Electrocoating or E-Coat

Publication Date: July 1999
Links Last Checked: July 2004

Introduction
Advantages/Limitations
Performance - Quick Reference Table
Case Study
More Resources

Introduction

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.

Anodic
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.

Cathodic
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.

Advantages
  • Environmental Friendly: The electocoating process is environmentally friendly due to the water-based paint system it uses and the reduction in heavy metals.
  • Corrosion Protection: In e-coat, the deposited paint film becomes part of the metal object until the entire workpiece is painted. Even intricate and hard to reach areas are painted.
  • Intricate Parts / Recessed Areas: Because electrodeposition continues until the entire workpiece is painted, even intricate and hard to reach areas are painted.
  • Reduces Paint Waste: Transfer efficiencies of better than 95% result in reduced paint waste, especially when compared with spray applied coatings.
  • Water-based: E-coat paint is 80% water with as little as 1-3% volatile organic solvent. It is environmentally friendly, not a fire hazard, and has low viscosity.
  • Fewer Defects: E-coat gives a better quality product: the uncured paint is dry enough to allow limited handling; during baking, the e-coat will not sag nor will the vapors cause the paint to wash off from recessed areas.
  • Predictable Costs: Paint costs are predictable because the film thickness does not vary from job to job. Costs in other painting processes (powder coat, wet spray) can vary according to operator technique or attention.
  • Better Finish: An even film build is possible over the entire surface of the part. In addition, the deposited film is reproducible from part to part and day to day.
  • Reduced Labor Costs: Even the largest e-coat system has only one person tending the e-coat equipment.
  •   Limitations
    • Substrates: Electrodeposition can only take place on electrically conductive substrates.
    • Color: Application of a variety of color coats requires separate dip tanks.
    • Capital Costs: Capital investment in an entire e-coat system can be substantial.

     

    Performance — Quick Reference Table
    Criteria Performance Achievements
    Adhesion 4B-5B (cross hatch adhesion)
    Corrosion Resistance 1000+ hours in a neutral salt spray bath
    Gloss (60 deg.) 65-85
    Film Thickness .6 - 1 mils
    Coverage 99.9% transfer efficiencies
    Heat Resistance Not measured
    Dry-Time Air dry: 3-5 hours to touch
    Oven: 20 - 40 minutes at 200 - 230 degrees F
    Zero T-Bend No loss
    Pencil Hardness 4H - 6H
    Impact Resistance 160 Inch-Pounds with no loss
    Chip Resistance >7+ (GM-9508-P)
    Surface Preparation Requirements Excellent cleaning and phosphate coating
    Thinning Requirements None
    Vendor(s)

    BASF Corp.
    DuPont, E.I., Inc.
    Eisenmann Corp.
    KMI Systems, Inc.
    PPG Industries
    Valspar Corp.
    Therma-Tron-X, Inc.

     

    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:

    1. Environmental benefits
    2. Cost
    3. Finish durability

    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:

    1. Labor costs have declined an estimated 30%. The E-coat line requires only one technician, whose job is to monitor the pretreatment, E-coat, DI water and wastewater treatment systems. Onan’s old system required personnel to program and operate the robotic system, as well as to perform manual spray finishing tasks.
    2. Disposal costs have declined significantly because Onan no longer has to collect, transport and dispose of large amounts of solid waste, primarily paint overspray. In addition, wastewater discharges, mostly from the old waterwash spray booths and pretreatment system, were cut by 83%. In total, Onan estimates savings in waste disposal costs of about $100,000 annually.
    3. Onan had been outsourcing some parts for E-coating, which it now does in-house, eliminating another expense. "In fact, our business has grown. If we had not converted when we did, there would have been a lot more out-sourcing of product to be painted. So we have avoided some additional costs," Knudtson notes.

    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.

     

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