HAPS- and Lead-Free Electrocoat for Automotive Applications

Article From: Products Finishing, , from Chrysler Corporation

Posted on: 4/1/1996

Cathodic electrocoat use has improved since its introduction in the 1970's because of custom-coater demands...

Cathodic epoxy electrocoat is the primer of choice for numerous automotive applications. Electrocoat is used on wheels, truck chassis, space frames, cross members, seat tracks, and other components. Auto bodies and service parts are primed almost exclusively with cathodic epoxy electrocoat. A properly formulated and applied electrocoat provides excellent corrosion and fluid resistance, appearance and topcoat adhesion.

Cathodic electrocoat was introduced to the component market in the mid-1970's. The first generation became the industry standard, and electrocoat use grew and dominated the custom coating market. No other finish could compete with electrocoat's properties:

  • Formation of protective films in recessed areas, providing corrosion protection in areas inaccessible by other coatings.
  • Elimination of fire hazards, since water is virtually the only carrier.
  • Pollution reduction, since water is the carrier, VOCs are driven to near zero. Waste is minimal and hazardous components have been virtually removed.
  • Transfer efficiencies of near 100 pct are an approximate threefold improvement over unreclaimed spray applications.
  • The easily automated process dramatically reduces labor costs and liabilities as well as providing a high level of consistency.
  • The consistent finish will not run or sag during cure.
  • Film build can be readily controlled.

Cathodic electrocoat was introduced to OEMs in the late 1970's, and the needs of OEM coaters drove electrocoat development. The third generation of electrocoat in the early 1980's offered improved topcoat adhesion, appearance and leveling and higher film-builds. This enabled auto makers the option of topcoating directly onto the electrocoat and eliminating spray primer operations.

The fourth generation of electrocoat, introduced in the mid-1980's provided reduced film shrinkage, reduced oven fouling and lower VOCs. The fifth generation in the early 1990's reduced VOCs further, while providing edge coverage and higher throwing power. A sixth generation was introduced in 1994, but the automotive parts industry proceeded directly from the fifth to the seventh generation in 1995.

The seventh generation coating was designed to meet the requirements of the parts market: heavy-metal free, zero HAPS (hazardous air pollutants), minimal VOCs and possibly one-feed component.

Performance requirements. Electrocoat's primary function is corrosion resistance. The most severe corrosion test is the "Volvo" outdoor scab test. The test is 12 months in South Florida at a 45 degree angle of exposure with twice daily salt spray applications. In addition to providing a pass/fail screen, South Florida Volvo testing reliably ranks materials, processes and variables of interest with regard to their impact on corrosion performance. Other less severe corrosion tests, such as scab and salt spray, are required to support the Volvo results.

Cyclic corrosion tests provide fast feedback, without which, a longer Volvo exposure would be required. Salt spray testing is only required on cold-rolled steel and is included to provide a baseline for process quality control purposes. Salt water soak is included to screen heavy-metal-free electrocoats.

Over and underbake cyclic corrosion tests evaluate the baked finish of the electrocoat. The cyclic scab over bare steel test evaluates the vulnerability of the electrocoat to pretreatment excursions. Low-film cyclic scab tests evaluate the vulnerability of the electrocoat at thinner films. The cyclic scab test for throwing power evaluates the actual protective throw of the electrocoat.

The physical properties of the electrocoat film are evaluated using gravel, shot, impact, thermal shock and fluid resistance and pencil hardness tests. These tests evaluate the film based on the expected field exposures.

Tests such as initial adhesion, humidity resistance and water soak are included, but seem rudimentary due to the performance of cationic epoxy electrocoats. These tests also serve the purpose of indicating potential pretreatment problems in the sample preparation. Appearance tests for gloss, film smoothness and color are included also, but are not as rigorous as a "Class A" decorative finish test.

Operational requirements for throwing power, bath stability, and crater resistance are included as well as cure curve, coulumbic yield, rupture voltage, horizontal settling, ultrafiltration requirements, pinholing tendency and pigment feed stability. These attributes may not appear to be appropriate within an automotive parts material specification, however, to include a material deficient in these areas could jeopardize just-in-time delivery of components. In addition, an OEM materials engineer should act with responsibility to the custom coater and provide a robust material. In further accordance with this objective, the final Chrysler requirement was a line trial with a custom coater buy-off and testing of line produced parts.

Performance results. Heavy-metal-free electrocoat technology has been driven by recent pending federal and local legislation. Through the course of the development effort, corrosion resistance has been the focal point. The learning curve has produced submissions of electrocoats that ranged from poor in general corrosion resistance, to poor corrosion protection over weaker pretreatment, and eventually to acceptable corrosion resistance versus the control (lead containing) materials.

The coating met all of the requirements of Chrysler specification MS-PB45-2, including a line trial at Metokote Corporation.

Electrocoat provides numerous advantages from an automotive material engineering perspective. Electrocoat has seen the continuous improvements necessary to continue its growth.

The capital required to build and run an electrocoat line, precludes "dabblers" from the application business. Consequently, electrocoaters are quality conscious. The level of automation associated with electrocoat produces a highly consistent finish. The electrocoat process was originally environmentally friendly. Technological improvements have made it the "greenest" high-performance coating available. As the automotive industry moves to life cycle risk management, electrocoat will continue to be the favored finish.

Electrocoat provides a relatively large window of bake and pretreatment variations under which it will still perform acceptably. High transfer efficiency, automation, low waste, and less hazards all reduce the total applied cost of the electrocoat film (ultimately passed on to the consumer). The development of seventh generation continues the tradition of high performance electrocoats, while providing a new industry standard for environmental compliance and responsibility.

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