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

Bogdan Dlugogorski, Ted H. Schaefer

    Research output: Contribution to journalArticlepeer-review

    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 3 M, 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 3 M Company.
    We review the foaming and filming technologies developed by 3 M, 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 3 M Company. Although PFOS has been banned internationally, Chinese manufacturers produce
    PFOS foams 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.
    Original languageEnglish
    Article number103288
    Pages (from-to)1
    Number of pages24
    JournalFire Safety Journal
    Early online date2 Feb 2021
    DOIs
    Publication statusE-pub ahead of print - 2 Feb 2021

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