Polymer Processing- Spinning Process of Polymers
According to Tadmor and Gogos, polymer processing are the "operations carried out on polymeric materials or systems to increase their utility." Therefore, polymer processing is done to increase the value of a polymer in order to obtain the desired shape, properties, and performance for a polymer article. As far as textile industry is concerned, polymer processing can be explained as the spinning process for manufacturing polymer fibers that are commonly known as synthetic fibers
Spinning of polymers
The polymers of synthetic fibers are almost always derived from by-products of petroleum and natural gas which include nylon, polyethylene terephthalate, as well as other compounds like acrylics, polyurethanes and polypropylene. Synthetic fibers, which are produced from these polymers are used for making various consumer and industrial textile products
ranging from clothing
, home furnishings
to conveyor belts
. Most of the cellulosic yarns
manufactured from synthetic fibers are produced by extrusion of a thick, viscous liquid through the tiny holes of an equipment called spinneret which forms continuous filaments of semi-solid polymer.
The polymer is at first converted into a fluid state. If it is a thermoplastic polymer then it is just melted otherwise it has to be dissolved in a solvent or has to be chemically treated in order to form soluble or thermoplastic derivatives. The fluid polymer is then forced through the spinneret. The polymer here cools to a rubbery state, and then finally solidifies. For making specialty yarn
, certain polymers have been technologically developed that do not melt, dissolve, or form appropriate derivatives. In such cases, the small fluid molecules are mixed and reacted to form the otherwise intractable polymers during the process of extrusion. There are typically four types of spinning for polymers-wet spinning, dry spinning, melt spinning, and gel spinning. However, before proceeding to know the actual processes of all these four types of spinning, one must know about the basic process of extrusion and the spinneret used for it.
Spinneret and Extrusion
The spinnerets used for manufacturing most of the synthetic fibers have many holes ranging from one to several hundred holes which gives it an appearance similar to that of a bathroom shower head. The fluid polymer is fed into these tiny openings. The liquid polymer emerging out of the holes of spinneret in the form of filaments, is converted first to a rubbery state and then into a solid state. This process of extrusion and solidification of innumerable filaments is called spinning of polymers. This spinning is different from the fiber spinning
of plant fibers
and animal fibers
, that are twisted into yarns
Extrusion is also of two types- single screw extrusion and twin screw extrusion.
Single screw extrusion:
is one of the basic operations of polymer processing. Single screw extrusion process builds pressure onto a polymer melt so that it can be forced through a die or can be injected into a mold. Most of the single screw extrusion machines are plasticating which bring in solids in pellet or powder form and melt them building pressure at the same time.
Twin screw extrusion:
is widely used for mixing, compounding, or reacting polymeric materials. The flexibility of twin screw extrusion tool lets this operation to be designed specifically for the formulation that is being processed, for instance- the two screws may be corotating or counterrotating, intermeshing or nonintermeshing. Additionally, the configuration of the screws themselves may be varied using forward conveying elements, reverse conveying elements, kneading blocks, and other designs which can help in obtaining particular mixing characteristics.
Types of Polymer Spinning Processes
As stated earlier, There are four types of spinning for polymers- wet spinning, dry spinning, melt spinning, and gel spinning.
Of all the four processes, wet spinning is the oldest process. It is used for polymers that need to be dissolved in a solvent to be spun. The spinneret remains submerged in a chemical bath that leads the fiber to precipitate, and then solidify, as it emerges out of the spinneret holes. The name of the process i.e. wet spinning has got its name from this "wet" bath only. Acrylic fiber
, rayon fiber
, aramid fiber
, modacrylic fiber, and spandex fibers
, all are manufactured through wet spinning.
It is also used for polymers that have to bedissolved in a solvent. However, solidification results from evaporation of the solvent. After dissolving the polymer in a volatile solvent, the solution is pumped through a spinneret. As the fibers
emerge from the spinneret, air or inert gas is used to evaporate the solvent which results in solidification of the fibers that can be collected on a take-up wheel. The fibers are stretched which provides for orientation of the polymer chains along the fiber axis. This technique is used only for polymers which cannot be melt spun because of the safety and environmental concerns concerned with solvent handling. Dry spinning may be used for manufacturing acetate fiber, triacetate fiber, acrylic fiber, modacrylic fiber, PBI, spandex fiber, and vinyon.
is used for the polymeric fibers or the polymers that can be melted. The polymer is melted and then pumped through a spinneret. The cooled and solidified molten fibers get collected on a take-up wheel. The fibers, when stretched in both, the molten and solid states, facilitate orientation of the polymer chains along the fiber axis. Melt spun fibers can be forced through the spinneret in different cross-sectional shapes such as round, trilobal, pentagonal, octagonal among others. Trilobal-shaped fibers are capable of reflecting more light which give a sparkle to the fabrics
. Pentagonal-shaped and hollow fibers are soil and dirt resistant and as such are used for making carpets and rugs
. Octagonal-shaped fibers offer glitter-free effects whereas hollow fibers trap air, creating insulation. Polymers like polyethylene terephthalate and nylon 6,6 are melt spun in high volumes. Nylon fiber
, olefin fiber, polyester fiber
, saran fiber, etc. are also manufactured through melt spinning.
It is also known as dry-wet spinning because the filaments first pass through air and then are cooled further in a liquid bath. Gel spinning is used to make very strong and other fibers having special characteristics. The polymer here is partially liquid or in a "gel" state, which keeps the polymer chains somewhat bound together at various points in liquid crystal form.
This bond further results into strong inter-chain forces in the fiber increasing its tensile strength. The polymer chains within the fibers also have a large degree of orientation, which increases its strength. The filaments come out with an unusual high degree of orientation relative to each other, further enhancing strength. The high strength polyethylene fiber
and aramid fibers are manufactured through this process.
Whatever spinning process is applied, the fibers are finally drawn to increase strength and orientation. This may be done while the polymer is still in the process of solidifying or after it has completely cooled down. Drawing pulls the molecular chains together and orients them along the fiber axis, resulting in a considerably stronger yarn.