Surface Modification of Sisal Fibers for Enhanced Interface and Performance in Bio‐Based Polyurethane Composites

ABSTRACT The increasing demand for sustainable materials has led to the development of bio‐based polymer composites reinforced with natural fibers. In this study, sisal fibers were surface‐modified using alkaline treatment (NaOH) and corona discharge and incorporated into castor oil–derived polyurethane matrices. The physicochemical, morphological, mechanical, thermal, and surface properties of the resulting composites were investigated. NaOH treatment significantly improved fiber morphology and reduced lignin and extractive content, resulting in a 52% increase in tensile strength (6.34 MPa) and a 66% increase in Young's modulus (24.97 MPa) compared to the control. Corona‐treated composites exhibited the lowest water absorption (0.43%) and enhanced interfacial compatibility, confirmed via SEM, AFM, and Raman spectroscopy. All composites showed low thermal conductivity (0.02–0.03 W/m K), indicating their potential use as lightweight thermal insulators. This study demonstrates the synergistic effect of chemical and physical surface modifications on fiber matrix interactions, opening avenues for high‐performance, eco‐friendly polymer composites.