An intumescent coating foams up when heated in a fire. Applied to heat sensitive materials, the resulting insulation protects them for a limited period from the heat of the fire. They are of 2 types: to protect structural steel and to protect wood and other combustible materials. Intumescent coatings are not fire retardant coatings which burn very slowly or not at all in a fire, but do not protect the subsrate from the heat of the fire.
The purpose of protecting structural steel is to give time for the building occupants to leave the building in case of fire, before it collapses, as the heat of a fire will soften the steel. Protection of woodwork is to stop it burning and adding to the heat of the fire; it may also be used to protect valuable woodwork.
Intumescent for steel protection is usually thick and opaque and finished with a thin protective fire retardant coat, similar to a gloss. For wood, the coating is usually transparent to allow the character of the wood to show through.
When dry, an intumescent coating is a reactive layer, so it is important to achieve the correct thickness of dry film to obtain the required fire resistance. This thickness will be specified by the coating manufacturer. For spraying, the film thickness is measured wet with a wet-film gauge, as a certain wet film thickness will dry to a specific dry film; this will also be specified by the coating manufacturer.
Because the wet film needs to be relatively thick, of several hundreds of microns according to the particular formulation, intumescent coatings are often thick to avoid slumping and runs while still wet. Several coats may need to be applied to build up to a total dry coat thickness to give the required heat protection. Typical spray tip sizes for spraying intumescent range from 27 thou’ to 36 thou’ and require a much more powerful airless unit to spray them, than require for general painting.
Until the last 10 years, the typical airless unit for spray painting intumescent coatings was a pneumatic pump having an air motor and capable of providing up to 4 - 5,000 psi at the pump. Such a unit requires a road compressor of about 150 cfm and is a heavy unit. The household name for these pneumatic pumps is the Graco King.
Improvements in intumescent formulation have reduced the weight of coatings and the need for very high pressures. Today, almost all intumescent coatings can be applied with a more compact self contained unit having either a petrol engine or an electric motor.
In the UK, petrol engined units are favoured over electric because electric power is often not available on site at the early stage of the life of a building when the intumescent is applied.