10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Measurements of the Physicochemical Properties of Nanoparticles Produced via Thermal Plasma Spraying Processes in a Precision Machining Workplace
Spyridon Bezantakos, Apostolos Salmatonidis, Mar Viana, GEORGE BISKOS, Université du Littoral Côte d' 'Opale, Dunkerque, France
Abstract Number: 1547 Working Group: Workplace Aerosol
Abstract Ceramic NPs (i.e., ceramic particles having sizes smaller than 100 nm) have received significant attention due to their unique properties that make them useful for a number of applications in diverse fields, including enhancing or altering the surface properties of engineering metallic parts. Concerns are raised regarding their human health implications due to the potential risk associated with their handling, manufacturing and from treating or manufacturing processes of bulk materials using ceramic NPs. In this study, we conducted measurements for determining the physicochemical properties (e.g., number concentration, size, shape, hygroscopicity and mixing state) of airborne particles in a machine shop that uses thermal plasma spraying for processing metallic engineering parts. In addition, samples of airborne NPs produced via the thermal plasma processes were collected in order to study their toxicity. The measurements were conducted at the workers breathing zone (i.e., inside the plasma spraying booths) when the plasma spraying processes were operational and during times of no activity (e.g., during night hours). Our first findings show:
1) The number concentration of fine airborne particles at the workers breathing zone when the plasma processes were active was by more than 100 times higher than that of the background, reaching some million particles/cm3. This significant increase in the number concentration of aerosols was caused by NPs produced via the plasma spraying processes.
2) The size of aerosols produced from the plasma spraying processes was residing in the ultrafine fraction, in contrast to the background aerosols whose size distributions were characterized mainly by accumulation mode particles.
3) NPs emitted directly from plasma spraying processes are hydrophobic.
4) NPs directly emitted from plasma spraying processes in the workers breathing zone are externally mixed with the background aerosols (i.e., a hydrophobic and a hygroscopic population of aerosols can be observed). The fraction of the hydrophobic aerosol population depends on the concentration of NPs produced via the plasma spraying process. For example the lowest fractions of hydrophobic aerosols were observed when the plasma spray was not operational. However, when the plasma spray was operational and the concentration of NPs that were produced was much higher than that of the background aerosol population, only the hydrophobic mode was observable.
5) Larger particles (i.e., having dry electrical mobility diameters >60 nm) of the NPs produced via plasma spraying processes exhibited an apparent shrinking (i.e., their measured size at elevated relative humidity conditions was lower than of that selected at dry conditions). This behavior can be explained if these NPs had an elongated shape at dry conditions, which changed to a more spherical one when they interacted with water vapor inside the instrument. Further analysis using transmission electron microscopy (TEM), will provide additional information regarding the shape of the NPs produced via these processes.