Natural fibre and carbonaceous filler-reinforced polymer composites: interphase engineering, modelling, and sustainability - A review

Natural fibre reinforced polymer composites (NFRPCs) have emerged as sustainable, recyclable, and lighter alternatives to conventional mineral-reinforced materials. This development is largely driven by global sustainability requirements. Despite this potential, the widespread industrial adoption of natural fibres is limited. By their inherent hydrophilicity, which causes extremely weak interfacial adhesion and poor micromechanical stress transfer when paired with non-polar, hydrophobic polymer matrices. This review examines engineering strategies overcome these thermodynamic and mechanical barriers. Chemical, physical, and biological surface modification protocols are discussed in detail These approaches mitigate fibre hydrophilicity, remove non-cellulosic impurities, and facilitate strong interfacial bonding. Furthermore, research on the engineering of the nanometric fibre/matrix boundary through hierarchical architectures, such as cellulose nanocrystals and Layer-by-Layer assembly, is reviewed, highlighting how a functionally graded transition zone drastically enhances mechanical interlocking and interfacial shear strength. The use of pyrolysed biochar is explored as a scalable carbon-negative structural reinforcement Material. Biochar can replace highly emissive inorganic fillers while enhancing micromechanical interlocking via its macroporous cellular architecture. The performance gains from interphase engineering and biochar integration are validated through multi-scale characterization techniques and coupled with advanced computational frameworks, including Molecular Dynamics (MD) and the Phase Field Method, to accurately predict non-linear fracture mechanics. Together hierarchical interphase engineering, scalable biochar integration, and high-fidelity predictive modelling provides a foundation for the immediate deployment of highly durable and sustainable advanced composite materials.