This year, electrically conducting polymer matrix composites were successfully fabricated at the NASA Glenn Research Center from bromine-intercalated graphite fibers and three different thermoplastic resins. All demonstrated relatively low electrical resistivities, comparable to those of composites made with thermosetting resins.
Increasing the electrical conductivity of graphite fibers through intercalation, the insertion of guest atoms or molecules between the graphene planes, is a technology that has been investigated since pitch-based graphite fibers first became available 25 years ago. Applications include electrically conductive composite structures for electrostatic dissipation, electromagnetic interference shielding, and ground return planes that could save between 30 and 90 percent of the mass of the structure, in comparison to aluminum (ref. 1). Composites made from intercalated graphite fibers have been fabricated since the late 1980s, but they have always been made with thermosetting resins. This is partially due to the fact that most applications for these materials were in the aerospace industry, where epoxies, polyimides, and polyisocyanates are the standard resins used with graphite fibers. In addition, most intercalation compounds are not very stable at high temperatures, so limited exposure to curing cycles is important.
However, with the advent of higher temperature stable intercalation compounds, like those intercalated with bromine, these considerations become irrelevant because these compounds are stable even at cure temperatures as high as 3000 °C (ref. 2). This technology is also becoming more attractive in applications outside of the aerospace industry in commodity products. Thermoplastic resins, which can be reheated and molded several times, have definite advantages over thermosetting resins for manufacturability. In addition, radiation shields made by combining materials with some high-atomic-mass atoms (which are most efficient at stopping x-rays and g-rays) with materials containing a lot of hydrogen (which is most efficient at stopping high- energy protons), are being considered for spacecraft that require extended human stays in space. Intercalated graphite fibers contain high-atomic-mass atoms, such as bromine; and thermoplastic polymers, such as polyethylene, contain a large fraction of hydrogen.
Glenn researchers made composites by combining bromine-intercalated graphite fibers with a variety of thermoplastics--including polyethylene, polystyrene, and acrylonitrile butadiene styrene copolymer (ABS)--and compared their electrical properties with those of composites made from the thermosetting resins epoxy and RS--3 (a polyisocyanate). As is illustrated in the graph, the results indicate that the electrical conductivity of thermoplastics is comparable to that of composites made with thermosetting resins. Thus, applications in which composites made with intercalated graphite fibers and thermosetting resin have been well suited should also work well with composites made with thermoplastic resins, but with lower cost and improved manufacturability.
Resistivity of composites made from varying resins as a function of the elastic modulus of the fiber in the composite; msi, millions of pounds per square inch.
Last updated: October 7, 2006
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