These intumescents produce a light char, which is a poor conductor of heat, thus retarding heat transfer. Typically the light char consist of microporous carbonaceous foam formed by a chemical reaction of three main components: ammonium polyphosphate, pentaerythritol and melamine. The reaction takes place in a matrix formed by the molten binder which is typically based on vinyl acetate copolymers or styreneacrylates. Ablative coatings contain a significant amount of hydrates. When the hydrates are heated, they decompose, and water vapour is released, which has a cooling effect. Once the water is spent, the insulation characteristics of the char that remains can slow down heat transfer from the exposed side to the unexposed side of an assembly. Soft char products are typically used in thin film intumescents for fireproofingstructural steel as well as in firestop pillows. Typically, the expansion pressure that is created for these products is very low, because the soft carbonaceous char has little substance, which is beneficial if the aim is to produce a layer of insulation.
Hard char
Harder chars are produced with sodium silicates and graphite. These products are suitable for use in plasticpipe firestops as well as exterior steel fireproofing. In those applications, it is necessary to produce a more substantial char capable of exerting quantifiable expansion pressure. In the case of firestops, a melting, burning plastic pipe must be squeezed together and shut so that there will be no opening for fire to propagate to an otherwise fire-resistance rated wall or floor assembly. In the case of exterior fireproofing, a hydrocarbon fire must be held off with quite potentially more kinetic energy than a house fire. Intumescents that produce hard chars are unsuitable for interior spray fireproofing.
Applications
Intumescents are used to achieve passive fire protection for such applications as firestopping, fireproofing, gasketing and window casings. Such applications are relevant for buildings, offshore constructions, ships and aircraft.
Problems
Some intumescents are susceptible to environmental influences, such as humidity, which can reduce or negate their ability to function. In Germany, the Deutsches Institut für Bautechnik, DIBt, quantifies the ability of intumescents to stand the test of time against various environmental exposures. DIBt-approved firestops and fireproofing materials are available in Canada and the U.S.
