Char formation in flame-retarded wool fibres. Part 1. Effect of intumescent on thermogravimetric behaviour

The char-forming characteristics of wool are well known but have been little studied during recent years, although flame-retardant activity may be associated with enhanced char formation in many cases. Recently, intumescent flame-retardants have been shown to react synergistically with flame-retarded cellulosic fibres and generate unexpectedly high levels of char. Extension of this concept to wool, a naturally intumescing fibre, shows that similar synergistic activity may be observed. This paper describes and discusses the initial work which has involved the application of a number of phosphate-based intumescents bound in a resin system to the surface of 100% wool fabrics in the absence and presence of zirconium-based flame-retardant (ZIRPRO®) and to unretarded wool/flame-retarded (FR) viscose blend fabrics. Thermal analytical studies (TGA) of treated fabrics and synthesised mixtures of respective powdered fibre/intumescent combinations demonstrate that enhanced char formation may occur and that char shows higher-than-expected oxidative resistance above 500°C. Scanning electron microscopy studies have demonstrated the physical nature of the interaction and show that, under the experimental heating conditions used to generate chars, the wool component appears to have passed through a semi-liquid state prior to or simultaneously with the onset of char formation. A qualitative model is proposed to explain the interactions taking place between fibre and intumescent char-forming reactions which compares well with earlier results observed for flame-retardant cellulosic fibre — intumescent interactions.