Case Study The Lifecycle of Conductive Plastics Based on Carbon Nanotubes - Safety of Nanomaterials along Their Lifecycle - page 407

soil layer (Cornelis et al. 2014). Details on the soil-nanoparticle interaction for CNTs
and other nanoparticles are well summarized by Cornelis et al. (2014).
Thus far, no definite predictions on the transport and fate of CNTs in water and
soils can be made to better describe the exposure in the environment. Previous stud-
ies showed that CNTs can be taken up by animals such as Daphnia magna (Petersen
et al. 2009). Uptake by plants is realistic taking the current general knowledge of
nanoparticle into account (Jacob et al. 2013; Ma et al. 2010) even though it was not
tested for CNTs to our knowledge.
18.4 HAZARD AND RISK POTENTIALS OF PURE CNTs
It appears from initial findings regarding the nanometer-scale materials that are
released from actual uses that risk evaluation for CNTs in conductive plastics should
be based on exposure and toxicity studies of the released particles, and not on extrap-
olation from studies of free CNT fibers. Furthermore, there is as yet no evidence to
assert that the released material (e.g., Figure 18.3 on primary particles with embed-
ded CNTs following abrasion/cutting or Table 18.1 on degradation products follow-
ing weathering or thermal processes) would be more hazardous than the material
released from the same polymer without added CNTs (Wohlleben et al. 2011, 2013).
More studies of such differential toxicity between material with and without embed-
ded CNTs are therefore needed.
Alternatively, risk evaluation could be based on the assumption that exposure
to free CNTs could occur as a small fraction of the released material or at some
later point in the environmental fate of the released material in which CNTs are
released from the polymer matrix as free fibers. In the first case, the fraction may
need to be determined on a case-by-case basis (e.g., for a type of CNT, composite,
and release scenario) and the difference between the released fibers and the added
fibers would need to be determined. In the second case, research is needed to iden-
tify fate and transport of the released particles, as well as points of eventual exposure
to free CNTs.
However, unfortunately, in lieu of such studies, the most relevant specific toxicity
information with which to consider hazard and risk for such released nanometer-
scale material comes from studies of free CNT fibers prior to their addition to any
composite (see Bonner 2014). It should be noted that application of toxicity evalu-
ation of these “prior to use” CNT fibers would be expected to greatly overestimate
“fiber-related” health risk for the release scenarios that have not yet shown release of
free CNT fibers. Furthermore, basing risk evaluation on assumed free CNTs would
provide poor discriminating utility across different use categories, because the pri-
mary exposure entity is not being evaluated.
Taking into account these caveats to use of toxicity information from the study
of free CNT fibers, the first challenge of drawing inference is again variation in the
possible entities to which exposure may occur. Many of the potentially risk-rele-
vant CNT characteristics, including thickness, length, and surface characteristics,
vary substantially from one CNT manufacturing process to the next. Furthermore,
research has shown that such characteristics do in fact affect toxicity (Aschberger
et al. 2010; Coccini et al. 2013; Donaldson et al. 2006, 2013; Liu et al. 2012).