Influence of heating conditions and initial thickness on the effectiveness of thin intumescent coatings

The study presented herein shows an experimental methodology aimed at analysing the effectiveness of intumescent coatings through detailed characterisation of their thermo-physical response for a range of heating conditions and applied initial dry film thickness (DFT). Steel plates coated with a commercial solvent-based thin intumescent coating were exposed to well-defined and highly-repeatable heating conditions in accordance with the H-TRIS test method. Experimental results emphasise that the swelling process and the resulting swelled thickness govern the thermo-physical response of intumescent coatings, thus their effectiveness. During swelling, the coated steel asymptotically tends to the temperature range 300–350 °C, regardless of the heating condition or DFT. Thermo-gravimetric analysis demonstrates that the coating undergoes the swelling reaction at this temperature range. Once the swelling process is completed, the steel temperature increases above 350 °C. The steel temperature acts as an indicator of the swelling process, as the reaction occurs near the steel-coating interface. The intumescent coating swells and insulates the steel substrate by displacing the already-swelled coating towards the direction of the heat source. Aiming at predicting the swelling of intumescent coatings, empirical correlations are derived: the swelling rate is governed by the heating conditions and the maximum swelled thickness is governed by the initial DFT.