Lab talk: Improving a super-strong material

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Carbon fibre threads woven together to make a tough carbon fibre mesh, used in strong yet feather-light reinforced plastics. Credit: Getty Images

When it comes to high-performance vehicles such as Formula 1 racing cars, lighter is better. But components made from metals such as aluminium are already about as light as they’ll ever get.

Enter carbon fibre composites – tough plastics reinforced by an internal mesh of carbon fibre threads. The material is about a third the weight of aluminium with strength that matches steel. That might sound impressive, but there are still many gains to be made. Theoretical calculations estimate that individual fibres within the material are performing at only 10% of their potential strength. I aim to bring that figure up.

A closer view of carbon fibre composites under a microscope reveals the important components that make up the structure: the individual fibres, each one 100 times smaller than the diameter of a human hair, are surrounded by a thin layer of epoxy adhesive, which is then set in a resin reinforcement called the matrix.

It’s the chemical bonds between the fibres and surrounding resin – called the interface – that give carbon fibre composites their super strength. If the resin attaches nicely all the way along a carbon fibre, the composite will be very strong. But when there are only a few connections, the bond is weak and microscopic cracks can appear in the matrix around the fibres. This weakens the structure and can cause what we call “permanent failure” – in other words, an unfixable break.

I discovered that large sections of the carbon fibre surface, which were previously thought to be chemically inert, are actually quite reactive. This means we can chemically graft molecules on to the carbon fibre surface that will react with the resin matrix and, like “molecular nails”, firmly anchor the fibre into the resin.

We don’t need to add many of these nails to increase composite strength – preliminary results show that the presence of only very small quantities of these molecular nails (a few parts per million) almost doubled the strength of the interface connection. This shows the impact atomic scale bond formation can have over composite performance, with many more unexplored chemistry techniques waiting in the wings. Maybe one day we’ll bring that strength percentage close to 100.

PAPER: A novel approach to functionalise pristine unsized carbon fibre using in situ generated diazonium species to enhance interfacial shear strength, Journal of Materials Chemistry: A, 2015, vol 3, p3360-3371.

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