Polypropylene (PP) and graphene nanoparticles (GNPs) were mixed in a twin-screw extruder equipped with a simple CO2 injection unit consisting of a standard gas cylinder, a regulator, a valve, and a metal hose. Nanocomposites were produced with a 0.5% by weight GNP loading level. The addition of CO2 into the melt at pressures below the supercritical point allowed for foaming to occur within the barrel of the extruder and upon exiting from the die. Foaming introduces an equibiaxial flow on the surface of the expanding bubbles. This extensional flow imparts a stress greater than the shear case alone, aiding in the breakup of GNP agglomerates. Samples processed with sub-critical CO2 showed increased exfoliation and dispersion as observed via electron microscopy, thermal analysis, Raman spectroscopy, and X-ray diffraction. This sub-critical gas-assisted processing (SGAP) method was introduced as an alternative to supercritical fluid-assisted processing. (C) 2017 Elsevier Ltd. All rights reserved.