How does intumescent paint work?

Three ingredients working together

A typical intumescent formulation contains an acid source (often a phosphate), a carbon source (a polyol) and a blowing agent (for example a melamine compound), held in a binder that softens when heated.

1. The binder softens and the acid source decomposes.
2. The acid reacts with the carbon source to form a carbon-rich char.
3. The blowing agent releases gases that foam the softened mass.
4. The result is a thick, porous char with low thermal conductivity.

The degree of expansion, the char strength and the insulation are specific to each product and are only known from its fire tests.

What the char does

The char is an insulating blanket between the fire and the steel. Heat still passes through, but much more slowly, so the steel temperature rises gradually. The coating is designed so that the steel stays below its critical temperature for the rated time.

Cellulosic and hydrocarbon fires

Building fires follow the standard (cellulosic) fire curve – ISO 834, ASTM E119, AS 1530.4. Hydrocarbon pool fires heat up much faster and hotter (UL 1709, EN 1363-2), and jet fires add erosion. Thin-film building intumescents are not designed for these conditions – epoxy systems tested for hydrocarbon and jet fires are.

What decides performance

• Dry film thickness – more coating, more char.
• Section factor – how fast the member heats up.
• Critical steel temperature – the target the steel must stay below.
• Section shape – open sections and hollow sections are tested separately.

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