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Release of Respirable Fibrous Dust from Carbon Fibers Due to Splitting along the Fiber Axis

Category: Air Toxics

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DOI: 10.4209/aaqr.2019.03.0149
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Dominic Kehren , Barbara Simonow, Daphne B├Ąger, Nico Dziurowitz, Daniela Wenzlaff, Carmen Thim, Judith Neuhoff, Asmus Meyer-Plath, Sabine Plitzko

  • Federal Institute for Occupational Safety and Health (BAuA), Berlin 10317, Germany

Highlights

  • High number concentrations of respirable fibrous dust released from carbon fibers.
  • Fiber splitting along the fiber axis observed for pitch based carbon fibers.
  • Carbon fiber microstructure influences the fracture behavior.
  • PAN based and pitch based carbon fibers differ greatly in their fracture behavior.

Abstract

The aim of this report is to sensitize to the fact that some types of carbon fibers can split along the fiber axis during machining processes, thus becoming a source of respirable fibrous fragments in high concentrations. The risk of releasing respirable fiber dust must be assessed both in terms of carbon fiber materials and handling processes. We present an analysis of fiber concentrations released during mechanical processing of carbon fiber reinforced polymers at two different workplaces together with measurements from abrasion testing in a closed laboratory setup with inherent bulk sample analysis. During workplace measurements of the machining of polymer composites that were reinforced with pitch-based carbon fibers, concentrations of 830,000 WHO-fiber m-3 were observed 15 cm from the source and of 33,000 WHO-fiber m-3 at about 3 m distance. An explanation for such a high release propensity for respirable fibers with diameters below 3 µm is a fiber splitting along the axis of the studied fiber type. Comparative abrasion experiments of pitch and polyacrylonitrile-based carbon fiber reinforced polymer composites revealed that their fracture behavior differs from another. The studied polyacrylonitrile-based carbon fibers broke primarily perpendicular to their axis, whereas the studied pitch-based carbon fibers show fiber splicing and splitting along the fiber axis. In order to predict and manage health risks of the large variety of existing carbon fiber materials, the understanding of the relation between fiber microstructure, fracture morphology and WHO-fiber release propensity must be urgently improved. In the meantime, all handling and processing steps of pitch-based carbon fibers have to be accompanied by precautionary or exposure measurement-controlled safety measures to protect the employees.

Keywords

Carbon fiber reinforced polymers Carbon fiber splitting WHO-fibers Occupational health Pitch CFRP


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