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rom powder to foam: What’s the surprising science behind the diversity of fire retardants
A fire retarder is a substance used to slow or stop the spread of fire or reduce the intensity of flames. This is usually done through chemical reactions that reduce the flammability of the fuel or delay its combustion. Fire retardants can also cool the fuel by physical action or endothermic chemical reaction. These products come in powder form and need to be mixed with water before use. They can also be used as firefighting foams and fireproof glues, and can also be applied to objects as coatings or sprays. Fire delay agents are widely used in firefighting work, and this type of product can be used both in the air and on the ground.
How it works
Generally speaking, fire retardants reduce the flammability of a material by either physically blocking the flame or triggering a chemical reaction.
Physical effects
Under physical action, this process can delay the combustion process by:
Cooling: Certain chemical reactions can actually cool a material.
Forms a protective layer: prevents underlying materials from igniting.
Dilution: Some retarders release water and/or carbon dioxide during combustion.
Commonly used fire retardants include aluminum hydroxide, magnesium hydroxide and some specific mixtures. When heated, aluminum hydroxide dehydrates to form aluminum oxide, releasing water vapor in the process. This reaction absorbs a large amount of heat, lowering the temperature of the added material, and the residue of aluminum oxide forms a protective layer on the surface of the material.
Chemical Action
The reaction can occur in both the gas phase and the solid phase:
Gas Phase Reactions: Chemical reactions in a flame can be interrupted by fire retardants. Such delay agents are generally organic halogenated agents, such as Halon and PhostrEx.
Solid Phase Reaction: Some retarders will decompose the polymer, causing it to melt and move away from flames. While this allows certain materials to pass certain flammability tests, it is not known whether fire safety is improved by the production of flammable plastic droplets.
For carbon-based fuels, solid phase fire retardants will form a carbonaceous char layer on the surface of the fuel, which is more difficult to burn and prevents further combustion process. Additionally, intumescent retarders cause expansion behind the protective char layer, providing better insulation. Such products can be used as additives for plastics and in coatings for protecting wood or steel structures.
Purpose
Fire Fighting Equipment
Class A foam is used as a fire retarding agent in 2.5-gallon APW and CAFS extinguishers to control incipient brush and grass fires by creating a firebreak. Other chemical retardants render Class A and Class B fuels non-combustible and extinguish Class A, Class B, and some Class D fires. Fire-retarding slurries dropped from aircraft are typically dropped before wildfires start to prevent ignition, while fire suppressants are used to extinguish open flames.
Surface coating
Objects may be coated with a fire retarding agent. For example, Christmas trees are sprayed with delay agents, which become a fire hazard because the trees become extremely flammable when they dry out. Steel structures are coated with fire-retardant paint around columns and vertical beams to prevent the structure from becoming vulnerable in a fire. In the United States, dormitory regulations require the use of these products. Many dormitories and older school buildings use intumescent coatings, especially historic buildings that lack fire sprinkler systems.
Forest fire fighting
Early fire retardants were a mixture of water and thickeners, and later borates and ammonium phosphates were added. Typically, fire retardants are dropped from aircraft or applied by ground teams at the edges of wildfires to slow the spread of the fire and give ground teams time to extinguish the flames.
Aerial firefighting: Aerial firefighting is a method of fighting wildfires using aerial vehicles, including fixed-wing aircraft and helicopters.
Aerial firefighting teams, called smoke jumpers and rappel teams, are dropped from a variety of aircraft by parachuting or rappelling, using chemicals such as water, water enhancers, or specially formulated fire retardants.
Application of text materials
Fire retardants used on wildland fires are typically a mixture of water and chemicals designed to wet the area and chemically retard the spread of fire in vegetation. Such products are often tinted to facilitate aerial visualization of the application area. Products based on nontoxic and biodegradable fire retardants that prevent wildland fires when used on U.S. federal lands must be included on the U.S. Forest Service’s Qualified Products List.
Environmental and health risks
While the forest fire retardants used are considered non-toxic, even low-toxic compounds can pose risks in the event of exposure in large quantities. Many of the fire retardants used release dioxins and furans when burned, chemicals that are toxic to fish and wildlife. The U.S. Forest Service is the agency that monitors and studies the effects of fire retardants on America’s wildland systems.
Long-term exposure to these chemicals may cause cancer or skin diseases, and potential contamination of the environment also becomes a safety issue.
As aerial firefighting becomes more common, these toxic chemicals have the potential to contaminate soil and water systems, which in turn can enter the human body, increasing the likelihood of developing long-term health issues such as respiratory disease or other health risks. These issues have prompted people to think about how to ensure the safety of our environment and human health as technology develops?
