Referring to the comparison of
the structural features in various types of yarn , it can be seen that, based
on the fiber geometric properties and inherent characteristics of the
processing systems, staple yarns have substantial fiber density, short fiber segment lengths between points of
entanglement, and minimal mobility of the fiber segments. This is the basis
for the retention of the intrinsic fiber
contiguity and thus the excellent dimensional stability of staple yarn
structures under low levels of stress. All yarns, except stretch or elastomeric, tend to have excellent dimensional
stability when loaded in the direction of the yarn axis. However, staple yarns
also have good dimensional stability when loaded or deformed cross-sectionally
or normal to the yarn axis This means that spun yarns retain their natural bulkiness, good covering power, and excellent hand under various extensional, compressional, and bending deformations to a much greater extent than would be expected in textured filament yarns.
Again the untwisted or slightly
entangled filament yarns have very long
filament segments length between points of entanglement and great lateral
mobility of the filament segments.
This combination of structural features allows the untwisted multifilament yarn to spread and flatten out under normal bending or compressional deformations. The spreading of the filaments changes the yarn cross-sectional shape to a rather flat, ribbon-like structure or one that is quite elliptical. This collapse of the filament bundle permits a much greater area of contact with other surfaces, resulting in greater friction and discomfort ( in apparel applications). Also, the filaments are easily snagged away from the main body of the yarn structure. When bending, compressional, or snagging stress is removed, the filaments often do not fall back into their original alignment, resulting in a false or pseudo-entanglement. With a rather small amount of twist in the filament yarn, however, most of these problems are minimized. With sufficient twist in the filament yarn structure, the problems are completely overcome.
This combination of structural features allows the untwisted multifilament yarn to spread and flatten out under normal bending or compressional deformations. The spreading of the filaments changes the yarn cross-sectional shape to a rather flat, ribbon-like structure or one that is quite elliptical. This collapse of the filament bundle permits a much greater area of contact with other surfaces, resulting in greater friction and discomfort ( in apparel applications). Also, the filaments are easily snagged away from the main body of the yarn structure. When bending, compressional, or snagging stress is removed, the filaments often do not fall back into their original alignment, resulting in a false or pseudo-entanglement. With a rather small amount of twist in the filament yarn, however, most of these problems are minimized. With sufficient twist in the filament yarn structure, the problems are completely overcome.
In filament high bulk yarns,
filament segments between points of entanglements are in the form of loops on
the surface of the yarn. Whereas filament segments in the yarn core have
minimal mobility, the loopy segments on the surface can easily rotate and
collapse. This means that the outer profile or loopy surface zone of filament
high-bulk yarns can be deformed by a rather slight compressional load, creating
a greater area of contact with another surface than a spun yarn of the same
size.
In stretch yarns, the packing
density of the filaments is very low and the filament segments between points
of entanglement are very long. This combination results in tremendous mobility
of filament segments along the yarn axis and in any direction away from the
yarn axis. The tremendous mobility of the filaments means that the stretch yarn
structure is easily deformed and that the yarn has poor dimensional stability,
in general. The flattening out of the yarn structure occurs rather easily,
causing almost as great an area of contact with other surfaces as found with
untwisted, un-textured multifilament yarns. In the textured filament yarns the
individual crimped filaments can move
laterally, rotate, or can de-crimp independently from other filaments in
the structure. Snagging of the individual filaments by a rough surface or edge
is greatly facilitated because of the mobility and crimp in the filaments.
In conclusion, it seems that many
of the desirable physical properties and
performance characteristics of a yarn are to the dimensional stability of the yarn cross-section under various types of
deformation. Unfortunately, not enough date is available on yarn
cross-sectional behavior, in the development of filament textured yarns. One
parameter that can be used to characterize yarn cross-sectional behavior under
stress is ellipticity. Ellipticity is the
ratio of the maximum yarn diameter to the minimum yarn diameter, where unity
indicates a perfectly circular cross-sectional shape.
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