Binderless Biocomposite Boards from Tomato and Eggplant Stalks: Synergistic Effects of Fiber Ratio and Structural Design

Agricultural waste is a renewable yet underutilized resource for producing binderless biocomposites. This study develops biocomposite boards from blended eggplant (ESP) and tomato (TSP) stalk fibers using single-material boards as references. Homogeneous and sandwich structures were fabricated at identical fiber ratios (75:25, 50:50, and 25:75) to distinguish compositional effects from structural contributions and reveal synergistic interactions. The optimal ESP/TSP ratio of 25:75 yielded a bending strength of 80.59 MPa in homogeneous boards and a tensile strength of 41.11 MPa in sandwich boards, which also showed superior water resistance. The 75:25 homogeneous board exhibited an initial water contact angle of 136.46°, with all samples maintaining a hydrophobicity of >100° after 10 s. Thermal conductivities of 0.0886–0.1076 W·m–1·K–1 indicated excellent insulation performance, and combustion tests demonstrated controllable burning behavior with potential for end-of-life energy recovery. The sandwich-structured biocomposite boards exhibited significantly enhanced tensile strength and water resistance due to effective interlayer synergy. Overall, synergistic optimization of the fiber ratio and structural configuration enabled high-performance adhesive-free biocomposites and provided a sustainable route for converting agricultural waste into functional materials for architectural and furniture manufacturing applications.