Combining plastics with carbon nanotubes

Paul Boughton
Researchers at the Fraunhofer Institute have found ingenious ways to combine plastics with nanoparticles to endow them with new properties. As an example of what might be possible, aircraft could in future be better protected against lightning strikes.
It is by no means a rare occurrence for aircraft to have to pass through storms, but when they do there is always one major danger: lightning. Naturally, aircraft manufacturers do everything they can to protect their machines against strikes, but even aircraft made of aluminium do not always escape entirely unscathed. And when polymer components – usually carbon fibre reinforced plastics (CFRPs) – are incorporated into the design as a weight-saving measure, the situation becomes even more problematic, because they do not conduct electrical current as well as aluminium.
At the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Bremen, Germany, researchers have developed a process for manufacturing new materials that should afford aircraft better protection against lightning strikes. They have been focusing on the unique material properties of carbon nanotubes (CNTs). CNTs are among the stiffest and strongest fibres known, and have particularly high electrical conductivity. In order to transfer their properties to CFRPs, the scientists have been combining these nanoparticles with plastics. Dr Uwe Lommatzsch, project manager at the IFAM, explains: "By mixing nanoparticles with plastics, we have been able to significantly enhance the material properties of the latter." To give just two examples, CNTs are being used to optimise the electrical conductivity of plastics, and their heat dissipation properties are likewise being improved by the addition of metal particles.
Lommatzsch says the trick is in the mixing process: "The micro- or nanoparticles must be highly homogeneous, and sometimes very closely bound to the polymer." To do this, the scientists employ plasma technology. They use an atmospheric plasma to alter the surface of the particles in such a way that they can be more readily chemically bound with the polymer. A pulsed discharge in a reaction chamber creates a reactive gas. Lommatzsch's colleague, Dr Jorg Ihde, explains: "We spray the particles – ie the nanotubes – into this atmospheric plasma." They immediately fall into the selected solvent, which can then be used to further process the polymer. The whole procedure takes just a few seconds – which is a huge advantage over the old method, in which CNTs were generally prepared in an acid bath using a wet chemical process. That took several hours or days, required considerably more chemicals, and generated significantly more waste.
In addition to improved CFRPs for use in aircraft, the IFAM researchers have several other potential applications in mind. Ihde outlines an example: "We can increase the heat dissipation properties of electrical components by giving metal particles of copper or aluminium an electrically insulating coating in the plasma and then mixing them into a polymer." This can be pressed onto an electronic component so heat is dissipated directly. He continues: "Overheating of elements is a major problem in the electronics industry." The researchers have also devised a way to reduce electromagnetic losses by using this plasma process to coat soft magnetic particles such as iron and then combine them with plastics. Built into electric motors, they cut eddy current losses, resulting in higher efficiency and a longer service life. IFAM experts will be exhibiting surface-modified carbon nanotubes – which demonstrate significantly enhanced miscibility with solvents – at the K 2010 trade fair in Dusseldorf, from 27 October to 3 November 2010 (Hall 3, Stand E91).
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