Manufacturing Process of Synthetic and Regenerated Fibers

What is Synthetic and Regenerated Fibers?
The term synthetic fiber relates to fibers formed from polymers constructed from chains grown via a controlled chemical process. The synthetic fibers are result of the extensive research to improve the properties of naturally occurring animal and vegetable fibers. These synthetic fibers are produced by the extrusion of a polymeric material having synthetic origin through spinneret into air or water. This fiber forming polymers are obtained generally from petro chemicals. Therefore, these fibers are called synthetic fibers. This category would include nylon, Kevlar, poly (ethylene terephthalate) (PET), and polyethylene, whereas fibers formed from so-called natural polymers are not considered to be true synthetic and are termed regenerated fibers. This latter category consists of viscose rayon and cellulose acetate along with some more recent developments such as chitosan, which is formed from the abundant chitin material found in sea crustaceans. Despite the now dominance of the true synthetics in the fiber market, it was the early development of regenerated cellulosics that would lay the ground work for many of the processes and techniques that are now used to make fibers from natural and synthetic feedstocks.

One of the key innovations in fiber science is the regenerated fibers. Regenerated fibers are produced from natural source with human interference. Several regenerated cellulosic fibers were produced from wood pulp such as viscose, lyocell, rayon, and modal, which have been used as reinforcements in composite processing. In the current context, this avenue shows more promise in terms of ecological impact and sustainable practices such as recyclability and reusability.

The basic element of a cellulose macromolecule is glucose. The empirical formula of cellulose is (C6H10O5)n, where n represents the number of glucose molecules constituting the cellulose macromolecule and is called the degree of polymerization (DP). The α-cellulose (insoluble in cold dilute NaOH) has a DP greater than 200, while β-cellulose (hemicelluloses soluble in cold dilute NaOH) has DP less than 200. Wood pulp is used as the raw material for cellulose, and is refined to increase the percentage of α-cellulose. The percentage of up to 99 % can be obtained depending on the cleaning method. The regenerated fibers are produced according to the viscose spinning method.

Synthetic and Regenerated Fibers Manufacturing Process:

Synthetic fiber is that which is made from different types of polymers. It is not cultivated as the natural fiber. Regenerated fiber is created by dissolving the cellulose area of plant fiber in chemicals and making it into fiber again. Synthetic fibers for textiles are all produced using the same fundamental processing techniques. In this article I will discuss synthetic and regenerated fibers manufacturing process with diagram.

In principle, a polymer fluid is forced through a series of fine holes that create the basic shape. The fluid is then encouraged to harden through cooling, chemical, or thermodynamic processes, which lead to a solid filament. There are four methods of spinning filaments of manufactured fibers: wet, dry, melt, and gel spinning.

Melt spinning:
In melt spinning, The polymer chips are melted in a large hopper and passed through a metered pump before reaching the spinneret. The filaments then pass through cold air, which solidifies the filament before it is drawn and wound onto bobbins. Thermoplastic fibers are mainly produced with melt spinning. Nylon, polyester, and liquid crystalline aromatic polyester are all manufactured with melt spinning. They are first heated above their melting temperatures, then extruded through spinneret to form continuous fiber, followed by drawing, cooling, and winding. This process
is schematically illustrated in Figure 1.

Schematic illustration of melt spinning process
Figure 1: Schematic illustration of melt spinning process

Melt spinning has the advantage of requiring no solvents and uses the polymer as is. Polymer melts are typically highly viscous and generate a phenomenon known as exudate swell on leaving the spinneret. Exudate swell is caused by an essentially elastic material recovering from a temporary compression as it exits the orifice. The practical implications of exudate swell are that the filaments will always deviate from the cross-sectional shape
of a complex orifice, meaning that overly complex fiber geometries may not be possible with melt spinning.

Wet spinning:
Solution spinning is typically used when melt spinning is not possible for non-thermoplastic and temperature-sensitive polymers. In this processing arrangement, the polymeric chains are dissolved in an appropriate solvent to form a viscous fluid. Typical solution concentrations can vary from 1%–25% depending on the polymer chain length, solvent system, and spin pack design. Once dissolved in solution, the chains are typically free to entangle and disentangle and move relative to each other. There are typically three variants of wet spinning, with the basic outline for each given in Figure. In all variants, a polymeric solution is pumped through a spinneret and filaments form through either evaporation or precipitation. In dry–wet spinning, the solvent is volatile enough to evaporate rapidly, leaving behind a gradually solidifying filament with only a small amount of residual solvent. In air-gap and coagulation spinning, the spinneret is submerged or suspended just above a spinning bath and the solvent is precipitated out of the filament using a coagulant or nonsolvent system. The filaments then harden and undergo several washing and drying steps before final winding. As the rate of diffusion of coagulant and solvent is critical, this variant of wet spinning is typically significantly slower than melt spinning.

Schematic diagram of wet spinning
Figure 2: Schematic diagram of wet spinning

The spinning of continuous filaments also allows for additives to be incorporated into the fibers at the time of formation. For example, spun-dyed fibers are created via the batchwise addition of colorants. This saves on any requirement to dye the fibers and is necessary for difficult to dye fibers such as polypropylene. However, the range of colors and minimum run quantity is often more limited.

An additional point on synthetic fibers is that the manufacturing process often locks in tension and strains within the fibers on a molecular level. When these fibers are subsequently heated for dyeing, bonding, or finishing, the fibers can contract as the strain is relaxed, which causes the yarn to shrink and the fabric to shrink in one if not two directions. This residual shrinkage is often removed through a finishing process known as heat setting. Here, fabrics are washed and allowed to shrink in a controlled manner through high-temperature ovens to remove as much as 20% shrinkage. This is a costly process, but the resulting fabric should be thermally stable in subsequent steps.

Dry spinning:
The polymer solution passes through a metered pump. Once through the spinneret, the filaments pass through warm air, which evaporates the solvent and dries the filament. The filament is then drawn and wound onto bobbins. Dry spinning is used to form polymeric fibers from solution. It is a direct process. Here a solvent and a solvent recovery plant are required. Washing is not done in this process. This process may be used for the production of Acetate, Tri-acetate, Acrylic, Modacrylic, PBI, Spandax and Vinyan.

Schematic diagram of dry spinning process
Figure 3: Schematic diagram of dry spinning process

Gel spinning:
Gel spinning is also known as dry-wet spinning because the filaments are cooled by passing through cold air first and then into a cooling liquid bath. This is a special process used to achieve high strength or special fiber properties. The polymer starts in a partially liquid or gel state, unlike the other three processes which causes the polymer chains to be bound together at intervals in liquid crystal form, which results in very strong, inter-chain forces. The polymer chains in the fibers have a high degree of orientation, which vastly increases their tensile strength. This process is used on aramid fiber and polyethylene.

Schematic diagram of gel spinning
Figure 4: Schematic diagram of gel spinning


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