Balancing mechanical properties of graphene nanoplatelet filled HDPE

Composites of high-density polyethylene (HDPE) and graphene nanoplatelets (GNP) were prepared by melt mixing and the effect of nanoplatelet geometry, (GNP-1, larger lateral size; nominal < 26.5 μm) and GNP-2 (smaller lateral size; nominal < 10 μm) and processing regime on mechanical properties studied. Increasing GNP-1 content had a pronounced stiffening effect at higher loadings (> 6 wt%), with tensile modulus increasing from ~ 1.40 GPa (neat HDPE) to ~ 3.55 GPa at 30 wt% (an increase of ~ 154%) and tensile strength rising by ~ 33% to ~ 35.5 MPa. Conversely, impact strength decreases continuously from ~ 28.5 to ~ 9.0 kJ m⁻² with increasing GNP loading (~ 68% decrease), while ductility shows a non-monotonic response. Elongation at break peaks at ~ 385% at 0.5 wt% and declines to ~ 40% at 30 wt% loading. The mechanical trends are consistent with interfacial slip-dominated deformation at low (< 6 wt%) GNP-1 loadings, whereas such mechanisms contribute less under rapid, crack-driven notched impact loading. At a fixed loading (1.25 wt%), platelet type strongly influences post-yield behaviour. HDPE/GNP-2 and HDPE/GNP-1 exhibit similar modulus (~ 1.4 GPa), yet elongation increases by 234% for HDPE/GNP-2 relative to HDPE/GNP-1, coinciding with an increase in crystallinity (from 72.5% to 76%). Additive selection further improves extensibility at constant GNP-1 content (1.25 wt%), the inclusion of a (PVCL–PVAc–PEG) copolymer when combined with a slip-agent (cis-13-docosenamide) results in an increase elongation (~ 150% to ~ 350%) without losses in tensile modulus and strength. Overall, the additive-enabled ductility enhancement is consistent with stabilization of necking and drawing under quasi-static tension via time-dependent interfacial shear and platelet sliding/pull-out in layered fracture regions.