Electrocoat Basics

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Electrocoat Basics

Heard of electrocoating but really don’t know much about

it? Well, here’s the place to begin your education . . .

By GEORGE LOVELL

BASF CorporationTroy, Michigan

Electrocoating, 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 the

process. After all, it’s been aroundfor over 25 years. Now you areready to take a closer look atelectrodeposition of paint. Perhapsyou want to consider this painting

method for your manufacturing op-eration. If you are ready to be

properly introduced to electrocoat-

ing, you’ve turned to the right page.Electrodeposition of paint is a

time-proven technology that allowsmost users to meet higher produc-tion requirements, lower appliedcosts, and improve the physical

properties of the end product. Andelectrodeposition of paint is envi-

1. MONORAIL electrocoating system moves parts continuously through pretreatment,

paint tank, rinses and bake oven.

58 PRODUCTS FINISHING APRIL, 1990



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 support the electrodeposition process.The main difference between the

two 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; and

6) bake oven.The auxiliary equipment neces-sary to support an electrodeposition

APR I L, 1990 PRODUCTS FINISHING 59



2. PROGRAMMED HOIST SYSTEM, also

called “square transfer,” raises and lowers

workbars into and out of individual cells.

Work is stationary at each stage. As a

result, hoist systems occupy less space

than monorail electrocoat lines.

Metal Pretreatment

changer. Each piece of equipment is

sized according to the volume of material processed.

The total volume or output vol-

system can be broken down into

four sub-systems.Circulating System. This system

provides agitation to keep the paintsuspended in the bath, provides for mechanical filtration of the bath and

provides temperature control of the

bath by removing heat caused by theelectrodeposition process and by the pumping process itself.

The circulation system will in-

clude pumps, filters and heat ex-

60 PRODUCTS FINISHING

ume of the various pumps on thesystem should turn the paint tank over a minimum of four times anhour. For example, if the total tank volume is 15,000 gal, a minimum of 60,000 gph must be pumped throughthe filters or 1,000 gpm.

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

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

APRIL, 1990



For example: 100 sq ft per min of product 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 to

purge the paint of water-solublecontaminants. The ultrafilter also provides 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 rinsing

problems.

flux, the amount of permeate pro-duced in a given period of time. Flux

is measured in gpm and in gallonsflux per day (gfd).A good rule of thumb is that you

will need 3.0 gpm of permeate at 70 pct of the ultrafilter flux rate for each 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 or cathode plates. In a cathodic elec-trodeposition system, the electricalsystem may require a sub-system, an

anolyte 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

An od e

Anolyte

Ano ly te Cel l

Cathode

Cations

Conductivity

Cream Coat

Deionized Water

Electrodeposition

Electroendosmosis

Electrolysis

Electrophoresis

Flux Rate

Hash Marking

PH

P/B

Permeate

Rectifier

Rupture

SolidsThrowing 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 the

influence 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 after electrocoating.



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. For

example, 100 sq ft would require 200amps. The rectifier should have afilter to keep ripple less than five

pct.

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. For example, with a cathodic system,100 sq ft per min of product pro-

cessed would require 25 sq ft of anode. The same size anodic systemwould require 25 sq ft of cathode.

Feed System. As the product is processed through the electrodeposi-tion system, paint from the tank isdeposited on the product. As the

paint 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 or two pumps and a static mixer that isconnected to the tank circulatingsystem. The feed materials are

pumped 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-volati le (% N.V.) or pctsolids; P/B or B/P, more commonlycalled Pigment-to-Binder or Binder-

APRIL, 1990

Milestones in Electrodepositionof Colloidal Dispersions

1920-Latex rubber electrodeposited onglove forms.

1939-Electrodeposition of emulsionson the inside of steel foodcontainers.

1959-First experiments on anodicallyelect rodeposi t ing paint byDr. 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 and

proceedings from technical confer-ences on electrocoating. Informa-tion is available; you don’t have tore-invent the wheel. PF

For more data circle 287 on Postpaid Card

PRODUCTS FINISHING 63

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Electrocoat Basics