Rayon


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. The types that resemble silk are often called artificial silk. It can be woven or knit to make textiles for clothing and other purposes.
Rayon production involves solubilizing cellulose fibers. Three common methods are as follows:
French scientist and industrialist Hilaire de Chardonnet 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 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.
English chemist Charles Frederick Cross and his collaborators, Edward John Bevan and Clayton Beadle, patented their artificial silk in 1894. They named it "viscose" because its production involved the intermediacy of a highly viscous solution. Cross and Bevan took out British Patent No. 8,700, "Improvements in Dissolving Cellulose and Allied Compounds" in May, 1892. In 1893, they formed the Viscose Syndicate to grant licences and, in 1896, formed the British Viscoid Co. Ltd.
The first commercial viscose rayon was produced by the UK company Courtaulds Fibres in November 1905. Courtaulds formed an American division, American Viscose, to produce their formulation in the US in 1910.
Manufacturers' search for a less environmentally-harmful process for making rayon led to the development of the Lyocell method for producing rayon. 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 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. The operating name for the fibre inside the Enka organization was "Newcell", and the development was carried through pilot plant scale before the work was stopped. The basic process of dissolving cellulose in NMMO was first described in a 1981 patent by Clarence McCorsley III for Akzona Incorporated. 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 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. By 2004, production had quadrupled to 80,000 tons.
Lenzing began a pilot plant in 1990, and commercial production in 1997, with 12 metric tonnes per year made in a plant in Heiligenkreuz im Lafnitztal, Austria. When an explosion hit the plant in 2003 it was producing 20,000 tonnes/year, and planning to double capacity by the end of the year. In 2004 Lenzing was producing 40,000 tons . In 1998, Lenzing and Courtaulds reached a patent dispute settlement.
In 1998 Courtaulds was acquired by competitor Akzo Nobel, 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. It took over the plants in Mobile and Grimsby, and by 2015 were the largest lyocell producer at 130,000 tonnes/year.

Process

Rayon is produced by dissolving cellulose, then converting this solution back to insoluble fibrous cellulose. Various processes have been developed for this regeneration. The current industrial methods for creating rayon are the cuprammonium method, the viscose method, and the Lyocell process. The cuprammonium and viscose methods have been practiced for more than a century with the viscose method contributing most of production.

Bernigaut's method (1884)

The first rayon was made by Louis Marie Hilaire Bernigaut in 1884. 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:
n + HNO3 ↔ n +H2O.
The sulfuric acid is used take up the water formed in the reaction, leaving nitrocellulose. The nitrocellulose was highly flammable and it took seven years of further development before Bernigaut brought the product to market. The nitrocellulose process had been fully superseded for industrial production of rayon by the cuprammonium and viscose methods by the early 20th century.

Cuprammonium method (1890)

Cuprammonium rayon has properties similar to rayon made using the more prevalent viscose method; 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 being produced in the United States. The process has been described as obsolete, but cuprammonium rayon is still made by one company in Japan.
Tetraamminecopper sulfate is also used as a solvent.

Viscose method (1891)

The viscose process builds 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 viscose method can use wood as a source of cellulose, whereas other routes to rayon require lignin-free cellulose as a starting material. The use of woody sources of cellulose makes the viscose process 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 the quality of the wastewater.
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 prepare viscose, pulp is treated with aqueous sodium hydroxide to form alkali cellulose, which has the approximate formula . This material is allowed to depolymerize to an extent. The rate of depolymerization 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:
Rayon fiber is produced from the ripened solutions by treatment with a mineral acid, such as sulfuric acid. In this step, the xanthate groups are hydrolyzed to regenerate cellulose and carbon disulfide:
Aside from regenerated cellulose, acidification gives hydrogen sulfide, sulfur, and carbon disulfide. The thread made from the regenerated cellulose is washed to remove residual acid. The sulfur is then removed by the addition of sodium sulfide solution, and impurities are oxidized by bleaching with sodium hypochlorite solution or hydrogen peroxide solution.
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:
  1. Immersion: The cellulose is treated with caustic soda.
  2. Pressing. The treated cellulose is then pressed between rollers to remove excess liquid.
  3. The pressed sheets are crumbled or shredded to produce what is known as "white crumb".
  4. The "white crumb" is aged through exposure to oxygen. This is a depolymerization step and is avoided in the case of polynosics.
  5. The aged "white crumb" is mixed in vats with carbon disulfide to form the xanthate. This step produces "orange-yellow crumb".
  6. The "yellow crumb" is dissolved in a caustic solution to form viscose. The viscose is set to stand for a period of time, allowing it to "ripen". During this stage the molecular weight of the polymer changes.
  7. After ripening, the viscose is filtered, degassed, and then extruded through a spinneret into 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 does not allow proper orientation of cellulose molecules. So to delay the process of regeneration, zinc sulfate is used in the bath, which converts cellulose xanthate to zinc cellulose xanthate, thus providing time for proper orientation to take place before regeneration.
  8. 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.
  9. Drawing. The rayon filaments are stretched, in a procedure known as drawing, to straighten out the fibers.
  10. Washing. The fibers are then washed to remove any residual chemicals from them.
  11. Cutting. If filament fibers are desired, then the process ends here. The filaments are cut down when producing staple fibers.