Phosphorus-based α-amino acid mimetic for enhanced flame-retardant properties in an epoxy resin

This work demonstrates the introduction of a phosphonate moiety into a commonly used curing agent, 4,4′-diaminodiphenylmethane (DDM), via an α-aminophosphonate. This compound (DDMP) can be prepared and isolated in analytical purity in under 1 h and in good yield (71 %). Thermoset polymer (epoxy-derived) samples were prepared using a room-temperature standard cure (SC) and a post-cured (PC) protocol to encourage incorporation of the α-aminophosphonate into the polymer network, with improved flammability properties observed for the latter. Thermogravimetric analysis under a nitrogen atmosphere showed increased char yield at 600°C, and similar observations were made when analysis was conducted in air. Significant reductions in flammability are observed at very low phosphorus content (P% = 0.16-0.49 %), demonstrated by higher char yields (25.5 from 14.0 % in air), decreased burn time from ignition (60 to 24 s), and decreased mass loss after ignition (87.6 to 58.5 %). Limiting Oxygen Index for the neat polymer (P% = 0 %, 20.3 ± 0.8 %) increased with increasing α-aminophosphonate additive (P% = 0.16 %, 20.8 ± 0.6 %; P% = 0.32 %, 21.4 ± 0.4 %; P% = 0.49 %, 22.6 ± 0.8 %).