BY JOHN SACHEN
Advertising in the late 1800s about the superior extinguishing power of “chemical extinguishers” on Class A and Class B fires later proved to be very enthusiastic, but truly effective water-based extinguishing agents have been in use since at least 1905, when a two-part foam agent was introduced.
Today, firefighting agencies still face a mountain of sales and engineering literature to evaluate each time a decision is made for water-based extinguishing agents. One of the most challenging areas involves determining which agent to use for ignitable liquids. As an example, when the National Institute of Standards and Technology (NIST) reviewed water-based extinguishing and mitigating agents in 1997, it studied 28 different additives.
Today, there are more than 30 agents available that foam, wet, emulsify, form a film, defoam, and float on a miscible (alcohol-like) liquid. Therefore, until public and private fire service agencies have fully studied the subject, the best advice is: Foam buyer, beware!
Surfactants. Virtually all of the functions of extinguishing and mitigating agents are produced by a group of additives called surfactants. These are the chemicals that make water foam, wet, form a film, emulsify, and defoam. Surfactants (surface active agents) change the way one material, such as water, interact with another. An easy way to see surfactant action is to pour fresh water onto a waxed car hood. The water forms well-defined beads and does not wet the hood because of surface tension.
Surface tension is the attraction between water molecules that makes water form beads. This happens at the surface of water because the molecules don’t have anything “above” them to hang on to, so their attraction is redirected to the molecules on each side and below. The resulting force pulls the water into drops that don’t effectively wet a clean smooth surface. This is surface tension, and it is very strong in pure water.
Surface tension causes uneven brush streaks or spray drops in fresh paint, keeps lubricants from wetting bearings and cylinder walls, and keeps water from wetting a piece of cloth (the water drops bridge across the individual fibers). Fortunately, wetting agents address this condition. They move to the surface of the water, paint, or oil and reduce the attraction between the molecules, allowing the liquid to wet the otherwise dry surface (if surface tension is reduced further, the liquid will hold air and foam). Thus: Water + a wetting agent (surfactant) = A wet hood!
Surfactants are used in very large quantities by the paint and coating industry to reduce brush marks, in lubricants to coat bearing surfaces, in natural-cover firefighting to mop up, and in laundry detergents to wet clothes. Wetting agents are the principal additives in Class A foam.
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To make intelligent selections and buying decisions, we must know that low-density hydrocarbons will float on water and high-density hydrocarbons will sink, but they won’t mix, even if they stand for years (Figure 1)!
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As previously mentioned, pure water does not foam. Water might be whipped in a blender for an hour, but when the blender is turned off, the surface tension in pure water will cause the bubbles to collapse. Modern firefighting foam is a mixture of water, wetting/foaming agents, and a gas (air) in cellular form (bubbles). It is made using mechanical energy from a pumper, an air compressor for compressed air foam (CAF), or the stored air pressure in an extinguisher to mix the air with the liquid (Figure 2).
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Important: Effective firefighting foams flow evenly over the fuel surface, insulate, resist burnback, suppress vapor pressure, and are tested and listed to Underwriters Laboratories 162, Standard for Safety for Foam Equipment and Liquid Concentrates. Do not confuse wetting and emulsifying agents (listed under UL 6N72) with firefighting foams (UL 162; see Table 1 for a comparison of testing procedures for UL-162 and UL 6N72 agents).
Interfacial tension. Before we leave foams, there is an important question: “Can surfactants be of any additional help to us?” They can with a property called “interfacial tension” (this is the heart and soul of AFFF, and we need to know this to intelligently evaluate Class B foams).
Water molecules have an -OH (hydroxyl) radical at each end (HOH) and hydrocarbon molecules have a -CH radical at each end (HC–CH). These different type radicals do not “like” each other, which creates a barrier, called interfacial tension, between the dissimilar liquids.
Of course, with firefighting foam, the water is actually on top of the hydrocarbon, so a higher interfacial tension between the water and the hydrocarbon will increase the vapor-sealing property of a foam blanket. Thus, some of the foam solution that drains will form a thin film on the surface of the product—this is the AFFF film!
Aqueous (water-based) films are formulated with the addition of special surfactants, which increase interfacial tension. They are usually more expensive than wetting, emulsifying, and foaming agents.
Interfacial surfactants are used in the textile industry to “soil-guard” cloth and also in the paper industry to waterproof sandwich bags. (Water and grease don’t mix unless something is added.)
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Although increasing interfacial tension dramatically improves the action of foam on hydrocarbon-like products, it does not work with the other family of ignitables, the miscibles. Miscible liquids, such as alcohols and acetones, destroy regular foams by mixing with the water in the foam cells at the fuel surface and destroying the cell structure (Figure 3).
To resist the attack by the polar solvents, one type of foam has a water-soluble polymeric (gum-like) material in the foam concentrate. The additive is not soluble in polar solvents, and as the foam blanket breaks down, the polymeric additive forms a floating gum-like layer. This floating layer slows the attack on the remaining foam.
Facts about polymeric Polar/AFFF foam:
- The gum layer does not form on hydrocarbons.
- The additive results in a longer-lasting, slower-draining foam.
- The application rate for most polar liquids is higher than for hydrocarbons.
- Application rates for hydrocarbon and polar solvents vary according to supplier. Be sure to use the rate specified by your supplier.
Note: UL has recently listed a new FFFP (fluoroprotein film-forming polar) agent that does not form a polymeric layer.
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A small number of surfactants, known as defoamers, destroy the cells of a foam. They are valuable in paints, lubricants, and food processing (particularly for cooking fruits, such as apples). Defoamers reduce the impact of firefighting foam during system tests at industrial plants. A combination of wetting surfactants and defoaming surfactants is often used in paint and lubricants. Important: Although most surfactants give more than one property to water, usually a given chemical has one very dominant property—wetting, defoaming, foaming, and emulsifying, for example (Table 2).
What about soaps and detergents? One of the most important uses of surfactants is for cleaning, especially removing greases, oils, and other hydrocarbon-like materials from cloth, skin, concrete, and metals. If we wanted to remove only common dirt (inorganic materials) from cloth, we could use an agent that reduces surface tension and allows the water to wet between the fibers and flush out the dirt—a wetting agent!
Unfortunately, soiled materials usually include oil and grease. Some surfactants allow water to mix with oil and grease. They are emulsifiers, which we call soaps, detergents, degreasers, soil-release agents, and the like.
Emulsifiers bridge between water and hydrocarbons by looking like water at one end of the molecule and like a hydrocarbon on the other.
The ethyl alcohol acts as the surfactant that allows water to mix with pentane, a hydrocarbon. Heet (an alcohol) is added to gasoline tanks to absorb the water condensation that forms in cold weather. Emulsification is not a desirable function for firefighting foam and is not included in the UL performance tests because foam should form a water vapor seal, not mix with the liquid.
As early as the 1950s, the formation of a ring of water molecules around hydrocarbon molecules through surfactant action was described in Canadian literature as a micelle (see previous paragraph on emulsification). Micelles form with all soaps, detergents, and degreasers. The more common and understandable term for this function is emulsification.
Knowing the properties of various types of foam, their associated additives, and their chemical interaction with different substances is essential. Relating that information to the firefighting needs of your organization will result in an appropriate, satisfactory purchase. n
JOHN SACHEN is the haz-mat/training officer for the Delta (MO) Fire Protection District. He is chief instructor and hazardous-materials specialist with Fire Technology, Ltd., which produces fire training videos and specializes in on-site fire safety and hazardous-materials training. He is a member of the Fire Engineering and FDIC advisory boards.
