Single-pellet, electrically-conductive PBT grade offers excellent EMI shielding performance in fuel exposure environments

Chomerics, a division of Parker Hannifin Corporation, has recently launched Premier PBT-250-FR, an electrically conductive, flame retardant polybutylene terephthalate (PBT) plastic compound formulated for EMI shielding applications. Premier PBT-250-FR is a single pellet material that delivers superior reliability where EMI shielding is required. The conductivity comes from 304 stainless steel (SS) long fibers which are cut to 7 mm length in each pellet. The pellets are manufactured using a long fiber pultrusion compounding process.

A comparison between a typical multi-pellet dry blend (left) and Chomerics’ single pellet solution (right). The single pellet material has a precise amount of long fiber SS encapsulated within each pellet.

Premier PBT-250-FR has been designated 5VA for 2.5 mm wall thickness, the highest flammability rating from UL 94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances. A 5VA rating means plaque specimens do not burn through when exposed to a flame 5 times more intense than the V0 test.

The single pellet system in Premier PBT-250-FR eliminates issues associated with multi-pellet dry blends (sometimes known as “cube” blends). This system provides unparalleled uniformity of conductive filler dispersion and prevents EMI shielding failures that can sometimes occur at random in production. “A notable percentage of parts molded from cube blends exhibit poor shielding performance related to "too low" a percentage of the conductive material pellets in that shot,” says Peter Torok, Product Line Manager, Chomerics Division at Parker Hannifin.

A single pellet system is not limited in packaging options. With cube blends, stainless steel loaded pellets are dry blended into a mixture with traditional plastic pellets – these blends will often see ‘settling’ during transportation and the heavier conductive pellets will tend to collect towards the bottom. Therefore, cube blends are typically only sold in 20 to 25 kg bag sizes. PBT-250-FR can be shipped and purchased in any container size with no such risk. “Single pellet material has no such limitations and can be sold in Gaylord or even larger delivery formats,” says Torok.

Another issue with the heavily loaded SS pellets is that they typically do not process the same as the unfilled plastic pellets (within the dry mix) and molded parts can sometimes exhibit slugs of SS fibers. These occurrences of SS fiber bundles are typically referred to as ‘agglomerations’ which are often the source of water intrusion and mechanical defects. Torok mentions that “Cube blends also exhibit a similar percentage of molded parts that have ‘too high’ a percentage of conductive pellets in the shot” and those will often lead to defective parts that contain fiber bundles or agglomerations.

“In ongoing discussions with our partners, one request we kept hearing over and over was how an electrically conductive plastic with a 5VA flammability rating would address a major need in the marketplace,” said John Beswick, engineered plastics solutions business unit manager, Parker Chomerics. This was the driving force that led to the development of PBT-250-FR.

The cutting-edge processability of Premier PBT-250-FR makes it a superior choice to other EMI plastic enclosure materials available to the market. Premier PBT-250-FR is manufactured at Parker Chomerics’ facility in Fairport, NY and is available immediately.

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