Electrocoat BasicsHeard of electrocoating but really don’t know much aboutit? Well, here’s the place to begin your education . . .

By GEORGE LOVELL

BASF CorporationTroy, Michigan

E lectrocoating, ELPO and E-Coat are some of the terms usedto refer to electrodepositing a

paint film onto a metal substrate.You’ve probably heard about theprocess. After all, it’s been aroundfor over 25 years. Now you areready to take a c loser look atelectrodeposition of paint. Perhapsyou want to consider this painting

method for your manufacturing op-eration. If you are ready to beproperly introduced to electrocoat-ing, you’ve turned to the right page.

Electrodeposition of paint is atime-proven technology that allowsmost users to meet higher produc-tion requirements, lower appliedcosts, and improve the physicalproperties of the end product. Andelectrodeposition of paint is envi-

1. MONORAIL electrocoating system moves parts continuously through pretreatment,paint tank, rinses and bake oven.

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Now lets take a look at what isinvolved in a system for electrode-positing paint.

Equipment. There are basicallytwo types of electrodeposition sys-tems based on how parts are pro-cessed through the system.

In a monorail system (Fig. 1) partsare suspended from a conveyor thatmoves at a constant line speed.

A hoist system (Fig. 2) uses acrane to lift and move the partsthrough each stage of the system.

Each kind of system requires the

same mechanical equipment to sup-port the electrodeposition process.The main difference between thetwo systems will be in the actual sizeof the equipment, a monorail systemrequiring larger equipment. Theequipment in an electrocoating facil-ity can be categorized as follows:1) pretreatment washer; 2) elec-trodeposition tank and its auxiliaryequipment (covered below); 3) post-rinse system; 4) waste treatment sys-tem; 5) deionized water system; and6) bake oven.

The auxiliary equipment neces-sary to support an electrodeposition

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2. PROGRAMMED HOIST SYSTEM, alsocalled “square transfer,” raises and lowersworkbars into and out of individual cells.Work is stationary at each stage. As aresult, hoist systems occupy less spacethan monorail electrocoat lines.

Metal Pretreatmentchanger. Each piece of equipment issized according to the volume ofmaterial processed.

The total volume or output vol-system can be broken down intofour sub-systems.

Circulating System. This systemprovides agitation to keep the paintsuspended in the bath, provides formechanical filtration of the bath andprovides temperature control of thebath by removing heat caused by theelectrodeposition process and by thepumping process itself.

The circulation system will in-clude pumps, filters and heat ex-

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ume of the various pumps on thesystem should turn the paint tankover a minimum of four times anhour. For example, if the total tankvolume is 15,000 gal, a minimum of60,000 gph must be pumped throughthe filters or 1,000 gpm.

The heat exchanger must controltemperature within a range specifiedby the paint supplier.

The circulating system is the heartof the system, pumping the paint

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For example: 100 sq ft per min ofproduct processed will require aminimum of 4.28 gpm of permeatefrom new or cleaned tubes.

twenty-four hours a day, seven daysa week, 52 weeks a year.

Ultrafiltration System. The UFsystem provides for a means topurge the paint of water-solublecontaminants. The ultrafilter alsoprovides rinse liquid (permeate) thatallows for a “closed-1oop” rinsingsystem. The ultrafiltration systemincludes pumps, filters, storagetanks and ultrafilter tubes.

The ultrafilter is rated in terms of

The pump size and filtration onthe ultrafilter will depend on thetype of ultrafilter used. The ultrafil-ter should be sized properly becausetoo little permeate can cause rinsingproblems.

flux, the amount of permeate pro-duced in a given period of time. Fluxis measured in gpm and in gallonsflux per day (gfd).

A good rule of thumb is that youwill need 3.0 gpm of permeate at 70pct of the ultrafilter flux rate foreach 100 sq ft of product processed.

Electrical System. The electricalsystem includes a rectifier thatchanges alternating current to directcurrent (AC to DC). The electricalsystem will also contain anode orcathode plates. In a cathodic elec-trodeposition system, the electricalsystem may require a sub-system, ananolyte system, that is used to con-trol the pH of the paint bath. Itconsists of a pump, anode plates(stainless steel) that are encased inan anolyte cell and a conductivitymeter.

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Basic Electrocoat Glossary

Amps (Amperes)

Anions

Anode

Anolyte

Anolyte Cell

Cathode

Cations

Conductivity

Cream Coat

Deionized Water

Electrodeposition

Electroendosmosis

Electrolysis

Electrophoresis

Flux Rate

Hash Marking

PHP/B

Permeate

Rectifier

Rupture

Solids

Throwing Power

Ultrafiltrate

Ultrafiltration

Flow of electricity.

Negatively charged ions that are attracted to the positivelycharged anode during electrodeposition.

Positively charged electrode.

Formed at the anode in the anolyte cell. It is sometimescalled solubilizer.

An anode enclosed in a plastic frame and separated from the paint witha semi-permeable membrane that allows the solubilizer to pass but notthe paint.

Negatively charged electrode.

Positively charged ions that are attracted to the negatively chargedcathode during electrodeposition.

Relative degree of electron mobility in solution.

Outer layer of paint that adheres to the deposited film when an article isremoved from the tank.

Water purified by the removal of conductive ions.

Forming a deposit on an electrode by electrolysis.

Movement of water away from the depositing electrode by diffusionthrough film formed during electrodeposition.

Oxidation/reduction reactions at the anode and cathode that make the flowof current possible.

Movement of charged particles in a conductive medium under theinfluence of an applied potential difference.

Rate at which permeate is formed.

Electrocoat film defect that sometimes occurs on vertical surfaces.

Relative degree of acidity or alkalinity.

Ratio of pigment to binder in the paint.

Liquid that passes through the membrane in ultrafiltration.

Device that converts alternating current to direct current.

Bursting of the deposited paint film by an excessive generation of gas atthe deposited electrode.

Non-volatile content of the paint.

Degree to which electrocoating paint covers recessed areas.

Permeate.

Process for removing liquid from the paint, using a selective semi-permeable membrane. The liquid or permeate is used for rinsing afterelectrocoating.

Typical voltage ranges from 350to 500 volts DC, depending on theamount of product being processed.The rectifier has an amperage ratingand a good rule of thumb is to have2.0 amps per sq ft processed. Forexample, 100 sq ft would require 200amps. The rectifier should have afilter to keep ripple less than fivepct.

Cathode-to-anode ratio for ananodic electrodeposition paint, anda n o d e - t o - c a t h o d e r a t i o f o r acathodic paint should be 4:1. Forexample, with a cathodic system,100 sq ft per min of product pro-cessed would require 25 sq ft ofanode. The same size anodic systemwould require 25 sq ft of cathode.

Feed System. As the product isprocessed through the electrodeposi-tion system, paint from the tank isdeposited on the product. As thepaint is depleted it is necessary toreplenish the tank with fresh paint.A common way to do this is to havea feed system consisting of one ortwo pumps and a static mixer that isconnected to the tank circulatingsystem. The feed materials arepumped into the tank through thestatic mixer and into the circulatingsystem.

An operating electrodepositiontank is in a dynamic state andrequires regular monitoring. Typicaloperational parameters that must bemeasured and controlled include:pct non-volatile (% N.V.) or pctsolids; P/B or B/P, more commonlycalled Pigment-to-Binder or Binder-

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Milestones in Electrodepositionof Colloidal Dispersions1920-Latex rubber electrodeposited on

glove forms.1939-Electrodeposition of emulsions

on the inside of steel foodcontainers.

1959-First experiments on anodicallye l e c t r o d e p o s i t i n g p a i n t b y Dr. George E. F. Brewer at theFord Motor Company.

1963-First anodic electrocoating pro-duction facility, Ford Motor Co.

1971 -Introduction of cathodic electro-coating technology to the automo-tive and appliance industries.

1976 -Introduction of high-film-buildcathodic technology.

to-Pigment, expressed as a ratio;paint pH; and paint conductivity.

Electrocoating is an excellent wayfor a manufacturer or coating facil-ity to comply with government regu-lations, improve its productivity,reduce costs and improve finishquality. Don’t be intimidated byelectrocoating. It may appear morecomplex and complicated than itreally is. Do your own investigation.There is plenty of information avail-able through a variety of sourcesincluding suppliers of pretreatmentchemicals, paints and equipment,consultants, trade publications andproceedings from technical confer-ences on electrocoating. Informa-tion is available; you don’t have tore-invent the wheel. PFFor more data circle 287 on Postpaid Card

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