The polymers of synthetic fibers are derived from by-products of petroleum and natural gas that include nylon, polyethylene terephthalate, and other compounds like acrylics, polyurethanes and polypropylene. The polymer is first converted into a fluid state. The thermoplastic polymer is just melted but if the polymer is not thermoplastic then it has to be dissolved in a solvent or has to be chemically treated to form soluble or thermoplastic derivatives. The fluid polymer is then forced through a spinneret, a bathroom shower head like equipment having multitude of holes.
The polymer extruded from spinneret cools to a rubbery state, and then to a solid state. In case of polymers for specialty yarns which do not melt, dissolve, or form appropriate derivatives, the small fluid molecules are mixed and reacted to form the otherwise intractable polymers during the process of extrusion. There are four basic types of spinning for polymers- wet spinning, dry spinning, melt spinning, and gel spinning.
Wet spinning is the oldest process and is used for polymers that need to be dissolved in a solvent to be spun. The spinneret is submerged in a chemical bath that leads the fiber to precipitate, and then solidify, as it comes out of the spinneret holes. Acrylic fiber, rayon fiber, aramid fiber, modacrylic fiber, and spandex fibers, are made through wet spinning.
Dry spinning is also used for polymers that have to be dissolved in a solvent but here solidification results from evaporation of the solvent. Air or inert gas is used to evaporate the solvent which results in solidification of the fibers. Acetate fiber, triacetate fiber, acrylic fiber, modacrylic fiber, PBI, spandex fiber, and vinyon are made through dry spinning.
Melt spinning is used for the polymers that can be melted. The molten polymer is pumped through a spinneret which subsequently gets cooled and solidified. Melt spun fibers can be forced through the spinneret in different cross-sectional shapes such as round, trilobal, pentagonal, octagonal etc. which give different qualities to the fabrics like insulation, sparkle, soil and dirt resistance among others.
Gel spinning, also known as dry-wet spinning as the filaments first pass through air and then are cooled further in a liquid bath, is used to make very strong and other fibers having special characteristics. Here polymer is partially liquid or in a "gel" state, which keeps the polymer chains somewhat bound together which leads to greater tensile strength. The high strength polyethylene fiber and aramid fibers are manufactured through this process.