Page 186 - Carbon Nanotube Fibres and Yarns

176 Carbon Nanotube Fibers and Yarns

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density of the idea material (e.g., carbon crystal density = 2.266 g/cm ) while
the values for commercial fibers are calculated using commercial fiber density
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(e.g., carbon fiber density = 1.8 g/cm ). The two columns on the right side of
the table give utilization ratios, which are the values of commercial fibers as a
percentage of the theoretical values in each row.
Table 7.4 divides the fibers into two sets. The first set are commodity
textile filaments for industrial applications, which are typically stronger than
their short fiber (staple fiber) counterparts, except for cotton, which is a staple
fiber consisting of over 90% cellulose. The cotton fiber strength and modulus
in the table are based on St Vincent cotton, the strongest type of commercial
cottons [109]. Table 7.4 shows that only 2.6%–4.3% of the theoretical strength
and 3.6%–15.8% of the theoretical modulus are utilized in these fibers.
The second set of fibers, high-performance fibers, including Kevlar,
Spectra, Dyneema, and carbon fibers, are extremely strong for high-value
end uses, such as personnel ballistic protection and challenging engineering
structures. They have utilized between 7.6% and 12.2% of their theoretical
strength and between 36.5% and 73.6% of their theoretical modulus, much
higher than the first set (commodity textile fibers).
The reason why textile fibers achieve only a small fraction of their the-
oretical strength and modulus can be ascribed to fiber structural defects,
such as practical limitations of molecular chain length and misalignment,
and manufacturing defects. All these can be attributed to deficiencies of ex-
isting manufacturing technology and insufficient level of care taken during
production. The high-performance fibers are typically manufactured from
ultrahigh molecular weight polymers using sophisticated processing meth-
ods, such as gel spinning and super-drawing, to achieve very high level of
molecular chain orientation. Commodity textile fibers are produced from
lower molecular weight polymers using less sophisticated processes than the
high-performance fibers, and they are likely to contain more defects than
high-performance fibers by using less sophisticated processing methods to
reduce production costs. The utilization of intrinsic polymer properties for
commodity textile fibers is, therefore, considerably lower than that for the
high-performance fibers.
For the purpose of comparison, we will use the strength and modulus
of monolayer graphene as the theoretical values for CNTs, which are gen-
erally agreed to be about 130 GPa and 1.0 TPa, respectively [110]. Based on
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the density of graphite (carbon crystal) 2.266 g/cm , the theoretical specific
strength (tenacity) and specific modulus for CNTs are 57.4 and 441 N/tex,
respectively.