Dry processed fabrics containing high aspect ratio carbon nanotubes could enable a new generation of products.
by Philip D. Bradford, Ph.D.
Carbon nanotube (CNT) research has received a lot of popular press recently because of the amazing potential applications of these unique nanofibers. Carbon nanotubes have been highly studied by materials scientists who see the potential in their amazing stiffness, strength, thermal conductivity, electrical properties and high surface area. However, CNTs have yet to make significant impacts in products produced by the textile industry. But this could change, as new methods to produce high aspect ratio carbon nanotubes, and nonwoven fabrics directly from them, are emerging.
What are carbon nanotubes and how are they currently used in the textile industry?
Carbon nanotubes are tubular fibrous structures composed entirely of graphitic carbon planes. The carbon-carbon bonds form a hexagon shape within the lamellar graphite planes that resembles common chicken wire. A common analogy to describe the structure of a CNT is to picture the same sheet of chicken wire rolled up into a cylinder. The orientation of the graphite planes, parallel to the fiber axis, along with the seamless nature of the tube structure, enables their extreme mechanical properties. The diameter of carbon nanotubes can vary, usually from 1-50 nanometers, which is significantly smaller than fibers produced by meltblowing or optimized electrospinning processes.
Most carbon nanotubes have lengths in the 1–10 micron range. Bulk nanotube material comes to the consumer as a black powder that resembles carbon black filler. Because of their high surface area, the tubes have a tendency to agglomerate or bundle together. Some fiber-producing companies that have explored the use of CNTs have done so through the compounding of CNT powder with polymer to be extruded into fibers. At low loading levels (less than a few weight percent), CNTs can be individualized from CNT agglomerates to increase the mechanical properties and electrical conductivity of polymer fibers. Higher CNT loading levels dramatically increase melt viscosity, while actually decreasing mechanical properties due to large CNT agglomerates.
What are the challenges?
The low CNT loading levels seen in polymer fibers create marginally improved properties and are a far cry from the applications imagined for CNTs. In addition, the CNTs within the fibers are encapsulated, which negates the potential of their high specific surface area. Producing fabrics entirely from CNTs is one possible way to produce materials with superior properties. A common process to make fabrics from CNTs is akin to producing wet-laid nonwovens. The CNTs are first dispersed in a suitable solvent using mechanical agitation. The CNTs are then filtered from the solution to produce a fabric, often referred to as “buckypaper.” The two major drawbacks of this route are:
- Short CNTs must be used to get a good dispersion in the solvent.
- The capillary forces between tubes developed during solvent drying creates significant bundling of CNTs.
Both of these factors limit the robustness of the fabrics. Two recently developed production methods overcome these shortfalls of CNT wet processing.
Dry CNT fabric formation processes
One major requirement for producing fabrics directly from CNTs is that the CNTs should have an extremely high aspect ratio. CNT lengths in the range of hundreds of microns to millimeters should be used. While this length would be considered very short in the case of traditional staple textile fibers, in this case the nanoscale CNT diameter means the aspect ratios are in the hundreds of thousands. This is important because the surface of CNTs are smooth with no chemical functional groups, so the fabrics made from them are held together by the weak secondary interactions among tubes. Increasing the length of the CNTs allows for more of these interactions to “add up” along their length to allow for the creation of stable fabrics.
Two processes have been shown to make dry CNT fabrics from high aspect ratio tubes:
- In the first process, carbon nanotubes are synthesized for an extended period of time within the reactor to form long CNTs that are entangled in an aerogel web (think black cotton candy). If one end of the reactor is open, the tangled web can be mechanically withdrawn from the reactor and spread out onto a takeup belt. In this process, the catalyst and carbon source gasses for CNT growth are continuously supplied to the reactor, making the process continuous. This extremely promising technique is in the process of commercialization by Nanocomp Technologies Inc. The main drawback of this technique is that the random entangled CNT growth morphology makes aligning the CNTs challenging.
- In the second process, carbon nanotubes are synthesized on a substrate instead of floating in the reactor. Deposition of catalyst nanoparticles with defined spacing produces “arrays” of carbon nanotubes with vertical alignment. Because of the alignment, and their similarity in appearance to a bamboo forest, they are also called CNT “forests.” With critical control of the CNT spacing and quality within the forest, they can exhibit a special property where sheets of horizontally aligned CNTs can be pulled from the vertically aligned array. These CNT sheets maintain the CNT alignment and can be taken up on takeup rolls or belts to produce CNT fabric. The main drawback to this technique is that the CNTs forests are usually produced in a batch process, slowing the production rate. The merits of this technology are being studied by a few research groups around the world and have not yet reached pilot scale production.
Where it could lead
Fabrics made entirely of carbon nanotubes have many potential applications. One of the most attractive is in high-strength composite materials. Fabrics with long, aligned, individualized CNTs can be used to produce resin pre-impregnated fabrics and high-fiber volume fraction composite materials with morphologies that resemble traditional carbon fiber materials.
With the multifunctional properties they would provide, CNT fabrics and composites produced from them may fill needs not met by carbon fiber composites in defense, aerospace, automotive and consumer markets. Their high specific surface area, chemical stability and thermal stability make them a great candidate for battery electrodes, catalyst supports, thermoelectric materials, and air and water filtration.