Biomaterial-Based Cu2O@Chitosan/Phosphorylated Cellulose Core–Shell Flame Retardant Fabricated via Layer-by-Layer Assembly for Enhanced Fire Safety of Epoxy Resin

Abstract Epoxy resins (EPs) are widely used in advanced engineering applications but suffer from inherent flammability and rapid heat release during combustion. A biomaterial-based core–shell flame retardant, Cu2O@(CH/PT), has been fabricated using a mild layer-by-layer (LbL) assembly strategy by alternately depositing chitosan (CH) and phosphorylated cellulose (PT) onto Cu2O particles. Scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) analyses provided complementary morphological and chemical evidence for the surface deposition and assembly of CH and PT on Cu2O particles. The resulting Cu2O@(CH/PT) was incorporated into epoxy resin blends to investigate its effects on thermal degradation and combustion behavior. The incorporation of 8 wt % Cu2O@(CH/PT) increased the residual mass of the EP blends at 700 °C during thermal degradation from 7.79% to 20.62% and increased the limiting oxygen index (LOI) for combustion from 23.3% to 31.0%. Microscale combustion calorimetry results showed that the peak heat release rate (PHRR), total heat release (THR), and heat release capacity (HRC) for combustion of EP-8 were reduced by 45.1%, 33.4%, and 44.0%, respectively, compared with the corresponding values for combustion of neat EP. The enhanced flame-retardant performance may be associated with the cooperative effects of the Cu2O catalytic core and the biomaterial-based phosphorus/nitrogen-containing CH/PT shell, where PT promotes condensed-phase charring, CH contributes to the development of protective char structures, and Cu2O facilitates the stabilization of carbonaceous residues. A sustainable strategy for constructing biomaterial-based core–shell Cu2O flame retardants through LbL assembly to improve the fire safety of epoxy resin blends has been developed.