The currently commercially available graphene materials in large quantities are typically graphene nanoplatelets (GnPs) with multiple-layer structures whose properties are not comparable to those of mono-layer or few-layer graphene. In this work, we demonstrated that high concentrations of commercial GnPs (15 wt% with potential to be higher) can be highly exfoliated in polypropylene (PP)/GnP nanocomposites using supercritical CO2 (ScCO2) in a foaming process. It was shown that although the exfoliation of GnP was increased with increasing expansion ratio in the foaming window of PP/GnP (159-163°C), high degree of GnP exfoliation can be maintained at elevated temperatures (e.g., 200°C) with minimized expansion ratios (< 1.3). The foaming pressure showed a critical influence on GnP exfoliation and X-ray diffraction intensity of GnP decreased significantly as the CO2 pressure increased from 500 to 3000 psi. The pressure threshold to delaminate graphene layers appears to be greatly reduced with the presence of PP matrix as compared to GnP powder. The results were attributed to PP chain penetration into GnP under ScCO2 and the prevention of quick loss of intercalating CO2 by the PP molecules surrounding the GnPs during pressure release.The electrical conductivity of the solid PP/15wt%-GnP was increased by almost 5 orders of magnitude after ScCO2 treatment. This study paves the road to develop highly exfoliated, low-expansion/solid, and versatile graphene/polymer nanocomposites for variuos downstream applications.