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Rayon is a manufactured regenerated cellulose fibre. It is made from purified cellulose, primarily from 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 synthetic fibres of nearly pure cellulose. Because rayon is manufactured from naturally occurring polymers, it is considered a semi-synthetic fibre. Specific types of rayon include viscose, modal and lyocell, each of which differs in manufacturing process and properties of the finished product.
Finally, in 1894, English chemist Charles Frederick Cross, and his collaborators Edward John Bevan, and Clayton Beadle patented their artificial silk, which they named "viscose", because the reaction product of carbon disulfide and cellulose in basic conditions gave a highly viscous solution of xanthate. The first commercial viscose rayon was produced by the U.K. company Courtaulds Fibres in 1905. Courtaulds formed an American division, American Viscose, (later known as Avtex Fibres) 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 U.S. Federal Trade Commission.
The method is able to use wood (cellulose and lignin) as a source of cellulose while the other methods need lignin-free cellulose as starting material. This makes it cheaper and therefore it was used on a larger scale than the other methods. Contamination of the waste water by carbon disulfide, lignin and the xanthates made this process detrimental to the environment. Rayon was only produced as a filament fibre until the 1930s when it was discovered that broken waste rayon could be used in staple fibre.
The physical properties of rayon were unchanged until the development of high-tenacity rayon in the 1940s. Further research and development led to the creation of high-wet-modulus rayon (HWM rayon) in the 1950s. Research in the UK was centred on the government-funded British Rayon Research Association.
MAJOR FIBRE PROPERTIES
Rayon is a versatile fibre and is widely claimed to have the same comfort properties as natural fibres, 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 fibres are easily dyed in a wide range of colours. Rayon fabrics are soft, smooth, cool, comfortable, and highly absorbent, but they do not insulate body heat, making them ideal for use in hot and humid climates, although also making their handfeel 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 fibre. 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.
Industrial applications of rayon emerged around 1935. Substituting cotton fibre in tires and belts, industrial types of rayon developed a totally different set of properties, amongst which tensile strength (elasticity) was paramount. Outperforming polyester, industrial yarns are still produced for high performance tires (e.g. Cordenka, Germany).
GALLERY OF TEXTURES
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 fibres range from 1.5 to 15 denier and are mechanically or chemically crimped. Rayon fibres are naturally very bright, but the addition of delustering pigments cuts down on this natural brightness.
Regular rayon (or viscose) is the most widely produced form of rayon. This method of rayon production has been utilized since the early 1900s and it has the ability to produce either filament or staple fibres. The process is as follows:
Cellulose: Production begins with processed cellulose (obtained from wood pulp and plant fibres).
Immersion: The cellulose is dissolved in caustic soda: (C6H10O5)n + nNaOH → (C6H9O4ONa)n + nH2O
Pressing: The solution is then pressed between rollers to remove excess liquid
White Crumb: The pressed sheets are crumbled or shredded to produce what is known as "white crumb"
Aging: The "white crumb" is aged through exposure to oxygen
Xanthation: The aged "white crumb" is mixed with carbon disulfide in a process known as Xanthation, the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulfide under controlled temperature (20 to 30 °C) to form cellulose xanthate: (C6H9O4ONa)n + nCS2 → (C6H9O4O-SC-SNa)n
Yellow Crumb: Xanthation changes the chemical makeup of the cellulose mixture and the resulting product is now called "yellow crumb"
Viscose: The "yellow crumb" is dissolved in a caustic solution to form viscose
Ripening: The viscose is set to stand for a period of time, allowing it to ripen: (C6H9O4O-SC-SNa)n + nH2O → (C6H10O5)n + nCS2 + nNaOH
Filtering: After ripening, the viscose is filtered to remove any undissolved particles
Degassing: Any bubbles of air are pressed from the viscose in a degassing process
Extruding: The viscose solution is extruded through a spinneret, which resembles a shower head with many small holes
Acid Bath: As the viscose exits the spinneret, it lands in a bath of sulphuric acid, resulting in the formation of rayon filaments: (C6H9O4O-SC-SNa)n + ½nH2SO4 → (C6H10O5)n + nCS2 + ½nNa2SO4
Drawing: The rayon filaments are stretched, known as drawing, to straighten out the fibres
Washing: The fibres are then washed to remove any residual chemicals
Cutting: If filament fibres are desired the process ends here. The filaments are cut down when producing staple fibres
High wet modulus rayon (HWM) is a modified version of viscose that has a greater strength when wet. It also has the ability to be mercerized like cotton. HWM rayons are also known as "polynosic" or can be 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 but during production, the cellulose is combined with copper and ammonia (Schweizer's reagent). Due to the environmental effects of this production method, cuprammonium rayon is no longer produced in the United States.
DISPOSAL AND BIODEGRADABILITY
The biodegradability of fibres in soil burial and sewage sludge was evaluated by Korean researchers, and are listed here in order of decreasing biodegradability: rayon, cotton, acetate. The more water-repellent the rayon-based fabric is, the more slowly it will decompose. Silverfish can eat rayon.
A recent ocean survey found that rayon contributed to 56.9% of the total microplastic fibres found in deep ocean areas.
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, companies such as Fortress Paper have been investing in pulp mills to increase production. Rayon now sells for as much as $2.70 per pound, which has led to an increase in the retail price of clothing made with rayon, yet rayon has a price advantage over cotton.
In early 2010, the U.S. Federal Trade Commission warned several retailers that six major manufacturers were falsely labelling rayon products as "bamboo", in order to appeal to environmentally conscious consumers. While rayon may be produced from bamboo as a raw material, and both rayon and bamboo may be used for similar fabrics (though natural bamboo is not as smooth), rayon is so far removed from bamboo by chemical processing that the two are entirely separate.
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. It performs much like viscose but has a smaller diameter and comes closest to silk in feel. Bemberg is now only produced in Italy due to United States Environmental Protection Agency regulations in the US. The fibres are finer than viscose rayon.
Modal and Tencel are widely used forms of rayon produced by Lenzing AG. Tencel, generic name lyocell, is made by a slightly different solvent recovery process, and is considered a different fibre by the US FTC. Tencel lyocell was first produced commercially by Courtaulds' Grimsby plant in England. The process which dissolves cellulose directly in amine oxide was developed by Courtaulds Research in Coventry.
Galaxy, Danufil, and Viloft are rayon brands produced by Kelheim Fibres.
Acordis was a major manufacturer of cellulose based fibres and yarns. Production facilities can be found throughout Europe, the U.S. and Brazil.
Visil rayon is a flame retardant form of viscose which has silica embedded in the fibre during manufacturing.
North American Rayon Corporation of Tennessee produced viscose rayon until its closure in the year 2000.
Grasim of India is the largest producer of rayon in the world (claiming 24% market share). It has plants in Nagda, Kharach and Harihar – all in India, as well as joint ventures in Canada, Laos and China.
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