Hydrothermal Durability of Microwave‐Processed Seashell‐Filled <scp>GFRP</scp> Composites: Diffusion, Microstructure, and Mechanical Performance

ABSTRACT This study investigates the hydrothermal durability of microwave‐cured glass fiber‐reinforced polymer composites (GFRPCs) incorporating seashell powder (SSP) as a sustainable filler. Composites containing 1, 1.5, 2, and 2.5 wt.% SSP were fabricated using the vacuum‐assisted resin infusion microwave curing (VARIMC) technique. Accelerated hydrothermal aging was performed in artificial seawater at 70°C until moisture saturation. Moisture uptake followed predominantly Fickian diffusion behavior; the optimized 1.5 wt.% SSP composite exhibited the lowest moisture uptake (1.7%) and diffusion coefficient (3.1 × 10 −12 m 2 /s), indicating improved resistance to moisture ingress. Tensile, interlaminar shear strength (ILSS), Izod impact, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and X‐ray micro‐computed tomography (micro‐CT) analyses were performed on both unaged and hydrothermally aged specimens. Mechanical evaluation showed a reduction in tensile strength of 51%–59%, whereas ILSS and impact strength were retained up to 93% after aging. DMA results showed that the optimized composite (1.5 wt.% SSP) retained 93% of its stiffness and exhibited a slight increase in T g (1.6°C), indicating improved thermal stability and restricted molecular mobility. Micro‐CT analysis confirmed enhanced structural integrity and reduced interfacial degradation at the optimum filler content. Overall, microwave‐assisted processing with SSP improves the hydrothermal durability of GFRPCs and provides a sustainable route for developing high‐performance marine composites.