THE GENERATION AND CHARACTERIZATION OF SILICON-CARBIDE WHISKERS (FIBERS) FOR INHALATION TOXICOLOGY STUDIES

Atmospheres of silicon carbide whiskers (wh) were required for a repeated dose lung deposition study and a subchronic inhalation toxicity study, each involving exposure to three concentrations of whiskers for 6 hr/day, 5 days/week for 1 or 13 weeks. Target concentrations were 0, 500, 1500, and 7500 wh/cc. By using the mean fiber length (10 mum), diameter (0.555 mum), and density (3.2 g/cc) of the bulk material, target mass concentrations were approximately 0, 4, 12, and 60 mg/m3. The Pitt III generator, developed for cotton fiber dispersion at the University of Pittsburgh, was selected. This instrument consists of a vertical 18-in. long cylinder, which is closed at each end with rubber dams and coupled at the base to a loudspeaker. The sonic energy from the loudspeaker fluidizes the test material and the whiskers in the air column are carried out with the exhaust air. The output of the Pitt III generator was altered by changing the sound energy input, the rate of introduction of the test material into the fluidizing cylinder, or the airflow through the cylinder. Separate generation systems were used for each inhalation chamber. Chamber atmospheres were characterized gravimetrically and samples were analyzed by scanning electron microscopy. Mean mass and number concentrations for the subchronic study were 0.09, 3.93, 10.7, and 60.5 mg/m3 and 0, 630, 1746, and 7276 wh/cc, respectively. Weighted mean values for whisker diameters, lengths, and aspect ratios were 0.560 mum, 4.53 mum, and 8.62, respectively. Although whisker lengths were less than half those of the bulk material, nearly 30% of the whiskers were greater than 5 mum long.