Electrocoating is a method of giving a metal an organic finish by using electrical current to deposit the paint. The process works on the simple principle of Opposites Attract.

Thus the fundamental principle of electrocoating is that materials with opposite electrical charges attract. An Electrocoat 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. At that thickness, the film insulates the part and the attraction of paint particle stops and electrocoating is complete.

Electrocoating could be protective, decorative and may combine the properties of both depending upon the end use

Depending upon the polarity of the charge, electrocoating is classified as either:

• Anodic, or
• Cathodic

ANODIC: In Anodic Electrocoating, the part to be coated is the anode with a positive electrical charge which attracts negatively charged paint particles in paint bath. In the anodic process, small amounts of metal ions migrate into the paint film, which limit the performance properties of these systems. Their main use is for products in interior or moderately aggressive exterior environments. Anodic coatings are economical and offers excellent color and gloss control.

CATHODIC: The cathodic process is just the opposite of anodic process. In Cathodic Electrocoating, the part to be coated is given a negative charge, attracting the positively charged paint particle. The reversing of anodic polarities greatly reduces the amount of ions entering the cured paint film and thus enhances the properties of cathodic electrocoated products. Cathodic electrocoating are high performance coatings with excellent corrosion resistance and can withstand the harshness of exterior environment.

Electrocoating offers some significant advantages over powder coating and liquid spray systems. They may be divided into two categories: Environmental and Techno-economic.

ENVIRONMENTAL: Electrocoat Technologies are Eco-friendly because:

• They are designed to be heavy metal free with little or no Hazardous Air Pollutants (HAPS) and very low levels of organic solvents.
• Since these are water based coatings, they have very low Volatile Organic Compounds (VOC). They range from 0.012 kg per litre up to 0.360 kg per litre with most products below 0.120 kg per litre.
• As solvent levels of electrocoat products have dropped over the year, so has the Biochemical Oxygen Demand (BOD).
• There is practically no solid waste
• Most electrocoat materials are not considered "RED LABEL" because they are formulated as water based materials. There is no fire hazard.
• Electrocoat is a clean system not requiring any disposable suits, respirators, and other paraphernalia to protect workers from airborne hazards.

TECNO-ECONOMIC: Electrocoat System also has the techno-economic edge over powder coating and liquid spray system. The important ones include:

• High Corrosion Protection Property: Cathodic epoxy electrocoating is highly corrosion resistant. A 20-micron dry film thickness can withstand as much as 1000 hours salt spray test in accordance to test conditions laid down in ASTM B117 standard.
• Uniform Coating: The ability to give very even coating with variances ranging to only 1-2 microns.
• Coating of Complex Parts: Electrocoat system has the versatility to coat highly recessed and complex surfaces, which are not amenable to powder coating or liquid spray system.
• Higher Transfer Efficiency: Closed loop rinsing allows transfer efficiency of more than 95% with dramatically reduced waste water streams.
• Very Low Incidence of Rejects: Since most of the key process parameters are electronically controlled, human errors play a very little role and hence the rejection rate could be as low as zero. As a corollary to it, no special skills are required for handling the plant and production.
• Much Higher Productivity: No other paint system can match the throughput capacity of an electrocoat system. For example, if product remains the same, an electrocoat system can give at least three-times more production with less manpower during the same period.
• Much Better Quality: Electrocoat films are controlled by voltage adjustments. Once the metal is insulated, deposition stops. There is no other organic coating system that can be controlled with the film uniformity as it is in Electrocoating system.
• Ease in Handling: Even the uncured paint is dry enough to allow human touch and careful handling.

Electrocoating has some limitations as well. They include:

• It can be done only on metals.
• Only one color is economically viable in one set-up/line.
• Its initial capital cost is higher in comparison to powder coating and liquid spray systems.
• It demands special care to prevent air entrapment during paint deposition process in paint bath.
• It is more energy intensive process than powder coating and liquid spray for the simple reason that the circulation pump to agitate the paint bath have to run 24 hours, irrespective whether there is production or not.
• It can not cover the surface defects of parts to be electrocoated.

The Electrocoat Process can be divided into the following four distinct sections:

• Pretreatment
• Electrocoat Bath and Ancillary Equipment
• Post Rinses
• Bake Oven

An overview of an electrocoat system is shown below

Pretreatment: The pretreatment section is where the metal surface is cleaned and Phosphated to prepare the part for electrocoating. Cleaning and phosphating are essential in achieving the desired quality of coating. Iron and Zinc Phosphates are common materials used in pretreatment.

Electrocoat Bath: The electrocoat bath consists of 80-90 % deionized water and 10- 20 % paint solids. The deionized water acts as a carrier for the paint solids that are under constant agitation. The paint solids consist of resin and pigment. Resin is the backbone of the final paint film and provides corrosion protection, durability, and toughness. Pigments are used to provide color and gloss.

POST RINSES: During the electrocoat process, paint is applied to a part to be coated at a controlled film thickness regulated by the voltage applied. Once the coating reaches the desired film thickness, the part gets insulated and the coating process slows down. As the part remains in the bath for few seconds after the coating before it is lifted, paint liquid clings to the surface and have to be rinsed off to maintain efficiency and aesthetics. The excess paint is called 'drag out' or 'cream coat'. These post rinses drag outs are returned to the bath tank to allow transfer efficiency greater than 95 percent.

BAKE OVEN: After rinsing, the coated part enters the bake oven. The bake oven crosslinks and cures the paint film to assure the maximum performance properties. Baking time and temperature would differ according to the weight of the coated part, on the one hand, and the desired gloss, on the other.

The Electrocoat System consists of a number of components that help maintain the line parameters. The critical components are:

RECTIFIER: The rectifier supplies DC electrical charge to the bath enabling the coating to take place. Unlike the low voltage rectifies used for electroplating, this is a more sophisticated version which delivers higher voltages and at a constant value.

CIRCULATION: Circulation pumps maintain proper paint mix uniformity throughout the electrocoat bath. Without constant circulation the paint solids settle down and begin to coalesce. Any attempt to try and dissolve them again leads to clogging of the pre filter. This has to be avoided at all times.

HEAT EXCHANGER/ CHILLER: A heat exchanger and a chiller provide Temperature control of the paint bath. It is essential that the temperature of the paint bath is maintained around 32 degrees Celsius in order to prevent the change of characteristics of the paint.

PRE FILTER: They remove dust particles that are introduced into the paint bath. Pre filters are of various design and media. The housings come in PVC, PP, and Stainless steel. The filter media is available in Nylon or Polypropylene. The media is normally disposable. The selection of the housing and the media is done on the basis of the design requirements. This plays a very important role in the final quality of finish. This removes all physical contaminants on a continuous basis. The cured film obtained is thus free from dust. It also plays an important role in protecting the ultrafilter from any damage due to ingress of particulate matter under high pressure. Dust particles settling on the ultrafilter membrane can cause fouling and reduce the permeate flux.

ULTRAFILTER: Ultrafilter is primarily used to produce permeate for rinsing and recovery of paint solids. Unlike dead end pre filters this is a cross flow filter of molecular size. The normal cut off value is 13000 Dalton. There are a number of different constructions of UF. However, in epoxy based ED systems the spiral is the most popular. The paint flows through the ultrafilter and water is removed from the paint. The concentrated paint goes back to the main bath. The clear permeate so obtained is sent to the ultrafiltrate rinse stage 2. The secondary use of the ultrafilter is to help maintain the conductivity of the main bath as and when required.

ANOLYTE CIRCUIT: Anolyte circuit is necessary to remove from the paint bath the acetic acid that is released in the deposition process

The coating process is an electro chemical reaction. At the cathode hydroxyl group is generated while at the anode the acidic hydrogen ion is released. This acid group if allowed to remain in the bath will cause an increase in the acidity of the bath and increase the conductivity. Both these factors will cause instability of the bath and coating defects in the finished product. For this reason the stainless steel anode is always separated from bath liquid. The anode cell serves this precise function.

The manually operated Control Panel provides the following main controls and monitoring devices:

• On/Off switch button for pumps
• Display for monitoring the temperature of the paint bath
• On/Off switch button for regulating the coating cycle
• Setting of Voltage
• Setting of coating time

In an automatic (PLC-based) control panel, all the above controls / monitoring is automatically programmed. In addition, it also has the facilities of:

• Alarm annunciation
• Monitoring of the paint bath parameters-pH and Conductivity
• Monitoring of pump pressure
• Monitoring of levels of liquid in all tanks
• Monitoring of flow in ultrafilter
• Control system for the transporter, if installed for material handling

In the table given below, an effort has been made to compare the various technologies of paint transfer with Electrocoating:

Electrocoat is utilized in a variety of industrial market segments. Each of these markets has specific performance requirements, leading to the development of a number of electrocoat technologies to meet their needs. Electrocoat has also become an accepted finishing method for new applications such as:

• Specialty clear finishes over Aluminum, Brass and Zinc plate.
• Extremely low gloss coating for military and photographic applications.
• Chemical resistant coating.
• Transparent metallic-type finishes over Nickel or Zinc plating.

The table given below provides more details about the application of Electrocoat in the industrial marketplace:

As has already been stated earlier, Electrocoat technology falls into two main categories:

• EPOXIES
• ACRYLICS

Both technologies are used extensively in anodic and cathodic systems technologies offer the following properties and end uses:

Electrocoat Technology has had significant advances since its first commercial installation in the year 1961. Equipment sophistication, new markets and products, operational efficiencies and government regulations on pollution control will continue to support the growth of Electrocoating industry.

Electrocoat Research has led to commercialization of one-coat-high-film-buildup products in the range of as high as 75 microns, two coat applications and clear coat.

Two-coat Electrocoat offers the ability to build high film thickness of coating, leading to outstanding corrosion protection and exterior durability. Performance has been observed to exceed 2000 hours salt spray. The primer coat is conductive epoxy electro deposition paint. After this has been cured it can be coated with a second coat of electro deposited paint. The second coat is usually of acrylic. Together they give the best of both the worlds – corrosion protection of epoxy and UV resistance of acrylic.

Clear Coat has been developed to allow an entrance into markets previously dominated by electroplating. Markets including hardware and bathroom fixtures have also been opened up for electrocoating. Gold, Silver, Brass, Copper, Zinc, Aluminum are coated with clear and dyed electro deposited lacquer. This protects the metal surface from tarnishing while giving excellent mechanical properties while protecting from solvent and chemical attacks.

Bulk or barrel electrocoat is also an emerging technology. This is the preferred system for paint application for small parts in high volumes like fasteners, hose clamps etc. The rotating barrel system is adapted with modifications to suit the nature of the coating.

These test results are supplied by the chemical supplier and have not been verified by us.

We strongly suggest that these be verified on actual components, coated by us, before committing these values to any other party.