Olive Stone Biochar for Tuning Mechanical, Electrical and Piezoresistive Properties of Flax Fiber-Reinforced Composites

Biochar powder from olive stones can be used as a functional additive to fabricate electrically conductive flax fiber-reinforced composites (FFRCs) for sustainable lightweight design. Additionally, the conductive composites can be used as self-strain sensing materials based on their good piezoresistive properties. For this purpose, different electrical particle network morphologies were prepared via two process routes. Firstly, biochar was mixed into the epoxy matrix (CiM). Secondly, biochar was deposited on the fiber fabric using an aqueous dispersion (CoF). As a local conductive network closer to the fibers is formed, electrical conductivity is enabled at a lower carbon content in the composite. The incorporation of biochar into FFRCs via CiM led to a 39% increase in Young’s modulus, improved the impact energy, did not cause embrittlement, but decreased the tensile strength. The effects were less pronounced with CoF. Accordingly, the mechanical properties can be tuned to optimize stiffness or tensile strength while maintaining electrical conductivity. The increase in Young’s modulus compensated the increased density of the composite that results from the biochar additive. Therefore, biochar from olive stones was identified as sustainable additive for use in FFRCs for stiffness-related lightweight applications. Biochar was compared with conductive carbon black as a fossil-based alternative. Carbon black has a lower percolation threshold than biochar, which results in a lower composite density, but showed no influence on the mechanical properties. It is therefore more suitable for tensile-, but not for stiffness- or bending-related applications than biochar. Biochar from olive stones was identified as an attractive multifunctional additive for use in flax fiber-reinforced composites (FFRCs). Biochar induces increased stiffness, impact toughness, electrical conductivity and good piezoresistive properties while maintaining ductility. By varying the incorporation methods, the conductive particle network morphology and the electrical and piezoresistive properties can be controlled.