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CHICAGO — Without using carbon-based additives, RTP Co. of Winona, Minn., has developed and evaluated thermoplastic compounds that can be spray-painted electrostatically.

``The quest [for such a system] has been out there for years,'' said engineer Larry Rupprecht.

RTP has found a new application for technology the company has been working on for some time, Rupprecht said.

The company blends a polymer and a proprietary conductive material to form the compound. Sample plaques of various base resins are displayed at the company's NPE exhibit (Booth N6508).

Several automotive, consumer-goods and medical-device customers are evaluating samples of the blended polymer material, which could open new markets for the specialty compounder.

The St. Paul, Minn., branch of Electro Painters Inc. tested the process using polypropylene, nylon 6 and 12, polycarbonate, acetal, polybutylene terephthalate, polyurethane, polyester and olefinic thermoplastic elastomers, acrylic and PC/ABS alloys.

Electro Painters originated the electrostatic paint process in 1957.

There was ``no overspray whatsoever,'' said branch manager James Bourque. ``A conventional system loses 60 percent of the spray.''

In April, using a 100 percent transfer-efficient unit, Electro Painters tested samples with a negative charge on the injection molded part and a positive charge on the paint.

``The paint wrapped around the edge just as if it was a metal sample,'' Bourque said.

The negative charge draws, rather than forces, paint particles to the part, improving control of volatile-organic-compound emissions. Additional tests are planned with other polymers.

The RTP process omits the use of carbon-based additives that can cause a plastic part to lose flexibility and impact resistance.

``The materials exhibit very uniform conductivity,'' Rupprecht said.

Compounds are colorable and can minimize the visibility of a paint scratch if the part is of a similar color. A scratch on a painted plastic with carbon additives may appear as a black or gray mark.

The technology ensures that paint gets to the part, but users need to verify the paint's compatibility with their resin systems.

Electrostatic painting with powders or liquids is common for metal parts in the automotive, appliance and electronics industries.

RTP employs 500 and operates facilities in Minnesota, Nevada, Virginia, Texas and France.