Engineering Biochar–Polymer Interactions for Sustainable Composite Applications: A Review

Rising demand for sustainable, high-performance polymer composites has driven the development of carbon-based fillers. This review article explores the recent advancements in the use of biochar, produced from pyrolyzed biomass, as a multifunctional filler in polymer composite materials. It also compares the performance of biochar with other established carbon-based fillers, such as carbon black, carbon nanotubes (CNTs), and graphene derivatives. These carbonaceous fillers are known to enhance the mechanical, thermal, electrical, rheological, and flame-retardant properties. Emphasis is placed on the molecular-level interactions between biochar and polymer matrices, particularly polymer polarity, as well as the effects of processing parameters and surface chemistry. The review also highlights the environmental benefits of biochar, including waste valorization and carbon sequestration, alongside its limitations in consistency, dispersion, and scalability. Critical research challenges remain to create a full picture of the relationship between feedstock composition, carbonization conditions, char physicochemical properties, the interfacial mechanisms governing polymer–biochar interactions, particularly in the context of surface functionalization strategies that can tailor compatibility and dispersion at the molecular scale and the resulting mechanical, thermal and stability properties of the resulting composite materials. Further, the performance optimization of biochar fillers within recycled polymer matrices, where degradation kinetics, polarity mismatch, and processing sensitivity play key roles, requires systematic investigation. This review incorporates recent advancements in biochar–polymer composite materials and provides future perspectives on positioning biochar as a renewable, high-performance filler with significant potential for future polymer engineering applications.