VISCOSERayon is a man made fibre which have a cellulosic base. During its development, it hasbeen designated by various names, such as artificial silk, wood silk, fibre silk and art silk.The implication in these names is that, it was originally intended to be a substitute forsilk.Rayon is classified as1. Viscose2. Acetate3. CuprammoniumThe raw material or the source from which the raw material can be extracted is celluloseor cellulose base. The regeneration of the fibre is a chemical process. In general,cellulose source, after the required pretreatment is dissolved in a proper reagent and thesolution is forced through spinnerets and coagulated in a bath, depending upon thesolvent.Cellulose occurs in many particles. The substances used for cellulosic material may be(1) Wood (2) Seed hair (3) Bast (4) Leaf (5) Grasses (6) Straws (7) Espatograss (7)Bamboo etc.Wood is the most widely used of all the cellulosic materials. It is found all over the world.The composition of wood isCellulose - 40-50 %Lignin - 20- 30 %Hemicelluloses - 10-30 %Except these three main components polysaccharides are also present in wood. Byremoving lignin, hemicelluloses etc, and cellulose can free as a raw material.The seed hairs like cotton contain cellulose with very less non- cellulosic substances.This makes purification simpler than for most cellulosic material. The best fibres are longfibres of the inner bark of various plants. They include flax, hemp, and ramie. Theycontain approximately 70-85 %.Further, cellulose is the major constituent of the cell wall of plant life. So those plantswhich have no industrial utilization, as well as the residual wastes from utilized plantsfrom a huge potential source of cellulose. The most important of these wastes belong tothe grass family like cereal straw, wheat straw and espatograss ( Alfa fibre). The husksof coconut, betel nut and other palms can be used as cellulosic source. The crushedstalks of sugar cane after the juice, can also be used for rayon. Other products may begrain and seed hulls, soybean stems, fruit pits which can serve as a cellulose source.MANUFACTURE OF VISCOSEViscose rayon is a regenerated cellulosic fibre and cellulose is the raw material forproducing this man made fibre. The raw material is obtained from special variety of wood(spruce). In addition to cellulose, wood also contains substances like lignin and otherimpurities. The first step in the manufacture of Viscose rayon consists of purification ofcellulose from wood. After purification, cellulose is treated with caustic soda solution toconvert it into soda cellulose, which is subsequently treated with carbon disulphide toproduce sodium cellulose xanthate. This is then dissolved in a dilute caustic sodasolution. This solution is forced through a spinning unit and brought into contact with anacid solution, cellulose is reprecipitated in the form of fine filaments called ViscoseRayon.PURIFICATIONThe cut pieces of a spruce trees are treated with solution of calcium bisulphate andcooked with steam under pressure for 14 hours. The cellulose component of wood isunaffected. The cementing material lignin is converted into a compound that is solublein water. By treating with excess of water cellulose can be separated. Then it is treatedwith bleaching agent (sodium hypochlorite) and finally converted into pulp sheets.CONDITIONINGThe wood pulp sheets contain 90-94% pure cellulose. They are cut to the requireddimension and are conditioned at a definite temperature and humidity in a special room.The sheet absorbs a definite amount of moisture from the air.STEEPINGThe conditioned pulp sheets are placed in a steeping tank which is provided with aseries of moveable, perforated steel plates. Caustic soda solution (17.5 %) is added intothe tank. The sheets are allowed to soak until they become dark brown in colour. Thistakes from 1-4 hours at 17C. The cellulose is converted into sodacellulose andhemicellulose dissolves in the caustic soda solution. The sheets are then pressed by themovable plates to squeeze the excess caustic soda liquor.SHREDDINGThe soda cellulose is then transferred to a shredding machine. The machine consist of adrum, inside which a pair of blades with serrated edges revolve at a high speed. Whenthey rotate the pulp sheet are cut into small bits. The temperature during shredding iscontrolled by passing water into the jacket of the machine. In 2-3 hours the pulp sheetsare broken into fine pieces called crumbs.AGEINGThe crumbs stored in small galvanized steel drums for about 72 hours at 22C. Duringthis ageing process the Degree of Polymerisation of sodacellulose is decreased from1000 to about 300 by oxygen present in the air (depolymerisation). Rapid ageing can becarried out at a higher temperature with the help of temperature control device.CHURNINGThis process is also called Xanthation. After ageing the crumbs are transferred torotating, air tight, hexagonal drum. About 10 % of their own (crumbs) weight of carbondisulphide is added. The drum is rotated at a speed of 2 revolution per minute for 3hours. Sodium cellulose xanthate is formed during this process. The colour of theproduct changes from white to light yellow, to deep yellow and then to reddish orange.The drums are double jacketed to control the temperature during Xanthation.DISSOLVINGThis sodium cellulose xanthate in the form of small balls fall into a mixer called dissolver,which is provided with a stirrer. A dilute solution of caustic soda is added and thecontents are stirred for 4-5 hours. At the same time the dissolver is cooled. The sodiumcellulose xanthate dissolves to give a clear brown, thick viscose liquor called viscose.BLENDING, FILTERATION AND DEAERATIONThe viscose solution is mixed or blended in a big mixer, in which solutions from 8 smalldissolvers are pumped. The blending is done to balance any irregularities present indifferent batches of viscose solution and to get uniform quality of the final product. Allthese solutions are mixed and stirred in the big mixer.The solution is then filtered to remove some undissolved cellulose and dirt. Removal ofair from the solutions is then carried out (Deaeration) in air- tight vessels by applyingvaccum.RIPENINGRipening is carried out by storing the viscose solution for 2-5 days at 10-18C. Duringthis period viscosity (a measure of thickness of solution) decrease and then rises to theoriginal value and remain constant. Ripening is done to give a solution having the bestspinning qualities.The ripened solution is again filtered carefully to remove any foreign matter anddeaerated. The solution is now ready for spinning. Pigments can be added to producedyed fibres delustering agents TiO2 is added for controlling the lustre and otherchemicals for special yarns.SPINNINGFor the production of Viscose filaments, the viscose solution is forced under pressure bya gear pump first through the filter (to remove fine impurities) and then through the veryfine orfies in the spinnerette. The spinnerettes have many fine holes (0.05mm diameter).They are immersed in a solution containing the following chemicals.Sulphuric acid - 10 %Sodium sulphate - 18 %Glucose - 2 %Zinc sulphate - 1 %Water - 69 %The viscose filaments emerging from the orifices of spinnerette are coagulatedimmediately and cellulose is regenerated.FUNCTIONS OF THE CHEMICALSThe sodium sulphate precipitates the sodium cellulose xanthate from the viscosesolution into filaments and sulphuric acid converts into cellulose, the glucose givespliability and softness to the filaments. Zinc sulphate is responsible for added strengthand serrated cross section.The fine viscose filaments from each spinneret are combined into a main thread. Theyare first guided round bottom and then top godet wheels. The top godet wheel runs at afaster rate than the bottom in order to give the desired stretch. The stretching of thefilaments is done in order to orient the molecules of cellulose along the direction of thefibre.POT SPINNING WITH TOPHAM BOXThe viscose filaments after being stretched between godet wheels is taken by a tophambox through a glass funnel. The filaments are twisted and build up in the form of cakesinside the topham box. The topham box is a plastic vessel, cylindrical and slightlytapered in shape, about 18cm. in diameter and rotates at about 7000 to 12000 rpm. Atraversing mechanism is provided to build a precise cake inside the box. The cakes areremoved from the box after a doff is full and are collected for further purification andprocessing.Purification consists of four operations namely 1) Washing 2) Desulphurising 3)Bleaching and 4) Washing . The cakes are first washed with water using a cake washingmachine to remove the chemicals. They are washed with sodium sulphide solution at 62-63C to free the filaments from the residual sulphur (desulphurising). At this stage thefilaments are bleached with a dilute solution of sodium hypochlorite and then treated withdilute hydrochloric acid and washed thoroughly to remove the acid and finally dried.PRODUCTION OF VISCOSE FIBREThe staple fibre of viscose is manufactured by cutting the rayon filaments into prescribedlength. The staple length ranges from 25mm to 75mm. The staple fibre may be spun intoyarn on cotton fibre spinning system or it may be blended with fibres like cotton, wool,terelyne and then spun into yarn. For producing viscose staple fibre large sizedspinnerets (with large number of orifices about 3000) and godet wheels are used. Thefilaments from a number of spinnerettes are combined together to get a rope like viscosecalled Tow.The tow, after stretching is crimped and then passed through the revolving blades of acutter. The cutter blades cut viscose. The cut staple fibre is then washed, givennecessary chemical treatments for desulphurising, bleaching, lubricating etc. It is finallydried and then baled.USES OF VISCOSEViscose rayon is a cheap man made fibre and has wide range of application like cotton.1. It is used practically in all types of apparel fabric as alone or in blends. Ladies dressmaterial such as georgettes, chiffons, satins, crepes, velvets etc. are commonly madefrom viscose rayon filament yarn.2. Home furnishings such as Curtains, Bedspreads, Cushions quilt covers, Table clothand Upholstery material are also made from Viscose filaments.3. Viscose filament is also used in knitted fabrics to some extent.4. Viscose staples, after blending with natural and synthetic staple fibres, is used for theproduction of apparel fabrics.5. Viscose staple is widely used in the production of nonwoven fabrics such as surgical,cosmetic and baby care products, hospital gowns, bed spread, sanitary napkins, pillowcases, mattress cover etc.POLYNOSIC RAYONThe polynosic fibres are a new kind of viscose rayon, in their manufacture steps aretaken to maintain a higher Degree of Polymerisation by reducing the severity ofchemical processing so that a value of 500-700 is retained, and also to obtain amicrofibrillar structure by precipitating the viscose gently or slowly, in particular bycarrying out the stretching before the xanthate has been reconverted to cellulose. Thefibres so made are much more like cotton, the degree of their disadvantage has beenvery greatly reduced, whilst they still retain their advantages of uniformity of size, length,luster and potential cheapness. Wet strength in polynosic rayon is higher than of normalviscose rayon.