Influence of processing methods on the mechanical and barrier properties of HDPE-GNP nanocomposites

In order to improve the barrier properties of Graphene nanoPlatelets (GnP)-High-Density PolyEthylene (HDPE) composites made by melt mixing, extra processing steps were investigated (solution mixing prior to melt mixing, cryomilling of the HDPE pellets into a fine powder, coating of the GnP with low molecular weight wax or with a thermoplastic elastomer, and microlayer co-extrusion). The associated mechanical properties were characterized by a flexural test and a notched Izod impact test. The barrier properties were evaluated by the permeation to oxygen. Each method resulted in a different state of dispersion and alignment of the graphene nanoplatelets, leading to a specific evolution of the barrier and mechanical properties. Microlayer co-extrusion led to a better relative improvement of the barrier properties versus neat HDPE. Solution mixing did result in an improvement of the barrier properties only if the composite was not further processed by melt mixing and extrusion. Starting with a fine HDPE powder did improve the barrier properties but affected the flexural properties. Lastly, a coating of the GnP with a thermoplastic elastomer enhanced the resistance to impact and barrier properties, but a coating with wax did not show any improvement.