Optimizing NaOH treatment of sisal fibers to improve interfacial adhesion and composite performance with unsaturated polyester matrix

The demand for sustainable materials has increased interest in natural fiber-reinforced composites as eco-friendly alternatives to synthetic counterparts. Among them, Agave sisalana (sisal) offers excellent mechanical properties and availability, but poor interfacial adhesion between untreated fibers and polymer matrices limits its reinforcing potential. This study investigates the effect of alkali treatment on the physicomechanical and interfacial adhesion of sisal fiber–reinforced unsaturated polyester composites. Sisal fibers were treated with 5% NaOH for 0, 2, and 4 h prior to composite fabrication. The 2-h treatment yielded the best performance, resulting in a tensile strength of 413.7 MPa, interfacial shear strength of 24.2 MPa, and composite tensile shear strength of approximately 35 MPa. FTIR analysis confirmed hemicellulose removal through the disappearance of the 1735 cm−1 carbonyl peak, while SEM images revealed a cleaner, rougher surface that enhanced matrix adhesion. However, excessive treatment (4 h) led to surface degradation and reduced performance. Overall, moderate alkali treatment significantly improves interfacial bonding and mechanical strength, making sisal fiber more suitable for polyester-based composite applications.