Asimov's New Guide to Science

by Isaac Asimov

  With the success of Bakelite, chemists naturally turned to other possible starting materials in search of more synthetic high polymers that might be useful plastics. And, as time went on, they succeeded many times over.

  British chemists discovered in the 1930s, for instance, that the gas ethylene (CH2 = CH2), under heat and pressure, would form very long chains. One of the two bonds in the double bond between the carbon atoms opens up and attaches itself to a neighboring molecule. With this happening over and over again, the result is a long-chain molecule called polythene in England and polyethylene in the United States.

  The paraffin-wax molecule is a long chain made up of the same units, but the molecule of polyethylene is even longer. Polyethylene is therefore like wax, but more so. It has the cloudy whiteness of wax, the slippery feel, the electrical insulating properties, the waterproofness, and the lightness (it is about the only plastic that will float on water). It is, however, at its best, much tougher than paraffin and much more flexible.

  As it was first manufactured, polyethylene required dangerous pressures, and the product had a rather low melting point—just above the boiling point of water. It softened to uselessness at temperatures below the melting point. Apparently this effect was due to the fact that the carbon chain had branches which prevented the molecules from forming close-packed, crystalline arrays. In 1953, a German chemist named Karl Ziegler found a way to produce unbranched polyethylene chains, without the need for high pressures. The result was a new variety of polyethylene, tougher and stronger than the old, and capable of withstanding boiling-water temperatures without softening too much. Ziegler accomplished this by using a new type of catalyst—a resin with ions of metals such as aluminum or titanium attached to negatively charged groups along the chain.

  On hearing of Ziegler’s development of metal-organic catalysts for polymer formation, the Italian chemist Giulio Natta began applying the technique to propylene (ethylene to which a small one-carbon methyl group, CH3–, is attached). Within ten weeks, he had found that, in the resultant polymer, all the methyl groups face in the same direction, rather than (as was usual in polymer formation before that time) facing, in random fashion, in either direction. Such isotactic polymers (the name was proposed by Natta’s wife) proved to have useful properties and can now be manufactured virtually at will. Chemists can design polymers, in other words, with greater precision than ever before. For their work in this field, Ziegler and Natta shared the 1963 Nobel Prize for chemistry.

  The atomic-bomb project contributed another useful high polymer in the form of an odd relative of polyethylene. In the separation of uranium 235 from natural uranium, the nuclear physicists had to combine the uranium with fluorine to form uranium hexafluoride. Fluorine is the most active of all substances and will attack almost anything. Looking for lubricants and seals for their vessels that would be impervious to attack by fluorine, the physicists resorted to fluorocarbons—substances in which the carbon has already combined with fluorine (replacing hydrogen).

  Until then, fluorocarbons had been only laboratory curiosities. The first (and simplest) of this type of molecule, carbon tetrafluoride (CF4), had been obtained in pure form only in 1926. The chemistry of these interesting substances was now pursued intensively. Among the fluorocarbons studied was tetrafluoroethylene (CF2 = CF2), which had first been synthesized in 1933 and is, as you see, ethylene with its four hydrogens replaced by four fluorines. It was bound to occur to someone that tetrafluoroethylene might polymerize as ethylene itself did. After the war, Du Pont chemists produced a long-chain polymer which was as monotonously CF2CF2CF2… as polyethylene was CH2CH2CH2… Its trade name is Teflon, the tefl being an abbreviation of tetrafluoro-.

  Teflon is like polyethylene, only more so. The carbon-fluorine bonds are stronger than the carbon-hydrogen bonds and offer even less opportunity for the interference of the environment. Teflon is insoluble in everything, unwettable by anything, an extremely good electrical insulator, and considerably more resistant to heat than is even the new and improved polyethylene. Teflon’s best-known application, so far as the housewife is concerned, is as a coating upon frying pans, thus enabling food to be fried without fat, since food will not stick to the standoffish fluorocarbon polymer.

  An interesting compound that is not quite a fluorocarbon is Freon (CF2Cl2), mentioned earlier in the book. It was introduced in 1932 as a refrigerant. It is more expensive than the ammonia or sulfur dioxide used in large-scale freezers; but, on the other hand, Freon is nonodorous, nontoxic, and nonflammable, so that accidental leakage introduces a minimum of danger. Midgley, its discoverer, demonstrated its harmlessness by taking in a deep lungful and letting it trickle out over a candle flame. The candle went out, but Midgley was unharmed. It is through Freon that room air conditioners have become a characteristic part of the American scene since the Second World War.

  GLASS AND SILICONE

  Plastic properties do not, of course, belong solely to the organic world. One of the most ancient of all plastic substances is glass. The large molecules of glass are essentially chains of silicon and oxygen atoms: that is, -Si-O-Si-O-Si-O-Si-, and so on indefinitely. Each silicon atom in the chain has two unoccupied bonds to which other groups can be added. The silicon atom, like the carbon atom, has four valence bonds. The silicon-silicon bond, however, is weaker than the carbon-carbon bond, so that only short silicon chains can be formed, and those (in compounds called silanes) are unstable. The silicon-oxygen bond is a strong one, however, and such chains are even more stable than those of carbon. In fact, since the earth’s crust is half oxygen and a quarter silicon, the solid ground we stand upon may be viewed as essentially a silicon-oxygen chain.

  Although the beauties and usefulness of glass (a kind of sand, made transparent) are infinite, it possesses the great disadvantage of being breakable. And in the process of breaking, it produces hard, sharp pieces which can be dangerous, even deadly. With untreated glass in the windshield of a car, a crash may convert the auto into a shrapnel bomb.

  Glass can be prepared, however, as a double sheet between which is placed a thin layer of a transparent polymer, which hardens and acts as an adhesive. This is safety glass, for when it is shattered, even into powder, each piece is held firmly in place by the polymer. None goes flying out on death-dealing missions. Originally, as far back as 1905, collodion was used as the binder, but nowadays that has been replaced for the most part by polymers built of small molecules such as vinyl chloride. (Vinyl chloride is like ethylene, except that one of the hydrogen atoms is replaced by a chlorine atom.) The vinyl resin is not discolored by light, so safety glass can be trusted not to develop a yellowish cast with time.

  Then there are the transparent plastics that can completely replace glass, at least in some applications. In the middle 1930s, Du Pont polymerized a small molecule called methyl methacrylate and cast the polymer that resulted (a polyacrylic plastic) into clear, transparent sheets. The trade names of these products are Plexiglas and Lucite. Such organic glass is lighter than ordinary glass, more easily molded, less brittle, and simply snaps instead of shattering when it does break. During the Second World War, molded transparent plastic sheets came into important use as windows and transparent domes in airplanes, where lightness and nonbrittleness are particularly useful. To be sure, the polyacrylic plastics have their disadvantages. They are affected by organic solvents, are more easily softened by heat than glass is, and are easily scratched. Polyacrylic plastics used in the windshields of cars, for instance, would quickly scratch under the impact of dust particles and become dangerously hazy. Consequently, glass is not likely ever to be replaced entirely. In fact, it is actually developing new versatility. Glass fibers have been spun into textile material that has all the flexibility of organic fibers and the inestimable further advantage of being absolutely fireproof.

  In addition to glass substitutes, there is also what might be called a glass compromise. As I said, each silicon atom in a silicon-oxygen chai~has two s
pare bonds for attachment to other atoms. In glass these other atoms are oxygen atoms, but they need not be. What if carbon-containing groups are attached instead of oxygen? You will then have an inorganic chain with organic offshoots, so to speak—a compromise between an organic and an inorganic material. As long ago as 1908, the English chemist Frederic Stanley Kipping formed such compounds, and they have come to be known as silicones.

  During the Second World War, long-chain silicone resins came into prominence. Such silicones are essentially more resistant to heat than purely organic polymers. By varying the length of the chain and the nature of the side chains, one can obtain a list of desirable properties not possessed by glass itself. For instance, some silicones are liquid at room temperature and change very little in viscosity over large ranges of temperature: that is, they do not thin out with heat or thicken with cold. This is a particularly useful property for a hydraulic fluid—the type of fluid used to lower landing gear on airplanes, for instance. Other silicones form soft, puttylike sealers that do not harden or crack at the low temperatures of the stratosphere and are remarkably water-repellent. Still other silicones serve as acid-resistant lubricants, and so on.

  Synthetic Fibers

  In the story of organic synthesis, a particularly interesting chapter is that of the synthetic fibers. The first artificial fibers (like the first bulk plastics) were made from cellulose as the starting material. Naturally, the chemists began with cellulose nitrate, since it was available in reasonable quantity. In 1884, Hilaire Bernigaud de Chardonnet, a French chemist, dissolved cellulose nitrate in a mixture of alcohol and ether and forced the resulting thick solution through small holes. As the solution sprayed out, the alcohol and ether evaporated, leaving behind the cellulose nitrate as a thin thread of collodion. (This is essentially the manner in which spiders and silkworms spin their threads: they eject a liquid through tiny orifices, and this becomes a solid fiber on exposure to air.) The cellulose-nitrate fibers were too flammable for use, but the nitrate groups could be removed by appropriate chemical treatment, and the result was a glossy cellulose thread resembling silk.

  De Chardonnet’s process was expensive, of course, what with nitrate groups being first put on and then taken off, to say nothing of the dangerous interlude while they were in place and of the fact that the alcohol-ether mixture used as solvent was also dangerously flammable. In 1892, methods were discovered for dissolving cellulose itself. The English chemist Charles Frederick Cross, for instance, dissolved it in carbon disulfide and formed a thread from the resulting viscous solution (named viscose). The trouble was that carbon disulfide is flammable, toxic, and evil smelling. In 1903, a competing process employing acetic acid as part of the solvent, and forming a substance called cellulose acetate, came into use.

  These artificial fibers were first called artificial silk, but were later named rayon because their glossiness reflects rays of light. The two chief varieties of rayon are usually distinguished as viscose rayon and acetate rayon.

  Viscose, by the way, can be squirted through a slit to form a thin, Aexible, waterproof, transparent sheet—cellophane—a process invented in 1908 by a French chemist, Jacques Edwin Brandenberger. Some synthetic polymers also can be extruded through a slit for the same purpose. Vinyl resins, for instance, yielded the covering material known as Saran.

  It was in the 1930s that the first completely synthetic fiber was born.

  Let me begin by saying a little about silk. Silk is an animal product made by certain caterpillars that are exacting in their requirements for food and care. The fiber must be tediously unraveled from their cocoons. For these reasons, silk is expensive and cannot be mass-produced. It was first produced in China more than 2,000 years ago, and the secret of its preparation was jealously guarded by the Chinese, so that it could be kept a lucrative monopoly for export. However, secrets cannot be kept forever, despite all security measures. The secret spread to Korea, Japan, and India. Ancient Rome received silk by the long overland route across Asia, with middlemen levying tolls every step of the way; thus, the fiber was beyond the reach of anyone except the most wealthy. In 550 A.D., silkworm eggs were smuggled into Constantinople, and silk production in Europe got its start. Nevertheless, silk has always remained more or less a luxury item. Moreover, until recently there was no good substitute for it. Rayon can imitate its glossiness but not its sheerness or strength.

  After the First World War, when silk stockings became an indispensable item of the feminine wardrobe, the pressure for greater supplies of silk or of some adequate substitute became very strong. This was particularly true in the United States, where silk was used in greatest quantity and where relations with the chief supplier, Japan, were steadily deteriorating. Chemists dreamed of somehow making a fiber that could compare with it.

  Silk is a protein (see chapter 12). Its molecule is built up of monomers called amino acids, which in turn contain amino (–NH2) and carboxyl (–COOH) groups. The two groups are joiried by a carbon atom between them; labeling the amino group a and the carboxyl group c, and symbolizing the intervening carbon by a hyphen, we can write an amino acid like this: a - c. These amino acids polymerize in head-to-tail fashion: that is, the amino group of one condenses with the carboxyl group of the next. Thus, the structure of the silk molecule runs like this:… a - c . a - c . a - c . a - c …

  In the 1930s, a Du Pont chemist named Wallace Hume Carothers was investigating molecules containing amine groups and carboxyl groups in the hope of discovering a good method of making them condense in such a way as to form molecules with large rings. (Such molecules are of importance in perfumery.) Instead, he found them condensing to form long-chain molecules.

  Carothers had already suspected that long chains might be possible, and he was not caught napping. He lost little time in following up this development. He eventually formed fibers from adipic acid and hexamethylenediamine. The adipic-acid molecule contains two carboxyl groups separated by four carbon atoms, so it can be symbolized as: c----c. Hexamethylenediamene consists of two amine groups separated by six carbon atoms, thus: a------a. When Carothers mixed the two substances together, they condensed to form a polymer like this:… a------a . c----c . a------a … The points at which condensation took place had the “c.a” configuration found in silk, you will notice.

  At first the fibers produced were not much good; they were too weak. Carothers decided that the trouble lay in the presence of the water produced in the condensation process. The water set up a counteracting hydrolysis reaction which prevented polymerization from going very far. Carothers found a cure: he arranged to carry on the polymerization under low pressure, so that the water vaporized and was easily removed by letting it condense on a cooled glass surface held close to the reacting liquid and so slanted as to carry the water away (a molecular still). Now the polymerization could continue indefinitely. It formed nice long, straight chains; and in 1935, Carothers finally had the basis for a dream fiber.

  The polymer formed from adipic acid and hexamethylenediamine was melted and extruded through holes. It was then stretched so that the fibers would lie side by side in crystalline bundles. The result was a glossy, silklike thread that could be used to weave a fabric as sheer and beautiful as silk, and even stronger. This first of the completely synthetic fibers was named nylon. Carothers did not live to see his discovery come to fruition, however. He died in 1937.

  Du Pont announced the existence of the synthetic fiber in 1938 and began producing it commercially in 1939. During the Second World War, the United States Armed Forces took all the production of nylon for parachutes and for a hundred other purposes. But after the war nylon completely replaced silk for hosiery; indeed, women’s stockings are now called nylons.

  Nylon opened the way to the production of many other synthetic fibers. Acrylonitrile, or vinyl cyanide (CH2 = CHCN), can be made to polymerize into a long chain like that of polyethylene but with cyanide groups (completely nonpoisonous in this case) attached to every other carbon. T
he result, introduced in 1950, is Orlon. If vinyl chloride (CH22 = CHCl) is added, so that the eventual chain contains chlorine atoms as well as cyanide groups, Dynel results. Or the addition of acetate groups, through the use of vinyl acetate (CH2 = CHOOCCH3), produces Acrilan.

  The British in 1941 made a polyester fiber, in which the carboxyl group of one monomer condenses with the hydroxyl group of another. The result is the usual long chain of carbon atoms, broken in this case by the periodic insertion of an oxygen in the chain. The British call it Terylene, but in the United States, it has appeared under the name of Dacron.

  These new synthetic fibers are more water-repellent than most of the natural fibers; thus they resist dampness and are not easily stained. They are not subject to destruction by moths or beetles. Some are crease-resistant and can be used to prepare “wash-and-wear” fabrics.

  Synthetic Rubber

  It is a bit startling to realize that humans have been riding on rubber wheels for only about a hundred years. For thousands of years they rode on wooden or metal rims. When Goodyear’s discovery made vulcanized rubber available, it occurred to a number of people that rubber rather than metal might be wrapped around wheels. In 1845, a British engineer, Robert William Thomson, went this idea one better: he patented a device consisting of an inflated rubber tube that would fit over a wheel. By 1890, tires were routinely used for bicycles; and in 1895, they were placed on horseless carriages.