Surface treatment of Alfa fibers to improve mechanical performance and matrix compatibility in sustainable bio-composites

Alfa fiber/unsaturated polyester resin (UPR) bio-composites were enhanced in this study using a dual chemical treatment consisting of sodium hydroxide (NaOH, 10%) and acetic acid (CH₃COOH, 20%). Fourier-transform infrared spectroscopy (FTIR) analysis indicated reduced hemicellulose bands with partial wax persistence, while scanning electron microscopy (SEM) revealed a roughened fiber surface. Composites (10–30 phr) were molded and post-cured at 45 °C. Relative to neat UPR (6.30 MPa), tensile strength peaked at 10.3 MPa at 15–20 phr ( ≈ + 63% vs. matrix), followed by a decline due to fiber agglomeration at higher loadings. Chemical treatment increased ductility, with elongation-at-break rising from 4.85% (10 phr, untreated) to 7.15% (10 phr, treated; ≈+47%). Unnotched Izod impact strength reached 2.5 kJ m⁻² at 20 phr, indicating optimal dispersion at intermediate contents. Treated composites exhibited lower water uptake than untreated ones at equal loading and showed increased Shore hardness with fiber addition. The results demonstrate that a simple alkali–acetylation sequence improves fiber/matrix compatibility and mechanical performance while defining processing windows (15–20 phr) that minimize clustering. This study brings something new by using a very simple dual treatment (NaOH + CH₃COOH) on Alfa fibers, which are still not much studied. The work shows that this approach can clearly improve the bond between fiber and resin, giving better tensile and impact strength. It also defines the best fiber loading range (15–20 phr), which can guide practical use of Alfa fibers in sustainable composites.