Spinning Processes
Synthetic and Cellulosic
Fiber Formation Technology
 Most
synthetic and cellulosic manufactured fibers are created by �extrusion� �
forcing a thick, viscous liquid (about the consistency of cold honey) through
the tiny holes of a device called a spinneret to form continuous filaments of
semi-solid polymer.
In their initial state, the fiber-forming polymers are solids and therefore
must be first converted into a fluid state for extrusion. This is usually
achieved by melting, if the polymers are thermoplastic synthetics (i.e., they
soften and melt when heated), or by dissolving them in a suitable solvent if
they are non-thermoplastic cellulosics. If they cannot be dissolved or melted
directly, they must be chemically treated to form soluble or thermoplastic
derivatives. Recent technologies have been developed for some specialty fibers
made of polymers that do not melt, dissolve, or form appropriate derivatives.
For these materials, the small fluid molecules are mixed and reacted to form the
otherwise intractable polymers during the extrusion process (if you are
interested in the latest information on extrusion,
The Spinneret
The spinnerets used in the production of most manufactured fibers are
similar, in principle, to a bathroom shower head. A spinneret may have from one
to several hundred holes. The tiny openings are very sensitive to impurities and
corrosion. The liquid feeding them must be carefully filtered (not an easy task
with very viscous materials) and, in some cases, the spinneret must be made from
very expensive, corrosion-resistant metals. Maintenance is also critical, and
spinnerets must be removed and cleaned on a regular basis to prevent clogging.
As the filaments emerge from the holes in the spinneret, the liquid polymer
is converted first to a rubbery state and then solidified. This process of
extrusion and solidification of endless filaments is called spinning, not to be
confused with the textile operation of the same name, where short pieces of
staple fiber are twisted into yarn. There are four methods of spinning filaments
of manufactured fibers: wet, dry, melt, and gel spinning.
Wet Spinning
Wet spinning is the oldest process. It is used for fiber-forming
substances that have been dissolved in a solvent. The spinnerets are submerged
in a chemical bath and as the filaments emerge they precipitate from solution
and solidify.
Because the solution is extruded directly into the precipitating liquid, this
process for making fibers is called wet spinning.
Acrylic,
rayon,
aramid,
modacrylic and
spandex can be
produced by this process.
Dry Spinning
Dry spinning is also used for fiber-forming substances in solution.
However, instead of precipitating the polymer by dilution or chemical reaction,
solidification is achieved by evaporating the solvent in a stream of air or
inert gas.
The filaments do not come in contact with a precipitating liquid, eliminating
the need for drying and easing solvent recovery. This process may be used for
the production of
acetate,
triacetate, acrylic,
modacrylic,
PBI,
spandex, and
vinyon.
Melt Spinning
In
melt spinning, the fiber-forming substance is melted for extrusion through the
spinneret and then directly solidified by cooling.
Nylon,
olefin,
polyester,
saran and
sulfar are produced
in this manner.
Melt spun fibers can be extruded from the spinneret in different
cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others).
Trilobal-shaped fibers reflect more light and give an attractive sparkle to
textiles.
Pentagonal-shaped and hollow fibers, when used in carpet, show less soil and
dirt. Octagonal-shaped fibers offer glitter-free effects. Hollow fibers trap
air, creating insulation and provide loft characteristics equal to, or better
than, down.
| Detailed production
flowcharts: |
| � Acrylic |
� Nylon
(Polyamide) |
� Polyester |
Gel Spinning
Gel spinning is a special process used to obtain high strength or
other special fiber properties. The polymer is not in a true liquid state
during extrusion. Not completely separated, as they would be in a true
solution, the polymer chains are bound together at various points in liquid
crystal form. This produces strong inter-chain forces in the resulting
filaments that can significantly increase the tensile strength of the
fibers. In addition, the liquid crystals are aligned along the fiber axis by
the shear forces during extrusion. The filaments emerge with an unusually
high degree of orientation relative to each other, further enhancing
strength. The process can also be described as dry-wet spinning, since the
filaments first pass through air and then are cooled further in a liquid
bath. Some high-strength polyethylene and aramid fibers are produced by gel
spinning.
Stretching and Orientation
While extruded fibers are solidifying, or in some cases even
after they have hardened, the filaments may be drawn to impart strength.
Drawing pulls the molecular chains together and orients them along the fiber
axis, creating a considerably stronger yarn.
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