Manufacturing of Viscose Rayon – Viscose Process

Viscose Process

It is the worldwide manufacturing process of "Viscose Rayon".

Raw Materials

• Wood Pulp
• Cotton Waste
• Cotton Linters

Viscose Process is as follows:

Preparation of Wood Pulp:

• The first step in the manufacturing of Viscose Rayon is preparation of wood pulp. Spruce trees are cut into timber.
• Their barks are removed and cut into pieces of about 7/8" x 1/2" x 1/4".
• These pieces are treated with a solution of calcium bisulphite [CS2] for 8-12 hours and cooked with steam under pressure for about 14 hours.
• The cellulosic component of the wood is unaffected by this treatment, but the cementing material called lignin, which is present in the wood, is converted into its sulphonated compound which is soluble in water.
• Sulphonated compound is washed off, hence purifying the remaining cellulose.
• After this it is treated with sodium hypochlorite solution and finally converted into paper boards or sheets. This is called wood pulp, which is normally purchased by the manufacturers of viscose rayon.

Steeping & Pressing:

• The wood pulp sheets are treated with 17-20% aqueous caustic soda at a temperature 18-25°C. It is called "Mercerizing or Steeping Process".
• The high DP cellulose (1000) is converted into soda cellulose. The sheets are allowed to soak until they become dark brown in colour. This takes about 1-4 hours.
• The excess caustic soda solution is drained off and sheets are pressed out by a hydraulic press to squeeze out excess caustic soda solution. This process is known as "Pressing".
• 100 kg of sulphite pulp gives about 310 kg of moist soda cellulose.

Recovery of NaOH:

• Pure caustic soda is recovered from the moist soda cellulose formed in the previous step by allowing diffusion of moist soda cellulose through paper membranes known as "parchment".
• Recovery of caustic soda and reuse of caustic soda makes it an economically viable process.
• To reuse the used caustic soda, it is concentrated and mixed with fresh caustic soda.
• When reused caustic soda is exhausted and can no longer give required result, it is sold to soap making industries and is used in soap manufacturing process.

Shredding:

• The pressed soda cellulose is shredded mechanically to yield finely divided, fluffy particles called "crumbs". This step provides increased surface area of the soda cellulose, thereby increasing its ability to react in the steps that follow.
• The wet, soft sheets of soda cellulose are passed through a shredding machine which cuts them into small bits.
• The sheets are broken into fine crumbs in 2-3 hours.

Aging:

• The soda cellulose is aged under controlled conditions of time and temperature (between 18 and 30°C) in order to depolymerize the cellulose to the desired degree of polymerization to obtain almost ideal solution of cellulose.
• The soda cellulose is stored in small galvanized drums for about 48 hours at 28°C. This process is called "Ageing".
• In this step, the average molecular weight of the original pulp is reduced by a factor of 2-3.
• Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration.
• During this process, the Degree of Polymerization of soda cellulose is decreased from 800 to about 350.

Xanthation:

• After ageing, the crumbs of soda cellulose are transferred to rotating, air tight, hexagonal drum called "Churner".
• In this step, the aged soda cellulose crumbs are placed in rotating drums and are allowed to react with carbon disulphide under controlled temperature to form sodium cellulose xanthate.
• Carbon disulphide about 10% of the weight of the crumbs is added to the churner and churned together for 3 hours by rotating the mixers at a slow speed of 2 revolutions per minute [rpm].
• Sodium cellulose xanthate is formed during this process and the colors of the product changes from white to reddish orange.
• Side reactions that occur along with the conversion of soda cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and the resulting viscose solution.

Dissolving:

• Sodium cellulose xanthate is in the form of small balls. This falls into a mixer called dissolver which is provided with a stirrer.
• A dilute solution of caustic soda is added, and the contents are stirred for 4-5 hours and at the same time, the dissolver is cooled.
• The sodium cellulose xanthate dissolves to give clear brown thick liquor, similar to honey. This is called 'viscose' and it contains about 6.5% caustic soda and 7.5% cellulose.

Ripening:

• Viscose solution requires ripening to give a solution having best spinning qualities.
• Two important processes occur during ripening: Redistribution and loss of xanthate groups.
• Ripening is carried by storing the viscose solution for 4-5 days at 10-18°C.
• The viscosity of the solution first decreases and then rises to its original value.
• The ripened solution is filtered carefully and is now ready for spinning to produce viscose rayon filaments.

Spinning:

• In spinning process, ripened viscose solution first goes through a centrifugal pump due to pressure exerted on the solution by compressed air.
• Then it is delivered to a "Filtering Media". From there, this solution goes into the "Glass Tubing" which has a spinneret.
• The viscose solution is forced through a spinneret, having many fine holes with diameters ranging from 0.05-0.1mm.
• The spinneret is submerged into a solution containing the following chemicals:
  • Water [H2O] = 69%
  • Sodium Sulphate [Na2SO4] = 18%
  • Sulphuric Acid [H2SO4] = 10%
  • Glucose [C6H12O6] = 2%
  • Zinc Sulphate [ZnSO4] = 1%
• The spinning solution is kept at 40-45°C.
• Sodium sulphate precipitates the dissolved sodium cellulose xanthate.
• Sulphuric acid converts xanthate into cellulose, carbon disulphide and sodium sulphate.
• Glucose gives softness and pliability to the filaments whereas zinc sulphate gives added strength.
• As soon as a number of filaments emerge from the spinneret, they are taken together to surface of the spinning bath and then guided to two rollers from where they are wound on to a spindle.