<![CDATA[The development of lightweight materials for Ocean Thermal Energy Conversion (OTEC) cold-water pipes requires an appropriate balance of mechanical strength and flexibility. This study investigated the mechanical properties of high-density polyethylene (HDPE) composites containing natural rubber (NR), polypropylene-graft-maleic anhydride (PP-g-MA), and palm oil boiler ash (POBA) to identify a promising formulation for further development. The composites were processed using a single-screw extruder at 60 rpm and subsequently fabricated by compression molding. We tested five specimens per composition for tensile properties and characterized fracture morphology and thermal behavior using scanning electron microscopy and thermogravimetric analysis, respectively. The HDPE/NR/PP-g-MA/POBA20 formulation exhibited the highest reported tensile strength and elastic modulus, reaching 18.93 MPa and 292.94 MPa, respectively, with an elongation at break of 101.5%. At higher filler loading, tensile strength decreased, potentially due to filler agglomeration, weak interfacial adhesion, and microvoid formation. These findings demonstrate that filler content influences the balance between strength, stiffness, and ductility. This study's novelty lies in evaluating boiler ash–filled polymer composites as potential materials for OTEC infrastructure. The results provide a basis for industrial waste valorization and further composite development. However, hydrostatic pressure testing, compressive evaluation, and long-term seawater durability studies are necessary before practical application can be established.]]>
