A theoretical and experimental investigation of intumescent behaviour in protective coatings for structural steel

Abstract

A mathematical model describing heat transfer and expansion processes within an experimental intumescent coating is described. The model has been developed alongside a relatively comprehensive experimental programme involving analytical methods, standard and non-standard furnace tests and mass-loss calorimeter (MLC) tests. The model is fully continuous (rather than semi-discrete as in other approaches) and uses a simple competitive reaction scheme to describe the kinetics of the initial gas-forming step of the coating degradation reaction. The degradation mechanism is coupled with a char expansion sub-model, where a fraction of the evolved gas is trapped causing expansion. This scheme incorporates endothermic and exothermic reactions, the heats of which have been estimated from DTA. Much effort has been expended on a realistic description of the heat transfer processes within the expanding char and a detailed composite thermal conductivity model including radiation transfer across pores is included. This has been calibrated for fully expanded chars using empirical temperature dependent thermal conductivity data. Model results compare well with furnace test results. However, results from MLC experiments demonstrate a larger than expected range in coating expansion than predicted by the model. These observations emphasise the importance of the basic expansion mechanism and demonstrate that this critical area requires more research.