Page 13 - Carbon Nanotube Fibres and Yarns

Introduction 5

have a nanoscale dimension, the van der Waals attraction (London disper-
sion force) between them plays an important role in the transfer of load
between the nanotubes in CNT yarns.
Unlike textile yarns, strong CNT yarns can be produced without the
insertion of twist. For example, twistless CNT yarns can be produced by
liquid shrinking without twist insertion [13, 14]. CNT yarns densified by
polar solvent ethylene glycol [13] demonstrated a strength of 1.45 GPa,
which is stronger than most twisted CNT dry spun yarns reported in the
literature [15]. Mechanical rubbing [10] can also produce a strong CNT
yarn without the insertion of twist.
Van der Waals forces are inversely proportional to the second power of
the distance between the surfaces of the particles so they become dominant
for collections of very small nanotubes where there are no capillary forces
present. Thus increasing the packing density of CNT is a very important
strategy for improving the strength of CNT yarns and fibers. Nanotubes
need to be very well aligned in order to pack them really close together.
Twisting is a reversible process, that is, the twist in a yarn can be removed
by inserting a twist of the same amount but in the opposite direction. When
the twist in a staple fiber textile yarn is removed by untwisting, the yarn
returns to a loose fiber strand with almost zero strength. On the other hand,
when the twist of a twist-spun CNT yarn is removed by untwisting, the
resulting yarn largely maintains its structural integrity and a major part of
its original strength [7].

1.3 Prospects of high-strength CNT yarns

The strength and modulus of monolayer graphenes (CNT shells) are gen-
erally considered to be about 130 GPa and 1.0 TPa, respectively [16], which
can be taken as the theoretical values for CNTs. These extraordinary values
drove the enthusiasm for extreme applications in the earlier days of CNT
research, such as space elevator cables [17]. Indeed, bundles of carefully
manufactured ultralong defect-free CNTs have been recently reported to
possess a tensile strength over 80 GPa [18].
In reality, all mass-produced materials always have defects and thus do
not achieve their theoretical strength. Although a CNT fiber with specific
strength as high as 9 N/tex at 1-mm gauge length was reported [19], most
commonly reported specific strength for CNT fibers is around 1 N/tex,
which is already above the specific strength of common synthetic fibers,
such as nylon and polyester. To put this in perspective, we plotted the specific