High-tech firefighting.

• Author: Thaler, Sebastian
• Position: Fire retardants - Research & Development

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A FIRE RETARDANT is any substance other than water that reduces the flammability of materials or delays their combustion. Fire and flame retardant technologies have been in existence for many years; as early as 1735, Englishman Obadiah Wild secured a patent for "preventing combustible substances from flaming," including canvas used in theaters and buildings. The U.S. Forest Service, meanwhile, which had been dropping water from airplanes onto forest fires since 1930, turned to chemicals in 1956. In the modern era, the spectacular destructive power of wildfires that strike the western U.S. each year has exposed the need for technological innovation in this area. With the National Interagency Fire Center warning of "higher than normal wildfire potential" across large stretches of the American West this year--what unfolded in California in early August may be a scary harbinger of things to come--a review of specific advances is timely.

There are distinct types of physical mechanisms that come into play when fire retardants are deployed. First, cooling of the combustible material can occur. For instance, one commonly used fire retardant is aluminum hydroxide. When heated, it dehydrates to form aluminum oxide, releasing water vapor in the process. This reaction absorbs a great deal of heat, cooling the material over which it is coated. That coat also forms a protective layer that prevents the underlying material from igniting. Then there is the process of dilution, as some retardants release water or carbon dioxide when exposed to flame, a process that may extinguish the latter.

As for the range of chemical reactions involved with fire retardants, at least four can come into play. One is the simple interruption of chemical reactions in the flame, extinguishing it; this can be triggered by contact with fire retardants such as Halon and PhostrEx. A second example concerns the ability of some polymer-based retardants to break down, melt, and flow away from the flame, allowing some materials to pass flammability tests. However, the resulting plastic droplets can be flammable, and there is argument over whether fire safety really is improved by their production. Another example is char formation; for carbon-based materials, some retardants can cause a layer of carbonaceous char to form on the fuel surface, inhibiting further burning. Finally, a class of retardant materials called intumescents incorporates chemicals that initiate swelling behind the protective char layer, in effect providing improved protection. In addition to being added to plastics, intumescents are available as paints for protecting wooden buildings and steel structures.

Attempts to mitigate the destructive power of wildfires can be waged on two fronts: from above and on the ground. The annual Fire Season Outlook released by the Predictive Services Group at the National Interagency Fire Center reports that, coming off a strong El Nino winter weather pattern, portions of the western U.S. entered the 2010 fire season drier than normal. According to the report, the drier conditions suggest...