Abstract
Ambient air sampling for nano-size particle emissions was performed during spot spray coating operations with a Sono-Tek Exactacoat Benchtop™ system (ECB). The ECB consisted of the application equipment contained within an exhaust enclosure. The enclosure contained numerous small access openings, including an exhaust hook-up. Door access comprised most of the width and height of the front. The door itself was of the swing-out type. Two types of nanomaterials, Cadmium selenide (Cd-Se) quantum-dots (QDs) and Gold (Au) QDs, nominally 3.3 and 5 nm in diameter respectively, were applied during the evaluation. Median spray drop size was in the 20 to 60 micrometer size range.1 Surface coating tests were of short duration, on the order of one-half second per spray and ten spray applications between door openings. The enclosure was ventilated by connection to a high efficiency particulate aerosol (HEPA) filtered “house” exhaust system. The exhaust rate was nominally 80 ft3 per minute producing about 5 air changes per minute.
Real time air monitoring with a scanning mobility particle size analyzer (SMPS™) with a size detection limit of 7 nm indicated a significant increase in the ambient air concentration upon early door opening. A handheld condensation particle counter (CPC) with a lower size limit of 10 nm did not record changes in the ambient background. This increase in the ambient was not observed when door opening was delayed for 2 minutes (~10 air changes). The ventilated enclosure controlled emissions except for cases of rapid door opening before the overspray could be removed by the exhaust. A time delay sufficient to provide 10 enclosure air changes (a concentration reduction of more than 99.99 %) before door opening prevented the release of aerosol particles in any size.2
Scanning-transmission electron microscopy (STEM) and atomic force microscopy (AFM) demonstrated the presence of agglomerates in the surfaces of the spray applied deposition. A filtered air sample of the enclosure overspray examined by AFM also demonstrated the presence of agglomerates for the Au QDs. The AFM system was not able to resolve individual QDs as was the STEM. Chemical fingerprinting of the QDs with STEM/EDS (energy dispersive spectroscopy) was performed for the Cd-Se surface deposition, but not the aerosol. Both STEM and AFM background characterization by morphology and chemical fingerprinting were performed throughout the laboratory for a period of about one year. Outdoor sources were primarily biological, combustion fume, salt and other crustal particles. Indoor sources were primarily paper/clothing fibers, spray-on insulation fragments, fiber glass, and human skin cells.