Rayon and cellulose

Rayon is the oldest of the artificial fibers, although it is not totally synthetic (but is made by chemically modifying natural cellulose). World production averages about 2.5 million short tons (2.3 million metric tons) a year, a quantity surpassed in the textile industry only by cotton. This success is partly due to the low cost and abundance of its raw material, cellulose, which constitutes 42 per cent of wood and 95 per cent of cotton. About three-quarters of rayon output is produced from highly refined wood pulp.

Viscose (left) is a sticky substance made by treating cellulose with various chemicals (see illustration on opposite page). To make rayon, the viscose is forced through tiny holes into a bath of acid, and the filaments combined and pulled from the bath as a yarn (right).

Viscose rayon

Most of the world’s rayon is now made by the viscose process. Sheets of wood pulp are first dissolved to form an alkali-cellulose slurry by steeping them in 18 per cent aqueous sodium hydroxide solution. After steeping, the cellulose is pressed under high pressure and as much caustic soda as possible is removed. It is then shredded into crumbs, which helps to distribute the caustic more evenly, and heated in a process called aging.

In an automatic aging room the crumbs are treated with oxygen to lower the degree of polymerization, which prevents premature hardening. Next follows the critical xanthation process, in which the mercerized alkali-cellulose is cooled and mixed with carbon disulfide to form cellulose xanthate. This yellow substance is dissolved in dilute caustic soda to form a clear syrup, known as viscose, which is filtered to remove any impurities.

Once the xanthate groups attached to the cellulose molecule have redistributed (in a “ripening” process), closer molecular chains can be formed, and the viscose is ready for spinning. First it is extruded through the holes of a spinnerette into a bath of a salt and an acid. This solution usually contains 15 per cent sodium sulfate, which removes water from the cellulose xanthate, and 10 per cent sulfuric acid, which helps to regenerate cellulose from the viscose. The extruded material forms a continuous filament containing 85-90 per cent cellulose. The size of the spinnerette determines the destiny of the yarn. A very narrow nozzle, for example, is used for making artificial hair, whereas a wide nozzle produces rayon for artificial leather fabrics. Extrusion through a narrow slit produces sheets of transparent viscose film, known as cellophane.

The fibers are spun continuously, reeled, then washed in water and bleached in sodium hypochlorite solution. After the fibers are cut to size, an appropriate finish may be applied to counteract any natural slipperiness or stickiness during weaving, before they are wound on to bobbins for shipping.

Viscose rayon fabrics—once marketed as “artificial silk”—are comfortable to wear, are cool, like cotton, but more absorbent, and can be brushed to make them warmer. Although viscose rayon creases and burns easily, it can be treated with a crease-resistant finish.

Cuprammonium rayon

Today, fibers made by the cuprammonium process, an expensive technique, are manufactured only for highly specialized purposes. Almost all artificial kidney machines, for example, use membranes prepared from cuprammonium rayon films and fibers. They are more supple, cause less bloodclotting, and have better dewatering properties than viscose rayon membranes.

In the cuprammonium process, wood pulp is mixed with aqueous ammonia, copper sulfate, and caustic soda until it produces a 10 per cent cellulose blue solution. Although it is wet spun, unlike viscose, coagulation is slow. The filaments are extruded into water, which also passes through the spinnerette, causing the fibers to elongate. As a result of this “stretch spinning,” the filament size is smaller than viscose, even though the spinnerette holes are larger. The threads are hardened as they pass through a dilute sulfuric acid solution, which also removes the ammonia and copper for recycling.

Acetate rayon

The first important application of cellulose acetate was as a nonflammable varnish for fabric-covered aircraft in World War I. The first acetate yarn (known as Celanese) was developed from the redundant stocks of acetate after the war. The cellulose is steeped in acetic acid, then treated with acetate anhydride and sulfuric acid. Each glucose unit in cellulose combines with three molecules of acetic acid and forms cellulose triacetate; but because it does not readily accept dyes in this form, one acetate molecule is removed (by adding water) to produce cellulose diacetate. The fibers are washed, dried, dissolved in acetone, and then extruded through a spinnerette sited 20 feet (6 meters) above the ground. As the solution descends, it is warmed by a current of hot air, which evaporates the acetone. The acetate fibers are then lubricated before twisting and winding.

The resulting cloth is more like natural silk than any other fiber. It is crease-resistant, mothproof, and water-repellent. Triacetate, sold as Tricel and now often combined with cotton, is popular for its drip-dry qualities.

The early commercial uses of cellulose acetate as photographic film, in shatterproof glass, and for contact lenses and varnishes are now threatened by acrylics, nylons, and other totally synthetic materials made from petroleum-based products. However, some experts speculate that a petroleum shortage and rising oil prices may lead to greater use of cellulose.

Cellophane is a transparent material mainly employed for wrapping, especially for foodstuffs such as candy. It is chemically identical to rayon, but is produced by extruding viscose through a narrow slit so that it takes the form of a thin film or sheet.

Nitrocellulose

The explosive properties of cellulose nitrate (often known as nitrocellulose or guncotton) are largely derived from the polymeric and fibrous structure of cellulose. It is used as a propellant explosive for rockets and guns and in cartridges for small arms. Its properties depend, however, on the extent of nitration, a nitrogen content of more than 13 per cent being necessary to make an explosive. When the nitrogen content is limited to 11 or 12 per cent, it may be used for lacquers and films, and at 10 per cent nitrogen, plastics may be manufactured from it. Low-viscosity nitrocellulose lacquers may also be produced by heating cellulose and nitrogen with water under pressure but, like other forms of cellulose nitrate, they suffer from the disadvantage of extremely high flammability.

In making rayon,
sheets of wood pulp are steeped in alkali (sodium hydroxide, NaOH), pressed damp dry and crumbled, and, after an aging process, dissolved in carbon disulfide (CS2) to form cellulose xanthate. This substance is reacted with more alkali to make viscose, which is filtered before being extruded into a bath containing acid. The resulting rayon yarn is washed, bleached, and dried before being wound on reels.