Chemical spinning | Describe different types of the chemical spinning

 

Chemical spinning

Chemical spinning

Chemical spinning processes are commonly used to make man-made filament fiber yarn. A smooth solution is extruded through a nozzle-like die called a spinneret. Man-made fibers are used to make yarn by chemical spinning.

Types of the chemical spinning process

There are three types of chemical spinning processes as below-

Melt spinning

Melt spinning is applicable to polymers that are capable of melting at advanced temperatures. This is the simplest and cheapest process, as it involves only melting, extrusion, and cooling. The polymer is melted, and extruded through a spinnerette hole which is a perforated metal plate of small diameter. The length of each hole can be 0.5 mm or less. Each spinnerette can have 1 to 100 holes depending on the type of yarn or picked product. A tow is a rope of fibers collected from various spinnerette that is used to make the staple fibers for blending.

In the case of making melt-spun filaments, the polymers in the form of chips are fed through a hopper and it falls into a hot grid where it melts. Molten polymers are collected and filtered so that suspended impurities block the flow of the polymer and a vacuum is also used to remove air bubbles, so as to ensure a continuous flow of the polymer liquid that produces continuous filament.

The polymer current collected in a tank enters a cooling chamber through the pores of the spinnerette which helps in the solidification of the filament. The fibers produced by molten extrusion are polyamide, polyester, and polypropylene.

The temperature used for molten extrusion is usually 30°C above the melting point of the polymer so that the viscosity of the fiber is low enough for extrusion. The viscosity of molten polymers is also a measure of the length of molecular chains. The higher the viscosity, the higher the polymer chain.

The polymer chains in the filaments produced in this way are arranged randomly. To improve its orientation, cool the filaments. For the convenience of cold drawing, the filaments are pulled between two sets of rollers which are operated at successively higher speeds. The second set rotates at least 4 times the speed of the first set so that the drawing of the filament occurs. The randomly arranged molecular chains face the fiber axis and the filament becomes finer and stronger. It also helps in the formation of crystals as the polymer chains come closer.

Solvent/ dry spinning

When a polymer cannot melt into a liquid by melting, an alternative method is employed in which the polymer dissolves in a suitable solvent. If the solvent is unstable, when the solvent dries it leaves the polymer in the desired shape. Hence this method is called dry spinning.

The polymer dissolves up to 25% in the solvent and is converted into a polymer solution. The solution is sometimes heated to ensure adequate flow of polymer between the tiny holes in the spinnerette. At this stage, certain additives like-colored pigments can be introduced. Delustering agents such as titanium dioxide can also be added to the solution to form a dull filament. The polymer solution is filtered, and the suspended particles are removed and treated with a vacuum that blocks the spinnerette holes and air bubbles that prevent the formation of uninterrupted filaments.

The solution from the storage tank is carried through tiny holes in the spinnerette to the heated air chamber where hot air or nitrogen is conducted. The heated air evaporates the solvent from the polymer and carries it to another chamber where the solvent is recovered. After spinning, the filaments need to be washed.

This process is considered expensive because it requires huge investments for solvent recovery plants. About 10% of the solvent is lost during this process, a portion of which may be present in this filament. Used solvents can be expensive, toxic, and flammable. Typically, 3 to 6 kg of solvent per kg of polymer is used. However, this process is considered to be fast.

Wet spinning

Wet spinning is applicable to polymers that cannot be dissolved by the volatile solvent in a liquid, it can change from solid to liquid. A non-volatile solvent is used to change it to a liquid state. The solvent cannot evaporate, it is made to pass through a chemical bath, where the filament leaves the solvent, hence the name wet spinning.

Wet spinning is a slow process (30-80 m / min) of regrowth by filament freezing. Since freezing occurs in the place of any chemical baths, spinnerettes should be made with a corrosion-resistant precious metal alloy, which again requires a huge investment. The holes used are much smaller than the other two methods. If the filament turns into a tow, there are about 20,000 holes present.

The polymer solution is filtered to be treated in a vacuum and then collected in tanks to keep it free from suspended particles and air bubbles. Additives such as pigments and confusing agents can be added. It is then mixed in a chemical bath through a spinnerette that contains a dilute acid and accumulates in the form of a filament. The filaments are washed and pulled to the desired diameter.

The fibers that are produced using this method include viscose rayon and polyacrylonitrile. The cross-sectional shapes of the filaments are often granular due to a chemical reaction that leads to the coagulation of the filament.

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