Four comparable phosphorus-containing compounds were synthesised, each with one central P atom and three hexenyl groups: an amide (P bonded to 3xN), a diamidate (P bonded to 2xN, 1xO), an amidate (P bonded to 1xN, 2xO) and a phosphate (P bonded to 3xO). These were tested for thermal stability and as FRs at 10% loading in an epoxy resin (DGEBA/DMC). The FR performance at 10% achieved only UL94-HB (4 mm) but were comparable to BDP at the same loading. The results demonstrated different mechanisms: the phosphate (P-O bonds) showed more gas phase action, whereas the amide (C-N bonds) showed more condensed action, and optimal FR performance resulted from combinations of these. pinfa comment: further work would be useful, in that only 4 molecules of similar structure is a small sample on which to base general conclusions, and it would also be important to study the potential toxicity / ecotoxicity of the different P-N structures.
“Systematically Controlled Decomposition Mechanism in Phosphorus Flame Retardants by Precise Molecular Architecture: P−O vs P−N”, J. Markwart et al., ACS Appl. Polym. Mater. 2019, 1, 1118−1128 http://dx.doi.org/10.1021/acsapm.9b00129