Coatings for passive fire protection

In the event of a fire, or merely temperatures in the range of 400 to 550 °C, the load bearing ability of steel is reduced significantly and this has implications for structures based on steel parts.

A cellulosic fire (e.g. buildings) can roughly be divided into three periods: 1) ignition, 2) developing fire and 3) fully developed fire. The temperature rises typically to 1100 °C in about 3 hours. To simulate and standardize the progress of a cellulosic fire, the cellulosic fire curve has been developed. It has been used for many years before the hydrocarbon fire curve was developed. Nowadays, the most commonly used fire curves (ISO 834, BS 476-20/22) within the fire protective products industry are:

  • The cellulosic fire curve
  • The hydrocarbon fire curve

The need for the hydrocarbon curve became apparent when it was clear that the burning rates of solid material like plastics, wood, paper, cotton or textiles did not match the burning rate of liquid or gaseous compounds like petrol and chemicals, which can be significantly higher. In the petrochemical industry, substrates may be exposed to hydrocarbon fires, which are quite different from cellulosic fires with rapid temperature rise to about 1100 °C within the first 5 minutes. In addition, erosive jet flames can reach 150 m/s and are used to simulate leakages from high pressure hydrocarbon gas containing vessels.

An efficient way to protect the steel structure and prolong the time before the problematic steel temperature is reached is by intumescent coatings. At elevated temperatures, the intumescent coating swells (about 40 times) to a thermally insulating char protecting the steel against the fire.

Intumescent coatings are comprised of five basic compound groups: blowing agent, acid- and carbon sources, and binder and pigments, which are all part of the char formation process. The interaction between the different compounds is complex, and various reaction sequences are reported in the literature. Research is ongoing to get a better understanding of the mechanisms for char formation including the important parameters providing a stable and mechanically strong char.

Disciplines involved in understanding the working mechanisms of intumescent coatings are chemical reaction kinetics, rheology, and mass and heat transport phenomena.