Abstract
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.
Highlights
Carbon fibers (CF) are known for their excellent mechanical, thermal and electrical properties
During workplace measurements of the machining of polymer composites that were reinforced with pitch-based carbon fibers, concentrations of 830,000 World Health Organization (WHO)-fiber m–3 were observed 15 cm from the source and of 33,000 WHO-fiber m–3 at about 3 m distance
Strategy for the Workplace Measurement The workplace measurements were performed in close accordance with established strategies for measuring airborne fibers, in particular the German Standard VDI 3492 “Measurement of inorganic fibrous particles – Scanning electron microscopy method” (VDI, 2013) and the recommendations given by the WHO ”Determination of airborne fibre number concentrations” (WHO, 1997)
Summary
Carbon fibers (CF) are known for their excellent mechanical, thermal and electrical properties. In a multitude of industrially relevant applications, for example in aerospace, automotive and wind energy sectors, a high and currently increasing demand for carbon fiber composites can be found. The clearance of insoluble WHO-fibers that entered the alveoli in the deep airways by lung self-cleaning mechanisms is considered impeded for morphological reasons, which may render them long term biopersistent. They can become the cause of chronic inflammation and may increase the risk to develop mesothelioma (Donaldson et al, 2010). One study described in Warheit (1994) and Warheit et al (2001) observed a dose-dependent but only transient inflammatory response in the lungs of exposed rats following acute
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