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In the process of yarn formation,
fibers are subjected to the twisting operation. The resistance to
twisting of a fiber, its torsional rigidity, is dependent on its shape,
density, shear modulus and fineness (diameter). That is for given fiber, the
amount of twist required to produce comparable internal stresses, which in turn
control the cohesion characteristics and yarn strength, with all other things
being equal, will be greater in a yarn composed of coarser fibers than in one
composed of fine fibers. This factor is of profound importance when considered
in terms of producing highly twisted yarns and sewing threads (snarls, kinks
can have detrimental effect on the quality of thread).
Further more, the cohesiveness of
fibers in a spun yarn depends on the inter fiber friction (influenced by
twist), which in turn is a function of the total of fiber contact (fiber
specific surface) and the fiber coefficient of friction. Thus fibers with
larger specific surface (finer fibers), all other things being equal, will
require less twist compared to those having smaller specific surface (coarse fibers)
to prevent slippage in a yarn.
In addition to fiber length, fiber
fineness also plays an important role in determining the processing efficiency
and spin ability – the optimum yarn size to which the raw material can be spun
– of a staple fiber stock. Moreover, the uniformity of a yarn is largely
dependent on the average number of fibers in its cross-section. Therefore, for
a given yarn linear density, the finer the fibers (meaning larger number of fibers in a cross-section),
the more regular the yarn. It is to be noted that the uniformity
characteristics has a considerable influence on such yarn properties as
strength, extensibility, appearance, feel, and end breakages in spinning,
winding, warping and weaving.
Fabric luster is yet another
property that has affected the fineness of fibers, since this determines the
number of individual reflecting surfaces per unit area of the fabric. In other
words, the fine fibers will produce soft sheen whereas coarse fibers
will generate hard glitter.
The rate of dye-uptake by
a fiber is also dependent on the total available surface for a given volume of
the fiber materials , that is specific surface of the fiber i.e. finer fibers
will require less time to exhaust the dye bath than coarse ones which will
affect the overall appearance of dyed fabrics.
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