Preparation of Viscose Rayon

Rayon fiber made from "viscose process" is called Viscose Rayon. Viscose Rayon is the oldest commercial manmade fiber. Viscose Rayon is a manufactured fiber composed of regenerated cellulose. Out of all the fibers produced, rayon is the most perplexing to consumers. It can be found in cotton-like end uses as well as silk-like end uses. It may function successfully in absorbent hygiene and incontinence pads and equally well providing strength in tire cords. Unlike most man-made fibers, rayon is not synthetic. It is generally made from wood pulp which is a cellulose-based raw material. Hence, properties of rayon are more like natural cellulosic fibers, such as lint or cotton, than those of synthetic fibers.

Viscose process was invented by English chemists, Charles Frederick Cross and Edward John Bevan, and their collaborator Clayton Beadle in 1894. They patented their artificial silk the same year and they named it "viscose". The first commercial viscose rayon was produced by the UK Company, Courtaulds Fibers in 1905.
Generally, in the manufacture of rayon, cellulose derived from wood pulp, cotton linters, or other vegetable matter is dissolved into a viscose spinning solution. The solution is extruded into an acid-salt coagulating bath and drawn into continuous filaments.

Methods of Preparation of Rayon

There are four big methods by which rayon can be produced. Out of these four processes, two are now obsolete and two are still practiced. "Viscose Process" is used on large scale as it is very inexpensive.

1. Nitrocellulose Process
2. Cuprammonium Method
3. Acetate Method
4. Viscose Method

Nitrocellulose Process

• The solubility of nitrocellulose in organic solvents, such as ether and acetone, made it possible for Georges Audemars to develop the first "artificial silk" in about 1855.
• Gorges' method was impractical for commercial use.
• Commercial production started in 1891, but the fiber produced was flammable and more expensive than acetate or cuprammonium rayon.
• Its production ended before World War I because of its flammability and expensiveness.
• It was briefly known as "mother-in-law silk."
• This method is now obsolete.

Acetate Method

• Paul Schützenberger discovered that cellulose could be reacted with acetic anhydride to form cellulose acetate which on saponification can give rayon.
• Initially, the cellulose triacetate formed by acetylation during this process was only soluble in chloroform making the method expensive.
• The discovery that hydrolyzed cellulose acetate is soluble in more polar solvents, like acetone, made production of cellulose acetate fibers cheap and efficient.
• This method is now obsolete.

Cuprammonium Method

• The Swiss chemist Matthias Eduard Schweizer discovered that tetraaminecopper dihydroxide could dissolve cellulose.
• Production of cuprammonium rayon for textiles started in 1899 in the Vereinigte Glanzstoff Fabriken AG in Oberbruch.
• It is produced by a solution of cellulosic material in cuprammonium hydroxide solution at low temperature in a nitrogen atmosphere, followed by extruding through a spinnerette into a sulphuric acid solution necessary to decompose cuprammonium complex to cellulose.
• This is a more expensive process than that of viscose rayon.
• It is used on low scale.

Viscose Process

It is the worldwide preparation process of rayon. Rayon made from as a result of this process is called viscose rayon. It is as follows:
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" is 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.
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.
Cutting: If filament fibers are desired the process ends here. The filaments are cut down when producing staple fibers.