Production Process of Different Synthetic Fibers

December 5, 2019

Synthetic fiber

A synthetic fiber is a man made fiber. But all man-made fiber are not synthetic fiber. Nylon, polyester are synthetic fiber. Rayon and acetate from cellulose or regenerated protein fibers from zein or casein are the man-made fiber but not synthetic fiber. 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.

Fig: Synthetic fiber

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.

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Spinneret:
A spinneret is a device used to extrude a polymer solution or polymer melt to form fibers. 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.

Fig: Spinneret

Production Process of Different Synthetic Fibers

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.

Fig: Different synthetic fibers production

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Wet spinning:
Wet spinning is the oldest process. It is used for polymers that need to be dissolved in a solvent frist.. 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:
In dry spinning the polymer is dissolved in its solvent and then extruded, as the fibres emerge through the spinneret the solvent is evaporated off with hot air, in most cases this is then collected and re-used. Dry spinning is also used for fiber-forming substances in solution. Dry spinning technique is used for such kind of Polymers which won’t melt but degrade on heating.

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 rapid cooling of liquids. 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, also known as dry-wet spinning, is used to obtain high strength or other special properties in the fibers. It is an old technique that has come into use commercially only since the 1980s. It 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:
After spinneret, while extruded fibers are coagulating, 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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