A pyrimidine-DOPO flame retardant improves the fire safety and toughness of epoxy thermosets

A phosphorus‑nitrogen additive denoted CTD was prepared from cytosine, terephthalaldehyde, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The resulting pyrimidine-containing compound was then introduced into epoxy resin (EP). CTD increased the flame resistance of EP while retaining favorable mechanical behavior. At 8 wt% CTD, the limiting oxygen index rose from 21.0% to 35.8%, equivalent to a 70.5% increase. This formulation also attained a UL-94 V-0 rating. Cone calorimetry gave a peak heat release rate of 562.3 kW m −2 and a total heat release of 33.08 MJ m −2 . These values were 44.9% and 39.8% below those of neat EP, respectively. The post-test residue simultaneously increased from 22.40 wt% to 39.90 wt%, supporting the enhanced carbonization induced by CTD. These findings identify CTD as an effective flame retardant for epoxy composites that combine improved fire safety with mechanical toughness.