The role of carbon nanotubes in polymers: Now and the future
By Dr. Brian P. Grady
According to a recent marketing project by TechNavio, the annualized growth rate of carbon nanotube production from now until 2015 is projected to be 10.6 percent. What benefits do carbon nanotubes bring to polymers, and – equally important – what can carbon nanotubes not do?
The most common use of inorganic materials (or fillers) in a polymer is to enhance stiffness of the solid polymer. Since carbon nanotubes are among the stiffest materials ever found, one would think that nanotubes would be used to increase the stiffness of polymers. In fact, such uses have been wildly unsuccessful for reasons that are not entirely understood.
For weak rubbery materials, the addition of nanotubes can cause a substantial stiffening effect, but for standard thermoplastics such as polycarbonate, polyethylene, polypropylene or polyamide or thermosets such as epoxy and polyester, the effect has been extremely underwhelming except at very low loading levels (less than 1 percent filler content). Such a large amount of research has gone into improving mechanical properties of polymers with nanotubes that it is doubtful such efforts will ever be successful. However, if nanotubes are successfully preformed into high-strength yarns or sheets, these could be chopped and added to a polymer to possibly significantly improve the stiffness.
Regarding the latter, the percolation threshold does depend on the type of polymer. For a melt-mixed polymer, the percolation threshold in general goes as glassy polymer < polar semicrystalline polymer < semicrystalline polyolefin. This lower percolation threshold vs. carbon black arises because of the high aspect ratio of carbon nanotubes.
Southwest Nanotechnologies has recently introduced to the market the highest aspect ratio MWCNTs currently commercially available (SMW-200) and unpublished work in the University of Oklahoma laboratory has found these nanotubes lower the percolation threshold vs. other nanotubes as much as an order of magnitude using standard twin-screw extrusion conditions.
Nanotubes are also being considered for use in fuel and solar cells where in some instances they will be mixed with specialty polymers. Piezeoelectric (e.g. strain-sensing) and electrostrictive (change in shape with applied voltage) applications are also being developed. An underdeveloped and less understood aspect of electronic applications is the role of nanotube orientation, and this area is expected to become more important in the future. Taking advantage of the electrical conductivity in nanotubes is currently the most important application for nanotubes in polymers and is expected to continue into the immediate future.
Nanotubes also have extremely high thermal conductivities – approximately a factor of five higher than copper! However, taking advantage of this high thermal conductivity has proven to be extremely problematic because of the high interfacial resistance to heat transfer between two nanotubes or between a nanotube or polymer. The increase in thermal conductivity when nanotubes are added to a polymer with no alignment is typically less than 50 percent.
Because of the lack of other materials that are able to transfer heat, nanotubes are still being considered. Longer nanotubes, if they can be processed without breakage, should benefit the thermal conductivity. Related to this thermal conductivity improvement is the fact that nanotubes have been shown to help reduce flammability of polymers, although there are no commercial uses of nanotubes in polymers in this regard.
Nanotubes have a significant effect on the rheology of the polymer, increasing the low-shear viscosity substantially. In terms of processing, nanotubes change the initial mixing stages most dramatically because in most processing operations, the shear rate is high enough that the viscosity is not that much higher than with the pure polymer. One area that will become very important in the future is the development of processing strategies to minimize nanotube breakage while maintaining good dispersion, since most properties will be improved by having longer nanotubes. Handling bulk nanotubes is also a safety and environmental concern, and if master batches are not used, then care must be taken when handling nanotubes because of their dispersability in air.
Probably more research effort has been expended to the modification of nanotubes than any other single aspect. Modification is said to be covalent, which involves attaching a molecule to the nanotube via covalent bonds; and noncovalent, which involves adsorption of a molecule to the nanotube surface. Modification usually aids dispersion, and modification can be used to change the properties of nanotubes.
From a commercial aspect, other than a few examples such as the one described earlier, nanotubes are typically used without modification. Commercial manufacturers might apply some light treatment to enhance dispersion and maintain this as a trade secret, but it is certainly true that no modification method is as ubiquitous as, for example, silane coupling is with silica. If water-dispersible tubes are to be used however, some modification is required since nanotubes without modification are not stable in aqueous solution.
Overall, it is fascinating that mechanical properties and thermal conductivity – of which nanotubes have just about the highest ever found – have not been utilized in most commercial applications. Instead, electrical conductivity, where nanotubes have values 1-2 orders of magnitude lower than most metals and equal to that of carbon black, have been the application where nanotubes have replaced other materials and where new applications have been developed. The reason for this dichotomy is obviously not the value, but the geometry of the carbon nanotube. Other than electrical properties, high-strength yarns, tapes and sheets made from nanotubes seem to show the most hope of becoming important in the future. The development of longer, higher aspect ratio nanotubes is a key element in all of this science and is expected.
Dr. Brian P. Grady is a Conoco-DuPont professor of chemical engineering in the School of Chemical, Biological and Materials Engineering at University of Oklahoma. Grady is also the author of "Carbon Nanotube-Polymer Composites: Manufacture, Properties, and Applications."