Cellulose-based nanohybrids enabling flame retardancy with mechanical reinforcement in poly(lactide) composites

Cellulose nanofibrils (CNFs) were used as a scaffold to construct CNF nanohybrids for simultaneous mechanical reinforcement and flame retardancy in poly(lactide) (PLA). A one-pot sol-gel route organized a silica framework, phosphate groups, and urea-derived nitrogen functionalities around the CNF scaffold, producing a structurally coupled organic-inorganic interphase. At optimized nanohybrid loadings, the PLA composites showed increased tensile and flexural strength while achieving UL-94 V-0 classification with effective suppression of melt dripping. Thermal and combustion analyses indicate a shift from volatile-dominated PLA depolymerization toward a condensed-phase-dominated degradation pathway. TG-IR showed reduced and delayed evolution of combustible volatile species. Cone calorimetry, post-combustion SEM-EDS, XPS, and Raman analysis support the formation of a silica-reinforced, heteroatom-rich char layer that acts as a thermal and mass-transfer barrier. These results show that CNF-templated silica-phosphate architectures can couple carbohydrate-based reinforcement with condensed-phase flame retardancy in fire-safe PLA composites.