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Manufacturing of Viscose Rayon: Artificial Silk

The viscose rayon is regenerated cellulosic fibre and cellulose is the raw material used for its production. Viscose rayon fibres are also called as artificial silk.

Introduction

The viscose rayon is regenerated cellulosic fibre and cellulose is the raw material used for its production. Viscose rayon fibres are also called as artificial silk. The required raw material is obtained from trees (spruce trees). Wood contains lignin besides cellulose. Hence to remove this impurity purification of wood is done. After purification cellulose is treated with caustic soda to convert it into soda cellulose. This further treated with carbon disulphide to produce sodium cellulose xanthate, and then dissolved in a dilute solution of caustic soda. This solution is forced through a spinning unit (Wet spinning) to get fibres. In the process of conversion of wood cellulose to the fibres cellulose undergoes different processing steps. These steps are as shown in fig. 1.

Fig.1 Manufacturing Process of Viscose
Detailed discussion regarding manufacturing process of viscose rayon fibre has been carried out in following points.

Manufacturing Process

Purification of Cellulose

Trees are cut into timber and transported to factories. In factories, their barks are removed and cut into pieces. These pieces are cooked with a solution of calcium bisulphate [Ca (HSO3)2] under pressure for 14 Hrs.
The lignin material present in the wood is converted into sulphonated compound. This sulphonated compound is soluble in water. After cooking wood with calciurn bisulphate large amount of water is added. When cellulose pulp floats on water it is then sucked. Then this sucked slurry of cellulose is treated with bleaching agent. Which is finally converted into paperboards or sheets.

Conditioning

Cellulose sheets/paperboards are kept in a room at a definite temperature and humidity for a sufficient time. When they absorb a definite amount of moisture from the air by storing the sheets under controlled conditions it is possible to take a known weight of cellulose for manufacturing of viscose rayon.

Steeping and Pressing

In this process, white sheets of board containing 90-94% pure cellulose are soaked in a 17.5 % caustic soda for one to four hours. This process is known as steeping or mercerizing. The boards become greatly swollen and the hemicellulose dissolves in the caustic soda. Here cellulose is simultaneously converted into soda cellulose. The excess caustic soda liquor along with dissolved hemi cellulose is removed by pressing in hydraulic press. This then transferred to shredding machine for the next operation.
Fig.2 Steeping

Shredding

The soda cellulose is mechanically shredded to increase surface area and make the cellulose easier to process. This shredded cellulose is often referred to as “white crumb”.

Ageing Process

After shredding, the crumbs are aged. Ageing is done to improve the spinning qualities of the solutions. The crumbs delivered by shredding process are stored in a galvanized vessel with a lid for about 48 hours at 230C. Through the oxidation with atmospheric oxygen, degradative changes set in. Some depolymerisation occurs, and degree of polymerization falls from 1000 to 300 (350 in some cases) in this process.
Fig. 3 Viscose Rayon Manufacturing Process

Churning/Xanthation

This process is also called as Xanthation or Sulpholising. After ageing process crumbs of-soda cellulose are transferred to rotating airtight hexagonal churners (mixers). Carbon disulphide is added to the mixer and churned together for 3Hrs. by rotating the mixer at a slow speed i.e. 2rpm. In this process soda cellulose xanthate is formed and color of the product changes from white to light yellow to deep blue and then to reddish orange. After churning is over vacuum is applied to remove the ill smelling vapours of carbon disulphide from the mixer.

Fig.4 Xanthation 

Dissolving/Mixing

In the mixers the sodium cellulose xanthate is stirred with dilute caustic soda solution for four to five hours, the vessel being cooled. The xanthate dissolves to a clear-brown, viscous liquid similar in appearance to honey.
Sodium cellulose Xanthate is taken in a mixer having stirrer. Dilute solution of caustic soda is added and the contents are stirred for 4-5 hrs. The soda cellulose Xanthate dissolves to give a clear brown thick liquor, which is called as viscose. Filtration is also done to remove undissolved portion from the wood pulp. During stirring air gets entrapped in the solution. Hence degassing is also carried out to remove entrapped air.
Fig. 5 Dissolving/Mixing

Ripening

Viscose solutions require to be ripened to give a solution having the best spinning qualities. The viscose solution is stored for 2 to 5 days at 10-18oC and during storage it ripens. In this period coagulation of sediment occurs. Viscosity of the
solution initially decreases and then rises to get original value. Ripened solution is again filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament. 

Spinning

In spinning process ripened viscose solution is forced by compressed air to spinning frame. From spinning frame it is distributed to different spinning units. And spinning solutions is discharged from nozzles to a spinning bath. Spinning bath contains chemicals which are represented in Table 1.
Table 1 Spinning Bath
Chemical
Amount (%)
Function
H2SO4
8-10
Xanthate to celllose glucose: Softness and Pliability
Na2SO4
16-24
Coagulates/ Precipitates SCX
ZnSO4
1-2
Additional Strength
Water
Remaining

 The spinning solution is kept in a lead tank at 40-450C sodium sulphate coagulates or precipitates the dissolved sodium cellulose Xanthate while Sulphuric acid converts Xanthate into cellulose glucose and gives softness and pliability to the filament. Zinc & Sulphate give additional strength. As a number of filaments emerge from the spinnerate they are taken together to an eye at the surface of the spinning bath and then guided to two rollers. From rollers, they are wound on a spindle to form viscose rayon. Finally washing is done.

Sequence of the reaction in the viscose process is given below:




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