Rayon is a cellulosic fiber that is made of regenerating natural cellulose from plants without modifying it. The degree of polymerization is 500, which is much lower and shorter than that of natural occurring cellulose. The viscose rayon polymer arrangement is more amorphous (65%) than crystalline (35%) because of the relative short polymers. When viewed under a microscope, the fiber appears smooth and rounded. Rayon fibers are rod-like with numerous longitudinal, thread-like striations or lines. In this article I will discuss introduction of viscose rayon, history of viscose rayon, appearance of viscose fiber, manufacturing of viscose rayon and its producing countries.
Viscose rayon, a man made fiber composed of 100% regenerated cellulose, was discovered in 1891, and the first commercial production was undertaken in 1905 by Courtaulds. It is made from cotton linters or wood pulp, usually obtained from spruce and pine trees. Initially, viscose was called “artificial silk” and later named as rayon because of its brightness and similarities in structure to cotton (rayon is a combination of sunray and cotton). The other two regenerated cellulose fibers are cuprammonium (cupro) and polynosic (modal). In the United States, these three regenerated cellulose fibers are still referred to collectively by the generic term rayon. But the International Organisation for Standardisation (ISO) prefers the name viscose and defines viscose as regenerated cellulose obtained by the viscose process. The name viscose was derived from the word viscous, which describe the liquid state of the spinning solution.
Viscose rayon is weak, with high elongation at break and a low modulus. It loses 30% to 50% of its strength when wet, and needs careful laundering. It also shrinks appreciably from washing. Viscose rayon is one of the most absorbent of all textiles. It is more absorbent than cotton or linen and is exceeded in absorbency only by wool and silk. A variation of rayon is classified as high wet modulus (HWM) rayon or polynosic rayon. This type of rayon is launderable.
The History of Viscose Rayon:
According to the U.S. Federal Trade Commission (FTC) regarding generic names and definitions for manufactured fibers, rayon is defined as:
A manufactured fiber composed of regenerated cellulose, as well as manufactured fibers composed of regenerated cellulose in which substituents have replaced not more than 15 percent of the hydrogens of the hydroxyl groups.
Viscose rayon fiber is regenerated cellulose fiber produced using the viscose technology and was originally patented by the British chemists Charles Cross, Edward Bevan, and Clayton Beadle in 1892. They discovered that cellulose xanthate could be formed by using raw cellulose from wood or cotton via reaction with an alkali and carbon disulphide. The resulting cellulose xanthate solution could also be precipitated in an ammonium sulfate solution and changed back to cellulose after neutralization using dilute sulfuric acid.
The spinning method for producing viscose fiber was developed by Charles Henry Stearn in cooperation with Charles Cross in 1898, and was successfully commercialized in 1904 when Samuel Courtauld & Co. Ltd in England acquired the viscose process patents. The first U.S. manufacturer, the American Viscose Company, was registered in 1910.
Appearance of Viscose Fiber:
Viscose rayon fiber used in the textile and apparel industries can be staple fiber or multifilament fiber. Figure 2 shows a typical fiber shape for a commercial viscose rayon fiber product with a 1.5-denier fineness and 1.5-in (38 mm) length.
The cross-section resembles a distorted circle with a serrated contour and the fiber surface is smooth but striated longitudinally, as shown in Figure 3. The luster of viscose rayon fiber can be bright, semi-dull, or dull. Commonly used fineness of viscose rayon fiber is in the range of 1.5–15 denier. Viscose rayon microfiber (fineness less than 1 denier) is also available for production of microfiber fabrics.
Manufacturing Process of Viscose Rayon:
While there are many variations in the manufacturing process that exploit the versatility of the fiber, the following is a description of the procedure that is used in making regular or viscose rayon.
Regardless of whether wood pulp or cotton linters are used, the basic raw material for making rayon must be processed in order to extract and purify the cellulose. The resulting sheets of white, purified cellulose are then treated to form regenerated cellulose filaments. In turn, these filaments are spun into yarns and eventually made into the desired fabric.
The process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: (1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7) Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12) Washing, (13) Cutting, (14) Winding. The various steps involved in the process of manufacturing viscose are explained below.
All steps of viscose rayon manufacturing are described below:
It involves treating the pulp boards with caustic soda solution of mercerizing concentration (about 17.5%) to form alkali cellulose. The operation is carried out in a long rectangular tank in which a hydraulic ram is incorporated. Thus when mercerization is complete the excess of caustic liquor can be pressed by the ram, leaving the softened boards of alkali cellulose containing known quantities of caustic soda water. This takes nearly 14 hr. The dark brown colored caustic soda solution is then drained off and the sheets pressed by the movable plates to squeeze the caustic soda solution to a given moist mass of soda cellulose, which is then transferred to a shredding machine for the next operation.
The swollen alkali cellulose mass is pressed to a wet weight equivalent of 2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of alkali to cellulose.
The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called “crumbs”. This step provides increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow.
The disintegrating machine or shredder consists of two arms, bearing toothed wings, which work against other similar teeth on the bottom of the vessel. When the vessel rotates, the pulp sheets are cut into small bits called crumbs.
The crumbs of soda cellulose are stored in small galvanized steel drums for about 48 hr at 23°C. The time and temperature depend on the nature of the pulp, degree of ageing (maturity) desired, spinning process to be employed, etc. In this step the average molecular weight of the original pulp is reduced by a factor of two to three. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration.
This process is also called sulphidizing. After the ageing process, the crumbs of soda cellulose are transferred to rotating, air tight chambers. These are double-jacketed, so as to maintain the required temperature during churning. Carbon disulphide is added to the mixer and churned together for 3 h by rotating the mixer at slow speed. Colour of the product changes from white to light yellow, light yellow to deep yellow and finally to reddish orange. Sodium cellulose xanthate formed during this process should not be allowed to form hard lumps as they are difficult to dissolve in the subsequent process. After the churning is over, vacuum is applied for removing the ill-smelling vapours of carbon disulphide from the mixer.
The yellow crumb is dissolved in aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to solution of the otherwise insoluble cellulose. Because of the blocks of un-xanthated cellulose in the crystalline regions, the yellow crumb is not completely soluble at this stage. Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed “viscose”.
As the alkali cellulose needs ageing, this viscose solution requires to be ripened to give a solution having the best spinning qualities. Ripening is carried out by storing the viscose solution for 2 to 5 days at 10–18°C. During this period, a kind of coagulation or throwing down of sediment occurs and the viscosity decreases and then rises to the original value. The sediment necessitates filtration. Hence, the ripened solution is again filtered carefully and deaeratred. The solution is now ready for spinning to produce viscose rayon filaments by wet spinning method.
8. Filtration and deaeration:
This is necessary before the spinning stage is reached, since the rayon is to be formed by forcing the solution through the spinneret having very small holes, all bubbles and solid particles likely to choke the spinneret holes must be removed. Hence, the viscous solution is passed through a series of filter presses on its way to the large tanks in which it is ripened, and later the solution is again filtered several times in the same manner while being transferred from one container to another. Finally, deaeration is accomplished by maintaining a partial vacuum over the viscose solution in the air-tight tanks in which it is kept during the last part of the ripening periods.
Bubbles of air entrapped in the viscose must be removed prior to extrusion or they would cause voids, or weak spots, in the fine rayon filaments.
10. Spinning – (Wet Spinning):
Production of Viscose Rayon Filament: The viscose solution is metered through a spinnerette into a spin bath containing sulphuric acid (necessary to acidify the sodium cellulose xanthate), sodium sulphate (necessary to impart a high salt content to the bath which is useful in rapid coagulation of viscose), and zinc sulphate (exchange with sodium xanthate to form zinc xanthate, to cross link the cellulose molecules). Once the cellulose xanthate is neutralized and acidified, rapid coagulation of the rayon filaments occurs which is followed by simultaneous stretching and decomposition of cellulose xanthate to regenerated cellulose. Stretching and decomposition are vital for getting the desired tenacity and other properties of rayon. Slow regeneration of cellulose and stretching of rayon will lead to greater areas of crystallinity within the fiber, as is done with high-tenacity rayons.
The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 –> (C6H10O5)n + nCS2 + (n/2)Na2SO4
Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fiber axis. As the chains become more parallel, interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibers.
The freshly regenerated rayon contains many salts and other water soluble impurities which need to be removed. Several different washing techniques may be used.
If the rayon is to be used as staple (i.e., discreet lengths of fiber), the group of filaments (termed “tow”) is passed through a rotary cutter to provide a fiber which can be processed in much the same way as cotton.
14. Winding and stretching:
Winding the stretching of the filaments is also done to orient the molecules in the direction of the fiber axis to improve the mechanical properties of the filaments. After that the filaments strands are wound and are further processed with following operations.
The cake is washed with water to remove the impurities using a cake washing machine. Mter that it is washed with sodium sulphide solution at 62-65°C to remove the residual sulphur. Then bleaching is done with sodium hypochlorite or with hydrogen per-oxide (H2O2) to remove the residual chlorine, cake is treated with mild HCI in the case of sodium hypochlorite bleaching and finally it is dried.
Major Viscose Rayon Manufacturing Countries:
The viscose rayon market was valued at over 4,750 kilo metric ton in 2020, and the market is projected to register a CAGR of over 4% during the forecast period (2021-2026). China is the largest producer and consumer of viscose staple fiber globally. The country accounts for ~65% of the total globally produced viscose staple fiber, owing to the rapidly increasing yarn production. Major viscose rayon manufacturing countries are shown in below graph.
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You can see complete manufacturing process in this video:
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.