10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Release of Respirable Fibrous Dusts during Abrasive Machining and Fatigue Testing of Carbon-Fiber Composites
DAPHNE BÄGER, Nico Dziurowitz, Judith Neuhoff, Barbara Simonow, Carmen Thim, Dominic Kehren, Daniela Wenzlaff, Asmus Meyer-Plath, Sabine Plitzko, Federal Institute for Occupational Safety and Health (BAuA)
Abstract Number: 729 Working Group: Workplace Aerosol
Abstract Carbon fibers are distinguished by their combination of low material density, electrical and thermal conductivity as well as high tensile strength and Young’s modulus. Their composites have found widespread application in high-performance or light-weight product components. Standard carbon fibers that generally exhibit a diameter of 5-10 µm are non-respirable and presumed to be uncritical in terms of inhalation toxicity. However, composite development and manufacturing involves abrasive machining and fatigue testing that may give rise to dust release. Here, we report the release aerosols containing respirable carbon fibers fragments from mechanical abrasion and failure of composites containing ultra-high modulus fibers.
Workplace emission measurements were conducted during mechanical processing and tensile fatigue testing of test specimen made of pitch-based carbon fiber composites. Aerosol release during mechanical abrasion of such composites was also studied systematically in laboratory tests using a zero particle background chamber performing grinding and milling under controlled conditions. Aerosols were characterized by means of condensation particle counts, aerosol spectrometry, lung-deposited surface area quantification as well as morphological classification and quantification of SEM images of track-etched membrane filters.
Different to previous studies on carbon fibers handling workplaces, our measurements revealed near-field concentrations up to 1,000,000 respirable fibrous objects of WHO fiber geometry per cubic meter. The observed asbestos-like morphology of the fragments together with the known high chemical inertness of graphitic carbon fibers, inducing long-term biopersistance, renders such types of aerosols very critical from the toxicological point-of-view.
In laboratory tests, strong differences in fracture mode were observed for polyacrylonitrile- and pitch-based carbon fibers. The ultra-high Young’s modulus of pitch-based fibers is therefore assumed to induce the observed strong propensity to release respirable fibrous fragments due to intrinsic anisotropic structure orientation during the graphitization step. Such material structure-related fracture modes must be considered for occupational health and safety strategies.
The work emphasizes the importance of detailed SEM-based morphological characterization and classification of aerosols found in workplace atmospheres to reveal the presence of unexpected, process-related toxic dusts components that are not detectable even by contemporary online techniques.