Complex char formation in flame-retarded fibre-intumescent combinations—II. Thermal analytical studies

Abstract

It is well known that flame retardant fibres, when heated, decompose differently when compared to untreated ones and this is especially so for char-promoting flame-retarding species such as Lewis-acid generators in cellulosic fibres. Certain intumescent systems when dispersed about flame retardant fibres, interact and develop a unique ‘char-bonded’ structure which has been shown to enhance flame and heat resistant properties. These interactive fibre-intumescent combinations require that both the flame retardant fibre and intumescent form chars by chemically and physically compatible mechanisms, usually by a semi-liquid intermediate phase. Examples analyzed to date are flame retarded viscose fibres (Visil, Kemira; Lenzing FR Viscose) which although comprising different flame-retarding species, generate polyacids on heating. Intumescents based on ammonium polyphosphate behave in a similar way thus enabling physicochemical interactions of both charring components to occur. Initial studies of char structure using scanning electron microscopy have already shown evidence for this kind of interaction. In this paper, the mode of thermal degradation of these systems has been studied by thermal analysis (DSC, TGA) and the nature of char determined by IR studies. The results of these investigations are used to more fully understand the mechanisms involved.