A phosphaphenanthrene-based derivative as multifunctional flame retardant for epoxy resins

In this work, the transparent, mechanically strengthened, flame retardant and smoke suppression epoxy resin (EP) composites was prepared by the addition of an indole/phosphaphenanthrene-based compound (IDT). The results presented that limiting oxygen index (LOI) of EP composite containing 5 wt% IDT raised to 32% and it passed a UL-94 V-0 rating. In the meantime, compared with EP, total smoke production (TSP) and the peak of heat release (PHRR) of EP/IDT5 were decreased by 51.9% and 32.1%, respectively. The flame-retardant mechanism of EP/IDT was also explored by pyrolysis/gas chromatography–mass spectrometry (Py-GC/MS), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Besides, the bending and tensile strengths were improved thanks to the presence of π-π conjugation and hydrogen bonds between IDT and EP. This work provided a promising way to meet the increasing multifunctional requirements of EP composites.