Wood-Derived Carbon/Phosphorylated Chitosan Hybrid Fillers for High-Performance Thermal Insulation and Flame-Retardant PLA Composites

Sustainable polymer composites with enhanced thermal insulation and flame retardancy are highly desirable for safety-critical applications. In this study, wood was sequentially delignified, treated with KOH, and carbonized to produce hierarchical porous carbonized wood (C-WD), which was subsequently surface-modified with phosphorylated chitosan (PCS) to yield PCS-C-WD. These functional fillers were incorporated into poly(lactic acid) (PLA) via melt compounding. BET analysis confirmed the formation of hierarchical porosity in C-WD, with a surface area of approximately 468.5 m 2 /g, while PCS-C-WD maintained a mesoporous structure. The PLA/C-WD composites exhibited reduced thermal conductivity down to 0.109 W·m –1 ·K –1, which further decreased to 0.070 W·m –1 ·K –1 for PLA/PCS-C-WD, consistent with the Agari–Uno model. Flame-retardant testing demonstrated that PLA/PCS-C-WD8 achieved a UL-94 V-0 rating with a limiting oxygen index (LOI) of 31.7%, compared with 20.5% for neat PLA. Mechanical characterization revealed simultaneous improvements in tensile strength (51.4 MPa) and elongation at break (5.4%). This work presents a straightforward strategy to fabricate PLA composites with excellent thermal insulation, superior flame retardancy, and well-balanced mechanical properties using wood-derived porous carbon and PCS hybrid fillers.