Fully Biobased Shellac/Latex Composites Reinforced with Modified Posidonia Oceanica Fibers: Mechanical and Acoustic Performance with Micromechanical Modeling

Fully biobased composites combining renewable matrices and marine biomass-derived reinforcements offer a sustainable route toward multifunctional materials with tailored mechanical and acoustic properties. In this study, fully biobased composites were fabricated using a shellac/natural rubber latex matrix reinforced with marine biomass-derived Posidonia oceanica (PO) waste fibers. To improve interfacial compatibility, fibers were modified with succinic acid, a bio-based platform chemical, under ultrasonic treatment, providing a green and efficient surface functionalization strategy for fully biobased composites. Tensile testing showed that at 0.5% fiber content, elastic modulus (4.64 MPa), tensile strength (0.55 MPa), and elongation at break (255%) all improved compared with the neat matrix (3.59 MPa, 0.48 MPa, 196%). At 1%, stiffness further increased (4.72 MPa), but strength and ductility decreased due to fiber agglomeration, while 2% yielded maximum elongation (286%) despite reduced stiffness. Acoustic characterization revealed superior absorption at intermediate loadings, with coefficients of 0.41–0.44 at 6300 Hz, and a maximum of 0.52 at 2%. Micromechanical modeling performed via finite element analysis supported these findings, confirming stress concentration transitions from the matrix to the fiber-matrix interface with increasing fiber content and validating the experimental tensile results with less than 7% deviation. Among the investigated compositions, composites containing 0.5–1 wt % PO fibers exhibited the most balanced combination of stiffness, ductility, and acoustic absorption, demonstrating the potential of ultrasonically succinic acid-modified marine-derived fibers for developing fully biobased lightweight materials with multifunctional performance.