Autodeposition of Organic Coatings

Publication Date: July 1999
Links Last Checked: July 2004

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

Autodeposition is a unique, waterborne organic coating process which uses chemical reactions to achieve deposition of a high quality finish on an assortment of substrates. The autodeposition process is simple in comparison to other processes used for applying corrosion-resistant coatings. Following conventional cleaning and rinsing stages, metal parts are immersed in a coating chemical bath where the deposition of pigment and resin particles takes place. The growth in thickness of the film is time dependent. There is an initially rapid deposition rate followed by a decrease in growth limited by the diffusion of ions on the metal surface and then "self-sealing" of the coating. A protective barrier film is formed wherever the coating bath makes contact with the metal workpiece.

The autodeposition coating process reduces the complexity and capital required by competing technologies. It requires no electric current, because self-limiting organic coatings are developed through a chemical reaction with the metal surface. Because the process coats anywhere it wets, it generates exceptionally uniform protective coatings even on difficult to coat parts. These parts are coated at a fraction of the conventional energy costs of competing technologies.

Autodeposited coatings can be uniquely applied on the same production line for parts with many different end-use market applications. No conversion coating is required; the only pretreatment necessary is cleaning. With no Volatile Organic Compounds (VOCs), no toxic heavy metals, and low-temperature curing, air emision are minimized and the work environment is improved.

Below are advantages and limitations of using the autodeposition 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 autodeposition process is environmentally friendly due to the water-based paint system it uses and the elimination and/or reduction in heavy metals.
  • Does Not Embrittle Steel Parts: The autodeposition process does not evolve hydrogen at the work surface so there is no danger of embrittling high carbon steels.
  • Reduced Frequency of Rack Stripping: After the first pass through the coating tank, the racks show no measurable build-up of coating thickness, and most commercial autodeposition operations clean the racks at a rate of about once every three years. This results in savings of energy, chemicals, labor, and waste disposal costs usually involved in stripping racks.
  • No Electrical Dependence: Immersion process uses a chemical reaction so coating does not rely on electrical contact and part conductivity.
  • Low Temperature Cure: Low temperature cure does not affect rubber bonded components.
  •   Limitations
    • Setup: Requires large dip tank for processing.
    • Curing: Requires oven or other heat source to cure the coating efficiently.
    • Colors: Limited color selection and low gloss.

     

    Performance — Quick Reference Table
    Criteria Performance Achievements
    Adhesion 100% tape adhesion
    Corrosion Resistance 600+ hours in a neutral salt spray bath
    Gloss Low
    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)
    Henkel Surface Technologies

     

    Case Study - Steelcase Replaces Zinc Plating Lines

    "We have a system now that is less labor-and-energy-intensive, that has, in one line, more than doubled our previous capacity from four lines, and that has almost entirely eliminated waste treatment costs. Most importantly, we have a process that gives us the ability to satisfy customer requirements while being sensitive to the environment."
         - David Rinard, Senior Environmental Engineer

    Steelcase Inc. replaced four zinc plating lines with a new Autophoretic/Autodeposition line at its Desk Division in Grand Rapids, Michigan.

    Steelcase has a corporate policy that encourages environmental initiatives. According to their plant manager, Steelcase is "committed to getting any problematic chemicals out of the workplace and out of the environment. No carcinogens, no toxins, lower VOC's."

    The autodeposition process utilizes a chemical reaction with the steel part to achieve deposition. The immersion bath consists primarily of a mildly acidic latex emulsion polymer in combination with deionized water and certain patented ingredients. The coating thickness is controlled by time and temperature, and ultimately is self-limiting, based on the release of iron ions from the steel surface.

    Steelcase evaluated several coating alternatives to their zinc plating process lines. Among the coating systems evaluated were: new plating lines, high solids, electrocoating, epoxy coating, and simply outsourcing the work. The manufacturing department had known about autodeposition since 1985 and knew "it had too many benefits to pass up."

    The various coating processes were tested for hardness, abrasion resistance and corrosion resistance, as well as finish consistency. Autophoretic coatings passed their standard of 144 hours salt spray exposure and had an excellent, uniform quality finish.

    Steelcase also did some additional testing outside of the performance parameters. They evaluated environmental impacts and associated waste treatment costs, as well as customer acceptance. Customer reaction to the uniform finish was very good, especially if it meant less of an impact to the environment.

    Payback in Two Years
    The autodeposition processing line accommodates parts that have dimensions of three feet by three feet by five feet deep, and is designed to run at 20 feet per minute. This gives the company the capability of processing over 12,000 square feet per hour. Capital costs for the autodeposition line was equivalent to that of new zinc plating lines, but the costs associated with waste treatment were substantially lower. According to David Rinard, Senior Environmental Engineer, "Rinse water can be discharged directly to the system without pretreatment," and "the only treatment required is the batch discharge of the coating stage." The use of an autodeposition processing line reduced total effluent by approximately 99%, generated no regulated wastes and eliminated VOCs. Although direct costs are being recorded and are showing a two-year payback period, indirect savings are being realized. These savings are from energy efficiency, consistent quality and not having to outsource any work.

    Processing Sequence
    The autodeposition processing sequence takes a total of one hour and forty minutes from racking through unracking. Including rinses and low-temperature curing, it involves the following stages:

    1. Spray cleaning with an alkaline cleaner, at 140 degrees F for 60 seconds
    2. Immersion cleaning with another cleaner at 140 degrees F for 120 seconds
    3. Immersion rinsing at ambient temperature using city water for 30 seconds
    4. Spray rinsing at ambient temperature, also with city water, for 30 seconds
    5. Deionized immersion rinsing for 30 seconds (Steelcase produces the water via reverse osmosis)
    6. Immersion coating, 70 degrees F for 90 seconds, developing a coating thickness of 0.6 to 0.8 mil.
    7. Immersion rinsing with city water for 60 seconds at ambient temperatures
    8. Immersion reaction rinsing at 70 degrees F for 60 seconds
    9. Deionized water misting during the transition to the low-temperature cure oven
    10. Curing for 28 minutes at 230 degrees F

     

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