Nylon
| Nylon | |
|---|---|
| Density | 1150 kg/m³ |
| Electrical conductivity (σ) | 10-12 m·Ω-1 |
| Thermal conductivity | 0.25 W/(m·K) |
Nylon represents a family of synthetic polymers, a thermoplastic material, invented in 1935 by Wallace Carothers at DuPont. The first product was a nylon-bristled toothbrush (1938), followed more famously by women's stockings (1940). It is made of repeating units linked by peptide bonds (another name for amide bonds) and is frequently referred to as polyamide (PA). Nylon was the first commercially successful polymer and the first synthetic fiber to be made entirely from inorganic ingredients: coal, water and air. These are formed into monomers of intermediate molecular weight, which are then reacted to form long polymer chains. 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 stocking hard to find until the war's end. Nylon fibers are now used in fabrics and ropes, and solid nylon is used for mechanical parts and as an engineering material.
Chemistry
Most nylons are condensation copolymers formed by reacting equal parts of a diamine and a dicarboxylic acid, so that peptide bonds form at both ends of each monomer in a process analogous to polypeptide biopolymers. The numerical suffix specifies the numbers of carbons donated by the monomers; the diamine first and the diacid second. The most common variant is nylon 6,6, also called nylon 66, which refers to the fact that the diamine (hexamethylene diamine) and the diacid (adipic acid) each donate 6 carbons to the polymer chain. As with other regular copolymers like polyesters and polyurethanes, the repeating unit consists of one of each monomer, so that they alternate in the chain. Since each monomer in this copolymer has the same reactive group on both ends, the direction of the amide bond reverses between each monomer, unlike natural polyamide proteins which have overall directionality with a C terminal and an N terminal. In the laboratory, nylon 6,6 can also be made using adipoyl chloride instead of adipic acid.
It is difficult to get the proportions exactly correct, and deviations can lead to chain termination at molecular weights less than a desirable 10,000 daltons (amu). To overcome this problem, a crystalline, solid "nylon salt" can be formed at room temperature, using an exact 1:1 ratio of the acid and the base to neutralize each other. Heated to 285°C, the salt reacts to form nylon polymer. Above 20,000 daltons, it is impossible to spin the chains into yarn, so to combat the reverse problem, some acetic acid is added to react with an amine end group and limit the molecular weight.
DuPont patented nylon 6,6, so in order to compete, other companies developed the homopolymer nylon 6, or polycaprolactam — not a condensation polymer, but formed by a ring-opening polymerization (alternatively made by polymerizing aminocaproic acid). The peptide bond within the caprolactam is broken with the active groups on each side re-forming two new bonds as the monomer becomes part of the polymer backbone. In this case, all amide bonds lie in the same direction, but the properties of nylon 6 are almost indistinguishable from those of nylon 6,6. There is also a nylon 9.
Nylon 5,10, made from pentamethylene diamine and sebacic acid, was studied by Carothers even before nylon 6,6 and has superior properties, but is more expensive to make. In keeping with this naming convention, "nylon 6,12" (N-6,12) or "PA-6,12" is a copolymer of a 6C diamine and a 12C diacid. Likewise for N-5,10 N-6,11; N-10,12, etc. Other nylons 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 (→ Kevlar) or isophthalic acid (→ Nomex), more commonly associated with polyesters. There are copolymers of N-6,6/N6; copolymers of N-6,6/N-6/N-12; and others. (Without punctuation, the designations for nylons like 5,10 and 6,12 would be ambiguous.)
The general reaction is:
A molecule of water is given off and the nylon is formed. Its properties are determined by the R and R' groups in the monomers. In nylon 6,6, R' = 6C and R = 4C alkanes, but then one has to include the two carboxyl groups to get the total number of carbons in the chain. In Kevlar, both R and R' are benzene rings.
Bulk properties
Above their melting temperatures, Tm, thermoplastics like nylon are amorphous solids or viscous fluids in which the chains approximate random coils. Below Tm, amorphous regions alternate with regions which are lamellar crystals.[1] The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity. The planar amide groups are very polar, so nylon forms multiple hydrogen bonds among adjacent strands. Because the nylon backbone is so regular and symmetrical, especially if the amide bonds are in the trans configuration, 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. Apparently it can never be quenched from a melt as a completely amorphous solid.
In nylon 6,6 parallel (but not antiparallel) strands have their peptide bonds at separations of exactly 6 and 4 carbons for the entire length, and nylon 5,10 has runs of 5 and 8 carbons between peptide bonds, so the carbonyl oxygens and amide hydrogens can line up to form interchain hydrogen bonds repeatedly, without interruption. Thus parallel strands can align in extended, multi-chain β-pleated sheets, a strong and tough supermolecular structure similar to that found in natural silk fibroin and the β-keratins in feathers. Nylon 6 will H-bond with both directionalities, as do polypeptide chains, but the β-sheet wrinkling is somewhat different.
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.[2] Block nylon tends to be less crystalline, except near the surfaces due to shearing stresses during formation. Nylon is clear and colorless, or milky, but is easily dyed. Multistranded nylon cord and rope is slippery and tends to unravel. The ends can be melted and fused with a match flame to prevent this.
There are carbon fiber/nylon composities with higher density than pure nylon.
Historical uses
During World War II, nylon replaced Asian silk in parachutes. It was also used to make tires, tents, ropes, ponchos, and other military supplies. 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% of all fibers used, and manufactured and wool fibers accounted for the remaining 20%. By August, 1945, manufactured fibers had risen to 25% and cotton had dropped.
Some conspiracy theorists surmise that cannabis sativa was made illegal because the fibers from the hemp plant, used for fabrics and ropes, were in strong competition with nylon. But nylon fiber is more than twice as strong as hemp and weighs 25% less. While hemp was originally used in climbing rope, this is no longer the case, including in countries where cannabis is legal.
Some of the terpolymers based upon nylon are used every day in packaging. One use of nylon polymers is in meat wrappings. This usage includes some sausage/meat sheaths.
Etymology
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" ("run" meaning "unravel"), but was modified to avoid making such an unjustified claim and to make the word sound better. The story goes that Carothers changed one letter at a time until DuPont's management was satisfied. But he was not involved in the nylon project during the last year of his life, and committed suicide before the name was coined. It was never trademarked.
Uses
- nylon fiber
- stockings
- leggings
- pantyhose (called tights in the UK)
- toothbrush bristles
- fishing lines
- nets
- carpet fiber
- airbag fiber
- auto parts: intake manifolds, gas (petrol) tanks
- slings and rope used in climbing gear
- machine parts, such as gears and bearings
- parachutes
- metallized nylon balloons
- classical and flamenco guitar strings
- jackets
- paintball marker bolts
- racquetball, squash, and tennis racquet strings
