Effect of phosphorus on soot formation and flame retardancy in fires

Abstract The mechanism by which phosphorus imparts flame retardancy to a burning polymeric solid remains controversial because of the complex coupling of gas and condensed phase reactions. To address that, an organophosphorous compound was mixed into epoxy at different concentrations and burned under diffusion flame conditions. The yield of carbon monoxide and soot was found to increase in the presence of phosphorus, significantly reducing the combustion efficiency. The reduction in heat release was attributed to a heterogeneous mechanism that promotes soot formation. Phosphorus does not exhibit a noticeable effect on soot morphology. Element analysis reveals that 75% of phosphorus in the virgin sample is incorporated into soot particles. More aromatic structures were identified under the influence of phosphorus. Soot surface reactivity was found to decrease as phosphorus loading increases. The main flame retardancy mechanism of phosphorus is through promoting soot formation in the gas phase. It is speculated that it increases direct condensation of aromatic structures in the soot formation path and decreases the number of the active sites on the particle surface for soot oxidation. The increased soot yield cools the flame by radiative heat loss, further hindering the dehydrogenation process that converts nascent soot to mature soot.