PRODUCTION OF RAYON VISCOSE
By using two different chemicals and manufacturing techniques, two basic types of rayon were developed - viscose rayon and cupramonium. Other processes for rayon include the polynosic (modal) process and the now obsolete nitrocellulose and saponified acetate processes. The nitrocellulose process is likely obsolete not only because the viscose and cupramonium processes are more effective, they are also safer; the nitrocellulose process results in a fiber with explosive properties.
As recently as 1992 there has been an entirely new process developed for producing regenerated cellulose fibers: the Lyocell process, developed by Courtaulds. While it is sufficiently different from rayon to almost be in a class by itself, the U.S. Federal Trade commission has formally amended the textile rules to add Lyocell as a subclass of rayon.
As viscose is the most common and recognized process for making rayon today, the process is outlined below. While the United States government considers fibers from all the above processes rayon, the International Organization for Standardization (ISO) prefers the name viscose for rayon (regenerated cellulose) obtained by the viscose process. The name viscose was derived from the word viscous, which describes the liquid state of the spinning solution.
The cellulosic raw materials for rayon are wood chips (usually from spruce or pine) or cotton linters. These are treated to produce sheets of purified cellulose containing 87-98% cellulose. They are then bleached with sodium hypochloride (NaOCl) to remove natural colour. These cellulose sheets are then soaked in 18% caustic soda for 1 to 2 hours producing sheets of alkali cellulose. Any excess alkali is pressed out. The substance is broken up into flakes or grains called cellulose crumbs, which are aged for two or three days under controlled temperature and humidity. Liquid carbon disulfide is added to the crumbs to change the cellulose into cellulose xanthate, a light orange substance that is still in crumb form. These crumbs are dissolved in a weak solution of caustic soda and transformed into a viscous solution called "viscose", honey-like in colour and consistency.
To produce the rayon filament, the viscose solution is aged, filtered, then vacuum-treated to remove any air bubbles that could weaken the filament and cause it to break. It is then pumped through spinnerets into a bath of sulfuric acid, which coagulates the cellulose xanthate to form regenerated filaments of 100% cellulose. The many variations and different properties of viscose such as luster, strength, softness and affinity for dyes, are influenced here by varying the technique and by the addition of external materials.
Once extruded, the freshly formed viscose must be purified and strengthened. It is thoroughly washed, treated with a dilute solution of sodium sulfide to remove any sulfur impurities. It may be bleached to remove a slight yellowness and to secure even white colour, and then given a final washing.
VISCOSE RAYON FIBER STRUCTURE
The structure of the rayon fiber is generally that of smooth, inelastic filaments like glass rods. However, different processes, additives and finishing techniques can vary the physical appearance and structure of the fiber.
VISCOSE RAYON FIBER IDENTIFICATION
In the burning test, rayon most resembles cotton. It ignites rapidly, sometimes even faster than cotton, burning with a large, bright, yellow flame. Burnt rayon leaves an odor like burnt paper, similar to cotton. The ash is also like cotton: light and feathery gray, which disintegrates rapidly.
In the feeling test, rayon is more difficult to identify. The variety of processes, modifications of technique and various treatments can make rayon look and feel like silk, cotton, wool or linen. In general, however, rayon has the smooth feeling of silk.
It is slippery because of the smoothness of the filaments, and has an almost brittle-feeling quality due to the fiber's inelasticity.
The breaking test can differentiate between rayon, cotton and linen yarns. Because of its inelasticity, rayon will tend to break shortly, with a short, uneven breaking pattern. It is even easier to distinguish when wet, as it breaks very easily then.
CARE OF VISCOSE RAYON
The critical element in successful care of rayon is the type of fiber.
- regular or viscose rayon usually require dry cleaning for best results
- high wet modulus, high wet strength or polynosic rayons, will normally machine wash and tumble dry satisfactorily
The low wet strength of untreated viscose rayon is likely to cause shrinkage and loss of body if fabrics made from these fibers are hand or machine washed. Viscose rayon are usually labeled "dry clean only." Check and follow garment labels for care and instructions. Consumers who do not follow care instructions will have no recourse to manufacturers should damage occur during laundering.
Since many dyes and finishes applied to viscose rayon are moisture-sensitive, consumers should protect garments from contact with moisture. Raincoats should be worn on stormy days. Also, care should be used when washing hands or working in the kitchen to avoid splashing water. The problem with sizing or dye migration increases the longer the area remains damp. Spills or moisture should be blotted with absorbent cloth to remove moisture. Avoid spilling acid or alkali, such as foodstuffs or perfume, as certain dyes used on rayon are especially sensitive to these substances. Use caution or avoid treating stained areas with water. Take the garment to a dry cleaner for stain removal. Rings or darker shading caused by sizing migration and lighter areas resulting from dye migration may be permanent.
When viscose rayon can be hand washed, do so with care. Always support wet fabrics, since rayon has low wet strength and is unstable when wet. Avoid wringing moisture from fabrics. Gently squeeze out moisture and roll in a towel. Smooth and shape, then lay flat to dry. If the garments are not too heavy when wet, they can be hung on a non-rusting hanger.
When pressing regular rayon garments at home, guard against spitting by steam irons that may cause water spots. Also, if possible, press on the wrong side or use a press cloth on the right side to avoid shine or iron imprints. Use a rayon or synthetic setting on the iron. When pressing blends, use the iron temperature for the most heat-sensitive fiber. Normally, a synthetic setting will be satisfactory.
High wet modulus or polynosic rayons can usually be machine washed or dried without special care. Blends, dark colors, and permanent press fabrics should be washed on a gentle cycle with warm water; however, 100 percent high wet modulus rayons are normally not damaged by hot water temperatures (more than 140°F). Tumble dry on a warm setting and remove immediately, or while slightly damp. Smooth and hang to reduce wrinkling. Polynosic rayons can be line dried; however, heavy items should be supported by clotheslines to avoid stretching or loss of shape.