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Post-spinning treatments to carbon nanotube fibers 129

composite ribbons show a slightly lower knot-strength efficiency of 78%.
However, their knot performance is still comparable to that of silk, wool,
nylon, and cotton, and much better than that of other high-strength com-
mercial fibers [9].

6.9 Conclusions and recommendations

We first focused on the dissolution of CNT fibers spun by the floating cat-
alyst method in chlorosulfonic acid (CSA). The as-spun CNT fibers were
not fully dissolved in the CSA because the impurities within the fibers
hindered the protonation effects of the superacid. After the removal of these
impurities by purification, the CNT fibers were dissolved successfully in
CSA, leading to the observation of isotropic, biphasic, and liquid crystal
phases of the CNT solutions [40, 44]. The length of the CNTs constituting
the fibers was characterized by extensional viscosity method [40, 47]. The
CNT length was determined to be 2 μm in average, which was still longer
than many commercial SWNTs and DWNTs, such as SweNT CG300,
UniDym OE, HiPco 188.3, and HiPco 183.6 (0.71–1.92 μm).
A second focus of the chapter is posttreatment methods based on mechani-
cal densification, which could significantly improve the mechanical and electri-
cal properties of the CNT fibers. The outstanding feature of this method is that
very high densifying force of 100 N could be applied directly to the CNT fibers
while minimizing the damage of the fiber structure. The method improved the
CNT alignment of the fibers and reduced spaces and pores within the fiber
structure. After the densification, the CNT fibers exhibited increases in strength
and Young’s modulus by 10 and 18 times, respectively. These are comparable to
the improved performance of the CNT fibers densified by pressurized rolling
process [26] and much better than twisting [22, 23], liquid densification [23],
drawing through dies [24], and rubbing [25]. The technique can also be applied
to aligned CNT fibers and CNT thin films fabricated from the wet-spinning
and array-spinning methods with many potential applications such as compos-
ite reinforcements and electrically conducting wires [31, 40, 69, 70].
Effects of different posttreatment methods, especially the hybrid post-
treatments, such as purification combined with epoxy infiltration and densi-
fication combined with epoxy infiltration, on the electrical and mechanical
performances of the CNT fibers spun from the floating catalyst method
were reviewed comprehensively. In particular, oxidative purification re-
moves catalyst impurity of the CNT fibers spun from toluene source by up
to 65%. The purified CNT fibers exhibited strength, Young’s modulus, and