Document g2xEpLKYk4pzbKQo9Dk9QoKDQ

Dr. Bradley A. Williams and Dr. Ronald S. Sheinson Environmental and Performance Issues in MilSpec AFFF ABSTRACT Aqueous Film-Forming Foam is an important component of shipboard fire protection, as it is in many land-based applications. A number of environmental issues have come to light in recent years which may impact AFFF use. The properties, uses, composition, military specifications, and environmental concerns regarding AFFF are reviewed here. Current research projects aimed at better understanding the performance of AFFF and finding more environmentally benign formulations are summarized. INTRODUCTION Aqueous film-forming foam (AFFF) is a highly efficient fire suppressant agent, used to combat flammable liquid fires shipboard and shore side. AFFF is used in the U.S. military, and in most civilian applications worldwide, as either a "3%" or a "6%" concentrate. These numbers refer to the percentage of foam concentrate mixed with either fresh water or seawater by a proportioner (e.g. a "6%" AFFF concentrate is nominally used as a mixture of 6% concentrate and 94% water). In the U.S. DoD, the 6% concentrate is used in most shipboard applications, while 3% is used in most land-based applications. A 1% AFFF concentrate is sold by some manufacturers for civilian uses, but the 1% concentrate is not included in the MilSpec. The discharge nozzle can either be handheld, or in cases such as the flight decks of aircraft carriers, built into the ship. Foam forms spontaneously upon ejection of the concentrate/water mix from the non-aspirated nozzle (Fig. 1). The concentrate/water mix coats the hydrocarbon fuel with a layer of foam, which Nozzle delivering Cjjjfentrate/water mixture Self-aspiration of AFFF concentrate/water mixture. Coverage offuel surface by film/foam layer Figure 1: Schematic of AFFF use against a liquid pool fire. 1 FOAM FILM FUEL US00007876 acts as a thermal barrier to inhibit and eventually extinguish combustion. The "film-forming" characteristic refers to the ability of the water/concentrate mixture to form a thin aqueous layer on top of the liquid hydrocarbon surface, even in the absence of foam formation. This film forming ability is not shared by all types of foams used in fire fighting; for instance standard protein foams do not possess this ability. The aqueous film coats and seals the liquid hydrocarbon surface, inhibiting fuel evaporation, even after the foam dissipates. Aqueous film-forming foam was first developed in the 1960s, as a more effective alternative to protein foams. AFFF formulations use the thenrecently developed fluorinated surfactants, which help them to achieve their foaming and film-forming characteristics (Tuve, Peterson, and Jablonski 1964). AFFF has been a very important component of fire safety and damage control both shipboard and in other civilian and military applications. As the case with many other technologies in long-time use, the increasing concern with environmental protection in recent decades, as well as advances in analytical techniques, have revealed a number of environmental concerns which raise questions about the future use of AFFF. This paper summarizes the characteristics of AFFF, the areas of environmental concern and the current situation regarding these issues, and current research directions aimed at better understanding the behavior of AFFF and finding more environmentally benign formulations capable of providing equally effective fire protection. MILSPEC AFFF BASICS AFFF used by the U. S. Department of Defense (DoD) must meet the requirements set forth in Military Specification MIL-F-24385F, which is under the control of the Naval Sea Systems Command, Code 05P9. The Naval Research Laboratory is the designated institution for certification evaluation for the DoD AFFF Qualifying Products List (QPL). The MilSpec AFFF formulations are effective against a wide variety of liquid hydrocarbon fire threats, although not against water-miscible fuels such as alcohols, for which alternate formulations (not covered by the MilSpec) are used. AFFF was used by the DoD before being employed in the civilian sector, and for this reason the AFFF MilSpec has importance as a performance standard beyond its stature as a requirement for the U.S. military. In particular, a MilSpec approved product is assured to offer a high and well-defined level of performance. Furthermore, one of the requirements of MIL-F24385F is that an AFFF concentrate must provide the same level of performance when mixed in any proportion with any other MilSpec qualified AFFF concentrate. Thus a user of a MilSpec product can replenish a store of AFFF concentrate with a MilSpec product from any other manufacturer and be guaranteed the same performance. For these reasons MilSpec AFFF concentrates are used in a number of civilian applications, even though MIL-F-24385F does not formally apply. The DoD has typically awarded contracts to a single supplier to cover military procurement needs for AFFF concentrate over a specified period of time. There can be MilSpec AFFF produced by manufacturers other than the current contract supplier in military systems, including cases where the AFFF is not procured directly by DoD but purchased from a subcontractor by the prime contracter of a system. 3M was the contract supplier of AFFF to DoD until 2002; as discussed below the company withdrew from the AFFF market following the completion of the contract with DoD. Following the withdrawal of 3M, there remain several manufacturers whose AFFF concentrate is MilSpec qualified. The requirements of MIL-F-24385F cover a number of areas. Some of the more important requirements are listed and discussed below: 1. Physical properties A number of physical characteristics are specified. A partial list is given below: Expansion ratio o ffoam AFFF is a "low expansion" foam intended to have good resistance to wind. Viscosity o ffoam concentrate Large variations in viscosity may impede proper 2 US00007877 functioning of proportioning equipment, causing improper concentrate/water mixtures. Spreading Coefficient on cyclohexane This parameter, discussed in more detail below, is meant to ensure film-formation. Index o f refraction offoam concentrate Currently an index of refraction measurement is used as a field determination of the proper concentrate/water proportions. Colorimetry of a dye added to the concentrate may be a preferable alternative method. 2. Fire suppression performance The AFFF, when mixed with water at its nominal concentration, must be able to extinguish a standard pool fire within 30 seconds when applied at a specified rate. One issue which has been raised with this test protocol is that the extinguishment time depends to a certain extent on the skill of the operator, so a fully automated test procedure might be expected to produce more repeatable results. 3. Suppression performance o f non-nominal mixtures An additional requirement is that the AFFF must be able to extinguish the standard fire when mixed at one-half of its nominal concentration (i.e. a 6% AFFF mixed in the proportions of 3% concentrate, 97% water). In this case a slightly longer time is allowed to extinguish the fire. Passing this test may in some cases require higher levels of surfactants than those required to pass the standard suppression test above. An additional test is conducted with the AFFF concentrate mixed at higher than its nominal proportion. The original rationale for these tests was to allow for improper performance of the proportioning equipment. They may also lead to an enhanced safety factor, since the limited testing matrix of the specification cannot capture all fire threats and operational conditions. 4. Reignition protection In addition to the fire extinguishments tests, the MilSpec contains requirements regarding burnback (the rate at which the fire advances across a foam covered pool when no additional foam is supplied). Also there are requirements for the pool not to exhibit sustained burning following attempted ignition in a small "hole" in the foam layer (sealability). These requirements are geared towards providing protection against reignition following extinguishment of the primary fire. 5. Materials compatibility Corrosion tests of several types of metals commonly used in construction are specified to assure that plumbing systems do not corrode following long term exposure to AFFF concentrate. Additionally, analytical tests are specified to assure that quantities of inorganic halides, which promote corrosion, do not exceed a certain concentration. The analytical test used is however insensitive to fluoride ion. Additionally, the use of a fluorinated surfactant is mandated in MIL-F-24385F, although there is no specification of the amount to be used. Composition of AFFF Concentrates The AFFF concentrates are mostly water, with other components such as glycol ethers or ethylene/propylene glycol added to extend the lifetime of the foam by increasing viscosity and thus lowering the water drainage rate from the foam. The lowering of surface tension to allow formation of foam and of a coverage film of water on hydrocarbon, is accomplished by use of Table 1 'Composition of 3M 3% AFFF concentrate (Moody and Field 2000)---------------------Water (69-71%) Diethylene glycol butyl ether (20%) Amphoteric fluoroalkamide derivative (fluorocarbon surfactant) (1-5%) Alkyl sulfate salts (hydrocarbon surfactants) (1-5%) Perfluoroalkyl sulfonate salts (fluorocarbon surfactants) (0.5-1.5%) Triethanolamine (0.5-1.5%) Tolyltriazole (corrosion inhibitor) (0.05%) 3 US00007878 both fluorocarbon and hydrocarbon surfactants. Additionally there may be additives to lower the freezing point of the concentrate to allow its use in lower temperature environments, and to minimize corrosion. The composition of a typical AFFF concentrate is given in Table 1. The exact details of the concentrate composition (particularly of the surfactants used) are proprietary and vary between manufacturers. Note that after dilution with water, the mixture from which the foam is formed will be more than 99% water, 0.6% glycol ether, and less than 0.15% of each of the other components. Roles of Surfactants in AFFF The surfactants used in AFFF fulfill two requirements. By reducing the surface tension, they facilitate the formation of foam, which has a large surface area per unit liquid volume. Were it not for the reduction of the surface energy due to the surfactant, foam formation would be difficult or impossible. Secondly, the reduction in surface tension permits the formation of the aqueous film on top of the hydrocarbon fuel surface. Since water is more dense than typical hydrocarbon fuels, a water layer will normally sink to the bottom of a hydrocarbon liquid. Surface tension can, under certain conditions, cause the aqueous layer to remain on top. The possibility of film formation can be characterized by the spreading AIR JUU1U1WFLUOROCARBON SURFACTANT AQUEOUS SURFACTANT SOLUTION wiiffirttHYDROCARBON SURFACTANT HYDROCARBON PHASE coefficient, S, defined as ^a/o -- To (.Y& ^ Y&lo)-> ( 1) where y0 is the surface tension of the underlying hydrocarbon, ;/a the surface tension of the aqueous film, and ya/0 the surface energy of the film-liquid interface. The spreading coefficient determines if it is energetically favorable for the fuel surface to be coated by a thin aqueous layer. If the spreading coefficient is negative, no film formation can occur. If the spreading coefficient is positive, film formation is energetically possible, although it does not always occur in practice. The fuel specified in the AFFF MilSpec is cyclohexane, which although not a typical fire threat itself, has a similar surface tension and thus will result in similar spreading behavior for AFFF as practical fuels such as gasoline and JP-5. Furthermore cyclohexane is an inexpensive, pure compound, facilitating a reproducible testing protocol. Achieving a positive spreading coefficient requires lowering the surface tension of the aqueous layer as well as the interface tension between the aqueous layer and the fuel. This is one reason why AFFF formulations contain both fluorocarbon and hydrocarbon surfactants. The fluorocarbon surfactants are thought to preferentially compose the water/air interface, since they are capable of achieving a lower surface energy than the hydrocarbon surfactants. The hydrocarbon surfactants are believed to dominate along the water/fuel interface, since the hydrophobic portion of the surfactant is in direct contact with the chemically similar hydrocarbon fuel (Figure 2). Achieving a positive spreading coefficient of at least 3 dynes/cm (the requirement of MIL-F24385F) on cyclohexane, which has a surface tension of 25 dynes/cm, requires lowering the surface tension of the aqueous layer to approximately 20 dynes/cm (a typical interface tension is 2 dynes/cm). Typical hydrocarbon surfactants do not produce values this low, but fluorocarbon surfactants and in some cases 4 FIGURE 2. Representation of location of surfactants in an AFFF film (Shinoda and Nomura 1980). US00007879 silicone surfactants can. Thus, it is the filming requirement, not foam formation, which leads to the use of the fluorocarbon surfactants in current AFFF formulations. AFFF Environmental Issues Environmental Persistence o f Fluorinated Surfactants The fluorinated surfactants used in AFFF arc produced by one of two synthetic processes. One class of fluorosurfactants is based on perfluorooctyl sulfonate (PFOS) and structurally related compounds; these were the principal fluorinated surfactants in the 3M AFFF formulations. These compounds were prepared by direct fluorination of a hydrocarbon precursor. Beginning in 1999, PFOS based surfactants were voluntarily removed from the market, after these chemicals were found to be environmentally persistent and widely distributed throughout the environment, as well as having some degree of toxicity. Alternative surfactants used in AFFF formulations of other manufacturers are made by telomerization (a polymerization process producing extremely short chains) and were not directly affected by the phase out of PFOS. The study conducted by 3M which identified the environmental issues with PFOS only considered those compounds; the environmental fate and effects of other classes of fluorinated surfactants were not investigated. Since the telomer-based surfactants are also highly fluorinated, they are likely to be environmentally persistent as well. The environmental effects of these surfactants and their decomposition products are still unknown at present, but are currently under investigation. Toxicity o f Glycols and Glycol Ethers Glycol and glycol ether compounds are added to AFFF concentrate to increase the liquid viscosity, and in some cases to lower the freezing point ("anti-freeze") to facilitate AFFF use in low-temperature environments. The higher viscosity extends the lifetime of the foam by reducing the rate at which liquid drains from the foam. Since many of the glycol compounds are toxic (and subject to environmental legislation such as SARA and RCRA), their presence can result in environmental restrictions on disposal of AFFF-containing waste or runoff. Disposal o fAFFF Waste AFFF emulsifies oil/water mixtures, which interferes with the functioning of oil separators used to remove oil from bilge water aboard Navy ships (Naval Sea Systems Command 1998). The MARPOL, which must be followed by U.S. Navy ships to the extent "reasonable and practical" does not allow discharge of oilcontaminated bilge water (which cannot be cleaned if it also contains AFFF) in "special areas", such as the Mediterranean and Baltic seas. Disposal of AFFF runoff from shore-based training exercises and from shipboard waste is also restricted due to the surfactants' interference with municipal wastewater treatment facilities (Army Corps of Engineers 1997). This is not only a problem with the fluorinated surfactants, but also of the persistence of the foam produced by the AFFF formulations. AFFF Research at the Naval Research Faboratory AFFF was first developed in the 1960s at the Naval Research Laboratory, in a collaborative research effort with 3M (Tuve, Peterson, and Jablonski 1964). Since that time, NRL has been the only facility to perform qualification testing for MilSpec AFFF. With the identification of the current environmental concerns regarding AFFF, new research programs have been initiated at NRL. Dynamics o f Foam Formation (Office o f Naval Research) The surface tension of a liquid containing surfactants is not a constant value, but changes with time. A surfactant requires a certain time to coat a newly created surface and so reduce the surface tension. Typically this equilibration time is on the order of seconds to minutes. The instantaneous surface tension as a function of the elapsed time following creation of a new surface is a monotonically decreasing function which 5 US00007880 asymptotically approaches the equilibrium value. The time for equilibration to occur depends on a number of factors, especially the concentration of the surfactant in the liquid. A higher surfactant concentration reduces the equilibration time, but this is undesirable in AFFF as the surfactant is one of the most expensive components of the product and causes undesirable environmental effects. The effects of varying surfactant concentration are seen in Figure 3, which shows measurements of the dynamic surface tension of 6% AFFF mixed with water for a range of concentrations. The time for the AFFF/water mix to reach the fire after being sprayed from a hose or sprinkler is on the order of a second. Furthermore the time allowed to extinguish a test fire in the MilSpec acceptance test is only 30 seconds. Thus, the equilibrium surface tension is not achieved during many aspects of AFFF use, particularly in the foam formation. The current MilSpec lists as product requirements a combination of physical properties (including the equilibrium spreading coefficient) and performance requirements, such as fire extinguishment time and burnback resistance. The goal of this research project is to characterize and understand the nonequilibrium behavior of AFFF, and its relationship to AFFF performance properties. That the nonequilibrium behavior of AFFF is important in influencing its performance is illustrated in Fig. 4. This shows the dynamic surface tension of two MIL-F-24385F qualified foam formulations from different manufacturers, mixed with water at their nominal concentrations of 6%, and also at 12%. The surface tensions at long surface ages are different for the two formulations, but the same for the 6% and the 12% mixtures of each formulation. Only at the lowest surface ages (less than 30 milliseconds) do the two formulations, both used at their nominal FIGURE 3. Dynamic Surface Tension of 3M 6% AFFF as a function of surface age, for concentrate mixes with water from 1.5% to 12% (0.25 to 2 times the nominal mixture ratio). Surface age (milliseconds) FIGURE 4. Comparison of dynamic surface tension of two 6% AFFF formulations from different manufacturers when mixed with water at 6% (their nominal concentration) and 12%. concentrations, show essentially the same dynamic surface tension. Since both products were formulated to meet the same specification, and were qualified, the performance seems to be largely determined by the behavior at short time scales. Similar results are obtained for other MIL-F-24385F qualified products; all show similar surface tension on short time scales, but not at long time scales. Assessment o f Current Alternative Foam Surface age (milliseconds) Technologiesfor DoD Use (Naval Facilities Engineering Service Center) This project evaluates environmentally friendly alternative (non MilSpec) foam technologies 6 US00007881 which have recently been or are now being commercialized, for their suitability as AFFF alternatives. A solicitation for producers to submit samples for testing appeared in Commerce Business Daily in October 2002; testing of submitted samples began in FY2003. The fire fighting performance (extinguishments time and burnback), sealability, foamability, and biodegradability of the submitted products will be evaluated to assess their suitability as alternatives. The testing will not comprise the entire requirements of MIL-F-24385F, but only a limited set of tests intended to quantify the performance of the alternative products compared to the current MilSpec-qualified formulations. CONCLUSIONS Given the issues surrounding AFFF use, exploration of alternatives to the current generation MilSpec AFFF products is prudent. Until serious environmental concerns were raised, there was little impetus to understand and improve its performance. Now, however, increasing United States and international environmental concerns regarding AFFF must be addressed as use restrictions can seriously impact operations. Generating formulations that will satisfy DoD fire protection needs and be more environmentally friendly requires an understanding of how AFFF functions. The MilSpec standard may well be updated as a consequence of the attention now being given to AFFF. Identifying and understanding the key parameters for successful AFFF extinguishment will guide development of replacement foams. Key research questions to be answered on AFFF, which will impact its formulation and use in the future include the following: 1. Do the environmental issues identified in PFOS extend to other fluorinated surfactants? 2. Flow do the physical and chemical properties of AFFF govern aspects of its performance in fire suppression? 3. Flow critical is the film formation aspect of AFFF to its performance, and in what situations? 4. Can a positive spreading coefficient be achieved with other types of surfactants with less environmental impact? Because fire threats against which AFFF must be effective are not always foreseeable in advance, it is necessary to assume a conservative approach in specifying the performance requirements. Some of the tests in MIL-F243 85F, such as the test at one-half the nominal concentration, may no longer be needed according to their original intent (modern proportioners are much more accurate). Nevertheless, they achieve the desirable effect of ensuring a more effective product. Reformulation of AFFF to meet the recent environmental concerns will be a challenge, but as the case with halon replacement, needs for effective damage control and operational capability can be met through improved understanding of the operational requirements and the technical issues. REFERENCES U. S. Army Corps of Engineers, "Containment and Disposal of Aqueous Film-Forming Foam (AFFF) Solution" Technical Letter ETL 1110 3-481, Appendix A, 23 May 1997. C. A. Moody and J. A. Field, "Perfluorinated surfactants and the environmental implications of their use in fire-fighting foams" Environ. Sci. Technol. 34: 3864-3870 (2000). Naval Sea Systems Command, "Guidebook for Oil Pollution Abatement Systems on Surface Ships" Publication S9593-CP-GYD-010, 1998. K. Shinoda and T. Nomura, "Miscibility of Fluorocarbon and Flydrocarbon Surfactants in Micelles and Liquid-mixtures - Basic Studies of Oil Repellent and Fire Extinguishing Agents,"J. Phys. Chem 84:365-369 (1980). R. L, Tuve, FI.B. Peterson, E. J. Jablonski, and R. R. Neill, "A New Vapor-Securing Agent For Flammable-Liquid Fire Extinguishment" Naval 7 US00007882 Research Laboratory Report #6957, March, 1964. ACKNOWLEDGMENT This work was funded by the Office of Naval Research under Project Number N0001402WX21212. Dr. Bradley A. Williams is a Research Physicist in the Combustion Dynamics Section o f the Naval Technology Centerfo r Safety and Survivability at the Naval Research Laboratory in Washington, DC. Dr. Williams received his Bachelors degree in physics from Stanford University, and his PhD in appliedphysics from Cornell University in 1992. He has worked at the NRL since that time. His research experience has involved laser spectroscopy, chemical kinetics, combustion chemistry, fire suppression andfire safety, instrumentation fo r laboratory andfield measurements, and computational modeling. Dr. Ronald S. Sheinson is the Combustion Dynamics Section Head o f the NTCSS, NRL. His expertise in combustion processes includes fire suppression, flammability limits, spontaneous ignition, plasma discharge induced oxidation fo r collective protection, combustion product analysis, and atmospheric habitability. His current research includes flame extinction, fire extinction dynamics, halon replacements and alternatives fire suppression systems development, and developing and optimizing replacements fo r Navy shipboardfire protection systems. He served as Principal Scientistfo r the DoD Halon 1301 Replacement Program fo r Manned Spaces Total Flooding Fire Suppression Systems, and is a Technical Coordinating Committee member o f the DoDSERDP Next Generation Fire Suppression Technology Program and US. Government Representative serving as a Technical Advisor on the UN Environment Program, Halon Technical Options Committee. Dr. Sheinson and NRL have received three EPA Stratospheric Ozone Protection Awards, including a "Best o f the Best "Award. He received his Ph.D. in Chemical Physics from MIT in 1970. 8 US00007883