|Density||1.15 grams per cubic centimeter|
|Electrical conductivity (σ)||10-12 Siemens per meter|
|Thermal conductivity||0.25 Watts/(m·K)|
|Melting points||463 K - 624 K
190 °C – 350 °C
374 °F – 663 °F
The name nylon is given to a family of synthetic polymers first produced on February 28, 1935, by Gerard J. Berchet of Wallace Carothers' research group at DuPont (E.I. du Pont de Nemours and Company) in Delaware. Nylon was the first commercially successful polymer and the first synthetic fiber to be made entirely from building blocks derived from coal, in the presence of water and air. Initially used to make nylon-bristled toothbrushes (in 1938), it was soon made into fabric suitable for women's stockings (in 1940). It was intended to be a synthetic replacement for silk and substituted for it in parachutes after the United States entered World War II in 1941, making stockings hard to find until the war's end. Nylon fibers are now used in clothing, ropes, carpets, guitar strings, racket strings, fishing lines, and nets, as well as for pantyhose and parachutes. In addition, solid nylon is used as an engineering material and for mechanical parts and gasoline tanks.
In 1940, John W. Eckelberry of DuPont stated that the letters "nyl" were arbitrary and the "on" was copied from the names of other fibers such as cotton and rayon. A later publication by DuPont (Context, vol. 7, no. 2, 1978) explained that the name was originally intended to be "No-Run" (where "run" means "unravel"), but it was modified to avoid making an unjustified claim and to make the word sound better. Another explanation is that the name nylon was derived from "New York and London," the hometowns of the chemists working on the materials sythesis. There is, however, no evidence that nylon was named after New York and London.
Nylons are composed of long-chain molecules, or polymers, made by linking smaller building blocks, or monomers. Most nylons are formed by reacting two types of building blocks: a diamine (which is a chemical base) and a dicarboxylic acid (which, as its name suggests, is an acid). Special types of bonds, called amide bonds (or peptide bonds), link up these monomers into long chains. The polymer is therefore classified as a polyamide (PA). The generalized reaction can be written as follows.
This diagram indicates that "n" molecules of a dicarboxylic acid (on the left) react with "n" molecules of a diamine, producing a long chain in which the two monomers take up alternate positions and are repeated "n" times. As each amide bond is formed, a molecule of water is given off, and the reaction is therefore categorized as a condensation reaction. The properties of the polymer are determined by the structures of the groups represented as R and R' in the monomers shown above.
The most common form of nylon is called Nylon 6,6, or Nylon 66, referring to the fact that the diamine (hexamethylene diamine) and the dicarboxylic acid (adipic acid) each contribute 6 carbon atoms to the polymer chain. (In the laboratory, Nylon 6,6 can also be made using adipoyl chloride instead of adipic acid.) The numerical suffixes specify the number of carbon atoms donated by each monomer—the diamine first, the dicarboxylic acid, second.
In synthesizing nylon, it is difficult to get the diamine (base) and diacid in exactly one-to-one proportion, and the reaction may terminate before the polymer chains are sufficiently long. To overcome this problem, a crystalline, solid "nylon salt" can be formed at room temperature, using an exact one-to-one ratio of the acid and base to neutralize each other. In practice, especially for Nylon 6,6, the monomers are often combined in a water solution. The water used to make the solution is evaporated under controlled conditions, and the increasing concentration of "salt" is polymerized by heating, until the molecules reach the desired molecular weight.
DuPont patented Nylon 6,6. Consequently, in order to compete, other companies (particularly the German firm BASF) developed Nylon 6, in which each chain is made from a single type of monomer called caprolactam. The properties of Nylon 6 are somewhat similar to those of Nylon 6,6—except for the melting temperature (N6 is lower) and some fiber properties in products like carpets and textiles.
A wide range of other nylons have been produced and are named using the above-mentioned convention. For instance, "Nylon 6,12" (N-6,12) or "PA-6,12" is a copolymer of a 6-carbon diamine and a 12-carbon diacid. Likewise, N-5,10, N-6,11, and N-10,12 have been made.
Additional varieties of nylon include copolymerized dicarboxylic acid/diamine products that are not based upon the monomers listed above. For example, some "aromatic" nylons are polymerized with the addition of diacids like terephthalic acid to produce Kevlar, or isophthalic acid to produce Nomex. Other nylons are copolymers of N-6,6/N6, or N-6,6/N-6/N-12, and so forth.
Given the way polyamides are formed, nylon would seem to be limited to unbranched, straight chains. Yet "star" branched nylon can be produced by the condensation of dicarboxylic acids with polyamines having three or more amino (NH2) groups.
Nylon is clear and colorless, or milky, but it is easily dyed. Multistranded nylon cords and ropes are slippery and tend to unravel. Their ends, however, can be melted and fused with a flame to prevent this.
Nylons are described as "thermoplastic" materials. Above their melting temperatures (Tm), they are amorphous solids or viscous fluids in which the chains are shaped approximately like random coils. Below Tm, the amorphous regions alternate with regions that are "lamellar" crystals (layered structures). The amorphous regions contribute elasticity, and the crystalline regions contribute strength and rigidity.
The nylon backbone is usually made to be regular and symmetrical. Consequently, nylons often have high crystallinity and make excellent fibers. The amount of crystallinity depends on the details of formation, as well as on the kind of nylon.
In addition, the amide (-CO-NH-) groups are very polar, so nylon forms multiple hydrogen bonds among adjacent strands. When the parallel strands in nylon 6,6 are aligned properly, the chains can be held together by repeated hydrogen bonds. In this manner, parallel strands can participate in extended, unbroken, multi-chain sheets, called "β-pleated sheets," forming a strong and tough supermolecular structure. Such a structure is similar to that found in natural silk fibroin and the β-keratins in feathers.
Engineering grade nylon is processed by extrusion, casting, and injection molding. When extruded into fibers through pores in an industrial spinneret, the individual polymer chains tend to align because of viscous flow. If subjected to cold drawing afterwards, the fibers align further, increasing their crystallinity, and the material acquires additional tensile strength (ability to resist breakage under stress). Block nylon tends to be less crystalline, except near the surfaces due to shearing stresses during formation.
During World War II, nylon replaced Asian silk in parachutes. It was also used to make tires, tents, ropes, ponchos, and other supplies for the military. It was even used in the production of a high-grade paper for U.S. currency. At the outset of the war, cotton accounted for more than 80 percent of all fibers used, and wool fibers accounted for the remaining 20 percent. By August 1945, manufactured fibers had taken a market share of 25 percent, and cotton had dropped.
Currently, various types of nylons are being manufactured in the form of fiber, sheets, and molded plastics. They are being used to make a wide range of products, such as those listed below.
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