Electrical Conductivity of Carbon-Filled Polypropylene-Based Resins Julia A. King, Beth A. Johnson, Michael D. Via, Charles J.Ciarkowski Department of Chemical Engineering, Michigan Technological University, Houghton, Michigan 49931-1295 Received 29 July 2008; accepted 12 October 2008 DOI 10.1002/app.29422 Published online 29 December 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Adding conductive carbon fillers to insulat- ing thermoplastic resins increases composite electrical con- ductivity.Often,as much of a single type ofcarbon filler is added to achieve the desired conductivity and still allow the material to be molded into a bipolar plate for a fuel cell. In this study,various amounts of three different carbons(carbon black,syntheticgraphite particles,and carbon nanotubes)were added to polypropylene resin. The resulting single-filler composites were tested for elec- trical resistivity (1/electrical conductivity).The effects of single fillers and combinations of the different carbon fill- ers were studied via a factorial design. The percolation threshold was 1.4 vol % for the compositescontaining only carbon black, 2.1 vol % for those containing only car- bon nanotubes, and 13 vol % for those containing only syn- thetic graphite particles. The factorialresults indicate that the compositescontainingonly single fillers (synthetic graphite followed closely by carbon nanotubes and then car- bon black) caused a statistically significant decrease in com- posite electrical resistivity. All of the composites containing combinations of different fillers had a statistically significant effectthat increased the electrical resistivity. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112: 425–433, 2009 Key words: composites;fillers; injection molding;nano- composites INTRODUCTION Most polymer resins are electrically insulating. Increasing the electrical conductivity ofthese resins allows them to be used in other applications.One emerging market for electrically conductive resins is that for bipolar plates for use in fuel cells. The bipo- lar plate separates one cell from the next,with this plate carrying hydrogen gas on one side and air (ox- ygen)on the other side.Bipolar plates require high thermal and electrical conductivities (to conduct heat and to minimize ohmic losses). One approach to improving the electrical conduc- tivity of a polymer is through the addition of a con- ductive filler material, such as carbon and metal. 1–14 Often, a single type of graphite powder is used in thermosetting resins (often a vinyl ester) to produce a thermally and electrically conductive bipolar plate material. 15–18 Thermosetting resins cannot be remelted.Recently,carbon-filled thermoplastic resins [e.g.,polypropylene (PP),liquid-crystalline polymer, polyphenylene sulfide, and polyethylene] have been considered for fuel cell bipolar plates. 19–23 In this study, we performed compounding runs followed by the injection molding of carbon-filled PP resins.PP has been studied by severalresearchers for possible use in fuel cell bipolar plates. 19,24 PP is a semicrystalline thermoplastic that can be remelted and used again.Three differentcarbon fillers [elec- trically conductive carbon black (CB), synthetic graphite (SG) particles, and carbon nanotubes (CNTs)] were studied.Composites containing vari- ous amounts ofa single type of carbon filler were fabricated and tested for electrical conductivity. Compositescontaining combinationsof the fillers were also investigated via a factorial design with a replicate.The goal of this projectwas to determine the effects and interactions of each filler on the com- posite electrical conductivity (1/electrical resistivity). EXPERIMENTAL Materials The matrix used for this project was Dow’s semi- crystalline homopolymer PP resin H7012-35RN (Midland, MI). The properties of this polymer are shown in Table I. 25 The first filler used in this study was Ketjenblack EC-600 JD. This is an electrically conductiveCB available from Akzo Nobel, Inc. (Chicago,IL) The highly branched, high-surface-areaCB structure allows it to contact a large amount of polymer, which results in improved electrical conductivity at Journal of Applied Polymer Science, Vol. 112, 425–433 (2009) V V C 2008 Wiley Periodicals, Inc. Correspondence to: J. A. King (jaking@mtu.edu). Contract grant sponsor: U.S. Departmentof Energy; contract grant number: DE-FG36-08GO88104. Contract grant sponsor: National ScienceFoundation; contract grant number: DMI-0456537.