|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.
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: Different synthetic fibers production|
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.
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.
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:
- Nylon (Polyamide)
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.