Viscose Rayon


Viscose is a viscous organic liquid used to make rayon and cellophane. Viscose is becoming synonymous with rayon, a soft material commonly used in shirt, coats, jackets, and other outer wear.

Viscose fibres are made from regenerated cellulose. The process of dissolving pulp (the very viscose solution of pulp gave the name to the fibre product) was an achievement of the industrial revolution of the 19th century. And this innovation triggered the discovery of full synthetic fibres during the 20th century. Many other cellulose dissolving and regeneration processes like cuprammonium rayon, which was the first process made available for manmade fibres and was already developed in the 1850s, or cellulose derivatives (like acetate) were in competition with the viscose process, but viscose proved to be superior based on process and product performance. Its booming period ended after World War 2 with the introduction of the synthetic competitive products. In the last decade, the production has stabilised at approximately 2.7 million tonnes worldwide (Europe
600000 tonnes).

It is preferably supplied to market end-uses (textile products and nonwovens) where the hydrophilic (moisture absorbing) properties of the material, for instance in direct contact with the skin or with mucous membranes, are relevant.

Currently (2005), about 85 % of the total viscose fibre production is produced as staple fibres and about 15 % as filaments.

It has to be mentioned that a substantial amount of regenerated cellulose in the form of films (cellophane) based on the viscose process are still applied in sausage encasings and other packaging foils.

In recent history in Europe, textile viscose filament end-uses are receiving increased competition (resulting in phasing out of capacity) by cheaper competitive yarns based on polyester and polyamide, whereas viscose staple fibre and viscose tyre cord keep a strong position.

Viscose currently is becoming less common also because of the polluting effects of carbon disulphide and other by-products of the process, forcing some factories to close.

  • Processes (staple fibres and filament yarn)

When producing viscose fibres, the pulp – which is mainly cellulose from wood – is dissolved and subsequently precipitated under controlled conditions. The most important process worldwide is the so-called ‘viscose process’ where the alkaline pulp is treated with carbon disulphide (CS2) and dissolved by adding sodium hydroxide solution. A viscous orange-brown solution called ‘viscose’ is formed which is ripened, degassed and then pressed through spinnerets into a highly acidic spinning bath. Here, the cellulose precipitates when CS2 and the by-product H2S is released. After this, the cellulose is stretched, washed and then undergoes further processing.

At this point, a distinction has to be made between staple fibres and filament yarn:

• Staple fibres are cut into short pieces after the spinning bath. These short fibres, which are each approximately 4 cm long, are spun into textile yarns or processed into ‘non-woven’ products later on.
• In contrast, filament yarns are spun into endless fibres which can be used immediately.

Viscose products for textile usage with certain improved product characteristics are called ‘modal fibres’.

Regular rayon (or viscose) is the most widely produced form of rayon. This method of rayon production has been utilized since the early 1900s and it has the ability to produce either filament or staple fibers. The process is as follows:

image

Figure 1.  gives a schematic view of both staple fibre and filament yarn production.

Cellulose: Production begins with processed cellulose

Immersion: The cellulose is dissolved in caustic soda: (C6H10O5)n + nNaOH —> (C6H9O4ONa)n + nH2O

Pressing: The solution is then pressed between rollers to remove excess liquid

White Crumb: The pressed sheets are crumbled or shredded to produce what is known as “white crumb”

Aging: The “white crumb” aged through exposure to oxygen

Xanthation: The aged “white crumb” is mixed with carbon disulfide in a process known as Xanthation, the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulfide under controlled temperature (20 to 30°C) to form cellulose xanthate: (C6H9O4ONa)n + nCS2 —> (C6H9O4O-SC-SNa)n

Yellow Crumb: Xanthation changes the chemical makeup of the cellulose mixture and the resulting product is now called “yellow crumb”

Viscose: The “yellow crumb” is dissolved in a caustic solution to form viscose

Ripening: The viscose is set to stand for a period of time, allowing it to ripen:

(C6H9O4O-SC-SNa)n + nH2O —> (C6H10O5)n + nCS2 + nNaOH

Filtering: After ripening, the viscose is filtered to remove any undissolved particles

Degassing: Any bubbles of air are pressed from the viscose in a degassing process

Extruding: The viscose solution is extruded through a spinneret, which resembles a shower head with many small holes

Acid Bath: As the viscose exits the spinneret, it lands in a bath of sulfuric acid, resulting in the formation of rayon filaments:

(C6H9O4O-SC-SNa)n + ½nH2SO4 —> (C6H10O5)n + nCS2 + ½nNa2SO4

Drawing: The rayon filaments are stretched, known as drawing, to straighten out the fibers

Washing: The fibers are then washed to remove any residual chemicals

The basic process ends here. Depending on the desired product it is continued optional by

Cutting: The filaments are cut down when producing staple fibers

Spinning: Filament yarns are spun into endless fibres Figure

  • Production of filament yarns

Until the spinning step, the process is very similar to the production of staple fibres

Long fibred pulp is used as the raw material. For the first step, it is treated with diluted sodium hydroxide solution (approximately 15 %). Afterwards, the liquid is removed by pressing and then it is recycled back into the process together with fresh NaOH. Next, the pulp sheets are defibrated, pre-ripened and put into CS2 for chemical conversion to xanthate. After the addition of aqueous NaOH, the viscose emerges which is ripened and degassed in vacuum prior to spinning.

Depending on the quality of the fibres, the spinnerets have different numbers of holes ranging from 30 to more than 2000. The spinning bath is sulphur acidic and contains high concentrations of sodium sulphate (Na2SO4) and zinc sulphate (ZnSO4).

Three different spinning methods are used:

  1. pot spinning – the viscose is pressed directly into the spinning bath. This is possible for threads from the size of 67 to 1330 dtex*
  2. continuous spinning – the viscose is pressed through the spinneret into a spinning tube where the flowing spinning bath picks up the coagulating fibre. This is again possible for threads from the size of 67 to 1330 dtex
  3. bobbin spinning – this process is similar to continuous spinning, but the fibre is fully coagulated. In order to achieve this, it is let into a second spinning bath where the coagulation is finished. This technique is possible for threads from the size of 1220 to 2440 dtex.

After spinning, the fibres are washed, finished, dried and spooled.
Currently, there are installations with integrated as well as batch washing

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