Page 172 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 163

There were several attempts to compare the properties of the yarns pro-
duced on the two systems. In general, the flyer-spun yarns tended to have a
tighter structure than the up-spun yarns, which was attributed to the higher
spinning tension in flyer-spinning. The elastic modulus of the flyer-spun
yarns tended to be higher but the strain to failure tended to be lower.
However, when the strength and modulus were converted into specific ten-
sile strength (tenacity) and work-to-rupture (toughness), the differences be-
tween the yarns spun on the two systems started to disappear.
Tran et al. [58] introduced a series of friction pins between the CNT
forest and the spindle on the flyer spinning machine. These pins affected
the spinning process in two ways, increasing the yarn tension and causing
the twist to be inserted to the yarn in steps along the zones separated by
the pins. The twist redistribution caused by the friction pins is known as
twist blockage [59]. The higher tension increases the yarn density, leading
to a more compact yarn (reduced yarn diameter) and higher stress-based
strength and modulus as well as lower breaking strain. Tran et al. [58] also
reported the benefits of using multiple narrow web strips, following the
Siro-spun yarn method used for wool fiber spinning, as well as heat treat-
ment of the CNT web during spinning. Siro-spun wool yarn spinning is an
alternative to twofold wool yarns, which can be woven into fabrics without
sizing because of lower yarn hairiness (protruding fiber ends) and higher
resistance to fiber attrition [60].

7.2.4 Densification methods
A number of methods have been used to densify CNT yarns, resulting in dif-
ferent yarn structures. The common parameter that can be used to quantify
the level of densification achieved by these methods is yarn density, which
directly affects voids and van der Waals force between nanotubes, and in turn
affects the yarn strength. Fig. 7.19 plots the yarn density-tenacity relation-
ships according to densification methods. Twist densification can provide a
3
wide range of yarn density from 0.1 to 1.3 g/cm by changing the level of
twist. However, the high yarn density obtained at very high twist is achieved
at the expense of large nanotube obliquity, which has a negative effect on
the yarn strength, as shown in Fig. 7.19A. By removing the twist from a
twisted yarn (or by forming a twistless yarn directly using the false-twist
method), the nanotube obliquity is eliminated, but this leads to loosening of
the initially tight yarn structure, especially at high twist levels, and limits the
highest density achievable, which is in turn reflected on the yarn tenacity
(Fig. 7.19B).