A methodology and a simple engineering fire performance model for Intumescent Fire Retardant coatings

Abstract

This paper presents a practical engineering model to determine the flammability properties and predict the fire performance of Intumescent Fire Retardant (IFR) coatings by performing experiments on a 5.0 mm inert steel substrate in the Cone Calorimeter. The present model simplifies the complete conservation equations governing the IFR system by utilizing the assumptions that the initial IFR coating layer (virgin plus melt) is thermally thin and its expanding char layer, which includes also the vapor and the foam, has negligible volumetric heat capacity. These simplifications eliminate the requirement in the existing models for determination of numerous expansion and thermal properties for the IFR coating and its resulting char. The thermal insulation effect of the expanded char is represented as a ratio between the heat flux received by the substrate and the heat flux imposed on the substrate surface for the case there is no char layer. This heat flux ratio is expressed as a function of the expanded char height history, which is related to the mass loss rate and total mass loss of the IFR coating as determined from modeling of mass loss rate based on TGA measurements. This type of modeling for the IFR expansion height is based on the observation that the mass loss as measured from the TGA experiments is in fact contributing, through chemistry and diffusion, to the formation of the gas bubbles in the expanded char. The accuracy and robustness of the model are validated by comparing its predictions of expansion height and steel plate temperature in Cone Calorimeter experiments at different heat fluxes for two commercial IFR coatings having different initial masses. Compared to other simple empirical models or complicated modeling efforts, our model includes the basic physics of IFR expansion and insulation in a comprehensive way. Improvements are also anticipated for oxidation or degradation of the expanded char at high temperatures over 1200 K.