A Nitrogen-Rich DOPO-Based Derivate for Increasing Fire Resistance of Epoxy Resin with Comparable Transparency

To endow synergistically epoxy resin (EP) with excellent fire resistance and high optical transparency, a nitrogen-rich DOPO-based derivate (named as FATP) was synthesized and incorporated into EP. It showed that the incorporation of the FATP reduced the fire hazard of the EP, as demonstrated by the fact that the EP/4% FATP blends gained a UL-94 V-0 rating and an LOI value of 35%, with the lowest values of the THR (86.7 MJ/m²), the PHRR (1059.3 kW/m²), and the TSP (89.6 MJ/m²). The presence of the FATP also reduced the thermal stability and the crosslinking density whilst improving the curing reaction and the storage modulus of the EP/FATP blends. The TG-FTIR spectra showed that •HPO/•PO free radicals and some nonflammable gases (HN₃ and NH₃) were produced during the pyrolysis, and the characterization (SEM, Raman spectroscopy, and XPS) of char residues confirmed that the FATP facilitated the formation of continuous and compact carbon layers of greater graphitization degree. It was thus concluded that the FATP played the flame-retardant roles in both the gas and condensed phases. Furthermore, the FREPs kept almost identical transparency as the pristine EP, and mechanical properties were also slightly enhanced. The FREPs presented in this work show promising applications in the fields of advanced optical technology.