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In-Depth Survey Report: Evaluation of Engineering Controls in a Manufacturing Facility Producing Carbon Nanotube-Based Products
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2012/10/01
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[PDF-1.94 MB]
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Description:This report summarizes the study results of an evaluation of engineering controls used by a secondary manufacturer (user) of carbon nanotubes (CNTs) to synthesize composite materials. Direct-reading instruments (including Fast Mobility Particle Sizer, or FMPS, Aerodynamic Particle Sizer, or APS, and DustTrak Aerosol Monitor) with continuous real-time measurements were used to monitor manufacturing processes. An assessment of existing exposure controls was conducted using hood capture and exhaust airflow measurements and smoke visualization techniques. The primary processes conducted at the plant were (1) weighing out CNTs and placing them in a slurry, (2) mixing the CNT slurry with other materials in large vats, (3) depositing the slurry materials on a substrate, and (4) cutting the final substrate to meet final product specifications. The task of weighing out the CNTs was performed in a Class II Biological Safety Cabinet (BSC), which was exhausted to the outside. The BSC was connected to a facility exhaust fan that ran continuously and had an integral supplemental fan that was used at the discretion of the worker. The face velocity of the BSC averaged 65 feet per minute (fpm) when the supplemental BSC fan was switched on and the external facility blower was running. When the supplemental blower was not switched on, the face velocity dropped to less than 10 feet per minute (fpm) across all external facility blower settings (low, medium, high). The results of this testing indicate that the supplemental BSC fan should always be turned on before working with any potentially hazardous materials inside the BSC. Particle emissions were also found during the mixing process. This process involved weighing out raw materials and mixing them in large drums into a solution. Raw material weigh-out and preparation should be performed in ventilated enclosures (such as a chemical fume hood or powder transfer station) to prevent particle emissions during transfer and weighing, even though these bulk powders are not nanomaterials. Two control measures were used in the cutting process: (1) a downdraft table for cutting CNT-deposited substrates with a rotary cutter, and (2) a canopy hood for controlling exposures during the cutting substrate rolls with a powered saw. The performance of the downdraft table was difficult to quantify because emissions from hand rotary cutting release were minimal. The capture of emissions during rotary cutting tasks could be improved by changing the existing downdraft table to a backdraft slotted hood design commonly used for welding operations [ACGIH 2010]. This design would allow for a solid work surface for cutting, while pulling the emissions away from the worker and into the exhaust system. The canopy hood did not effectively collect particle emissions nor prevent worker exposure to airborne particles released from the powered saw cutting task. The monitoring data showed that operating the canopy hood (which was normally turned off) resulted in 15%-20% higher nanoparticle concentrations in the worker's breathing zone. The primary reason for this result is due to the positioning of the worker between the source of emission and the exhaust. This configuration is not recommended because it can cause saw emissions to be pulled through the worker's breathing zone. For this powered cutting process, the optimum control approach is to contain emissions at the source. The use of a ventilated shroud on the saw or the use of a ventilated enclosure around the process could effectively collect saw emissions and reduce the potential for worker exposure during this task. Although there is currently no regulatory occupational exposure limit (OEL) for CNTs, the use of engineering controls is recommended to reduce the potential risks associated when working with these materials. This report presents the findings of our control assessment and provides recommendations on approaches to contain process emissions and reduce the potential for worker exposure.
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DOI:10.26616/NIOSHEPHB35613a
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Pages in Document:1-38
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NIOSHTIC Number:20042040
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NTIS Accession Number:PB2013-104685
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Citation: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 356-13a, 2012 Oct; :1-38
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Federal Fiscal Year:2013
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Peer Reviewed:False
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Resource Number:EPHB-356-13a;B20130124;
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