Spinning process(synthetic)
In Textiles, the
polymer to be converted into fiber must first be converted to a liquid or
semiliquid state, either by dissolving in a solvent or by heating until it
melts. This process frees the long molecules from close association with each
other, allowing them to move freely. The resulting liquid is extruded through
small holes in a device known as a spinneret, which emerges as a fine jet of
liquid that solidifies to form a solid rod with all the surface characteristics
of very long fibers or filaments. This extrusion of liquid fiber-forming
polymer, which is then solidified to form filaments, the process is called the spinning process. There are many types of spinning processes: wet spinning, dry spinning, dry
jet-wet spinning, melt spinning, gel spinning, and electrospinning.
Types of the spinning process
Solution Spinning
One
of the oldest methods of producing man-made fibers is solution spinning, which
was introduced into the industry in the late 19th century. The solution spinning process includes both wet spinning and dry spinning. In both methods, a viscous
solution of polymer is pumped through a filter and then passed through fine
holes in a spinneret. The solvent is then removed, leaving a fiber.
a. Wet spinning
During
wet spinning the spinneret is usually placed in a spin bath, a coagulant bath
in which the solvent is dispersed out of the extrudate, and a non-solvent,
usually, water is dispersed into the extrudate. The resulting gel may be
oriented by stretching at this stage, as the polymer remains solidified, or the
newly formed fibers may stretch after removal from the spin bath. At this
point, the fiber containing the solvent and non-solvent is washed with more non-solvent-like
water. A lubricant called a fiber finish, is usually applied before drying the
fibers on large, heated drum rolls. The fibers are then wound onto spindles.
Advantages
a.
It can be used for any polymer.
b.
Fiber can attain maximum strength.
c.
The process can be continuous.
Disadvantages
a.
The production rate is low
b.
Post-spinning operations are lengthier
c.
It is more costly.
b. Dry spinning
During
dry spinning, the polymer solution is pushed through a spinnerette into a
heated column called a spinning tower, where the solvent evaporates, leaving
behind a fiber. The emerging fiber may contain a solvent that may need to be
removed by further heating or washing. This operation is followed by
stretching, application of finish, and fiber taking on a spindle or cutting on
staples.
Advantages
a.
It is suitable for heat-sensitive polymers
b.
Higher spinning speeds can be easily achieved
c.
The post-spinning operation is simple
d.
A moderate concentration of polymer is required.
Disadvantages
a.
Flammable solvent hazards.
b.
Solvent recovery is required.
Melt spinning
The
most economical method of spinning is melt spinning, primarily because there is
no solvent to recover as in solution spinning and the spinning rate is much
higher. In this process, a viscous melt of polymer is spun through a spinneret
in a process zone called a spinning tower, with a number of porous pores. There
the molten polymer is solidified by a blast of cold air, and after the application
of the finish, numerous fibers are collected at high speed. In a process known
as spin drawing, fibers can be drawn into lines several times their original
length. The packages may be collected directly from the spinning tower into
what is called a continuous filament, or several lines of fiber may be
collected into a large tow for staple cutting.
Advantages
a.
Direct and simple process.
b.
High production speed.
c.
Low investment cost.
d.
No environmental pollution.
Disadvantages
a.
It is suitable for thermoplastic fiber.
b.
Maximum strength can’t be achieved easily.
c.
The heat of the input is high.
Gel spinning
In
modern adaptations of this process, a very high molecular weight polymer is
dissolved in a solvent of low concentration, producing a very viscous solution.
This solution is either dry- or wet on the fiber, which still retains most of
the solvent, and is actually a gel of polymer and solvent. While in the gel state,
the fiber can be stretched to pull the polymer molecules into an elongated
state rather than the normal solid state of the chain-folded molecules.
Ultra-high-strength, high-tough polyethylene fibers, marketed under trademarks
such as Spectra, is produced commercially using gel-spinning techniques.
Advantages
a.
It gives high-strength and high-mechanical-property fibers
b.
It gives ultra-high extension in fiber.
c.
It can promote the polymer of fiber's molecular weight, degree of orientation,
crystallinity, and fiber density effectively.
d.
Technology is relatively simple.
Emulsion spinning
Some
non-melted and insoluble polymers can be mixed with a finely divided powder,
while others are mixed with a polymer solution and cut into fibers in the
solution. The soluble polymer can be removed by a solvent or by burning and
collecting residual fibers. Such processes can be used to make fibers of
fluorocarbons such as Teflon, which have very high melting points.
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