Objective

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 pool (Class B) 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 synthesize and/or identify fluorine-free materials to replace perfluoroalkyl surfactants in fire-fighting foams. The new materials will be made to operate equally well to quickly extinguish hydrocarbon pool fires. The new materials could become drop-in replacements to existing fire-fighting technology to allow the DoD to continue its mission with less ecosystem impact and maintain a high degree of environmental stewardship.

Technical Approach

Silicon is the second most abundant element in the Earth’s crust and so new surfactants made from this element might be environmentally-friendly materials. Polyhedral oligosilsesquioxanes are thermally stable silicate materials that could be made into surfactants by asymmetric attachment of poly(ethylene)glycol ‘tails’ to increase water solubility. Aminopropylsiloxanes were another platform that could be chemically modified by salt formation or quaternization to create fluorine-free surfactants. The surfactant properties of these new silicon-containing materials will be measured such as surface and interfacial tensions against cyclohexane, specified by MIL-F-24385F.  Silicon-based surfactants with spreading coefficient > +3 mN/m will be tested for extinguishing small-scale hydrocarbon pool fires.  

Results

An incomplete, polyhedral oligosilsesquioxane trisilanol was successfully modified with three poly(ethylene)glycol chains by a platinum-catalyzed hydrosilation reaction. The pegylated-polyhedral oligosilsesquioxane was soluble in water and its surface tension in air decreased with increasing concentration. Unfortunately, its lowest surface tension in air was only 48 mN/m at 9%-weight loading meaning it was a poor surfactant.  Interfacial tension against cyclohexane was 18.5 mN/m, which calculated to a spreading coefficient of –49.8 mN/m meaning it would not spread on the surface of this hydrocarbon, and that the surfactant did not meet the mil spec requirements. The ascorbic acid salt of 3-aminopropyl-methylbis-(trimethylsiloxy)silane (APS/AA) was a better surfactant with positive spreading coefficients on both cyclohexane (+4.7 mN/m) and heptane (+0.74 mN/m). Unfortunately, foamed APS/AA was unable to extinguish a heptane pool fire even at the high delivery rate of 1500 mL/min.  Quaternary ammonium salts of 3-aminopropylmethylbis(trimethylsiloxy)silane were easy to synthesize and appeared to be strong surfactants requiring further study. The derivatives of aminopropylsiloxanes are not stable in water indefinitely but decomposed fairly quickly.

Benefits

The surface tension of polyhedral oligosilsesquioxane surfactant would have to be greatly decreased to have a chance as a fire-fighting foam. This might be accomplished by decreasing the hydrocarbon content of the silicate cage. The aminopropylsiloxanes appear to be excellent lead compounds with low surface tensions. These compounds could be readily derivatized by a high throughput screen (HTS) to discover new surfactants with tailored properties for fire-fighting foams. However, favorable tensiometry measurements do not necessarily equate to a good fire-fighting foam as the aminopropylsiloxane salt demonstrated.

  • AFFF ,

  • PFAS-Free Alternative