Rayon


Rayon is a manufactured fiber made from natural sources such as wood and agricultural products that are regenerated as cellulose fiber. The many types and grades of rayon can imitate the feel and texture of natural fibers such as silk, wool, cotton, and linen.
Rayon is not considered to be synthetic. Technically, the term synthetic fiber is reserved for fully synthetic fibers. In manufacturing terms, rayon is classified as "a fiber formed by regenerating natural materials into a usable form". Specific types of rayon include viscose, modal and lyocell, each of which differs in the manufacturing process and the properties of the finished product.
The raw material for viscose is primarily wood pulp, which is chemically converted into a soluble compound. It is then dissolved and forced through a spinneret to produce filaments which are chemically solidified, resulting in fibers of nearly pure cellulose. Unless the chemicals are handled carefully, workers can be seriously harmed by the carbon disulfide used to manufacture most rayon.
To safeguard the workers from the hazards of the chemicals, new technologies are now applied by the leading manufacturers of viscose to efficiently capture the emissions and recover and recycle the carbon disulfide. These technologies have significantly reduced the risks and have addressed the safety concern related to exposing workers to chemicals.

Rayon and its variants

Nitrocellulose

, a chemical derivative of cellulose produced with nitric acid, was first discovered in 1832 by Henri Braconnot but was unstably explosive until the 1846 process of Christian Schönbein. The degree of nitrification determined its explosiveness, solubility in organic solvents such as ether and acetone, and mechanical properties when dry. Its solubility was the basis for the first "artificial silk" by Georges Audemars in 1855, which he called "Rayon".
However, Hilaire de Chardonnet was the first to patent a nitrocellulose fiber marketed as "artificial silk" at the Paris Exhibition of 1889. Commercial production started in 1891, but the result was flammable and more expensive than cellulose acetate or cuprammonium rayon. Because of this, production ceased early in the 1900s. Nitrocellulose was briefly known as "mother-in-law silk".
Frank Hastings Griffin invented the double-godet, a special stretch-spinning process that changed artificial silk to rayon, rendering it usable in many industrial products such as tire cords and clothing. Nathan Rosenstein invented the "spunize process" by which he turned rayon from a hard fiber to a fabric. This allowed rayon to become a popular raw material in textiles.

Acetate

In 1865, Paul Schützenberger discovered that cellulose reacts with acetic anhydride to form cellulose acetate. The German chemists Arthur Eichengrün and Theodore Becker invented the first soluble forms of cellulose acetate in 1903.
By 1910, Camille Dreyfus and his brother Henri were producing acetate film for the motion picture industry. In 1913, after some twenty thousand separate experiments, they produced excellent laboratory samples of continuous filament yarn, something that had eluded others in the cellulose acetate industry.
After World War I, attention turned to the production of acetate fibers. The first yarn was of fair quality, but sales resistance was heavy, and silk associates worked zealously to discredit acetate and discourage its use. However, the thermoplastic nature of acetate made it an excellent fiber for moiré because the pattern was permanent and did not wash away. The same characteristic also made permanent pleating a commercial fact for the first time, and gave great style impetus to the whole dress industry.
Today, acetate is blended with silk, cotton, wool, nylon, etc. to give fabrics excellent wrinkle recovery, good heft, handle, draping quality, quick drying, proper dimensional stability, cross-dye pattern potential, at a very competitive price.
Acetate shares many similarities with viscose rayon, and was formerly considered as the same textile. However, rayon resists heat while acetate is prone to melting. Acetate must be laundered with care either by hand-washing or dry cleaning, and acetate garments disintegrate when heated in a tumble dryer. The two fabrics are now required to be listed distinctly on garment labels.
Furthermore, viscose rayon production requires carbon disulfide in production, while acetate uses safer solvents such as acetone. However, because viscose rayon is a stronger and more robust fiber than the otherwise similar acetate, it has come to dominate the market.
Cellulose triacetate is a similar cellulose derivative to acetate. Acetate fiber is a modified or secondary acetate having two or more hydroxyl groups; triacetate is a primary acetate containing no hydroxyl group. Triacetate fibers contain a higher ratio of acetate-to-cellulose than do acetate fibers.
Triacetate is significantly more heat resistant than cellulose acetate; articles containing triacetate fibres require little special care, and are ironable up to 200 °C.

Cuprammonium method

Swiss chemist Matthias Eduard Schweizer discovered that cellulose dissolves in tetraaminecopper dihydroxide. Max Fremery and Johann Urban developed a method to produce carbon fibers for use in light bulbs in 1897. Production of cuprammonium rayon for textiles started in 1899 in the Vereinigte Glanzstoff Fabriken AG in Oberbruch near Aachen. Improvement by J. P. Bemberg AG in 1904 made the artificial silk a product comparable to real silk.

Viscose method

English chemist Charles Frederick Cross and his collaborators, Edward John Bevan and Clayton Beadle, patented their artificial silk in 1894. They named their material "viscose" because its production involved the intermediacy of a highly viscous solution. The process built on the reaction of cellulose with a strong base, followed by treatment of that solution with carbon disulfide to give a xanthate derivative. The xanthate is then converted back to a cellulose fiber in a subsequent step.
The first commercial viscose rayon was produced by the UK company Courtaulds Fibres in 1905. Courtaulds formed an American division, American Viscose, to produce their formulation in the United States in 1910. The name "rayon" was adopted in 1924, with "viscose" being used for the viscous organic liquid used to make both rayon and cellophane. In Europe, though, the fabric itself became known as "viscose", which has been ruled an acceptable alternative term for rayon by the US Federal Trade Commission.
The viscose method can use wood as a source of cellulose, whereas other routes to rayon require lignin-free cellulose as starting material. The use of woody sources of cellulose makes viscose cheaper, so it was traditionally used on a larger scale than the other methods. On the other hand, the original viscose process generates large amounts of contaminated wastewater. Newer technologies use less water and have improved quality of the wastewater. Rayon was produced only as a filament fiber until the 1930s, when methods were developed to utilize "broken waste rayon" as staple fiber.
The physical properties of rayon remained unchanged until the development of high-tenacity rayon in the 1940s. Further research and development led to high-wet-modulus rayon in the 1950s. Research in the UK was centred on the government-funded British Rayon Research Association.
Industrial applications of rayon emerged around 1935. Substituting cotton fiber in tires and belts, industrial types of rayon developed a totally different set of properties, amongst which tensile strength and elastic modulus were paramount.

Lyocell

The Lyocell process relies on dissolution of cellulose products in a solvent, N-methylmorpholine N-oxide. The process starts with woody sources of cellulose and involves dry jet-wet spinning. It was developed at the now defunct American Enka and Courtaulds Fibres. Lenzing's Tencel and Birla Cellulose's Excel brands are examples of currently available lyocell fibers.

Modal

Modal is a type of rayon, a semi-synthetic cellulose fiber made by spinning reconstituted cellulose. Modal is used alone or with other fibers in clothing and household items like pajamas, underwear, bathrobes, towels, and bedsheets.
Modal is processed under different conditions to produce a fiber that is stronger and more stable when it is wet than standard rayon, yet has a soft feel, similar to cotton. It can be tumble dried without damage due to its increased molecular alignment. The fabric has been known to pill less than cotton due to fiber properties and lower surface friction.

Major fiber properties

Rayon is a versatile fiber and is widely claimed to have the same comfort properties as natural fibers, although the drape and slipperiness of rayon textiles are often more like nylon. It can imitate the feel and texture of silk, wool, cotton and linen. The fibers are easily dyed in a wide range of colors. Rayon fabrics are soft, smooth, cool, comfortable, and highly absorbent, but they do not always insulate body heat, making them ideal for use in hot and humid climates, although also making their "hand" cool and sometimes almost slimy to the touch.
The durability and appearance retention of regular viscose rayon are low, especially when wet; also, rayon has the lowest elastic recovery of any fiber. However, HWM rayon is much stronger and exhibits higher durability and appearance retention. Recommended care for regular viscose rayon is dry-cleaning only. HWM rayon can be machine-washed.

Gallery of textures

Physical structure

Regular rayon has lengthwise lines called :wikt:striation|striations and its cross-section is an indented circular shape. The cross-sections of HWM and cupra rayon are rounder. rayon yarns vary from 80 to 980 per yarn and vary in size from 40 to 5000 denier. Staple fibers range from 1.5 to 15 denier and are mechanically or chemically crimped. Rayon fibers are naturally very bright, but the addition of delustering pigments cuts down on this natural brightness.

Production method

Regular rayon is the most widely produced form of rayon. This method of rayon production has been used since the early 1900s and it has the ability to produce either filament or staple fibers. The process is as follows:

The steps

  1. Cellulose. Production begins with processed cellulose. The cellulose content in the pulp should be around 87% to 97%.
  2. Immersion. The cellulose is dissolved in caustic soda:
+ NaOH → + H2O
is the chemical reaction. The reaction converts cellulose to alkali cellulose and removes impurities.
  1. Pressing. The solution is then pressed between rollers to remove excess liquid. The pressing is done in accordance with the press-weight ratio. Press weight ratio is the ratio between the pressed alkali cellulose sheet weight to the weight of air dried pulp. It is adjusted to 2.7-3.
  2. "White Crumb." The pressed sheets are crumbled or shredded to produce what is known as "white crumb."
  3. Ageing. The "white crumb" is aged through exposure to oxygen. This is a depolymerization step and is avoided in case of polynosics.
  4. "Xanthation." The aged "white crumb" is mixed with carbon disulfide in a process known as "Xanthation," in which the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulfide under controlled temperature to form cellulose xanthate:
  5. : + CS2 →
is the reaction.
  1. "Orange-Yellow Crumb." The Xanthation changes the chemical makeup of the cellulose mixture, and the resulting product is now called "yellow crumb." This orange coloration is due to the formation of sodium trithiocarbonate.
  2. Viscose. The "yellow crumb" is dissolved in a caustic solution to form viscose.
  3. Ripening. The viscose is set to stand for a period of time, allowing it to ripen:
  4. : + H2O → + CS2 + NaOH
is the reaction.
  1. Filtering. After ripening, the viscose is filtered to remove any undissolved particles.
  2. Degassing. Any bubbles of air are pressed from the viscose in a degassing process.
  3. Extruding. The viscose solution is extruded through a spinneret, which resembles a shower head with many small holes.
  4. Acid Bath. As the viscose exits the spinneret, it lands in a bath of sulfuric acid, resulting in the formation of rayon filaments. The acid is used as a regenerating agent. It converts cellulose xanthate back to cellulose. The regeneration step is rapid which doesn't allow proper orientation of cellulose molecules. So to delay the process of regeneration, zinc sulphate is used in the bath which converts cellulose xanthate to zinc cellulose xanthate thus providing time for proper orientation to take place before regeneration.
  5. : + H2SO4 → + CS2 + Na2SO4
is the reaction.
  1. Spinning. The spinning of viscose rayon fiber is done using a wet spinning process. The filaments are allowed to pass through a coagulation bath after extrusion from the spinneret holes. Two-way mass transfer takes place.
  2. Drawing. The rayon filaments are stretched, in a procedure known as drawing, to straighten out the fibers.
  3. Washing. The fibers are then washed to remove any residual chemicals from them.
  4. Cutting. If filament fibers are desired, then the process ends here. The filaments are cut down when producing staple fibers.
High wet modulus rayon is a modified version of viscose that is stronger when wet. It also has the ability to be mercerized like cotton. HWM rayons are also known as "polynosic." Polynosic fibers are dimensionally stable, and do not shrink or get pulled out of shape when wet like many rayons. They are also wear resistant and strong while maintaining a soft, silky feel. They are sometimes identified by the trade name Modal.
High-tenacity rayon is another modified version of viscose that has almost twice the strength of HWM. This type of rayon is typically used for industrial purposes such as tire cord.
Cuprammonium rayon has properties similar to viscose; however, during its production, the cellulose is combined with copper and ammonia. Due to the detrimental environmental effects of this production method, cuprammonium rayon is no longer produced in the United States.

Environment and sustainability

Highly toxic carbon disulfide is used in the production of viscose, leading to many incidents and legal cases in the early years of viscose technology development. However, the volatile carbon disulfide is lost before the rayon gets to the consumer; the rayon itself is basically pure cellulose. Studies from the 1930s show that 30% of American rayon workers suffered severe effects. During the Second World War, political prisoners in Nazi Germany were made to work in appalling conditions at the Phrix rayon factory in Krefeld, where workers died, went blind, and developed horrible skin conditions. This case was publicized after the war by the French Resistance member Agnès Humbert who was held there for four years and survived to tell the tale, though her health was permanently impaired.
Rates of disability in modern factories are unknown. In the last two decades, new technologies have been deployed in viscose manufacturing process which are highly efficient in capturing the carbon disulfide released in the manufacturing process. The newer technologies referred to as the closed loop manufacturing process of viscose, can recover most of the sulfur in a usable form and recycle it. Leading manufacturers of viscose now apply these latest technologies in the viscose process for effective control of hazards from these chemicals.
The impact of the textile value chain on the environment has become a major concern globally. The most important impacts include: millions of tons of textile waste going to landfill annually, plastic microfibers flowing into our waterways and oceans, pesticides and fertilizers use on fiber crops, excessive use of fresh water, greenhouse gas emissions, air and water pollution and depleted forest cover.
Rayon has historically had many of these issues, especially air and water pollution and depleted forest cover.  Recently, rayon producers have focused on sourcing from 100% certified forests and applying closed-loop manufacturing technologies, limiting the release of pollutants to water and air. Canopy's offers a detailed overview of the largest global producers of viscose, ranking their raw material sourcing practices, documenting their risk of sourcing from Ancient and Endangered Forests, highlighting research and development investment towards lower impact fibers and new disruptive innovation, and noting when producers are showing leadership in supporting global forest conservation solutions.

Disposal and biodegradability

The biodegradability of various fibers in soil burial and sewage sludge was evaluated by Korean researchers. Rayon was found to be more biodegradable than cotton, and cotton more than acetate. The more water-repellent the rayon-based fabric, the more slowly it will decompose. Silverfish like the firebrat can eat rayon, but damage was found to be minor, potentially due to the heavy, slick texture of the tested rayon. Another study states that "artificial silk readily eaten" by Ctenolepisma longicaudata. Rayon has been tested and found compostable and in soil, aqueous and marine conditions.
A 2014 ocean survey found that rayon contributed to 56.9% of the total fibers found in deep ocean areas, the rest being polyester, polyamides, acetate and acrylic. A 2016 study found a discrepancy in the ability to identify natural fibers in a marine environment via Fourier transform infrared spectroscopy.

Alternative to cotton

Rising cotton prices in 2010 led clothing makers to begin replacing cotton with rayon in their fabrics. As demand for rayon increases, some companies have been investing in pulp mills to increase production. Rayon prices do vary with changes in supply and demand in the market, but rayon generally has a price advantage over natural fibers such as cotton and silk.

Mislabelling

See also Bamboo textile
In 2010, the U.S. Federal Trade Commission issued letters informing over 100 companies that they were mislabeling products made of rayon as being made from bamboo, deceiving environmentally conscious consumers. In 2015, the FTC filed complaints against Bed Bath & Beyond, Nordstrom, J.C. Penney, Backcountry.com, and their subsidiaries, for continuing to deceptively sell rayon mislabeled as bamboo. The four companies were required to pay civil penalties totaling for violating the "Textile Act and the Textile Rules" and Section 5 of the FTC Act. Similar action took place in Canada.

Impact on U.S. textile industry

Rayon contributed partly to the decline of the US textile industry in the 1920s. It is far cheaper to produce than wool, cotton, or silk. It also requires less processing and hence fewer workers. In addition, it was 50% cheaper than silk during the 1920s in the US. Then, it was used initially for men's socks but later for lingerie and women's stockings. Currently rayon is only 5-6% of the total fibers used in the , which is dominated by polyester and cotton.

Producers

In 2018, viscose fiber production in the world was approximately 5.8 million tons, and China was the largest producer with about 65% of total global production.
Trade names are used within the rayon industry to label the type of rayon in the product. Viscose Rayon was first produced in Coventry England in 1905 by Courtaulds.
Bemberg is a trade name for cupramonium rayon developed by J. P. Bemberg. Bemberg performs much like viscose but has a smaller diameter and comes closest to silk in feel. Bemberg is now only produced in Japan. The fibers are finer than viscose rayon.
Modal and Tencel are widely used forms of rayon produced by primarily by Lenzing AG and . Tencel, generic name lyocell, is made by a slightly different solvent recovery process, and is considered a different fiber by the US FTC. Tencel lyocell was first produced commercially by Courtaulds' Grimsby plant in England. The process, which dissolves cellulose without a chemical reaction, was developed by Courtaulds Research.
Accordis was a major manufacturer of cellulose based fibers and yarns. Production facilities can be found throughout Europe, the U.S. and Brazil.
Visil rayon and HOPE FR are flame retardant forms of viscose that have silica embedded in the fiber during manufacturing.
North American Rayon Corporation of Tennessee produced viscose rayon until its closure in the year 2000.
Birla Cellolose of India is one of the largest producers of rayon, model and lyocell in the world. It has plants in Nagda, Kharach, Vilayat, and Harihar – and one each in Thailland, Indonesia and China along with pulp facilities in Canada and Sweden.
Indonesia is one of the largest producers of rayon in the world, and Asia Pacific Rayon of the country has annual production capacity of 0.24 million tons.

Cited sources