Mechanical properties of vapor-grown carbon fiber composites with thermoplastic matrices
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Mechanical properties of vapor-grown carbon fiber composites with thermoplastic matrices Gary G. Tibbettsa) and John J. McHugh Physics and Physical Chemistry Department, General Motors Research and Development Center, Warren, Michigan 48090 (Received 26 June 1998; accepted 19 April 1999)
This article discusses the mechanical properties of vapor-grown carbon fiber (VGCF)/nylon and VGCF/polypropylene composites. Fibers in the as-produced condition yielded composites with marginally improved mechanical properties. Microscopic examination of these composites clearly showed regions of uninfiltrated fibers, which could account for the unsatisfactory mechanical properties. The infiltration of the fibers by both polymers was improved by carefully ball milling the raw fiber so as to reduce the diameter of the fiber clumps to less than 300 m. Properties of composites made with ball-milled material were improved in every respect. VGCF reinforcement in nylon slightly improved the tensile strength and doubled the modulus, while VGCF in polypropylene doubled the tensile strength and quadrupled the modulus compared to unreinforced material. Moreover, the composites were sufficiently improved that differences in fiber surface preparation became important. For example, air-etched fibers and fibers covered with low concentrations of aromatics produced polypropylene composites with significantly better mechanical properties than did fibers whose surfaces were heavily coated with aromatics. Both the tensile strength and the modulus of the composites fabricated with clean fibers exceeded theoretical values for composites made with fibers randomly oriented in three dimensions, indicating that the injection-molding process oriented the fibers to some extent.
I. INTRODUCTION
Vapor-grown carbon fibers (VGCF) may be produced catalytically on metallic (mostly iron) particles from gaseous hydrocarbons using a technique described previously.1 Although transmission electron microscopic observations show that the fibers are quite graphitic, they are two small for direct measurement of mechanical properties.2 However, direct measurements on fibers grown to macroscopic dimensions (which, because of their greater pyrolytic carbon coating should be less graphitic than their microscopic counterparts) yielded excellent values: a strength of 2.92 GPa and a tensile modulus of 240 GPa.3,4 Therefore, there has been recent interest in investigating the potential of VGCF as a composite reinforcement.5,6 However, in the most practical production mode, the fibers flow through the reactor with the feedstock gas and emerge with a diameter of 0.2 m in a highly entangled state.7,8 Furthermore, the bulk density of the a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 7, Jul 1999
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product mass is very low. For these reasons, conventional composite fabrication techniques are difficult to apply to VGCF. A few wor
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