Stabilization of the cellular structure of polypropylene foams and secondary nucleation mechanism in the presence of graphene nanoplatelets

Graphene nanoplatelets (GNPs) consisting of few-layered graphene (FLG) with surface area of about 400 g/cm(2) were prepared from expanded graphite (EG) using a shear-induced thermomechanical exfoliation process. Melt compounded composites of EG with polypropylene (PP) exhibited good filler dispersion with no visible aggregation, whereas PP-GNP composites contained finely-dispersed nanometer-scale FLG. Foams were prepared through a batch foaming process using nitrogen as the physical blowing agent, at temperatures ranging from 140 to 180 degrees C. The presence of EG particles and finely-dispersed GNPs promoted heterogenous nucleation, resulting in cell densities of the order of 10(7) cells/cm(3), and a fine micro-cellular structure with an average cell diameter in the range of 20 mu m-60 mu m. It was further noted that the foams exhibited secondary nucleation effects stemming from the presence of very finely dispersed platelets, resulting in a secondary population of smaller cells in the range of 1 mu m-10 mu m. The compressive modulus of the composite foams containing 9 wt% GNPs increased five-fold compared to the neat PP foams.