Two novel phosphrous–nitrogen-containing flame retardants (3) and (4) were successfully synthesized from two steps: condensation reaction between 4-aminopenol and 1,4-phthaladehyde (or 4,4′-biphenyldicarboxaldehyde) and addition reaction between the above condensation compounds and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. The structures of (3) and (4) were characterized by Fourier transform infrared, nuclear magnetic resonance, and mass spectroscopy methods. The results show that (3) and (4) can serve as co-curing agents of diaminodiphenylmethane (DDM) for the curing reaction of epoxy resin. The epoxy thermosets-containing DDM and (3) or (4) exhibit moderate changes in glass transition temperature, thermal stability, and show improvement in flame retardancy: UL94 V-0 grade can be obtained when phosphorus content of the epoxy thermoset is only 1.0%; the limiting oxygen index of epoxy resin system containing (4) reaches 40.9 when phosphorus content is up to 1.5%.
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