Flame retardant flexible polyurethane foams from novel DOPO-phosphonamidate additives

In this work we report synthesis and flame retardant application of novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) based phosphonamidates. Two mono and one bis DOPO-phosphonamidates were synthesized and incorporated in polyether based polyurethane (PU) manufacturing process. For the comparison of fire performance of these DOPO-phosphonamidates, we have chosen commercially available flame retardants (TCPP, DOPO and Exolit® OP 560) commonly used in flexible PU foam manufacturing. UL94 HBF fire results of various PU foam formulations indicate that DOPO-phosphonamidates exhibit superior fire performance as compared to the commercial flame retardants. 6,6′-(ethane-1,2-diylbis-(azanediyl)-bis-(6H-dibenzo[c,e][1,2]-oxaphosphine-6-oxide (EDAB-DOPO))/foam formulations exhibit the best fire performance as compared to other DOPO-phosphonamidate/foam formulations. A concentration of only 5% EDAB-DOPO on wt. of polyol is needed to achieve a HF1 rating. Thermal decomposition studies of the DOPO-phosphonamidate/foam formulations indicate their limited condensed phase interaction. In TGA experiments, a very small residue (<5%) was observed at 800 °C for all PU formulations. Additionally EDAB-DOPO showed evidence of intermediate condensed phase interaction in the first stage of PU foam decomposition. Direct insertion probe (DIP) - MS studies indicate that mono DOPO-phosphonamidates volatilize primarily in the first stage of thermal decomposition of PU foams whereas the EDAB-DOPO, being thermally more stable is only detected in gas phase in the second stage. Like in case of TCPP/PU foam formulation, the increase in the CO/CO2 ratio in the cone calorimeter experiments for the DOPO-phosphonamidate/foam formulation further proves the gas phase activity (flame inhibition) of these DOPO-phosphonamidates. EDAB-DOPO being thermally more stable seems to offer more sustained gas phase activity during the entire burning stage of PU foams in cone calorimeter experiments. Higher thermal stability of EDAB-DOPO may explain its superior flame retardant efficacy among all the DOPO-phosphonamidates investigated in this study.