Scanning electron microscopic studies of wool/intumescent char formation

Thermal analytical studies of synthesized mixtures of powdered wool, wool/flame retardant viscose blended fibre/intumescent combinations and intumescent-coated fabrics have shown previously that while enhanced volatilization occurs during char formation below 400 °C, the chars formed show higher-than-expected oxidative resistance up to temperatures beyond 700 °C. In this paper, scanning electron microscopy has been used to study these interactions and show that over the temperature range 250–450 °C, charred residues from wool-containing samples coated with intumescent and without intumescent exhibit significant changes in wool fibre morphology. Wool fibres first appear first to swell and then develop an axial defect. Further swelling occurs to yield a swollen ‘pod-like’ morphology at higher temperatures, and splitting of the cuticle along the axial defect allows the largely liquid degraded wool keratin interior to escape and wet neighbouring surfaces. The presence of intumescent flame retardant appeared to inhibit the rupture of the wool cuticular membrane, possibly via enhanced crosslinking of the keratinous protein accompanied by enhanced volatile formation in the 250–350 °C region. The subsequent rupture stage (>350 °C) enables the intumescent to act upon the emerging liquefied pyrolysing keratin core of the fibres, providing a second high volatilization stage to accompany complex char formation with adjacent cellulosic fibres.