Components of an intumescent paint coating system

Intumescent paint coating systems generally have three components:

• A primer
• The intumescent coating or basecoat
• A sealer or topcoat

Of these three it is the basecoat that is the most important (in terms of passive fire protection)... this is the component that reacts to a fire.

Intumescent paint coating anatomy

This photo above shows the recommended coatings over mild steel. Consult with the intumescent paint coating manufacturer for their approved primer and topcoat... these vary from product to product.

All surfaces should receive an abrasive blast to SA2.5 prior to applying the approved primer.

Always refer to the relevant technical datasheet prior to use to ensure a product is suitable for your application.

Visit the manufacturer's website to find the relevant datasheet or check our own library of datasheets that are regularly udated.

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The primer

The primary purpose of the primer is to assist with the adhesion of the basecoat to the steel. It should be applied in strict accordance to the manufacturer's data sheet in order to function correctly. Primer coats generally have a dry film thickness (DFT) of between 75 and 100 microns.

The dry film thickness of the primer should be measured to ensure compliance with the specification. This thickness should also be subtracted from the subsequent basecoat thickness in order to accurately assess the basecoat thickness.

Newer products such as Nullifire SC902 and Interchar 3120 for internal steel applications can go over blasted steel that has tightly adhered rust with no primer.

Note: this is only for internal structural steel elements within the enclosed building elements.

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The basecoat - the intumescent coating itself

As already mentioned, the basecoat is the component that does all the heavy lifting.

It achieves its protective properies through these components:

• A catalyst, which decomposes to produce a mineral acid like phosphoric acid. A common catalyst is ammonium polyphosphate.
• A carbohydrate which combines with the mineral acid to form a carbonaceous char. Starch is a typical carbohydrate used.
• A binder (or resin) which softens at a predermined temperature within the upper limits of the reaction temerature range.
• A spumific agent which, together with the melting of the binder, decomposes to liberate large volumes of non-flammable gases. These gases include carbon dioxide, ammonia and water vapour. The production of these gases cause the binder to foam and expand, providing an insulating char many times the thickness of the original coating.

Basecoat wet film thickness (WFT) and dry film thickness (DFT) is determined by product loading schedules that take into account the steel section factor (Hp/A).

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The topcoat (or sealer)

The primary purpose of the sealer is to protect against degredation of the basecoat. Thick film intumescent coatings may also need a sealer to prevent chalinking in situtions where the structure is exposed to ultraviolet or moist conditions.

Sealers also perform a decorative function in some applications. Various levels of finish can be applied depending on aesthetic requirements and budget.

In some instances, however, a sealer may not be necessary.