Most synthetic fibers are polymer-based, and are produced by a process known as spinning. This process involves extrusion of a polymeric liquid through fine holes known as spinnerets. After the liquid has been spun, the resulting fibers are oriented by stretching or drawing. This increases the polymeric chain orientation and degree of crystallinity, and has the effect of increasing the modulus and tensile strength of the fibers. Fiber manufacture is classified according to the type of spinning that the polymer liquid undergoes: this may be melt spinning, dry spinning, or wet spinning.
Melt spinning is the simplest of these three methods, but it still requires that the polymer constituent be stable above its melting temperature. In melt spinning, the polymer is melted and forced through the spinnerets, which may contain from 50-500 holes. The diameter of the fiber immediately following extrusion exceeds the hole diameter. During the cooling process, the fiber is drawn to induce orientation. Further orientation may later be achieved by stretching the fiber to a higher draw ratio. Melt spinning is used with polymers such as nylon, polyethylene, polyvinyl chloride, cellulose triacetate, and polyethylene terephthalate, and in the multifilament extrusion of polypropylene.
In dry spinning, the polymer is first dissolved in a solvent. The polymer solution is extruded through the spinnerets. The solvent is evaporated with hot air and collected for reuse. The fiber then passes over rollers, and is stretched to orient the molecules and increase the fiber strength. Cellulose acetate, cellulose triacetate, acrylic, modacrylic, aromatic nylon, and polyvinyl chloride are made by dry spinning.
In wet spinning, the polymer solution is spun into a coagulating solution to precipitate the polymer. This process has been used with acrylic, modacrylic, aromatic nylon, and polyvinyl chloride fibers. Viscose rayon is produced from regenerated cellulose by a wet spinning technique.
Table 1 (Artificial Polymeric Fibers) provides detailed information about each of the important classes of spun fibers.