TECHNOLOGY UPDATE

Water Quality Products March 200442 www.wqpmag.com

There are many niche marketsfor the water treatment dealerto pursue. A multitude of

industries rely on a consistent sourceof deionized water for manufacturingand production, and as a source ofmakeup water for power generationand process water.

Elecrocoating is a painting method usedby many manufacturers today. It is anelectrodeposition process that appliescoatings to conductive surfaces using anaqueous paint bath. The process useswater-based paints and primers, andrelies on pure water for the rinsing steps.

The electrocoat (or ecoat) process worksby putting an electrical charge on apart and immersing it in a bath withpaint particles of an opposite charge.Since opposite electrical charges attract,the part is completely covered with alayer of the paint particles. The beautyof elecrocoating is that the part iscompletely and evenly coated. Thevoltage applied during the processcontrols the thickness of the coating.

The advantages that ecoat has overtraditional spray applications of paint

is that it evenly coats any irregularshapes, nooks and crannies. The uniformcoating is free of drip marks or sag lines.There is no paint booth so there are noproblems with overspray and fumes.Also, without paint vapor present, VOCsare eliminated or present only at verylow levels. Most times operators do nothave to wear special breathing appara-tus. Fire hazards are greatly reducedsince it is a waterborne system.

Advantages• Environmentally Friendly—The

water-based system minimizes excessvolatile organics and heavy metals. Thepaint used is about 80 percent water.

• Better Corrosion Protection—Allareas of the parts are totally covered.This is particularly advantageous incomplex parts.

• Less Paint Waste—Efficientapplication can reduce paint wasteby up to 95 percent.

• Better Finish—The ecoat techniqueis a consistent, repeatable processthat applies an even, reproduciblecoat from part to part.

• Predictable Costs—The repeatabilityof the coating is not susceptible tooperator variables, so the sameamount of paint and water is usedfor each piece of work. The totalapplied costs are relatively low.

Disadvantages • Conductive Surfaces Only—The

parts to be painted must be able toconduct an electrical charge. Someacceptable materials include copper,aluminum, gold, silver, brass, zinc-rich metals, galvinized/galvaneal,CRS/HRS (cold rolled steel and hotrolled steel).

• Color—Each different color to beapplied requires a separate diptank system.

• Capital Costs—The initial investmentof an ecoat system is substantial.Most systems are fully automated.

Water Use During the Ecoat ProcessThe parts to be coated are first cleanedof any oils, greases, polishing compounds,

Electrocoating Requires DI WaterDeionization Provides Quality Water for Painting MethodElectrocoating Requires DI WaterDeionization Provides Quality Water for Painting Method

By Frank DeSilva, ResinTech

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Table 1. Electrocoating Uses by MarketEnd Use Properties Added By Electrocoat

Agriculture Equipment High Gloss, Color Control, Weatherability,Corrosion Resistance

Appliance Corrosion and Stain Resistance, Color Control

Automotive Corrosion and Chip Resistance, Weatherability

Automotive Components Corrosion, Chemical and Chip Resistance

Containers (Can Coatings) Barrier and Chemical Resistance, FDA-Approved, No Effect on Flavor

Electrical Switchgear Corrosion Resistance and U.L. Approval

Fasteners Corrosion and Edge Coverage

Heating, Ventilation and Cooling Corrosion Resistance, Color Control and Weatherability

Laboratory Furniture Chemical, Stain, and Corrosion Resistance,Color Control

Lawn and Garden Corrosion Resistance and Weatherability

Printed Circuit Boards Edge Coverage and Hardness

Shelving and Furniture Color Control, Hardness and Stain Resistance

Wheels Corrosion and Chip Resistance, Weatherability

Reprinted with permission from Total Source From Product Finishing magazine, “Electrocoat—TheNext Step, The advantages of electrocoating and its new Association.” By Cindy J. Goodridge,executive director, The Electrocoat Association, Cincinnati, Ohio.

etc., by alkaline cleaners and vigorousrinsing. Different coating chemicalsrequire different degrees and amountsof pretreatment. Some systems requirean activator or phosphate dip beforethe part undergoes it’s final rinsebefore ecoat.

Since the ecoat process is an aqueous-based process, there is no drying off ofthe parts required after the final pre-treatment rinse with deionized (DI)water. The wet parts can go directlyinto the ecoat bath.

After the ecoat deposition, the partsundergo additional DI water rinses. Thismay include a static DI rinse, counter-flow DI rinse and virgin DI rinse. Thestatic rinse is merely a DI water rinsetank that the part is first passed throughto rinse off the bulk of any dragout. Thecounterflow DI rinses are recirculatingrinse tanks that have the freshest (orvirgin) DI water entering the final tank.The dragout and initial rinse waterscan be cycled back to the paint bath tomaintain a high rate of transfer ofpaint solids to the parts being coated.

In addition to the ecoat rinses, the ecoatpaint itself requires DI water as it’sprimary constituent. A “premix,” highsolids paint concentrate is blended withDI water to form the final paint bath.The bath final makeup is in the range of

• 80–90 percent deionized water (acts as carrier for paint solids)

• 0–5 percent solvent • 1–10 percent pigment (provides

color and gloss)• 10–15 percent resin polymer (not

IX resins—the polymer is meltedduring the bake stage to form aprotective coating)

The final step of the ecoat process is abaking at elevated temperatures. Itallows the process to fully cure andprovides the ultimate in corrosionprotection by melting the resin polymerpresent in the paint mixture.

Water QualityThe deionized water specifications will vary according to the paintmanufacturer’s requirements but,generally speaking, DI water with aspecific conductance of less than 10micromhos (equivalent to a resistanceof 100,000 ohms) is needed. On mostcity water supplies, this can be achievedusing a two-bed deionizer or a reverseosmosis system. The paint manufacturer also may havea silica requirement that would neces-sitate the use of strong base anion resinsor mixed-bed resins.

Separate bed deionizers (cation followedby anion) offer the most efficient method

of ion removal on low to moderate TDSwaters. Consider RO for higher TDSwaters. Separate bed resins have highercapacity than mixed beds and also offeradvantages of easier regenerationcompared to mixed beds.

In a normal operating two-bed deminer-alizer, the effluent quality—measured asconductivity or resistivity—primarilyis indicated by the sodium leakage of

Water Quality ProductsMarch 2004 43www.wqpmag.com

About the AuthorFrank DeSilva is the national sales manager for ResinTech, Inc., West Berlin, N.J. He hasbeen in the water treatment industry for more than 20 years. ResinTech manufactures abroad range of ion exchange resins for water and wastewater treatment and alsoactivated carbon and inorganic selective exchangers.

the cation unit. The sodium exits a weakbase anion unit two-bed system as a salt,thereby having a neutral effluent pH.A two-bed deionizer that uses a strongbase anion resin for the second bed willhave a slightly higher pH, depending onthe sodium leakage, because the sodiumion from the cation unit leakage exitsthe deionizer as sodium hydroxide.

Weak-base anion resins have highoperating capacity and good regenerationefficiency. And since they only neutralizeacids, they do not excessively increase theeffluent pH returning to the rinse tanks.

Strong-base anion resins offer completeremoval of all anions including carbondioxide and silica. The use of strong-base anion resin results in higherquality water than can be produced byweak-base anion resins. The completeremoval capability comes at theexpense of operating capacity andregeneration efficiency.

Mixed beds (cation and anion resinmixed in a single tank) offer the simplicityand economy of a single tank systemand also offer neutral pH and excep-tional water quality. The advantagesare counterbalanced by somewhatpoorer operating capacity and greatlycomplicated regeneration. Mixed bedsare popular with service exchangecompanies because of their simplicityof operation and the elimination of theneed for a second tank.

Your resin supplier can predict whicheffluent quality can be achieved by atwo-bed deionizer. The informationneeded to calculate the anticipatedquality of treated water includes

• Cations(Calcium, Magnesium, Sodium)

• Anions(Alkalinity, Sulfate, Chloride, Silica)

If an uninterrupted flow of water isrequired, any permanent inhousesystem would require a two train ionexchange system or a means of storingDI water to accommodate times when atrain is in regeneration. For low flowrequirements, the water treatmentdealer may want to propose service DItanks or a combination of RO polishedby service DI tanks. WQP

Water Quality Products March 200444 www.wqpmag.com

Table 2. Approximate Conductivity, Resistivity and pH of From Two Bed Deionizers

Leakage from Weak Base Anion Exchange Resin Strong Base Anion Exchange Resin the cation unit (leakage as NaCl) (leakage as NaOH)

Na Resistance Conductivity Resistance Conductivity(ppm as CaCO3) Ph (ohm-cm) (micromhos) pH (ohm-cm) (micromhos)

0.02 neutral 10,000,000 0.1 7.5 5,000,000 0.2

0.14 neutral 2,500,000 0.4 8.4 1,000,000 1.0

0.4 neutral 1,000,000 1.0 9.0 500,000 2.0

0.8 neutral 500,000 2.0 9.5 250,000 4.0

4.0 neutral 100,000 10 10.0 50,000 20

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