Waste-Derived Oil Palm Mesocarp Fiber in LLDPE Biocomposites: Low-Loading Mechanical and Moisture Response

Waste-derived oil palm mesocarp fiber (OPMF) has potential as a lignocellulosic reinforcement for thermoplastic composites. The same trend in strength, stiffness, and moisture-related properties does not always hold at relatively low fiber loadings. This study examined these property changes in linear low-density polyethylene (LLDPE) biocomposites. Composites containing 0, 5, 10, and 15 wt% alkali-treated OPMF were produced by hot compression molding. Tensile and flexural properties, density, 24 h water absorption, and water contact angle were evaluated, supported by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The 5 wt% composite showed the highest mean tensile and flexural strengths of 16.16 ± 1.14 MPa and 29.33 ± 3.57 MPa, while the highest apparent tensile modulus (521.25 ± 38.84 MPa) and flexural modulus (0.888 ± 0.122 GPa) occurred at 10 wt%. Water absorption increased from 0.06 ± 0.01% to 1.19 ± 0.12%, while the contact angle decreased from 97.78 ± 2.04° to 65.18 ± 5.21°. Strength, stiffness, and moisture response followed different loading-dependent trends, indicating a composition-dependent strength–stiffness–moisture trade-off within the investigated range.