New alternatives in conductive plastics.

A new highly conductive graphitic microfiber has become available in a range of resins and reportedly offers some important performance advantages over standard conductive fillers. Conductive Graphite Fibrils, supplied by Hyperion Catalysis International, Cambridge, Mass., are just now seeing their first commercial automotive and electrical/electronics applications. They offer significantly higher conductivity than traditional carbon fibers and are also less prone to sloughing and particulate contamination.

TINY SUPER-CONDUCTIVE TUBES

Typically 1000 times smaller in diameter than conventional carbon fibers, Graphite Fibrils are vapor-grown hollow carbon nanotubes 0.01 microns in diameter. Produced as agglomerates that under a microscope resemble steel-wool pads, Fibrils exist as entanglements of curved, intertwined strands rather than long, straight fibers. With fiber lengths of 1 to 10 microns, Fibrils' high aspect ratio is said to be a key to their greatly improved conductivity over conventional carbon fibers.

The key advantage of Graphite Fibrils is that they add conductivity to the matrix without sacrificing any other physical properties, according to GE Plastics, Pittsfield, Mass., which is developing Fibril-filled grades in its Noryl GTX line of PPO/nylon alloys. Although their conductive efficiency varies according to the application, Fibrils can be as much as three times as efficient as carbon fiber or carbon powder, according to research by GE Plastics.

"The focus taken with Fibrils is to develop materials that mold and behave like untilled resins in every regard," says John Getz, GE's industry manager of custom engineered products. Lower loading levels required to achieve desired conductivity, as well as the smaller size of the Fibrils, means that they are less likely to sacrifice moldability or physical properties such as ductility. The result is easier mold filling, lower fill pressures, and longer flow lengths. Another benefit is better part finish, with no sanding or polishing needed to achieve Class A finish, according to GE sources.

AUTOMOTIVE DEBUT

Hyperion is working with several major resin companies to introduce Fibrils into the automotive market. It has established supplier agreements with GE Plastics and Huls America, Inc., Piscataway, N.J.

GE Plastics has commercialized one grade, Noryl GTX 990EP, for electrostatic painting of automotive parts and plans to develop additional grades in the next year. The first automotive application using Graphite Fibrils is the exterior rearview mirror housing on the 1997 Ford Taurus/Mercury Sable, injection molded by United Technologies Automotive in Berne, Ind. After about two years of development, the Taurus mirror housing has been in production for just eight weeks.

The application illustrates the potential benefits of Fibril technology, which enabled UTA to eliminate the primer step before electrostatic painting. Prior to converting to the conductive GTX material, the mirror housing required a conductive primer coat to be applied to all surfaces including body mount areas to ensure grounding, as well as a second, manually applied "bridge" coat of color, according to UTA technical manager Ron Hendricks.

Even without the primer and bridge coats, electrostatic paint-transfer efficiency has improved 20-25% with the conductive substrate, says Hendricks, and his measurements so far indicate a 35% reduction in VOC emissions. Hendricks' goal is to eliminate the primer step on all existing parts by the end of 1999. The resulting VOC reductions should allow the plant to increase its total volume output by as much as 50% without violating environmental codes.

In molding, Hendricks reports that conductive GTX was "not as forgiving" as neat resin, requiring a somewhat fighter processing window. However, shrinkage properties are virtually the same as neat resin, and the conversion was made using existing tools with no modifications.

Meanwhile, Huls America, Inc. has commercialized three Fibril-filled conductive grades of its Vestamid nylon 12 resin. Designed for automotive fluid lines, these nylon grades achieve ESD properties with Fibril loadings of 3-5%, versus up to 15% for carbon black, according to Kevin Bonhoff, marketing manager for engineering plastics. The three grades include an extrudable version for multi-layer fuel hose, a glass-filled injection grade for quick connectors for fuel-line bundles, and an unreinforced injection grade for fuel filter bodies.

NEW E/E APPLICATIONS

Hyperion is also developing its own conductive engineering compounds for electrical/electronics uses. So far, the line includes PC, PBT, and PEEK grades. The first commercial application of Hyperion's SR600-PC polycarbonate grade is a computer disk drive. Hyperion says parts molded of Fibril-based SR compounds exhibit a particle-free surface, unlike conventional carbon-fiber compounds, in which fibers exposed on the surface may cause fissures or cracks or may break or wear off, causing particle contamination. SR compounds are also said to reduce particle contamination caused by sloughing or part wear. Hyperion is developing other E/E grades based on PP, ABS, and GE's Ultem polyetherimide.

RELATED ARTICLE: Conductivity Without Fillers

A line of permanently antistatic specialty compounds from RTP Co., Winona, Minn., is based on inherently conductive polymeric additives that eliminate the need for conductive fillers such as carbon powders or fibers. These compounds are available in a full range of colors and also include translucent grades. Another advantage of these proprietary materials is that they are non-sloughing - that is, abraded material will not carry an electrical charge, RTP says.

Available previously in compounds based on low- and moderate-temperature resins, the PermaStat line now includes some higher-performance materials, including polycarbonate and Shell Chemical Co.'s Carilon aliphatic polyketone.

Conductivity of these materials is extremely consistent through the part surface and from part to part, according to Larry Rupprecht, senior development engineer for conductive plastics. PermaStat products meet MIL Spec B-81705C, which makes the materials suitable for packaging electronic devices. Applications include automotive engine components, chip carriers, and gears.

RTP also has developed a line of relatively new electrostatically paintable thermoplastic compounds without carbon. So far, the materials tested include PC, acetal, PBT, acrylic, PC/ABS, TPU, TPO, and other TPEs. The compounds are fully colorable, so that the substrate minimizes the visibility of a scratch.