Adding conductive carbon fillers to insulating thermoplastic polymers increases the resulting composite's electrical conductivity. Carbon nanotubes (CNTs) are very effective at increasing composite electrical conductivity at low loading levels without compromising composite tensile and flexural properties. In this study, varying amounts (2-8 wt %) of CNTs were added to polycarbonate (PC) by melt compounding, and the resulting composites were tested for electrical conductivity (1/electrical resistivity), thermal conductivity, and tensile and flexural properties. The percolation threshold was less than 1.4 vol % CNT, likely because of CNTs high aspect ratio (1000). The addition of CNT to PC increased the composite electrical and thermal conductivity and tensile and flexural modulus. The 6 wt % (4.2 vol %) CNT in PC resin had a good combination of properties for electrical conductivity applications. The electrical resistivity and thermal conductivity were 18 X-cm and 0.28 W/m Á K, respectively. The tensile modulus, ultimate tensile strength (UTS), and strain at UTS were 2.7 GPa, 56 MPa, and 2.8%, respectively. The flexural modulus, ultimate flexural strength, and strain at ultimate flexural strength were 3.6 GPa, 125 MPa, and 5.5%, respectively. Ductile tensile behavior is noted in pure PC and in samples containing up to 6 wt % CNT.
Conductive fillers are often added to insulating polymers to increase the composite conductivity. Adding fillers often increases the viscosity, which can make the material more difficult to process. In this study, 2-8 wt % multiwalled carbon nanotubes or 2-8 wt % carbon black were added to polycarbonate. The effects on composites' viscosities were studied with small-amplitude oscillatory shear and capillary rheometer testing. The addition of carbon nanotubes and carbon black created yield-stress materials with the yield-stresses increasing with increased filler loadings. The addition of carbon black increased the steadyshear viscosity of the composite at all shear rates for all loadings. The addition of the carbon nanotubes reduced the steady-shear viscosity of the composite at high shear rates for all loadings. This is thought to be due to an internal lubrication of the flow (enhanced disentanglement) due to the presence of the carbon nanotubes. The lubrication effect saturates once 3 wt % carbon nanotubes has been reached. The observed rheological behavior of the carbon nanotube composites is markedly different than usually seen in filled systems. A yield-stress-modified Carreau-Yasuda model was used to interpret the effects of carbon nanotube and carbon black fillers on carbon nanotube/polycarbonate and carbon black/polycarbonate composite viscosity.
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