Case Study The Lifecycle of Conductive Plastics Based on Carbon Nanotubes - Safety of Nanomaterials along Their Lifecycle - page 399

Recycling

Excluded until
compounding

Starting with
pelletizing

CNT
production

Masterbatch Manufacturing Use End of life

Injection molding
Extrusion
Cutting
Drilling

Outside of scope of
nanorelease

Incineration
landfill

Skin contact

Professional user Consumer Environment

FIGURE 18.1 Lifecycle map for carbon nanotube composites used in electronics. The thick-
ness of the lines corresponds to the likelihood and magnitude of release based on the general
release scenario with all possible lifecycle steps and stressors. (Adapted from Environ. Int .,
Nowack, B. et al., Potential release scenarios for carbon nanotubes used in composites, 1-11,
Copyright 2013, with permission from Elsevier.)

For conductive polymer applications such as consumer electronics and automotive
fuel system components, releases to consumers are essentially zero simply because
of lack of access to the materials in normal use and lack of off-gassing for the mate-
rials (e.g., in comparison to volatile organic chemical constituents of the materials).
However, professional users (automobile mechanics, electronics repair technicians)
will have a higher contact with the materials and possibly under conditions of abra-
sion, and thus exposure to released materials is possible. The nature of the released
materials is dependent on physical-chemical factors of the CNT, the matrix mate-
rial, as well as the actual mechanisms involved in the release process.
For example, Figures 18.2 and 18.3 illustrate outcomes for mechanical stress pro-
cesses such as abrasion affecting the attachment of CNTs to the polymer. Initially,
the following steps may occur: the CNT is fixed in the matrix (a) and shear forces (b)
may loosen at one end from the matrix, rupture the fiber into two attached strands (c),
“pull-out” inner sheets of the CNT (d), or elongate connecting across cracks in the
matrix (e). A fifth option of course is the release of free CNT fibers, resulting from
simultaneous release of the CNT from all matrix surfaces. This release of free CNTs
seems less likely in cases in which the matrix is not completely degraded due to the
need for simultaneous disengagement along the entire length of the fiber. In fact,
observation of released fragments independently across laboratories rarely shows