Applied Polymer Science - ACS Publications - American Chemical

34

Electrodeposition of Paint

Downloaded by SUNY STONY BROOK on October 15, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch034

GEORGE Ε. F. BREWER Coating Consultants, Birmingham, MI 48010

Background The Practice of Electrocoating Equipment and the Industrial Painting Process Advantages

In the late 1950s a group of chemical engineers of Ford Motor Co. began to experiment with a new paint application process that they called "electrocoating." The process resembles metal plating in as much as an electric direct current causes the paint to deposit on conductive surfaces (Figure 1). Quickly, the many advantages of the process were recognized and summarized as "Electrocoating... The greatest breakthrough since the invention of the spray gun...." Background Corrosion of metals results in enormous damage to our economy, and a great need exists for improved corrosion protection. A study of the deplorable junk piles reveals that discarded merchandise exhibits shiny, almost new looking surfaces in certain of its areas, while other sections of the same piece are completely corroded. Typical examples for the selective action of corrosion are automobile bodies. The roof of a car, the hood, and the trunk lid show usually very little corrosion, while other areas of a car body are completely destroyed. A study of cut open inner surfaces of new automotive bodies revealed that an incomplete paint coat existed inside those areas that subsequently corroded. The insufficient paint coat resulted from the inability of the spray painting process to reach highly recessed areas, like the insides of doors or the capillary recesses of butted joints or flanges. The dip-coating process does form a wet paint coat in all recessed areas, but during the subsequent 0097-6156/85/0285-0827$06.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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A P P L I E D P O L Y M E R SCIENCE

paint bake the paint i s p a r t i a l l y removed through a phenomenon c a l l e d "reflux damage" or "solvent wash," due to temperature differences between outer and inner layers of metal that cause vapor condensation in certain areas of the merchandise. What was needed was a painting process that allows the application of corrosion-protective films even in most recessed areas in v i r t u a l absence of solvents. The awareness of the shortcomings of the then existing paint application processes led to the concept and development of the electrocoating process, which is a dip coating process using waterborne paint compositions. An e l e c t r i c direct current deposits the paint s o l i d s (essentially resins and pigments) on the e l e c t r i c a l l y conductive surfaces of merchandise. The migration of c o l l o i d a l materials toward an electrode of the opposite polarity is called electrophoresis, which can be either cataphoresis (cathodic deposition) or anaphoresis (anodic deposition). The migration of c o l l o i d a l l y dispersed p a r t i c l e s in a direct current f i e l d was reported as early as 1809. From then on the phenomenon received attention only every 30 or 50 years as an analytical method, culminating in 1948 when Arne Tiselius received the Nobel P r i z e for h i s experimentations, particularly electrophoretic separation of proteins. Electrophoretic deposition seems not to have been observed before 1905, and the earliest patented use of electrodeposition was made in 1919 by Wheeler P. Davey, who deposited bituminous material on electric wires as insulation. Subsequently, patents were granted for electrodeposition of rubber latex (1923) and for deposition of bees wax and other materials as protective coatings in food cans (1937 and 1943). A l l of these processes for electrophoretic deposition were using naturally occurring materials, and none of these processes seem to have been in operation by 1950. The technology that i s now known as electrodeposition of coatings, electropainting, electrocoating, etc., uses synthetic, waterborne oligomers. (See References 1-10.) The Practice of Electrocoating Figure 1 shows the essential steps of the process: the merchandise receives f i r s t the usual metal treatment and then enters the electrocoating tank where i t i s e l e c t r i c a l l y connected to one polarity of a power source, while the tank or electrodes in the tank are used as counterelectrodes. Within 1 or 2 min the desired film thickness of usually 1 mil (25 um) i s formed. The current consumption ranges from 2 to 3 A/ft . The process depends on the existence in the coatings bath of about 5 to 20 wt % of film-forming electrodepositable macroions of the general formula RC00~ for anodic deposition and R^NH+ or RqS+ for cathodic deposition. The freshly deposited coat consists or water-indispersible material, since the action of the e l e c t r i c current has converted the macroions into molecules, somewhat resembling the conversion of soluble metal ions into insoluble metal atoms during the electroplating process. The freshly painted merchandise is then rinsed with water for removal of adhering bath droplets. More recently, some of the f l u i d i s separated from the paint by u l t r a f i l t r a t i o n , since the use of ultrafiltrate as rinse fluid and subsequent return to the coating



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Power Supply

Figure 1.

Rinse Deck

The electrocoating process.


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tank results in higher efficiency. The rinsed merchandise is then moved to the usual paint bake or cure, as indicated in Figure 1. Thus, an existing metal finishing department can be converted from spray painting or dip coating by replacing the paint transfer equipment with an electrodeposition tank plus a rinse deck. Electrocoating materials are acid oligomers RC00H or more frequently R(C00H)n for anodic deposition or RoN for cathodic deposition. These - oligomers are water insoluble but can be dispersed in water through the action of acids or bases, used as external solubilizers. The solubilization and deposition processes can be symbolized as follows: water insoluble oligomer

aqueous + external solubilizer

dispersion

deposition

dispersed macroions

counter ions

RCOOH

BOH

RC00"

B + + aq

R3N

HX

R3NH"*"

X~ + aq

It may be noted that the resinous moiety "R" may contain the chemical groupings characteristic for practically any known film former, such as acrylics, alkyds, epoxies, phenolics, polyesters, etc. The electrodeposited cured films exhibit essentially the same properties as their basic resins. Thus, there is a large variety of formulations possible. Electrocoating paints are usually sold as approximately 40 wt % dispersions of paint nonvolatiles in the presence of 10 wt % v o l a t i l e organic cosolvents and 50 wt % water. The paint s o l i d concentration in the coating tank i s selected to give the best performance and varies for i n d i v i d u a l i n s t a l l a t i o n s from 5 to 20 wt %. As a guideline, most electrocoating paints require approximately 50 C for the deposition of 1 g of paint, or 1 Faraday for 2000 g. The t y p i c a l molecular weight of these oligomers is approximately 10,000, and not a l l of the ionizable groups are neutralized, since a high degree of neutralization results in an increased "wash off" of the freshly deposited resin, while a low degree of neutralization reduces the stability of the aqueous dispersion. The oligomers deposit on the electrode of opposite p o l a r i t y , while the counterion is discarded or reused for solubilization of replenishment resin (see Figures 2 and 3). The electrodeposition process is currently used for the coating of merchandise ranging from structural s t e e l , automotive bodies, furniture, coil stock, appliances, and toys to nuts and bolts. The use of c a t i o n i c r e s i n s of the RoNH+ type i s widely advocated, since their cured films result in high corrosion protection. A new type of cationic resin is based on "onium bases," p a r t i c u l a r l y sulfonium bases, R3S"** 0H~. These do not require the control of an external solubilizer and lose their ionizable group during deposition on the cathode. The properties of the f i n a l , polymerized f i l m w i l l largely depend upon the chemical nature of R. Thus, i f the R carries many



BREWER


B

Figure 2.

+

CATHOLYTE (SOLUBILIZER) DISPOSAL

Solubilizer removal (completely solubilized feed).

—1 POWER SUPPLY

BRUSH

| ^ ^ ^ CONVEYOR fa GROUNDED •^INSULATOR

CATHODIC GROUND

/ BUSS

PART RCOO-

RCOO-

~B+

Figure 3.

Solubilizer reuse (solubilizer-deficient feed).



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epoxy groups, the final film w i l l have essentially the properties of epoxy spray or dip primers; i f the R carries many a c r y l i c groups, the electrodeposited f i l m w i l l have conventional a c r y l i c coat properties. The paint formulator has additional latitude in the choice of resins because dispersed macroions can be used as emulsifying agents for a variety of resins. These, as well as many pigments, w i l l electrodeposit together with the macroions. In addition to water dispersibility, the process requires other special properties: low redispersibility of the freshly deposited f i l m , since the work piece, on removal from the bath, w i l l be covered with bath droplets that tend to redissolve the fresh deposit; lower resin viscosity (Gardner Scale Z3, for instance), when compared with conventional paint resins, since a desirable electrodeposit is virtually free from viscosity reducing solvents; comparatively high e l e c t r i c a l resistance (10 -ICP ohm cm) of the freshly deposited film to obtain "throwing power"; designing of the contact of counterion and the counter electrode to produce soluble products that do not interfere with the continuing operation. If, for instance, in the cathodic deposition of a film-former RgNH"1", acetate ion CH^COO" is used as the counterion, water-soluble acetic acid w i l l be generated near the anode, while oxalate ions w i l l similarly give rise to the liberation of CO2. In the case of anodic deposition of RC00~ as film-forming macroions, the counterions carry positive e l e c t r i c charges (B + ). A l k a l i metal ions, ammonia, and particularly organic amines are most widely used as counterions. The most unconventional yet most important feature to be built into electrocoating materials is "throwing power", or the ability to form electrodeposited films of such uniform thickness that even the most recessed areas of a workpiece are covered. Electrocoats are known to deposit f i r s t on areas that are closest to the counterelectrode. However, the electrodeposit is of high electrical resistance, and the current has to seek the nearest s t i l l a v a i l a b l e path, namely, bare metal, u n t i l a l l the metal surface is covered. The o r i g i n a l e l e c t r i c a l resistance between two electrodes immersed in the coating bath i s comparatively low, but the r e s i s tance rises during the deposition process to 100 and more times its original value, with a related drop in current flow. Thus, paint is deposited quickly until a l l bare metal areas are covered. The film build is self-limiting: areas of thinner film offer less resistance against current flow, so deposition continues there until the film thickness has evened out. The application of higher voltage produces higher film thickness and more throwing power. There exists, however, for each individual paint a maximum applicable voltage: beyond this point the freshly deposited film ruptures, causing unsightly blemishes, lowered corrosion protection, etc. Other factors that increase throwing power are higher bath conductivity, which makes i t easier to get e l e c t r i c i t y into recesses, and higher electric equivalent weight of the paint, which gives more deposit per unit of electricity (coulomb).


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Equipment and the Industrial Painting Process M e t a l l i c objects are f i r s t cleaned to remove impurities and materials associated with metal forming, welding, temporary rust protection, etc. In many instances phosphate and/or chromate treatments are applied. O v e r a l l , any e l e c t r i c a l l y conductive surface preparation w i l l impart benefits to electropainted articles similar to the benefits imparted to spray painted or dip-coated articles. The workpieces go from pretreatment to the electrocoating tank either water wet or dried. Most baths are operated at 5-20% nonvolatile concentration. A low s o l i d l e v e l is desirable, since the paint loss on drag out i s smaller and the paint s o l i d s dwell a shorter time in the bath ( c a l l e d faster "turnover"). The bath pH ranges from 3 to 6 + for cathodic deposition and from 7 to 9 for anodic deposition. The majority of e l e c t r o c o a t i n g i n s t a l l a t i o n s take the merchandise through two to nine stages of cleaning and phosphating under continuous movement through the electrodip, rinse, and bake, followed by spray coating i f so desired. Energy savings are accomplished by so c a l l e d batch-type operation. Here one or more parts are located over a dip tank and enter through vertical downward motion. The parts exit vertically upward and are then moved horizontally until located over the next tank. Very small parts, like fasteners, are coated in bulk, sometimes through the use of three short endless conveyors. The f i r s t conveyor receives the phosphated pieces and drops them into the electrocoating tank; a second conveyor located in the tank and completely submerged i s e l e c t r i c a l l y energized to accomplish the electrodeposition. The pieces drop then onto a third conveyor, which moves the pieces upward and out of the bath. Symmetrically designed pieces, like cans, are sometimes held and rotated by a mandrel. An e l e c t r i c a l l y energized and perforated tube supplies the paint to the vicinity of the surfaces to be coated. Electrocoats are also applied by fast-running c o i l coaters (6001000 ft/min). The majority of i n s t a l l a t i o n s uses conveyorized tanks. Recently, however, batch-type tanks are sometimes selected for small- and medium-sized production. Tanks may be classed into two groups, (a) The tank wall i s lined with an e l e c t r i c a l l y insulating coat (Figure 2), while the counterelectrodes are inserted in the tank and then positioned according to size or shape of workpiece. (b) The tank wall is used as counterelectrode (Figure 3). When lined tanks are used (Figure 2), the workpieces can be grounded through the conveyor. In many cases, however, the hanging device (paint hook) carries an electric contactor (brush) sliding along a grounded r a i l (bus bar) to ensure electrical ground. In any event, electrical insulation has to be provided between the positive and the negative sides of the system. In the case of the lined tank, i t is the "liner" that insulates the bath fluid from the ground. If the entire tank wall is used as an electrode, i t is grounded and an "insulating l i n k " i s provided that separates the upper, grounded section of the paint hook from i t s lower part. The lower insulated section of the hook is in electrical contact with



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the power source (Figure 3) and energized only inside the tank enclosure. Pumps, draft tubes, l i n e shafts, and ejector-nozzle systems capable of moving or turning over the entire bath volume in 6-30 min are used to prevent the paint from settling in the tank. As a r u l e , 50-75 ym pore size f i l t e r s are used to pass the entire paint volume through the f i l t e r in 30-120 min. The feed materials are manufactured and shipped at paint solid concentrations ranging from 40% to 99+%. In some installations, the feed is metered into the tank in the form of two or more components, one component being the resin, the other component being a pigment slurry, etc. To keep a bath in operating condition, removal of leftover solubilizer is accomplished through electrodialysis (Figure 2), ion exchange, or dialysis methods. If the dispersed, electrodepositable resin i s symbolized as RC00" + Y + , i t is implied that the RC00~ moiety w i l l be removed from the bath in the form of a paint coat that is anodically deposited on the workpiece. If the s o l u b i l i z e r moiety Y + would remain in the bath, i t would accumulate and eventually interfere with the coating operation. The electrodialysis method uses tanks lined with an insulating layer. From 5 to 30 properly spaced counterelectrodes are surrounded by membranes (Figure 2), which separate the coating bath from the counterelectrode by means of a membrane permeable to the solubilizer Y+. A plumbing system flushes the solubilizer out of the counterelectrode compartment (Figure 2). A widely practiced method is the reuse of leftover solubilizer for the dispersion of the feed material. In this case, the feed consists of p a r t i a l l y s o l u b i l i z e d resin that i s reacted with a s o l u b i l i z e r - r i c h bath through the use of an ultrasonic or a highshear homogenizer. An interesting variation of the electrocoating process, the soc a l l e d exhaustion method, has been developed for bulk coating of fasteners, electronic components, fine mechanical components, etc. These are used for very small tanks which use very l i t t l e paint; thus, the cost of monitoring the tank for replenishment i s comparatively expensive, while the cost of discarding the tank f i l l is low. Modified electrocoating paints are, therefore, placed into the tank at approximately 12% paint solids and 35% neutralization. The solids are gradually used up through painting u n t i l only 4% solids at 70% neutralization remain. The tank is then r e f i l l e d with fresh paint at 12% solids. The f i l l of an electrocoating dip tank i s actually an inventory of diluted paint. Concentrated paint i s added to the tank to replace the paint that is removed from the tank on the surfaces of painted merchandise. The time interval in which the paint additions to the tank equal the original f i l l is called the turnover period. Electrocoating tanks are reported to have turnover periods ranging from several days to several months. Since both freshly added paint and older paint are coated onto the surfaces of merchandise, sizable quantities of paint reside in the tank for long periods, as predictable from the law of averages. Electrocoating tanks are heavily agitated to prevent pigment settling. Thus, high-shear s t r e s s r e s i s t a n c e i s r e q u i r e d .



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Furthermore, as in a l l dip processes, the paint is subject to a long residence period in the tanks requiring high resistance against saponification, oxidation, bacteria, etc. Experience t e l l s that electrocoating baths should have a pumping stability exceeding four turnover periods. In general, a higher pigment/binder ratio is observed in the anodically deposited films than exists in the a l k a l i n e bath. A t y p i c a l bath may, for instance, contain 25 g of inorganic pigment combined with 75 g of resin, while the film deposited from this bath may contain 28 g of pigment with 72 g of resin. It i s easy to maintain the bath in proper working condition by feeding a composition containing 28 g of pigment plus 72 g of resin. Similar differences between bath and film may exist regarding other bath components. In other words, the bath is formulated to give the desired film properties, while the feed is formulated to replace the materials that were coated out or otherwise removed from the tank. Rectifiers that deliver direct current of less than 5% ripple factor are usually specified. Various output voltage controls are in use, such as tap switches, induction regulators, saturable core reactors, etc. Voltages in the 50-500 V range are usually provided. The current requirement is calculated from the weight of coating to be applied in the available time. For example, i f 1 lb of coating is to be deposited in 2 min, by using a paint that requires 75 C/g, then an average of 453 g χ 75 C/g / 120 s = 283 A i s required. Entering merchandise demands a large amperage that diminishes as the insulating paint coat forms. The current fluctuations in the tank can be mitigated through the incorporation of current limiting devices. Suppose that an entering workpiece produces a peak current draw of 705 A; i f the current draw i s limited to 500 A, a smaller power source but a somewhat longer coating time w i l l be required. Practically a l l of the applied electrical energy is converted into heat. Cooling equipment must be adequate to maintain the desired bath temperature, usually between 70 and 90 °F as specified by the paint suppliers. The cleaned or pretreated workpieces may enter the tank carrying an e l e c t r i c charge (sometimes c a l l e d " l i v e entry," "energized entry," or "power in"). Figure 4 shows the time/amp r e l a t i o n observed in the case of " l i v e entry" of workpieces. Suppose that the conveyor l i n e enters one workpiece into the tank with a charge of 250 V. Suppose further that i t takes 30 s for complete submersion. The current draw w i l l r i s e during that time (Area A). If the workpiece remains submerged in the bath for 1 min, then the current w i l l diminish during that period (Area B) and then f a l l rapidly when the workpiece leaves the tank. Workpieces may enter the tank without applied voltage and, therefore, without current draw (sometimes c a l l e d "dead entry"). When the submerged pieces are then energized, a large current surge w i l l be observed. This may be mitigated through gradually raising the applied voltage or through successively engaging different power sources with b u i l t - i n maximum amperage settings in addition to maximum voltage settings (see right half of Figure 4). Although "energized entry" at full-coating voltage is preferred, some electrocoating baths, on energized entry, produce merchandise showing peculiar patterns or streaks, c a l l e d "hash marks" or



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Figure 4.

Time-amperage study.



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"wisps." In this case, pieces are entered into the bath while unenergized or low energized to eliminate these defects. Users of the electrocoat process provide for various methods of energizing through the i n s t a l l a t i o n of two or more smaller power sources rather than one source of the f u l l required size. Multiple power sources also protect against complete shutdown in case of breakdown of one of the power sources. Amperage limitation, current c y c l i n g , or intermittent current application lengthens the required coating time, since i t i s the applied ampere seconds (Coulombs) that produce the electrodeposit. Current consumption ranges from about 15 C/g of finished coat up to 150 C/g. After an i n i t i a l amperage surge, the high e l e c t r i c a l resistance of the freshly deposited f i l m diminishes the current flow, resulting in an overall requirement of 2-4 A/ft for 1-3 min, or between 1 and 3 kWh/100 f t 2 . The coating time ranges usually from 1 to 3 min. For some special work, such as wires, steel bands, etc., coating times as low as 6 s are reported. The voltage requirement is largely dictated by the nature of the dispersed resin in the bath. Installations are usually operated at between 200 and 400 V, although some are reportedly operated as low as 50 V and others as high as 1000 V. Freshly coated pieces, when l i f t e d from the bath, carry bath droplets and even puddles of paint. A high concentration of paint solids exists in the vicinity of a workpiece that is being coated. It i s estimated that an automotive body may carry about 1 gal of bath. At 10 wt % nonvolatiles this is approximately 1 lb of solids. Considering the migration of solids toward surfaces that are being coated, s o l i d concentrations of up to 35% are expected in their vicinity. Thus, i t is evident that the recovery of the lifted paint bath is necessary, and a lucrative way has been found in the form of an "ultrafiltrate rinse" or ultrafiltrate dip. Ultrafiltration uses membranes that allow the passage of water and t r u l y dissolved substances, such as solvents, s o l u b i l i z e r s , s a l t s (impurities), etc. Dispersed paint resins, pigments, etc., are retained by the membrane. One hundred or more gallons of bath pass on one side of the membrane under pressure, while 1 gal of clear aqueous fluid passes through the membrane. The fluid, called permeate or ultraf i l t r a t e , is collected and used as rinse fluid. A three-stage rinse system recovers approximately 85% of the paint solids that were lifted from the bath. Some of the ultraf i l t r a t e (permeate) is occasionally discarded to remove low molecular weight ionized materials. The high b i o l o g i c a l oxygen demand of the permeate makes i t s disposal expensive. Subjecting the u l t r a f i l t r a t e to reverse osmosis (RO) lowers the disposal cost for the R0 concentrate, while the R0 permeate is returned to the rinse tank. The conventional type of bake oven is used. Electrodeposited coats are usually baked from 5 to 30 min at temperatures varying from 225 to 400 ° F . The a i r v e l o c i t y through the oven can be comparatively low, due to the very small quantities of organic volatiles in the paint coat. The time/temperature requirements are dictated by the resin system and are similar to those required for conventional dip or spray paints—usually 5-25 min at 225-400 ° F air


838

APPLIED POLYMER SCIENCE

temperature. Ambient temperature drying electrocoats are on the market.


Advantages The speedy acceptance of electrodeposition is due to the fact that it combines many advantages of other painting methods with new and desirable features: Formation of protective films in highly recessed areas such as cavities, box sections, creases, and flanges results in very much improved corrosion protection. High transfer efficiency results in up to 50% lower paint consumption. Use of water as practically the only carrier virtually eliminates the fire hazard. Low paint bath viscosity—approximately equal to that of water—results in ease of bath agitation and pumping and allows fast entry and drainage of merchandise. Freshly deposited coats allow rinsing and some handling immediately and have the lowest known tendency to sag or wash during cure. A second coat, or color coat, of waterborne or solventborne spray paint can be applied directly over the uncured or semicured electrocoat. Overall savings, including materials, labor, facilities investment, electrical power, etc., are reported to reach 20-50% when compared with spray, electrostatic spray, or dip-coat painting. Literature Cited 1. Raney, M. W. "Electrodeposition and Radiation Curing of Coatings"; Noyes Data Corp.: Park Ridge, N.J., 1970. 2. Yeates, R. L. "Electropainting"; Robert Draper Ltd.: Teddington, England, 1970. 3. Brewer, G. E. F.; Hamilton, R. D. "Paint for Electrocoating"; ASTM Gardner-Sward Paint Testing Manual, 13th ed.; ASTM: Philadelphia, 1972; pp. 486-89. 4. "Electrodeposition of Coatings"; Brewer, G. E. F., Ed.; ADVANCES IN CHEMISTRY SERIES No. 119, American Chemical Society: Washington, D. C., 1973. 5. Machu, W. "Handbook of Electropainting Technology"; Electrochemical Publication, Ltd.: Ayr, Scotland KA7 1XB, 1978. 6. Brewer, G. E. F., Chairman. "Organic Coatings and Plastics Chemistry"; American Chemical Society: Washington, D.C., 1981; Vol. 45, pp. 1-22, 92-113. 7. "Advances in Electrophoretic Painting"; Chandler, R. H., Ed.; Bi- or Triannual Abstracts since 1966; Braintree: Essex, England. 8. "Electrodeposition Processes and Equipment"; Duffy, J. I., Ed.; Noyes Data Corp.: Park Ridge, N.J., 1982. 9. Kardomenos, P. I.; Nordstrom, J. D. J. Coat. Technol. 1982, 54(686), 33-41. 10. Robison, T., Organizer of "Electrocoat" Conferences (even years, since 1982), Products Finishing, Cincinnati, Ohio.


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