bath and drains directly from said coated electrode by gravity ilow onto drain means constructed and arranged to cause said liquid bath fraction to drain therefrom by gravity ow into said coating bath.

References Cited UNITED STATES PATENTS 3,663,400 5/1972 Christenson 204-181 3,419,488 12/1968 Cooke 204-181 3,444,064 5/ 1969 Johnson 204-181 3,444,066 5/ 1969 Brewer et al. 204-181 3,492,213 1/1970 JohnsOn 204-181 3,556,970 1/1971 Wallace et al. 204-181 FOREIGN PATENTS 1,073,965 6/ 1967 Great Britain 204-181 1,071,458 6/ 1967 Great Britain 204-181 HOWARD S. WILLIAMS, Primary Examiner U.S. Cl. X.R.