Page 37 - Carbon Nanotube Fibres and Yarns

30 Carbon Nanotube Fibers and Yarns

CNTs are hydrophobic but can be wetted by a wide range of solvents.
The surface tension pulls the CNT bundles together as the yarn leaves
the meniscus of liquid (Fig. 2.13B and C). The volatile solvent evaporates,
leaving a dry densified CNT yarn. Solvents with a low boiling point like
ethanol and acetone are often used for CNT densification.
The capillary force is influenced by the solvent’s volatility (boiling
point), surface tension, and interaction with CNT surfaces [47]. The most
important parameter of the solvent for efficient infiltration was found
to be polarity. Despite of their high boiling points and low evaporation
rates, DMF (dimethylformamide), DMSO (dimethyl sulfoxide), and NMP
(N-methyl-2-pyrrolidone) have better infiltration than ethanol. Ethylene
glycol offered the best infiltration due to its two OH groups.

2.4 Production scale-up

CNT webs can be drawn from CNT forests at rates as high as 10–16 m/s
(600–960 m/min) [48, 49], which enters the low end of the range for syn-
thetic fiber extrusion (up to 10,000 m/min). For a typical CNT yarn with
a 30-μm diameter and 25 degree twist angle, 5000 turns of twist is required
per meter of yarn. To produce yarns at 960 m/min, the twist insertion rate
required would be 5,000,000 turns/min, which is hundreds of times faster
than any twist-insertion system in the textile industry. However, other
methods of densification, including solvent densification, mechanical rub-
bing, and die-drawing, may match this production rate.
A major challenge for commercial scale CNT yarn spinning from CNT
forests is the small forest size which is limited by the dimension of the
furnace. Approximately each millimeter of CNT forest length provides a
meter-long web [7, 9]. CNT forests are typically grown on stiff substrates
that have to be fitted into the furnace. This sets a limit to the size of the
forests that can be produced and in turn the length of a continuous yarn
produced without replenishing the feedstock forest. Frequent replenishing
of feedstock forest and yarn package can seriously decrease production ef-
ficiency, and post-spinning yarn joints can become a source of yarn quality
issues, such as weak spots and structural irregularity. Several methods have
been proposed to scale-up CNT yarn production.

2.4.1 Automated spinning

Because of its engineering simplicity, the CSIRO up-spinner can be run at
high spindle speeds up to 18,000 rpm and operations required to start a new