An Introduction to Firefighting Foams

Sometimes early in this new century, someone will write about significant innovations and technologies of the latter half of the 20^th century. The invention of the microprocessor will no doubt dominate the discourse concerning its influence and effect on everyday life. Other chapters could include bio-technology, advancements in material science, and telecommunications.

Hopefully, as a discrete nod to fire safety, a footnote will appear somewhere to note the following:

fire fighting foam: "Bleeding edge" technology added to water in order to extend the fire fighting capabilities of water. Reduces water usage. Finally gained acceptance in the late 1990's.

The fire fighting industry is not the first industry one thinks of as being innovative. "Leading Edge" is not a word you associate with this industry. It has the same importance as the lettuce at your local grocery store. After all, the use of water as the prime fire extinguishing agent has been around forever. Why innovate?

In fact, there actually has been some innovations. We're not talking rocket science here; nevertheless, the use of foam over the last twenty years has made inroads in providing a better extinguishing agent than straight water.

We know that by converting water into steam, air and heat is displaced - the volume of air (oxygen) is reduced. Water adsorbs the most heat when it is converted into steam, and it will be converted into steam more easily from droplets than from a solid stream. Converting straight water stream to droplets increases the surface area that can be converted to steam. Converting droplets to bubbles further increases this surface area and conversion.

One way to convert a water stream into small droplets is through specially designed nozzles. The other way is to add foaming agents to create foam bubbles. In a general way, the addition of these agents extends the extinguishing properties of water by:

A. Creating a foam blanket to cover the fire.

B. Reducing the surface tension of water in order to allow water to spread and penetrate combustible materials

C. Creating a thin film over the fuel surface, thereby enhancing burn-back and vapour protection.

Although foam has been around for at least 50 years, its use in the fire service has been limited to fighting hydrocarbon flammable fires, or Class B fires. It is only later, within the last decade or so, in which foam has also found use for Class A fires (combustibles, wood, paper). Even with these application, a vast majority of fire service personnel are still ignorant of the different types of foams and their use.

Over the years, fire fighting foam has evolved from two different methodology: Chemically produced foams and Mechanically produced foams.

Chemical foams were the first type of foams produced for fighting flammable fuel fires (Class B fires), consisting of a chemical reaction between solutions of sodium bicarbonate and saponin with a solution of aluminum sulphate. This reaction produced a stiff, fuel resistant foam. It was commonly referred to as "Foam Charge A & B"; however it is no longer being used.

Mechanically produced foams are foams in which a foam concentrate is proportioned through a mixing device with water and discharged through a special nozzle in which outside air is drawn in inorder to aerate the solution and produce foam. Depending on the type of nozzles being used as well as the kind of foam concentrate, a wide range of foam expansion ratios can be obtained.

Mechanical foams are derived from two different formulation: protein based and synthetic based.

Protein foams first appeared before the Second World War. These foams are made from protein-rich materials such as blood meal, chicken feathers, hoof and horn, and soy meal which have undergone a process known as hydrolisis inorder to extract the protein component. Protein foam produces a low expansion foam with good heat resistance, but stiff in structure. The addition of small amounts of fluorosurfactants enables the foam to form a film over the fuel surface inorder to further resist fuel contamination and improve vapour sealing. All-purpose protein foams were developed in order to extinguish both hydrocarbon and polar solvent (alcohols) fuel fires. An early form of this foam was a product known as Unifoam NN in which the protein concentrate was hydrolized "in line" with the addition of water, producing an alcohol resistant foam. The current method usually involves the addition of various polymers in order for the foam to resist the foam destructive effects of alcohols.

Another recent development within the Protein family is a product known as Film Forming Fluoroprotein foam or FFFP for short. FFFP provides similar film forming characteristics as the synthetic based AFFF (see below) but with better heat resistance and greater tolerance to fuel contamination.

Synthetic based foams were introduced in the early 1950's with the development of hydrocarbon based surface active agents (surfactants) for the production of detergents. Over the years, various kinds of synthetic based foam have evolved: from high expansion foams for use in flooding aircraft hangars or mine tunnels, to low expansion film forming foams for flammable fuel fires.

In particular, low expansion foams, in which foam is expanded from 1 to 50 times, using a variety of air-aspirated nozzles, were developed with the addition of a fluorine based surface active agent (fluorosurfactant) creating a very fluid foam, which despite its lower heat resistant than regular protein foam was able to "knock down" hydrocarbon fuel fires much faster with less foam. This kind of foam, more commonly known as AFFF (Aqueous Film Forming Foam) also provides a very thin film which floats on top of the fuel and help seal any vapours. The addition of special polymers to the AFFF allowed another kind of film to be released onto the fuel; in this case on polar solvent. This polymer protects the foam blanket from being destroyed by the effects of alcohols. This kind of foam is more commonly known as "Alcohol Resistant" AFFF.

Within the last 10 years, a new type of foam has appeared which has been making inroads in the fire service. Originally developed and used for forestry applications, Class A foam brings a serious challenge to straight water usage in that this kind of foam can be used at very low application rates - as low as 0.1%.

Actually, what has made Class A foam more acceptable has more to do with the way foam is created. The developments of compressed air foam systems in which compressed air is used to create the foam instead of an air- aspirating nozzle, has enabled the foam to be used at very low application rates, yet still provide all of the characteristics of foam that has been previously stated. Depending on the application, a range of foam structure can be produced: from a wet foam, in which the foam liquid is used for fast spreading and penetration; or to a dry foam, in which very little water is used, producing a thick foam blanket for exposure protection. In certain cases, Class A foam can also be used on flammable fuels, provided there is sufficient AFFF for burn-back protection, since a Class A foam has no film-forming capabilities.

Acceptance of foam has been slow going. The three "E's": Education, Economics, and Effort are still the major stumbling blocks; however, in time these will be overcome and foam will be part of the vocabulary in the next century as water is in this one.

George Vestergom Jr. is Product Developer at Unifoam Company Limited. Unifoam has been manufacturing fire fighting chemicals for over 40 years. He can be reached at: unifoam@total.net

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