When PET meets phosphorus flame retardants: A ReaxFF molecular dynamics study

Phosphorus-based flame retardants (P-FRs) are widely recognized as effective halogen-free additives for flammable thermoplastics, such as polyethylene terephthalate (PET), offering both strong flame resistance and relatively low toxicity. Due to its broad applications, it is crucial to pin down the degradation behavior of PET in the presence of P-FRs to enhance fire safety and polymer circularity. During conventional mechanical recycling, the underlying chemistry in the PET/P-FR materials may cause adverse reactions, leading to deteriorated mechanical properties of recycled products. This study employed reactive molecular dynamics (ReaxFF-MD) simulations based on reactive force field (ReaxFF) to explore the degradation of the PET containing two model P-FRs, specifically DOPO-PEPA (DP) and Aflammit PCO 900 (AF), at elevated temperatures. The predicted thermal behavior of PET was validated against experimental data, and the degradation mechanisms of PET were scrutinized through the analysis of degradation products, bonding evolution, and extensive trajectory analysis. The theoretically predicted thermal decomposition mechanisms of P-FRs were successfully verified by experiment, which is also consistent with existing research. Compared with DP, the molecule AF shows a retarded decomposition during the heat-up before a rapid fragmentation occurs, which can be attributed to its low-energy chair conformation; DP decomposes earlier due to the weaker C-O bond linkage and availability of protons via hydrogen abstraction. Our ReaxFF-MD simulations are based on quantum mechanical calculations and allow for an explicit investigation of the interactions between PET and P-FRs by including polymeric chains and additives in the same simulation. The reactions involving phosphorus species in the PET/P-FR were identified; notably DP fragment that can combine with the polymeric chain-end, as well as gasification effects from AF, which together aids in the comprehensive understanding of their different modes of action. In addition to the temperature effects, the oxidative conditions were included in this study to determine the thermo-oxidative degradation behavior. In this study, ReaxFF-MD simulations provide valuable insights into how thermal and thermo-oxidative degradation pathways evolve and control the fragmentation of PET and PET/P-FR systems. This methodology is proposed as a foundation for future research aimed at understanding complex reaction networks and improving the recycling quality of PET/P-FR materials.