Page 167 - Carbon Nanotube Fibres and Yarns

158 Carbon Nanotube Fibers and Yarns

The oscillating nature of the rubbing action for producing rub-densified
yarns raised a concern about the variability of its tensile properties. In all, 23
specimens taken randomly from a 5 m long rub-densified CNT yarn were
tested [29]. The Weibull scale parameter T 0 obtained was 54.74 cN/tex with
a shape parameter m of 9.72, higher than the values for the above twisted
CNT yarns with and without solvent densifying.

7.2.3 Influencing factors

There are a large number of nanotube, processing, and yarn constructional
parameters that can affect the strength of CNT yarns. Many researchers
compiled strength results from CNT yarns produced in different laborato-
ries around the world in order to understand the contributions of individual
parameters [4, 33, 46–49]. Unlike textile fibers and yarns, testing the me-
chanical properties of single CNTs is rather complicated and is thus rarely
carried out; relatively small samples are usually used in yarn tensile testing;
and testing conditions (e.g., gauge length) and yarn structural characteris-
tics (e.g., yarn porosity, diameter, twist) are often not standardized. All these
contribute to the difficulties of isolating the contribution of an individual
parameter. Here, we summarize some of the conclusions made by different
research groups, some of which may contradict one another.

7.2.3.1 Nanotube strength
In the textile industry, there are well-established relationships between fiber
strength and yarn strength. Fiber strength can be obtained by testing single
fibers or fiber bundles. Fiber bundles used in testing usually contain about
1000 fibers held by a clamp at one end and are combed at the other end to
align the fibers and to remove short fibers that are not held by the clamp.
The position of the other clamp, which gives the testing gauge length,
is usually from 3 to 9 mm for cotton which typically has a staple length
of more than 25 mm. This means that the vast majority of the fibers are
gripped at both ends during fiber bundle testing. The ratio between cotton
fiber bundle tenacity and average single fiber tenacity is usually in the range
from 0.426 to 0.52 [50]. The tenacity of a medium count cotton yarn (27
tex) is about 50% of the fiber bundle tenacity [51].
Experimentally determined tensile strength and Young’s modulus of sin-
gle CNTs vary in a wide range between 11 and 150 GPa and between 200
a
and 1000 GPa, respectively. These values (intrinsic strength) were calculated

a https://en.wikipedia.org/wiki/Mechanical_properties_of_carbon_nanotubes.