In 1928 E. I. du Pont de Nemours & Company (Du Pont) launched one of its first basic research programs and hired Wallace Hume Carothers to run it. He was brought to Du Pont in part because his fellow researchers at Harvard University and the University of Illinois had called him the best
synthetic chemist they knew. The program he supervised was designed to investigate the composition of natural polymers such as silk, cellulose, and rubber. Many of Carothers's efforts related to condensation polymers were based on his deduction that if a monofunctional reactant reacted in a certain manner in forming a small molecule, then similar reactions that employed a comparable reactant, but with two reactive groups, would form polymers. (See Figure 1.)
The amide unit (found in polyamides) shown in Figure 2 is the same connective grouping that is found in proteins.
Although the Carothers group had worked with both polyesters and polyamides, they initially emphasized their work on the polyesters, as polyesters were more soluble and easier to work with. Julian Hill, a member of the Carothers team, noticed that he could form fibers if he separated a portion of a soft polyester material using a glass stirring rod and pulled it away from the clump. But because the polyesters had softening points that were too low for their use as textiles, the group returned to its work with the polyamides. The researchers found that fibers could also be formed from the polyamides, similar to those formed from the polyesters. The strength of these fibers approached, and in some cases surpassed, the strengths of natural fibers. This new miracle fiber (nylon) was introduced at the 1939 New York World's Fair, in an exhibit that announced the synthesis of a wonder fiber from "coal, air, and water"—an exaggeration but nevertheless eye-catching. When the nylon stockings were first offered for sale in New York City, on May 15, 1940, over four million pairs were sold in the first few hours. Nylon stocking sales took a large drop during World War II when it became publicized that nylon was needed to make parachutes.
The polyamides (nylons) were given a special naming system. Nylons made from diamines and dicarboxylic acids are designated by two numbers, the first representing the number of carbons in the diamine chain (a) and the second the number of carbons in the di carboxylic acid (b). (See Figure 3.)
The nylon developed by Carothers at Du Pont was nylon 6,6. Because of the importance of starting out with equal amounts of the two reactants, salts of the diamine and of the diacid are made and then used in the commercial synthesis of nylon 6,6. (See Figure 4.)
STEPHANIE KWOLEK (1923–)
Stephanie Kwolek has seventeen patents, the first of which is for Kevlar. After creating a new polymer, she would spin them into fibers for strength and flexibility testing. Material for fibers of Kevlar was cloudy white instead of molasses brown, the first indication that she had uncovered an exceptional polymer.
Nylon 6,6 (or simply nylon 66) is the largest volume nylon used as fiber, film, and plastic. About 1,134 million kilograms (2,500 million pounds) of nylon 66 were produced for fiber applications in 2000. Nylon 66 is used to make tire cord, rope, clothing, thread, hose, undergarments, rug filament,
socks, dresses, and more. Because of the presence of polar units in nylons, similar to the presence of polar units in proteins, materials made from nylon have a nice "feel" to them. Nylon materials also attract odors (many everyday odors are polar in nature) and are easily stained. Most textile and fabric products are treated to repel unwanted odors and stainmaking materials.
Nylon 66 was the first engineering thermoplastic, and up until 1953 represented all of engineering thermoplastic sales. The term "thermoplastic" denotes a material that can be melted through heating. The term "engineering thermoplastics" describes a plastic material that can be cut, drilled, or machined. About 680.4 million kilograms (1,500 million pounds) of nylons were produced in the United States in 2000 for thermoplastic use. Nylon 66 plastic is tough and rigid. It has a relatively high use temperature (to about 270°C or 518°F), and is used in the manufacture of products ranging from automotive gears to hairbrush handles. Molded nylon 66 is used to make skate wheels, motorcycle crank cases, bearings, tractor hood extensions, skis for snowmobiles, lawnmower blades, bicycle wheels, and so on.
Most polymers, when heated, progress from a glasslike solid to a softer solid, and then to a viscous "taffylike" material that is most amenable to heat-associated fabrication. In the case of nylon 66, the transition from the solid to the soft stage is abrupt, requiring that fabrication be closely watched.
The presence in nylons of polar groups results in materials that have a relatively high glass transition temperature (T g , the point at which segmental mobility begins) and high melting point (the point at which entire polymer chain mobility begins), so that, unlike many vinyl polymers such as polyethylene and polypropylene (which must be at temperatures above their glass transition temperatures to possess needed flexibility), nylons, and many other condensation polymers, function best in contexts in which strength, and not flexibility, is the desired attribute.
Because they have these polar groups that also allow for hydrogen bonding , nylons and most condensation polymers are also stronger, more rigid and brittle, and tougher in comparison to most vinyl polymers. Nylons are also "lubrication-free," meaning they do not need lubricant for easy mobility; thus they can be used to make mechanical bearings and gears that do not need periodic lubrication.
During the early 1950s George deMestral, after walking in the Swiss countryside, noticed that he had cockleburs caught in his jacket. He examined the cockleburs and noticed that they had tiny "hooks." His cotton jacket had loops that "held" the cockleburs. He began playing with his observations and making combinations of materials—one having rigid hooks and the other having flexible loops or eyes. Today, Velcro, the name given to the nylon-based hook-and-eye combination, uses nylon as both the hook material and the eye material. Polyester is sometimes blended with the nylon to make it stronger. (Polyesters have also been used to make hook-andeye material.) Velcro is used to fasten shoes, close space suits, and it has many other applications.
Nylon 6, produced via the ring-opening reaction of the compound caprolactam is structurally similar to nylon 66 and has similar properties and uses. It is widely used in Europe in place of nylon 66, but not in the United States. (See Figure 5.)
Nylon 6,10 and nylon 6,12 are also commercially available. Because of the presence of the additional methylene (–CH 2 –) groups that are hydrophobic (water-hating), these nylons are more resistant to moisture and more ductile than nylon 66.
DSM (once called Dutch State Mines) introduced nylon 4,6 (Stanyl) in 1990. It is produced via the condensation reaction between adipic acid and 1,4-diaminobutane, produced from renewable resources. Stanyl can withstand temperatures up to about 300°C (570°F), allowing it to occupy a niche position—between conventional nylons and high-performance materials. (See Figure 6.)
Several new commercial ventures are based on using natural, renewable starting materials (instead of petrochemicals). These products are known as "green" products because they are made from renewable resources and can be composted. The compound 1,4-butanediamine, used to produce nylon 4,6 from natural material, is such a green product.
In general, more crystalline nylons are fibrous whereas less crystalline nylon materials are more plastic in behavior.
Several aromatic polyamides, called aramids, have been produced. These materials are strong, are stable at high temperatures, and have good flame-resistance properties. Nomex (made from m -diaminobenzene and isophthalic acid) is used to make flame-resistant clothing and the thin pads used in space shuttles to protect sintered silica-fiber mats from stress and vibration during flight. Kevlar (made from p -diaminobenzene and terephthalic acid) is structurally similar to Nomex and by weight is stronger than steel. It is used in the manufacture of so-called bullet-resistant clothing. Because of its outstanding strength to weight ratio, it was used as the skin covering of the human-powered Gossamer Albatross, flown over the English Channel.
Aramids are also widely used as the fibers that are part of space-age composites and in the manufacture of tire cord and tread. (See Figure 7.)
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