© 2019 IOP Publishing Ltd. Polypropylene (PP) nanocomposites with different low concentrations (0.05-1.0 wt%) of reduced graphene oxide (rGO) were prepared in this work at large-scale, i.e., several kilograms/hour, by a clean method. For this, graphene oxide (GO) was reduced by microwave irradiation and melt mixed with PP via twin-screw extrusion. Good dispersion and breakage of rGO agglomerates was achieved by introducing extensional flow components in the processing line. For the rGO contents used in this work, the nanocomposites exhibited a shear flow behavior similar to that of the neat PP with minimal changes in viscosity. However, the tensile properties and hardness of the nanocomposites were improved significantly as compared to the neat PP. This improvement in the mechanical properties is explained by the enhancement in the dispersion of the rGO due to the combination of shear and extensional flows during processing, as well as to good compatibility of rGO with the matrix. Finally, it is shown how rGO microagglomerates in the nanocomposites deform and break under tension due to poor cohesion among them.