What is Wet Spinning | Principle and Uses of Wet Spinning Method

What is Wet Spinning?
Wet spinning is a fiber-forming process in which the polymer is dissolved in a solvent and the solution is extruded into a chemical bath. It is a traditional technique to develop polymer-based fibers. The wet spinning method was first used in the manufacture of rayon (Chardonnet Silk) by extruding the alcoholic solution of the cellulose nitrate through a mouthpiece dipped in cold water.

In wet spinning technique, a polymer-based solution is run through a syringe using a syringe pump into a coagulation or cross-linking solution to form continuous fibers. The coagulation or cross-linking solution is comprised of a non-solvent and/or a poor solvent depending on the polymer characteristics. Using this technique, fibrous scaffolds with relatively large pore size (~250–500 μm) and fairly thick fibers (30–600 μm) with various morphologies could be fabricated. The fiber’s diameter could be varied by varying the flow rate of solution or needle size. Wet spinning technique could be applied for the development of fibers from a wide range of polymers, including natural polymers such as alginate, collagen, chitosan, silk and gelatin and synthetic polymers such as poly-ε-caprolactone (PCL) and PEG. This technique is more useful when the cross-linking process of prepolymer solution is fast. For instance, alginate with rapid cross-linking process by calcium chloride (CaCl2) is the most applied polymer in wet spinning process. Since the prepolymer solutions as well as the coagulation bath are often compatible with cells, this technique could be applied to encapsulate live cells and provide 3D scaffold for cells. For instance, Arumuganathar and Jayasinghe developed cellladen multi-compositional fibrous structures using a three-needle pressure assisted spinning approach in which living cells were spread in the inner layer.

The earliest man-made fiber was proceed using the wet spinning method. Through this process, fiber is produced by spinning liquids in a coagulating or regenerating bath. Solution spinning techniques require dissolving of polymer in a solvent without degrading the polymer. The wet spinning process involves filament extrusion into a nonsolvent. Since it is a gentle process, applying lower temperatures, it has been the preferred method for production of fibers that cannot be melt spun. The wet spinning process offers the advantage of producing a wide variety of fiber cross-sectional shapes and sizes. Wet spinning imparts a round or lima bean shape to some fibers. Higher production speeds have been successfully attained in wet spinning over the last few years.

In wet spinning, the polymer is dissolved in solvent, extruded into a coagulating bath, dried, crimped, and collected as tow for use in the high-bulk process or cut into staple and baled. Among the various fiber manufacturing processes available, wet spinning has the potential to convert biomolecules into fibers without need for high voltage during manufacture and is less likely to be associated with denaturation. Wet spinning is conventional methods to produce man made fibers such as viscose rayon. This fiber spinning technology is based on nonsolvent-induced phase separation, whereby polymer dope solutions are extruded through a spinneret into a nonsolvent coagulation bath in which it continuously polymerizes so as to form a solid long fiber.

Equipment Required for Wet Spinning:
The equipment required for wet spinning consists of a supply tank for the spinning solution, pump, filter, spinneret, and coagulating bath. Viscose rayon is the fiber that is spun in greatest quantity using this method. High tenacity rayons and polynosic (cotton-like) rayons are produced in a similar way, with different formulae for the composition of the coagulating bath and varying stretching techniques. Calcium alginate and polyvinyl chloride are other fibers in this category. Most acrylic fiber such as acrilan and courtelle are wet spun.

components of wet spinning
Figure 1: Components of wet spinning

Principle of 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.

In the wet spinning process, the filament is passed through a bath in which the solvent is washed out of the fibrous material. The solvent trapped in the coagulated filaments is first removed and then the filaments stretched to many times their original length at temperatures of 140 and 180°C. This causes orientation of the crystalline areas of the fiber and improves the strength properties of the fiber markedly. Wet spinning differs from dry spinning primarily in the way solvent is removed from the extruded filaments. The solvent is the same as the dope solvent and the nonsolvent is usually water. Filament fusion is less of a problem in wet spinning, and so the number of capillaries in wet spinning spinnerets is much larger than in dry spinning. The spinnerets in commercial processes may have anywhere from 3000 to 100,000+ capillaries, which may range in diameter from 0.05 to 0.25 mm; it is common to use multiple spinnerets in a single spinbath. The critical part of this process is the transition from a liquid to a solid phase within the filaments. Precipitation is favored when the solvent is organic and the nonsolvent is water, as the solubility of polymer decreases abruptly with water concentrations of only a few per cent.

The fiber emerging from the spin bath is a highly swollen gel containing both solvent and nonsolvent from the spin bath. The fibers are essentially unrented except at the fiber skin. The microstructure consists of a febrile network. The spaces between fibrils are called micro voids. Depending on the conditions of coagulation, the filaments may also contain large voids radiating out from the center of the fiber. The best combination of tensile properties, abrasion resistance and fatigue life is realized when the coagulated fiber has a homogeneous, dense structure with small fibrils and no macro voids. fiber cross-sectional shape is determined by the coagulation conditions.

After the spin-bath or spin-tower step, the tow processing is similar for both wet and dry spun yarns. Wet-spun tows, however, may contain 100– 300% of solvent/no solvent. Therefore, the initial washing steps differ in their details. The key wet-spinning steps are washing, stretching, finish application, collapse, drying, crimping and relaxing. The washing step consists of several countercurrent stages, with the effluent being recycled to a solvent recovery process. The washing step may be followed by additional stretching.

Organic solvent wet spinning process
Figure 2: Organic solvent wet spinning process

The porous febrile structure of wet-spun fibers increases in density with stretching. After the fiber is washed, stretched and optionally dyed, finish may be applied using a bath or similar device. If drying is accomplished on heated rolls (see Figure 2), a prettying finish is required to prevent fiber fusion. In other processes (see Figure 3), finish application may be postponed until the fiber is dried and collapsed. After drying/collapsing, the tow is relaxed. Relaxation is essential because it reduces the tendency for fibrillation and increases the dimensional stability of the fiber. fiber shrinkage during relaxation ranges from 10% to 40% depending on the temperature, the polymer composition and the amount of prior orientation. fiber crimping using a stuffer box device may be done before in-line relaxation or before autoclaving. The relaxation process tends to ‘set’ the crimp. Process speeds for wet spinning vary from 55 to 260 m/min. The limitations are the speed at which the fiber can move through the spin bath without filament breakage and the equipment line length required to complete the washing and drying processes. A single machine may have up to 48 spinnerets (six rows of eight) with a total productivity of 50 ton/day.

NaSCN salt wet spinning process
Figure 3: NaSCN salt wet spinning process

In wet spinning the spinning compound is spun out into a bath of chemicals (Figure 4). The solvent of the spinning compound (such as dimethylformamide) is neutralized by chemicals in the bath, which results in the solidification of the fibers. In this case, a costly regeneration of the solvent is mandatory. Polyacrylonitrile is produced according to this principle.

Principle of wet spinning
Figure 4: Principle of wet spinning

Viscose and cupro are also spun by means of the wet-spinning method. In both cases, an intermediate product (derivate) is spun that is transformed into regenerated cellulose in the bath.

The spinnerets are made of various materials depending on the spinning solutions. Precious metal alloys of gold, platinum, iridium, as well as rhodium, tantalum, and glass are used. The shape and size of the boreholes differ. For the production of viscose fibers, spinnerets have 30,000 to 90,000 capillaries. Acrylic fibers are spun according to the wet-spinning method with 40,000 to 150,000 holes per spinneret.

Uses of Wet Spinning:
Wet spinning is the most often used to produce rayon. Wet spinning is also used to produce lyocell. Besides, acrylic, aramid, modacrylic and spandex can be produced by this process. Alginate fibers are also produced by the wet spinning method.


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Author of this Article:
Mustaque Ahammed Mamun
Department of Textile Engineering
Dhaka University of Engineering & Technology (DUET)
Email: mamuntex09@gmail.com

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