A Versatile Filler for Plastics
The mechanical (stiffness, strength, toughness), thermal and electrical properties of pure buckytube materials enable a multitude of applications, from batteries and fuel cells to fibres and cables to pharmaceutics and biomedical materials. Scores of additional applications emerge when one thinks of blending nanotubes with other materials to improve existing properties or to provide new ones. Using nanotubes as fillers in thermoplastics and thermosets, for example, has been discussed for several years, and is only recently undergoing rapid investigation and development as sufficient quantities of high-quality buckytube material is becoming available to enable such investigations.
One of the most important technology developments of the last half of the twentieth century was the substantial replacement of metals with plastics. Most of this replacement has been in structural applications, where plastics have been engineered to outperform steel and other structural metals by providing adequate strength or stiffness at lower weight and cost. A key property that metals will always have over plastics, however, is in electrical conductivity. Plastics are amazingly good electrical insulators; in fact, this property gives rise 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.
These application areas 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, imposing substantial weight and manufacturing expense.
Electrically Conductive Plastics
For some applications, plastics have been loaded with conductive materials for years to provide conductivity. The most common filler is carbon black, which is relatively inexpensive and works well in many applications.
One drawback to 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. That is, up to a critical loading, the bulk conductivity changes little, but increases very rapidly upon adding just a bit more filler. 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 actually many times higher than would be required if somehow these particles could be placed in the optimal positions to form long chains with the minimal loading. But, of course, being sub-micron in size, this can’t be done. A great deal of carbon black is wasted in the redundancy required to build up above the threshold level where these long chains form.
Also well known is the fact that 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 of the conductive pathway, whereas carbon black, which is spheroidal, has to form a chain of touching particles to cover the distance that a fibre-shaped filler would cover by itself.
Why is filler loading important? Aside from weight savings, when a plastic is loaded with carbon black at 30, 40, even 50% by volume, which is often the level needed to reach the desired bulk conductivity, the mechanical properties of the composite are severely degraded. Often it is not usable at all, and typically it is no longer mouldable, which is frequently the most critical property of plastic parts.
Buckytubes As A Solution
Buckytubes offer a solution. First, Buckytubes are terrific electrical conductors, as described above. No polymer is a better conductor and none better is likely to be found. (So-called conductive polymers, a class of long-chain molecules with a conjugated backbone, would be better described as molecular resistors; they are intrinsically semiconducting.) Second, 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 fibres), and 100-1000 nm in length. Thus, the aspect ratio of buckytubes is around 100-1000, compared with about 1 for carbon black particles. This already changes the game entirely, by pushing the critical loading level downward, as described above. Finally, buckytubes naturally form, in fact are born with, a morphology that is probably ideal for conductive filler applications. Buckytubes self-assemble into “ropes” of tens to hundreds of aligned tubes, running side by side, branching and recombining. When examined by electron microscopy, it is exceedingly difficult to find the end of any of these ropes. Thus, ropes form naturally occurring very long conductive pathways that can be exploited in making electrically conductive filled composites. Initial indications are that dramatically lower loadings of buckytubes are required to reach a given level of conductivity than for any other conductive fillers.
The opportunities for conductive plastics, as well as thermosets, filled with buckytubes are abundant. Very low loadings (<0.1%) provide for antistatic and electrostatic dissipative applications. One example is in painting automobile body parts, which are increasingly made of plastics. Because they are insulators, plastic parts charge up, which cause them to repel electrostatically the paint droplets formed in spray-painting of the body parts. This results in a great deal of wasted paint, which is both an economic and an environmental problem. A conductive primer coat can be applied, but that extra processing step is also quite costly. the ideal situation is to make the part itself sufficiently conductive to drain away charge build up by connecting the part to ground during the painting process.
Another broad area of application for buckytube-filled plastics is in EMI/RFI shielding, which has uses, as described above, in portable electronics, and defence applications. If, as appears likely, good attenuation of electromagnetic radiation can be attained at buckytube loadings on the order of 1% or less, good mechanical stability should be maintained, allowing it to be moulded. This would represent a significant breakthrough in plastics and enable broadening of their uses. Other defence uses of buckytube composites are similarly significant such as radar-absorbing or modifying material for aircraft and missiles.
The application areas just described make 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 mind boggling. Stay tuned!