Stay Home. Stay Safe. Stay Informed: www.sacoronavirus.co.za

All about nylon

The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Nylon

NYLON

Nylon is a generic designation for a family of synthetic polymers, more specifically aliphatic or semi-aromatic polyamides. They can be melt processed into fibres, films or shapes. The first example of nylon (nylon 66) was produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station. Nylon polymers have found significant commercial applications in fibres (apparel, flooring and rubber reinforcement), in shapes (moulded parts for cars, electrical equipment, etc.), and in films (mostly for food packaging)

OVERVIEW


Wallace Carothers


Nylon stockings being inspected in Malmö, Sweden in 1954

Nylon is a thermoplastic, silky material, first used commercially in a nylon-bristled toothbrush (1938), followed more famously by women's stockings ("nylons"; 1940) 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 a type of polyamide and is frequently referred to as such  Nylon was the first commercially successful synthetic thermoplastic polymer. Commercially, nylon polymer is made by reacting monomers which are either lactams, acid/amines or stoichiometric mixtures of diamines (-NH2) and diacids (-COOH). Mixtures of these can be polymerized together to make copolymers. Nylon polymers can be mixed with a wide variety of additives to achieve many different property variations.

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.

After initial commercialization of nylon as a fibre, applications in the form of shapes and films were also developed. The main market for nylon shapes now is in auto components, but there are many others.

CHEMISTRY

Nylons are condensation copolymers, formed by reacting difunctional monomers containing equal parts of amine and carboxylic acid, so that amides are formed at both ends of each monomer in a process analogous to polypeptide biopolymers. Most nylons are made from the reaction of a dicarboxylic acid with a diamine (e.g. PA66) or a lactam or amino acid with itself (e.g. PA6). In the first case, the structure is so-called ABAB similar to 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 second case (so called AA), the repeating unit corresponds to the single monomer.

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 (u). 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. The salt is crystallized to purify it and obtain the desired precise stoichiometry. Heated to 285 °C (545 °F), the salt reacts to form nylon polymer with the production of water.

Wallace Carothers at DuPont patented nylon 66, but overlooked the possibility to use lactams. That synthetic route was developed by Paul Schlack at IG Farben, leading to nylon 6, or polycaprolactam — 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.

The 428 °F (220 °C) melting point of nylon 6 is lower than the 509 °F (265 °C) melting point of nylon 66.

Nylon 510, made from pentamethylene diamine and sebacic acid, was studied by Carothers even before nylon 66 and has superior properties, but is more expensive to make. In keeping with this naming convention, "nylon 6,12" or "PA 612" is a copolymer of a 6C diamine and a 12C diacid. Similarly for PA 510 PA 611; PA 1012, etc. Other nylons include copolymerized dicarboxylic acid/diamine products that are not based upon the monomers listed above. For example, some fully aromatic nylons (known as "aramids") are polymerized with the addition of diacids like terephthalic acid (→ Kevlar, Twaron) or isophthalic acid (→ Nomex), more commonly associated with polyesters. There are copolymers of PA 66/6; copolymers of PA 66/6/12; and others. In general linear polymers are the most useful, but it is possible to introduce branches in nylon 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.

Industrial synthesis is usually done by heating the acids, amines or lactams to remove water, but in the laboratory, diacid chlorides can be reacted with diamines. For example a popular demonstration of interfacial polymerisation (the "nylon rope trick") is the synthesis of nylon 66 from adipoyl chloride and hexamethylene diamine

Nomenclature
The nomenclature used for nylon polymers was devised during the synthesis of the first simple aliphatic nylons and uses numbers to describe the number of carbons between acid and amine functions (including the carbon of the carboxylic acid). Subsequent use of cyclic and aromatic monomers required the use of letters or sets of letters. One number after "PA" for a homopolymer based on one monomer, and two numbers or sets of letters where there are two monomers. For copolymers the comonomers or pairs of comonomers are separated by slashes, as shown in the examples below.

  • homopolymers :
    • PA 6 : [NH−(CH2)5−CO]n made from ε-Caprolactam ;
    • PA 66 : [NH−(CH2)6−NH−CO−(CH2)4−CO]n made from hexamethylenediamine and adipic acid;
  • copolymers :
  • PA 6/66 : [NH-(CH2)6−NH−CO−(CH2)4−CO]n−[NH−(CH2)5−CO]m made from caprolactam, hexamethylenediamine and adipic acid ;
  • PA 66/610 : [NH−(CH2)6−NH−CO−(CH2)4−CO]n−[NH−(CH2)6−NH−CO−(CH2)8−CO]m made from hexamethylenediamine, adipic acid and sebacic acid.

In common usage, the prefix 'PA' or the name 'Nylon' are used interchangeably and are equivalent in meaning.

Monomers
Nylon momomers are manufactured by a variety routes, starting in most cases from crude oil but sometimes from biomass.

  • ε-Caprolactam: Crude oil → benzene → cyclohexane → cyclohexanone → cyclohexanone oxime → caprolactam
  • 11-aminoundecanoic acid: Castor oil → ricinoleic acid → methylricinoleate → methyl-11-undecenoate → undecenoic acid → 11-undecenoic acid → 11-bromoundecanoic acid → 11-aminoundecanoic acid
  • Laurolactam: Butadiene → cyclododecatriene → cyclododecane → cyclododecanone → cyclododecanone oxime → laurolactam
  • Adipic acid: Crude oil → benzene → cyclohexane → cyclohexanone + cyclohexanol → adipic acid
  • Hexamethylene diamine (HMD): Crude oil → butadiene → adiponitrile → hexamethylene diamine
  • Sebacic acid (decanedioic acid): Castor oil → ricinoleic acid → sebacic acid

Polymers
Due to the large number of diamines, diacids and amino acids that can be synthesized, many nylon polymers have been made experimentally and characterized to varying degrees. A smaller number have been scaled up and offered commercially, and these are detailed below.

Homopolymers
Homopolymer nylons derived from one monomer

Examples of these polymers that are or were commercially available

  • PA6 Lanxess Durethan B
  • PA11 Arkema Rilsan
  • PA12 Evonik Vestamid L

Homopolymer polyamides derived from pairs of diamines and diacids (or diacid derivatives). Shown in the table below are polymers which are or have been offered commercially either as homopolymers or as a part of a copolymer.

Examples of these polymers that are or were commercially available

  • PA46 DSM Stanyl
  • PA410 DSM Ecopaxx
  • PA4T DSM Four Tii
  • PA66 DuPont Zytel

Copolymers
It is easy to make mixtures of the monomers or sets of monomers used to make nylons to obtain copolymers. This lowers crystallinity and can therefore lower the melting point.

Some copolymers that have been or are commercially available are listed below:

  • PA6/66 (DuPont Zytel)
  • PA6/6T (BASF Ultramid T)
  • PA6I/6T (DuPont Selar PA]
  • PA66/6T (DuPont Zytel HTN)
  • PA12/MACMI (EMS Grilamid TR)

Blends
Most nylon polymers are miscible with each other allowing a range of blends to made. The two polymers can react with one another by transamidation to form random copolymers.

According to their crystallinity, polyamides can be:

  • semi-crystalline:
  • high crystallinity : PA46 et PA 66 ;
  • low crystallinity : PA mXD6 made from m-xylylenediamine and adipic acid;
  • amorphous : PA 6I made from hexamethylenediamine and isophthalic acid.

According to this classification, PA66, for example, is an aliphatic semi-crystalline homopolyamide.

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 (-CO-NH-) 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 fibres. 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.


Hydrogen bonding in Nylon 6,6 (in mauve)

Nylon 6,6 can have multiple parallel strands aligned with their neighbouring 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 (see the figure opposite). Nylon 5,10 can have coordinated runs of 5 and 8 carbons. Thus parallel (but not antiparallel) 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. (Proteins have only an amino acid α-carbon separating sequential -CO-NH- groups.) 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 fibres through pores in an industrial spinneret, the individual polymer chains tend to align because of viscous flow. If subjected to cold drawing afterwards, the fibres align further, increasing their crystallinity, and the material acquires additional tensile strength. In practice, nylon fibres 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 colourless, 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.

Characteristics

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 (256 °C/492.8 °F)
  • Superior colourfastness
  • 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.

Variation of lustre: nylon has the ability to be very lustrous, semi lustrous or dull.

Durability: its high tenacity fibres are used for seatbelts, tire cords, ballistic cloth and other uses.

High elongation

Excellent abrasion resistance

Highly resilient (nylon fabrics are heat-set)

Paved the way for easy-care garments

High resistance to insects, fungi, animals, as well as moulds, 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 (−12 dB)

USES

Fibres


These worn out nylon stockings will be reprocessed and made into parachutes for army fliers c. 1942


Blue Nylon fabric ball gown by Emma Domb, Chemical Heritage Foundation

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 fibres used and manufactured, and wool fibres accounted for nearly all of the rest. By August 1945, manufactured fibres 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.

Nylon 6 and 66 fibres are used in carpet manufacture.

Nylon is one kind of fibre used in tire cord.

Shapes
Nylon resins are widely used in the automobile industry especially in the engine compartment.

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 moulding. Type 6,6 Nylon 101 is the most common commercial grade of nylon, and Nylon 6 is the most common commercial grade of moulded 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 sulphide-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 modelling 3D printers. Nylon can be used as the matrix material in composite materials, with reinforcing fibres like glass or carbon fibre; such a composite has a higher density than pure nylon. Such thermoplastic composites (25% to 30% glass fibre) 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.

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.

Food packaging
Nylon resins are used as a component of food packaging films where an oxygen barrier is needed. Some of the terpolymers based upon nylon are used every day in packaging. Nylon has been used for meat wrappings and sausage sheaths. The high temperature resistance of nylon makes it useful for oven bags.

Filaments
Nylon filaments are primarily used in brushes especially toothbrushes and 'strimmers'. They are also used as monofilaments in fishing line. Nylon 610 and 612 are the most used polymers for filaments.

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 modelling 3D printers.

Other forms

Extruded Profiles
Nylon resins can be extruded into rods, tubes and sheets.

Powder Coating
Nylon powders are used to power coat metals. Nylon 11 and nylon 12 are the most widely used.

Instrument strings
In the mid-1940s, classical guitarist Andrés Segovia mentioned the shortage of good guitar strings in the United States, particularly his favourite 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, sceptical of the idea, agreed to supply the nylon if Augustine would endeavour 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.

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 (such as nylon 6) are affected more than higher members such as nylon 12. This means that nylon parts cannot be used in contact with sulphuric acid for example, such as the electrolyte used in lead–acid batteries. When being moulded, nylon must be dried to prevent hydrolysis in the moulding 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 5.43 kg 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.

Data published by PlasticsEurope indicates for nylon 66 a greenhouse gas footprint of 6.4 kg CO2 equivalent per kilo, and a energy consumption of 138 kJ/kg. When considering the environmental impact of nylon, it is important to consider the use phase. In particular when cars are lightweighted, significant savings in fuel consumption and CO2 emissions are reduced.

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. Nylon is a robust polymer and lends itself well to recycling. Much nylon resin is recycled directly in a closed loop at the injection moulding machine, by grinding sprues and runners and mixing them with the virgin granules being consumed by the moulding machine.

CURRENT MARKET AND FORECAST

As one of the largest engineering polymer families, the global demand of nylon resins and compounds was valued at roughly US$20.5 billion in 2013. The market is expected to reach US$30 billion by 2020 by following an average annual growth of 5.5%.

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 fibres 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. Since the products were 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".

An alternative but apocryphal explanation for the name is that it is a combination of New York and London: NY-Lon.

To read more about nylon, please click on the following link: http://en.wikipedia.org/wiki/Nylon

Wikipedia: