Influence of Hemp Biochar on the Rheological, Mechanical, and Sustainability Performance of EPDM-Based Rubber Compounds

Abstract

Reducing the carbon footprint (CF) of elastomers used in the automotive sector requires alternatives to fossil-derived carbon black (CB), which is widely used as a reinforcing filler in rubber formulations. Bio-derived carbonaceous materials, such as hemp biochar (HB), may help reduce this dependence while valorizing biomass resources. To this end, industrially produced HB was investigated as a partial and full replacement for CB in ethylene propylene diene monomer (EPDM)-based compounds. The effects of 0–100% HB substitution were systematically evaluated through physicochemical, rheological, mechanical, electrical, thermal, and sustainability analyses. Increasing HB content weakened filler networking, reducing minimum cure torque, Mooney viscosity, and Payne effect. At full replacement, tensile strength decreased from 8.45 to 1.21 MPa, while elongation at break reached 514% at intermediate HB contents. Thermal stability remained largely unaffected. HB substitution also reduced the normalized processing-energy demand index and lowered the cradle-to-gate CF from 2.12 to 0.74 kg CO2-eq kg–1 under the baseline HB emission-factor scenario, corresponding to a reduction of approximately 65% at full replacement. Among the tested formulations, 12.5–25% HB provided the most favorable balance between performance and environmental benefit. Within this range, tensile strength remained at 5.4–6.3 MPa, compression set did not differ significantly from the reference compound and the calculated cradle-to-gate CF decreased by 8.2–16.3%. These results identify a practical window for the partial replacement of conventional CB in EPDM sealing compounds. The study clarifies the tradeoff between reinforcement and sustainability across the full replacement range.