Biomedical Engineering Reference
In-Depth Information
collection of spinning orifices for synthetic fiber
manufacturing is called a
spinneret
. The spinneret
resembles a showerhead. After the polymer fluid
passes out of the spigot or spinneret, the natural
and synthetic processes begin to diverge. In the
natural process, the filament is immediately put
to work; it is in its final, useable form, having
solidified just before exiting the spigot. In syn-
thetic fiber manufacturing, the filament remains
fluid close to the face of the spinneret. The fila-
ment is stretched and solidified (by cooling or
solvent removal) prior to being wound up on a
package. There are more details to this process,
but that is the process at its most basic.
Orientation of the chains is achieved by the
applied longitudinal stress and the extensional
flow field during the fluid or semifluid phase of
fiber production. Crystallization is a consequence
of annealing (aided by applied stress, in some
instances). Both of these material structural
attributes are critical to the final fiber having the
desired properties, and they must be carefully
controlled
[15]
.
The interesting science and engineering from
the standpoint of structure development occur
at distinctly different points in the fiber produc-
tion process, natural vs. synthetic. In natural
systems, the development of structure occurs
within the host. In synthetic extrusion, with
apologies to polymer extruder engineers, very
little of interest occurs prior to the face of the
spinneret. It is in this region that, under the
influence of mechanical and thermal stresses,
the fiber structure is developed.
Since the next step, yarn production, was devel-
oped using natural fibers, which have restricted
lengths and some degree of roughness to their
surface, and since neither property occurs auto-
matically in synthetic fiber production, the syn-
thetics must be made to have more natural
features: They are cut to short lengths to yield
staple fibers, and they are crimped to be not
smooth or straight. Then they can be processed
into spun yarns. It is important to note that fila-
ment yarns of silk and synthetic fibers are useful
in their own right. Fabrics made from filament
yarns have excellent drape characteristics. Fila-
ment yarns are also useful as the reinforcement
phase in composites.
10.3.1.2 Yarn
The next step up in the hierarchy of textiles is the
ordered collection of fibers into a yarn, which
is achieved by twisting, called
spinning
by
practitioners, of the staple fibers into a yarn. The
feedstock for yarn spinning is a several hundred
pound bale of randomly oriented fibers, either
natural or synthetic, which must be opened and
individual fibers separated and aligned. Hence,
at the most fundamental, all the yarn-forming
processes have the goal of entropy reduction
and the result of improvement in strength. The
fibers are twisted so as to increase the strength of
the interfiber cohesion. The most common of the
spinning processes currently in use is
ring spin-
ning
, so named because of the ring that carries the
nascent yarn in a circle as it is being twisted. The
degree of fiber-to-fiber overlap, the surface char-
acteristics of the fibers, and the degree of twist
and the tightness of the twist, in addition to the
fiber strength, all contribute to the yarn strength
and flexibility. There have been and continue to
be many developments in yarn spinning. I com-
mend the interested reader to Goswami
et al
.
[16]
.
10.3.1.3 Fabric
Fabric as a concept has many facets. The common
thread running through the manifestations of
the concept is that the elemental component is
a fiber. The fiber is paramount, but the interme-
diate structures, if any, are critical in their own
right. The properties of the individual fibers do
have an impact, albeit limited, on the ultimate
mechanical properties of the fabric and strongly
influence its chemical and thermal properties.
The properties of a fabric that influence comfort
are strongly influenced by both the fiber type
and the fabric structure.
The earliest type of loom, and still the most
commonly used in the production of fine fabrics,
