Chemistry Spins a Yarn (Dec, 1947)

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Chemistry Spins a Yarn

By Kenneth M. Swezey

TRANSFORMATION of tree fibers or cotton linters into rayon fabrics is one of the greatest achievements of modern industrial chemistry. Chemically, rayon is almost pure cellulose, the same as cotton and linen. But instead of using cellulose as found in nature, the rayon chemist starts with cheap and plentiful spruce and hemlock trees, or the fuzz that clings to cotton seed after it has been ginned. He chops these up, dissolves them, and then causes the cellulose to reappear in silky filaments that may be spun, twisted, knit, or woven into forms that compete successfully with cotton, silk, linen, or wool.

Although the rayon industry is less than 60 years old, and modern rayons less than 15, the idea that man might imitate the silkworm dates back to the 17th Century. In 1664, Robert Hooks, an English scientist, wrote that natural silk is merely a “glutinous excrement” from the body of a worm, and he wondered why men couldn’t do as well by squeezing a suitable gum through tiny holes.

Count Hilaire de Chardonnet was the first to accomplish this on a commercial basis. One day while he was coating some photographic plates with collodion, the bottle slipped and broke on the table. When he tried to clean up the mess some time later, the partly dried collodion stretched into long threads. With this as a beginning, Chardonnet worked 30 years to perfect a synthetic yarn. His “Chardonnet silk” was the hit of the Paris Exposition in 1889.

Other methods of taking cellulose apart and putting it together again as textile fibers were soon perfected. Early types were called artificial silk. In 1924, the name “rayon” was adopted for all textiles made artificially from cellulose.

Most rayon is now made by the viscose process. Acetate rayon comes next, while cuprammonium, a superior rayon for certain special uses, is in third place. Although complex machinery and precise technical control are required to produce usable rayon yarn, you can readily demonstrate how cellulose in one form can be broken down and then “regenerated”.in a form entirely different. The cuprammonium process has been chosen as the one most easily paralleled at home.

First, make some copper hydroxide by dissolving 5 grams of copper sulphate crystals in 100 ml. water and adding slowly a 10 percent solution of sodium hydroxide until the pale blue precipitate stops forming. Wash this precipitate by decantation— mixing with water in a tall glass, allowing to settle, pouring off the clear upper liquid, mixing with more water, and repeating the process five or six times.

Filter the precipitate and dissolve the gelatinous solid in the smallest possible amount of 28 percent ammonium hydroxide. With the ammonium hydroxide, the copper hydroxide forms a complex deep-blue compound. The resulting solution (called Schweitzer’s reagent from the German chemist who discovered it) dissolves paper, cotton, wood, and other forms of cellulose.

Commercially, cuprammonium rayon is made from cotton linters. If you wish, you may use absorbent cotton, but filter paper —a pure form of cellulose—will dissolve much faster. Tear up a 15 cm. sheet and put it into the blue solution. Stir occasionally until it has completely dissolved. This may take several hours.

In commercial practice, the solution is now filtered and air bubbles removed. Then it is pumped to the spinnerets—disks with tiny holes through which the solution is forced. The spinnerets are located at the upper end of a cylinder through which water is flowing downward. This water starts to coagulate the threads and stretches them. The filaments are completely coagulated in dilute sulphuric acid.

In your home setup, the spinneret can be a glass tube drawn to a 1-mm. opening at one end and fitted into a stopper that is inserted, in turn, into the lower end of a tube about 1″ in diameter and 6″ long. The large tube serves as a tank from which the solution can be gravity-fed to the coagulating bath, a 5 percent solution of sulphuric acid in a large cylinder.

Before lowering the outlet tube beneath the surface of the coagulating bath, half fill the tank with the cellulose solution. Be sure the solution is free from undissolved cellulose as this would clog the opening. As a further precaution, run a fine copper wire down through the opening. By manipulating the wire, you can clear the opening if it becomes clogged.

To complete the experiment, lower the filled spinneret until the delivery tube is several inches below the surface of the bath. If the solution doesn’t flow, give it a few prods with the wire. At first, the issuing thread may tend to rise, but gradually it will sink. Commercial cuprammonium filaments are much finer, of course, and are stretched during the spinning to give them greater strength. The final treatment consists of washing, drying, and twisting into yarn or thread.

Viscose rayon usually is made from cellulose derived from wood pulp. After being steeped in sodium hydroxide solution, this pulp is squeezed almost dry and allowed to age for several days. Treated with carbon disulphide, the alkali cellulose changes to cellulose xanthate. Dissolved in sodium hydroxide solution, this forms a viscous amber-colored solution that is forced through spinnerets in much the same way as cu-prammonium solution.

Unlike cuprammonium or viscose rayon, acetate rayon is not merely cellulose that has been taken apart and put together again. Instead, it is a real compound of cellulose—cellulose acetate, the material of which most photographic film is made. All you must do to make filaments of this substance is to dissolve it in acetone, or other suitable solvent, squirt it through little holes, and finally pass it through warm air. The heated air rapidly evaporates the solvent, leaving a solid thread.

You can make cellulose acetate by dissolving 1 gram of filter paper in a mixture of 40 ml. of glacial acetic acid and 12 ml. of acetic anhydride, with 6 drops of concentrated sulphuric acid added as a catalyst. Complete solution will take at least eight hours. Stir it occasionally.

When the paper has been fully dissolved, you can recover the cellulose acetate by pouring the solution in a thin stream into a large volume of water, stirring constantly. Then separate the compound by filtering it, press out the excess water by squeezing the compound between filter papers, and dry-either in the open air or by directing upon it the warm blast from an electric drier.

When dry, dissolve in the smallest possible amount of acetone. Place the resulting solution in the “tank” used in the previous experiment. This time the solution just flows into the air where heat from the drier solidifies it.

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