Evaluation of Aerosol Release During the Handling of Unrefined Single Walled Carbon Nanotube Material

Carbon nanotubes represent a new form of carbon that has several unique properties, including great tensile strength, high conductivity (in some states), high surface area, unique electronic properties, and potentially high molecular adsorption capacity. A number of laboratories are generating small quantities of this material, and commercial interest in the substance is motivating the rapid development of large-scale production facilities. However, little is known of the potential toxicity of the material, or how it should most appropriately be handled to minimize exposure.
In a collaborative effort the National Institute for Occupational Safety and Health (NIOSH), National Aeronautics and Space Administration (NASA), Rice University, and Carbon Nanotechnologies Inc. (CNI) are investigating the nature of the aerosol released when unrefined nanotubes are handled—that is, the material that is aerosolized during the production process, prior to its being purified. Two separate studies have been undertaken on single-walled carbon
nanotubes (SWCNT)—a specific form of the material comprised of carbon tubes around 1.5 nanometers (nm) in diameter and up to a millimeter or more in length, and having a single layer of carbon atoms that form the tube wall. Two sources of the material were considered: (1) laser ablation, which leads to a relatively compact powder, and (2) nanotubes formed using the High-Pressure Carbon Monoxide process (HiPCO®), leading to an expanded material that has very low bulk density. Primarily, the propensity for the material to form an aerosol while being agitated was investigated in the laboratory. Aerosol size distribution between 4 nm and 20 μm was measured when the material was agitated in a number of ways. At high agitation levels, a bimodal or trimodal aerosol was formed below 10 μm, depending on the material source, and HiPCO®-generated material led to the release of particles smaller than 10 nm in diameter. However, it was unclear whether these particles represented nanotubes, catalyst particles (used in the manufacturing process), or compact
carbonaceous particles. Generation rates were typically two orders of magnitude below those for a similar volume of fumed alumina—chosen because it represents another material that has a very low bulk density and is formed from nanometer-sized primary particles. ...