Electrical conductivity of vapor-grown carbon fiber/thermoplastic composites
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Conducting polymers are required for applications such as radio frequency interference shielding, primerless electrostatic painting, and static discharge. We have used vapor-grown carbon fiber (VGCF) as an additive to investigate conducting thermoplastics for these applications. The electrical properties of VGCF/polypropylene (PP) and VGCF/nylon composites are very attractive compared with those provided by other conventional conducting additives. Because of the low diameter of the VGCF used, the onset of conductivity (percolation threshold) can be below 3 vol%. Because of the highly conductive nature of the fibers, particularly after a graphitization step, the composites can reach resistivities as low as 0.15 ⍀ cm.
I. INTRODUCTION
Electrically conducting polymer-based composites are required in many applications yet are difficult and expensive to make. At present, some of the materials one might add are, in order of decreasing cost, hyperion vapor-grown carbon fibers,1 conventional carbon fibers,2 metallic particles,3 or carbon black.4 Figure 1 shows the approximate resistivities required for the more common applications for which electrically conducting plastics are sought: static dissipation; electrostatic painting with no primer coat; radio frequency interference (RFI) shielding.1 The exact values required depend, of course, on the specific geometry of the pieces and the details of the application. In the following report, we will show that relatively small quantities of graphitized vapor-grown carbon fibers (VGCF)5,6 can allow the fabrication of extremely low resistivity composites of the thermoplastics polypropylene and nylon. We will demonstrate how the small fiber diameter of these VGCF makes these encouraging results possible. A recent paper by Gordeyev, Macedo, Ferreira, van Hattum, and Bernardo7 featured resistance measurements on polypropylene filled with similar fibers. They observed initial time-dependent changes in resistance with applied voltage, necessitating a settling period before a stable resistance could be measured. They also observed a much lower percolation threshold if resistance was measured at an applied voltage of 1000 V than at 0.1 V. They gave more complete observations on resistance as a 1668
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 12 Nov 2014
function of applied voltage in a more recent publication.8 We will show how their observations are consistent with ours in the Discussion section.
II. EXPERIMENT
The nylon used in this study was DuPont Zytel 103 HSL (Parkersburg, WV), in pellet form. It was vacuum baked immediately before use at 90 °C for 8 h to remove absorbed water. The polypropylene, Montell Pro-Fax 6301 (Montell Polyolefins, Wilmington, DE), in flake form, was not baked before use due to its smaller tendency to absorb water. The fibers used in this study were produced by Applied Sciences, Inc., in Cedarville, OH (www.apsci.com). These “PYROGRAF III” fibers were approximately 0.2 m in diameter and were formed by iron-based c
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