The objective of this project was to develop an environmentally-friendly, fluorine-free, firefighting surfactant formulation to replace aqueous film forming foam (AFFF) by reducing the fuel permeation through foam and foam degradation and increasing surfactant oleophobicity. The project team also conducted measurements of aquatic toxicity and biodegradation to assess the environmental impact of the fluorine-free surfactants in comparison to the fluorinated surfactants.
The project team chose surfactant molecules based on hydrophobic and oleophobic balance. Commercial sources and chemical synthesis were used to obtain candidate surfactants. The project team pursued parallel efforts to quantify firefighting performance and assess environmental and health impacts. Surfactants were down-selected by utilizing molecular dynamics simulations, and measurements of surfactant-solution properties. The first tier of evaluation was based on the measurements of the short-term acute toxicity using algae, invertebrates, and fish. The first tier also included the measurements of fuel transport through the foam, foam degradation, and foam spread rates. The second tier of evaluation included the measurements of chronic toxicity using algae, invertebrates, and fish. The second tier also included measurements of fire extinction dynamics. The final formulations’ compositions were optimized at bench-scale prior to large scale MILSPEC testing.
The chemistry of surfactants and oleophobicity were found to play a very important role in controlling pool fires. A reference AFFF formulation containing a fluorocarbon surfactant, a hydrocarbon surfactant such as alkylpolyglycoside (APG), and a solvent was shown to extinguish heptane and gasoline pool fires as quickly as a commercial AFFF at bench (19-cm diameter pool) and large (28 ft2 pool) scales. By substituting different fluorine-free surfactants for the fluorocarbon and different hydrocarbon surfactants for APG in the RefAFFF, it was found that varying the composition significantly affected the fire suppression. The bench scale measurements revealed that synergistic interactions between the siloxane and APG surfactants reduced foam degradation and fuel transport which caused the observed higher fire suppression performance.
In addition to the synergism, surfactant’s amphiphilicity and polydispersity was found to affect fire suppression significantly. The project team synthesized trisiloxane polyoxyethylene surfactants with different distributions of oxyethylene head sizes with the average values ranging from 0 to 50. The project team quantified the effects of varying the oxyethylene head size of the trisiloxane surfactant on the degree of synergism, solution and foam properties, foam degradation rate, fuel transport rate, foam spread rate, and heptane-fire suppression. To obtain even larger improvement, the project team varied the glycoside head size with the tail size fixed and varied the alkyl tail size with the head size fixed using monodispersed APGs. The project team found slight minima in fire extinction time as the head or tail size is varied. Varying head or tail size, one at a time, may have reduced amphiphilicity. Also, the lack of polydispersity of APG could have decreased the degree of synergism.
The work revealed a few general principles: (1) surfactant’s chemical structure has a significant effect on foam’s ability to suppress fires, (2) when varying surfactant’s chemical structure, head’s hydrophilicity should be balanced with tail’s hydrophobicity to preserve amphiphilicity, which affects interfacial activity and fire suppression, (3) increasing synergism between two types of surfactants adsorbed at an interface is the key to enhancing fire suppression, (4) surfactant head structures that suppress surfactant extraction by fuel can increase fire suppression, (5) surfactant structures that decrease fuel permeation through foams enhance fire suppression, and (6) foams cool pool surface rapidly and reduce fuel vapor pressure leading to enhanced fire suppression for high flash point fuels, whose fire extinction times are close to their pool coverage times.
Biological endpoints were assessed to determine whether the mixture exhibited additive effects. The acute studies (toxicity based upon 48-hour or 96-hour survival) demonstrated that the mixture tested had additive or more-than-additive effects. In the chronic tests (72 hours or 7 days), the scenario resulted in additive or less-than-additive effects. When comparing each species based upon measured mixture concentrations, P. promelas and C. dubia were more sensitive to chronic exposures versus acute exposures, while the algae, R. subcapitata was the least sensitive to the mixture tested.
Further work is needed to improve the fire suppression of siloxane based formulation on gasoline and heptane. If successful, it has the potential to be a drop-in replacement for AFFF because of low viscosity and homogeneity of the 3% concentrate, unlike many of today’s commercial fluorine-free formulations. The new surfactants and foams have the potential to be effective on different fuel-fires and meet MIL-F-24385F performance standards, and have acceptable persistence and aquatic toxicity. (Project Completion - 2022)