A study of the global kinetics of thermal degradation of a fibre-intumescent mixture

Abstract

A mathematical model is being developed to predict the behaviour of an intumescent flame retardant within a rigid, resin-impregnated, textile-reinforced composite during exposure to heat. As part of a larger programme of research into flame retardant composites, there exist considerable empirical data showing the enhancement of char formation, at temperatures greater than 400 °C, when an intumescent based on melamine phosphate is dispersed on the surface of a flame-retardant, cellulosic fibre. Visil (Sateri, Finland), is a flame-retardant viscose fibre containing polysilicic acid which interacts with phosphate or polyphosphate components of the intumescent to form a resistant char-bonded structure. When introduced into a composite either as a pulverised additive, or as additional layers with glass fabric in the composite structure, it has been shown to promote enhanced char formation. In order to model the thermal decomposition of a cellulosic fibre and intumescent mixture, a number of different decomposition schemes, involving competitive and successive reactions, have been considered. The thermogravimetric mass loss curve displays at least three waves of overlapping reactions, so that the kinetics cannot be estimated by the usual differential or integral methods. A simple, reduced kinetic scheme has been compared with a temperature dependent scheme to determine the effect of increased heating rates on the yield of solid residue. The results will be used in later work for the interpretation of Arrhenius parameters estimated from thermogravimetric data by the method of minimisation of least squares.