<i>Posidonia oceanica</i> Leaves as a Natural Filler for Poly(Butylene Succinate‐Co‐Adipate) Composites: Characterization and Biodegradation Assessment in Seawater

ABSTRACT This study investigates the use of Posidonia oceanica ( PO ) leaves, derived from coastal banquette residues, as a filler in PBSA, aiming to valorise a natural biomass typically treated as waste within a circular materials approach, while reducing production costs and enhancing seawater biodegradation of the polymer matrix. Composites containing 5 and 10 wt.% of PO, with and without micro‐talc, were prepared via extrusion and injection molding. Materials were characterized morphologically, chemically, thermally, and mechanically, while biodegradation was assessed in a controlled seawater environment. Thermal analyses confirm that both PO and micro‐talc do not compromise PBSA thermal stability, with degradation temperatures remaining at approximately 403°C. Calorimetric results highlighted an increment of crystallization temperature and the appearance of a secondary melting peak at ~79°C, while the main PBSA melting peak remains at ~85°C and overall crystallinity degree is unchanged. Chemical and morphological analyses reveal no chemical interactions and confirm homogeneous filler dispersion with good interfacial adhesion. Mechanical testing shows a moderate stiffening and embrittling effect with the addition of PO, with elastic modulus increasing from 0.32 to 0.51 GPa and elongation at break decreasing from 450% to 325%, while impact toughness is markedly reduced at high filler loadings. Seawater exposure demonstrates accelerated biodegradation for Posidonia‐filled composites, showing a ~20% mass loss after 4 months and rapid mechanical deterioration. The incorporation of PO fillers constitutes an effective approach to modulate and accelerate degradation under marine conditions.