<![CDATA[Recycled high-density polyethylene (rHDPE) offers a sustainable alternative to virgin plastics, but repeated processing can degrade its mechanical properties, motivating reinforcement with natural fibers and fillers. This study investigated the combined effect of coir fiber and rice husk ash (RHA) on the mechanical and burning-rate characteristics of rHDPE composites using a two-stage experimental design. In Stage 1, coir fiber content (5, 10, and 15 vol.%) was varied to determine the optimum reinforcement level based on impact strength. In Stage 2, the selected optimum fiber content was combined with RHA (5 and 10 vol.%) to evaluate tensile strength and burning behavior, tested according to ASTM D638, ISO 179, and ASTM D635, respectively. Results showed that 10 vol.% coir fiber produced the highest impact strength (16.36 kJ/m²), while higher fiber loading reduced toughness due to agglomeration and void formation. The composite with 5 vol.% RHA exhibited superior tensile strength (10.46 ± 1.23 MPa), whereas 10 vol.% RHA improved ignition resistance, extending ignition time to 14 s. These findings demonstrate that alkali-treated coir fiber and RHA can enhance rHDPE performance, supporting the valorization of plastic and agricultural waste into sustainable composite materials.]]>
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