Electrocoat is a technology used worldwide to achieve high-quality, low-cost finishes at a level of efficiency and environmental compliance no other finishing method approaches. The performance advantages, along with reduced finishing costs, have made it a growing technology that will continue long into the future as new uses and technologies are developed.

Electrocoating is a process in which electrically charged particles are deposited out of a water suspension to coat a conductive part. During the electrocoat process, paint is applied to a part to produce a film thickness, that is regulated by the amount of voltage applied. The deposition is self-limiting and slows down as the applied coating electrically insulates the part. Electrocoat solids deposit initially in the areas closest to the counter electrode and, as these areas become insulated to current, solids are deposited in more recessed bare metal areas to provide complete coverage. This phenomenon is known as trowing power and is a critical aspect of the electrocoat process.

The Electrocoat Process
The electrocoat process can be divided into four distinct sections:

• Pretreatment
• Electrocoat bath and ancillary equipment
• Post rinses
• Bake oven.

Figure 1. Typical electrocoat system.

An overview of an electrocoat system is shown in Figure 1. Parts are first cleaned and pretreated to produce a phosphate conversion coating. Parts are then immersed into a paint bath where direct current is applied between the parts and a “counter” electrode. Paint is attracted to the part by the electric field and is deposited on the part. Parts are removed from the bath, rinsed to reclaim undeposited paint solids, and then baked to cure the paint.

Pretreatment
Prior to paint film application, most metal surfaces receive a surface pretreatment that usually includes application of a conversion coating.

The typical pretreatment process consists of the following steps:

1. Cleaning (one or more stages)
2. Rinsing
3. Conditioning
4. Conversion coating
5. Rinsing
6. Post-treatment
7. Deionized water rinsing

Figure 2. Typical organic coating composition.

Phosphating processes can be separated into two types: iron phosphate and zinc phosphate. Iron phosphating has been the process of choice for applications where overall cost considerations override all maximum performance needs. Since iron phosphates are thinner coatings than zinc phosphates and only contain the metal ion of the substrate being processed, they provide reduced corrosion resistance compared with a zinc phosphate. However, with environmental restrictions becoming increasingly tight with respect to heavy metals, an iron phosphate coating coupled with thorough post treatment may offer a viable alternative while still meeting required corrosion-resistance specifications. Zinc phosphates have become the preferred prepaint treatment in the products finishing industry, especially with the use of electrocoat paint systems. The reason is that they provide better corrosion resistance and paint adhesion than iron phosphates, even under more demanding conditions.

Electrocoat Paint
All organic coatings, including electrocoatings, are made from 1) polymeric resin or binder, 2) pigments, and 3) solvents and diluents (Figure 2). The resin (typically 10-20%) is the backbone of the final paint film and provides properties such as corrosion resistance and ultraviolet durability. Pigments provide color, gloss, and corrosion resistance. Deionized water is the major component of an electrocoat bath, making up 80-90% of the electrocoat bath. The deionized water acts as the carrier for paint solids, which include resins, pigments, and small amounts of solvent. Solvents also help to ensure smooth film appearance and application.

Anodic vs. Cathodic Electrocoat
Electrocoat systems are referred to as either anodic or cathodic, indicative of where coating deposition takes place. The process works on the principle of “opposites attract.” Figure 3 is a comparison of anodic and cathodic electrocoat processes.

Figure 3. Anodic vs. cathodic electrocoating.

Anodic electrocoating involves the use of negatively charged paint particles that are deposited onto positively charged metal substrates. During the anodic process, small amounts of metal ions migrate into the paint film. These ions become trapped in the depositing paint film, and, due to their ability to interact with moisture, limit the corrosion performance of the films. The main use of anodic electrocoat is for products used in interior or moderate exterior environments. Anodic coatings are economical systems and offer excellent color and gloss control.

In cathodic deposition, positively charged paint particles are attracted to a negatively charged part. Much less iron is incorporated into the depositing film and consequently corrosion resistance is better. Cathodic coatings are high-performance coatings with excellent corrosion resistance and they are often formulated for exterior durability.

Electrocoat Resin Types
Acrylics, epoxies and hybrid formulations are used to match the desired quality, performance, cost, and environmental objectives that electrocoat paint manufacturers demand. Table I illustrates the properties of the four categories of electrocoats. Epoxy polymers are known for their corrosion and chemical resistance. Acrylic polymers are known for their ultraviolet durability and color control. Hybrids are a combination of epoxy and acrylic polymers that give a combination of properties.

Anodic epoxy electrocoatings are the most corrosion-resistant anodic electrocoat that can be cured at less than 200ºF. The low-cure temperature attributes of anodic epoxies make these electrocoat products an excellent finish for castings, engines, and temperature-sensitive substrates or assemblies.

Anodic acrylic electrocoatings offer a single-coat application used for both interior and exterior environments and are available in numerous colors and glosses. In applications requiring a cost-effective way of applying a decorative or functional coating with good color control, anodic acrylic electrocoatings usually offer the best value. These products are used as a one-coat finish on toolboxes, air diffusers, hangers, and other indoor or mild exterior environments.

Cathodic epoxy electrocoatings are the benchmark for corrosion resistance. Widely used in the automotive and automotive parts industries, they provide superior salt spray, humidity, and cyclic corrosion resistance. However, the cathodic epoxy technologies generally require a topcoat to be protected from sunlight. Aromatic epoxy-type coatings are particularly prone to chalking and degradation by the UV components of sunlight.

Cathodic acrylic electrocoatings are available in a wide range of glosses and colors to maximize exterior durability, gloss retention, color retention and corrosion resistance. These products are used as a one-coat finish in the agricultural, lawn and garden, appliance, and air-conditioning industries. Cathodic acrylic electrocoatings are typically used in applications where both UV durability and corrosion resistance on ferrous substrates (steel) are desired. They are also used in applications where light colors are desired. Some major markets that use electrocoating are listed in Table II.

Electrocoat offers both one- and two-coat applications for optimal performance. More and more manufacturers are using electrocoat to apply primers, and many are even using it for the topcoat. Some manufacturers apply an electrocoat primer augmented with a liquid, powder, or electrocoat topcoat.

Benefits of Electrocoat
Electrocoat offers finishers cost efficiencies, line productivity and environmental advantages. The cost efficiencies include higher transfer efficiency, precise film-build control and low manpower requirements. Increased line productivity in electrocoat results from faster line speeds, dense racking of parts, non-uniform line loading, and reduced human fatigue or error. The environmental advantages are no or low VOC and HAP emissions, heavy-metal-free products, reduced exposure of workers to hazardous materials, reduced fire hazards, and minimum waste discharge.

Testing Electrocoat Paint Finishes
An important aspect of the electrocoating process is the final performance of the applied finish. A variety of tests measure the attributes of a paint films for different end-use markets. There are three major appearance characteristics: finish uniformity, color and gloss. A correctly applied electrocoat finish should have a smooth and continuous appearance. Electrocoat materials come in various colors and are color controllable. Gloss is a measure of the reflectivity of the coating surface.

There are many cured paint tests: pencil hardness, flexibility, adhesion and resistance to abrasion, humidity, salt spray, stains and detergents. Pencil-hardness tests determine the actual hardness of the coating. Impact testing determines the ability of the coating to resist strikes. Flexibility is the ability of the electrocoat paint film to bend and stretch somewhat to maintain adhesion under hot and cold conditions or during manipulation of the product. Adhesion testing measures the “sticking power” of the electrocoat paint—to the phosphate coating that, in turn, adheres to the metal surface. Abrasion testing measures the surface-wear characteristics of the paint film. Humidity tests determine the coatings reaction to water and condensation. Salt spray is a harsh test for evaluating corrosion resistance. The automotive industry performs the “cyclic corrosion” variation of this test, in which the panels are cycled through various testing conditions, such as salt fog, heating, freezing, and humidity. It is believed that cyclic corrosion testing better simulates “real world” exposure for severe-use applications. Stain resistance tests use foods, chemicals, and other substances to determine if stains will be left on the paint surface. Detergent immersion tests suspend parts in a detergent bath for a specified time period; they are evaluated with the tape pull method for creepage and lifting.

Economics of Electrocoat
Electrocoat is often the lowest cost finishing application. The key word is “application.” Anytime one evaluates the cost to paint parts, he or she needs to look beyond just the material cost per square foot or the cost per gallon. Some of the major factors that enter into the equation for the application selection are:

• Part Complexity: Parts come in all shapes and sizes. Electrocoat excels over other technologies in this category because all surfaces receive a consistent film thickness due to the electrical insulating effect of the electrocoat as it deposits onto the part.
• Production Volume: As manufacturers increase production quantities above 2 million square feet annually per shift, electrocoat becomes a more preferred application method. The dense rack loading achieved with electrocoat allows manufacturers to produce greater volumes of parts.
• Energy: All coating operations require energy. Liquid and powder lines require daily rack cleaning and an additional dry-off oven after pretreatment. Powder lines normally require an environmental room to maintain air quality for consistent paint application to the part. Electrocoat has its share of energy requirements as well requiring recirculating pumps, a chiller, and a rectifier. The energy differences are not normally great when all factors are considered.
• Repair and maintenance: There is normally more mechanical equipment associated with electrocoat; however, the labor requirements to maintain a liquid or powder system are usually greater.
• Paint material: Electrocoat is usually the most cost-effective when comparing the costs of applied paint material. This is due to the high transfer efficiency (95-98%) and the self-limiting ability of the electrocoating process, which results in uniform film builds..
• Capital: In most cases, electrocoat lines require more intial capital expenditure. But after other cost variables (film thickness, transfer efficiency, and labor requirements) are taken into consideration, the electrocoat process produces the cheapest coating on an applied cost per square foot basis.
• Miscellaneous items: There will be additional overhead expenses such as insurance, building lease or mortgage, material handlers, shipping, etc. that the manufacturer needs to consider when determining the “final applied cost” for parts. The choice of electrocoat over other coating technologies is usually driven by the lower total applied cost of electrocoat compared to technologies like liquid spray and powder coatings. The low total applied cost is achieved in part through more consistent deposition of paint film on parts (particularly complex geometries), high transfer efficiencies, and the lower costs associated with labor.

Considerations
Different coaters have different reasons for adding electrocoat. Contract shops generate new revenue streams with electrocoating by better meeting the needs of their old customers while expanding their client base and becoming a single stop for a wider variety of coating options. OEMs are turning to in-house electrocoating to improve their products—and many companies who outsource their coating operations are beginning to specify electrocoating as well, making it an essential element in the highest-quality finishes available today. Manufacturers are making products that require a range of coatings, and they will look for coaters that can supply all of their needs. The days of shipping parts from shop to shop for multiple coatings are ending primarily because transportation costs are becoming prohibitive. Coaters that can offer multiple coating options for their customers will have the best chance to maintain or gain business.

Waste Treatment
Electrocoating is an extremely efficient process in which 95% or more of the paint components (resin, pigment, and other additives) entering the paint tank will eventually be applied and cured on the product due to the recycling of paint through ultrafiltration. Except in rare cases of catastrophic tank contamination, the amount of paint solids requiring waste treatment is typically very small. Generally, discharge standards for wastewater become more stringent as they become more localized. Federal standards can be found in the Code of Federal Register (CFR), Part 433—“Metal Finishing Paint Source Category.” This resource lists all registered substances that must be controlled at a national level.

Local standards can vary greatly from municipality to municipality. In any case, they may be more demanding than the federal limits. Information on allowable contaminant levels may be obtained by contacting the local waste treatment facility. The permitting process can be quite complicated. It is suggested that a local environmental consultant be included in the permitting process to ensure that it is carried out completely and correctly.

Air Abatement
Electrocoat paint formulas have greatly reduced volatile organic compound (VOC) and hazardous air pollutant (HAP) content. Although solvent loading is greatly reduced, a source of VOC emissions will still exist as a result of the curing process. Ovens must be continuously exhausted to maintain the oven atmosphere below the lower explosion limit (LEL) and to prevent smoke buildup in the plant. The resulting exhaust contains the released VOCs, smoke, and odor form the cured paint, which may require treatment prior to release into the atmosphere. The selection of the proper abatement system is critical to the safe operation of the manufacturing facility, meeting air discharge standards, and minimizing odors for the surrounding area.

In any case, electrocoat paints have much lower VOC levels than solvent-based spray. And, just as some minute levels of contaminants may be legally discharged into sewer facilities, companies are allowed to exhaust some amount of VOCs into the atmosphere. This discharge is based on pounds released per year, and is set by federal and state Environmental Protection Agencies (EPA). In addition, localities may establish further limits on emissions. Generally, requirements tend to be more stringent as the controlling body becomes more localized.

The Future
The future of electrocoating is wide open. Advances in electrocoat chemistries are making applications extremely user-friendly and cost-efficient. Equipment sophistication, new markets and products, operational efficiencies, and governmental regulations will continue to support the growing electrocoating industry.

Low-cure technology for more temperature-sensitive substrates, bulk coating formulations for small parts such as fasteners, and clear electrocoat technology for applications such as aluminum extrusion, automotive trim, hardware, and jewelry are revolutionizing the e-coat industry. Applicators are continuing to find more uses for this technology as a primer, topcoat, and in conjunction with other finishing methods. PFD