FTIR Spectra and Tensile Strength Analysis of Alkali-Treated Alfa Fiber (Stipa tenacissima L.) Reinforced Polyester Composites with Uncertainty Quantification Due to Natural Fiber Variability

This study investigates the mechanical performance of eco-friendly polyester composites reinforced with Alfa fibers. The fibers were extracted by retting and treated with 5 wt.% NaOH solution for 24 h to improve fiber – matrix adhesion. Composite laminates with a fiber volume fraction of approximately 45% were fabricated using vacuum-assisted resin transfer molding (VARTM). ATR-FTIR analysis confirmed that alkali treatment effectively removed non-cellulosic components from the fiber surface. Tensile tests conducted according to ASTM D3039 revealed significant improvement after treatment. Tensile strength increased from approximately 27–28 MPa for untreated composites to 42–47 MPa for treated composites, corresponding to an improvement of nearly 60–65%. Young’s modulus and elongation at break also increased, indicating improved stiffness and ductility. To account for the intrinsic variability of plant-based fibers and the limited number of specimens, a bootstrap statistical approach was applied to estimate confidence intervals for the mechanical properties. The statistical analysis confirmed the robustness of the observed improvements and highlighted the influence of plant variability on composite behavior. Alkali treatment enhances the reinforcing efficiency of Alfa fibers for eco-friendly polymer composites.