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 1h 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 24s), 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%).