Case Study The Lifecycle of Conductive Plastics Based on Carbon Nanotubes - Safety of Nanomaterials along Their Lifecycle - page 408

Accordingly, risk evaluation should consider such characteristics in evaluating the
potential for adverse health effects.
For example, a study by Poland et al. (2008) compared the toxic responses of
thin and shorter CNTs to thicker and longer CNTs. The thin and short fibers were
described as forming “tightly packed spherical agglomerates” and “tangled agglom-
erates,” and the thicker fibers were described as forming “dispersed bundles and
singlets” or “regular bundles and ropes.” The toxic effects noted by Poland et al.
were caused by the thick and longer CNTs. Toxicity has also been observed for
these thicker and longer CNTs in other studies (Liu et al. 2008; Sakamoto et al.
2009; Sargent et al. 2014; Takagi et al. 2008). The CNTs that did not show toxic
effects in the study by Poland et al. (2008) were those that were described as tangled
agglomerates.
Thinner and shorter CNTs also did not show cancer effects in a 2-year bioassay
(Muller et al. 2009). However, studies of some thinner CNTs have shown potential
for other effects (Liu et al. 2012; Ma Hock et al. 2009; Mitchell et al. 2007). Adverse
effects have also been shown for high-dose exposures (high relative to dust levels
in occupational settings) to the thin and flexible CNT agglomerates (Ma Hock et al.
2009; Pauluhn 2009). The dose-response range for these effects relative to possible
exposure levels has not been fully evaluated. Therefore, more information is needed
before broad generalizations about the biological interactions of CNTs are under-
stood well enough to make risk management decisions without individual studies of
toxicity. At this time, a generalized model for risk management of CNTs cannot be
generated. In lieu of such a generalized model, it is essential to generate information
specific to each CNT, if there is demonstration or expectation of fiber release to an
exposure pathway.
18.5 CONCLUSIONS AND RECOMMENDATIONS
Specific combinations of factors such as aging, polymer additives, CNT sur-
face modification, media contact, UV exposure, and abrasion will dictate where
matrix degradation is more likely to occur for composites containing CNTs
and the nature of the released material. Therefore, an understanding of these
release-related characteristics through the lifecycle (product manufacture, use,
and disposal/reuse) is necessary to evaluate the likelihood and nature of materi-
als entering into exposure pathways. Through this kind of structured review of
data regarding environmental conditions and the underlying material properties
of CNT-polymer composites, some generalizations and therefore some exposure
likelihoods can be bounded through evaluation of release likelihood. Such evalu-
ations indicate that release of free CNTs is expected to be rare and below likely
toxicity levels for many of the specific release and exposure scenarios occurring
during the lifecycle, and a consequent low likelihood of human health risk is
expected based on the added CNT for those exposure scenarios (Kingston et al.
2014; Nowack et al. 2013).
For other release scenarios, studies of the actual released material are needed
before assessment of risk can be undertaken. Hazard inference for “pure” CNTs
varies greatly, from essentially no evidence of toxicity for some CNT forms