Can Waterborne Intumescent Coatings Be Used Externally?

It is vitally important that products used for fire protection applications, such as intumescent coatings used for structural steel fire protection, are correctly specified for the particular operating environment. In general terms there are two product categories that intumescent coatings fit into, waterborne or solvent borne. Waterborne intumescent coatings are traditionally formulated for use in dry, internal controlled environments whereas solvent borne intumescent coatings are generally suitable for use in both internal and external locations.

When exposed externally to rain and UV, the durability performance of any coating system is put to the test. In the case of intumescent coatings the incorrect specification of the coating for the particular exposure conditions can ultimately lead to a severe degradation in the fire performance of the coating. This is most likely to occur when a waterborne intumescent coating has been applied in an external location.

The specification of a waterborne intumescent coating, including application of a top seal (finish coat), does not automatically qualify the system as being suitable for use in external environments.

By their very nature, waterborne intumescent coatings are hygroscopic i.e. they will absorb water from their surroundings. The application of a top seal will act as a surface sealant initially but as this seal coat ages and weathers its ability to prevent moisture intrusion will be reduced and surface degradation of the paint film will be even further accelerated with exposure to UV.

Over time the top seal will oxidize and micro-fine cracking will occur allowing moisture to penetrate into the intumescent coating. Any imperfections, or damage to, the top seal film will accelerate this process and localised areas of damage can occur within a matter of weeks.

In an external location, the inevitable absorption of moisture will leach vital ingredients from the intumescent film and quickly reduce the fire performance of the system. The next stage is a visible swelling of the intumescent, bubbling and ultimately disbondment of the coating from the primer.

To protect the building owner, and the occupants, the supplier/manufacturer of the intumescent coating system must provide qualified testing evidence that the coating system specified will be suitable for external application, for example, ETAG 018 part 2 exterior “X” classification. Nullifire SC902 has been tested in accordance with ETAG 018 durability classifications and meets all categories > X: exposed, Y: semi-exposed, Z1: humid internal and Z2: dry internal. In addition, it has been tested “Prohesion” Cyclic Corrosion Test to ASTM G85:2009 Annex A5 – At 1000 hours total exposure in accordance with BS EN ISO 12944-6 test procedures for water condensation /neutral salt spray.

It is not merely a case of providing a statement of suitability without supporting evidence, as now more than ever before, the onus of responsibility rests with the specifier in assuring that ‘fitness for purpose’ is a prime requisite for products used in ‘life safety’ applications.