ELECTROCOATING REQUIRES DI WATER 

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Deionization provides quality water for painting method

There are many niche markets for the water treatment dealer to pursue. A multitude of industries rely on a consistent source of deionized water for manufacturing and production, and as a source of makeup water for power generation and process water.

Electro coating is a painting method used by many manufacturers today. It is an electrode position process that applies coating to conductive surfaces using an aqueous paint bath. The process uses water –based paints and primers, and relies on pure water for the rinsing steps.

The electro coat (or coat) process works by putting an electrical charge on a part and immersing it in a bath with paints particles of an opposite charge. Since opposite electrical charges attract, the part is completely covered with a layer of the paints particles. The beauty of electro coating is that the part is completely and evenly coated. The voltage applied during the process controls the thickness of the coating.

The advantages that .coat has over traditional spray applications of paint is that it evenly coats any irregular shapes, nooks and crannies. The uniform coating is free of drip marks or sag lines. There is no paint booth so there are no problems with overspray and fumes. Also, without paints vapor present, VOCs are eliminated or present only at very low levels.  Most times operators do not have to wear special breathing apparatus. Fire hazards are greatly reduced since it is waterborne system.

Advantages

Environmentally Friendly –The water- based system minimizes excess volatile organics and heavy metals. The paint used is about 80 percent water.

Better Corrosion protection- All areas of the parts are totally covered. This is particularly advantageous in complex parts.

Less Paints Waste-Efficient application can reduce paint waste by up to 95 percent.

Better Finish-The .coat technique is a consistent, repeatable process that applies an even, reproducible coat from part to part.

Predictable Costs- The repeatability of the coating is not susceptible to operator variables, so the same amount of paint and water is used for each piece of work. The total applied costs are relatively low.

Disadvantages

Conductive Surfaces Only-The parts to be painted must be able to conduct an electrical charge. Some acceptable materials include copper, aluminum, gold, silver, brass, Zinc-rich metals, galvanized/galvaneal, CRS/HRS (cold rolled steel and hot rolled steel).

Color- Each different color to be applied requires a Separate dip tank System.

Capital Costs- The initial. Investment of an coat System is Substantial. Most Systems are fully automated.

Water Use During the Ecoat Process

The parts to be coated are first cleaned of any oils, greases, polishing compounds, etc., by alkaline cleaners and vigorous rinsing. Different coating chemicals require different degrees and amounts of pretreatment. Some systems require an activator or phosphate dip before the part undergoes its final rinse before ecoat.

Since the ecoat process is an aqueous-based process, there is no drying off of the parts required after the final pretreatment rinse with deionized (DI) water. The wet parts can go directly into the ecoat bath.

After the ecoat deposition, the parts undergo additional DI water rinses. This may include a static DI rinse, counter flow DI rinse and virgin DI rinse. The static rinse is merely is DI water rinse tank that the part is first passed through to rinse off the bulk of any dragout. The counter flow DI rinses are recirculating rinse tanks that have the freshest (or virgin) DI water entering the final tank. The drag out and initial rinse waters can be cycled bank to the paints bath to maintain a high rate of transfer of paint solids to the parts being coated.

In addition to the ecoat rinses, the ecoat paint itself requires DI water as its primary constituent. A “premix,” high solids paint concentrate is blended with DI water to from 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 melted during the bake stage to from a protective coating)

The final step of ecoat process is a baking at elevated temperatures. It allows the process to fully cure and provides the ultimate in corrosion protection by melting the resin polymer present in the paint mixture.

Water Quality

The deionized water specifications will specification will vary according to the paint manufacturer’s requirements but, generally speaking, DI water with a specific conductance of less than 10 micromhos (equivalent to a resistance of 100,000 ohms) is needed. On most city water supplies, this can be achieved using a two-bed deionizer or a reverse osmosis system.

The paints manufacturer also may have a silica requirement that would necessitate the use of strong base anion or mixed –bed resins.

Separate bed deionizers (cation followed by anion) offer the most efficient method of ion removal on low to moderate TDS waters. Consider RO for higher TDS waters. Separate bed resins have higher capacity than mixed beds and also offer advantage of easier regeneration compared to mixed beds.

In a normal operating two-bed demineralizer, the effluent quality-measured as conductivity or resistivity-primarily is indicated by the sodium exits a weak base anion unit two-bed system as a salt, thereby having a neutral effluent pH. A two-bed deionizer that uses a strong base anion resin for the second bed will have a slightly higher pH, depending on the sodium leakage, because the sodium ion from the cation unit leakage exits the deionizer as sodium hydroxide.

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