Initial Steps- Even if early indications for the existence of carbon nanotubes (CNT) date back to a Russian publication from the 1950s, detailed research on carbon nanotubes is not considered to have really started until 1991. At that time, Prof. Sumio Iijima observed carbon nanotubes with electron microscopy, and a new field of carbon research began. Nowadays, carbon nanotubes are increasingly losing their status as a curiosity. Today's production capacity has increased enormously over the last few years, and hundreds of tons are produced to satisfy market demand.

Carbon nanotubes can be described as sheets of graphene that are rolled to form a tube. Depending on the synthesis and reaction parameter, single-wall carbon nanotubes (SWCNTs) as well as multi-walled (MWCNTs) can be produced. In the case of MWCNTs, several independent tubes are grown in concentric circles. Their outer diameter is usually in the range of 20 nm, about 2,000 times thinner than a human hair. Their length can be around 10 µm, and thus their aspect ratio can be in the range of a few hundreds to thousands.

Superior Properties of Carbon Nanotubes

Due to the high aspect ratio of carbon nanotubes and given their low density, huge surface areas of a few hundred m2/g are usually obtained. We can imagine one gram of material exhibiting a surface area similar to that of an average semidetached house.

Because of these parameters, carbon nanotubes seek to reduce their surface energy and tend to easily agglomerate in the form of bundles and skeins. This entanglement is already encouraged during their synthesis. To derive the full benefit from carbon nanotubes, one has to break off these agglomerates to achieve a good dispersion. We have already reported on how to disperse carbon nanotubes and how complicated the process is (CHEManager Europe 9/2009). Not only the right amount of shear forces is needed, but also optimized wetting and dispersing additives are required to stabilize the carbon nanotubes.

Dispersion Into Individual Tubes

Since most coating producers do not want to spend several years and thousands of euros of research costs developing a process to disperse carbon nanotubes, they are focusing on already pre-dispersed carbon nanotubes in the form of aqueous or solvent-borne dispersions.

This not only offers comfortable access to carbon nanotubes; it also gives advantages in respect of safety. Most concerns about the health hazards of carbon nanotubes are related to their powder form. To avoid the creation of carbon nanotube dust, dispersions using polymeric wetting and dispersing additives of high molecular weight are beneficial. If the dispersion dries to a solid residue, a sticky film will be formed instead of a dusty powder.

Choosing The Right Carbon Nanotubes Is Key

Depending on their diameter and length, carbon nanotubes are expected to behave differently in human bodies if inhaled. Most producers and users are focusing on thin and flexible carbon nanotubes. This is due to the fact that thick and rigid tubes are assumed to show some kind of asbestos-like behavior. In this case, human macrophages cannot digest the fibrous particles and fail in phagocytis. This would also mean that the particles would not be removed from the human body and could cause inflammation. To avoid such problems, people should therefore focus on thin and flexible carbon nanotubes with diameters well below 50 nm.

Price Considerations

While single-wall carbon nanotubes exhibit superior properties, their synthesis is still extremely complicated, and yields are small. Because of this, single-wall carbon nanotubes are still much more expensive than multi-wall carbon nanotubes. Consequently, most companies focus on multi-wall carbon nanotubes for industrial applications in coatings and plastics. But it is not only the difference in price between single-wall and multi-wall carbon nanotubes one has to consider; it is also the comparison to other conductive materials such as carbon black or copper or silver particles that should be taken into account.

Comparing the prices of different conductive materials, we can assume the following order of increasing prices: carbon black ≈ copper < multi-wall carbon nanotubes < silver < conductive polymers≈ single wall carbon nanotubes. Multi-wall carbon nanotubes can be considered as a medium-priced conductive material. This order can, of course, change depending on both quality and particle size, and given that copper or silver nanoparticles are much more expensive compared to the bulk metal.

Size Matters

There are many ways cited in the literature for how to disperse carbon nanotubes into a matrix. Most procedures use high shear forces that can break the carbon nanotubes into short fragments during the dispersion process. Very often, dispersions of carbon nanotubes with a reduced length of up to a mere 200 nm are obtained. In such cases, the aspect ratio would have been reduced from about 1,000 to about 10, and the unique properties of carbon nanotubes would have been diminished. It is therefore very important for the coating manufacturer to choose the right dispersion process or, even better, to use already pre-dispersed carbon nanotubes.

Keeping a high aspect ratio also guarantees lower percolation thresholds, which also lead to higher transparency. From all the conductive pigments used today, carbon nanotubes offer the highest aspect ratio and provide, for example, higher transparency and better mechanical properties than carbon black or metal particles. There is only one drawback to be considered. Carbon nanotubes are black and have a high level of jetness. Another point of importance is to keep the concentration of carbon nanotubes in application as low as possible.