Potential release scenarios for carbon nanotubes used in composites

Abstract

The expected widespread use of carbon nanotube (CNT)-composites in consumer products calls for an assessment of the possible release and exposure to workers, consumers and the environment. Release of CNTs may occur at all steps in the life cycle of products, but to date only limited information is available about release of CNTs from actual products and articles. As a starting point for exposure assessment, exploring sources and pathways of release helps to identify relevant applications and situations where the environment and especially humans may encounter releases of CNTs. It is the aim of this review to identify various potential release scenarios for CNTs used in polymers and identify the greatest likelihood of release at the various stages throughout the life-cycle of the product. The available information on release of CNTs from products and articles is reviewed in a first part. In a second part nine relevant release scenarios are described in detail: injection molding, manufacturing, sports equipment, electronics, windmill blades, fuel system components, tires, textiles, incineration, and landfills. Release from products can potentially occur by two pathways; (a) where free CNTs are released directly, or more frequently (b) where the initial release is a particle with CNTs embedded in the matrix, potentially followed by the subsequent release of CNTs from the matrix. The potential for release during manufacturing exists for all scenarios, however, this is also the situation when exposure can be best controlled. For most of the other life cycle stages and their corresponding release scenarios, potential release of CNTs can be considered to be low, but it cannot be excluded totally. Direct release to the environment is also considered to be very low for most scenarios except for the use of CNTs in tires where significant abrasion during use and release into the environment would occur. Also the possible future use of CNTs in textiles could result in consumer exposure. A possibility for significant release also exists during recycling operations when the polymers containing CNTs are handled together with other polymers and mainly occupational users would be exposed. It can be concluded that in general, significant release of CNTs from products and articles is unlikely except in manufacturing and subsequent processing, tires, recycling, and potentially in textiles. However except for high energy machining processes, most likely the resulting exposure for these scenarios will be low and to a non-pristine form of CNTs. Actual exposure studies, which quantify the amount of material released should be conducted to provide further evidence for this conclusion.

Uncertainty communication in the environmental life cycle assessment of carbon nanotubes

“…Arc discharge 3.2 × 10 5 Healy et al [17] 2.2 × 10 3 Kushnir and Sanden [13] CVD (VGCNF) 1.1 × 10 4 Khanna et al [12] CVD (floating bed) 4.8 × 10 2 Kushnir and Sanden [13] HiPCO and CoMoCAT 5.8 × 10 3 Kushnir and Sanden [13] 1.6 × 10 5 Healy et al [17] …”

“…This has a large effect on the overall life cycle impacts attributed to CNT production. The modelling of CNT emissions to the environment has also been studied by several authors [3][4][5]. A better understanding of this is needed to inform risk and life cycle impact assessment.…”

Carbon nanotubes: Impacts and behaviour in the terrestrial ecosystem - A review

“…Chronic dissemination corresponds to the contamination by low doses of CNTs, but over a long period of time. Direct release (chronic dissemination) has been considered as very low for most of the scenarios, except for tires [38]. For example, CNTs can be accumulated in soil due to the rubbing of CNT-containing tires on roads [38].…”

Carbon nanotube buckypaper reinforced polymer composites: a review

“…For example, sports equipment, such as tennis racquets containing CNTs, have been produced and marketed. With CNTs becoming easier to produce and cheaper to buy, the CNT industry could potentially overtake that of the carbon fiber industry and become one of the major additives for polymer-composite fabrication [18,22,23] . Similar to mechanical properties, electrical conductivity of CNTs are quite varied, probably due to varying levels of defects as well as an unknown distribution of chiralities.…”