Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB 356-19a, 2013 Sep; :1-55
This case study was completed in response to a company request to investigate the release of carbon nanotubes (CNTs) during cutting and sanding of composite panels containing CNTs. To evaluate the release of CNTs, a ventilated enclosure was built to capture and mix the emissions from sanding and cutting composite panels. The ventilation system consisted of a partial enclosure around the band saw, a metal duct, and a clean air machine that provided air flow and filtration. The volumetric air flow through the test system was 1,072 cubic feet per minute (cfm), with a hood face velocity of 197 feet per minute (fpm). The panel sanding and cutting was performed inside the hood face to reduce workers' dust exposure and to capture emissions for characterization. The process emissions were well mixed because of the inclusion of a baffle and a 90 degrees elbow upstream of the sampling location. A portable air exhaust system exhausted air through the enclosure, the 90 degrees elbow, and the 12-inch-diameter duct. Process emission rates were computed on the basis of air flow and concentration. To determine dust concentrations, air samples were collected 15 duct diameters from the elbow. It was expected that at this location the emissions would be well mixed, and that samples collected from the center core of the duct would be representative. The number concentration of particles in the range of 7 to 560 nanometers (nm) was measured with a Fast Mobility Particle Sizer (TSI Inc., Shoreview, MN). Isokinetic sampling was conducted to obtain samples for determining the following: 1) particle number concentrations (from 0.5 to 20 micrometers, um), by means of an Aerodynamic Particle Sizer (TSI Inc., Shoreview, MN), 2) aerosol mass concentration, based upon light scattering, by means of an aerosol photometer, 3) elemental carbon in a filter sample, per NIOSH Method 5040, and 4) fiber concentration in a filter sample, measured by transmission electron microscopy (TEM), per NIOSH Method 7402. Four different composite panels were tested: 1) panel A, an IM7 graphite fiber/BMI (bismaleimide) composite panel, 2) panel B, an IM7 graphite fiber/epoxy composite panel, 3) panel C, an IM7 graphite fiber/epoxy composite panel with a carbon-based nonwoven mat (fiber diameter of 7.5 microm) as a surface ply, 4) panel D, an IM7/epoxy panel with multiwalled CNT-coated carbon-based nonwoven mat as a surface ply. Cutting the composite with the band saw did not result in the detection of CNTs or other fibers by TEM. In addition, the presence of CNTs did not greatly increase measured amounts of elemental carbon, aerosol mass, and aerosol number. However, the number concentration of particles smaller than 560 nm exceeded 105 particles per cubic meter (cm3), as compared with background concentrations, which averaged 1.09 × 104 particles/cm3. The calculated emission rates per volume of material removed during cutting ranged from 70 to 230 milligrams per cubic meter (mg/cm3) of cut volume (the product of blade diameter, panel thickness, and cut length) and from 2.95 × 1013 to 6.72 × 1014 particles/cm3 of cut volume. The high number concentration and emission rates may have been caused by the formation of nano-aerosols generated by frictional heating and did not appear to be elevated by the presence of CNTs. Sanding the composite panel D, which contained CNTs, generated fiber emission rates of 1.9 × 108 and 2.8 × 106 fibers per second. No measurable fiber concentrations were generated from panels that contained graphite and carbon fibers (panels A, B, and C). The particle number concentration in the duct was about 103 particles/cm3; this may be largely attributed to the carbon brushes that are part of the sander's motor. Mass emission rates were between 0.01 and 0.36 milligrams per second (mg/sec). However, the pressure applied to the sander by the worker was not controlled, and this may have caused a wide range in the mass and fiber emission rates. The ventilated enclosure used in this study effectively contained the emissions from panel cutting and sanding processes. Because sanding composites containing CNTs resulted in noticeable emissions of fibers, careful and aggressive control of exposure to CNTs is recommended. These emissions could probably be controlled by either a conventional local exhaust ventilation hood or a high-velocity, low-volume ventilation system. Local exhaust ventilation can also be used to capture and collect the aerosol and debris generated by the band saw.