Page 33 - Carbon Nanotube Fibres and Yarns

26 Carbon Nanotube Fibers and Yarns

The CNTs forming the sheath are drawn from the CNT forest in the form
of a continuous web. The core material is pulled out from a supply bobbin
to merge with the CNT web at the center. The twisting action of the spin-
dle at the right-hand side causes the metal filament and the CNT web to
rotate together, resulting in wrapping of the CNT web around the metal
filament to form a core/sheath structured yarn (Fig. 2.10B). Because of the
very large width of the CNT web in relation to the diameter of the metal
filament, the core is completely covered by the CNT sheath in the resulting
core-spun yarn.

2.3.2 Rubbing densification

Mechanical rubbing action can be used to produce a highly densified CNT
yarn (Fig. 2.11A) [42]. The main working parts of the machine are a pair
of padded rollers that participate in both rotation and axial oscillation, as
shown in Fig. 2.11B. The rotational motion of the two rollers transports the
CNT web drawn from the CNT forest to the yarn collection bobbin. The
axial oscillations of the two rollers work in opposite directions to apply a
rubbing action that densifies the CNT web into a yarn.
During rubbing, the CNT yarn is deformed under the pressure be-
tween two elastomeric surfaces moving in opposite directions, as shown
in Fig. 2.11C. The action of the moving surfaces is to rotate the yarn
about its axis, causing the fibers to move around in a “race-track” fashion.
CNTs in the race-track are unable to retain their positions relative to
others in the yarn cross section, and the jockeying for position results in
relative movement of CNTs in the yarn. On the other hand, the strong
van der Waals attraction restricts the free relative movements between the
individual nanotubes. The jockeying for positions caused by the rubbing
action leads to the filling of voids between CNT bundles, forming a closely
packed yarn structure.
As illustrated in Fig. 2.11C, the outer layer on two sides of the yarn are di-
rectly driven by the two rubbing surfaces and thus are forced to move in oppo-
site directions, so large-scale shears must take place in the intervening region,
which causes the CNTs in the yarn core to be torn apart repeatedly as the pro-
cess continues. When the yarn moves past the roller nip, the lateral compression
applied to the yarn is released. The flattened yarn cross section opens up due
to elastic recovery, leading to the formation of voids in the yarn core. The re-
sulting yarn structure has thus a high-density sheath and a low-density core, as
shown in the cross-sectional image in Fig. 2.11D. A high roller pressure causes
severe yarn flattening at the roller nip, leading to a ribbon-like yarn structure
without a distinctive porous core, as shown in Fig. 2.11E.