An intumescent is any material that expands when exposed to heat. Intumescent materials typically are used in construction to provide passive fire protection — in some cases delaying the spread of fire by as long as one hour. Care must be taken when selecting an intumescent, because some are known to degrade over time, particularly because of humidity.
When exposed to heat over 300 degrees Fahrenheit (149 degrees Celsius), intumescents generate a thick layer of ash or foam, which conducts heat poorly, and burns slowly. Although the expansion of intumescent materials generally is 15-30 times their original thickness, some manufacturers claim expansions of as much as 200 times. After the fire burns through the first layer of insulative foam, it might reach another layer of intumescent, which will again form additional ash or foam.
By this cycle, the fire might be retarded until all the intumescent material is spent. This process might last for several minutes or as long as an hour before the fire spreads further. If the fire is extinguished before the intumescent material is exhausted, then the foam can be scraped off with minimal damage to the original structure.
Many intumescents contain chemically bound water and are endothermic. In this case, when exposed to fire, the intumescent will release water. This will help cool the surrounding areas.
There are a variety of ways that intumescent materials can be used to provide fire protection. Intumescent gaskets used in electrical boxes and fire doors provide fireproofing by significantly slowing down the spread of a fire. Whereas normal paint is highly flammable, an intumescent coating can be added to paint to give significant fire protection. The fire-resistance rating of structural steel can be raised through the application of intumescent coatings. Intumescent materials are also used within floor or wall assemblies as a fire blanket.
When exposed to elevated humidity, some materials will lose their intumescent properties — for example, they will not generate as much foam or char in the event of a fire. In fact, normal humidity is enough to degrade some intumescents. Some intumescents can also degrade from exposure to ultraviolet radiation or heat. These sensitive intumescents can still be useful, but they must have a protective coating at all times; if the coating wears off, the intumescent might degrade immediately. Several independent agencies test intumescent coatings and rate them based on their tolerance of environmental stimuli as well as their fire-resistive performance.