ENGINEERING APPROACH TO TEXTILE STRUCTURES
Man-made fibers must be drawn to
orient their constituent molecules into an orderly arrangement parallel to the
fiber axis, and this leads to higher tenacity. The greater the draw ratio, the
greater the degree of molecular orientation (i.e. crystallization), and higher the fiber tenacity. Most man-made
fibers must be drawn to some degree to give them adequate tenacity to fit them
for commercial applications, but that having been said, there is still room for
producing a range of fibers of different tenacities from the same material to
suit different end-use. For example, engineering uses require fibers of higher tenacity
than normally needed for apparel fabrics or carpets.
If one uses the analogy of yarn
in which fiber are parallel to one another (Staple fiber yarns) and those in
which fibers are very much crumpled and distorted (textured yarns) , it is not
difficult to see why drawn, and therefore strong fibers with good molecular
orientation are much less extensible than un drawn fibers with little molecular
orientation. In fact, as tenacity increases due to drawing, extensibility
decreases. This means that Young’s
modulus for the fiber increases and the fibers becomes stiffer; this will have
an effect on the aesthetic properties of fabrics made from the fibers, such as
handle, softness, drape and also on weaving property.
The length of the molecules also
affects the fiber tenacity, and the means of modifying tensile properties vary
a great deal from one fiber material to another. In wet spinning, the
constitution of the coagulating bath may be modified to slow down the rate of
polymer formation – conditions of formation affect molecular orientation; in
melt spinning some drawing may take place in the spinning bath; in other cases
it takes place after wards; in other cases again it takes place in hot liquid
baths.. In the case of melt spinning drawing may be done cold, as in the case
of nylon or hot spinning hot spinning drawing may be done cold, as in the case
of polyesters. It may take place in one stage, but in some cases it is done in
two stages, and these factors all affect the manner and degree of molecular orientation.
Polypropylene crystallizes so
rapidly that un drawn filaments are highly crystalline.. In this, it is
different from other fibers that are melt spun, and the production of this
fiber is very sensitive to spinning conditions; the ability to control these
conditions can be used to produce fibers of engineering and textile-end uses
with a wide range of crystallinity and physical properties.
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