What Is Viscose Rayon? Manufacturing, Properties, Uses & Care

What Is Viscose Rayon?

Viscose rayon fiber has been one of the most important manufactured cellulosic fibers in the textile industry for over a century. It is widely used in apparel, home textiles, and nonwoven products because of its softness and good drape. Performance of viscose rayon depends a lot on how it is made. The same basic process can be adjusted to produce regular rayon, high-wet-modulus rayon, and high-tenacity rayon. This is why viscose rayon remains an important fiber in both textile and industrial applications. Understanding its history, production process, properties, and end uses gives a complete picture of why this fiber remains widely used in global textile markets today.

History of Viscose Rayon

Viscose rayon fiber is regenerated cellulose produced using the viscose process, first patented by British chemists Charles Cross, Edward Bevan, and Clayton Beadle in 1892. They discovered that cellulose xanthate could be formed from raw cellulose, sourced from wood or cotton, through reaction with an alkali and carbon disulfide. This cellulose xanthate solution could then be precipitated in ammonium sulfate and converted back to cellulose after neutralization with dilute sulfuric acid.

The spinning method was developed by Charles Henry Stearn in collaboration with Charles Cross in 1898. Commercial production followed in 1904 when Samuel Courtauld and Co. Ltd in England acquired the viscose process patents. The first U.S. manufacturer, the American Viscose Company, was established in 1910.

According to the U.S. Federal Trade Commission (FTC), rayon is defined under 16 CFR (2011) as a manufactured fiber composed of regenerated cellulose, including fibers in which substituents have replaced no more than 15 percent of the hydrogens of the hydroxyl groups.

Raw Materials

Wood pulp is the major source of cellulose used to produce viscose rayon and other manufactured cellulosic fibers. Cotton fibers, especially the short cotton linters, can also be used. Most of the timber used comes from eucalyptus, beech, or pine trees that are raised in nurseries. Harvested trees are stripped of their bark, sun-dried, then cut into strips, and finally reduced to chips. Chips are treated to remove lignin (binding agents) and to remove as much of the resin as possible.

The resulting dissolving pulp is more than 95 percent cellulose, a purer grade than the wood pulp used for making paper. It is pressed and cut into blotter-like sheets for further processing.

Manufacturing Process

Rayon fiber is made from pure cellulose, often derived from wood pulp. Because cellulose is not easily dissolved, it is treated to produce a soluble form that can be spun into fibers. The following steps are employed in the process for manufacturing viscose rayon.

Steeping

The quantity of cellulose is measured carefully by weight then placed in a soaking press, or slurry press, where it remains immersed in a solution of sodium hydroxide (caustic soda) for about an hour. The action of the sodium hydroxide swells the structure and separates the polymer chains. The excess solution is pressed out of the pulp material, leaving a substance called alkali cellulose.

Aging

Alkali cellulose is shredded into small, fluffy particles called white crumbs. These are aged under carefully controlled conditions for several days. During this time the cellulose chains are broken into shorter polymers that can be dissolved more easily.

Xanthation

Carbon disulfide is added to the white crumbs, which produces sodium cellulose xanthate and changes the color to bright orange-yellow. The large xanthate groups push the polymers farther apart, and the cellulose xanthate becomes soluble in sodium hydroxide.

Dissolving and Ripening

The orange-yellow crumbs are placed in dissolving tanks of dilute sodium hydroxide and aged for a time. The resulting solution is thick and viscous and is known as viscose, the term from which this fiber process derives its name. In color, the solution is gold with a consistency similar to that of honey.

Filtration

The viscose solution is filtered to remove any insoluble particles. This is important because the particles would clog the spinneret holes and interfere with the spinning process. It is at this point that delustering agents such as titanium dioxide or pigments for coloring the fiber may be added.

Extrusion and Fiber Formation

The viscous liquid is forced through the spinneret and wet spun into a dilute sulfuric acid bath. The acid hydrolyzes the xanthate, reversing the xanthate formation and regenerating the cellulose in the form of long, continuous filaments that are then washed to remove chemicals and other impurities.

The continuous fiber tow is drawn through the first and second drawing units for stretching. Between these two drawing areas is a tow washing step that removes carbon disulfide. The second drawing unit feeds the washed tow into a cutter to cut the fiber into staple lengths between 25 and 152 mm. The rayon staple fiber is then laid on a wash belt to pass through a series of washing steps that include a hot water wash to remove residual acid, a sulfide wash, a bleaching bath, and a final hot water wash.

After washing, the rayon staple fiber is dried, opened, and sent to a bale press for baling. Rayon filament fiber can also be wound directly onto a bobbin if the cutting step is skipped. In today’s viscose rayon market, 85 percent of rayon fiber production is staple fiber.

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Modified Viscose Rayon Fibers

With changes to the viscose reaction conditions and control parameters, viscose fiber properties vary. This results in the production of modified viscose rayon fibers, among which high-wet-modulus (HWM) viscose and high-tenacity viscose are the two most important.

High-Wet-Modulus Rayon

A Japanese researcher, S. Tachikawa, modified the viscose process to develop a fiber with a physical structure more like that of cotton. In this process the alkali cellulose is not aged after the steeping step, the cellulose xanthate is dissolved in water rather than in caustic soda, and the ripening step is eliminated. A lower concentration of acid is used in the spinning bath with little or no salts. The fibers are stretched more during spinning than regular rayon fibers. These rayons are known as high-wet-modulus (HWM), which means that they, like cotton, have greater resistance to deformation when wet.

To produce HWM viscose fiber, aging in cellulose alkalization and ripening in xanthation is no longer required and chemical concentrations are reduced, so that cellulose coagulation speed is reduced, allowing more time for fiber stretching. HWM viscose fiber has two commonly used brand names: Modal and Polynosic. The Lenzing Group of Austria, a leading producer of cellulosic fibers worldwide, manufactures Modal fibers under its own brand name.

High-Tenacity Rayon

For the production of high-tenacity viscose fiber, cellulose regeneration speed needs to be reduced by increasing zinc sulfate concentrations in the spin bath. There are also high-tenacity rayons made primarily for tire cords.

Molecular Structure and Fiber Shape

Viscose rayon fiber used in the textile and apparel industries can be staple fiber or multifilament fiber. A typical commercial viscose rayon fiber product has a 1.5-denier fineness and 38 mm length. The cross-section resembles a distorted circle with a serrated contour and the fiber surface is smooth but striated longitudinally.

The striations on the surface of viscose rayon are a visual effect caused by the appearance of the serrations created when the skin of the fiber formed at a faster rate than the core. When the core is formed, the skin, which is larger, collapses slightly to form an irregular, wrinkled surface. HWM rayons have a less crenulated cross-section because the regeneration of the cellulose is retarded in the spinning bath, resulting in less of a skin and core structure.

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 to 15 denier. Viscose rayon microfiber with fineness less than 1 denier is also available for production of microfiber fabrics.

Rayon is composed of cellulose, like cotton. In viscose rayon, however, the cellulosic chains are much shorter. The degree of polymerization is only about 400 to 700, compared to 6,000 to 10,000 for cotton. The molecules are also less ordered, the crystalline areas are smaller in size, and the fiber has a higher amount of amorphous material. HWM fibers have longer polymer chains and a more crystalline structure, but the degree of polymerization is still less than one thousand.

Properties of Viscose Rayon

In the production of manufactured fibers, many qualities can be built into the fiber. For this reason, it is difficult to generalize about all rayon fibers because each manufacturer may produce viscose rayons that differ somewhat.

Physical Properties

As spun, rayon fibers are white in color. Their luster can be modified by adding titanium dioxide, a delustering agent, to the solution before the fibers are extruded. The specific gravity of rayon is 1.51, comparable to that of cotton and linen at 1.54, which are also composed of cellulose.

Mechanical Properties

Strength: The strength of viscose rayon is low because of its lower polymer chain length when compared with cotton and flax. Rayon is weaker than cotton because its physical structure is different. During the growing process, cotton develops a fibril structure, the layers of which protect the fiber and provide greater strength. Ordinary rayon has no fibril layers, and instead has a more amorphous inner structure. There is a considerable decrease in strength when the fiber is wet. The tenacity of regular rayon ranges from 1.0 to 2.5 g/d for dry fibers and 0.5 to 1.5 g/d for wet fibers. HWM rayons have higher strengths: from 3.0 to 5.7 g/d dry and 1.9 to 4.3 g/d wet. While these fibers are stronger than regular rayons, their tenacities are similar to or lower than those of most nylon and polyester fibers.
Modulus: Viscose rayon fibers have a fairly low resistance to stretching. HWM fibers were specifically designed to have a high modulus, more comparable to that of cotton and polyester, making HWM rayon a good choice for blending with polyester.
Elongation and Recovery: Viscose rayons stretch and, having low elastic recovery, tend to remain stretched. For some time after stretching, the distorted fabric tends to creep toward, but not completely to, its original length.
Resilience: The elastic recovery of rayon is low, as is its resiliency. Untreated rayons tend to stretch and wrinkle badly. Wrinkle-resistant finishes are generally not applied to viscose rayon because they weaken the fiber.
Abrasion Resistance: The resistance of rayon fibers to abrasion is extremely low, and the resistance decreases when the fibers are wet. Durability is not a quality generally associated with rayon.
Dimensional Stability: Rayon fabrics may be stretched during processing and exhibit relaxation shrinkage upon first laundering. Because the moisture regain of rayon is high, the amount of shrinkage is usually fairly high as well. Fibers may continue to shrink in subsequent launderings. Special finishes can be given to viscose to overcome some of the problems of shrinkage.

Chemical Properties

Absorbency and Moisture Regain: The molecular structure of viscose is more amorphous than that of cotton or linen, making viscose fibers more absorbent than the natural cellulosics. Moisture regain is 11 percent. Viscose accepts dyes readily because of its increased accessibility. Fibers with larger surface areas dye more readily, and the serrated edge of the rayon fiber provides greater surface area.
Heat and Electrical Conductivity: The conductivity of both heat and electricity of viscose rayon is satisfactory, so the fiber is reasonably comfortable in hot weather and does not build up static electricity.
Effect of Heat and Combustibility: Viscose fabrics must be ironed at lower temperatures than cotton. Too-high ironing temperatures will produce scorching; rayons should be ironed at temperatures below 350 degrees Fahrenheit. Long exposure to high temperatures deteriorates the fiber. The fibers burn with characteristics similar to those of cotton. Viscose rayon fabrics continue to burn after the source of the flame has been removed, and burning fabrics have the odor of burning paper. A soft, gray ash remains after burning.
Chemical Reactivity: The amorphous molecular structure of viscose makes it more susceptible to the actions of acids and bases. Acids attack viscose more readily than cotton or other cellulosic fibers. Viscose is also more susceptible to damage from bases.

Environmental Properties

Resistance to Microorganisms and Insects: Viscose is subject to damage from mildew and rot-producing bacteria. Silverfish will attack the fiber. Care in storage is necessary to prevent exposure of the fabric to conditions that encourage mildew and silverfish.
Resistance to Sunlight: Exposure to sunlight will deteriorate viscose rayons more rapidly than cotton. Although it is often used in curtains and draperies, viscose rayon is not especially satisfactory for these products unless they are lined to protect against sunlight.

Uses of Viscose Rayon

Viscose rayon is used in fashionable wearing apparel, ranging from lingerie to suits, dresses, and sportswear. Its use is dictated by its aesthetic rather than its durability properties. Especially notable is the drape of rayon fabrics. Because rayon is among the denser of the commonly used fibers and has a lower modulus, the weight of the fabric and its low stiffness help to develop the pleasing folds associated with good drapability.

To compensate for its lower strength and abrasion resistance, viscose rayon is often blended with other fibers that may be more durable. Rayon and polyester blends are seen in soft, drapable fabrics for apparel and in men’s dress pants where these blends provide a worsted effect at a lower price point. Another common blend is rayon with acetate fiber for women’s apparel. Modal fibers are blended with cotton for knits and towels. The HWM rayon Modal, increasingly produced in finer deniers, is marketed in intimate apparel and loungewear for its softness.

Regular and HWM rayons are not distinguished in U.S. labels, so without a brand name, it is not always possible to know which rayon is contained in a blend.

In the home, rayon fabrics or blends of rayon with other fibers can be found in tablecloths, slipcovers, upholstery, bedspreads, blankets, curtains, and draperies. Because of their absorbency, rayon fibers are used in the manufacture of a wide variety of nonwoven fabrics, both disposable and durable. They are often used in cotton balls sold for cosmetic uses and are being blended with other fibers in diapers and disposable hygiene products. In the industrial realm, rayon fibers are also used in tire cord.

Care Procedures

Dry cleaning is usually recommended for regular viscose fabrics because of their low strength, abrasion resistance, and dimensional stability. Some rayon fabrics may be washed, but caution should be observed in laundering because the fiber is weaker when wet and can be damaged more easily by rough handling. Hand washing is often recommended. HWM rayons have better wet strength and are more suitable for regular laundering.

Drying in an automatic dryer may accentuate rayon’s tendency to shrink. When pressing rayon fabrics, use lower ironing temperatures. Chlorine bleaches must be controlled carefully to avoid fabric deterioration. Oxygen or perborate bleaches are safer for rayon fabrics if bleaching must be done.

Environmental Considerations

Because of environmental problems associated with the process, viscose rayon is no longer manufactured in the United States. The evolution of hydrogen sulfide and carbon disulfide gas during the regeneration step and the runoff from the sulfuric acid bath are difficult and expensive to control. The viscose process is still, however, the most commonly used and economical production method worldwide. As sustainability demands grow across the textile industry, managing the environmental impact of viscose production will remain an important challenge for manufacturers in the years ahead.

Conclusion

Viscose rayon is a useful cellulose-based fiber with a long history, a flexible manufacturing route, and a wide range of end uses. Its softness, absorbency, and drape make it especially valuable in apparel and home textiles. At the same time, its lower wet strength, limited abrasion resistance, and shrinkage behavior call for careful processing and care. Modified forms such as HWM and high-tenacity rayon show how the viscose process can be adapted to meet different textile needs.