Performance Enhancement of PLA Hybrid Biocomposites Using Flax Fiber and Agricultural Waste Biofillers: A Comparative Study with Jute-Based Systems Supported by Fuzzy CRITIC–COPRAS Analysis

The development of high-performance, sustainable biocomposites requires biodegradable matrices and optimized natural reinforcements. In this study, flax fiber-reinforced polylactic acid (PLA) hybrid biocomposites incorporating waste pistachio nut shells (WPNS), waste tea leaf fiber (WTLF), and waste quail eggshell (WQES) were developed and evaluated, with direct comparison to previously reported jute-based hybrid systems to assess the benefits of fiber substitution. The composites were fabricated via compression molding and characterized for their mechanical, thermal, acoustic, surface, and moisture-related properties. Replacing the jute with flax resulted in a consistent performance enhancement. Among the hybrids, the flax–WPNS composite exhibited the highest tensile and flexural performance, achieving tensile strength improvements of approximately 30–40% over neat PLA due to effective stress transfer and crack deflection. The flax–WTLF composite showed superior acoustic behavior, attaining a maximum sound absorption coefficient of approximately 0.65–0.70 at mid-to-high frequencies, attributed to its porous microstructure. In contrast, the flax–WQES composite demonstrated the highest thermal conductivity (0.54 W/(mK)) and apparent density (2.24 g/cm3), reflecting dense packing and the presence of CaCO3-rich particles. Scanning electron microscopy revealed distinct microstructural mechanisms governing these property-specific responses, including differences in interfacial bonding, void distribution, and filler packing efficiency. An integrated fuzzy CRITIC–COPRAS multicriteria decision-making approach identified the flax–WPNS hybrid as the optimal overall formulation. The results clearly demonstrate that flax fibers outperform jute as a reinforcement in PLA-based hybrid biocomposites, and that targeted combinations of flax and waste-derived fillers enable multifunctional performance optimization for sustainable engineering applications.