Current aqueous film-forming foams (AFFFs) employed for liquid fuel fire suppression by the military utilize perfluorinated compounds (PFCs) as surfactants. PFCs exhibit toxicity, bioaccumulation, and persistence in the environment, resulting in fluoro-containing surfactants in regions not directly exposed to PFCs but rather through secondary exposure by chemical migration. While fluorinated compounds provide desirable thermo-oxidative stability and excellent fire retardancy, the environmental impact imposed by these chemicals spurs research that targets the complete removal of PFCs in conventional surfactant formulations.
A new fluorine-free foam platform will be developed through the proposed collaborative effort that is capable of meeting the fire suppression and environmental performance in MIL-F-24385F (NAVSEA, 1992). This work describes a new modular family of sulfonated polyimides containing flame-resistant aromatic sequences and imide linkages coupled with sulfonated units to impart water dispersibility/foaming and photo-crosslinkable sites to influence long-range flow. Testing is conducted to assess the foam fire suppression performance and environmental impact.
In view of this, the design and synthesis of a series of wholly aromatic polyimides aims to replace PFCs in liquid fuel fire suppression (Scheme 1). The synthesized polyimides comprise highly thermally stable moieties that provide excellent fire resistance, high char yields, and highly rigid polymer backbones, yielding infusible materials. Likewise, the incorporation of metal-substituted sulfonate pendent groups enables water solubility for the rigid-rod polymers. Tailoring of the polyimide backbone through copolymerization with sulfonated and non-sulfonated monomers permits a family of polyimides to exhibit a balance of water solubility and flame suppression.
When combined with non-toxic surfactants and salts in water, these sulfonated polyimides (sPI) have a high propensity for stable foam formation. The MIL-F-24385F performance requirement evaluates foam quality/stability, drainage time, and burn-back resistance to access viability and provides a comparison to other systems. To remove small molecule additives, sPI foam systems with backbone-tethered ammonium and phosphonium ion surfactants are also investigated.
Tailoring of the polymer backbone through copolymerization with sulfonated and non-sulfonated monomers permitted production of a modular family of poly(imide)and poly(amic acid)s exhibiting a balance of water solubility and synthetic ease. Formulation of the sPIs or sulfonated poly(amic acid) salts (sPAAs) with organic surfactants, glycolic ethers, and inorganic salts permits formation of stable aqueous foams. Analysis of the firefighting capacity and environmental impact of the sPI and sPAAs formulations has been undertaken to ascertain their performance as AFFFs. Overall, the investigated systems performed well as stand-ins to AFFF but will require further development and optimization to realize a commercially accessible product.
Assessment of the modified sPI and sPAAs at Jensen Hughes and additional large-scale synthesis, now that large-scale sPAAS and sPI techniques are in place, are paramount for determining optimal backbone structure for foam performance. With current synthetic techniques recently developed, rapid evaluation of aromatic polymeric surfactants is envisioned. Expansion of the currently employed monomeric units for sPI and sPAAS could lead to a host of polymeric surfactants with highly variable and advantageous properties as surfactants or waterborne coatings. Due to the highly charged nature of sPAAS, coupling of monomers which provide water solubility in the sPAAS state, but not when thermally converted to sPIs, may allow for coatings applied from water, which once cured, would display insolubility. Likewise, the inclusion of siloxane or other highly non-polar oligomeric diamines would allow for the synthesis of segmented sPIs and sPAAS, which may boast improved surfactant performance.