Viscose rayon is a regenerated cellulosic fiber and cellulose is the raw material for producing this man made fiber. The raw material is obtained from a special variety of wood called spruce. It is the oldest commercial manmade fiber. The name viscose was derived from the word “viscose” which describe the liquid state of the spinning sun. A viscose solution is as thick as honey. Flow Chart of Viscose Rayon Manufacturing Process is as follows:
Flow Chart of Viscose Rayon Manufacturing Process:
Preparation of wood pulp
Steeping & Pressing
(Formation of soda cellulose &excess alkali is pressed out)
(In two or three hours formation crumbs)
(In atmospheric oxygen D.P fall of 800-350)
(Xanthation or Sulphidising)
Preparation of Wood Pulp:
The first step in the manufacturing of Viscose Rayon is the preparation of wood pulp. Spruce trees are cut into timber. These pieces are treated with a solution of calcium bisulfite for 8-12 hrs and cooked with steam under pressure for about 14 hours.
The cellulosic component of the wood is unaffected by this treatment. But cementing material called lignin, which is present in the wood. It is converted into its sulfonated 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. Finally converted into paper boards or sheets. This is called the wood pulp.
Steeping & Pressing:
Wood pulp sheets are treated with 17-20% aqueous caustic soda at an 18-25°C. It is called the “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 color. 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”.
The pressed soda cellulose is shredded mechanically to yield finely divided. The fluffy particles is called “crumbs”. This step provides an 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.
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 the 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”.
Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration. After aging, the crumbs of soda cellulose are transferred to a 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 disulfide 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 rpm.
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.
Viscose solution requires ripening to give a solution having the 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.
In the spinning process, the 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. Then it goes for drying.
Athor of this Article: Rana Sohel Executive Engineer Sunman Textile Mills Ltd. Chittagong Cell: +8801912-420118 Email: [email protected]