Metallic nanoparticles have received considerable attention for their applications as flame retardants in different polymer matrices. Metallic nanoparticles exhibit different reaction mechanisms against fire according to their structure; some metal nanoparticles (metal hydroxide particles) utilize hydrated minerals and release water molecules as they decompose in the presence of fire and provide an endothermic reaction. In this case, the cooling effect would increase the self-extinguishing ability in nanocomposites. Aluminum tri-hydroxide (ATH) and magnesium hydroxide (MH) are two non-halogen fire retardant additives that undergo endothermic reaction and interfere with the combustion process when exposed to heat (Equation 6 and Equation
Incorporation of the metal hydroxide nanoparticles in polymer composites would result in a noticeable increase in limiting oxygen index (LOI). This phenomenon is due to yielding a barrier on the polymer surface, which in turn can lower the heat flux provided by flame and improve the fire retardancy. The formation of char is another mechanism in some fire retardant materials, such as, alumina trihydrate (ATH) that delays ignition and fire development.41
In addition, some metal hydroxide FRs release water when decomposing at elevated temperatures, which leads to the cooling the substrate below the flash point. Subsequently, water formation favors the dilution of combustible gasses, reduces the oxygen effect, and decreases the flame spread rate. Xi et al reported a flame-retardant behavior in polyurethane foams due to the endothermic decomposition and water release reaction from adding 8 to 14 wt% of ATH filler in a synergetic system. They also reported that the water released from ATH reacted with the decomposition products and formed polyphosphate, increasing the barrier effect of the char layer to heat and flame.42 However, contradictory results were reported for release of ammonia and water owing to the incorporation of ATH in polymer matrices, which led to an earlier degradation of EVA43 and poly(1,4-butanediol succinate) (PBS)44 due to hot water hydrolysis.
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