Page 134 - Carbon Nanotube Fibres and Yarns

126 Carbon Nanotube Fibers and Yarns

main reason for their increased cross-sectional area [31]. Although the breaking load
of the CNT ribbons increased by more than 100% after the infiltration treatment,
their strength only showed a slight net increase of more than 20% to 3.6 ± 0.16 GPa.
The enhancement in Young’s modulus by epoxy-coated CNT rib-
bons was remarkable although the improvement in the strength of the
epoxy-coated CNT ribbons was much lower than that of the CNT fibers
treated by combined treatment of liquid densification and polymer infil-
tration [22, 27]. Specifically, their Young’s modulus increased by more than
three times with an average value of 266 ± 14.47 GPa after the infiltration
treatment. These results may be attributed to significant enhancement in
the interfacial load transfer between the CNTs and between the CNT
bundles. The epoxy infiltrated into the CNT structures, cross-linked the
CNTs and CNT bundles with strong bonds, and minimized the inter-tube
slippage [64]. Consequently, the inter-tube interactions of the cross-linked
CNT ribbons were significantly enhanced, resulting in the large increase in
their Young’s modulus. Moreover, the reductions in both failure elongation
and plastic region of the coated CNT ribbons can be attributed to the
hindering effects of the infiltrated epoxy on the inter-tube slippage [64].

6.8.3 Advantages of hybrid posttreatments

Mechanical densification has been used in many studies to densify different
CNT assemblies such as CNT arrays with biaxial densification [65] and
rolling [66], and CNT fibers with pressurized system [26]. In the densifica-
tion technique described above, compressive stress is applied perpendicular
the fiber axis to densify the CNT fibers into highly densified structures.
The resulting shear stress during the treatment causes CNTs to slide slightly
along the fiber axis, improving the CNT alignments. Moreover, the use of
two protective layers can prevent damage caused by the shear and compres-
sive effects during the treatment while maximizing the packing effects [28].
The thickness of the densified samples is determined by the gap between
the two protective sheets under the compressive forces during the densifica-
tion treatment. There was no damage observed in the CNT fiber structures
although excessive densification forces of 100 N were applied to the samples
many times. This technique can be used to densify CNT fibers of continu-
ous length using a mechanism similar to the pressurized rolling system [26].
The hybrid posttreatment described in this chapter achieved excellent
improvement in the mechanical performance of the as-spun MWNT fibers.
Fig. 6.16A compares the improvement factors in the mechanical performance
of CNT fibers using different posttreatments, including liquid densification