Composite plastics have high conductivity and strength

The material, which can be made into fibres or sheets, could find a use in strain monitoring and has already been tested to this end in the sails of high-end yachts competing in the Americas Cup.

Conductive polymers have existed for some time, but are generally made from exotic semi-conducting organics, and restricted to organic solar cells, printing electronic circuits, and organic light-emitting diodes.

A team at NanoForce, a spin-off from Queen Mary University of London, set about creating robust plastics that could conduct at near-metallic levels.

‘You can just use normal polypropylene or polyamide, so they’re much more stable than these fancy semi-conducting polymers,’ Ton Peijs, technical director, told The Engineer.

The team uses additive multi-walled carbon nanotubes at a weight percentage of around one per cent. In isolation, the nanotubes show excellent metallic conduction, but the challenge has been to incorporate them into composites.

‘If I want to make a conductive polymer composite I need to mix in these nanoparticles and you don’t want them to be all agglomerated here and all agglomerated there because then they are too far apart and never form a network — but if they are all perfectly and evenly dispersed, then they are also quite far apart,’ Peijs said.

NanoForce’s solution was a post-processing technique that involves annealing and hot pressing — basically re-melting the polymer after extrusion — allowing the nanotubes to migrate into a self-organising ‘dynamic network’ that is conductive. The process can be tuned by temperature and time of annealing, and can also align the polymer units to increase the strength.  

Crucially, when the resulting fibre or sheet is stretched and placed under strain, the nanotube networks that has been in place previously gets pulled apart and deforms, breaking down the connections and creating electrical resistance of several orders of magnitude. This is particularly useful for in situ monitoring of strain in various critical structures.

Indeed, NanoForce has recently done some work with North Sails for certain teams competing in the Americas Cup yacht competition.

‘It’s sort of the Formula One of sailing — actually they spend more than than Formula One — and they do a lot of analysis,’ Peijs said. ‘Using our technology they could analyse the load in the sails, what are the local strains… then, of course, you can optimise performances.’

Applied as a thin-film layer it could also be used for condition monitoring in the aerospace industry and wind turbines, for example.