Nanoclays act as flame-retardant fillers by moving to the surface of the polymer, forming a coating that acts as a heat shield. But cracks often appear in the coating, causing the polymer beneath to bubble.

The team found that nanocomposites containing 0.5% by mass single-walled carbon nanotubes in poly(methyl methacrylate) PMMA formed a heat-shield layer that did not crack. The scientists believe this is because the nanotubes assembled into a jammed-network structure in the polymer matrix.

As well as providing protection against fire damage, the network caused the material to behave like a gel at 200 °C, rather than like a liquid. As a result, conducting viscoelastic tests on a nanocomposite sample should indicate its flame-retardant abilities.

Multiwalled carbon nanotubes did not provide the same level of flammability protection as single-walled nanotubes until they were present in PMMA at a concentration of 1%. Carbon nanofibres, meanwhile, required a concentration of 4% to give the same effect. These were the levels at which the filler formed a jammed network.

Nanoclays and nanotubes could take the place of halogenated flame-retardant additives, a number of which are banned because of the damage they cause to the environment. The resulting nanocomposites could have applications in packaging, electronics, construction and the aerospace industry.

Now the team plans to investigate the effect of network formation in plate and carbon-sheet additives on flammability, as well as to look at factors such as particle flexibility and size distribution.

The researchers reported their work in Nature Materials.