Editorial Feature

Buckytubes - An Additive for Plastics

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The mechanical properties; i.e. stiffness, strength, toughness; and the thermal and electrical properties of pure Buckytube materials, enable a multitude of applications. These include batteries, fuel cells, fibers and cables, pharmaceutics and, biomedical materials.

A number of other applications will be available when blending nanotubes with other materials to improve existing properties or to provide new ones. The use of nanotubes as fillers in thermoplastics and thermosets, for example, has been developed as sufficient quantities of high-quality Buckytube material became available.

One of the most important technological developments in the last half of the twentieth century was the significant replacement of metals with plastics in applications. Most of this has occurred in structural applications, where plastics have been engineered to outperform steel and other structural metals. It can provide sufficient strength or stiffness at a lower weight and cost.

A key property that metals will always outperform when compared to plastics, however, is in electrical conductivity. Plastics are very good electrical insulators, this property lends itself to many of the most widespread and important uses of plastics.

Nevertheless, the applications for plastics would be broadened substantially if good solutions existed to make these materials conductive. For some applications, plastics have been loaded with conductive materials - the most common filler is carbon black, which is relatively inexpensive and works well in many applications.

New Applications

Applications include antistatic, electrostatic dissipative, and, electromagnetic shielding and absorbing materials. Electromagnetic interference and radiofrequency interference (EMI/RFI) shielding, for example, is essential in laptop computers, cell phones, pagers and, other portable electronic devices to prevent interference with - and from, other electronic equipment.

At present, there is no suitable plastic material for this purpose and metal, in one form or another, is typically added to provide this function in electronic equipment cases, resulting in substantial weight and manufacturing expense.

Critical Loading

One drawback for using carbon black as a conductive filler, however, is the high loading required to provide the desired level of conductivity. It is well known that conductivity of a filled insulator, such as a polymer resin, increases with filler loading in a classic S-curve; i.e. up to a critical loading, the bulk conductivity changes little, but increases very rapidly when a further small amount is added.

This is because high bulk conductivity requires the presence of many long conductive pathways, which are obtained only when the loading is so high that when randomly distributed, the conductive particles (e.g., carbon black) are likely to form long chains.

This critical loading threshold is many times higher than would be required if these particles could be placed in the optimal positions to form long chains with minimal loading. However, being sub-micron in size, this can’t be done. A great deal of carbon black is wasted to build up above the threshold level where these long chains form.

In addition, the critical loading threshold decreases dramatically as the aspect ratio (length to width) of the filler particles increases. This is because longer particles cover a greater distance in the conductive pathway, whereas carbon black, which is spheroidal, has to form a chain of touching particles to cover the distance that a fiber shaped filler would cover by itself.

Filler loading is important because, aside from weight savings, when plastic is loaded with carbon black at 30, 40, or even 50% in volume - which is often the level needed to reach the desired bulk conductivity - the mechanical properties of the composite are severely degraded. Often the material is not able to be used and typically it is no longer malleable, which is frequently the most critical property of plastic parts.

Buckytubes as a Solution

Buckytubes offer a solution. Firstly, their electrical conductivity is excellent, as described above, and cannot be equaled by any other polymer. Conductive polymers, a class of long-chain molecules with a conjugated backbone, would be better described as molecular resistors, as they are intrinsically semiconducting.

Secondly, Buckytubes have a phenomenally high aspect ratio. Individual tubes are about 1 nm in diameter (about half the diameter of DNA, and about 1/10,000th the diameter of graphite fibers), and 100-1000 nm in length. Therefore, the aspect ratio of Buckytubes is around 100-1000, compared with about 1 for carbon black particles.

Finally, Buckytubes naturally form and they have a morphology from the outset that is ideal for conductive filler applications. Buckytubes self-assemble into “ropes” of tens to hundreds of aligned tubes, running side by side, branching and, re-combining. When examined under an electron microscope, it is very difficult to find the end of any of these ropes.

As a result, the ropes form naturally in long conductive pathways that can be exploited to make electrically conductive filled composites. Indications are that much lower loadings of Buckytubes are required to reach a given level of conductivity when compared to other conductive fillers.

Opportunities

There are many opportunities for conductive plastics as well as thermosets filled with Buckytubes. Anti-static and electrostatic dissipative applications can be achieved through very low loadings (<0.1%). One example is in painting auto body parts, which are increasingly made of plastics.

Due to their insulating properties, plastic parts charge up which causes them to electrostatically repel the paint droplets formed during the spray painting process, which results in wasted paint, which is both an economic and an environmental problem. A conductive primer coat can be applied, but that extra processing step is costly. The ideal situation is to make the part itself sufficiently conductive to drain away the build-up of charge, by connecting the part to the ground during the painting process.

Another application area for Buckytube filled plastics is in EMI/RFI shielding, which has uses, as described above, in portable electronics and defense applications – for example radar, absorbing or modifying material for aircraft and missiles. Good attenuation of electromagnetic radiation can be attained at Buckytube loadings on the order of 1% or less.

Good mechanical stability should be maintained, which allows it to be molded. This would represent a significant breakthrough in plastics to enable their broader use. All of these application areas make effective use of the electrical conductivity of Buckytubes where they contribute the highest added value. However, there is great promise in the development of applications that exploit the thermal and mechanical properties of Buckytubes as well. Like the electrical applications described here, the possibilities are significant.

This article was updated on the 3rd September, 2019.

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