Mechanically Superior HDPE Composites Reinforced with Alkali-Treated Saccharum Munja Fibers for Sustainable Engineering Applications

Abstract The global transition towards sustainable materials demands high-performance composites derived from naturally abundant resources. Although natural fiber-reinforced polymers have gained attention, the development of scalable, industry-ready alternatives to synthetic fillers remains limited. To the best of our knowledge, this work presents the first systematic investigation of Saccharum munja-reinforced HDPE composites, an under-utilized natural fiber that grows abundantly in rural regions. This study evaluates injection-molded HDPE composites with untreated and alkali-treated munja fibers at 5, 10, and 20 wt % loadings. Alkali treatment removes surface impurities to improve fiber−matrix adhesion, dispersion, and interfacial bonding. Comprehensive characterizationsFTIR (Fourier transform infrared spectroscopy), CHNS (carbon−hydrogen−nitrogen−sulfur elemental analysis), XRD (X-ray diffraction), DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), SEM (scanning electron microscopy), FDXM (four-dimensional X-ray microscopy), rheology, and thermal expansion measurementswere conducted to correlate fiber introduction, modification, and content with composite structure and properties. The results show that alkali treatment increases the crystallinity and thermal stability of the munja fibers. In HDPE composites, fiber loading up to 20 wt % improves tensile modulus and strength by up to ∼45% and ∼24%, respectively, compared to pristine HDPE, with treated fiber composites outperforming untreated counterparts at equivalent loadings due to reduced voids and improved dispersion (FDXM). Rheological measurements show that alkali-treated fiber composites with higher fiber loading exhibit increased complex viscosity, particularly at lower oscillation strains, due to enhanced network formation; however, complex viscosity decreases at higher strains because of progressive network breakdown, reflecting improved melt processability and stability during processing. Fractography confirms improved interfacial bonding and enhanced stress transfer in treated-fiber composites, with increased fiber breakage and reduced interfacial debonding versus untreated composites, particularly at 10−20 wt % fiber loadings. Overall, results demonstrate that surface modification of munja fibers is a viable route to tailor HDPE-based natural fiber composites toward higher stiffness, strength, and dimensional stability while maintaining processability.