Page 28 - Carbon Nanotube Fibres and Yarns

Yarn production from carbon nanotube forests 21

yarn forming zone. As a continuous flat ribbon of CNT web is pulled from
the forest, it gradually converges into a triangle shape (starting from Position
A in Fig. 2.5A). The ribbon starts to curve out of plane at approximately
the middle line (position B) due to the torque transmitted by the already
formed yarn (position D) from the twisting element. As the ribbon moves
forward further, the middle part of the ribbon rolls further into a nuclei
(Position C), on which neighboring nanotubes from both sides gather and
wrap following the direction of the twisting action. Finally, the rest of the
nanotubes are wrapped onto the growing nuclei to form a round yarn at
the apex of the triangle zone (Position D). A SEM image of the final twisted
yarn is shown in Fig. 2.5C.
The spinning method initially used by Zhang et al. [9] was a
motor-assisted hand-spinning process. Twist was applied by the rotation of
a motor while it is being moved away from the nanotube forest by slid-
ing it along a table top. The achieved yarn length was limited to about 1
m, or the arm length of the person holding the motor. Other methods
for spinning similar yarn lengths have been reported. Zhu’s group built a
CNT yarn spinning machine from what appeared to be a modified lathe
[34]. In this case, the yarn length is limited to the travel distance of the tool
rest. University of Cincinnati [14] and Chinese Academy of Sciences [35]
built continuous CNT yarn spinning machines with yarn collection bobbin
perpendicular to the spindle (Fig. 2.6). The twisting and winding assembly
consisted of a small motor, a number of pulleys, and a yarn winding bobbin.
The bobbin shaft was carried on the spindle that inserts twist to the yarn.
The yarn throughput and twist insertion speeds were regulated by two
separate motors. The unbalanced design of the twisting-winding assembly
results in a side to side movement of the forming CNT yarn, which de-
stabilizes the spinning condition and is a source of yarn irregularity [14].
Because of the cumbersome design, the machines could run only at rela-
tively low spindle speeds.

2.3.1.1 Flyer spinning
Research efforts at the Commonwealth Scientific and Industrial Research
Organization (CSIRO) resulted in two high-speed CNT yarn spinning ma-
chine designs. The first automated continuous CNT yarn spinning machine
was built based on the conventional flyer spinning principle (Fig. 2.7). The
twisting and winding operations are realized by two coaxial shafts that rotate
at differential speeds, causing the yarn to be wound onto the yarn collection
bobbin carried on the spindle. The spindle also performs a linear motion to