In-Depth Survey Report: Assessment of Engineering Controls for Reducing Contaminant Emissions from Producing Carbon Nanotubes.

摘要

This report summarizes the survey results from a joint study conducted by the National Institute for Occupational Safety and Health (NIOSH) Engineering and Physical Hazards Branch (EPHB), Purdue University, and the University of Massachusetts Lowell. The study site produced carbon nanotubes (CNTs) using the chemical vapor deposition (CVD) process. Four major processes/tasks were evaluated during the site visit, including: (1) product harvesting; (2) product sieving or sizing; (3) product milling, and; (4) material handling. After the completion of the CVD process inside a furnace, the product is harvested in a custom-built glove box to prevent nanomaterial emissions during product harvest. Following product harvesting, sieving was done by a shaker inside an unventilated, enclosed cabinet to grade the product. If required, the product was also milled using a small ball mill in a separate room. The emission from these processes and the performance of control measures on mitigating exposures during the conduct of these tasks were evaluated using directing-reading instruments and collecting air filter samples for transmission electron microscope (TEM) inspection. Three fume hoods were available for the handling of precursor and product materials, but only one of them was used to handle and transfer nanomaterials. The survey results showed that the fume hood effectively controlled the release of nanoparticles. The glove box also effectively controlled particle release from the unloading of the production furnace during CNT harvesting. Air samples showed evidence of very low levels of airborne CNTs around the product furnace. No release of nanoparticles was detected during the ball milling process using direct-reading instrumentation, while the TEM analysis of air filters collected in the ball mill room showed the presence of CNTs in the room air at very low levels. During the sieving process, particle emissions were detected by the direct-reading instruments and area air filter samples when the worker opened the cabinet following completion of the process. Increasing the waiting time before opening the cabinet did not help reduce particle emissions. Overall, few airborne CNTs were identified in area air samples at the furnace location, near the sieving operation and in the ball mill room. Based on the findings in this study, recommendations can be made to control containment of emissions from the sieving process. The addition of local exhaust ventilation (LEV) on the sieving cabinet would help reduce the potential for release of product into the work environment when unloading the sieve. This improvement could also lower the risk of transporting contaminants to other working areas. Besides, the total exhaust flow for each of the three laboratory fume hoods should be increased to provide a minimum average face velocity of 100 feet per minute (fpm). This may require the upgrading of the exhaust fan and reconfiguration of the exhaust ducting from the hoods.