Silk and Nylon

Silk and Nylon

     Silk has always been regarded as a fabric for the wealthy, and it is considered irreplaceable. The desirable properties of silk- its warmth in cold weather and coolness in warm weather, its ability to take dyes so well, its smooth feel, and its luster- are all results of its chemical structure. The chemical structure of silk ultimately led to new trade routes between the East and the rest of the world.

     There is a legend that around 2640 B.C., Princess Hsi-ling-shih found that a thread of silk could be unwound from an insect cocoon that fell into her tea. It is unknown whether this story is true, however silk production did begin in China with the cultivation of the silkworm. The silkworm is a small gray worm that only feeds on the leaves of mulberry trees. The silkworm moth can lay approximately 500 eggs in a five-day period, and then it dies. A single gram of these eggs can produce over a thousand silkworms, which together produce around 200 grams of raw silk. The worm spends several days spinning its cocoon with a long, continuous strand of silk that comes from its jaw. It creates a dense cocoon, and gradually changes into a chrysalis. To extract the silk, the cocoon is heated to kill the chrysalis inside, and then submerged in boiling water to dissolve the secretion that holds the threads together. The silk is unwound from the cocoon and wound onto reels, with a thread length ranging from 400 to over 3,000 yards. 

      Although trade routes known as the Silk Road were opened throughout central Asia, China kept silk production details secret. Silk trade had a slow spread, but regular shipments were arriving in the West by 1 B.C.. The Chinese desperately wanted to maintain their silk monopoly, so they created new laws. Any attempt to smuggle silkworms, silkworm eggs, or white mulberry seeds out of China was punishable by death. According to legend, in the year 522, two monks managed to smuggle silkworm eggs and mulberry seeds in hollowed-out canes, opening silk production to the West. By the 14th century, silk production became a major industry in Italy, which became one of the financial bases of the Renaissance movement. There were various attempts to produce silk in North America, but they were commercially unsuccessful. However, the United States became one of the largest silk manufacturers in the world in the 20th century.
     Silk is a protein, which is made of amino acids. The three main amino acids found in silk are glycine, alanine, and serine, which have the smallest side groups of all the amino acids. Together, these amino acids consitute 85% of silk's structure. The very small side groups of these amino acids contribute to the smooth texture of silk. Silk is a polymer in which the constituent amino acids have varying side groups. Amino acids are linked by peptide bonds between the carbon of one amino acid and the nitrogen of the other amino acid. An estimated 80-85% of the amino acids in silk form a repeating pattern of glycine-serine-glycine-alanine-glycine-alanine. A pleated sheet structure formed by adjacent chains of amino acids accounts for the smoothness as well as the luster of silk. The amino acids that are not part of the pleated sheet structure are responsible for the brightness or "sparkle" of silk and its ability to easily absorb dyes.

     Silk is difficult to replicate due to all of its properties, however many attempts to produce a synthetic version were made beginning in the late 19th century. Although silk is a fairly simple molecule, it is chemically challenging to attach the amino acids in the combinations that occur in natural silk. In the late 1870s, while pursuing photography, French count Hilaire de Chardonnet found that spilled collodion (nitrocellulose material that coats photographic plates) stuck to a sticky mass. Chardonnet was able to pull long, silk-like threads from this mass. He had spent a lot of time studying the silkworm and the way it spins its silk fiber. Chardonnet tried forcing collodion through tiny holes, producing the first reasonable facsimile of silk fiber.
     Although the words synthetic and artificial are often used interchangeably, they have different meanings. Synthetic means that a product is man-made through chemical reactions. Artificial means that a compound has a different chemical structure than another compound, but it has properties that can mimic its role. Chardonnet actually produced artificial silk, even though it was synthetically made. This Chardonnet silk was soft and had luster, but it was also highly flammable. This silk was spun from nitocellulose, which is explosive. Chardonnet began manufacturing this material in 1891, but its flammable nature led to its downfall. Several incidents with the fabric caused the Chardonnet factory to be shut down. However, Chardonnet continued his production of artificial silk with a denitrating agent that made the fabric safer and only about as flammable as cotton. In 1901, Charles Cross and Edward Bevan produced viscose. The name was derived from the high viscosity of the substance. Viscose liquid forced through a spinnerette and into an acid bath produced viscose silk. This process was used by the American Viscose Company and the Du Pont Fibersilk Company (later becoming the Du Pont Corporation). By 1938, approximately 300 million pounds of viscose silk were produced each year. The viscose process is still used today to produce rayons- artificial forms of silk with threads composed of cellulose.
     In 1938, an organic chemist hired by the Du Pont Fibersilk Company created nylon, which was not cellulose based. In the 1920s, Wallace Carothers, an organic chemist from Harvard University who was 31 years old, was given the opportunity to do an independent study for Du Pont on a virtually unlimited budget. Carothers made the decision to work with polymers. At the time, it was unclear what the structure of polymers actually was. One theory was that they were groups of molecules grouped together (colloids). Another theory, proposed by Hermann Staudinger, was that polymers were very large molecules. Within a year of working, Carothers produced a polyester molecule that had an atomic weight larger than 5,000. He later increased the weight to 12,000, providing more support for the giant molecule theory. Staudinger won the Nobel Prize in chemistry for this theory in 1935.
     For four years, Carothers and his colleagues created different types of polymers and studied their properties, until they finally produced nylon. This fabric is man-made and is the most similar to silk in its properties, worthy of being called "artificial silk". Nylon is a polyamide, having the same amide linkages as silk. However, the monomer units are different. Rather than having an acid and amine on each end like silk does, nylon has units with either two acids or two amines. Nylon was used for the first time as toothbrush bristles in 1938. A year later (1939) nylon stockings were brought onto the market. It proved to be an excellent material for stockings because it did not sag or wrinkle, and it was far cheaper than silk. Only a year after being introduced, approximately 64 million pairs of "nylons" were sold.

                         

     Nylon's properties allowed it to find uses in other products, including fishing nets and lines, surgical sutures, strings for tennis and badminton rackets, and coating for electrical wires. During World War II, Du Pont shifted its focus to military-related products. Main uses of nylon became tire cords, mosquito netting, ropes, weather balloons, and parachute shrouds. After the war, civilian products were made again. In the 1950s, silk was commonly used for skiwear, clothing, sails, furnishings, and carpets. Nylon was also able to be molded and known as the "engineering plastic" because it was strong enough to be a replacement for metal. In 1953, over 10 million pounds of nylon were produced for that single purpose. Wallace Carothers became depressed and committed suicide in 1937. He swallowed a vile of cyanide, and never lived to see the success of his polymer.