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Fire retardant foam, or fire suppression foam, is a foam used for fire suppression. Its role is to cool down the fire and to coat the fuel, preventing its contact with oxygen, resulting in suppression of the combustion. The surfactants used have to produce foam in concentration of less than 1%.
Other components of fire retardant foams are organic solvents (eg. trimethyltrimethylene glycol and hexylene glycol. Foam stabilizers are used as well, eg. lauryl alcohol. Other chemicals are used as well, eg. corrosion inhibitors.
Low-expansion foams are foams with expansion rate lower than 20 times. Foams with expansion ratio between 20-200 are medium expansion. The low-expansion foams, eg. AFFF, are low-viscosity, mobile, able to quickly provide coverage of large areas.
High-expansion foams have expansion rate over 200. They are suitable for cases when an enclosed space, eg. a hanger, has to be quickly filled.
Alcohol-resistant foams contain a polymer that forms a protective layer between the burning surface and the foam, preventing the foam breakdown by alcohols present in the burning fuel. Alcohol resistant foams should be used in fighting fires of fuels containing oxygenates, eg. MTBE, or fires of liquids based on or containing polar solvents.
Class A foams
Class A foams were developed in mid 1980s for firefighting wildfires. Favorable practical experiences led to its increasing acceptance for fighting other types of class A fires, including structure fires. * Class A foams facilitate wetting of the class A fuels, assisting with saturating them with water, which helps suppressing the fire and preventing reignition.
Class B foams
Class B foams are designed for firefighting of class B fires - burning flammable liquids. Using class A foam for extinguishing of a class B fire may lead to unexpected results, as the class A foams are not designed to contain the explosive vapors produced by the flammable liquids. Class B foams have two major subtypes.
Synthetic foams
Synthetic foams are foams based on synthetic surfactants. Synthetic foams provide better flow, faster knockdown of the flames, but limited post-fire security.Aqueous film forming foams (AFFF) are water-based foams, frequently containing alpha-olefin sulfonates, and/or perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) as surfactants. They have the ability to spread over the surface of hydrocarbon-based liquids. Alcohol-resistant aqueous film forming foams (AR-AFFF) are foams resistant to the action of alcohols, able to form the protective film even in their presence.
Protein foams
Protein foams contain naturally occurring proteins as the foaming agents. Protein foams flow and spread slower, but provide a foam blanket that is more heat resistant and more durable.Protein foams include regular protein foam (P), fluoroprotein foam (FP), alcohol resistant fluoroprotein foam (AR-FP), film forming fluoroprotein (FFFP), and alcohol-resistant film fluoroprotein (AR-FFFP).
Applications
Every type of foam has its best application. High-expansion foams are used when an enclosed space, eg. a basement or a hanger, has to be quickly filled. Low-expansion foams are used on burning spills. AFFF is best for spills of jet fuels, FFFP is better for cases where the burning fuel can form deeper pools, AR-FP is suitable for burning alcohols. The most flexibility is achieved by AR-AFFF or AR-FFFP. AR-FFFP has to be used in areas where gasolines are blended with oxygenates, which prevent the formation of the film between the AFFF foam and the gasoline and break down the foam, rendering the AFFF foam virtually useless.
See also
"Fire retardant foam"