Thermal Degradation of Organophosphorus Flame Retardants: Impact of Oxygenation-Level at Phosphorus on Mode of Degradation

The present study highlights the thermal Degradation of Organophosphorus Flame Retardants. The need to replace more conventional materials with ones that are more effective and have relatively low toxicity is driving the development of novel organophosphorus flame retardants for polymeric materials. These substances need to break down in a degrading polymer matrix in order to produce species that either encourage solid phase modification or produce active radical moieties that escape into the gas phase and halt processes that propagate combustion. An understanding of the decomposition process for these compounds may provide insight into the nature of flame retardant action which they may offer and suggest parameters for the synthesis of effective new organophosphorus flame retardants. The thermal degradation of a series of organophosphorus esters varying in the level of oxygenation at phosphorus—alkyl phosphate, aryl phosphate, phosphonate, phosphinate—has been examined. Initial degradation in all cases corresponds to the elimination of a phosphorus acid. However, the facility with which this occurs is strongly dependent on the level of oxygenation at phosphorus. For alkyl phosphates elimination occurs rapidly at relatively low temperature. Degradative elimination occurs much more readily for these compounds (high level of oxygenation at phosphorus) than for compounds containing low levels of oxygenation at phosphorus, phosphonate, and phosphinate. For aryl phosphates, the same procedure takes place at a slightly higher temperature. Phosphorous acid is removed from phosphonate or phosphinate at a temperature that is significantly higher and more slowly. Additionally, the acids created by removal break down quickly to produce new volatile species.