Improved Thermoplastic Composite by Alignment of Vapor Grown Carbon Fiber

This paper discusses the properties of an improved thermoplastic composite by alignment of vapor grown carbon fiber (VGCF) suspended in a polypropylene matrix. VGCF provides improved mechanical and electrical properties in composites. In this study an extruder was used to shear mix and extrude VGCF/polypropylene mixtures containing fiber volume fractions of 2.5%, 7% and 11% through a converging-annular die which produces a high degree of fiber alignment along the flow direction. X-ray diffraction analysis performed on the extruded composite strands showed that the fibers were oriented approximately ± 23.7, ± 28.15 and ± 30.0 degrees along the preferred direction for the 2.5%, 7% and 11% specimens, respectively. Tensile tests were done in both the preferred and transverse directions of samples reinforced with pyrolytically stripped VGCF. When compared to polypropylene, there was a 36.5%, 69.4% and 82.0% increase in tensile strength, and a 94.9%, 173.7% and 218.2% increase in modulus for the 2.5%, 7% and 11% VGCF mixtures along the preferred direction, respectively. The tensile strength and modulus in the transverse direction increased as the fiber volume content increased, however, all values were well below that of polypropylene. This behavior could be attributed to stress concentrations in the composite material. Electrical resistivity measurements were made on samples reinforced with two types of VGCF. The results concluded that the electrical conductance of the polymer strands reinforced with a heat-treated VGCF was far superior to those reinforced with a pyrolytically stripped VGCF.