Nanoclays are ubiquitous nanofillers that are isolated from naturally occurring clays through energetic stirring, followed by centrifugation and freeze-drying, centrifugation and cross-flow filtration, or ultracentrifugation. Nanoclays are composed of stacked mineral silicates layers, forming complex clay crystallites. Three main mechanisms have been reported for fire retardancy in composite materials containing clay particles, including migration, barrier, and paramagnetic mechanisms.
In the combustion process of the composites containing clay particles, the bubbles that are formed during polymer degradation push the clay nanoparticles from interior layers to the surface of the composites. The clay migration to the outer surface could be attributed to different factors; the difference in surface free energy between polymer and polymer-clay blend, the temperature and viscosity gradient during directional heating, and the formation of rising gasses during combustion process. The aggregation of clay particles results in more hydrophilicity (i.e. less compatible with common hydrophobic matrices) due to the degradation of the organic treatment on the clay interlayers. Consequently, a clay-rich barrier layer will form and reduce the rate of weight loss in the composite combustion.
Barrier mechanism during the condensed phase suggests the formation of a char layer in composites containing clay particles while exposing to fire. The insulating char barrier prevents the mass transfer of degradation products to the combusting polymer surface. This, in turn, would limit further exposure to heat and oxygen and hinder the burning process.
Paramagnetic radical trapping is another suggested fire retardancy mechanism of nanoclays. This mechanism proposes that structural metals in clay particles, such as, iron, can trap the radicals formed during polymer combustion and reduce the degradation rate.
Nanoclays are widely used as Flame Retardant additives in polymer nanocomposites, owing to their incombustible properties. Contributions of nanoclays can result in superior fire retardancy in polymer matrices through noticeable improvement in peak heat release rate, ignition time, and fire growth index. Ahmed et al observed a protective char layer on the surface of polystyrene upon the incorporation of 1 to 3 wt% nanoclays, which resulted in higher fire retardancy and lesser emission of smoke, CO, and CO2 than the pure polystyrene.39 Achieving nano-dispersion of clay in the polymer matrices is an essential parameter, which affects the fire retardancy of polymer-clay nanocomposites. Lu et al reported different methods in improving the nanodispersion of clay in LLDPE/PA6 blends. They observed that the blends with clay localized in the LLDPE phase, rather than PA6 phase, exhibited better flame retardancy.
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