Eco-smart PLA/carbon fiber/brine-salt composites: a triple waste-valorization route toward high-performance thermal, acoustic, and fire-resistant materials

This study presents a novel eco-smart composite fabricated entirely from recycled PLA waste, reclaimed aerospace-grade carbon fibers, and multifunctional inorganic fillers recovered from desalination brine. Brine-derived salts were obtained via an electrocoagulation–modified Solvay hybrid process and incorporated into a PLA matrix containing 10 wt% carbon fibers and 1–5 wt% mineral fillers. Composites were produced through melt compounding and injection molding and evaluated for mechanical, thermal, acoustic, moisture, and fire performance. The optimized 85% PLA–10% CF–5% brine formulation achieved a thermal resistance of 5.59 m·K/W (thermal conductivity: 0.179 W/(m·K)), alongside an 88% reduction in water absorption (0.086%) compared with neat PLA. Fire performance improved significantly, reaching a UL-94 V-2 rating and a limiting oxygen index (LOI) of 26%. TGA revealed enhanced thermal stability and increased char yield, with residual mass rising from ~1.2 wt% for neat PLA to ~5.3 wt% at 700 °C. FTIR analysis confirmed filler incorporation primarily through physical interfacial interactions. Acoustic testing showed a peak absorption coefficient of 0.93 at 2 kHz. Tensile properties improved up to 3 wt% brine loading, beyond which filler agglomeration reduced uniformity. This triple-waste composite demonstrates a scalable circular-economy pathway for sustainable, multifunctional building materials.