Eggshell‐Derived Surface‐Modified Hydroxyapatite as a Sustainable Filler for Styrene–Butadiene Rubber Composites

ABSTRACT This study demonstrates the conversion of eggshell waste into hydroxyapatite (HAP) as a sustainable bio‐derived reinforcing filler for styrene–butadiene rubber (SBR) composites. HAP was synthesized from eggshell‐derived calcium oxide and surface‐modified by ball milling with polyethylene glycol (PEG, P‐HAP) or bis‐(3‐triethoxysilylpropyl) tetrasulfide (TESPT, T‐HAP) to improve filler dispersion and interfacial compatibility. Unmodified and modified HAP fillers were incorporated into SBR at 10–40 phr, and their effects on rubber–filler interaction, curing behavior, and mechanical performance were investigated. Bound rubber content increased with HAP loading, indicating enhanced interaction between HAP and SBR. T‐HAP showed the highest bound rubber content, followed by P‐HAP and unmodified HAP, suggesting the beneficial role of TESPT in promoting interfacial adhesion. HAP incorporation reduced scorch time and optimum cure time, indicating its possible contribution to sulfur vulcanization. The torque difference increased after HAP addition, particularly in the modified systems, suggesting increased compound stiffness, restricted rubber chain mobility, and improved rubber–filler interaction within the SBR matrix. Mechanical properties improved with filler loading. At 40 phr, tensile strength increased from 15.8 MPa for unfilled SBR to 19.4, 20.8, and 24.2 MPa for HAP‐, P‐HAP‐, and T‐HAP‐filled composites, respectively. These findings highlight TESPT‐modified eggshell‐derived HAP as an effective sustainable filler for reinforcing SBR composites.