The effects of chemical structure for phosphorus-nitrogen flame retardants on flame retardant mechanisms

To evaluate the effect of chemical structure on flame-retardant mechanisms, two types of flame retardants (FRs) containing both phosphorus and nitrogen groups such as N-(2-methoxyethyl)-N-methyl-P,P-diphenylphosphinic amide (PNOFR) and 2-(dimethylamino)ethyl diphenylphosphinate (PONFR) were synthesized and applied to polyvinyl chloride (PVC). PNOFR and PONFR exhibited the typical flame-retardant mechanism of FRs containing phosphorus and nitrogen compounds. It was observed that PNOFR and PONFR have different main flame-retardant mechanisms depending on the element combined with phosphorus in their chemical structures. In the condensed phase, PNOFR induced a denser crosslinked char layer with a higher phosphorus content compared with PONFR. By contrast, in the gas phase, PONFR released more PO2∙ and PO∙ radicals compared with PNOFR. Therefore, while PNOFR and PONFR have flame-retardant properties through both mechanisms, the dominant mechanisms for PNOFR and PONFR are different. This study presents an effective strategy for designing attractive chemical structures to prepare FRs for applications in various polymers.