Graphene Nanoplatelets Produced by Thermomechanical Exfoliation of Graphite, and their use in Polyamide Composites
This thesis presents a new thermomechanical exfoliation (TME) method to produce graphene nanoplatelets (GNPs) from flake graphite. GNPs having specific surface area up to ~430 m2/g are produced under the action of shear forces, which serve to delaminate graphite, using a lab-scale polymer batch mixer equipped with roller rotors at a set temperature of 250 oC and 150 rpm. The corresponding energy requirement is 35-60 kJ/g for an operation that lasts 90-100 minutes. Bulk characterization showed the resulting product comprises a mixture of up to 68 wt% GNPs, the rest of it being residual micrographite. Electron microscopy images and atomic force microscopy (AFM) revealed that the lateral dimensions of the GNP flakes are 0.1 – 1.5 µm, with thickness as low as 0.9 nm, which suggests the presence of few-layered graphene with aspect ratios up to 250. Graphite and GNPs were used as fillers in a polyamide 6,12 (PA) matrix. Graphite increased the thermal conductivity more than GNP, up to 3-fold higher than neat PA, due to its larger particle size and therefore lower interfacial thermal resistance. GNP composites had an electrical percolation threshold of ~30 wt%, and maximum electrical conductivity of 10-1 S/m, whereas graphite did not percolate. Both fillers increased the flexural modulus up to 240% and decreased the flexural strain to 20% of the neat PA, at 40 wt% filler loading. Both fillers also increased the impact strength, but 20 wt% GNP saw the largest increase of 300%, compared to neat PA. To further improve the impact strength and the flexural strain, a maleated ethylene-octene copolymer (EOC) was added to 30 wt% GNP/PA composites. The resultant blend was still electrically conductive and up to 700% tougher than neat PA.