The role of graphene nanoplatelets and carbon black hybrid fillers on rheological and mechanical properties and sagging behavior of high-density polyethylene nanocomposite

Carbon black (CB) is widely employed in high-density polyethylene (PE100) pipe-grade materials to mitigate ultraviolet (UV) degradation. However, the melt-state and solid-state properties of polymer nanocomposites are highly sensitive to the dispersion state of their constituents. This study investigates the effect of graphene nanoplatelets (GNPs) as secondary nanofillers in PE100/CB systems, with particular emphasis on melt sagging behavior governed by creeping flow. Two types of GNPs—few-layer graphene (FLG) and multi-layer graphene (MLG)—were incorporated to form hybrid nanocomposites with total filler contents of 2.25 wt.% (1.95 wt.% CB + 0.3 wt.% GNPs) and 2.55 wt.% (2.25 wt.% CB + 0.3 wt.% GNPs). Morphological observations using optical microscopy (OM) and field-emission scanning electron microscopy (FESEM) revealed that FLG, characterized by a higher structural defect density, achieved superior dispersion, whereas MLG showed a limited ability to disrupt CB agglomerates within the PE100 matrix. Rheological characterization, including small-amplitude oscillatory shear, shear-induced time-dependent measurements, and melt creep tests, demonstrated that FLG-containing hybrids exhibit enhanced melt viscosity and increased resistance to creep flow. Consequently, the addition of 0.3wt.% FLG caused a reduction in the sag index by up to 31.16% in compared with 2.25wt.% CB-filled systems. Tensile testing showed that FLG incorporation leads to slight improvements in break-point properties (e.g., a 14.6% increase in elongation at break for the 2.55 wt.% filled samples). This behavior is attributed to reduced stress concentration sites resulting from improved filler dispersion in the amorphous phase. These results demonstrate that CB/FLG hybridization provides an effective materials-based strategy for improving sagging resistance in PE100 while maintaining mechanical integrity