A primary component in aqueous fire-fighting film-forming foams (AFFF) are per- and polyfluoroalkyl substances (PFAS), the addition of which significantly enhances the ability of extinguishing hydrocarbon fires. Despite their robustness in high temperature applications, PFAS are known to cause adverse health effects, be resistant to remediation, and are global environmental contaminants. Therefore, it is important to devise effective PFAS alternatives for AFFF applications that are easier to degrade in the environment. However, although there exist effective fluorine-free surfactants as lower tier flame retardants, there are no known surfactants which are free of fluorine and exhibit the same behavior and properties as PFAS for higher tier military-grade firefighting foams. Promising formulations containing trimethylsiloxanes (TriSil) in tandem with polyethylene glycol (PEG) are fluorine-free and have had limited success extinguishing heptane pool flames; however, the degradation mechanisms of these surfactants at high temperatures are not well understood. We investigated the thermal decomposition of a truncated TriSil-PEG using density functional theory (DFT) by calculating unimolecular and bimolecular reaction pathways at 298 K and 1200 K. Formation and propagation of radicals from both pathways occurred along the PEG tail and all end products resulted in small organic molecules (such as ethylene, formaldehyde, and acetaldehyde) as well as thermally stable polydimethylsiloxanes (PDMS). The products generated would be more environmentally friendly than those produced by the incineration or combustion of PFAS shown by previous research, encouraging the use of trimethylsiloxanes. Our work aids in fundamentally understanding the degradation mechanism of alternative, fluorine-free trimethylsiloxane surfactants for high temperature applications to facilitate the replacement of PFAS in AFFF.

High temperature decomposition chemistry of trimethylsiloxane surfactants

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