By intercalating and exfoliating natural graphite flakes, expanded graphite was obtained and used as the additive for making composites. The expanded graphite was composed of partly connected graphite nanosheets. Three types of composites were made, representing three levels of dispersing the graphite nanosheets. The first was the impregnation of epoxy resin to the expanded graphite by resin transfer molding. No dispersion was applied, and the expanded graphite can retain its original shape. The second was the use of a high-power sonication to break apart the expanded graphite. The thickness of the sonicated expanded graphite was reduced to about 100 times. The third method was to use a high-shear strain rate to separate the graphite nanosheets from the expanded graphite and to disperse them into the resin. The thickness range of the graphite nanosheets was 20-50 nm, about 100 times thinner than the sonicated ones. Compression and impact tests were conducted. The influence of dispersion on the material behavior was studied. Some fracture modes associated with the layered microstructures of the graphite nanosheets were observed. A simple model was used to study the stress transfer and frictional energy consumption of the pullout of the nanosheet.
Three sets of expanded graphite-filled polymers, having three different particle sizes, were reinforced with 1-5% by weight. The structural, mechanical, electrical, and thermal properties of these composites were studied and compared. After dispersion, the particles were reduced to nanometer size through exfoliation, sonication, and high-shear strain rate mixing, which further breaks and delaminates them. In addition, scanning electron microscope characterizations were performed. The expanded graphite-filled polymer material could be tailored to be high conducting. Compared with the pure polymer, the polymers filled with 5 wt% expanded graphite have seen a significant reduction in electrical resistivity by orders. The thermal expansion coefficient and water absorption for the weight containing 5 wt% expanded graphite has also been drastically improved, decreasing with the weight percentage of graphite content. Compression and impact tests were conducted. The influence of dispersion on the material behavior was studied. Some fracture modes associated with the layered microstructures of the graphite nanosheets were observed. POLYM. COMPOS., 32:305-313, 2011. ª
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