Waste Oyster Shell‐Derived CaO ‐Rich Powder as a Multifunctional Filler for Polyamide 6: Interfacial Reinforcement, Moisture‐ and Gas‐Barrier Regulation, Thermal Stability, and Antibacterial Activity

Calcium‐rich biowastes offer low‐cost functional fillers for engineering polymers, but their use in polyamide 6 requires balanced control of interfacial contact, morphology, moisture response, transport behavior, crystallization, and thermal stability. Waste oyster shells were calcined at 1000°C to obtain CaO‐rich thermally treated oyster shell powder (TOS), which was incorporated into polyamide 6 by melt blending and compression molding at 5–25 wt%. Comparative characterization against commercial CaO confirmed that TOS contained 98.70 mass% CaO on an oxide‐equivalent basis and retained a CaO/Ca(OH) 2 ‐rich mineral structure with minor Mg‐containing species. The composites showed loading‐dependent mechanical, morphological, moisture‐related, oxygen‐barrier, thermal, and antibacterial responses. The 10 wt% composite gave the best overall balance, increasing tensile strength from 25.0 to 30.5 MPa while maintaining compact fracture morphology, controlled surface roughness, and the lowest moisture content and water absorption. Surface microscopy and elemental mapping confirmed the distribution of Ca‐rich domains, whereas excessive TOS caused particle exposure, surface heterogeneity, and local defects. The oxygen transmission coefficient decreased at moderate TOS contents but increased at 25 wt% because of defect‐assisted transport. Thermal and crystalline analyses confirmed crystallization regulation and improved early weight‐loss resistance. The 10 wt% composite showed antibacterial efficiencies of 95.45% ± 1.70% against Escherichia coli and 89.41% ± 1.46% against Staphylococcus aureus . A screening cradle‐to‐gate assessment estimated a 7.7% material‐level reduction in global warming potential, linking waste valorization to verified multifunctional performance.