Introduction

Over the past decades, nanoclays have been widely used as additives to improve the strength as well as the fire performance of polymers, as evidenced by applications and a large number of studies reported in the literature. The mechanism of action of nanoclays is now relatively well understood, despite some aspects remaining unclear, such as the phenomena controlling ignition time. During the burning of polymer nanocomposites, a surface layer is formed on top of the virgin polymer, which acts as a mass and heat shield slowing down mass transfer of pyrolyzed gas to the surface, because less heat is transferred to unpyrolyzed material. Furthermore, in the presence of nanoparticles, the temperature at the surface of the surface layer increases far beyond the so-called ignition temperature of the polymer, which results in increased surface reradiation losses and, hence, decreased heat transfer to the solid. The formation of this surface layer has been observed in a number of studies using the cone calorimeter, where a significant reduction of the peak heat release rate (PHRR) compared with the corresponding pure polymer was observed for relatively thin samples. Zhang, Delichatsios, and Bourbigot also studied the effect of the surface layer numerically, finding that the reduction in heat transfer at the interface of the surface layer and the virgin polymer is inversely proportional to the number of nanoparticles that remain on the surface after degradation of the polymer (if the concentration of nanoparticles is less than about 10%).