Exfoliated graphite nanoplatelets (GNP) can be added polymers to produce electrically conductive composites. In this study, varying amounts (2–15 wt%) GNP were added to polycarbonate (PC) and the resulting composites were tested for electrical conductivity (1/electrical resistivity), thermal conductivity, and tensile, flexural, and rheological properties. The percolation threshold was approximately 4.0 vol% (6.5 wt%) GNP. The addition of GNP to polycarbonate increased the composite electrical and thermal conductivity and tensile and flexural modulus. The 8 wt% (5.0 vol%) GNP in polycarbonate composite had a good combination of properties for electrostatic dissipative applications. The electrical resistivity and thermal conductivity were 4.0 × 107 ohm-cm and 0.37 W/m · K, respectively. The tensile modulus, ultimate tensile strength, and strain at ultimate tensile strength were 3.5 GPa, 58 MPa, and 3.5%, respectively. The flexural modulus, ultimate flexural strength, and strain at ultimate flexural strength were 3.6 GPa, 108 MPa, and 5.5%, respectively. Ductile tensile behavior is noted in pure polycarbonate and in samples containing up to 8 wt% GNP. PC and GNP/PC composites show shear-thinning behavior. Viscosity of the composite increased as the amount of GNP increased dueto a volume-filling filler effect. The viscosity of the GNP/PC composites are well described by a Kitano-modified Maron-Pierce model.
Abstract. This work is concerned with the modeling of in-machine fiber breakage during injection molding. A lab-scale single screw extruder is used to evaluate fiber breakage in the screw. Our experiments show that as the initial glass fiber pellets length is reduced relative to the channel width, the overall percent of breakage is reduced. We believe that the ratio of initial pellet length to the screw channel width, or diameter, is an important parameter determining the percent of fiber breakage. Data have been fit using an exponential decay model with a kinetic decay constant and a critical length value. This empirical model has been tested on an injection molding machine with a screw 1.6 times larger than that of our single screw extruder. The predicted average fiber length leaving the nozzle shows reasonable agreement with the measured value. For the injection molded end-gated-plaque produced by the same machine, additional fiber breakage has been observed in the runner. A hydrodynamic force based breakage model is combined with mold filling simulation to simulate the fiber breakage in the runner. For carbon fibers, similar breakage trends were observed in the single screw extruder indicating the applicability of our empirical model to carbon fiber.
a b s t r a c tIn order to use rheological measurements as a tool to investigate fiber orientation in simple flows, the relationship between stress and fiber orientation must be understood. In this work, a sliding plate rheometer was used to measure the shear stress growth during the startup of simple shear flow of a polymer melt containing long glass fibers. The concentrations of the suspensions were varied from 10 to 40 wt% and tested over three shear rates spanning an order of magnitude. Significant shear thinning was observed in the suspension as concentration increased. Additionally, the magnitude of stress and breadth of the stress growth overshoot increased with concentration. A larger distinction between the different concentrations is observed in the shear stress growth than the measured evolution of fiber orientation. Measured values of fiber orientation were used with a semi-dilute stress equation to show that the fiber motion in these experiments was not responsible for the stress overshoot and that additional stress contributions must be considered.
Conductive fillers are often added to thermoplastic polymers to increase the resulting composite's electrical conductivity (EC) which would enable them to be used in electrostatic dissipative and semiconductive applications. The resulting composite also exhibits increased tensile modulus. The filler aspect ratio plays an important role in modeling composite EC, and tensile modulus. It is difficult to measure the filler aspect ratio after the manufacturing process (often extrusion followed by injection molding) in the composite, especially when nanomaterials are used. The EC percolation threshold is a function of the filler aspect ratio; hence, knowledge of this percolation threshold provides a means to extract the filler aspect ratio. In this study, the percolation threshold of the composite was determined from EC measurements and modeling, which in turn was used to determine the filler aspect ratio for tensile modulus modeling. Per the authors' knowledge, this approach has not been previously reported in the open literature. The fillers; carbon black (CB: 2-10 wt %), multiwalled carbon nanotubes (CNT: 0.5-8 wt %), or exfoliated graphite nanoplatelets (GNP: 2-12 wt %); were added to polycarbonate (PC) and the resulting composites were tested for EC and tensile modulus. With the filler aspect ratio determined from EC values for CNT/PC and GNP/PC composites, the three-dimensional randomly oriented fiber Halpin-Tsai model accurately estimates the tensile modulus for the CNT/PC composites and the Nielsen model predicts the tensile modulus well for the CB/PC and GNP/PC composites.
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