Page 290 - Handbook of Properties of Textile and Technical Fibres

264 Handbook of Properties of Textile and Technical Fibres

and optimum fiber compactness. This means that twist level should be determined in
view of two critical characteristics: strength and comfort. The true practical optimum
twist level is determined by the intersection of the strengthetwist curve and the
comfortetwist curve. We should point out that in both curves the parameter is
expressed as an index. In the case of comfortetwist curve, comfort is expressed by
an index from zero to one, with zero indicating high discomfort and one indicating
the highest possible comfort level. In the case of the strengthetwist curve, the strength
index implies the ratio between the actual strength produced at a certain twist level and
the maximum strength that can be obtained from the spinning system.
The effects of other fiber properties on the strengthecomfortetwist relationship are
illustrated in Fig. 7.17. It is well known that the yarn strengthetwist relationship is
influenced by many fiber properties, including fiber length, fiber strength, interfiber
friction, and fiber fineness. For a given yarn count, the maximum strength can be
obtained at a lower optimum twist when fibers of high length, high strength, high
friction, and low coarseness are used. This is due to the fact that longer, stronger,
and finer fibers will provide higher yarn strength by virtue of their effective contribu-
tion to yarn structure. In addition, long and fine fibers are flexible under torsion. This
means that they provide minimum resistance to the mechanical twist applied during
spinning. With regard to the comfort-twist relationship, many fiber characteristics
contribute to the make of such relationship. In general, longer and finer fibers are
expected to produce yarn and fabric of better handle due to high fiber flexibility,
particularly under bending. In addition, fibers of low flexural and torsion rigidity
will also result in better fabric handle. On the thermophysiological side, appropriate
levels of fiber length and fineness should be used to provide optimum fiber compact-
ness. Other properties related to this aspect of comfort include moisture content and
fiber density.
The second most commonly used spinning system is open-end spinning. In general,
it is well known that fiber strength is one of the primary fiber characteristics in open-
end spun yarns (Elmogahzy and Chewning, 2001). This fact has nothing to do with the
spinning principle of open-end spinning, as it imposes minimum spinning tension on
fibers during spinning, and there are no battlefields in open-end spinning in which the
fibers must withstand high spinning tension. The need for higher fiber strength in open-
end spun yarn is a direct result of the fact that open-end spun yarns are weaker than
ring-spun yarns, made from the same fibers, by about 10%e20%. It is critical,
therefore, to use fibers of higher tensile strength and higher breaking elongation.

7.17 The fiber-to-yarn relationships in the context of the
tensile behavior of cotton fibers

The ultimate benefit of understanding the tensile behavior of cotton fibers is reflected
in how the key tensile parameters such as fiber strength, breaking elongation, work
of rupture (toughness), and flexibility (elastic modulus) are transmitted to a fiber
assembly (yarn, fabric, and end product). In practice, the quality of a staple-fiber
yarn may be defined in many ways. In the spinning mill, yarn quality is defined as