Polymer that is to be converted into fibre must first be converted to a liquid or semiliquid state, either by being dissolved in a solvent or by being heated until molten. This process frees the long molecules from close association with one another, allowing them to move independently. The resulting liquid is extruded through small holes in a device known as a spinnerette, emerging as fine jets of liquid that harden to form solid rods with all the superficial characteristics of a very long fibre, or filament. This extrusion of liquid fibre-forming polymer, followed by hardening to form filaments, is called spinning (a term that is actually more properly used in connection with textile manufacturing). Several spinning techniques are used in the production of man-made fibre, including solution spinning (wet or dry), melt spinning, gel spinning (a variant on solution spinning), and emulsion spinning (another variation of solution spinning).
One of the oldest methods for the preparation of man-made fibres is solution spinning, which was introduced industrially at the end of the 19th century. Solution spinning includes wet spinning and dry spinning. The former method was first used to produce rayon fibres, and the latter method was used to spin cellulose triacetate to acetate fibres. In both methods, a viscous solution of polymer is pumped through a filter and then passed through the fine holes of a spinnerette. The solvent is subsequently removed, leaving a fibre.
During wet spinning the spinnerette is generally (but not always) placed in the spin bath, a coagulation bath in which solvent diffuses out of the extruded material and a nonsolvent, usually water, diffuses into the extrudate. The resulting gel may be oriented by stretching during this stage, as the polymer is coagulated, or the freshly formed fibres may be stretched after they are removed from the spin bath. At this point the fibre, containing solvent and nonsolvent (e.g., water), is washed with more nonsolvent (again, usually water). A lubricant, referred to as the fibre finish, is generally applied before the fibre is dried on large, heated drum rolls. The fibre is then wound onto spindles or sent to a cutter. The cutter produces fibre in lengths of 2.5 to 15 cm (1 to 6 inches) known as staple. A spindle that has been fully wound with continuous fibre is called a package.
In dry spinning, the solution of polymer is pushed through a spinnerette into a heated column called the spinning tower, where the solvent evaporates, leaving a fibre. The emerging fibre may contain solvent that may have to be removed by further heating or by washing. This operation is followed by stretching, application of finish, and either take-up on a spindle or cutting to staple.
The wet-spinning method is capable of spinning a large number of fibres at a time because several thousand holes may be present in a single spinnerette. The large bundle of emerging fibres, known as tow, can be spun at rates slow enough to make possible the use of a large spin bath and large washing rolls, drying rolls, and other processing equipment. Wet spinning is thus highly economical, the low spinning rates being compensated for by the large tows to give high overall productivity. In dry spinning, on the other hand, the rate of spinning is much higher, but relatively small bundles of fibre are extruded in order to achieve adequate solvent removal and drying. As a consequence, productivity is lower than in wet spinning. Dry spinning is being phased out for most commodity fibres and is used only for expensive specialty fibres, such as spandex, that cannot be spun by any other process.
The use of solvents that can be recovered from the spin bath is becoming more common in solution spinning. Acrylic fibres are an example of this trend. In some older acrylic processes the solvents were salts such as sodium or ammonium thiocyanates, but the preferred method now is to use an amide-type solvent—e.g., N,N-dimethylacetamide (DMAc)—which can be recovered from the spin bath by distillation. Amide solvents are also used for the spinning of some aramids—e.g., for the trademarked fibres Nomex and Conex.
Rayon fibres traditionally have been spun from xanthate solutions, as noted above, but this process has been abandoned in developed countries owing to environmental problems caused by the carbon disulfide ingredients and also by salts produced in treating the xanthate with acid. Newer plants use an inorganic solvent, morpholine N-oxide, which can be recovered by distillation of the spin bath.