Enhancing the mechanical performance of sustainable hemp–bamboo hybrid epoxy composites reinforced with microcrystalline cellulose

This study examines the effect of microcrystalline cellulose (µCC) on the mechanical behavior and fracture characteristics of an epoxy composite at filler loadings of 0, 3, 6, and 9 wt%. Incorporation of µCC resulted in significant improvements across all measured properties compared to the neat epoxy, including tensile strength, flexural strength, modulus, hardness, and impact energy. The highest absolute mechanical values were recorded at 9 wt% µCC; however, the additional gains relative to 6 wt% were modest, with increases of only ∼2% in tensile strength, ∼2–5% in flexural properties, ∼1% in hardness, and ∼7% in impact toughness. Scanning electron microscope (SEM) analysis revealed uniform dispersion and strong matrix–particle interfacial bonding at 3 and 6 wt%, while slight agglomeration and microvoid formation became evident at 9 wt%, potentially acting as stress concentrators. Considering both performance enhancement and microstructural stability, 6 wt% µCC offers the most favorable balance, providing near-maximum mechanical improvements without the drawbacks associated with higher filler concentrations. These findings demonstrate that controlled µCC incorporation can significantly strengthen epoxy composites, with 6 wt% identified as the practical optimal loading for achieving enhanced stiffness, strength, and toughness while maintaining effective dispersion.