A Brief History of E-coat

Origin

Even though E-coat has been with us since the 1930, it is mainly due to large interest and capital investment in the 70's by the automobile industry for primers that made it popular. Since then the technology has found its way into the more decorative and functional (non primer) single coat application like CLEARCLAD.

Timeline

1940's - Experimentation into electodepositing phenolic resin coatings onto electrical wire on a continuous basis.

1950's - Full scale development of electrodeposition of anti-corrosive paint primers onto automobile bodies.

1960's - Development of exterior durable, light colored electropaint resin systems suitable for domestic appliances, architectural aluminum etc/(Principle technology so far - anodic)

1970's -Cathodic technology displaces anodic as the principle system in the automobile industry. Such systems are adapted for small scale use in the electroplating industry (circa 1978).

1980's - Technology continues to evolve as protective coatings for the metal finishing industry.

 

So, what is E-coat?

Process Mechanism

E-coat is an emulsion of organic resins and de-ionized water, which is in a stable condition. The e-coat solution also comprises of some solvent and some ionic components. When a D.C. voltage is applied across two immersed electrodes, the passage of current is accompanied by electrolysis of water. This results in oxygen gas being liberated at the anode (positive electrode) and hydrogen gas liberated at the cathode (negative electrode). The liberation of these gases disturbs the hydrogen ion equilibrium in the water immediately surrounding the electrodes. This results in a corresponding pH change and this in turn de-stabilizes the paint components of the solution and they coagulate onto the appropriate electrode.

- Cathodics electropaints are stable except at high (alkaline) pH. Anodics are stable except at low (acid) pH

- Electrolysis of water causes the cathode to become alkaline and the anode to become acid

Electrolysis of Water 

Electrophoresis is a well documented process whereby electrically charged particles in a conductive medium will migrate to the electrode bearing the opposite charge under the influence of D.C. voltage. Although many technical descriptions of electropaint ascribe electrophoresis to the deposition process it is not the predominant mechanism. However, it is very common to refer to electropaint as "Electrophoretic"

 

How is it applied?

Application

An unfinished product is immersed in a bath containing the electrophoretic paint emulsion, and then an electric current is passed through both the product and the emulsion. The paint particles that are in contact with the product adhere to the surface, as described in the above mechanism, and build up an electrically insulating layer. This layer prevents any further electrical current passing through, resulting in a perfectly level coating even in the recessed parts of complex-shaped goods. The product is then removed from the paint bath and baked in an oven.

 

How does this compare to plating?

Due to the insulating nature of the deposit as described above, it is possible to accurately control the thickness over the part. Whereas with plating and anodizing thickness is controlled by amp/time relationship.

 

E-Coat vs. electroplating and anodizing - amp/time relationship.

With e-coat the thickness is controlled by voltage. Time is not as critical, as once the part is coated and insulated, no more coating will take place. Depending on surface area and complexity of the parts, most coating is easily accomplished with 2 minutes. This highlights one of the big equipment differences. Plating and anodizing require low voltage and high amperage rectification. E-coat requires high voltage and low amperage (1 sq. ft. draws 1.5 amps max) rectification.

Electrocoating is a method of painting which uses electrical current to deposit the paint.  The process works on the principal of "Opposites Attract".  An e-coat system applies a DC charge to a metal part immersed in a bath of oppositely charged paint particles.  The paint particles are drawn to the metal part and paint is deposited on the part, forming an even, continuous film over every surface, in every crevice and corner, until the coating reaches  the desired thickness.

The electrocoat process can be divided into four distinct steps:

1.        Pretreatment cleaning & phosphating cycle .

2.        Electrocoat bath cycle.

3.        Post rinse cycle.

4.        Baking and curing cycle.

 

ELECTROCOATING ADVANTAGES

1.        Uniform coating thickness over all areas including sharp corners, recesses and areas that would be hard to reach with spray painting.

2.        Electrocoating is automatic and labor saving, requiring little maintenance.

3.        Electrocoating saves the costs and operating expenses of air supply systems, fire protection equipment, respiratory hazards and costly cleanup. The paint material is water-based and nontoxic.

4.        Approximately 95% utilization of paint with no overspray, drip or drain losses.

5.        Complete paint coverage - no touchup ever required.

6.        Parts may be racked on the conveyor, one on top of the other, with no concern for dripping.

7.        Primers applied by electrocoating come out smooth and may be top coated without sanding.

ELECTROCOAT DETAILS

The system offers better uniformity, higher density and less permeable coating than spray applications, saving up to 50% on coating materials. It is environmentally friendly, reducing emissions up to 70% and achieving nearly 100% coating utilization. Electocoat also eliminates expenses associated with overspray cleanup and disposal.

 

DIAL YOUR COATING

The main factors controlling film thickness are the applied voltage and the film resistance. Increasing the coating voltage or lowering the specific film resistance causes an increase in film thickness. You simply dial the desired coating thickness.

The electrocoating process will continue until the organic film deposited provides an electrical insulating resistance which prevents further current flow.

When the coated parts are removed from the bath, they are rinsed in permeate and deionized water to remove non-deposited paint particles.

 

TANK DESIGN

Electrocoat tanks are designed for an immersion time of 1-1/2 to 2 minutes. It is possible to deposit approximately 1.0 mil organic coating in the first 15 seconds. However, for heavier film deposit, a longer time is required.

Tank equipment includes dual pumps, with each pump able to maintain the bath and prevent the setting of paint solids.

Plate and frame heat exchangers are used with chiller units to maintain proper tank temperature.

 

TANK DESIGN IS VITAL

In the design of the electrocoat tanks, some of the most important items are circulation rate, circulation flow, and density of the paint. With the paint solids normally at 8 to 10 percent density, a flow rate and pattern is determined to prevent setting.

The flow rate in the average tank is accomplished by the use of headers with eductors. The flow pattern in the bottom of the tank is opposite that of the conveyor movement and with the conveyor at the top of the tank. The exit end of the tank is equipped with an overflow weir tank, designed to prevent foaming without dropping or aerating the paint. The recirculating pump suctions are also connected to this tank.

 

FILTER SYSTEMS

Conventional filter systems are provided with approximately 50 micron filter media to remove foreign debris that may enter the bath.

An ultrafiltration system will be used to remove soluble salt and water carried into the bath from the cleaning process by the parts being coated. Ultrafiltration may also be used to recover paint solids from the post rinse so they may be returned to the bath. A virtually closed system exists when ultrafiltration is used to provide rinse water in the place of deionized water. This arrangement will aid considerably in the prevention of water pollution