Controlling the Flammability of Polymeric Materials: Organophosphorus Flame Retardants—Synthesis, Structure, Toxicity, Mode of Action

The availability of polymeric materials has dramatically impacted societal development. However, in the main, these materials are highly flammable and for most applications must be flame‐retarded. This usually requires the presence of an additive to, by one means or another, reduce flammability to acceptable limits. While many substances have been used as flame retardants, the most effective have been organohalogen, particularly brominated aromatics, and organophosphorus compounds. With recognition of the hazards associated with the use of organohalogen compounds, organophosphorus flame retardants have become prominent. Both the toxicity and effectiveness of these materials are strongly dependent on structure. In particular, both seem to be largely dictated by the level of oxidation at phosphorus. Compounds with a high oxidation level at phosphorus (e.g., phosphates) undergo decomposition at relatively low temperature in a thermally degrading polymer matrix to form acidic substances which promote cationic crosslinking and char formation at the surface of the degrading polymer. Those with a low oxidation level at phosphorus (e.g., phosphonates) decompose at higher temperatures to generate PO radicals. Some of these escape reaction with components of the degrading polymer matrix and are evolved to the gas phase. There they efficiently scavenge combustion propagating radicals.