Regenerated Fibres a6w1i
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REGENERATED FIBRES -: A fiber manufactured by dissolving a natural material, such as cellulose, and then restoring or regenerating the material through extrusion and precipitation. Cellulose, for example, is commonly regenerated to form rayon. VISCOSE RAYON Polymer Chemistry : The raw material for the production of Viscose rayon is wood pulp. However, the manufacturing process emits toxic effluents which have adverse environmental effects. Because of its cellulosic base , it has some properties which are similar to those of cotton and any other natural cellulosic fibres. It has moderate resistance to acids and alkalis. However, it may be damaged by relatively weak acids. Moreover, due to the inelasticity of fibers, it has poor abrasion resistance. Technology (Figure 40.2): The raw materials used in this process are cellulose wood pulp sheets or cotton linters. These are placed in a steeping press with in aqueous NaOH Solution for a period of 2 – 4 hours at normal room temperature. A hydraulic ram presses out the excess alkali and the sheets are shredded to crumbs and aged for 2 – 3 days. The aging process has its direct consequence on the viscosity of the solution. Later CS2 is added in a rotating drum mixer over a period of 3 hours. The orange cellulose xanthate which forms is transferred to a solubilizer , wherein it is mixed into dilute caustic . The mixing of cellulose xanthate and dilute caustic yields a orange colored viscous solution which contains 7 – 8 % cellulose and 6.5 – 7 % NaOH. It is digested at room temperature for 4-5 days. Thereafter , the solution is filtered and fed to spinning machines. The spinning is carried out in extrusion spinnerettes , which are made up of platinum or gold alloys. The orifices of these spinnerets have a diameter of 0.1 – 0.2 mm. for continuous filament yarns and of diameter 0.05 – 0.1 mm for short fibre shapes. The solution extruded from the spinnerette is ed with an acid bath which precipitates the filaments without causing them to break or stick together. The processing treatments of washing, desulphurizing, bleaching and conditioning takes place continuously and in order after the filaments are wound on a series of plastic rolls.
Figure 40.2 Manufacture of Viscose Rayon
Cellulose fibers are fibers from some plant or plant-based materials. They are usually categorized as "natural" or "manufactured". Natural Natural cellulose fibers are fibers that are still recognizable as being from a part of the original plant because they are only processed as much as needed to clean the fibers for use. For example, cotton fibers look like the soft fluffy cotton balls that they come from. Linen fibers look like the strong fiberous strands of the flax plant. All "natural" fibers go through a process where they are separated from the parts of the plant that are not used for the end product, usually through harvesting, separating from chaff, scouring, etc. The presence of linear chains of thousands of glucose units linked together allows a great deal of hydrogen bonding between OH groups on adjacent chains, causing them to pack closely into cellulose fibers. As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Since cellulose does not have a helical structure like amylose, it does not bind to iodine to form a colored product. They are usually categorized into two categories Manufactured Manufactured cellulose fibers come from plants that are processed into a pulp and then extruded in the same ways that synthetic fibers like polyester or nylon are made. Rayon or viscose is one of the most common "manufactured" cellulose fibers, and it can be made from wood pulp. Textile In the textile industry regenerated cellulose is used as fibers such as rayon, (including modal, and the more recently developed Lyocell). Cellulose fibers are manufactured from dissolving pulp. Cellulose-based fibers are of two types, regenerated or pure cellulose such as from the cupro-ammonium process and modified cellulose such as the cellulose acetates. The first artificial fiber, known as artificial silk, became known as viscose around 1894, and finally rayon in 1924. A similar product known as cellulose acetate was discovered in 1865. Rayon and acetate are both artificial fibers, but not truly
synthetic, being made from wood. Although these artificial fibers were discovered in the mid-nineteenth century, successful modern manufacture began much later. Environmental issues What is often marketed as "bamboo fiber" is actually not the fibers that grow in their natural form from the bamboo plants, but instead a highly processed bamboo pulp that is extruded as fibers. Although the process is not as environmentally friendly as "bamboo fiber" appears, planting & harvesting bamboo for fiber is much more sustainable and environmentally friendly than harvesting more slow growing trees and clearing existing forest habitats for timber plantations.
RAYON FIBERS Updated: April, 2004 - Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath (Praveen Kumar Jangala, Haoming Rong) 1. INTRODUCTION Rayon is the oldest commercial manmade fiber. The U. S. Trade Commission defines rayon as "manmade textile fibers and filaments composed of regenerated cellulose". The process of making viscose was discovered by C.F.Cross and E.J.Bevan in 1891. The process used to make viscose can either be a continuous or batch process. The batch process is flexible in producing a wide variety of rayons, with broad versatility. Rayon's versatility is the result of the fiber being chemically and structurally engineered by making use of the properties of cellulose from which it is made. However, it is somewhat difficult to control uniformity between batches and it also requires high labor involvement. The continuous process is the main method for producing rayon. Three methods of production lead to distinctly different rayon fibers: viscose rayon, cuprammonium rayon and saponified cellulose acetate. Of the methods mentioned, the viscose method is relatively inexpensive and of particular significance in the production of nonwoven fabrics. According to the latest data from the fiber Economics Bureau, domestic producers shipments of rayon staple to nonwoven roll goods are shown in table 1. Table 1: Shipments of Rayon Staple to Nonwoven roll goods [11]
Years
Millions pounds
1989
98
1990
72
1991
70
1992
70
1993
70
1994
64
1995
60
1996
57
1997
58
1998
60
of
2. VISCOSE RAYON The process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: (1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7)Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12) Washing, (13) Cutting. The various steps involved in the process of manufacturing viscose are shown in Fig. 1, and clarified below.
Figure 1: Process of manufacture of viscose rayon fiber
Steeping: Cellulose pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a temperature in the range of 18 to 25C in order to swell the cellulose fibers and to convert cellulose to alkali cellulose. (C6H10O5)n + nNaOH ---> (C6H9O4ONa)n + nH2O (2) Pressing: The swollen alkali cellulose mass is pressed to a wet weight equivalent of 2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of alkali to cellulose. (3) Shredding: The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called "crumbs". This step provides increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow. (4) Aging: The alkali cellulose is aged under controlled conditions of time and temperature (between 18 and 30 C) in order to depolymerize the cellulose to the desired degree of polymerization. In this step the average molecular weight of the original pulp is reduced by a factor of two to three. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration. (5) Xanthation: In this step the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulphide under controlled temperature (20 to 30C) to form cellulose xanthate. (C6H9O4ONa)n + nCS2 ----> (C6H9O4O-SC-SNa)n Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution. The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20 C under high-shear mixing conditions to obtain a viscous orange colored solution called "viscose", which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated. (6) Dissolving: The yellow crumb is dissolved in aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to solution of the otherwise insoluble cellulose. Because of the blocks of un-xanthated cellulose in the crystalline regions, the yellow crumb is not completely soluble at this stage. Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed "viscose"[13].
(7) Ripening: The viscose is allowed to stand for a period of time to "ripen". Two important process occur during ripening: Redistribution and loss of xanthate groups. The reversible xanthation reaction allows some of the xanthate groups to revert to cellulosic hydroxyls and free CS2. This free CS2 can then escape or react with other hydroxyl on other portions of the cellulose chain. In this way, the ordered, or crystalline, regions are gradually broken down and more complete solution is achieved. The CS2 that is lost reduces the solubility of the cellulose and facilitates regeneration of the cellulose after it is formed into a filament. (C6H9O4O-SC-SNa)n + nH2O ---> (C6H10O5)n + nCS2 + nNaOH (8) Filtering: The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament[13]. (9) Degassing: Bubbles of air entrapped in the viscose must be removed prior to extrusion or they would cause voids, or weak spots, in the fine rayon filaments[13]. (10) Spinning - (Wet Spinning): Production of Viscose Rayon Filament: The viscose solution is metered through a spinnerette into a spin bath containing sulphuric acid (necessary to acidify the sodium cellulose xanthate), sodium sulphate (necessary to impart a high salt content to the bath which is useful in rapid coagulation of viscose), and zinc sulphate (exchange with sodium xanthate to form zinc xanthate, to cross link the cellulose molecules). Once the cellulose xanthate is neutralized and acidified, rapid coagulation of the rayon filaments occurs which is followed by simultaneous stretching and decomposition of cellulose xanthate to regenerated cellulose. Stretching and decomposition are vital for getting the desired tenacity and other properties of rayon. Slow regeneration of cellulose and stretching of rayon will lead to greater areas of crystallinity within the fiber, as is done with high-tenacity rayons. The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation. (C6H9O4O-SC-SNa)n + (n/2)H2SO4 --> (C6H10O5)n + nCS2 + (n/2)Na2SO4 Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
(11)Drawing: The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fiber axis. As the chains become more parallel, interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibers[13]. (12) Washing: The freshly regenerated rayon contains many salts and other water soluble impurities which need to be removed. Several different washing techniques may be used [13]. (13) Cutting: If the rayon is to be used as staple (i.e., discreet lengths of fiber), the group of filaments (termed "tow") is ed through a rotary cutter to provide a fiber which can be processed in much the same way as cotton [13]. 3. CUPRAMMONIUM RAYON It is produced by a solution of cellulosic material in cuprammonium hydroxide solution at low temperature in a nitrogen atmosphere, followed by extruding through a spinnerette into a sulphuric acid solution necessary to decompose cuprammonium complex to cellulose. This is a more expensive process than that of viscose rayon. Its fiber cross section is almost round [14].
Fig. 2: Cupro flow chart
4. SAPONIFIED CELLULOSE ACETATE Rayon can be produced from cellulose acetate yarns by saponification. Purified cotton is steeped in glacial acetic acid to make it more reactive. It is then acetylated with excess of glacial acetic acid and acetic anhydride, with sulphuric acid to promote the reaction. The cellulose triacetate formed by acetylation is hydrolysed to convert triacetate to diacetate. The resultant mixture is poured into water which precipitates the cellulose acetate. For spinning it is dissolved in acetone, filtered, deaerated and extruded into hot air which evaporates the solvent. A high degree of orientation can be given to the fiber by drawing because of the fact that cellulose acetate is more plastic in nature. Its fiber cross section is nearly round, but lobed[15]
Fig. 3: Acetate flow chart
5. STRUCTURE OF RAYON The unit cell of cellulose is shown in Fig. 4.
Fig 4. Structure of unit cell of cellulose In regenerated celluloses, the unit cell structure is an allotropic modification of cellulose I, designated as cellulose II (other allotropic modifications are also known as cellulose III and cellulose IV). The structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than definite polymorphic forms of cellulose which depend on the conditions by which the fiber is made. Rayon fiber properties will depend on: how cellulose molecules are arranged and held together; the average size and size distribution of the molecules. Many models describe ways in which the cellulose molecules may be arranged to form fiber fine structure. The most popular models of fiber fine structure are the fringed micelle and fringed fibrillar structures. Essentially, they all entail the formation of crystallites or ordered regions. The skin-core effect is very prominent in rayon fibers. Mass transfer in wet spinning is a slow process (which s for the skin-core effect) compared to the heat transfer in melt spinning. The skin contains numerous small crystallites and the core has fewer but larger crystallites. The skin is stronger and less extensible, compared to the core. It also swells less than the core; hence, water retention is lower in the skin than in the core although moisture regain is higher in the skin. This is explained by an increased number of hydroxyl groups available for bonding with water as a result of a larger total surface area of the numerous small crystallites.
Fig. 5: Cellulose structure When rayon fibers are worked in the wet state,the filament structure can be made to disintegrate into a fibrillar texture. The extent to which this occurs reflects the order that exists in the fiber structure, as a consequence of the way in which the cellulose molecules are brought together in spinning. Another important structural feature of rayon fiber is its crosssectional shape. Various shapes include round, irregular, Y-shaped, E-shaped, U-shaped, T-shaped and flat. 6. PROPERTIES OF RAYON Variations during spinning of viscose or during drawing of filaments provide a wide variety of fibers with a wide variety of properties. These include: Fibers with thickness of 1.7 to 5.0dtex, particularly those between 1.7 and 3.3 dtex, dominate large scale production. Tenacity ranges between 2.0 to 2.6 g/den when dry and 1.0 to 1.5 g/den when wet. Wet strength of the fiber is of importance during its manufacturing and also in subsequent usage. Modifications in the production process have led to the problem of low wet strength being overcome. Dry and wet tenacies extend over a range depending on the degree of polymerization and crystallinity. The higher the crystallinity and orientation of rayon, the lower is the drop in tenacity upon wetting.
Percentage elongation-at-break seems to vary from 10 to 30 % dry and 15 to 40 % wet. Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon. Thermal properties: Viscose rayon loses strength above 149C; chars and decomposes at 177 to 204C. It does not melt or stick at elevated temperatures. Chemical properties: Hot dilute acids attack rayon, whereas bases do not seem to significantly attack rayon. Rayon is attacked by bleaches at very high concentrations and by mildew under severe hot and moist conditions. Prolonged exposure to sunlight causes loss of strength because of degradation of cellulose chains. Abrasion resistance is fair and rayon resists pill formation. Rayon has both poor crease recovery and crease retention. 7. Rayon Fiber Characteristics Highly absorbent Soft and comfortable Easy to dye Drapes well The drawing process applied in spinning may be adjusted to produce rayon fibers of extra strength and reduced elongation. Such fibers are designated as high tenacity rayons, which have about twice the strength and two-third of the stretch of regular rayon. An intermediate grade, known as medium tenacity rayon, is also made. Its strength and stretch characteristics fall midway between those of high tenacity and regular rayon[13]. 8. Some Major Rayon Fiber Uses Apparel: Accessories, blouses, dresses, jackets, lingerie, linings, millinery, slacks, sportshirts, sportswear, suits, ties, work clothes; Home Furnishings: Bedspreads, blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery; Industrial Uses: Industrial products, medical surgical products, nonwoven products, tire cord
Other Uses: Feminine hygiene products[13]. 9. DIFFERENT TYPES OF RAYONS Rayon fibers are engineered to possess a range of properties to meet the demands for a wide variety of end uses. Some of the important types of fibers are briefly described. High wet modulus rayon: These fibers have exceptionally high wet modulus of about 1 g/den and are used as parachute cords and other industrial uses. Fortisan fibers made by Celanese (saponified acetate) has also been used for the same purpose. Polynosic rayon: These fibers have a very high degree of orientation, achieved as a result of very high stretching (up to 300 %) during processing. They have a unique fibrillar structure, high dry and wet strength, low elongation (8 to 11 %), relatively low water retention and very high wet modulus. Specialty rayons: Flame retardant fibers: Flame retardance is achieved by the adhesion of the correct flame- retardant chemical to viscose. Examples of additives are alkyl, aryl and halogenated alkyl or aryl phosphates, phosphazenes, phosphonates and polyphosphonates. Flame retardant rayons have the additives distributed uniformly through the interior of the fiber and this property is advantageous over flame retardant cotton fibers where the flame retardant concentrates at the surface of the fiber. Super absorbent rayons: This is being produced in order to obtain higher water retention capacity (although regular rayon retains as much as 100 % of its weight). These fibers are used in surgical nonwovens. These fibers are obtained by including water- holding polymers (such as sodium polyacrylate or sodium carboxy methyl cellulose) in the viscose prior to spinning, to get a water retention capacity in the range of 150 to 200 % of its weight. Micro denier fibers: rayon fibers with deniers below 1.0 are now being developed and introduced into the market. These can be used to substantially improve fabric strength and absorbent properties. Cross section modification: Modification in cross sectional shape of viscose rayon can be used to dramatically change the fibers' aesthetic and technical properties. One such product is Viloft, a flat cross sectional fiber sold in Europe, which gives a unique soft handle, pleasing drape and handle. Another modified cross section fiber called Fibre ML(multi limbed) has a
very well defined trilobal shape. Fabrics made of these fibers have considerably enhanced absorbency, bulk, cover and wet rigidity all of which are suitable for usage as nonwovens [10]. Tencel rayon:Unlike viscose rayon, Tencel is produced by a straight solvation process. Wood pulp is dissolved in an amine oxide, which does not lead to undue degradation of the cellulose chains. The clear viscous solution is filtered and extruded into an aqueous bath, which precipitates the cellulose as fibers. This process does not involve any direct chemical reaction and the diluted amine oxide is purified and reused. This makes for a completely contained process fully compatible with all environmental regulations. Lyocell: A new form of cellulosic fiber, Lyocell, is starting to find uses in the nonwovens industry. Lyocell is manufactured using a solvent spinning process, and is produced by only two companies -- Acordis and Lenzing AG. To produce Lyocell, wood cellulose is dissolved directly in n-methyl morpholine n-oxide at high temperature and pressure. The cellulose precipitates in fiber form as the solvent is diluted, and can then be purified and dried. The solvent is recovered and reused. Lyocell has all the advantages of rayon, and in many respects is superior. It has high strength in both dry and wet states, high absorbency, and can fibrillate under certain conditions. In addition, the closed-loop manufacturing process is far more environmentally friendly than that used to manufacture rayon, although it is also more costly[12]. 10. MARKET POTENTIAL: The market share of rayon in the nonwovens area dropped has decreased since 1987 but has gradually picked up since 1990. Rayon was a predominant fiber used in the nonwovens industry until 1985. After 1985[3] the production of rayon decreased considerably in the US and Western Europe because of the increasing cost of the fiber. Wipes represent the largest nonwovens market for rayon. Fabric softeners represent the second largest, despite rayon's loss of market share to PET. Rayon is the fiber of choice in many medical applications such as surgical packs, drapes and gowns where hand, absorbency and sterilizability are important[7]. Cellulose acetate is a soft, supple fiber of low modulus and low sticking point of 180oF and thus, can be used as a binder fiber in the manufacture of nonwovens[8]. The development and expansion of hydroentanglement coupled with growing importance of disposability is now beginning to turn rayon properties into powerful advantages. The biodegradability and compatibility with both septic tank and main sewage systems enables them to be used in the manufacture of disposables. Recent trials have shown that in the sludge digestion plant where sludge is held for about 3 weeks for cleanup and stabilization prior to disposal, the rayons biodegrade totally within a week.[9]
Rayon with its unique characteristics has the potential to become the leading fiber used in the nowovens industry, if the inherent pollution in the manufacturing process can be corrected.
Introduction Of all the fibers, rayon is probably the most perplexing to consumers. It can be found in cotton-like end uses, as well as sumptuous velvets and taffetas. It may function successfully in absorbent hygiene and incontinence pads and equally well providing strength in tire cords. What is this fiber that has so many faces? Rayon was the first manufactured fiber. The term rayon was officially adopted by the textile industry. Unlike most man-made fibers, rayon is not synthetic. It is made from wood pulp, a naturally-occurring, cellulose-based raw material. As a result, rayon's properties are more similar to those of natural cellulosic fibers, such as cotton or linen, than those of thermoplastic, petroleum-based synthetic fibers such as nylon or polyester. Although rayon is made from wood pulp, a relatively inexpensive and renewable resource, processing requires high water and energy use, and has contributed to air and water pollution. Modernization of manufacturing plants and processes combined with availability of raw materials has increased rayon's competitiveness in the market.
History Rayon is the generic term for fiber (and the resulting yarn and fabric) manufactured of regenerated cellulose by any one of six processes. Its importance as a fiber lies in its versatility, and in the fact that it was the first viable manufactured fiber.
As far back as 1664, English naturalist Robert Hooke theorized that artificial filaments might be spun from a substance similar to that which silkworms secrete to make silk. This was often tried by scientists in the ensuing years who sought an "artificial silk", yet no one was to succeed until in 1855 the Frenchman did so, George Audemars. By dipping a needle into a viscous solution of mulberry bark pulp and gummy rubber, he was able to make a thread. While interesting from a scientific standpoint, this process was hardly viable economically - it was very slow, and required a great deal of skill and precision. The first commercial synthetic fiber was produced by Hilaire de Bernigaud, Count of Chardonnay (1839-1924) after 29 years of research, was patented in 1884, and manufactured by him in 1889. This cellulose-based fabric known as Chardonnay silk was pretty but very flammable, it was removed from the market. Soon after, the English chemist Charles Frederick Cross and his collaborators Edward John Bevan and Clayton Beadle discovered the viscose process in 1891 (1892?). Courtaulds Fibers produced the first commercial viscose rayon in 1905; the first in the United States was in 1910 by the American Viscose Company. Initially rayon was called "Artificial Silk", and many other names. In 1924 (1926?), a committee formed by the U.S. Department of Commerce and various commercial associations decided upon the name "rayon". It was called "rayon" for one of two reasons: either because of its brightness and similarities in structure with cotton (sun = ray, -on = cotton). Or because the naming committee couldn't find a name from the thousands entered in a contest they sponsored, and who hoped to shed a "ray of light" on the subject (from rayon, French for ray).
Properties Viscose Rayon has a silk-like aesthetic with superb drape and feel and retains its rich brilliant colors. Its cellulosic base contributes many properties similar to those of cotton or other natural cellulosic fibers. Rayon is moisture absorbent (more so than cotton), breathable, comfortable to wear, and easily dyed in vivid colors. It does not build up static electricity, nor will it pill unless the fabric is made from short, lowtwist yarns. Rayon is comfortable, soft to the skin, and has moderate dry strength and abrasion resistance. Like other cellulosic fibers, it is not resilient, which means that it will wrinkle. Rayon withstands ironing temperatures slightly less than those of cotton. It may be attacked by silverfish and termites, but generally resists insect damage. It will mildew, but that generally is not a problem. One of rayon's strengths is its versatility and ability to blend easily with many fibers—sometimes to reduce cost, other times for luster, softness, or absorbency and resulting comfort. Rayon has moderate resistance to acids and alkalis and generally the fiber itself is not damaged by bleaches; however, dyes used in the fabric may experience color change. As a cellulosic fiber, rayon will burn, but flame retardant finishes can be applied. Fiber Properties overview
General Characteristics: Rayon as a cloth is soft and comfortable. It drapes well, which is one of the reasons it is so desirable as an apparel fabric. Most characteristics are variable depending on processing, additives and finishing treatments, not to mention fabric construction.
Absorbency: Rayon is the most absorbent of all cellulose fibers, even more so than cotton and linen. Because of this, rayon absorbs perspiration and allows it to evaporate away from the skin, making it an excellent summer fabric. Its high absorbency applies equally to dyes, allowing beautiful, deep, rich colours. Strength: It loses a great deal of strength when wet. Because of this, it stretches and shrinks more than cotton. Abrasion resistance: Poor due to inelasticity of the fibers. It is easily damaged by scraping and will pill on the surface of the cloth. Flammability: Because of its excessive flammability, it inspired the Flammable Fabrics Act. The FFA was enacted by the U.S. Department of Commerce in 1953 in response to public concern over a number of serious burn accidents involving brushed rayon high pile sweaters (referred to as "torch sweaters") and children's cowboy chaps which could easily catch fire and flash burn. Static: No static build-up. Chemical reactions: Because it is a cellulose fiber, it is damaged by even relatively weak acids.
Comparative Rating Parameters Cotton
Viscose
Polyester
Moisture Regain
Good
Very good
Poor
Thermal protection
Good
Very good
Poor
Very good
Good
Poor
Softness
Good
Very good
Poor
Smoothness
Poor
Good
Very good
Static dissipation
Good
Very good
Poor
Drape
Good
Very good
Poor
Luster
Poor
Very good
Very good
Crease recovery
Poor
Poor
Very good
Uniformity
Poor
Very good
Good
Comfort
Air permeability
Aesthetic
Utility Performance
Antipilling
Good
Very good
Poor
Wash & wear
Good
Poor
Very good
Background For centuries humankind has relied upon various plants and animals to provide the raw materials for fabrics and clothing. Silkworms, sheep, beaver, buffalo deer, and even palm leaves are just some of the natural resources that have been used to meet these needs. However, in the last century scientists have turned to chemistry and technology to create and enhance many of the fabrics we now take for granted. There are two main categories of man-made fibers: those that are made from natural products (cellulosic fibers) and those that are synthesized solely from chemical compounds (noncellulosic polymer fibers). Rayon is a natural-based material that is made from the cellulose of wood pulp or cotton. This natural base gives it many of the characteristics—low cost, diversity, and comfort—that have led to its popularity and success. Today, rayon is considered to be one of the most versatile and economical man-made fibers available. It has been called "the laboratory's first gift to the loom." In the 1860s the French silk industry was being threatened by a disease affecting the silkworm. Louis Pasteur and Count Hilaire de Chardonnet were studying this problem with the hope of saving this vital industry. During this crisis, Chardonnet became interested in finding a way to produce artificial silk. In 1885 he patented the first successful process to make a useable fiber from cellulose. Even though other scientists have subsequently developed more cost-effective ways of making artificial silk, Chardonnet is still considered to be the father of rayon. For the next forty years this material was called artificial or imitation silk. By 1925 it had developed into an industry unto itself and was given the name rayon by the Federal Trade Commission (FTC). The term rayon at this time included any man-made fiber made from cellulose. In 1952, however, the FTC divided rayons into two categories: those fibers consisting of pure cellulose (rayon) and those consisting of a cellulose compound (acetate). By the 1950s, most of the rayon produced was being used in industrial and home furnishing products rather than in apparel, because regular rayon (also called viscose rayon) fibers were too weak compared to other fibers to be used in apparel. Then, in 1955, manufacturers began to produce a new type of rayon—high-wet-modulus (HWM) rayon—which was somewhat
stronger and which could be used successfully in sheets, towels, and apparel. The advent of HWM rayon (also called modified rayon) is considered the most important development in rayon production since its invention in the 1880s. Today rayon is one of the most widely used fabrics in our society. It is made in countries around the world. It can be blended with natural or man-made fabrics, treated with enhancements, and even engineered to perform a variety of functions. Raw Materials Regardless of the design or manufacturing process, the basic raw material for making rayon is cellulose. The major sources for natural cellulose are wood pulp—usually from pine, sprucs—
To make rayon, sheets of purified cellulose are steeped in caustic soda, dried, shredded into crumbs, and then aged in metal containers for 2 to 3 days. The temperature and humidity in the metal containers are carefully controlled. After ageing, the crumbs are combined and churned with liquid carbon disulfide, which turns the mix into orange-colored crumbs known as sodium cellulose xanthate. The cellulose xanthate is bathed in caustic soda, resulting in a viscose solution that looks and feels much like honey. cotton linters. Cotton linters are residue fibers which cling to cotton seed after the ginning process. Strictly defined, rayon is a manufactured fiber composed of regenerated cellulose. The legal definition also includes manufactured fibers in which substitutes have not replaced more than 15 percent of the hydrogens. While the basic manufacturing process for all rayon is similar, this fabric can be engineered to perform a wide range of functions. Various factors in the manufacturing process can be altered to produce an array of designs. Differences in the raw material, the processing chemicals, fiber diameter, post treatments and blend ratios can be manipulated to produce a fiber that is customized for a specific application. Regular or viscose rayon is the most prevalent, versatile and successful type of rayon. It can be blended with man-made or natural fibers and made into fabrics of varying weight and texture. It is also highly absorbent, economical and comfortable to wear. Regular viscose rayon does have some disadvantages. It's not as strong as many of the newer fabrics, nor is it as strong as natural cotton or flax. This inherent weakness is exacerbated when it becomes wet or overexposed to light. Also, regular rayon has a tendency to shrink when washed. Mildew, acid and high temperatures such as ironing can also result in damage. Fortunately, these disadvantages can be countered by chemical treatments and the blending of rayon with other fibers of offsetting characteristics. High-wet-modulus rayon is a stronger fiber than regular rayon, and in fact is more similar in performance to cotton than to regular rayon. It has better elastic recovery than regular rayon, and fabrics containing it are easier to care for—they can be machine-washed, whereas fabrics containing regular rayon generally have to be dry-cleaned.
The Manufacturing Process While there are many variations in the manufacturing process that exploit the versatility of the fiber, the following is a description of the procedure that is used in making regular or viscose rayon. Regardless of whether wood pulp or cotton linters are used, the basic raw material for making rayon must be processed in order to extract and purify the cellulose. The resulting sheets of white, purified cellulose are then treated to form regenerated cellulose filaments. In turn, these filaments are spun into yarns and eventually made into the desired fabric. Processing purified cellulose
1 Sheets of purified cellulose are steeped in sodium hydroxide (caustic soda), which produces sheets of alkali cellulose. These sheets are dried, shredded into crumbs, and then aged in metal containers for 2 to 3 days. The temperature and humidity in the metal containers are carefully controlled. 2 After ageing, the crumbs are combined and churned with liquid carbon disulfide, which turns the mix into orangecolored crumbs known as sodium cellulose xanthate. The cellulose xanthate is bathed in caustic soda, resulting in a viscose solution that looks and feels much like honey. Any dyes or delusterants in the design are then added. The syrupy solution is filtered for impurities and stored in vats to age, this time between 4 and 5 days.
Producing filaments
3 The viscose solution is next turned into strings of fibers. This is done by forcing the liquid through a spinneret, which works like a shower-head, into an acid bath. If staple fiber is to be produced, a large spinneret with large holes is used. If filament fiber is being produced, then a spinneret with smaller holes is used. In the acid bath, the acid coagulates and solidifies the filaments, now known as regenerated cellulose filaments.
Spinning
4 After being bathed in acid, the filaments are ready to be spun into yarn. Depending on the type of yarn desired, several spinning methods can be used, including Pot Spinning, Spool Spinning, and Continuous Spinning. In Pot Spinning, the filaments are first stretched under controlled tension onto a series of offsetting rollers called godet wheels. This stretching reduces the diameter of the filaments and makes them more uniform in size, and it also gives
the filaments more strength. The filaments are then put into a rapidly spinning cylinder called a Topham Box, resulting in a cake-like strings that stick to the sides of the Topham Box. The strings are then washed, bleached, rinsed, dried, and wound on cones or spools. Spool Spinning is very similar to Pot Spinning. The filaments are ed through rollers and wound on spools, where they are washed, bleached, rinsed, dried, and wound again on spools or cones. In Continuous Spinning, the filaments are washed, bleached, dried, twisted, and wound at the same time that they are stretched over godet wheels.
5 Once the fibers are sufficiently cured, they are ready for post-treatment chemicals and the various weaving processes necessary to produce the fabric. The resulting fabric can then be given any of a number of finishing treatments. These include calendaring, to control smoothness; fire resistance; pre-shrinking; water resistance; and wrinkle resistance.
High-Wet-Modulus Rayon Manufacture The process for manufacturing high-wet-modulus rayon is similar to that used for making regular rayon, with a few exceptions. First, in step #1 above, when the purified cellulose sheets are bathed in a caustic soda solution,
After the syrupy viscose solution is prepared, it is forced through a spinneret into an acid bath. The resulting strings or filaments are then stretched on godet wheels to strengthen them and put into a spinning Topham box. This method produces cake-like strings of rayon, which are washed, rinsed, and dried before being wound on spools or cones. a weaker caustic soda is used when making HWM rayon. Second, neither the alkali crumbs (#1 above) nor the viscose solution (step #2) is aged in the HWM process. Third, when making HWM rayon, the filaments are stretched to a greater degree than when making regular rayon. Quality Control As with most chemically oriented processes, quality control is crucial to the successful manufacture of rayon. Chemical make-up, timing and temperature are essential factors that must be monitored and controlled in order to produce the desired result.
The percentages of the various fibers used in a blended fabric must be controlled to stay within in the legal bounds of the Textile Fiber Identification Act. This act legally defines seventeen groups of man-made fibers. Six of these seventeen groups are made from natural material. They include rayon, acetate, glass fiber, metallics, rubber, and azion. The remaining eleven fabrics are synthesized solely from chemical compounds. They are nylon, polyester, acrylic, modacrylic, olefin, spandex, anidex, saran, vinal, vinyon, and nytril. Within each generic group there are brand names for fibers which are produced by different manufacturers. Private companies often seek patents on unique features and, as could be expected, attempt to maintain legal control over their competition. Byproducts As one of the industry's major problems, the chemical by-products of rayon have received much attention in these environmentally conscious times. The most popular method of production, the viscose method, generates undesirable water and air emissions. Of particular concern is the emission of zinc and hydrogen sulfide. At present, producers are trying a number of techniques to reduce pollution. Some of the techniques being used are the recovery of zinc by ion-exchange, crystallization, and the use of a more purified cellulose. Also, the use of absorption and chemical scrubbing is proving to be helpful in reducing undesirable emissions of gas. The Future The future of rayon is bright. Not only is there a growing demand for rayon worldwide, but there are many new technologies that promise to make rayon even better and cheaper. For a while in the 1970s there was a trend in the clothing industry toward purely synthetic materials like polyester. However, since purely synthetic material does not "breath" like natural material, these products were not well received by the consumer. Today there is a strong trend toward blended fabrics. Blends offer the best of both worlds. With the present body of knowledge about the structure and chemical reactivity of cellulose, some scientist believe it may soon be possible to produce the cellulose molecule directly from sunlight, water and carbon dioxide. If this technique proves to be cost effective, such hydroponic factories could represent a giant step forward in the quest to provide the raw materials necessary to meet the world wide demand for man-made fabric.
ACETATE RAYON INTRODUCTION Initially invented in Europe as a varnish for airplane wings, acetate was first produced in the United States in 1924 making it the second oldest manufactured fiber. Like rayon, acetate is a cellulose-based fiber that was initially only an experimental fiber. Before mass production could occur, modifications to the dyeing process was necessary before it could be usable. Acetate was the first thermoplastic (heat sensitive) fiber to be introduced to consumers. Originally, consumers were surprised that a fabric could melt when ironed.
In addition to the early discovery that acetate’s chemical structure was not compliant with the current dyes, it was also soon discovered that pollutants and fumes in the air caused acetate fabrics to discolor.The dyeing process was once again changed to help correct this problem that resulted in solution dyeing, a process that can now be used on all manufactured fibers . However, acetate fabrics are still somewhat sensitive to discoloration due to air pollutants .Like rayon, acetate is
often used as a substitute for silk, but acetate is much weaker than either rayon or silk and is generally used for apparel that will not be worn often . Specifically, special occasion clothing is made from acetate fabric. ADVANTAGES OF ACETATE FABRIC: •Excellent drape (a fabric’s ability to fold while worn) •Luxurious hand (the feel of a fiber, yarn, or fabric to the wearer) •High luster (reflection of light on fabric) •No pilling •Little static •Low cost •Holds white very well DISADVANTAGES OF ACETATE FABRIC: •Poor resiliency (fabric does not return to original shape or form after being altered) •Poor elasticity •Poor tenacity (ability to withstand stress) •Low abrasion resistance •Poor strength •Thermoplastic (melts when heated) •Susceptible to wrinkles •Most often needs to be dry clean END USES OF ACETATE FABRIC: •Apparel: lining, graduation gowns, neck ties, special occasion wear, dresses, ribbons, blouses, carves •Interior: draperies, upholstery, quilted items
Figure 40.2 Manufacture of Viscose Rayon
Cellulose fibers are fibers from some plant or plant-based materials. They are usually categorized as "natural" or "manufactured". Natural Natural cellulose fibers are fibers that are still recognizable as being from a part of the original plant because they are only processed as much as needed to clean the fibers for use. For example, cotton fibers look like the soft fluffy cotton balls that they come from. Linen fibers look like the strong fiberous strands of the flax plant. All "natural" fibers go through a process where they are separated from the parts of the plant that are not used for the end product, usually through harvesting, separating from chaff, scouring, etc. The presence of linear chains of thousands of glucose units linked together allows a great deal of hydrogen bonding between OH groups on adjacent chains, causing them to pack closely into cellulose fibers. As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Since cellulose does not have a helical structure like amylose, it does not bind to iodine to form a colored product. They are usually categorized into two categories Manufactured Manufactured cellulose fibers come from plants that are processed into a pulp and then extruded in the same ways that synthetic fibers like polyester or nylon are made. Rayon or viscose is one of the most common "manufactured" cellulose fibers, and it can be made from wood pulp. Textile In the textile industry regenerated cellulose is used as fibers such as rayon, (including modal, and the more recently developed Lyocell). Cellulose fibers are manufactured from dissolving pulp. Cellulose-based fibers are of two types, regenerated or pure cellulose such as from the cupro-ammonium process and modified cellulose such as the cellulose acetates. The first artificial fiber, known as artificial silk, became known as viscose around 1894, and finally rayon in 1924. A similar product known as cellulose acetate was discovered in 1865. Rayon and acetate are both artificial fibers, but not truly
synthetic, being made from wood. Although these artificial fibers were discovered in the mid-nineteenth century, successful modern manufacture began much later. Environmental issues What is often marketed as "bamboo fiber" is actually not the fibers that grow in their natural form from the bamboo plants, but instead a highly processed bamboo pulp that is extruded as fibers. Although the process is not as environmentally friendly as "bamboo fiber" appears, planting & harvesting bamboo for fiber is much more sustainable and environmentally friendly than harvesting more slow growing trees and clearing existing forest habitats for timber plantations.
RAYON FIBERS Updated: April, 2004 - Raghavendra R. Hegde, Atul Dahiya, M. G. Kamath (Praveen Kumar Jangala, Haoming Rong) 1. INTRODUCTION Rayon is the oldest commercial manmade fiber. The U. S. Trade Commission defines rayon as "manmade textile fibers and filaments composed of regenerated cellulose". The process of making viscose was discovered by C.F.Cross and E.J.Bevan in 1891. The process used to make viscose can either be a continuous or batch process. The batch process is flexible in producing a wide variety of rayons, with broad versatility. Rayon's versatility is the result of the fiber being chemically and structurally engineered by making use of the properties of cellulose from which it is made. However, it is somewhat difficult to control uniformity between batches and it also requires high labor involvement. The continuous process is the main method for producing rayon. Three methods of production lead to distinctly different rayon fibers: viscose rayon, cuprammonium rayon and saponified cellulose acetate. Of the methods mentioned, the viscose method is relatively inexpensive and of particular significance in the production of nonwoven fabrics. According to the latest data from the fiber Economics Bureau, domestic producers shipments of rayon staple to nonwoven roll goods are shown in table 1. Table 1: Shipments of Rayon Staple to Nonwoven roll goods [11]
Years
Millions pounds
1989
98
1990
72
1991
70
1992
70
1993
70
1994
64
1995
60
1996
57
1997
58
1998
60
of
2. VISCOSE RAYON The process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: (1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7)Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12) Washing, (13) Cutting. The various steps involved in the process of manufacturing viscose are shown in Fig. 1, and clarified below.
Figure 1: Process of manufacture of viscose rayon fiber
Steeping: Cellulose pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a temperature in the range of 18 to 25C in order to swell the cellulose fibers and to convert cellulose to alkali cellulose. (C6H10O5)n + nNaOH ---> (C6H9O4ONa)n + nH2O (2) Pressing: The swollen alkali cellulose mass is pressed to a wet weight equivalent of 2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of alkali to cellulose. (3) Shredding: The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called "crumbs". This step provides increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow. (4) Aging: The alkali cellulose is aged under controlled conditions of time and temperature (between 18 and 30 C) in order to depolymerize the cellulose to the desired degree of polymerization. In this step the average molecular weight of the original pulp is reduced by a factor of two to three. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration. (5) Xanthation: In this step the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulphide under controlled temperature (20 to 30C) to form cellulose xanthate. (C6H9O4ONa)n + nCS2 ----> (C6H9O4O-SC-SNa)n Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution. The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20 C under high-shear mixing conditions to obtain a viscous orange colored solution called "viscose", which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated. (6) Dissolving: The yellow crumb is dissolved in aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to solution of the otherwise insoluble cellulose. Because of the blocks of un-xanthated cellulose in the crystalline regions, the yellow crumb is not completely soluble at this stage. Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed "viscose"[13].
(7) Ripening: The viscose is allowed to stand for a period of time to "ripen". Two important process occur during ripening: Redistribution and loss of xanthate groups. The reversible xanthation reaction allows some of the xanthate groups to revert to cellulosic hydroxyls and free CS2. This free CS2 can then escape or react with other hydroxyl on other portions of the cellulose chain. In this way, the ordered, or crystalline, regions are gradually broken down and more complete solution is achieved. The CS2 that is lost reduces the solubility of the cellulose and facilitates regeneration of the cellulose after it is formed into a filament. (C6H9O4O-SC-SNa)n + nH2O ---> (C6H10O5)n + nCS2 + nNaOH (8) Filtering: The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament[13]. (9) Degassing: Bubbles of air entrapped in the viscose must be removed prior to extrusion or they would cause voids, or weak spots, in the fine rayon filaments[13]. (10) Spinning - (Wet Spinning): Production of Viscose Rayon Filament: The viscose solution is metered through a spinnerette into a spin bath containing sulphuric acid (necessary to acidify the sodium cellulose xanthate), sodium sulphate (necessary to impart a high salt content to the bath which is useful in rapid coagulation of viscose), and zinc sulphate (exchange with sodium xanthate to form zinc xanthate, to cross link the cellulose molecules). Once the cellulose xanthate is neutralized and acidified, rapid coagulation of the rayon filaments occurs which is followed by simultaneous stretching and decomposition of cellulose xanthate to regenerated cellulose. Stretching and decomposition are vital for getting the desired tenacity and other properties of rayon. Slow regeneration of cellulose and stretching of rayon will lead to greater areas of crystallinity within the fiber, as is done with high-tenacity rayons. The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation. (C6H9O4O-SC-SNa)n + (n/2)H2SO4 --> (C6H10O5)n + nCS2 + (n/2)Na2SO4 Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
(11)Drawing: The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fiber axis. As the chains become more parallel, interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibers[13]. (12) Washing: The freshly regenerated rayon contains many salts and other water soluble impurities which need to be removed. Several different washing techniques may be used [13]. (13) Cutting: If the rayon is to be used as staple (i.e., discreet lengths of fiber), the group of filaments (termed "tow") is ed through a rotary cutter to provide a fiber which can be processed in much the same way as cotton [13]. 3. CUPRAMMONIUM RAYON It is produced by a solution of cellulosic material in cuprammonium hydroxide solution at low temperature in a nitrogen atmosphere, followed by extruding through a spinnerette into a sulphuric acid solution necessary to decompose cuprammonium complex to cellulose. This is a more expensive process than that of viscose rayon. Its fiber cross section is almost round [14].
Fig. 2: Cupro flow chart
4. SAPONIFIED CELLULOSE ACETATE Rayon can be produced from cellulose acetate yarns by saponification. Purified cotton is steeped in glacial acetic acid to make it more reactive. It is then acetylated with excess of glacial acetic acid and acetic anhydride, with sulphuric acid to promote the reaction. The cellulose triacetate formed by acetylation is hydrolysed to convert triacetate to diacetate. The resultant mixture is poured into water which precipitates the cellulose acetate. For spinning it is dissolved in acetone, filtered, deaerated and extruded into hot air which evaporates the solvent. A high degree of orientation can be given to the fiber by drawing because of the fact that cellulose acetate is more plastic in nature. Its fiber cross section is nearly round, but lobed[15]
Fig. 3: Acetate flow chart
5. STRUCTURE OF RAYON The unit cell of cellulose is shown in Fig. 4.
Fig 4. Structure of unit cell of cellulose In regenerated celluloses, the unit cell structure is an allotropic modification of cellulose I, designated as cellulose II (other allotropic modifications are also known as cellulose III and cellulose IV). The structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than definite polymorphic forms of cellulose which depend on the conditions by which the fiber is made. Rayon fiber properties will depend on: how cellulose molecules are arranged and held together; the average size and size distribution of the molecules. Many models describe ways in which the cellulose molecules may be arranged to form fiber fine structure. The most popular models of fiber fine structure are the fringed micelle and fringed fibrillar structures. Essentially, they all entail the formation of crystallites or ordered regions. The skin-core effect is very prominent in rayon fibers. Mass transfer in wet spinning is a slow process (which s for the skin-core effect) compared to the heat transfer in melt spinning. The skin contains numerous small crystallites and the core has fewer but larger crystallites. The skin is stronger and less extensible, compared to the core. It also swells less than the core; hence, water retention is lower in the skin than in the core although moisture regain is higher in the skin. This is explained by an increased number of hydroxyl groups available for bonding with water as a result of a larger total surface area of the numerous small crystallites.
Fig. 5: Cellulose structure When rayon fibers are worked in the wet state,the filament structure can be made to disintegrate into a fibrillar texture. The extent to which this occurs reflects the order that exists in the fiber structure, as a consequence of the way in which the cellulose molecules are brought together in spinning. Another important structural feature of rayon fiber is its crosssectional shape. Various shapes include round, irregular, Y-shaped, E-shaped, U-shaped, T-shaped and flat. 6. PROPERTIES OF RAYON Variations during spinning of viscose or during drawing of filaments provide a wide variety of fibers with a wide variety of properties. These include: Fibers with thickness of 1.7 to 5.0dtex, particularly those between 1.7 and 3.3 dtex, dominate large scale production. Tenacity ranges between 2.0 to 2.6 g/den when dry and 1.0 to 1.5 g/den when wet. Wet strength of the fiber is of importance during its manufacturing and also in subsequent usage. Modifications in the production process have led to the problem of low wet strength being overcome. Dry and wet tenacies extend over a range depending on the degree of polymerization and crystallinity. The higher the crystallinity and orientation of rayon, the lower is the drop in tenacity upon wetting.
Percentage elongation-at-break seems to vary from 10 to 30 % dry and 15 to 40 % wet. Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon. Thermal properties: Viscose rayon loses strength above 149C; chars and decomposes at 177 to 204C. It does not melt or stick at elevated temperatures. Chemical properties: Hot dilute acids attack rayon, whereas bases do not seem to significantly attack rayon. Rayon is attacked by bleaches at very high concentrations and by mildew under severe hot and moist conditions. Prolonged exposure to sunlight causes loss of strength because of degradation of cellulose chains. Abrasion resistance is fair and rayon resists pill formation. Rayon has both poor crease recovery and crease retention. 7. Rayon Fiber Characteristics Highly absorbent Soft and comfortable Easy to dye Drapes well The drawing process applied in spinning may be adjusted to produce rayon fibers of extra strength and reduced elongation. Such fibers are designated as high tenacity rayons, which have about twice the strength and two-third of the stretch of regular rayon. An intermediate grade, known as medium tenacity rayon, is also made. Its strength and stretch characteristics fall midway between those of high tenacity and regular rayon[13]. 8. Some Major Rayon Fiber Uses Apparel: Accessories, blouses, dresses, jackets, lingerie, linings, millinery, slacks, sportshirts, sportswear, suits, ties, work clothes; Home Furnishings: Bedspreads, blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery; Industrial Uses: Industrial products, medical surgical products, nonwoven products, tire cord
Other Uses: Feminine hygiene products[13]. 9. DIFFERENT TYPES OF RAYONS Rayon fibers are engineered to possess a range of properties to meet the demands for a wide variety of end uses. Some of the important types of fibers are briefly described. High wet modulus rayon: These fibers have exceptionally high wet modulus of about 1 g/den and are used as parachute cords and other industrial uses. Fortisan fibers made by Celanese (saponified acetate) has also been used for the same purpose. Polynosic rayon: These fibers have a very high degree of orientation, achieved as a result of very high stretching (up to 300 %) during processing. They have a unique fibrillar structure, high dry and wet strength, low elongation (8 to 11 %), relatively low water retention and very high wet modulus. Specialty rayons: Flame retardant fibers: Flame retardance is achieved by the adhesion of the correct flame- retardant chemical to viscose. Examples of additives are alkyl, aryl and halogenated alkyl or aryl phosphates, phosphazenes, phosphonates and polyphosphonates. Flame retardant rayons have the additives distributed uniformly through the interior of the fiber and this property is advantageous over flame retardant cotton fibers where the flame retardant concentrates at the surface of the fiber. Super absorbent rayons: This is being produced in order to obtain higher water retention capacity (although regular rayon retains as much as 100 % of its weight). These fibers are used in surgical nonwovens. These fibers are obtained by including water- holding polymers (such as sodium polyacrylate or sodium carboxy methyl cellulose) in the viscose prior to spinning, to get a water retention capacity in the range of 150 to 200 % of its weight. Micro denier fibers: rayon fibers with deniers below 1.0 are now being developed and introduced into the market. These can be used to substantially improve fabric strength and absorbent properties. Cross section modification: Modification in cross sectional shape of viscose rayon can be used to dramatically change the fibers' aesthetic and technical properties. One such product is Viloft, a flat cross sectional fiber sold in Europe, which gives a unique soft handle, pleasing drape and handle. Another modified cross section fiber called Fibre ML(multi limbed) has a
very well defined trilobal shape. Fabrics made of these fibers have considerably enhanced absorbency, bulk, cover and wet rigidity all of which are suitable for usage as nonwovens [10]. Tencel rayon:Unlike viscose rayon, Tencel is produced by a straight solvation process. Wood pulp is dissolved in an amine oxide, which does not lead to undue degradation of the cellulose chains. The clear viscous solution is filtered and extruded into an aqueous bath, which precipitates the cellulose as fibers. This process does not involve any direct chemical reaction and the diluted amine oxide is purified and reused. This makes for a completely contained process fully compatible with all environmental regulations. Lyocell: A new form of cellulosic fiber, Lyocell, is starting to find uses in the nonwovens industry. Lyocell is manufactured using a solvent spinning process, and is produced by only two companies -- Acordis and Lenzing AG. To produce Lyocell, wood cellulose is dissolved directly in n-methyl morpholine n-oxide at high temperature and pressure. The cellulose precipitates in fiber form as the solvent is diluted, and can then be purified and dried. The solvent is recovered and reused. Lyocell has all the advantages of rayon, and in many respects is superior. It has high strength in both dry and wet states, high absorbency, and can fibrillate under certain conditions. In addition, the closed-loop manufacturing process is far more environmentally friendly than that used to manufacture rayon, although it is also more costly[12]. 10. MARKET POTENTIAL: The market share of rayon in the nonwovens area dropped has decreased since 1987 but has gradually picked up since 1990. Rayon was a predominant fiber used in the nonwovens industry until 1985. After 1985[3] the production of rayon decreased considerably in the US and Western Europe because of the increasing cost of the fiber. Wipes represent the largest nonwovens market for rayon. Fabric softeners represent the second largest, despite rayon's loss of market share to PET. Rayon is the fiber of choice in many medical applications such as surgical packs, drapes and gowns where hand, absorbency and sterilizability are important[7]. Cellulose acetate is a soft, supple fiber of low modulus and low sticking point of 180oF and thus, can be used as a binder fiber in the manufacture of nonwovens[8]. The development and expansion of hydroentanglement coupled with growing importance of disposability is now beginning to turn rayon properties into powerful advantages. The biodegradability and compatibility with both septic tank and main sewage systems enables them to be used in the manufacture of disposables. Recent trials have shown that in the sludge digestion plant where sludge is held for about 3 weeks for cleanup and stabilization prior to disposal, the rayons biodegrade totally within a week.[9]
Rayon with its unique characteristics has the potential to become the leading fiber used in the nowovens industry, if the inherent pollution in the manufacturing process can be corrected.
Introduction Of all the fibers, rayon is probably the most perplexing to consumers. It can be found in cotton-like end uses, as well as sumptuous velvets and taffetas. It may function successfully in absorbent hygiene and incontinence pads and equally well providing strength in tire cords. What is this fiber that has so many faces? Rayon was the first manufactured fiber. The term rayon was officially adopted by the textile industry. Unlike most man-made fibers, rayon is not synthetic. It is made from wood pulp, a naturally-occurring, cellulose-based raw material. As a result, rayon's properties are more similar to those of natural cellulosic fibers, such as cotton or linen, than those of thermoplastic, petroleum-based synthetic fibers such as nylon or polyester. Although rayon is made from wood pulp, a relatively inexpensive and renewable resource, processing requires high water and energy use, and has contributed to air and water pollution. Modernization of manufacturing plants and processes combined with availability of raw materials has increased rayon's competitiveness in the market.
History Rayon is the generic term for fiber (and the resulting yarn and fabric) manufactured of regenerated cellulose by any one of six processes. Its importance as a fiber lies in its versatility, and in the fact that it was the first viable manufactured fiber.
As far back as 1664, English naturalist Robert Hooke theorized that artificial filaments might be spun from a substance similar to that which silkworms secrete to make silk. This was often tried by scientists in the ensuing years who sought an "artificial silk", yet no one was to succeed until in 1855 the Frenchman did so, George Audemars. By dipping a needle into a viscous solution of mulberry bark pulp and gummy rubber, he was able to make a thread. While interesting from a scientific standpoint, this process was hardly viable economically - it was very slow, and required a great deal of skill and precision. The first commercial synthetic fiber was produced by Hilaire de Bernigaud, Count of Chardonnay (1839-1924) after 29 years of research, was patented in 1884, and manufactured by him in 1889. This cellulose-based fabric known as Chardonnay silk was pretty but very flammable, it was removed from the market. Soon after, the English chemist Charles Frederick Cross and his collaborators Edward John Bevan and Clayton Beadle discovered the viscose process in 1891 (1892?). Courtaulds Fibers produced the first commercial viscose rayon in 1905; the first in the United States was in 1910 by the American Viscose Company. Initially rayon was called "Artificial Silk", and many other names. In 1924 (1926?), a committee formed by the U.S. Department of Commerce and various commercial associations decided upon the name "rayon". It was called "rayon" for one of two reasons: either because of its brightness and similarities in structure with cotton (sun = ray, -on = cotton). Or because the naming committee couldn't find a name from the thousands entered in a contest they sponsored, and who hoped to shed a "ray of light" on the subject (from rayon, French for ray).
Properties Viscose Rayon has a silk-like aesthetic with superb drape and feel and retains its rich brilliant colors. Its cellulosic base contributes many properties similar to those of cotton or other natural cellulosic fibers. Rayon is moisture absorbent (more so than cotton), breathable, comfortable to wear, and easily dyed in vivid colors. It does not build up static electricity, nor will it pill unless the fabric is made from short, lowtwist yarns. Rayon is comfortable, soft to the skin, and has moderate dry strength and abrasion resistance. Like other cellulosic fibers, it is not resilient, which means that it will wrinkle. Rayon withstands ironing temperatures slightly less than those of cotton. It may be attacked by silverfish and termites, but generally resists insect damage. It will mildew, but that generally is not a problem. One of rayon's strengths is its versatility and ability to blend easily with many fibers—sometimes to reduce cost, other times for luster, softness, or absorbency and resulting comfort. Rayon has moderate resistance to acids and alkalis and generally the fiber itself is not damaged by bleaches; however, dyes used in the fabric may experience color change. As a cellulosic fiber, rayon will burn, but flame retardant finishes can be applied. Fiber Properties overview
General Characteristics: Rayon as a cloth is soft and comfortable. It drapes well, which is one of the reasons it is so desirable as an apparel fabric. Most characteristics are variable depending on processing, additives and finishing treatments, not to mention fabric construction.
Absorbency: Rayon is the most absorbent of all cellulose fibers, even more so than cotton and linen. Because of this, rayon absorbs perspiration and allows it to evaporate away from the skin, making it an excellent summer fabric. Its high absorbency applies equally to dyes, allowing beautiful, deep, rich colours. Strength: It loses a great deal of strength when wet. Because of this, it stretches and shrinks more than cotton. Abrasion resistance: Poor due to inelasticity of the fibers. It is easily damaged by scraping and will pill on the surface of the cloth. Flammability: Because of its excessive flammability, it inspired the Flammable Fabrics Act. The FFA was enacted by the U.S. Department of Commerce in 1953 in response to public concern over a number of serious burn accidents involving brushed rayon high pile sweaters (referred to as "torch sweaters") and children's cowboy chaps which could easily catch fire and flash burn. Static: No static build-up. Chemical reactions: Because it is a cellulose fiber, it is damaged by even relatively weak acids.
Comparative Rating Parameters Cotton
Viscose
Polyester
Moisture Regain
Good
Very good
Poor
Thermal protection
Good
Very good
Poor
Very good
Good
Poor
Softness
Good
Very good
Poor
Smoothness
Poor
Good
Very good
Static dissipation
Good
Very good
Poor
Drape
Good
Very good
Poor
Luster
Poor
Very good
Very good
Crease recovery
Poor
Poor
Very good
Uniformity
Poor
Very good
Good
Comfort
Air permeability
Aesthetic
Utility Performance
Antipilling
Good
Very good
Poor
Wash & wear
Good
Poor
Very good
Background For centuries humankind has relied upon various plants and animals to provide the raw materials for fabrics and clothing. Silkworms, sheep, beaver, buffalo deer, and even palm leaves are just some of the natural resources that have been used to meet these needs. However, in the last century scientists have turned to chemistry and technology to create and enhance many of the fabrics we now take for granted. There are two main categories of man-made fibers: those that are made from natural products (cellulosic fibers) and those that are synthesized solely from chemical compounds (noncellulosic polymer fibers). Rayon is a natural-based material that is made from the cellulose of wood pulp or cotton. This natural base gives it many of the characteristics—low cost, diversity, and comfort—that have led to its popularity and success. Today, rayon is considered to be one of the most versatile and economical man-made fibers available. It has been called "the laboratory's first gift to the loom." In the 1860s the French silk industry was being threatened by a disease affecting the silkworm. Louis Pasteur and Count Hilaire de Chardonnet were studying this problem with the hope of saving this vital industry. During this crisis, Chardonnet became interested in finding a way to produce artificial silk. In 1885 he patented the first successful process to make a useable fiber from cellulose. Even though other scientists have subsequently developed more cost-effective ways of making artificial silk, Chardonnet is still considered to be the father of rayon. For the next forty years this material was called artificial or imitation silk. By 1925 it had developed into an industry unto itself and was given the name rayon by the Federal Trade Commission (FTC). The term rayon at this time included any man-made fiber made from cellulose. In 1952, however, the FTC divided rayons into two categories: those fibers consisting of pure cellulose (rayon) and those consisting of a cellulose compound (acetate). By the 1950s, most of the rayon produced was being used in industrial and home furnishing products rather than in apparel, because regular rayon (also called viscose rayon) fibers were too weak compared to other fibers to be used in apparel. Then, in 1955, manufacturers began to produce a new type of rayon—high-wet-modulus (HWM) rayon—which was somewhat
stronger and which could be used successfully in sheets, towels, and apparel. The advent of HWM rayon (also called modified rayon) is considered the most important development in rayon production since its invention in the 1880s. Today rayon is one of the most widely used fabrics in our society. It is made in countries around the world. It can be blended with natural or man-made fabrics, treated with enhancements, and even engineered to perform a variety of functions. Raw Materials Regardless of the design or manufacturing process, the basic raw material for making rayon is cellulose. The major sources for natural cellulose are wood pulp—usually from pine, sprucs—
To make rayon, sheets of purified cellulose are steeped in caustic soda, dried, shredded into crumbs, and then aged in metal containers for 2 to 3 days. The temperature and humidity in the metal containers are carefully controlled. After ageing, the crumbs are combined and churned with liquid carbon disulfide, which turns the mix into orange-colored crumbs known as sodium cellulose xanthate. The cellulose xanthate is bathed in caustic soda, resulting in a viscose solution that looks and feels much like honey. cotton linters. Cotton linters are residue fibers which cling to cotton seed after the ginning process. Strictly defined, rayon is a manufactured fiber composed of regenerated cellulose. The legal definition also includes manufactured fibers in which substitutes have not replaced more than 15 percent of the hydrogens. While the basic manufacturing process for all rayon is similar, this fabric can be engineered to perform a wide range of functions. Various factors in the manufacturing process can be altered to produce an array of designs. Differences in the raw material, the processing chemicals, fiber diameter, post treatments and blend ratios can be manipulated to produce a fiber that is customized for a specific application. Regular or viscose rayon is the most prevalent, versatile and successful type of rayon. It can be blended with man-made or natural fibers and made into fabrics of varying weight and texture. It is also highly absorbent, economical and comfortable to wear. Regular viscose rayon does have some disadvantages. It's not as strong as many of the newer fabrics, nor is it as strong as natural cotton or flax. This inherent weakness is exacerbated when it becomes wet or overexposed to light. Also, regular rayon has a tendency to shrink when washed. Mildew, acid and high temperatures such as ironing can also result in damage. Fortunately, these disadvantages can be countered by chemical treatments and the blending of rayon with other fibers of offsetting characteristics. High-wet-modulus rayon is a stronger fiber than regular rayon, and in fact is more similar in performance to cotton than to regular rayon. It has better elastic recovery than regular rayon, and fabrics containing it are easier to care for—they can be machine-washed, whereas fabrics containing regular rayon generally have to be dry-cleaned.
The Manufacturing Process While there are many variations in the manufacturing process that exploit the versatility of the fiber, the following is a description of the procedure that is used in making regular or viscose rayon. Regardless of whether wood pulp or cotton linters are used, the basic raw material for making rayon must be processed in order to extract and purify the cellulose. The resulting sheets of white, purified cellulose are then treated to form regenerated cellulose filaments. In turn, these filaments are spun into yarns and eventually made into the desired fabric. Processing purified cellulose
1 Sheets of purified cellulose are steeped in sodium hydroxide (caustic soda), which produces sheets of alkali cellulose. These sheets are dried, shredded into crumbs, and then aged in metal containers for 2 to 3 days. The temperature and humidity in the metal containers are carefully controlled. 2 After ageing, the crumbs are combined and churned with liquid carbon disulfide, which turns the mix into orangecolored crumbs known as sodium cellulose xanthate. The cellulose xanthate is bathed in caustic soda, resulting in a viscose solution that looks and feels much like honey. Any dyes or delusterants in the design are then added. The syrupy solution is filtered for impurities and stored in vats to age, this time between 4 and 5 days.
Producing filaments
3 The viscose solution is next turned into strings of fibers. This is done by forcing the liquid through a spinneret, which works like a shower-head, into an acid bath. If staple fiber is to be produced, a large spinneret with large holes is used. If filament fiber is being produced, then a spinneret with smaller holes is used. In the acid bath, the acid coagulates and solidifies the filaments, now known as regenerated cellulose filaments.
Spinning
4 After being bathed in acid, the filaments are ready to be spun into yarn. Depending on the type of yarn desired, several spinning methods can be used, including Pot Spinning, Spool Spinning, and Continuous Spinning. In Pot Spinning, the filaments are first stretched under controlled tension onto a series of offsetting rollers called godet wheels. This stretching reduces the diameter of the filaments and makes them more uniform in size, and it also gives
the filaments more strength. The filaments are then put into a rapidly spinning cylinder called a Topham Box, resulting in a cake-like strings that stick to the sides of the Topham Box. The strings are then washed, bleached, rinsed, dried, and wound on cones or spools. Spool Spinning is very similar to Pot Spinning. The filaments are ed through rollers and wound on spools, where they are washed, bleached, rinsed, dried, and wound again on spools or cones. In Continuous Spinning, the filaments are washed, bleached, dried, twisted, and wound at the same time that they are stretched over godet wheels.
5 Once the fibers are sufficiently cured, they are ready for post-treatment chemicals and the various weaving processes necessary to produce the fabric. The resulting fabric can then be given any of a number of finishing treatments. These include calendaring, to control smoothness; fire resistance; pre-shrinking; water resistance; and wrinkle resistance.
High-Wet-Modulus Rayon Manufacture The process for manufacturing high-wet-modulus rayon is similar to that used for making regular rayon, with a few exceptions. First, in step #1 above, when the purified cellulose sheets are bathed in a caustic soda solution,
After the syrupy viscose solution is prepared, it is forced through a spinneret into an acid bath. The resulting strings or filaments are then stretched on godet wheels to strengthen them and put into a spinning Topham box. This method produces cake-like strings of rayon, which are washed, rinsed, and dried before being wound on spools or cones. a weaker caustic soda is used when making HWM rayon. Second, neither the alkali crumbs (#1 above) nor the viscose solution (step #2) is aged in the HWM process. Third, when making HWM rayon, the filaments are stretched to a greater degree than when making regular rayon. Quality Control As with most chemically oriented processes, quality control is crucial to the successful manufacture of rayon. Chemical make-up, timing and temperature are essential factors that must be monitored and controlled in order to produce the desired result.
The percentages of the various fibers used in a blended fabric must be controlled to stay within in the legal bounds of the Textile Fiber Identification Act. This act legally defines seventeen groups of man-made fibers. Six of these seventeen groups are made from natural material. They include rayon, acetate, glass fiber, metallics, rubber, and azion. The remaining eleven fabrics are synthesized solely from chemical compounds. They are nylon, polyester, acrylic, modacrylic, olefin, spandex, anidex, saran, vinal, vinyon, and nytril. Within each generic group there are brand names for fibers which are produced by different manufacturers. Private companies often seek patents on unique features and, as could be expected, attempt to maintain legal control over their competition. Byproducts As one of the industry's major problems, the chemical by-products of rayon have received much attention in these environmentally conscious times. The most popular method of production, the viscose method, generates undesirable water and air emissions. Of particular concern is the emission of zinc and hydrogen sulfide. At present, producers are trying a number of techniques to reduce pollution. Some of the techniques being used are the recovery of zinc by ion-exchange, crystallization, and the use of a more purified cellulose. Also, the use of absorption and chemical scrubbing is proving to be helpful in reducing undesirable emissions of gas. The Future The future of rayon is bright. Not only is there a growing demand for rayon worldwide, but there are many new technologies that promise to make rayon even better and cheaper. For a while in the 1970s there was a trend in the clothing industry toward purely synthetic materials like polyester. However, since purely synthetic material does not "breath" like natural material, these products were not well received by the consumer. Today there is a strong trend toward blended fabrics. Blends offer the best of both worlds. With the present body of knowledge about the structure and chemical reactivity of cellulose, some scientist believe it may soon be possible to produce the cellulose molecule directly from sunlight, water and carbon dioxide. If this technique proves to be cost effective, such hydroponic factories could represent a giant step forward in the quest to provide the raw materials necessary to meet the world wide demand for man-made fabric.
ACETATE RAYON INTRODUCTION Initially invented in Europe as a varnish for airplane wings, acetate was first produced in the United States in 1924 making it the second oldest manufactured fiber. Like rayon, acetate is a cellulose-based fiber that was initially only an experimental fiber. Before mass production could occur, modifications to the dyeing process was necessary before it could be usable. Acetate was the first thermoplastic (heat sensitive) fiber to be introduced to consumers. Originally, consumers were surprised that a fabric could melt when ironed.
In addition to the early discovery that acetate’s chemical structure was not compliant with the current dyes, it was also soon discovered that pollutants and fumes in the air caused acetate fabrics to discolor.The dyeing process was once again changed to help correct this problem that resulted in solution dyeing, a process that can now be used on all manufactured fibers . However, acetate fabrics are still somewhat sensitive to discoloration due to air pollutants .Like rayon, acetate is
often used as a substitute for silk, but acetate is much weaker than either rayon or silk and is generally used for apparel that will not be worn often . Specifically, special occasion clothing is made from acetate fabric. ADVANTAGES OF ACETATE FABRIC: •Excellent drape (a fabric’s ability to fold while worn) •Luxurious hand (the feel of a fiber, yarn, or fabric to the wearer) •High luster (reflection of light on fabric) •No pilling •Little static •Low cost •Holds white very well DISADVANTAGES OF ACETATE FABRIC: •Poor resiliency (fabric does not return to original shape or form after being altered) •Poor elasticity •Poor tenacity (ability to withstand stress) •Low abrasion resistance •Poor strength •Thermoplastic (melts when heated) •Susceptible to wrinkles •Most often needs to be dry clean END USES OF ACETATE FABRIC: •Apparel: lining, graduation gowns, neck ties, special occasion wear, dresses, ribbons, blouses, carves •Interior: draperies, upholstery, quilted items
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