Development of Zwitterionic Additives to Aqueous Foams to Enhance Suppression of Aromatic and Aliphatic Fuel Pool-Fires

Ramagopal Ananth | U.S. Naval Research Laboratory

WP22-3391

Objective

The objective of this project is to develop siloxane-betaine zwitterionic additives to foams to enhance gasoline-fire suppression and to understand mechanisms of synergism and suppress surfactant extraction by fuel. The project team previously discovered that addition of a small amount of siloxane-sulfobetaine to an alkylpolyglycoside surfactant formulation reduces bench-scale gasoline-fire extinction time by a factor of three over the previous nonionic siloxane-polyoxyethylene formulation. The increased fire suppression is due to increased synergistic interactions between the sulfobetaine and alkylpolyglycoside surfactant and due to reduced extraction of the siloxane by gasoline, which contains aromatic and aliphatic components. However, the extinction times are still 60% longer than aqueous film forming foam (AFFF). Both extinction and burnback performance must be improved for gasoline fires. This can be achieved by systematically varying the siloxane-sulfobetaine additive’s molecular structure through synthesis. By varying the molecular structure, the project team can develop a new understanding of the synergistic intermolecular interactions and their effects on surfactant bilayer formation, packing density, and oleophobicity at an interface. Varying molecular structure can affect hydrolysis in water and biodegradability as well as toxicity. The project team will evaluate the environmental impact of the novel formulations and a life cycle analysis for the formulation with acceptable fire performance.

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Technical Approach

The project team will synthesize novel siloxane-sulfobetaine chemical structures, characterize them by nuclear magnetic resonance and liquid chromatography-mass spectrometry, and explore commercial hydrocarbon-betaines as additives. As measures of oleophobicity/hydrophobicity, the project team will measure solubilities or partition coefficients and measure contact angles of surfactant-laden water and fuel on Teflon plate relative to the contact angles of pure water and pure fuel. The project team will form surfactant layers on water and fuel in a Langmuir-Blodgett Trough and measure surfactant’s area per molecule (packing density), surface pressure versus area per molecule isotherms, and surface elasticity. They will characterize the size and type (segregated versus mixed) of micelles using Dynamic Light Scattering based on differences in micelle diffusion. They will measure 19-centimeter diameter bench scale fire-extinction times, foam spread times, and burnback times. They will also quantify acute lethality endpoints (LC50), chronic sublethal toxicity endpoints (EC50, IC50), and biodegradation using standard methods. Leading surfactant candidates identified by the bench-scale tests will be transitioned to  WP20-1507 project, where the project team will scale-up synthesis to generate the amount needed for a 28ft2 MilSpec pool fire testing.

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Benefits

A fluorine-free alternative to the current AFFF will eliminate the bio-accumulative, environmentally persistent, and toxic perfluoro and polyfluoro alkyl surfactants. The fluorine-free firefighting foam will lead to lower projected risks to the aquatic environment and human health. In addition, the fluorine-free formulation will meet MilSpec performance standards for gasoline, maintaining essential fire suppression performance of AFFF, critical to Department of Defense applications. The fluorine-free formulation will be a drop-in replacement for AFFF, having low viscosity and be compatible with existing hardware.

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Points of Contact

Principal Investigator

Ramagopal Ananth

U.S. Naval Research Laboratory

Phone: 202-767-3197

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