RAYON
Rayon is a manufactured regenerated cellulose fiber. 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 fibers of nearly pure cellulose.[1] Because rayon is manufactured from naturally occurring polymers, it is considered a semi-synthetic fiber.[2] Specific types of rayon include viscose, modal and lyocell, each of which differs in manufacturing process and properties of the finished product.
Three methods of production lead to distinctly different rayon fibers: 1. viscose rayon, 2. cuprammonium rayon 3. saponified cellulose acetate. 1. STRUCTURE OF RAYON The unit cell of cellulose is shown in Fig. 2. 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 149°C; chars and decomposes at 177 to 204°C. 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. 3.. 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]. 4. 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 |
5. 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]. |