Abstract
A model is constructed for intumescent paints in which it is assumed that the transition to the swollen state occurs at a very thin zone or front. Across this front there is a discontinuity in the density, the velocity, and the temperature gradient. The temperature is continuous and fixed at the front at a value that is a property of the paint. All of the mass loss through outgassing occurs at the front, and this gives rise to a large increase in the volume of the paint particles as they are traversed by the front. On each side of the front, the physical processes consist exclusively of heat conduction. In the context of this model, a finite coating subject to a heat flux contains two moving boundaries: the front, and the outer surface of the coating at which the flux is applied. Through an elementary mathematical transformation, this problem is reduced to one in which the outer surface is fixed and only the inner front moves. This simplified problem is of Stefan type, easily amenable to numerical analysis. The time-dependent substrate temperature, when calculated in this way, tends to level off at a constant value until the front has completely traversed the coating; then it sharply increases again. This is a characteristic of much of the available experimental data and, combined with visual evidence, lends credence to our model.
Published Version
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