Electrocoating Requires DI Water
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.
Elecrocoating is a painting method used by many manufacturers today. It is
an electrodeposition process that applies coatings 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 electrocoat (or ecoat) process works by putting an electrical charge on
a part and immersing it in a bath with paint particles of an opposite charge.
Since opposite electrical charges attract, the part is completely covered with
a layer of the paint particles. The beauty of elecrocoating is that the part is
completely and evenly coated. The voltage applied during the process controls
the thickness of the coating.
The advantages that ecoat 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 paint 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 a waterborne system.
* 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 Paint Waste--Efficient application can reduce paint waste by up to 95
* Better Finish--The ecoat 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.
* 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, galvinized/galvaneal, CRS/HRS (cold rolled
steel and hot rolled steel).
* Color--Each different color to be applied requires a separate dip tank
* Capital Costs--The initial investment of an ecoat 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 it's 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, counterflow DI rinse and virgin DI rinse.
The static rinse is merely a DI water rinse tank that the part is first passed through
to rinse off the bulk of any dragout. The counterflow DI rinses are
recirculating rinse tanks that have the freshest (or virgin) DI water entering
the final tank. The dragout and initial rinse waters can be cycled back to the
paint bath to maintain a high rate of transfer of paint solids to the parts
In addition to the ecoat rinses, the ecoat paint itself requires DI water as
it's primary constituent. A “premix,” high solids paint concentrate
is blended with DI 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 melted during
the bake stage to form a protective coating)
The final step of the 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.
The deionized water specifications 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 paint manufacturer also may have a silica requirement that would
necessitate the use of strong base anion resins 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
advantages 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 leakage of
the cation unit. 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.
Weak-base anion resins have high operating capacity and good regeneration
efficiency. And since they only neutralize acids, they do not excessively
increase the effluent pH returning to the rinse tanks.
Strong-base anion resins offer complete removal of all anions including
carbon dioxide and silica. The use of strong-base anion resin results in higher
quality water than can be produced by weak-base anion resins. The complete
removal capability comes at the expense of operating capacity and regeneration
Mixed beds (cation and anion resin mixed in a single tank) offer the
simplicity and economy of a single tank system and also offer neutral pH and
exceptional water quality. The advantages are counterbalanced by somewhat
poorer operating capacity and greatly complicated regeneration. Mixed beds are
popular with service exchange companies because of their simplicity of
operation and the elimination of the need for a second tank.
Your resin supplier can predict which effluent quality can be achieved by a
two-bed deionizer. The information needed to calculate the anticipated quality
of treated water includes
* Cations (Calcium, Magnesium, Sodium)
* Anions (Alkalinity, Sulfate, Chloride, Silica)
If an uninterrupted flow of water is required, any permanent inhouse system
would require a two train ion exchange system or a means of storing DI water to
accommodate times when a train is in regeneration. For low flow requirements,
the water treatment dealer may want to propose service DI tanks or a
combination of RO polished by service DI tanks.