Theoretical study on the effect of oxidation states of phosphorus flame retardants on their mode of action

Polymers are ubiquitous and find applications in various facets of daily life and industrial production. However, the fire incidents induced by polymers jeopardize the safety of human life and property. Improving the fire resistance of polymeric materials through halogen-free flame retardants has become a prominent direction guided by the regulatory framework. In this paper, the decomposition pathways and mechanisms of five phosphorus-containing flame retardants, PPh3, DPPO, PDPP, TPPO and TPP, were investigated. The reaction pathways primarily encompassed the generation of PO, PO2, PO3, PO4, phenyl, and phenoxy radicals. Through a comprehensive analysis of the dissociation energies of P-C, P-O, and C-O bonds under diverse chemical environments, phenoxy radicals was the principal decomposition products, with the cleavage of P-O bonds being the predominant choice. Increasing phosphorus oxidation states favors the accumulation of a carbon layer, tending to retard flame propagation through condensed-phase mechanisms. Phosphorus oxidation states primarily facilitated secondary dissociation reactions. The presence of oxygen favored the cleavage of P-C bonds, offering diverse flame retardant mechanisms that concurrently mitigated flame spread. This work provided an in-depth exploration of the flame retardant mechanisms of phosphorus-containing agents and summarized the impact of phosphorus oxidation states on flame retardants.