Intumescent coating surface temperature measurement in a cone calorimeter using laser-induced phosphorescence

Intumescent coating acts as a thermal barrier for construction materials during fire hazards. The coating has the ability to expand by a factor of 40 or more during heat exposure. However, due to its fragile consistence, no physical or optical measurement technique has previously been able to measure the coating surface temperature during expansion. This is undesirable since without accurate information about the coating surface temperature, it will always be difficult to fully understand the heat transfer processes from the fire exposure to the coating and within the coating. Thermographic phosphor technique makes it possible to reliably measure intumescent coating surface thermometry. This paper presents the results of an experimental study using this technique. It compares experimental results with predictions of a calculation model for measurements made inside a cone calorimeter in well-controlled conditions.The coating was applied to steel plates having different thickness, in layers of 1000 g/m2 by means of a brush. Thermographic phosphor particles were seeded on the surface of the coating. The coated plate was placed in a non-standard electric cone calorimeter, which was calibrated to give a constant radiant heat flux of 50 kW/m2, measured by a Gardon-type Medtherm heat flux meter at a height of 40 mm from the surface of the coating before expansion, being about the expected position after the intumescent coating had fully expanded. The third harmonic generation of a pulsed Nd:YAG laser at 355 nm was used to excite the phosphor particles. A photomultiplier detector monitored the subsequent emission and an ICCD camera was used to measure the expansion rate of the intumescent coating. A comparison between measurement results using this technique and predictions of a model that adequately describes the heat transfer condition of the test setup shows that the measured and predicted surface temperatures were generally in good agreement. The inaccuracy in the predicted results is due to the uncertainty over the emissivity value of the intumescent coating surface. The results of this study clearly indicate that thermographic phosphor technique is promising.