Characterizing the Role of Fluorocarbon and Hydrocarbon Surfactants in Firefighting-Foam Formulations for Fire-Suppression

Fluorosurfactants used in current firefighting foams must be replaced with environmentally-friendly surfactants; however, current fluorine-free surfactants have subpar fire performance. Understanding how a surfactant affects fire performance of a foam is essential for developing new fluorine-free replacement surfactants. Surfactants can affect foam’s ability to form a barrier between a fuel-pool surface and the fire, thereby suppressing the fuel feeding the fire and extinguishing it. Fire extinction performance, fuel transport through a foam layer, foam degradation by fuel, and foam spread rate were measured for solutions containing a mixture of fluorocarbon and hydrocarbon surfactants as well as the individual surfactants to understand the roles of different surfactants in a formulation. The measurements show that surfactants that reduce foam degradation and fuel transport rates through a foam covering a hot pool result in faster fire extinction. Small synergistic effects for the mixture of fluorocarbon surfactant (Cap) and a hydrocarbon surfactant (Gluc215) were found in foam degradation rate and fuel transport rate through a foam layer. The surfactant mixture had 30% longer foam lifetime and two times smaller fuel flux through a foam layer than the fluorosurfactant solution (Cap) and differed with the hydrocarbon surfactant solution (Gluc215) by more than a factor of ten. However, when a different hydrocarbon surfactant (TX100) was used in a mixture with the fluorocarbon surfactant, synergism was not observed. The small synergistic effects resulted in a small reduction in fire extinction times at high foam application rates (> 500 mL/min) at which foams can cover the pool surface quickly. At small foam application rates (< 500 mL/min), effects of foam spread rates become significant; the fluorocarbon surfactant (Cap) formed wetter foam that spread quicker and led to smaller fire extinction times than those for the surfactant mixture (Cap + Gluc215). Thus, the foam spread, foam degradation, and fuel transport rate measurements performed in the presence of a hot fuel but in the absence of a fire can explain the surfactant’s effects on fire extinction times for the six formulations. Traditionally, aqueous film formation was thought to be the key for fire suppression of AFFF rather than the foam dynamics described in this work. Additional properties such as static and dynamic surface tensions, foam bubble diameter distribution, bubble coarsening, liquid drainage from the foam, and expansion ratio were also measured in the absence of a fuel, but did not explain the trends in extinction performance among the surfactants.