Synergistic Flame Retardancy of Epoxy Resin with Aminated Multi-Walled Carbon Nanotubes and Ammonium Polyphosphate

Epoxy resins have been extensively applied in aerospace and automotive fields. Nevertheless, their inherent flammability significantly restricts broader applications. In this study, carboxylated multi-walled carbon nanotubes (COOH-MWCNTs) were first aminated to obtain aminated Multi-Walled Carbon Nanotubes (NH2-MWCNTs). Subsequently, NH2-MWCNTs and ammonium polyphosphate (APP) were incorporated into the epoxy resin via mechanical stirring, thereby constructing a phosphorus–carbon synergistic flame-retardant system. Compared with the neat epoxy thermoset, the EP/17.5APP/0.1NH2-MWCNTs composite showed a limiting oxygen index (LOI) value of 29.6% and attained a UL-94 V-0 rating. In addition, for the modified composite material, the maximum thermal decomposition rate (RTmax) is 12.4 wt%/min, the char residue at 600 °C (C600) reaches 44.2%, and the smoke density is 425.8. The impact strength and tensile modulus are increased to 10.1 Mpa and 3.0 Gpa, respectively, while the compressive strength remains essentially unchanged. Furthermore, the synergistic flame-retardant mechanism between phosphorus and carbon was investigated by analyzing the char residues of the epoxy resin and its composites. This study offers a promising approach for designing epoxy composites with improved flame retardancy and enhanced thermal stability for high fire-safety applications, such as electronic encapsulation and structural materials.