Electrocoating or E-Coat
Publication Date: July 1999
Links Last Checked: July 2004
Electrocoating is a method of organic finishing which uses electrical current to deposit the paint. The process works on the principal of "Opposites Attract".
The fundamental physical principle of electrocoating is that materials with opposite electrical charges attract each other. 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, so attraction stops and electrocoating is complete. Depending on the polarity of the charge, electrocoating is classified as either anodic or cathodic.
In anodic electrocoating, the part to be coated is the anode with a positive electrical charge, which attracts negatively charged paint particles in the paint bath. During the anodic process, small amounts of metal ions migrate into the paint film, which limits the performance properties of these systems. Their main use is for products in interior or mild exterior environments. Anodic coatings are economical systems and offer excellent color and gloss control.
In cathodic electrocoating, the product is given a negative charge, attracting the positively charged paint particles. The negative electrical charge of the metal part attracts positively charged paint particles. Reversing the polarities used in the anodic process greatly reduces the amount of iron entering the cured paint film and enhances the properties of cathodic products. Cathodic coatings are high-performance coatings with excellent corrosion resistance and can be formulated for exterior durability.
Below are advantages and limitations of using the electrocoating process. This report also provides a Quick Reference table that helps an applicator identify the performance achievements of this coating process and a case study of how this process saved money, achieved environmental compliance and improved product quality.
|Case Study - Onan Corporation
The Onan Corporation, manufacturer of automotive parts, faced environmental regulatory requirements to lower air and waste emissions from their finishing operations. The company carefully reviewed all coating alternatives to conventional spray finishing and selected a two-coat electrocoating (E-coat) process.
E-coat was chosen as the best option for three basic reasons:
Onan’s original finishing line included an aging pretreatment system, conveyor and curing oven, which needed replacement. The company had invested in spray booth improvements and robotic spray equipment a few years earlier, however, which made the decision to drop spray finishing a little more difficult.
Installing a new E-coat line was also a more expensive proposition than other options, especially with the 11-stage pretreatment system Onan felt was needed to ensure excellent finish quality.
"At that time, the capital investment in E-coat was a little bit higher than alternative coating systems, but we felt the additional cost was justified, looking at the long-term payback," Knudtson says.
Indeed, in the last four years, Onan’s E-coat system has nearly paid for itself due to savings in several areas:
From an environmental standpoint, Onan had been spraying medium- to high-solids solvent-borne coatings with about 4.0 pounds of Volatile Organic Compounds (VOC) per gallon. E-coat, which contains small amounts of co-solvent, still emits some VOCs and HAPs into the atmosphere, but well within regulatory limits. "HAPs-free materials are becoming more and more popular. That’s a consideration for us in the future," Knudtson says.
Onan was one of the first manufacturers in the world to use a two-coat E-coat process. A special epoxy primer is applied first. When it is cured, it becomes conductive, allowing a second E-coat application of a green acrylic topcoat. The epoxy offers excellent corrosion resistance, and the acrylic excellent weatherability. The quality of finish gives Onan products a clear advantage over the competition’s, Knudtson says.
Several factors—the pressure to reduce VOCs, the aging conveyor and pretreatment system, the size and shape of the parts and the need to apply only one color—all pointed to E-coat as the logical choice for Onan. For other companies, that may not be the case. Knudtson recommends that a firm first undertake a careful, introspective analysis by representatives from all departments before converting to a new finishing process.
"Every situation is different. There are many answers to the same question. E-coat is not necessarily the right answer for everyone," he says.
- National Defense Center for Environmental Excellence - Technology Report www.ndcee.ctc.com/b3_eq3.html
- E-Coat Process Description, End Uses and Publications www.ppg.com/_private/FrameResult.asp?f=/car_indcoat/e_articles.htm
- E-Coat Association www.electrocoat.org
- Electrocoat - The Next Step www.pfonline.com/articles/089702.html
- Ready for A Challenge www.pfonline.com/articles/059601.html
- Metal Worker Chooses Electrocoating for First Finishing System www.pfonline.com/articles/089706.html
- Trends and Developments in Electrocoat www.pfonline.com/articles/029601.html
- An End User Perspective www.pfonline.com/articles/049603.html