Page 170 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 161

On the other hand, Zhao et al. [57] reported a general increasing trend
of yarn strength as the yarn diameter increased, which was attributed to a
lower radial pressure inside finer yarns.

7.2.3.4 Twist
In textile spun yarns, fibers are held together by the fiber-fiber friction
derived from the inward pressure generated by the coaxial fiber helices in
the twisted yarn. At a low twist level, due to low pressure and low friction
between the fibers, the yarn fails by fiber slippage. At a high twist, fiber slip-
page is largely prevented by high interfiber friction and the strain experi-
enced by fibers differs greatly according to their radial positions in the yarn.
High twist yarns fail mainly due to fiber breakage, first occurring at the
outer ring which has the highest strain, and then propagating to the entire
yarn [8]. In addition, high twist reduces the contribution of fiber strength to
yarn strength due to fiber obliquity. Therefore, the maximum yarn strength
is achieved at an intermediate twist level.
Most researchers agree that CNT yarns follow a similar twist-strength
(and modulus) relationship as textile spun yarns [14, 41, 46, 54] (Fig. 7.18A
and B). Liu et al. [9] produced yarns by twisting and acetone densifying of
webs drawn from CNT forests and found a decrease of CNT yarn strength
and modulus with an increasing twist angle. As a CNT yarn without being
twisted or densified in other ways has a very low strength, the complete
relationship between twist and yarn strength/modulus should assume a par-
abolic shape. Liu et al. also demonstrated that the breaking strain of the yarn
increased as a higher twist was applied, which is in general agreement with
textile spun yarns. The explanation used in textile spun yarns, therefore, can
be adapted to explain the relationship of CNT yarns by replacing the fric-
tional force between fibers with van der Waals force between nanotubes [5].
Insertion of higher twist to CNT yarns provides greater densification effect
and thus stronger inter-tube van der Waals forces.
However, Zhao et al. [57] reported a double-peak relationship between
twist and yarn strength. The second peak was attributed to CNT collapse
due to high internal pressure in the yarn caused by very high twist. The
collapse of CNTs would increase the contact between CNTs, which would
allow the yarn to densify further.
When twist in a twist-spun yarn is removed by applying opposite twist,
the untwisted yarn retains most of its original strength [6]. The twist-untwist
action is also known as false twist. The resulting false-twisted yarn had a larger
diameter (Fig. 7.18C) but followed a similar twist-strength relationship as the