Rayon

thumb|Rayon is made by dissolving cellulose and then precipitating it into a fiber. One method is shown here using a blue solution of cellulose ([[Schweizer's reagent|cuprammonium hydroxide used to dissolve) and colorless sulfuric acid to precipitate. The blue color fades from the fiber over time.]]

Rayon, also called viscose, is a semi-synthetic fiber made from natural sources of regenerated cellulose, such as wood and related agricultural products. It has the same molecular structure as cellulose. Many types and grades of rayon fibers and films exist. Some imitate the feel and texture of natural fibers such as silk, wool, cotton, and linen. It can be woven or knitted to make textiles for clothing and other purposes.

Rayon production involves solubilizing cellulose fibers. Three common methods are as follows:

• The cuprammonium process (obsolete), using ammoniacal solutions of copper salts
• The viscose process (most common) using alkali and carbon disulfide
• The Lyocell process, using amine oxide, avoids neurotoxic carbon disulfide but is more expensive

History

French scientist and industrialist Hilaire de Chardonnet (1838–1924) is known as the "father of rayon" for his early development and commercialization of nitrocellulose rayon, the first commercial semi-synthetic fiber. He patented his production process in 1885. Rayon was and still is also known by the name artificial silk.

Swiss chemist Matthias Eduard Schweizer (1818–1860) discovered that cellulose dissolved in tetraamminecopper dihydroxide. Max Fremery and Johann Urban developed a method to produce carbon fibers for use in light bulbs in 1897. Improvement of cuprammonium rayon for textiles by J. P. Bemberg in 1904 made the artificial silk a product comparable to real silk. In 1893, they formed the Viscose Syndicate to grant licences and, in 1896, formed the British Viscoid Co. Ltd. Courtaulds formed an American division, American Viscose (later known as Avtex Fibers), to produce their formulation in the US in 1910. For the next decade, American Viscose was the sole producer of rayon in the United States.

By the 1920s, the United States was the largest producer of rayon in the world. The Lyocell process was under development in the late 1970s by a team at the now defunct American Enka fibers facility near Asheville, North Carolina, with a landmark patent granted in 1979 to employees Neal E. Franks and Julianna K. Varga. In recognition of this work, the American Association of Textile Chemists and Colorists (AATCC) awarded Neal E. Franks their 2003 Henry E. Millson Award for the Invention for Lyocell. In 1966–1968, D. L. Johnson of Eastman Kodak Inc. studied NMMO solutions. In the decade 1969 to 1979, American Enka tried unsuccessfully to commercialize the process. In the 1980s the patent was licensed by Akzo to Courtaulds and Lenzing. The fibre was developed by Courtaulds Fibers under the brand name "Tencel" in the 1980s. In 1982, a 100 kg/week pilot plant was built in Coventry, UK, and production was increased tenfold (to a ton/week) in 1984. In 1988, a 25 ton/week semi-commercial production line opened at the Grimsby, UK, pilot plant. The process was commercialized at Courtaulds' rayon factories at Mobile, Alabama. In January 1993, the Mobile Tencel plant reached full production levels of 20,000 tons per year, by which time Courtaulds had spent £100 million and 10 years on Tencel development. Tencel revenues for 1993 were estimated as likely to be £50 million. A second plant in Mobile was planned. In 2004 Lenzing was producing 40,000 tons [sic, probably metric tonnes]. which combined the Tencel division with other fibre divisions under the Accordis banner, then sold them to private equity firm CVC Partners. In 2000, CVC sold the Tencel division to Lenzing AG, which combined it with their "Lenzing Lyocell" business, but maintained the brand name Tencel. Bernigaut's rayon was made of nitrocellulose produced from cellulose, where the cellulose was obtained from cotton and then reacted with a mixture of sulfuric and nitric acid. Nitration occurs as:

[C₆H₇O₂(OH)₃]_n + HNO₃ ↔ [C₆H₇O₂(NO₃)₃]_n +H₂O.

The sulfuric acid is used take up the water formed in the reaction, leaving nitrocellulose.

Tetraamminecopper(II) sulfate is also used as a solvent.

Viscose method (1891)

thumb|A device for spinning viscose rayon dating from 1901

thumb|Simplified view of the xanthation of cellulose

The raw material for viscose is primarily wood pulp (sometimes bamboo 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 prepare viscose, pulp is treated with aqueous sodium hydroxide (typically 16–19% by mass) to form alkali cellulose, which has the approximate formula [C₆H₉O₄−ONa]. This material is allowed to depolymerize to an extent. The rate of depolymerization (ripening or maturing) depends on temperature and is affected by the presence of various inorganic additives, such as metal oxides and hydroxides. Air also affects the ripening process, since oxygen causes depolymerization. The alkali cellulose is then treated with carbon disulfide to form sodium cellulose xanthate:

Production begins with processed cellulose obtained from wood pulp and plant fibers. The cellulose content in the pulp should be around 87–97%.

The steps: "Lyocell" has become a genericized trademark, used to refer to the Lyocell process for making cellulose fibers.

the Lyocell process is not widely used, because it is still more expensive than the viscose process.

The durability and appearance retention of regular viscose rayons are low, especially when wet; also, rayon has the lowest elastic recovery of any fiber. However, HWM rayon (high-wet-modulus 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.

Regular rayon has lengthwise lines called striations and its cross-section is an indented circular shape. The cross-sections of HWM and cupra rayon are rounder. Filament rayon yarns vary from 80 to 980 filaments 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. High-wet-modulus rayon is a modified version of viscose that is stronger when wet. It can 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. Modal is used alone or with other fibers (often cotton or spandex) in clothing and household items like pajamas, underwear, bathrobes, towels, and bedsheets. Modal can be tumble-dried without damage.

Producers and brand names

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 cuprammonium 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.