Hazelnut Shell-Talc-Carbon Black Hybrid High-Density Polyethylene Biocomposites: Formulation-Property Relationships for Potential Rigid Food-Packaging Applications

Agricultural residues offer opportunities to reduce virgin polymer consumption, although balancing mechanical performance, thermal stability, and processability in multicomponent composites remains challenging. This study developed high-density polyethylene (HDPE) hybrid biocomposites incorporating hazelnut-shell (HS) powder, talc, a carbonaceous additive, and maleic-anhydride-modified linear low-density polyethylene (LLDPE-g-MA) through sequential formulation. Mechanical, physical, processing, thermal, spectroscopic, and morphological properties were evaluated. The formulations exhibited distinct property trade-offs. The HS/talc formulation (PEHSCB4) achieved the highest flexural strength (38.87 MPa), 10.6% above neat HDPE, while maintaining comparable tensile strength. PEHSCB5 exhibited the highest tensile modulus (963.3 MPa) and Shore D hardness (71.8), whereas PEHSCB2 and PEHSCB3 achieved the highest V-notched impact strengths (~29 kJ/m2), compared with 12.78 kJ/m2 for neat HDPE. Melt flow rates ranged from 0.22 to 3.90 g/10 min, with all formulations remaining processable by extrusion and injection molding. Water absorption increased with HS incorporation but remained below 0.36% for all formulations after 240 h of immersion. Thermal analysis revealed formulation-dependent changes in crystallinity and initial degradation behavior. FTIR and SEM confirmed the polyethylene structure and heterogeneous multiphase morphologies. Overall, sequential formulation enabled property tailoring, supporting further evaluation for potential rigid food-packaging applications.