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Advice Centre

What is Intumescent? Your questions answered...

What is Intumescent?

A common question we get asked is “What is intumescent”, or “What does intumescent do?”  Let’s explain…

An ‘intumescent’ is a material that swells up and expands on exposure to heat.  There are a few main types of intumescent, but most can be classified as ‘soft char’ or ‘hard char’.

Soft char intumescents, such as ammonium polyphosphate, produce a light, fluffy char that produces very little pressure but is an excellent insulator.  This is often used in fire protection for glazing, as high pressure could shatter the glass and make the situation a lot worse.  Soft char intumescent materials are also suitable for spraying so are commonly found in spray mastic and fire protective coatings for structural steel.

Hard char intumescents are the most common, and include sodium silicate and graphite based materials.  These produce a much higher pressure and substantial char, and are used in applications like plastic pipe fire stopping and intumescent strip for around fire doors.


How do they work?

In the case of an ammonium polyphosphate based intumescent, this is combined with other starch-based materials to cause swelling.  When the ammonium polyphosphate is exposed to heat, it starts to decompose and produce CO2, helping to prevent it from burning.  The resulting char protects the underlying layers from heat and fire.  It also allows for quite significant movement of components during a fire without compromising the protection.

Sodium silicates and graphite based intumescent materials contain hydrates, that turn to gas when they get hot, thus expanding and forming a seal.  Sodium silicate intumescents have a very low activation temperature of just 110-120°C, whilst intercalated graphite intumescents start activating at around 200°C.  Graphite intumescents are extremely flexible, so are commonly used in pipe wraps and collars.


What do they do?

Intumescent products form a vital part of fire protection in a building.  Fire travels extremely rapidly, and finds any small gap or hole to spread through.  These gaps or holes must be protected, and by doing so with an intumescent material the passage of smoke and fire through a building is limited or even prevented, helping to contain the fire and ensure safe and rapid evacuation of the building.