Page 31 - Carbon Nanotube Fibres and Yarns

24 Carbon Nanotube Fibers and Yarns

yarn collection is carried out by the slow rotating horizontal yarn bobbin.
Additionally, a yarn guide or the bobbin itself traverses longitudinally to
spread the yarn along the bobbin. The up-spinner is a much simpler ma-
chine design than the flyer spinner. The mass carried on the high-speed
spindle is immensely smaller than that of the spindle assembly in the flyer
spinning method. The yarn path on the up-spinner is essentially a straight
line, resulting in much lower yarn tension during spinning so that delicate
CNT webs can be spun into yarns at a high speed. For high-speed opera-
tion, the mass of the CNT forest and its holder must be counterbalanced by
a weight. A liquid applicator is fitted next to the yarn guide for application
of solvent, polymer, or other additives during spinning. The machine was
run at spindle speeds up to 18,000 rpm during trials.

2.3.1.3 False-twisting
Twisting is a reversible process, that is, the twist in a yarn can be removed by
inserting a twist of the same amount but in the opposite direction (i.e., un-
twisting). When the twist in a conventionally spun textile yarn is removed
by untwisting, the yarn returns to a loose fiber bundle with virtually zero
strength. However, when the twist in a twisted CNT yarn is removed by
untwisting, the resultant twistless yarn largely maintains its structural integ-
rity due to the van der Waals force between the nanotubes [39]. Fig. 2.9
shows SEM images of (A) a twisted yarn and (B) an essentially twistless yarn
obtained by untwisting the twisted yarn.

Fig. 2.9 Production of false-twisted CNT yarns: (A) Twisted yarn 5000 T/m. (B) Twist-
untwisted yarn 5000 T/m [39]. (C) Schematic of false-twist CNT yarn spinning. (Panels (A
and B) reprinted with permission from M. Miao, The role of twist in dry spun carbon nano-
tube yarns, Carbon 96 (2016) 819–826.)