Perfluoroalkyl surfactants are the key ingredients in aqueous film-forming foams (AFFF) which are used by the Department of Defense (DoD) and others to fight hydrocarbon (Class B) pool fires. Perfluoroalkyl surfactants work extremely well for this application, however there are growing concerns about these materials because they are highly persistent in the environment and may be toxic to plants and animals or increase their risk to certain diseases. The objective of this project was to explore glycosiloxane surfactants as replacements of perfluoroalkyl surfactants found in current AFFF concentrates used in fire-fighting by the DoD. The new, stable siloxane surfactants produced in this research contain only the elements carbon, silicon, hydrogen and oxygen.  The goal is for formulations based on the new new surfactants to approach the requirements of MIL-PRF-32725.

In previous work, it was discovered that aminoalkylsiloxanes can be converted into surfactants by modification with carbohydrate groups. The new glycosiloxanes had the ability to extinguish jet fuel (F24) pool fires as fast as legacy fluorinated AFFF in a 20 in² pan fire. More research was carried out to understand why the mixture of glycosiloxane and alkyl polyglucoside worked so well to extinguish the jet fuel fire. Glycosiloxane surfactants with hydrolysis-resistance were made by derivatizing cyclosiloxanes and carbosilanes. The project team quantified the effects of changes to the surfactant chemical structures on surfactant/fuel diffusion at fuel-aqueous interfaces, micelle diffusion, and shelf-stability, in addition to fire extinction for gasoline, jet fuels and diesel at bench-scale. The leading glycosiloxane surfactants were scaled-up to generate multi-gallon surfactant premixes for 28 ft² MilSpec hydrocarbon pool fire testing. Glycosiloxanes and their mixtures were screened for ecotoxicity (LC50) in aquatic animal models. 

New siloxane surfactants were synthesized by a four-step route starting from an inexpensive, commercially available silane and silanol reagents. The new siloxanes had tertbutyldimethylsilyl rather than trimethylsilyl groups as the terminus. For water solubility of the new siloxanes, it was critical to incorporate the proper number of aldonic acid substituents. The stability to hydrolysis was tested by dissolving two structurally similar siloxane surfactants in water. After 29 days, the siloxane surfactant capped with tert-butyldimethylsilyl groups continued to make foam when shaken indicating surfactant properties were intact. In contrast, the siloxane surfactant capped with trimethylsilyl groups no longer produced any foam when shaken and had obviously lost surfactant activity. Surface tension measurements showed that the new siloxane surfactants had critical micelle concentrations around 25 mN/m or less, which was consistent with hydrocarbon surfactants. Certain derivatives of the new siloxane surfactants displayed spreading behavior when their aqueous solutions were deposited on the surface of a pool of cyclohexane.

The fire extinguishment results showed that the experimental mixture containing the hydrolysis-resistant surfactant required 61 seconds, or four times longer application time to reach 100% extinction versus military specification AFFF solution which required only 11 seconds. It was also shown that the hydrolysis-resistant surfactant was a vital component in the experimental mixture because in its absence the mixture of glucopon 225DK and DGBE was unable to extinguish the fire. Remarkably, the siloxane surfactant mixture composed of 2062–139 was able to extinguish the fire in 11 seconds just like the AFFF solution, albeit at a slightly higher application rate.

This new technology could help develop improved fluorine-free formulations according to MIL-PRF-32725.  Using glycosiloxane surfactants, it may be possible to extinguish fires with smaller quantities of chemicals than previous fluorinated-AFFF formulations. (Project Completion - 2022)