Viscose rayon is the regenerated cellulosic fiber obtained from the wood pulp.
Flow diagram of production of viscose rayon:
Purification of cellulose:
Cellulose is derived from wood pulp. So, the first step is that trees are cut into timber and barks are removed which are measuring 7/8 inch×1/2 inch×1/4 inch. The pieces of wood should be treated with calcium bisulphite and cooked with steam under pressure up to 14 hrs. By this process the lignin present in the wood turn into soluble form and can be washed off. After this process the cellulose is converted into sheets. The sheets are cut to the required dimension and conditioned at 30 ͦc to had the desired moisture content.
Steeping means to soak in water or other liquid to extract its impurities. The wood pulp sheets which contains 87 to 98% cellulose is steeped with 17.5 to 18% NaOH for 30 to 60 min, preferably a temperature of 18 ͦ c. It removes hemicelluloses and resinous impurities from the wood pulp and alkali cellulose which is also known as soda cellulose is generated in this operation.
The alkali cellulose sheets are converted into fluffy crumbs by this process called shedding and it is accomplished through rotating serrated blades for 1 to 3 hrs. at 25 to 35 ͦ c.
This is an oxidative depolymerization step and needs to be carefully controlled to obtain the desired degree of depolymerization. The crumbs which are obtained in shredding process are stored in rectangular galvanized steel containers for 1-3 days at a temperature of 25 to 30ͦ c. and. The steel containers must be covered with lids as oxygen available from the air within the fluffy crumbs is sufficient and exposure to outside air leads to formation of carbonates which affects xanthation. This process is done to reduce the average molecular weight of cellulose and to get right viscosity and cellulose concentration.
The alkali cellulose crumbs obtained at the ageing process are transferred into hermetically sealed churns where carbon disulphide is introduced in a weight proportion of 32 to 35% of the weight of cellulose in alkali cellulose. The churn is rotated slowly at 1 to 4 revolutions/min for a period of 1 to 3 hrs. at a temperature of 20 to 30ͦ c. The alkali cellulose is reacted with carbon disulphide and forms sodium cellulose xanthate.
During the process of dissolution, the cellulose xanthate is transferred to cylindrical vessels equipped with agitators and can react with 4 to 6% solution of NaOH. And during this process a considerable amount of air is whipped into solution and which is removed by evacuation process.
Filtration is done by using filter cloth and other filter aids to remove undissolved cellulose particles and impurities.
To get homogenized solution and to remove batch t batch variations blending can be done as the solution comes from several dissolving tanks.
Ripening is nothing but storing of viscose solution in a large tank for 1 to 3 days at a temperature of 15 to 25 ͦc. In this process the viscosity of xanthate solution initially drops as redistribution of xanthate groups and then increases as dexanthation. Ripeness can be measured by using two tests named as salt index and hottenroth number.
During ripening precise temperature control is necessary because temperature is the most convenient for obtaining proper degree of ripening.
The following ingredients should present in the bath to spin the viscose rayon.
Cellulose is recovered from cellulose xanthate by an acid decomposition method. And the process can be done by diffusing the acid into the filament. Stretching of filament takes place to orient the cellulose molecules in the direction of fiber axis. Sulphur is evolved in the bath when the filaments are being formed, that evolved Sulphur deposited on the surface of the filaments. That can be removed by subjecting the filaments to washing and desulphurization, i.e., to treatments with a solution of sodium Sulphide or sulphite.
Furthermore bleaching, soaping or oiling can be done to soften the filaments.
Drying and winding:
Filaments are dried in air quench chamber where it gives cool air to the filaments and facilitates solidification.
After drying, winding of filaments to take-up roller takes place.