Spinning (polymers)

Spinning is a specialized manufacturing process used to create polymer fibers. These fibers are long, continuous filaments that serve as the building blocks for many materials. The process is a specific type of extrusion. Extrusion involves pushing a material through a tool to change its shape. In spinning, this tool is called a spinneret. A spinneret contains many tiny openings known as capillaries. By forcing material through these capillaries, manufacturers can create many filaments at once.

One primary method is melt spinning, which is used for thermoplastic polymers. A thermoplastic is a material that becomes liquid when heated. In this process, the molten polymer is forced through the spinneret capillaries. As the liquid emerges, it is cooled to solidify into a filament. This method is commonly used to produce materials like nylon, polyester, and olefin. It is also used for making saran and sulfar.

There are variations in how the polymer is prepared before it reaches the spinneret. Extrusion spinning begins with solid polymer pellets or granules. These are fed into an extruder where an extrusion screw compresses, heats, and melts them. The melted material is then sent to a spinning pump and the spinneret. Direct spinning is a more efficient alternative for certain materials. It skips the solid pellet stage by pumping the polymer melt directly from the finisher to the spinning mill. This method is mainly used for polyester and supports very high production capacities, exceeding 100 tons per day.

Sometimes, a polymer cannot be melted because its melting point is higher than its degradation temperature. If the heat required to melt the polymer would destroy it, manufacturers use solution spinning. In solution spinning, the polymer is dissolved in a liquid called a solvent. This creates a spinning solution, which is often referred to as a "dope." There are several different techniques used to turn this dope into solid fibers. These include dry spinning, wet spinning, dry-jet wet spinning, gel spinning, and electrospinning.

Dry spinning and wet spinning are two distinct ways to solidify a solution. In dry spinning, the spinning solution is extruded into an evaporating chamber. A stream of hot air hits the liquid jets to evaporate the solvent. This leaves behind solid filaments. Wet spinning is the oldest of the five solution spinning processes. Here, the solution is extruded into a coagulation bath filled with nonsolvents. This bath causes the polymer to precipitate into fiber form. This process produces materials like acrylic, rayon, aramid, modacrylic, and spandex.

Advanced techniques like dry-jet wet spinning and gel spinning offer even more control. Dry-jet wet spinning involves passing the solution through an air-gap before it enters the coagulation bath. This is used in Lyocell spinning and can create higher polymer orientation. Gel spinning, or semi-melt spinning, is used to create fibers with very high strength. Instead of relying on precipitation, it uses temperature-induced physical gelation to solidify the material. This process is used for high strength polyethylene and polyacrylonitrile fibers. For example, it can produce ultra-high molecular weight polyethylene (UHMWPE) fibers, such as Spectra, which have a high degree of orientation.

Electrospinning is a unique method that uses an electrical charge to draw fibers. It can pull very fine fibers from either a polymer solution or a polymer melt. These fibers are often on the micro or nano scale. Electrospinning is useful for large and complex molecules because it does not require high temperatures or coagulation chemistry. A version called melt electrospinning ensures that no solvent remains in the final product. Finally, most fibers undergo a post-spin process called drawing. Drawing involves pulling the fibers to increase their strength and orientation. This can occur while the polymer is still solidifying or after it has cooled completely.