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.
THE DESIGN OF OTHER FIBER
ATTRIBUTES:
Many of the physical properties
can be modified to confer desirable properties on man-made fibers. For example,
other chemical may be added to modify their composition and properties; these
can open-up the structure and make the fibers more accessible to water and
dyestuff. To modify the aesthetics of fabrics made from man-made fibers,
manufactures have also altered linear density, shrinkage, crimp level and
character, surface charge characteristics, cross-sectional shape and many
fibers can be produced with built-in-color. Variable fiber shrinkage gives
bulking properties, crimped fibers and springy and bulky, and straight fibers
are soft and smooth to the hand. Surface friction is also important; fine
fibers with smooth surfaces can provide a soft, luxurious handle. The shape of
the fiber cross-section can be modified by spinning from spinnerets with
non-circular orifices. The degree of departure from the circularity and
differences in shape offer a large variety of possibilities. Flat fibers have a
high luster and tend to glitter; they are said to have harsh handle. Fibers
with lobed cross-sections have high luster; this type of cross-section affects
bending of the fibers and is said to give increased firmness and crispness of
handle. Tailored nylon fibers are similar in cross-section to silk fibers, and
the fabric made from them is then said to be closely resemble silk in handle.
Such fibers, it is also claimed to show soiling so readily.
Thermal stability of fibers is
also important for such domestic processes as ironing, and the related property
of flammability is even more crucial. These matters are largely concerned with
the fiber material; new types of nylon have much higher dangerous temperature
and are much more heat resistance; they are used in fire resistant clothing.
There is, therefore, a very wide range of technology involving many factors of
Material and methods, and within it the properties of man-made fibers can be
modified in fundamental ways that enables them to be design for particular
engineering and textile-end uses. Also because of the possible variations of
many of the processing factors, what may appear to be the same sorts of fiber
made by different manufacturers but may have different physical properties and
may behave differently in processing.
FIBER
GEOMETRY
All ordinary yarns, either
man-made or natural, consist of a number of fibers or filaments. In the case of
man-made fibers for textile uses, this number is usually 15 to 100 i.e. most
yarns will be composed of not fewer than 15 and not more than 100. But there
are, of course, some variations to these numbers which largely depends on its
property and the end-products–use. The multifilament construction in yarn is
adopted to confer pliability and flexibility on the yarn- a yarn composed of a
number of fine filaments is much more flexible than a solid, thick filament of
the same diameter as the yarn.
Textile materials are generally
soft to the touch, flexible, capable of being transformed into desired shapes
without resistance and durable over a reasonable period of wear. They derive
these properties from fibers and yarns that form the building units arranged or
interlaced in various forms. The yarn, in turn, is formed by twisting a bundle
of fibers together, therefore, the
properties of the yarn, for that matter the properties of the ultimate
textile structure will depend very largely on the ch
most Essential And Other Desirable Properties that must be taken into
account in making a choice of a fiber for use as textile material:-
aracteristics of the fibers
from which they are made and how they are made. The following are some of the- dimensional and physical characteristics, such as, length, fineness, cross-sectional shape, crimp, density.
- mechanical properties, such as, strength, elasticity, extensibility, rigidity (stiffness).
- general properties , such as, surface characteristics, frictional properties, softness, environmental stability, resistance to sunlight,thermal stability, resistance to chemical and organic-solvents, pliability, durability, abrasion resistance, dimensional stability, moisture absorption, resistance to bacteria, fungi, mildew, moths, etc, static charge build-up, colour, wetting characteristics etc.
the technical significance of some of the most important properties of
fibers that have a profound influence on the processing behavior and the
end-use characteristics of yarns, are, fiber length, fiber fineness i.e. width,
area of cross section,
wall thickness, linear density,fiber strength and
extensibility and related miscellaneous properties.
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