Rayon, also calledviscose[1] and commercialised in some countries assabra silk orcactus silk,[2] is asemi-synthetic fiber[3] made from natural sources of regeneratedcellulose, such as wood and related agricultural products.[4] It has the same molecular structure as cellulose. Many types and grades of viscose fibers and films exist. Some imitate the feel and texture ofnatural fibers such assilk,wool,cotton, andlinen. The types that resemble silk are often calledartificial silk. It can be woven or knit to make textiles for clothing and other purposes.[5]
Rayon production involvessolubilizing cellulose to allow turning the fibers into required form. Three common solubilization methods are:
French scientist and industrialistHilaire de Chardonnet (1838–1924) invented the first artificial textile fiber,artificial silk.[11]
SwisschemistMatthias Eduard Schweizer (1818–1860) discovered that cellulose dissolved intetraamminecopper dihydroxide.Max Fremery andJohann Urban developed a method to producecarbon fibers for use inlight bulbs in 1897.[12] Production ofcuprammonium rayon fortextiles started in 1899 in theVereinigte Glanzstoff Fabriken AG inOberbruch (nearAachen).[citation needed][13] Improvement byJ. P. Bemberg AG in 1904 made the artificial silk a product comparable to real silk.[14][15]
English chemistCharles 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.[16] In 1893, they formed the Viscose Syndicate to grant licences and, in 1896, formed the British Viscoid Co. Ltd.[11][17]
The first commercial viscose rayon was produced by the UK companyCourtaulds Fibres in November 1905.[18] Courtaulds formed an American division,American Viscose (later known as Avtex Fibers), to produce their formulation in the US in 1910.[19] The name "rayon" was adopted in 1924[citation needed], with "viscose" being used for the viscous organic liquid used to make both rayon andcellophane. In Europe, though, the fabric itself became known as "viscose", which has been ruled an acceptable alternative term for rayon by the USFederal Trade Commission (FTC).[citation needed]
Rayon was produced only as a filament fiber until the 1930s, when methods were developed to utilize "broken waste rayon" asstaple fiber.[citation needed]
Manufacturers' search for a less environmentally-harmful process for making Rayon led to the development of the lyocell method for producing Rayon.[20] The lyocell process was developed in 1972 by a team at the now defunctAmerican Enka fibers facility atEnka, North Carolina.[citation needed] In 2003, theAmerican Association of Textile Chemists and Colorists (AATCC) awarded Neal E. Franks their Henry E. Millson Award for Invention for lyocell.[21] 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.[20] 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 inNMMO was first described in a 1981 patent by Mcorsley for Akzona Incorporated[20][22] (the holding company of Akzo). In the 1980s the patent was licensed byAkzo to Courtaulds and Lenzing.[23] The fibre was developed byCourtaulds Fibres 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 theGrimsby, UK, pilot plant.[24][20] The process was first[citation needed] commercialized at Courtaulds' rayon factories atMobile, Alabama[25] (1990[citation needed]), and at the Grimsby plant (1998)[citation needed]. 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.[25] By 2004, production had quadrupled to 80,000 tons.[23]
Lenzing began a pilot plant in 1990,[20] and commercial production in 1997, with 12 metric tonnes/year made in a plant inHeiligenkreuz im Lafnitztal, Austria.[20][23] 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.[26] In 2004 Lenzing was producing 40,000tons [sic, probably metric tonnes].[23] In 1998, Lenzing and Courtaulds reached a patent dispute settlement.[23]
In 1998 Courtaulds was acquired by competitorAkzo Nobel,[27] which combined the Tencel division with other fibre divisions under the Accordis banner, then sold them to private equity firmCVC Partners. In 2000, CVC sold the Tencel division toLenzing AG, which combined it with their "Lenzing Lyocell" business, but maintained the brand name Tencel.[23] It took over the plants in Mobile and Grimsby, and by 2015 were the largest lyocell producer at 130,000 tonnes/year.[20]
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Rayon is produced by dissolving cellulose, then converting this solution back to insoluble fibrous cellulose. Various processes have been developed for this regeneration. The most common methods for creating rayon are thecuprammonium method, the viscose method, and the lyocell process. The first two methods have been practiced for more than a century.
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Cuprammonium rayon has properties similar to viscose; however, during its production, the cellulose is combined withcopper andammonia (Schweizer's reagent). Due to the detrimental environmental effects of this production method, cuprammonium rayon is no longer being produced in theUnited States.[28] The process has been described as obsolete,[6] but cuprammonium rayon is still made by one company in Japan.[29][better source needed]
Tetraamminecopper(II) sulfate is also used as a solvent.
The viscose process builds on the reaction ofcellulose with a strong base, followed by treatment of that solution withcarbon disulfide to give axanthate derivative. The xanthate is then converted back to a cellulose fiber in a subsequent step.
The viscose method can usewood as a source of cellulose, whereas other routes to rayon requirelignin-free cellulose as a 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 the quality of the wastewater.
The raw material for viscose is primarilywood pulp (sometimesbamboo pulp), which is chemically converted into a soluble compound. It is then dissolved and forced through aspinneret to produce filaments, which are chemically solidified, resulting in fibers of nearly pure cellulose.[30] Unless the chemicals are handled carefully, workers can be seriously harmed by thecarbon disulfide used to manufacture most rayon.[31][32]
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]n. This material is allowed todepolymerize 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 cellulosexanthate:[5]
Rayon fiber is produced from the ripened solutions by treatment with a mineral acid, such assulfuric acid. In this step, the xanthate groups are hydrolyzed to regenerate cellulose and carbon disulfide:
Aside from regenerated cellulose, acidification giveshydrogen sulfide (H2S), 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 ofsodium sulfide solution, and impurities are oxidized by bleaching withsodium hypochlorite solution orhydrogen peroxide solution.[17]
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:[30]
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Thelyocell process relies on dissolution of cellulose products in a solvent,N-methyl morpholine N-oxide (NMMO).
The process starts with cellulose and involvesdry-jet wet spinning. It was developed at the now defunctAmerican Enka Company andCourtaulds Fibres. Lenzing's Tencel is an example of a lyocell fiber.[14] Unlike the viscose process, the lycocell process does not use highly toxic carbon disulfide.[9][10] "Lyocell" has become a genericized trademark, used to refer to the lyocell process for making cellulose fibers.[10]
As of 2018[update], the lyocell process is not widely used, because it is still more expensive than the viscose process.[10][9]
Rayon is a versatile fiber and is widely claimed to have the same comfort properties as natural fibers, although thedrape and slipperiness of rayon textiles are often more likenylon. It can imitate the feel and texture ofsilk,wool,cotton, andlinen. The fibers are easilydyed 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" (feel) cool and sometimes almost slimy to the touch.[33]
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.[34]
Regular rayon has lengthwise lines calledstriations and its cross-section is an indented circular shape. The cross-sections of HWM and cupra rayon are rounder.Filament rayonyarns vary from 80 to 980 filaments per yarn and vary in size from 40 to 5000denier.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 delusteringpigments cuts down on this natural brightness.[34]
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 (HWM rayon) in the 1950s.[34] Research in the UK was centred on the government-fundedBritish Rayon Research Association.
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.[28]
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.
Modal is agenericized trademark ofLenzing AG, used for (viscose) rayon which is stretched as it is made, aligning the molecules along the fibers. Two forms are available: "polynosics" and "high wet modulus" (HWM).[35][36][better source needed]High-wet-modulus rayon is a modified version of viscose that is stronger when wet. It can bemercerized like cotton. HWM rayons are also known as "polynosic".[contradictory] 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.[28] Modal is used alone or with other fibers (often cotton orspandex) in clothing and household items like pajamas, underwear, bathrobes, towels, and bedsheets. Modal can betumble-dried without damage.[37] The fabric has been known topill less than cotton due to fiber properties and lower surface friction.[36] The trademarked Modal is made by spinning beech-tree cellulose and is considered a more eco-friendly alternative to cotton, as the production process uses on average 10–20 times less water.[38]
In 2018, viscose fiber production in the world was approximately 5.8 million tons, andChina was the largest producer with about 65% of total global production.[39]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 forcuprammonium rayon developed byJ. 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.[29] The fibers are finer than viscose rayon.[15][failed verification]
Modal andTencel are widely used forms of rayon produced byLenzing AG. Tencel, generic namelyocell, 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 byCourtaulds' Grimsby plant in England. The process, which dissolves cellulose without a chemical reaction, was developed by Courtaulds Research.
Birla Cellulose is also a volume manufacturer of rayon. They have plants located inIndia,Indonesia andChina.
Accordis was a major manufacturer of cellulose-based fibers and yarns. Production facilities can be found throughout Europe, the U.S. andBrazil.[40]
Visil rayon and HOPE FR areflame retardant forms of viscose that havesilica embedded in the fiber during manufacturing.
North American Rayon Corporation ofTennessee produced viscose rayon until its closure in the year 2000.[41][42]
Indonesia is one of the largest producers of rayon in the world, andAsia Pacific Rayon (APR) of the country has an annual production capacity of 0.24 million tons.[43]
Thebiodegradability 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 thanacetate. The more water-repellent the rayon-based fabric, the more slowly it will decompose.[44] Subsequent experiments have shown that wood-based fibres, like Lyocell, biodegrade much more readily thanpolyester.[45]Silverfish—like thefirebrat—can eat rayon, but damage was found to be minor, potentially due to the heavy, slick texture of the tested rayon.[46] Another study states that "artificial silk [...] [was] readily eaten" by thegrey silverfish.[47]
A 2014 ocean survey found that rayon contributed to 56.9% of the total fibers found indeep ocean areas, the rest being polyester,polyamides,acetate andacrylic.[48] A 2016 study found a discrepancy in the ability to identify natural fibers in a marine environment viaFourier transform infrared spectroscopy.[49] Later research of oceanic microfibers instead found cotton being the most frequent match (50% of all fibers), followed by other cellulosic fibers at 29.5% (e.g., rayon/viscose, linen, jute, kenaf, hemp, etc.).[50] Further analysis of the specific contribution of rayon to ocean fibers was not performed due to the difficulty in distinguishing between natural and man-made cellulosic fibers using FTIR spectra.
For several years, there have been concerns about links between rayon manufacturers and deforestation. As a result of these concerns,FSC andPEFC came on the same platform with CanopyPlanet to focus on these issues. CanopyPlanet subsequently started publishing a yearly Hot Button report, which puts all the man-made cellulosics manufacturers globally on the same scoring platform. The scoring from the 2020 report scores all such manufacturers on a scale of 35, the highest scores having been achieved byBirla Cellulose (33) andLenzing (30.5).
Carbon disulfide ishighly toxic.[51] It is well documented to have seriously harmed the health of rayon workers in developed countries, and emissions may also harm the health of people living near rayon plants[51] and their livestock.[52] Rates of disability in modern factories (mainly in China, Indonesia, and India) are unknown.[32][7] This has raised ethical concerns over viscose rayon production.[8][7][9][31] As of 2016[update], production facilities located in developing countries generally do not provide environmental or worker safety data.[53]
Most global carbon disulfide emissions come from rayon production, as of 2008.[54] As of 2004[update], about 250 g of carbon disulfide is emitted per kilogram of rayon produced.[55]
Control technologies have enabled improved collection of carbon disulfide and reuse of it, resulting in a lower emissions of carbon disulfide.[5] These have not always been implemented in places where it was not legally required and profitable.[52]
Carbon disulfide isvolatile and is lost before the rayon gets to the consumer; the rayon itself is basically purecellulose.[31]
Studies from the 1930s show that 30% of American rayon workers experienced significant health impacts due tocarbon disulfide exposure.Courtaulds worked hard to prevent this information being published in Britain.[8]
During theSecond World War, political prisoners inNazi Germany were made to work in appalling conditions at the Phrix rayon factory inKrefeld.[56] Nazis usedforced labour to produce rayon across occupied Europe.[8]
In the 1990s, viscose rayon producers faced lawsuits for negligentenvironmental pollution.Emissions abatement technologies had been consistently used.Carbon-bed recovery, for instance, which reduces emissions by about 90%, was used in Europe, but not in the US, by Courtaulds.[52]Pollution control and worker safety started to becomecost-limiting factors in production.
Japan has reduced carbon disulfide emissions per kilogram of viscose rayon produced (by about 16% per year), but in other rayon-producing countries, including China, emissions are uncontrolled. Rayon production is steady or decreasing except in China, where it is increasing, as of 2004[update].[55]
Rayon production has largely moved to the developing world, especially China, Indonesia and India.[7][8] Rates of disability in these factories are unknown, as of 2016[update],[32][7] and concerns for worker safety continue.[53]
Studies have found the production of rayon can be harmful to the health of factory workers.[57][58] Workers in factories utilizing the viscose process may be exposed to high levels of carbon disulfide, which can cause coronary heart disease, retinal damage, behavioral changes, impaired motor function, and various fertility and hormonal effects.[59]
Related materials are not regenerated cellulose, butesters of cellulose.[60][61]
Nitrocellulose is a derivative of cellulose that is soluble in organic solvents. It is mainly used as an explosive or as alacquer. Many early plastics, includingcelluloid, were made from nitrocellulose.
Cellulose acetate shares many traits with viscose rayon and was formerly considered 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 atumble dryer.[62][63] The two fabrics are now required to be listed distinctly on USA garment labels.[64]
Cellophane is generally made by the viscose process, but dried into sheets instead of fibers.
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