Page 10 - Carbon Nanotube Fibres and Yarns

Introduction 3

Without considering their detailed atomic structures, CNTs are na-
noscale fibers that resemble nanofibrils in plant and animal fibers, such as
cotton and wool. The CNTs are very long relative to their diameters, with
aspect ratios one order of magnitude greater than common natural textile
fibers. It is therefore a logical approach to align the CNTs in the form of
a fiber or yarn that is expected to outperform conventional textile fibers.
This book deals with the various aspects of such fibers or yarns produced
from CNTs.

1.2 CNT yarns versus conventional textile yarns

The use of the terms “CNT fiber” and “CNT yarn” has now become
rather arbitrary. The term “CNT yarn” can be related to its manufactur-
ing from vertically aligned CNT arrays, which bears close similarity to
the production method used in traditional textile yarn spinning [2, 3]
(Chapter 2). In comparison, the direct-spinning method [4] (Chapter 3)
and the solution-spinning method [5, 6] (Chapter 4) more closely resem-
ble reaction spinning and wet spinning of synthetic fibers, respectively.
However, nowadays use of term “fiber” or “yarn” is more of a personal
choice than a reference to its method of manufacture. The two terms are
used interchangeably in this book.
CNT yarns, especially twist-spun CNT yarns, are often compared with
textile yarns in the analysis of their structure and tensile properties [3]. The
insertion of twist to a textile yarn places individual fibers in approximately
coaxial helix configuration and the fibers are pressed together because of
the inward pressure generated by the tension in the helically disposed fibers.
In a conventional textile yarn, interconnection between fibers relies on the
fiber-fiber friction that arises from the pressure between fibers, which in-
creases with the external tensile load applied to the yarn [7]. At low twist
levels, due to low fiber-fiber friction, the yarn failure mechanism is domi-
nated by fiber slippage. At high twist, fiber slippage is largely prevented by
high fiber–fiber friction and thus the yarn fails due to fiber breakage. On
the other hand, high twist reduces the contribution of fiber strength to the
yarn strength due to fiber obliquity in the yarn. Therefore the maximum
yarn specific strength is usually achieved at an intermediate level of twist, as
illustrated in Fig. 1.2A.
A twisted CNT yarn has a similar twist-strength relationship as the con-
ventional textile yarns [12]. There is, however, a major difference in the
mechanism of fiber-fiber (CNT-CNT) interaction. Because the nanotubes