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Electrocoat, also known as E-coat, is a widely used coating technology that has provided superior levels of performance on a vast array of industrial metal objects for over 40 years. E-coat technology has evolved dramatically since the early 1960s, when it was first commercialized as an automobile body primer. Today, over 98% of all car bodies produced globally utilize an e-coat primer. New end uses for e-coat continue to be found annually; examples include automotive parts and accessories, appliances, heat exchangers, decorative plated objects, and heat-sensitive components.
Current commercially available e-coat systems offer superior performance using processes and materials that are both environmentally friendly and economically efficient. Major advantages of the e-coat process include:
• Total coverage of complex parts with unsurpassed film uniformity
• Material transfer efficiencies routinely in 95-99% range
• Highly automated, closed-loop system with excellent productivity and low operating costs
• Fast line speeds and high part racking densities
• Very low air and wastewater emissions that foster environmental compliance
• Totally enclosed system leading to a cleaner and sager paint application methods.
THE E-COAT PROCESS
CLEANING AND PRETREATING
The cleaning and pretreating of metal prior to the e-coat bath are critical steps in the process of providing a high-performance paint finish. Alkaline cleaners are employed frequently to remove dirt and oils found on industrial metal parts in manufacturing. Aluminum objects generally receive a conversion coating prior to e-coat and can be e-coated simultaneously with ferrous parts.
Phosphate (iron or zinc) pretreatments are used to provide adhesion between the e-coat and the substrate and to enhance corrosion protection. A final deionized water rinse is applied to the parts prior to the e-coat tank. Dry-off ovens are generally not required.
The e-coat bath consists of 80-90% deionized water and 10-20% paint solids. The deionized water acts as the carrier for paint solids, which consists of resins, pigment, and small amounts of solvents. The resin is the backbone of the final paint film and provides properties such as corrosion protection and ultraviolet durability. Pigments provide color, gloss, and corrosion protection as well. Solvents help ensure smooth film appearance and application.
During the e-coat process (see Fig. 2), paint is applied to a part at a certain film thickness, which is regulated by the amount of voltage applied. The deposition is self-limiting and slows down as the part becomes electrically insulated by the applied coating. E-coat solids deposit initially in the areas closest to the counterelectrode and, as these areas become insulated to current, solids are forced into more recessed, bare metal areas to provide complete coverage. This phenomenon is known as throw power and is a critical aspect of the e-coat process and materials. E-coat bath solids are deposited electrically via a system that includes a number of components: the rectifier, which supplies a DC charge to the bath enabling deposition of ionic species; circulation pumps to maintain proper paint bath uniformity; a heat exchanger and chiller to control the temperature of the bath; filters, which remove dirt particles introduced into the system; and ultrafilters that produce permeate for rinsing and allow for recovery of excess paint solids.
As the part exits the bath, excess paint solids not deposited electrically cling to the part and must be rinsed off to maintain process efficiency and optimal aesthetics. Rinse material is supplied from the ultrafilters and is called permeate. The permeate, containing low molecular weight organics and some solvent, is used to rinse the drag-out from the parts; the excess solids and permeate are returned to the bath in a counterflow fashion, affording superior levels of transfer efficiency.
After exiting the postrinses, the coated parts enter the bake oven for curing and crosslinking of the paint film, resulting in a high quality finish void of runs, drips, and sags (see Fig. 3). Bake temperatures range form 180 to 375ºF depending on the type of e-coat applied.
Cathodic deposition, where positively charged paint particles are attracted to a negatively charged part, involves much less iron incorporation into the depositing film and consequently offers substantially improved corrosion resistance. Additionally, the polymer species are amine functional and acid solubilized, with the alkaline nature of the polymer leading to better inherent corrosion resistance than can be obtained with acid functional species.
Whenever high coating performance is required, cathodic e-coat systems are generally specified. Market penetration of these coatings into the appliance and automotive industries over the last 30 years bears evidence to the attractiveness of these coatings.
CURRENT TECHNOLOGY CAPABILITIES
E-coat research and development has fostered many exciting advancements over the past 40 years. Some of the significant advancements enjoyed today by end users include closed-loop systems with close to 100% material efficiency and little wastewater discharge; near zero VOC e-coats; HAPs-free (hazardous-air-pollutants) anodic and cathodic products; lead-free cathodic e-coat with corrosion protection equal to prior lead containing products; cathodic acrylic coatings with corrosion and ultraviolet protection as a single coat finish; anodic e-coats with cure capabilities below 200ºF; two-coat e-coat systems for ultimate primer plus topcoat performance; exceptional coverage of sharp metal edges; decorative clear e-coats that can be water-white or tinted to simulate plating; and photoimageable e-coats.
As with any coating process, e-coat has inherent limitations and is not suited for all applications. Low production levels of multiple colors favor other coating application methods over e-coat, which requires a separate tank and postrinse system for each color. High production levels, however, can economically favor the use of multiple e-coat tanks to handle different colors.
Initial capital outlays for an e-coat system are often higher than for other types of coating methods, such as dip or liquid spray. Justification of the capital to install an e-coat system has become easier with advances in more efficient equipment design and closed-loop operation.
Because e-coat is a total coverage process, applications where coating is not desired on all areas of a part can be problematic. Masking of areas to be left uncoated can be costly and time consuming in production.
Estimated Salt Spray Results = 1000+ hours to red rust