Compatibility of aqueous film-forming foams (AFFF) with sea water

Abstract

Abstract This contribution combines the observations from fire experiments with modelling of ionisation of fluorosurfactants to explain the compatibility of AFFF with sea water. We report detailed chemical structures of fluorosurfactants introduced into the firefighting foam market since the development of the so-called light water by the US Naval Research Laboratory (NRL) in late 1960s and study their ionisation constants (i.e., pKa). The analyses of recent formulations of foam concentrates indicate that, almost all of them include four species of high purity C6 fluorotelomer and non-fluorotelomer surfactants, which have their roots in fluorosurfactant chemistries developed by 3M, Ciba-Geigy and Elf Atochem (and its predecessors). The earliest fluorosurfactants emerged as derivatives of perfluorocarbon acyl and sulfonyl fluorides, from a series of discoveries made by the 3M Company. We review the foaming and filming technologies developed by 3M, derived from the products of electrochemical fluorination (ECF), comprising acidic, propane-sultone, acrylic and amine-oxide concentrates, with potassium perfluorooctane sulfonate (K-PFOS) filmers used together with the sultone and acrylic foamers. To the best of our knowledge, these chemistries have not been documented in one reference in the open literature, and some of the chemicals used have not yet been identified in the environment. While we do not cover the development of fluorotelomer chemistries in the same detail, we discuss the fluorotelomer surfactants that have proven themselves to be successful in compatibility with sea water and compare their structure with those of the ECF-derived chemicals. We discovered that, the chemical compatibility with sea water is related to the formation of the specific ionised species that combine with divalent alkaline-earth metal cations to form ionic assemblies in the premix (solution made by mixing foam concentrates with water). These species arise at high pH values that are characteristic of sea water. We also reveal that, changes in the ionisation state of amine-oxide and tertiary-amine head groups, as pH varies between that of fresh and sea-water premix, may impact the performance of fluorosurfactants. The physical compatibility of fluorosurfactants with sea water manifests itself by shielding of the ionised head groups by metal cations decreasing the surface tension and modifying the size, shape and diffusion of micelles. For foam concentrates that satisfy the necessary condition of chemically compatible with sea water, the physical effect usually improves the foam quality and the fire-suppression performance of AFFF. We analyse in detail the patent literature and the early NRL reports to gain an intimate understanding of the first formulations introduced by the 3M Company. Although PFOS has been banned internationally, Chinese manufacturers produce PFOS-type concentrates for the domestic market, in addition to fluorotelomer surfactants destined for export. We provide detailed formulations of all types of concentrates that employed the ECF-based fluorosurfactant technologies, including hydrocarbon surfactants, solvents, corrosion inhibitors and buffers and link the chemical composition of the concentrates with the firefighting performance of the foams. We outline the correlations between the ionisation states of fluorosurfactants and their fundamental properties, such as surface packing and interfacial tension and comment that very little is known about these critical structure-activity relationships. Finally, we stress the need to reboot the progress in the field and give directions for future research.