Property relationships study of sugar palm fiber-reinforced recycled HDPE bottle-cap composites

Post-consumer high-density polyethylene (rHDPE) bottle caps represent an abundant waste stream with potential for use in semi-structural composite applications, although their mechanical performance remains limited. This study investigated the feasibility of reinforcing recycled HDPE bottle-cap composites with alkali-treated sugar palm (Arenga pinnata) fiber at loadings of 0, 10, 15, and 20 wt%. The composites were fabricated by manual melt mixing followed by hot pressing and evaluated in terms of density, 24 h water absorption, tensile, flexural (dry and water-conditioned), and Charpy impact properties, together with FTIR, DSC, and SEM characterization. The density increased from 0.949 to 0.983 g/cm3 with increasing fiber content, while the tensile strength improved markedly from 11.52 MPa for neat rHDPE to 20.92 MPa at 20 wt% fiber. Water absorption increased from 0.00% to 1.16% as fiber loading increased. The impact strength reached its maximum at 15 wt% (15.75 kJ/m2) before decreasing at 20 wt%, indicating an optimum fiber content for toughness. Under water-conditioned flexural testing, composites with 10–15 wt% fiber exhibited better wet/dry strength retention (63.25–63.35%) than neat rHDPE (52.16%), whereas 20 wt% fiber resulted in reduced retention. FTIR confirmed the presence of lignocellulosic constituents in the composite, while DSC showed only minor variation in melting temperature, suggesting that the mechanical response was governed mainly by reinforcement efficiency and microstructural features. Overall, 10–15 wt% alkali-treated sugar palm fiber provided the best balance of impact resistance and wet-strength retention, demonstrating a practical route for upgrading recycled HDPE bottle caps into lightweight engineering composites.