Substitution of carbon black with wood biochar in natural rubber composites: A study on reinforcement and sustainability

• Biochar enhances tensile stiffness at low strain • Tensile properties decrease linearly with increasing biochar content • Processing is more energy-intensive at higher biochar content • The addition of biochar significantly reduces the carbon footprint The increasing demand for sustainable materials in the rubber industry has prompted the search for renewable alternatives to carbon black (CB). Biochar (BC), a carbon-rich byproduct of biomass pyrolysis, offers a promising solution. In this study, we investigate the substitution of CB with wood-based BC at different ratios in natural rubber (NR) composites, and we evaluate its effect on mechanical properties, processability, and environmental performance. To assess sustainability, we quantified CO₂ emissions and energy requirements based on material inputs and processing parameters during the mixing and curing stages. With increasing BC contents, tensile strength, strain at break and modulus at 300 % strain decreased linearly. However, the initial tensile stiffness increased indicating that BC-filled NR composites may be advantageous for applications under small deformations. Higher BC contents also led to higher viscosity, which increases the energy requirements for processing. Overall, at a BC content of 20 %, tensile strength, modulus at 300 % strain and strain at break was reduced by 13 % (from 24.3 to 31.1 MPa), 16 % (from 12.6 to 10.6 MPa) and 3.6 % (from 500 % to 482 %), respectively, while CO₂ emissions were reduced by 20 % and energy requirements remained low. The results indicate that integrating BC as a partial substitute for CB can help reduce environmental impact while maintaining satisfactory rubber performance.