Blending for reinforcements

UK researchers are attempting to improve the strength and wear resistance of composites by combining them with carbon nanotubes.

A team led by Dr Milo Shaffer, lecturer in materials at Imperial College, plans to fill the matrix of conventional fibre-reinforced composites with nanotubes to improve their performance in a four-year, EPSRC-funded project that began this month.

Carbon fibre composites are increasingly used in aircraft, sports cars and offshore oil industry pipelines. But there is concern, particularly in the aerospace industry, about the material’s compressive strength, which can be weakened when struck by birds or stones, or in taxiing accidents. There are also fears that the material presents a greater fire risk.

‘We hope to improve the composites’ compressive strength, which is often a problem,’ said Shaffer. ‘We should also improve their wear properties, raise their operating temperature and the temperature at which they degrade, and improve their fire-retardant qualities. We also hope to improve toughness.’

Nanotubes have approximately the same rigidity as diamonds and are around 10 times stronger than any other material available. But a number of issues remain to be resolved before they can be used in bulk form, including whether nanotubes of sufficiently highquality can be produced in large numbers.

Shaffer and his colleagues plan to make use of those nanotube properties that can be exploited now, by using them to reinforce fibre composites. ‘Rather than saying “let’s go out and make the strongest material that has ever been made just using nanotubes”, which has been shown to be very challenging, let’s use the properties we know we do have already to improve the performance of conventional high-performance materials,’ he said.

Nanotubes have a diameter around 1,000 to 10,000 times smaller than that of the matrix, creating a more complex structure and thereby improving the composite’s properties, he said.

Filling the matrices with nanotubes will increase the weight of the composites very slightly, but the improvement in strength will allow less material to be used.

‘In the sense that we are going to fill the matrix it will increase the density of the material a little, although not by much,’ said Shaffer. ‘But we expect to improve the properties of the composite, so ultimately the amount of material needed would be reduced, so this would save weight overall.’

The researchers are working with UK companies Insys – which makes composite parts and systems for the aerospace and defence industries – and Security Composites, which is developing thermoplastic materials for safety applications such as reinforced toecaps.

Surface Measurement Systems is supplying an instrument to allow the researchers to closely study the surface properties of the nanotubes, which is vital to ensuring they combine well with the fibre.

The team plans to produce prototype test panels to demonstrate the material’s improved strength and wear resistance by the end of the project. These could then be taken up by a composite manufacturer to produce components, although the researchers have no plans to commercialise the material themselves.