Thermal decomposition mechanisms of phosphorus flame retardants: A combined theoretical and experimental approach

• Thermal decomposition pathways of two phosphorus flame retardants are summarized. • A combination of DFT calculations and DIP-MS analysis cross-validates the findings. • Bond dissociation and protonation are identified as the major thermal decomposition paths. • Stabilizing mechanisms are considered to investigate the possibility of reducing the high reaction energies of bond dissociation. Phosphorus flame retardants (PFRs) are one of the main candidates for a fully organic, sustainable replacement to traditional halogenated FRs. However, their unexpected decomposition remains an open challenge hindering their full potential for sustainable applications in polymers. To address this issue, thermal decomposition of two common PFRs, namely 6H-Dibenzc,e1,2oxaphosphorin, 6-(1-oxido-2,6,7-trioxa-1-phospha bicyclo[2.2.2oct-4-yl)methoxy]-, 6-oxide (DOPO-PEPA) and Aflammit PCO 900 (AF), was studied using density functional theory (DFT) coupled with experimental methods, particularly the direct inlet probe-mass spectrometry (DIP-MS). The DIP-MS spectra were processed and analyzed using algorithms to identify potential decomposition products of DOPO-PEPA and AF. Under inert atmospheric conditions, bond dissociation and proton attack were identified as the predominant decomposition pathways. Geometries of intermediates, transition states, and products along potential energy surfaces were identified through DFT calculations. For DOPO-PEPA, the dissociation of the C-O bond linking the DOPO and PEPA moieties was identified as the most kinetically favored dissociation pathway; while for AF, bond dissociation was found energetically demanding. On the other hand, protonation processes demonstrate more dependence on the availability of protons. Cross-validating computational results with experimental observations verified the pathways through which DOPO-PEPA and AF release phosphorus-containing species and other decomposition products. Benefitting from a combination of fundamental molecular models and experimental evidence, this study provides new insights into the molecular mechanisms of thermal degradation of PFRs, thus providing a template for developing new FRs with enhanced thermal stability.