Upcycling <scp>PET</scp> and Eggshell Waste Into <scp>BHET</scp> ‐Based Unsaturated Polyester/Hydroxyapatite Composites

ABSTRACT This study reports the preparation and characterization of composites based on unsaturated polyester (UP) resins derived from post‐consumer PET and reinforced with hydroxyapatite (Hap) obtained from eggshell waste. Hap was synthesized via calcination and phosphate reaction, exhibiting characteristic FTIR and XRD features and a Ca/P ratio indicative of Ca‐rich phases. BHET, a PET glycolysis product, was used as a monomeric precursor to synthesize UPs with 0, 15, and 30 wt% BHET content. These resins showed similar molecular weights and conversions, with nuclear magnetic resonance confirming BHET incorporation and revealing its impact on side reactions. All UPs displayed Newtonian behavior when dissolved in N‐vinylpyrrolidone (VP), indicating good processability. UP/VP/HAp pastes cured with benzoyl peroxide exhibited typical exothermic profiles, and the curing rate decreased slightly at higher BHET content, resulting in maximum temperatures of 71°C–76°C and setting times of 200–260 s. FTIR analysis showed ~89% overall double bond conversion. Thermal analysis revealed low glass transition temperatures (60°C–64°C), while mechanical testing showed composites with tunable properties: elongation at fracture ranging from 5.7% to 9.4%, fracture strength remained constant (~1.6–1.7 MPa), and compressive strength varied from 3.6 to 7.0 MPa. These findings demonstrate the feasibility of integrating PET and eggshell waste into composite materials for in situ curing applications; however, further optimization of setting time and mechanical properties is required to enhance their potential for biomedical use.