Preparation and synergistic effect of aluminum hydroxide nanoplates on the fire resistance and thermal stability of the intumescent flame retardant epoxy composite

Aluminum hydroxide nanoplates (nATH), with an average particle size of about 350–450 nm and a thickness of 30 nm, were successfully synthesized through a hydrothermal process using an Al(OH)₃ gel precursor. The ATH nanoplates were then surface-treated with organic compounds and incorporated into an intumescent flame-retardant epoxy system containing polyethyleneimine-modified ammonium polyphosphate (APP@PEI). Among them, the combination of APP@PEI and PEI-treated nATH (nATH_PEI) exhibited the highest synergistic effect on the fire resistance and thermal stability of epoxy resin due to the superior dispersion of the nanoplates. Additionally, the optimal mass ratio of two flame retardant additives was examined. As a result, a nanocomposite containing 3 wt% nATH_PEI and 7 wt% APP@PEI exhibited the best flame resistance and thermal-oxidative stability. This nanocomposite reached a V-0 rating in the UL-94 vertical burning test with a high limiting oxygen index value of 31.1%, and a substantial char yield of 17.98% at 900 °C. Char residues of samples were analyzed by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy coupled with energy dispersive X-ray analysis to investigate the reasonable flame retardant mechanism. The results demonstrated that the formation of highly thermally stable aluminum phosphates played an important role in the augmentation of the flame retardancy in the condensed phase. Furthermore, tension and Izod impact tests indicated that the presence of nATH_PEI notably increased the mechanical properties of composite loading APP@PEI. This combination provides a promising approach for intumescent flame retardant applications in epoxy resin.