Generation and Characterization of Aerosolized Particles from Nanoclay Composites During Sanding

1105 

Professional Poster 

E Lee¹

¹NIOSH, Morgantown, WV

Dr Eun Gyung (Emily) Lee  
NIOSH

The objective of this study was to simulate industrial sanding of nanoclay polypropylene composites and to characterize particles generated to estimate occupational exposure levels. The study was conducted to characterize the aerosolized particles from nanoclay composite materials during sanding by employing various sanding belt types and different percent mass loading of nanoclay in a base material.

Nanoclay composite materials continue to rapidly grow in novel applications including food and beverage packaging, biomedical tools, fire retardants, automobile/aerospace parts industry, etc. Previous studies have found adverse toxicological effects due to the exposures to raw nanoclay materials: a) pulmonary health effects (e.g., respiratory tract irritation); b) hemolysis; c) cytotoxicity effects (e.g., decreased cellular proliferation); d) mitochondrial and membrane damage; e) reactive oxygen species generation; and f) genotoxic effects. The life cycle of nanocomposites (e.g., manufacturing, use, machining and disposal) could lead to the potential release of aerosolized particulate including intact nanocomposite, nanocomposite with surface nanoclay protrusions, or free release of nanoclay particles from the polymer matrix. All conditions could potentially cause adverse health effects via inhalation exposure. Occupational exposures to airborne nanocomposites are poorly understood.

Two types of nanoclay material (Cloisite 25A and Cloisite 93A) were dispersed into polypropylene (PP) at 0%, 1%, and 4% by weight. Virgin PP (used in making agricultural film, packaging film, and automotive panels) was selected as the comparative control. For each nanocomposite material, mechanical properties of Young's modulus, tensile strength, toughness, and elongation at break were determined. Crystallinity of each composite, the degree of dispersion of nanoclay within the PP matrix and the visualization of dispersed nanoclay were examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM).

In an automated, controlled exposure chamber to generate sanding particles, each composite was sanded with two types of sanding belts, (zirconium aluminum (ZrAl) oxide and silicon carbide). Particles released during sanding were measured with the following direct-reading instruments (DRIs): a) a condensation particle counter to determine particle number concentrations, b) a scanning mobility particle sizer and c) an aerodynamic particle sizer to measure size distributions. A cascade impactor was used for gravimetric analysis. Particles released from nanocomposite sanding were collected with inhalable samplers loaded with polycarbonate filters for electron microscopy (EM) analysis. Nanocomposite particulate in collected dust samples was positively identified using computer-controlled FESEM/EDX scans by comparing the morphology and elemental composition against sandpaper dust. Protruding or embedded nanoclay particles was also identified by detecting similar aluminum, silica, and magnesium EDX spectra compared to nanoclays alone. For each test condition, the temperature on the surface where a composite block contacted the sandpaper was measured with a traceable infrared thermometer to determine temperature changes during the sanding work. The height of each material was measured pre- and post sanding work to determine the amount of material abraded after sanding.

Data collected with DRIs were averaged from the three replicates and adjusted by subtracting the background concentrations. Statistical analysis was performed to compare the particle number and respirable mass concentrations among different composites and between sandpaper grit sizes. For the EM analysis, the results of particle elemental composition by number and weight percent and size distribution frequency by number and weight were reported for each composite material.

ZrAl oxide sandpaper released substantially more particles than silicon carbide during sanding of the virgin PP. The addition of nanoclay influenced composite tensile strength and toughness, which correlated with particle release. The nanocomposite with 1% Cloisite 25A, 1% Cloisite 93A, and 4% Cloisite 93A loading generated higher particle number concentrations (1.3-2.6 times) and respirable mass concentrations (1.2–2.3 times) relative to the virgin PP, while the 4% Cloisite 25A composite produced comparable results with virgin PP, regardless of sandpaper type. P100 sandpaper produced higher particle number and respirable mass concentrations than P180 irrespective of composite material type. For the size distributions by number, all composites showed the peak number concentrations < 15 nm and the majority of particles < 30 nm, without noticeable shift of the diameter of peak concentration with added nanoclay materials. Particle emission rates were positively associated with the amount of nanocomposite mass abraded from sanding (rp = 0.972 for P100 and 0.817 for P180). For all composites, size distributions by mass revealed that the majority of mass was dominated by the inhalable fraction. The change of temperature before and during sanding was minimal showing the highest temperature of 32°C. This is considerably lower than the PP melting point (130°C~171°C) and thus, no generation of semivolatile organic compounds due to thermal degradation is expected.

The findings indicate that during abrasion, the majority of the inhalable particles originated from nanocomposite materials. In addition, a significant number of the composite particles displayed platelet-shaped protrusions with an elemental composition and morphology indicative of nanoclay (18–59% for all nanocomposites).

This study indicates that the percent loading and dispersion of nanoclay in the PP modified the matrix structure, strength, and toughness, thus affecting the number of particles abraded and released during sanding, along with type of sandpaper. These results may have implications for the potential toxicity of these nanocomposite dust particles generated as higher particle concentrations and/or particles with protruding nanoclays could elicit more severe adverse health effects after inhalation. Currently, in vivo/in vitro toxicity studies are underway with collected inhalable fractions.

Nanotechnology

Aerosols