Role and potential of biochar as a sustainable alternative reinforcing filler to carbon black in rubber composites

Traditionally, carbon black (CB) has been the predominant reinforcing filler in rubber composites. This preference is due to superior mechanical properties of CB, which stem from its unique high carbon-based structure and high purity. However, the production of CB is energy-intensive, poses significant health risks, and is environmentally detrimental, releasing significant amounts of CO₂. Consequently, there is a growing interest in finding sustainable alternatives to CB. Biochar (BC), another carbon-rich solid produced through the pyrolysis of biomass under limited oxygen, continues to be studied as a promising, eco-friendly, and low-cost reinforcement filler for rubber composites. BC exhibits high surface area, stability, and carbon content, making it a contender for enhancing the mechanical properties of rubber. Various biomass materials, including rice husks, corncobs, nutshells, grain husks, and palm kernel shells, have been used to produce BC, with its reinforcement potential influenced by production conditions and feedstock type. However, the variation in the feedstock and production protocol has been found to produce BC with different compositional and morphological features which affect the properties of the final vulcanizate in a detrimental manner. Therefore, it is essential to understand the governing factors that optimize the performance of BC in rubber composites. While BC is increasingly viewed as a promising replacement for CB as a reinforcement filler, a detailed evaluation of its reinforcement capabilities remains lacking. This review explores the viability of BC as an alternative to CB or a partial replacement CB, highlighting its environmental benefits and effectiveness in rubber composite applications. Rheological and mechanical properties of BC-incorporated rubber composites never reached the standards of CB. However, a mixture of BC and CB, replacing CB content partially showed promising results. Therefore, further studies are required to find optimum properties for BC to incorporate into the rubber matrix to replace CB.