Nylon is a generic designation for a family of synthetic polymers known generically as
aliphatic polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's
research facility at the DuPont Experimental Station. Nylon is one of the most commonly
used polymers. Key representatives are nylon-6,6; nylon-6; nylon-6,9; nylon-6,10; nylon-6,12;
nylon-11; nylon-12 and nylon-4,6.
Overview
Nylon is a thermoplastic, silky material, first used commercially in a nylon-bristled
toothbrush, followed more famously by women's stockings after being introduced as a fabric
at the 1939 New York World's Fair. Nylon is made of repeating units linked by amide bonds
and is frequently referred to as polyamide. Nylon was the first commercially successful
synthetic thermoplastic polymer. There are two common ways of making nylon for fiber
applications. In one approach, molecules with an acid group on each end are reacted with
molecules containing amine groups on each end. The resulting nylon is named on the basis
of the number of carbon atoms separating the two acid groups and the two amines. These
are formed into monomers of intermediate molecular weight, which are then reacted to form long
polymer chains. Nylon was intended to be a synthetic replacement
for silk and substituted for it in many different products after silk became scarce during World
War II. It replaced silk in military applications such as parachutes and flak vests, and was
used in many types of vehicle tires. Nylon fibers are used in many applications,
including clothes fabrics, bridal veils, package paper, carpets, musical strings, pipes, and
rope. Solid nylon is used in hair combs and mechanical
parts such as machine screws, gears and other low- to medium-stress components previously
cast in metal. Engineering-grade nylon is processed by extrusion, casting, and injection
molding. Type 6,6 Nylon 101 is the most common commercial grade of nylon, and Nylon 6 is
the most common commercial grade of molded nylon. For use in tools such as spudgers,
nylon is available in glass-filled variants which increase structural and impact strength
and rigidity, and molybdenum sulfide-filled variants which increase lubricity. Its various
properties also make it very useful as a material in additive manufacturing; specifically as
a filament in consumer and professional grade fused deposition modeling 3D printers.
Chemistry Nylons are condensation copolymers formed
by reacting equal parts of a diamine and a dicarboxylic acid, so that amides are formed
at both ends of each monomer in a process analogous to polypeptide biopolymers. Chemical
elements included are carbon, hydrogen, nitrogen, and oxygen. 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 which refers to the fact that the diamine and the diacid
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: C terminal → 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. 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 this, some acetic acid is added to react with a free
amine end group during polymer elongation to limit the molecular weight. In practice,
and especially for 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 to the final 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.
The peptide bond within the caprolactam is broken with the exposed active groups on each
side being incorporated into 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 sometimes indistinguishable from those of nylon 6,6 — except for melt
temperature and some fiber properties in products like carpets and textiles. There is also nylon 9.
The 428 °F melting point of nylon 6 is lower than the 509 °F melting point of nylon 6,6.
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" or "PA-6,12" is a copolymer of a 6C diamine
and a 12C diacid. Similarly 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 or isophthalic acid, more commonly associated with polyesters. There are copolymers of N-6,6/N6;
copolymers of N-6,6N-12; and others. Because of the way polyamides are formed, nylon would
seem to be limited to unbranched, straight chains. But "star" branched nylon can be produced
by the condensation of dicarboxylic acids with polyamines having three or more amino
groups. The general reaction is:
Two molecules of water are 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 = 4C and R' = 6C alkanes,
but one also has to include the two carboxyl carbons in the diacid to get the number it
donates to the chain. In Kevlar, both R and R' are benzene rings.
Concepts of nylon production The first approach: combining molecules with
an acid group on each end are reacted with two chemicals that contain amine groups on
each end. This process creates nylon 6,6, made of hexamethylene diamine with six carbon
atoms and adipic acid. The second approach: a compound has an acid
at one end and an amine at the other and is polymerized to form a chain with repeating
units ofx. In other words, nylon 6 is made from a single six-carbon substance called
caprolactam. In this equation, if n = 5, then nylon 6 is the assigned name.
The characteristic features of nylon 6,6 include: Pleats and creases can be heat-set at higher
temperatures More compact molecular structure
Better weathering properties; better sunlight resistance
Softer "Hand" Higher melting point
Superior colorfastness Excellent abrasion resistance
On the other hand, nylon 6 is easy to dye, more readily fades; it has a higher impact
resistance, a more rapid moisture absorption, greater elasticity and elastic recovery.
Characteristics Variation of luster: nylon has the ability
to be very lustrous, semilustrous or dull. Durability: its high tenacity fibers are used
for seatbelts, tire cords, ballistic cloth and other uses.
High elongation Excellent abrasion resistance
Highly resilient Paved the way for easy-care garments
High resistance to insects, fungi, animals, as well as molds, mildew, rot and many chemicals
Used in carpets and nylon stockings Melts instead of burning
Used in many military applications Good specific strength
Transparent to infrared light 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. 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 all 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.
Nylon 6,6 can have multiple parallel strands aligned with their neighboring peptide bonds
at coordinated separations of exactly 6 and 4 carbons for considerable lengths, so the
carbonyl oxygens and amide hydrogens can line up to form interchain hydrogen bonds repeatedly,
without interruption. Nylon 5,10 can have coordinated runs of 5 and 8 carbons. Thus
parallel strands can participate in extended, unbroken, 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 form uninterrupted H-bonded sheets with
mixed directionalities, but the β-sheet wrinkling is somewhat different. The three-dimensional
disposition of each alkane hydrocarbon chain depends on rotations about the 109.47° tetrahedral
bonds of singly bonded carbon atoms. 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. In practice, nylon fibers are most
often drawn using heated rolls at high speeds. 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 heat source such as a flame or electrode
to prevent this. When dry, polyamide is a good electrical insulator.
However, polyamide is hygroscopic. The absorption of water will change some of the material's
properties such as its electrical resistance. Nylon is less absorbent than wool or cotton.
Historical uses
Bill Pittendreigh, DuPont, and other individuals and corporations worked diligently during
the first few months of World War II to find a way to replace Asian silk and hemp with
nylon 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 nearly all of the rest. By August 1945, manufactured fibers
had taken a market share of 25%, at the expense of cotton. After the war, because of shortages
of both silk and nylon, nylon parachute material was sometimes repurposed to make dresses.
Some of the terpolymers based upon nylon are used every day in packaging. Nylon has been
used for meat wrappings and sausage sheaths. Nylon was used to make the stock of the Remington
Nylon 66 rifle. The frame of the modern Glock pistol is made of a nylon composite.
Instrument strings In the mid-1940s, classical guitarist Andrés
Segovia mentioned the shortage of good guitar strings in the United States, particularly
his favorite Pirastro catgut strings, to a number of foreign diplomats at a party, including
General Lindeman of the British Embassy. A month later, the General presented Segovia
with some nylon strings which he had obtained via some members of the DuPont family. Segovia
found that although the strings produced a clear sound, they had a faint metallic timbre
which he hoped could be eliminated. Nylon strings were first tried on stage by
Olga Coelho in New York in January, 1944. In 1946, Segovia and string maker Albert Augustine
were introduced by their mutual friend Vladimir Bobri, editor of Guitar Review. On the basis
of Segovia's interest and Augustine's past experiments, they decided to pursue the development
of nylon strings. DuPont, skeptical of the idea, agreed to supply the nylon if Augustine
would endeavor to develop and produce the actual strings. After three years of development,
Augustine demonstrated a nylon first string whose quality impressed guitarists, including
Segovia, in addition to DuPont. Wound strings, however, were more problematic.
Eventually, however, after experimenting with various types of metal and smoothing and polishing
techniques, Augustine was also able to produce high quality nylon wound strings.
Use in composites Nylon can be used as the matrix material in
composite materials, with reinforcing fibers like glass or carbon fiber; such a composite
has a higher density than pure nylon. Such thermoplastic composites are frequently used
in car components next to the engine, such as intake manifolds, where the good heat resistance
of such materials makes them feasible competitors to metals.
Hydrolysis and degradation All nylons are susceptible to hydrolysis,
especially by strong acids, a reaction essentially the reverse of the synthetic reaction shown
above. The molecular weight of nylon products so attacked drops fast, and cracks form quickly
at the affected zones. Lower members of the nylons are affected more than higher members
such as nylon 12. This means that nylon parts cannot be used in contact with sulfuric acid
for example, such as the electrolyte used in lead–acid batteries. When being molded,
nylon must be dried to prevent hydrolysis in the molding machine barrel since water
at high temperatures can also degrade the polymer. The reaction is of the type:
Environmental impact, incineration and recycling Berners-Lee reckons the average greenhouse
gas footprint of nylon in manufacturing carpets at 5.43kg CO2 equivalent per kilo, when produced
in Europe. This gives it almost the same carbon footprint as wool, but with greater durability
and therefore a lower overall carbon footprint. Various nylons break down in fire and form
hazardous smoke, and toxic fumes or ash, typically containing hydrogen cyanide. Incinerating
nylons to recover the high energy used to create them is usually expensive, so most
nylons reach the garbage dumps, decaying very slowly. Some recycling is done on nylon, usually
creating pellets for reuse in the industry. Etymology
In 1940, John W. Eckelberry of DuPont stated that the letters "nyl" were arbitrary and
the "on" was copied from the suffixes of other fibers such as cotton and rayon. A later publication
by DuPont explained that the name was originally intended to be "No-Run", but was modified
to avoid making such an unjustified claim. Since the products was not really run-proof,
the vowels were swapped to produce "nuron", which was changed to "nilon" "to make it sound
less like a nerve tonic". For clarity in pronunciation, the "i" was changed to "y".
Alternative but apocryphal explanations for the naming are:
- it is a combination of the names of the two cities New York and London, NY-Lon;
- it is an acronym from WWII, a challenging slogan by the allies to their enemy at that
time: Now You Lazy Old Nippon, Nippon being an alternative name for Japan, which is the
country known for the traditional culture of making silk from silkworms.
See also
References
Further reading Textiles by Sara J. Kadolph, ISBN 0-13-118769-4
External links For historical perspectives on nylon, see
the Documents List of "The Stocking Story: You Be The Historian" at the Smithsonian website,
by The Lemelson Center for the Study of Invention and Innovation, National Museum of American
History, Smithsonian Institution. A chemical demonstration of the synthesis
of nylon in Carleton University's CHEM 1000 course.
Typical properties of Nylon / Polyamide Polyamide material description
Discussion of nylon synthesis and properties "How Nylon Yarn Is Made", January 1946, Popular
Science from raw material to shipment article with drawings and illustrations