6. SILK AND NYLON
EXPLOSIVE MOLECULES may seem very remote from the images of luxury, softness, suppleness, and sheen conjured up by the word silk. But explosives and silk have a chemical connection, one that led to the development of new materials, new textiles, and by the twentieth century, a whole new industry.
Silk has always been prized as a fabric by the wealthy. Even with the wide choice of both natural and man-made fibers available today, it is still considered irreplaceable. The properties of silk that have long made it so desirable—its caressing feel, its warmth in cool weather and coolness in hot weather, its wonderful luster, and the fact that it takes dyes so beautifully—are due to its chemical structure. Ultimately it was the chemical structure of this remarkable substance that opened trade routes between the East and the rest of the known world.
THE SPREAD OF SILK
The history of silk goes back more than four and a half millennia. Legend has it that around 2640 B.C., Princess Hsi-ling-shih, the chief concubine of the Chinese emperor Huang-ti, found that a delicate thread of silk could be unwound from an insect cocoon that had fallen into her tea. Whether this story is myth or not, it is true that the production of silk started in China with the cultivation of the silkworm, Bombyx mori, a small gray worm that feeds only on the leaves of the mulberry tree, Morus alba.
Common in China, the silkworm moth lays around five hundred eggs over a five-day period and then dies. One gram of these tiny eggs produces more than a thousand silkworms, all of which together devour about thirty-six kilograms of ripe mulberry leaves to produce around two hundred grams of raw silk. The eggs must initially be kept at 65°F, then gradually rise to the hatching temperature of 77°F. The worms are kept in clean, well-ventilated trays, where they eat voraciously and shed their skins several times. After a month they are moved to spinning trays or frames to start spinning their cocoons, a process that takes several days. A single continuous strand of silk thread is extruded from the worm’s jaw, along with a sticky secretion that holds the threads together. The worm moves its head constantly in a figure-eight pattern, spinning a dense cocoon and gradually changing itself into a chrysalis.
To obtain the silk, the cocoons are heated to kill the chrysalis inside and then plunged into boiling water to dissolve the sticky secretion holding the threads together. Pure silk thread is then unwound from the cocoon and wound onto reels. The length of a silk thread from one cocoon can be anywhere from four hundred to more than three thousand yards.
The cultivation of silkworms and the use of the resulting silk fabric spread rapidly throughout China. Initially, silk was reserved for members of the imperial family and for nobility. Later, though its price remained high, even the common people were allowed to wear garments made of silk. Beautifully woven, lavishly embroidered, and wonderfully dyed, silk fabric was greatly prized. It was a highly valued commodity of trade and barter and even a form of currency—rewards and taxes were sometimes paid in silk.
For centuries, long after the opening of the trade routes through Central Asia collectively known as the Silk Road, the Chinese kept the details of silk production a secret. The path of the Silk Road varied over the centuries, depending mostly on the politics and safety of the regions along the way. At its longest it extended some six thousand miles from Peking (Beijing), in eastern China, to Byzantium (later Constantinople, now Istanbul) in present-day Turkey, and to Antioch and Tyre, on the Mediterranean, with major arteries diverting into northern India. Some parts of the Silk Road date back over four and a half millennia.
Trade in silk spread slowly, but by the first century B.C. regular shipments of silk were arriving in the West. In Japan sericulture began around A.D. 200 and developed independently from the rest of the world. The Persians quickly became middlemen in the silk trade. The Chinese, to maintain their monopoly on production, made attempts to smuggle silkworms, silkworm eggs, or white mulberry seeds from China punishable by death. But as the legend goes, in 552 two monks of the Nestorian church managed to return from China to Constantinople with hollowed-out canes that concealed silkworm eggs and mulberry seeds. This opened the door to silk production in the West. If the story is true, it is possibly the first recorded example of industrial espionage.
Sericulture spread throughout the Mediterranean and by the fourteenth century was a flourishing industry in Italy, especially in the north, where cities like Venice, Lucca, and Florence became renowned for beautiful heavy silk brocades and silk velvets. Silk exports from these areas to northern Europe are considered to be one of the financial bases of the Renaissance movement, which started in Italy about this time. Silk weavers fleeing political instability in Italy helped France become a force in the silk industry. In 1466, Louis XI granted tax exemptions for silk weavers in the city of Lyons, decreed the planting of mulberry trees, and ordered the manufacture of silk for the royal court. For the next five centuries European sericulture would be centered near Lyons and its surrounding area. Macclesfield and Spittalfield, in England, became major centers for finely woven silks when Flemish and French weavers, escaping religious persecution on the Continent, arrived in the late sixteenth century.
Various attempts to cultivate silk in North America were not commercially successful. But the spinning and weaving of silk, processes that could be readily mechanized, were developed. In the early part of the twentieth century the United States was one of the largest manufacturers of silk goods in the world.
THE CHEMISTRY OF SHEEN AND SPARKLE
Silk, like other animal fibers such as wool and hair, is a protein. Proteins are made from twenty-two different α-amino acids. The chemical structure for an α-amino acid has an amino group (NH2) and an organic acid group (COOH) arranged as shown, with the NH2 group on the carbon atom of the alpha carbon—that is, the carbon adjacent to the COOH group.
Generalized structure for an α-amino acid
This is often drawn more simply in its condensed version as
The condensed structure of the generalized amino acid structure
In these structures R represents a different group, or combination of atoms, for each amino acid. There are twenty-two different structures for R, and this is what makes the twenty-two amino acids. The R group is sometimes called the side group or the side chain. The structure of this side group is responsible for the special properties of silk—and indeed for the properties of any protein.
The smallest side group, and the only side group consisting of just one atom, is the hydrogen atom. Where this R group is H, the name of the amino acid is glycine, and the structure is as shown as follows.
The amino acid glycine
Other simple side groups are CH3 and CH2OH, giving the amino acids alanine and serine respectively.
These three amino acids have the smallest side groups of all the amino acids, and they are also the most common amino acids in silk, constituting together about 85 percent of silk’s overall structure. That the side groups in the silk amino acids are physically very small is an important factor in the smoothness of silk. In comparison, other amino acids have much larger, more complicated side groups.
Like cellulose, silk is a polymer—a macromolecule made up of repeating units. But unlike the cellulose polymer of cotton, in which the repeating units are exactly the same, the repeating units of protein polymers, amino acids, vary somewhat. The parts of the amino acid that form a polymer chain are all the same. It is the side group on each amino acid that differs.
Two amino acids combine by eliminating water between themselves, an H atom from the NH2 or amino end and an OH from the COOH or acid end. The resulting link between the two amino acids is known as an amide group. The actual chemical bond between the carbon of one amino acid and the nitrogen of the other amino acid is known as a peptide bond.
Of course, on one end of this new molecule there is still an OH that can be used to form another peptide bond with another amino acid, and on the other end there is an NH2 (also written H2N) that can form a peptide b
ond to yet another amino acid.
The amide group
is usually shown in a more space-saving way as
If we add two more amino acids, there are now four amino acids joined through amide linkages.
As there are four amino acids, there are four side groups, designated above as R, R’, R” and R”’. These side groups could all be the same, or some of them could be the same, or they could all be different. Despite having only four amino acids in the chain, a very large number of combinations is possible. R could be any of twenty-two amino acids, R’ could also be any of the twenty-two, and so could R” and R”’. This means there are 224 or 234,256 possibilities. Even a very small protein such as insulin, the hormone secreted by the pancreas that regulates glucose metabolism, contains 51 amino acids, so the number of combinations possible for insulin would be 2251 (2.9 x 1068), or billions of billions.
It is estimated that 80 to 85 percent of the amino acids of silk is a repeating sequence of glycine-serine-glycine-alanine-glycine-alanine. A chain of the silk protein polymer has a zigzag arrangement with the side groups alternating on each side.
The silk protein chain is a zigzag; the R groups alternate on each side of the chain.
These chains of the protein molecule lie parallel with adjacent chains running in opposite directions. They are held to one another through cross-attractions between the molecular strands, as shown by dotted lines below.
Attractions between side-by-side protein chains hold silk molecules together.
This produces a pleated sheet structure, where alternate R groups along the protein chain point either up or down. It can be shown as:
The pleated sheet structure. The bold lines represent the protein amino acid chains. Here R represents groups that are above the sheet, while R’ groups (where shown) are below. The narrow and dotted lines show the attractive forces holding the protein chains together.
The flexible structure resulting from the pleated sheet structure is resistant to stretching and accounts for many of the physical properties of silk. The protein chains fit together tightly; the small R groups on the surfaces are relatively similar in size, creating a uniform surface responsible for the smooth feel of silk. As well, this uniform surface acts as a good reflector of light, accounting for silk’s characteristic luster. Thus many of the highly valued qualities of silk are due to the small side groups in its protein structure.
Connoisseurs of silk also appreciate the fabric’s “sparkle,” which is attributed to the fact that not all silk molecules are part of a regular pleated sheet structure. These irregularities break up reflected light, creating flashes of brightness. Often considered unsurpassed in its ability to absorb both natural and man-made dyes, silk is easy to color. This property is again due to the parts of the silk structure that are not included in the regular repeating sequence of pleated sheets. Among these remaining 15 to 20 percent or so of amino acids—those that are not glycine, alanine, or serine—are some whose side groups can easily chemically bond with dye molecules, producing the deep, rich, and colorfast hues for which silk is famous. It is this dual nature of silk—the repetitive small-side-group pleated sheet structure responsible for strength, sheen, and smoothness, combined with the more variable remaining amino acids, giving sparkle and ease of dyeing—that has for centuries made silk such a desirable fabric.
THE SEARCH FOR SYNTHETIC SILK
All of these properties make silk difficult to replicate. But because silk was so expensive and the demand for it so great, numerous attempts were made, beginning in the late nineteenth century, to produce a synthetic version. Silk is a very simple molecule—just a repetition of very similar units. But attaching these units together in the random and non-random combination found in natural silk is a very complicated chemical problem. Modern chemists are now able, on a very small scale, to replicate the set pattern of a particular protein strand, but the process is time consuming and exacting. A silk protein produced in the laboratory in this way would be many times more expensive than the natural article.
As the complexities of the chemical structure of silk were not understood until the twentieth century, early efforts to make a synthetic version were largely guided by fortunate accidents. Sometime in the late 1870s a French count, Hilaire de Chardonnet, while pursuing his favorite hobby of photography, discovered that a spilled solution of collodion—the nitrocellulose material used to coat photographic plates—had set to a sticky mass, from which he was able to pull out long threads resembling silk. This reminded Chardonnet of something he had seen a number of years previously: as a student, he had accompanied his professor, the great Louis Pasteur, to Lyons, in the south of France, to investigate a silkworm disease that was causing enormous problems for the French silk industry. Though unable to find the cause of the silkworm blight, Chardonnet had spent much time studying the silkworm and how it spun its silk fiber. With this in mind, he now tried forcing collodion solution through a set of tiny holes. He thus produced the first reasonable facsimile of silk fiber.
The words synthetic and artificial are often used interchangeably in everyday language and are given as synonyms in most dictionaries. But there is an important chemical distinction between them. For our purposes synthetic is taken to mean that a compound is man-made by chemical reactions. The product may be one that occurs in nature or it may not occur naturally. If it does occur in nature, the synthetic version will be chemically identical to that of the natural source. For example, ascorbic acid, vitamin C, can be synthesized in a laboratory or factory; synthetic vitamin C has exactly the same chemical structure as naturally occurring vitamin C.
The word artificial refers more to the properties of a compound. An artificial compound has a different chemical structure from that of another compound, but it has properties similar enough to mimic the other’s role. For example, an artificial sweetener does not have the same structure as sugar, but it will have an important property—in this case, sweetness—in common. Artificial compounds are often man-made and are therefore synthetic, but they need not necessarily be synthetic. Some artificial sweeteners are naturally occurring.
What Chardonnet produced was artificial silk, not synthetic silk, although it was made synthetically. (By our definitions, synthetic silk would be man-made but chemically identical to real silk.) Chardonnet silk, as it came to be known, resembled silk in some of its properties but not in all of them. It was soft and lustrous, but unfortunately it was highly flammable—not a desirable property for a fabric. Chardonnet silk was spun from a solution of nitrocellulose, and as we have seen, nitrated versions of cellulose are flammable and even explosive, depending on the degree of nitration of the molecule.
A portion of a cellulose molecule. The arrows on the middle glucose unit indicate the OH groups where nitration could take place on each glucose unit along the chain.
Chardonnet patented his process in 1885 and started manufacturing Chardonnet silk in 1891. But the flammability of the material proved to be its downfall. In one incident a cigar-smoking gentleman flicked ash on the Chardonnet silk dress of his dancing partner. The garment was reported to have disappeared in a flash of flame and a puff of smoke; there was no mention of the fate of the lady. Although this event and a number of other disasters at the Chardonnet factory led to its closure, Chardonnet did not give up on his artificial silk. By 1895 he was using a somewhat different process, involving a denitrating agent that produced a much safer cellulose-based artificial silk that was no more flammable than ordinary cotton.
Another method, developed in England, in 1901, by Charles Cross and Edward Bevan produced viscose, so named because of its high viscosity. When viscose liquid was forced through a spinnerette into an acid bath, cellulose was regenerated in the form of a fine filament called viscose silk. This process was used by both the American Viscose Company, formed in 1910, and by the Du Pont Fibersilk Company (later to become the Du Pont Corporation), formed in 1921. By 1938, 300 million pounds of viscose silk were being pr
oduced annually, supplying a growing demand for the new synthetic fabrics with the desired silky gloss so reminiscent of silk.
The viscose process is still in use today, as the principal means of making what are now called rayons—artificial silks, like viscose silk, in which the threads are composed of cellulose. Although it is still the same polymer of β-glucose units, the cellulose in rayon is regenerated under a slight tension, supplying a slight difference in twist to rayon threads that accounts for its high luster. Rayon, pure white in color and still having the same chemical structure, can be dyed to any number of tints and shades in the same manner as cotton. But it also has a number of drawbacks. While the pleated sheet structure of silk (flexible but resistant to stretching) makes it ideal for hosiery, the cellulose of rayon absorbs water, causing it to sag. This is not a desirable characteristic when used for stockings.
NYLON-A NEW ARTIFICIAL SILK
A different type of artificial silk was needed, one that had rayon’s good characteristics without its weaknesses. Noncellulose-based nylon arrived on the scene in 1938, created by an organic chemist hired by the Du Pont Fibersilk Company. By the late 1920s Du Pont had become interested in the plastic materials coming into the market. Wallace Carothers, a thirty-one-year-old organic chemist at Harvard University, was offered the opportunity to perform independent research for Du Pont on a virtually unlimited budget. He began work in 1928 at the new Du Pont laboratory dedicated to basic research—itself a highly unusual concept, as within the chemical industry the practice of basic research was normally left to the universities.
Penny le Couteur & Jay Burreson Page 10