Long Way Education

By Eman Abdallah Kamel

Eman is a writer and textile engineer. She obtained her bachelor’s degree in textile science from the Faculty of Applied Arts, Egypt.

This article discusses the history of rayon, its types, manufacturing methods, properties, and how to care for rayon fabrics.

Rayon

Rayon is a cellulose-based material. But the way rayon is made makes a difference in the final product. Rayon fabrics resemble natural fibers like cotton, wool, silk, or linen.

The rayon production process causes some negative environmental impacts due to the carbon disulfide used in the manufacture of most rayon fabrics.

The major manufacturing countries for rayon are China, Indonesia, India, America, Brazil, and some European countries.

In 2018, the world’s viscose production reached about 5.8 million tons. China was the largest producer, with about 65% of the total global production.

Bemberg Rayon, or Cuprammonium Rayon, has a similar feel to silk. This type of rayon is currently produced in a limited number of countries.

The United States Environmental Protection Agency prohibits the use of cuprammonium silk.

Did You Know?

Cellulose, copper, and ammonia are combined to make cuprammonium rayon. A solution of this substance in caustic soda is passed through a mandrel, and cellulose is regenerated in stiffening baths that remove copper and ammonia and neutralize caustic soda.

A Brief History

In 1664, artificial silk was first introduced. It was suggested that synthetic thread could be spun from a material similar to that used to make silk by silkworms. Many scientists often tried this in the following years but were unsuccessful. In 1855, the Frenchman Georges Audemarte managed to create a thread by dipping a needle into a viscous solution of mulberry bark pulp and gum rubber. While this process was interesting from a scientific perspective, it was not economically viable because it was slow and required accuracy and skill.

Rayon’s first patent was issued in 1855 by Swiss chemist George Audemars. A French chemist named Hilaire de Chardonnier, who built a commercial production plant for Chardonnett silk near Besancon, France, displayed rayon at the Paris Exhibition in 1889.

In 1905, the English company Courtaulds produced a regenerated cellulose called viscose rayon. Around 1908, French chemist Louis-Henri patented cuprammonium rayon. Courtaulds formed a branch in America called the American Viscose Company, which began producing rayon in 1910.

Around 1918, the acetate silk was manufactured at the British Celanese plant in Derbyshire, England. Roughly 24% of the world’s rayon produced in the 21st century comes from Grasim in India.

Modal rayon was developed in Japan in 1951. This type of rayon is almost similar to viscose silk.

Courtaulds Research developed Lyocell Rayon in the 1980s as an environmentally friendly alternative.

Types of Rayon and the Manufacturing Process of Each Type

There are many types of rayons:

1. Viscose
2. Acetate
3. Lyocell
4. Modal 

Each type has a manufacturing method that gives it the final shape.

1. Viscose Rayon

Viscose rayon has a silk-like aesthetic and retains its rich, bright colors.

The Manufacturing Process of Viscose:

• Soaking: The cellulose core is dipped in 17–20% aqueous sodium hydroxide (NaOH) at a temperature of 18–25°C to swell the cellulose fibers and convert the cellulose into alkaline cellulose.
• Compression: A swollen alkali cellulose mass is pressed to 2.5 to 3.0 times the original core’s weight to obtain an accurate proportion of alkali to cellulose.
• Shredding: Pressed alkali cellulose is mechanically shredded into fine, well-divided particles called crumbs.
• Aging: To depolymerize the alkali cellulose to the required degree of polymerization, the cellulose is aged under controlled conditions of time and temperature (18–30°C). During this stage, the average molecular weight of the original core is reduced by a factor of two to three. For a viscose solution to have the correct viscosity and composition, cellulose must be reduced.
• Xanthation Formation: At this stage, the aged alkali cellulose crumbs are placed in basins to react with carbon disulfide at a temperature of 20 to 30°C to form cellulose xanthate.
• Solubility: The crumbs are dissolved in a caustic solution. The large xanthine substitutes in cellulose drive the chains to separate, reducing the hydrogen bonds between the chains and allowing water molecules to dissolve and separate the chains, resulting in an insoluble cellulose solution. Because of the non-xanthate cellulose clumps in the crystalline regions, the crumb is not completely soluble because the cellulose xanthate solution has a very high viscosity.
• Ripening: The viscose is allowed to stand for some time to ripen. Two major processes take place during ripening: the redistribution of xanthate groups and the loss of xanthate groups. The reversible xanthation reaction allows some xanthate groups to revert to hydroxyl cellulose and free CS₂. This free CS2 can interact with another hydroxyl in other parts of the cellulose chain. By doing so, crystal regions become gradually subdivided. The loss of CS₂ reduces the solubility of cellulose and facilitates the regeneration of cellulose after it has been formed into filaments.
• Filtering: The viscose is filtered to remove unsolved substances that may disrupt the spinning process.
• Degassing: Before viscose extrusion, all trapped air bubbles must be removed to avoid weak spots in the rayon threads.
• Spinning: A solution of viscose is measured through a spinneret into a bath containing sulfuric acid (to acidify sodium cellulose xanthate), sodium sulfate (to impart a high salt content to the bath, which is useful in the rapid coagulation of viscose), and zinc sulfate (exchanged with sodium xanthate to form zinc xanthate). Once the cellulose xanthate has been neutralized and acidified, the rayon filaments are rapidly coagulated, followed by simultaneous expansion and hydrolysis of the cellulose xanthate to produce regenerated cellulose.

2. Acetate Rayon

While viscose silk is a regenerated fiber, rayon acetate is a regenerated modified fiber.

The Manufacturing Process of Acetate:

• Acetylation Process: Purified cotton linter is fed into a closed container containing a mixture of acetic anhydride, glacial acetic acid, and concentrated sulfuric acid. For every 100 kg of cotton linter, 300 kg of glacial acetic acid, and 500 kg of acetic anhydride. The closed container consists of a metal tank with a round door at the top. The door is closed after adding the mixture of chemicals and cotton lint. The motor with many blades rotates in the acetylation to mix the ingredients well. An acetylation reaction is an exothermic reaction. The heat is removed by circulating cold water through a jacket attached to the closed container. At 25–30°C, the acetylation reaction takes 7–8 hours to complete. Triacetate is formed at this stage as a suspension in an acetylated mixture called an acid dope.
• Hydrolysis: The acid dope is stored in jars for the aging process. After that, acetic acid, water, and sulfuric acid are added and left for 10–20 hours. During the ripening period, the partial conversion of the acetate groups into the hydroxy groups occurs. Afterward, the mixture is diluted with water and continuously stirred until white flakes of rayon acetate precipitate. Flakes are placed in a centrifuge, and excess water is ejected through holes in the cage. Then the flakes are dried.
• Spinning Process: The dope is spun into a rayon acetate yarn by a dry spinning process. The dope is fed from the spinning tank into spinning cabinets. The dope exiting the spindle travels 2–5 meters vertically downwards to the feed roller, where it is directed to the bobbin much faster than the spinning speed. This lends a twist to the filaments.

3. Lyocell Rayon

Lyocell fiber is one of the most popular regenerated cellulose fibers. It is produced based on the N-methyl morpholine-N-oxide (NMMO) dissolution method. It has unique properties in comparison to viscose fiber.

The Manufacturing Process of Lyocell:

The manufacturing process of Lyocell consists of five steps:

• Dissolution: This step involves breaking down the pulp fibers and mixing them with the solvent. The cellulose is dissolved in an aqueous system containing NMMO to form a highly viscous dope. The dissolving of the pulp of the lyocell is much simpler than that of the viscose process, as the dissolution of the dissolved pulp consists of the mercerization phase (soaking with sodium hydroxide), aging, and xanthation using carbon disulfide.
• Filtration: Dope is filtered to remove coarse components.
• Spinning Regeneration: The dope is extruded through the spinneret nozzle into an air gap and then regenerated in a coagulation bath.
• Washing: The resulting lyocell fibers are then washed. The remaining NMMO is recovered and recycled.
• Finishing: This step includes bleaching, finishing, and drying.

4. The Modal Rayon

Modal rayon is also known as HWM rayon. This type is commonly used for consumer textiles. The main component of modal rayon is cellulose, which is derived from hardwood trees such as oak and birch.

The Manufacturing Process of Modal:

• Harvesting of Trees: Trees like oak are split into small pieces roughly the size of a postage stamp and then transported onto the manufacturing floor. Then, these chips are purified to extract their cellulose content.
• Steeping the Sheets: Cellulose is extracted and formed into sheets. Then, the sheets are soaked in sodium hydroxide. Compared to viscose silk, modal rayon is created by using lower concentrations of NaOH, resulting in less toxic waste.
• Breaking into Crumbs: Sheets are broken into crumbs after being soaked. These crumbs are directly submerged in carbon disulfide, which turns the white crumbs into an orange substance called sodium cellulose xanthate. At this stage, tree-derived cellulose can no longer be considered purely organic matter.
• Forming into Yarn: After that, cellulose xanthate is dipped in caustic soda again to create a syrup solution. Once again, the modal rayon process bypasses the aging process that typically occurs at this stage. This syrup is immediately forced through a spinneret to form fibers.
• Finishing: The fibers are then dipped in sulfuric acid, stretched, and formed into yarn. Then the resulting strands are washed, bleached, rinsed, dried, and loaded onto reels. Modal yarn is ready for pre-shrinking, fire-proofing, wrinkle-proofing, and any other treatment it requires before it is woven into fabric.

Properties of Rayon

• Rayon is soft, comfortable on the skin, and resistant to abrasion. It is not elastic, which means it will wrinkle.
• Rayon withstands lower ironing temperatures than cotton. The fiber is generally resistant to insect damage, though termites and silverfish may attack it. Rayon is prone to rot.
• Rayon fibers are easily blended with many fibers to reduce costs. Or for luster, softness, and absorbency.
• Rayon is moderately resistant to acids and alkalis. In general, bleaching does not harm the fibers themselves. However, the color of the dyes used in the fabric may change.
• Viscose rayon has silk-like aesthetics and keeps its rich, bright colors. It is more moisture-absorbent than cotton. It is also dyed easily with bright colors and does not produce static electricity.

Uses of Rayon

Rayon fibers are used to make shirts, blouses, dresses, jackets, underwear, scarves, suits, socks, towels, and hats.

Rayon is also used for furnishings such as bed sheets, blankets, curtains, and upholstery.

Tips to Care for Rayon Fabrics

1. Some rayon manufacturing processes produce durable, fast-color fabrics that can be machine-washed. But other processes produce a fabric that is best handled with care to prevent damage to the garment. You can find this on the care label.
2. When washing in the washing machine, turn the rayon garment inside out and wash in cold water with a low spin cycle.
3. It is best not to dry rayon fabrics in the washing machine, as they may be damaged. If you must, dry it on the lowest possible temperature cycle.
4. During ironing, use medium heat, as high temperatures can burn cellulose fibers. Always iron on the wrong side of the fabric. For added protection, use a zipped cloth between the iron and the rayon fabric.
5. When storing rayon fabrics, it is preferable to use materials that are resistant to moths and mold.

Sources

• www.ijera.com/pdf. Viscose Rayon: A Legendary Development in the Manmade Textile.
• www.researchgate.net… Regenerated cellulose by the Lyocell process: a brief review of the process and properties.
• www.patagonia.com/ bamboo rayon.pdf. Bamboo and Rayon.

©Eman Abdallah Kamel, 2022

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