Chemical spinning
Chemical
spinning processes are commonly used to make man-made filament fiber yarn. A
smooth solution is extruded through a nozzle-like die called a spinneret.
Man-made fibers are used to make yarn by chemical spinning.
Types of the chemical spinning process
There
are three types of chemical spinning processes as below-
Melt spinning
Melt
spinning is applicable to polymers that are capable of melting at advanced
temperatures. This is the simplest and cheapest process, as it involves only
melting, extrusion, and cooling. The polymer is melted, and extruded through a spinnerette
hole which is a perforated metal plate of small diameter. The length of each
hole can be 0.5 mm or less. Each spinnerette can have 1 to 100 holes depending
on the type of yarn or picked product. A tow is a rope of fibers collected from
various spinnerette that is used to make the staple fibers for blending.
In the case of making melt-spun filaments, the polymers in the form of chips are
fed through a hopper and it falls into a hot grid where it melts. Molten
polymers are collected and filtered so that suspended impurities block the flow
of the polymer and a vacuum is also used to remove air bubbles, so as to ensure
a continuous flow of the polymer liquid that produces continuous filament.
The
polymer current collected in a tank enters a cooling chamber through the pores
of the spinnerette which helps in the solidification of the filament. The
fibers produced by molten extrusion are polyamide, polyester, and
polypropylene.
The temperature used for molten extrusion is usually 30°C above the melting point
of the polymer so that the viscosity of the fiber is low enough for extrusion.
The viscosity of molten polymers is also a measure of the length of molecular
chains. The higher the viscosity, the higher the polymer chain.
The
polymer chains in the filaments produced in this way are arranged randomly. To
improve its orientation, cool the filaments. For the convenience of cold
drawing, the filaments are pulled between two sets of rollers which are
operated at successively higher speeds. The second set rotates at least 4 times
the speed of the first set so that the drawing of the filament occurs. The
randomly arranged molecular chains face the fiber axis and the filament becomes
finer and stronger. It also helps in the formation of crystals as the polymer chains
come closer.
Solvent/ dry spinning
When
a polymer cannot melt into a liquid by melting, an alternative method is
employed in which the polymer dissolves in a suitable solvent. If the solvent
is unstable, when the solvent dries it leaves the polymer in the desired shape.
Hence this method is called dry spinning.
The
polymer dissolves up to 25% in the solvent and is converted into a polymer
solution. The solution is sometimes heated to ensure adequate flow of polymer
between the tiny holes in the spinnerette. At this stage, certain additives
like-colored pigments can be introduced. Delustering agents such as titanium
dioxide can also be added to the solution to form a dull filament. The polymer
solution is filtered, and the suspended particles are removed and treated with a
vacuum that blocks the spinnerette holes and air bubbles that prevent the
formation of uninterrupted filaments.
The solution from the storage tank is carried through tiny holes in the spinnerette
to the heated air chamber where hot air or nitrogen is conducted. The heated
air evaporates the solvent from the polymer and carries it to another chamber
where the solvent is recovered. After spinning, the filaments need to be
washed.
This process is considered expensive because it requires huge investments for
solvent recovery plants. About 10% of the solvent is lost during this process,
a portion of which may be present in this filament. Used solvents can be
expensive, toxic, and flammable. Typically, 3 to 6 kg of solvent per kg of
polymer is used. However, this process is considered to be fast.
Wet spinning
Wet
spinning is applicable to polymers that cannot be dissolved by the volatile solvent in a liquid, it can change from solid to liquid. A non-volatile solvent
is used to change it to a liquid state. The solvent cannot evaporate, it is
made to pass through a chemical bath, where the filament leaves the solvent,
hence the name wet spinning.
Wet
spinning is a slow process (30-80 m / min) of regrowth by filament freezing.
Since freezing occurs in the place of any chemical baths, spinnerettes should be
made with a corrosion-resistant precious metal alloy, which again requires a
huge investment. The holes used are much smaller than the other two methods. If
the filament turns into a tow, there are about 20,000 holes present.
The polymer solution is filtered to be treated in a vacuum and then collected in
tanks to keep it free from suspended particles and air bubbles. Additives such
as pigments and confusing agents can be added. It is then mixed in a chemical
bath through a spinnerette that contains a dilute acid and accumulates in the
form of a filament. The filaments are washed and pulled to the desired
diameter.
The fibers that are produced using this method include viscose rayon and polyacrylonitrile. The cross-sectional shapes of the filaments are often granular due to a chemical reaction that leads to the coagulation of the filament.
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