Page 169 - Carbon Nanotube Fibres and Yarns

160 Carbon Nanotube Fibers and Yarns

in control experiments is in general agreement with the fiber length-yarn
strength relationship of conventional textile yarns [55].
However, when CNTs synthesized in different laboratories are com-
pared, yarns spun from long CNTs made in one laboratory does not neces-
sarily show higher strength than yarns from shorter CNTs made in another
laboratory. For example, optimized yarns spun from 5 mm long CNTs in
one laboratory exhibited a strength of 280 MPa [56], which was lower than
the strength of CNT yarns produced from much shorter CNTs in other
works [14, 41, 53, 54].

7.2.3.3 Yarn diameter
It is well understood in the textile industry that yarn production cost goes
up dramatically as yarn count becomes finer. This is primarily because finer
count yarns require more twists per meter and suffer higher end-down rate
during processing due to their higher mass irregularity and lower breaking
force. The effect of linear density (tex) on strength of textile yarns is gener-
ally considered to be negligible provided the yarns are of good evenness. A
small negative effect on strength observed for a finer yarn can be attributed
to the deterioration of yarn evenness, which leads to more weak spots in
the yarn.
Several CNT yarn researchers reported significant benefits on yarn
strength by spinning finer yarns. Zhang et al. [13] produced yarns of dif-
ferent diameters using 650 μm long CNTs and found that the finer yarn
with a diameter of 4 μm (0.85 GPa) was five times as strong as the thicker
yarn with a 13 μm diameter (0.17 GPa). However, no details were given on
other aspects of the two yarns, for example, whether the two yarns were
spun to a similar twist angle. Ghemes et al. [54] reported decreases of both
strength and modulus of twisted CNT yarns with increasing yarn diame-
ter. However, in their experiment, the twist level (turns/m) was kept at a
constant of 4000 turns/mm irrespective of the yarn diameter, which ranged
from 10 to 60 μm. Fang et al. [41] carried out a controlled experiment on
the influence of yarn diameter. Two sets of yarns were produced with dif-
ferent yarn diameters by varying the web width while twist angle was kept
to an approximately constant level (~15 degrees). The strength was 430 MPa
for a 5 μm diameter yarn but only 160 MPa for a 60 μm diameter yarn. Liu
et al. [9] also reported a general improvement of strength for finer yarns, but
reducing yarn diameter below 10 μm displayed a negative effect. Deng et al.
[43] reported a statistical relationship showing a decrease of yarn strength
(σ) with an increasing yarn diameter (d), σ = 22,617.5d −1.42 .