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Stability of Fluorine-Free Foams with Siloxane Surfactants for Improved Pool Fire Suppression
Ramagopal Ananth | U.S. Naval Research Lab
Based on preliminary research, the research team found that the foams generated from aqueous solutions of commercial siloxane surfactants were particularly vulnerable to degradation preventing them from covering the burning and non-burning fuel-pools. The degradation appeared to be sensitive to the variations in the structure of the head group of a surfactant. The research team plans to synthesize siloxane surfactants with a systematic structural variation of the head group and quantify the effects on foam degradation, fire extinction, and environmental impact by quantitative structure property relationships. This knowledge will be used to achieve full coverage of burning pool surface with a siloxane foam.
The objective of this project is to develop new surfactants whose foams can potentially meet the performance requirements defined in sections 3.4, 188.8.131.52, and 184.108.40.206 of MIL-F-24385F and have acceptable persistence and aquatic toxicity in order to replace current environmentally hazardous Aqueous Film Forming Foam (AFFF).
The research team is building on the research community’s understanding and research experience on fuel-pool fire suppression mechanisms, laboratory experimental methods, and computational models at the U.S. Naval Research Laboratory (NRL), and Oregon State University’s expertise in toxicology measurements. The researchers have been conducting research to identify and develop fluorine-free surfactants having both high fire suppression effectiveness and low environmental impact. The evaluation of several commercial fluorine-free siloxane surfactants in the last several months has shown that foams made from several of these surfactants exhibit more rapid degradation relative to AFFF containing fluorocarbon surfactants. The rapid degradation prevents these siloxane-based fluorine-free foams from completely covering the liquid fuel surface; full coverage is necessary but not sufficient to extinguish the fire because the foam layer must also block the diffusion of fuel vapors through the foam. Quantifying the effects of systematic and fundamental variations in surfactant structure on foam stability is essential to achieve foam’s full coverage of the fuel pool surface.
This research will synthesize fluorine-free siloxane-based surfactants by attaching different head groups (cationic, anionic, non-ionic, zwitterionic) to a fixed tail group because the solubility of surfactant in fuel (versus water phase) and stability of the lamellae (bubble walls) within the foam are affected by the charge or polarity of the surfactant’s head group. Researchers will also attach different tail groups (straight chain siloxane, trisiloxane with methyl pendant groups, and a trisiloxane with phenyl pendant groups) to the most promising head group to vary the packing density and stiffness of the tail at the lamella surface. They will quantify the effect of both head group and tail group substitution on foam stability. They will also synthesize a straight chain siloxane with a sulfonate head group and compare its performance with a hydrocarbon analogue (e.g., sodium dodecyl sulfonate); they will test the basic hypothesis that siloxane-based surfactant tails are more effective than hydrocarbon tails for suppressing fuel transport and thus more effective at fire suppression. The research team will use Quantitative Structure Activity Relationships (QSAR) and United States Environmental Protection Agency (USEPA) models to assess the environmental impact of the promising siloxane-based fluorine-free surfactants.
Effective and environmentally acceptable fluorine-free surfactants will contribute to improvements in the impact on healthy aquatic and terrestrial environments while maintaining critically needed pool firefighting performance to ensure the safety and continuing operability of Department of Defense personnel and facilities including airfields and onboard ships.