The specific objective of this project was to suppress surfactant extraction from foams by fuels to improve foam stability and gasoline fire suppression. In the expanded scope, the project team included synthesis of ionic siloxanes and an alkylpolyglycoside surfactant with increased head size, 28 ft2 pool fire testing of fluorine-free foams at different foam application rates, design of a miniature nozzle using the principle similar to the MIL-SPEC nozzle, and quantification of acute and chronic toxicities for siloxane surfactants.
The project team had a two prong approach; (1) synthesize and evaluate anionic, cationic, and siloxane surfactants and increase the head size of hydrocarbon surfactant (alkyltriglycoside in collaboration with, Naval Air Warfare Center Weapons Division, China Lake, CA), (2) collaborate in the technical approach with Dow Silicones Co., Midland, MI for developing novel siloxane surfactants. Dow developed the samples for the Naval Research Laboratory's (NRL) bench and large scale evaluations. The evaluations were done by measuring solution, foam, and fuel-pool fire suppression properties on heptane and alcohol-free gasoline pools. A binary mixture of 1,2,4-trimethylbenzene in heptane can be used as a well-defined surrogate fuel to gasoline as a reference in evaluating fluorine-free surfactant formulations to eliminate seasonal and other variations in commercial gasoline.
The Dow sample C has foam and extinction properties that are superior to the NRL formulation for gasoline but not for heptane. The 19-centimeter diameter pool fire extinction times are about the same for gasoline and heptane fires for Dow sample C unlike the NRL formulation, which has much longer extinction times for gasoline compared to heptane fire. The extinction times for reference aqueous film forming foam (RefAFFF)* are 3 to 4 times smaller than those for sample C, at foam flow rates greater than 350 mL/min.
The project team synthesized anionic (Trisiloxane-polyoxyethylene Sulfate), cationic (Trisiloxane-Polyoxyethylene Quaternary Amine), and zwitterionic (Trisiloxane-Polyoxyethylene Sulfobetaine) surfactants, which are analogues of the non-ionic trisiloxane polyoxyethylene with a terminal hydroxyl group (Dowsil 502Watts [W]). The project team also increased the glycoside head size by synthesizing alkyltriglycoside. These results suggest that the oxyethylene head length should be decreased to restore the amphiphilicity by performing further synthesis.
To evaluate novel, synthesized, surfactants’ fire suppression using very small quantities (gram scale) of surfactant, the project team designed and built a miniature nozzle. The nozzle is based on a similar principle used in a MIL-SPEC nozzle for mixing air and liquid to make foam so that the bench scale measurements correlate better with the large scale. To quantify the effects of fuel and liquid application rate on the fire suppression performance of fluorine-free formulations, the project team conducted 28 ft2 fire extinction and burnback tests on heptane and gasoline pool fires at 2, 2.5, and 3 gallons per minute (gpm) solution flow rates. All three fluorine-free foams had extinction times greater than the required 30 seconds for gasoline fire unlike aqueous film forming foam (AFFF) at 2 gpm. The results show that extinction times for gasoline fires are longer than heptane fires confirming the conclusion from previous bench scale tests. The extinction times decreased with increased solution flow rate but to a different degree depending on the surfactant formulation used.
The project team also quantified acute and chronic toxicities of individual siloxane, hydrocarbon, fluorocarbon surfactants and RefAFFF using P. promelas and C. dubia species following standard methods (in collaboration with Dr. David Moore, United States Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, MS). Based on results to date with both species, the relative order of toxicity of the tested compounds from least toxic to most toxic is as follows: DGBE<, Capstone 1157<, 502W<, Glucopon 215UP<, Glucopon 225DK<, Silwet L-77,< HMTS-PEO-TMACl, <<RefAFFF.
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 of the 3% concentrate, unlike many of today’s commercial fluorine-free formulations. The new surfactants and foams have the potential to meet MIL-F-24385F performance standards and have acceptable persistence and aquatic toxicity.
*NRL's custom formulation containing 0.3 wt% Capstone1157+ 0.2 wt% Glucopon 215UP+ 0.5 wt% diethyleneglycol-butylether(DGBE)