Nanocomposites and flame retardancy

Nanocomposites are polymers reinforced with nanomaterials. They are highly praised and a subject of intensive research, because they show impressive performance for multifunctional applications at a relatively low filler level of 2 to 5 percent by weight. While carbon nanotubes and nanofibres play a secondary role, nanoclays are the most dominant commercial nanomaterials; they are defined by the particle size of the dispersed phase having at least one dimension less than 100 nm. Polymer layered silicates provide enhanced mechanical, thermal and barrier properties to polymers. The enhancement in properties is largely attributed to the aspect ratio and intercalation ability of the layered silicates, resulting in high specific surface area and hence more potential for polymer filler interaction.

 The effect of nanocomposites in improving the fire safety of polymers is of increasing interest to researchers in academia and industry. While the significant reduction in heat release rate during the combustion process of nanocomposites was reported for quite some time, the flame retardancy of polymer filled with nanoclays has not further improved to generally achieve industrial acceptance. It appears that nanoclays enhance the thermal stability, prevent dripping, and form a solid char, but they do not improve fire retardancy on their own. However, in conjunction with mineral flame retardants such as alumina trihydrate (ATH), magnesium hydroxide, zinc borate, or brominated flame retardants, they act as synergists and the addition of 2 to 5 wt% of montmorillonite allows to dramatically reducing the amount of these flame retardants without affecting the flame retardancy of the polymer. Unfortunately, there are some restrictions for their use: only polar polymers like ethylene-vinyl acetate (EVA) or polyamides are suitable for dispersing the nanoclays, and montmorillonite nanoclays already begin to degrade at around 200°C, due to the rapid thermal decomposition of their quaternary ammonium components.

The only commercially available flame retarded polymers containing nanoclays to date are polyolefin cable applications based on polyethylene and EVA with montmorillonite and ATH as the flame retardant system. They have a good fire safety performance, but are rather expensive, as the cheapest component of the formulation, ATH, is dramatically reduced in this formulation.       

Therefore, much work remains to be achieved for developing nanocomposites addressing the needs of the flame retardant markets.