How They Work: Retardants Drive Wood Decomposition To Dehydration, Hinder Ignition

Trees and brush don’t just ignite from a stray spark. Wood itself, composed of cellulose, a natural polymer, doesn’t burn.

For wood to catch fire, heat—such as that generated by an approaching wildfire—must first thermally degrade cellulose. Decomposition, however, happens via two processes, one of which results in fire and the other imparts a protective carbon coating to wood.

The predominate decomposition process is depolymerization, which breaks the molecular bonds of cellulose. This initially produces a tarry mixture of which levoglucosan is a major constituent. Further heating turns this goo into volatile compounds that include alcohols, aldehydes, hydrocarbons, and ketones. The gases escape from the tree and can react—ignite—with atmospheric oxygen in the exothermic, light-emitting process called fire.

A smaller amount of decomposition takes place through dehydration. In this process, cellulose breaks down to water vapor and a solid that becomes slow-burning or smoldering char, a carbon-based solid that can eventually turn to ash. Char also forms an insulating barrier on wood that can slow further thermal degradation of cellulose.

Treating vegetation with ammonium phosphate-based fire retardants alters the balance of decomposition processes, leading to more dehydration and less depolymerization, explains Edward Goldberg, ICL Performance Products LP business director for fire safety.

The firm produces the aerially applied fire retardant used by the U.S. Forest Service. This happens because at the high temperatures generated by forest fires, the phosphates in fire retardants decompose to phosphoric acid and then to water and phosphorus pentoxide, Goldberg says.

P₂O₅ causes dehydration of cellulose and thus promotes formation of char instead of flammable tars. The water vapor released in dehydration both helps cool the fire and reacts with P₂O₅ at lower temperatures to regenerate phosphoric acid, he says.

Because the phosphoric acid is regenerated repeatedly, Goldberg says, “a small amount of retardant will treat a large amount of fuel.”