Material of the month - Nylon

Materials World magazine
1 Mar 2014

In less than 80 years, nylon has become a material so commonplace, and with applications so widespread, that it touches our lives with unbelievable frequency, diversity and significance. Anna Ploszajski investigates.  

Nylon’s short history is a tumultuous story of warfare, liberation and tragedy. The break-up of international trade routes during the Second World War inspired heightened research efforts to find synthetic alternatives to imported commodities. Before war broke out, Japan was the principle supplier of silk to the USA – an essential component of military supplies, including parachutes, tents and vehicle tyres. However, international relations were deteriorating and, in 1935, Wallace Carothers, leader of DuPont’s research division, produced and patented a synthetic silk substitute at DuPont Experimental Station in Delaware, USA. It was the start of something big – before the war, 80% of all fibres produced were cotton, with the remainder mainly wool, but by mid-1945, 25% of all fibres produced were manufactured.

In 1938, nylon first hit the shelves in the bristles of Dr West’s Miracle Tuft toothbrush, followed more famously by women’s stockings, which even became synonymous with their new polymeric ingredient. DuPont did not trademark the name, choosing instead ‘to allow the word to enter the American vocabulary as a synonym for stockings’. Nylon hosiery was first introduced to the world at the 1939 New York World’s Fair and was immediately a storming success. Sixty four million pairs of stockings were sold in the first year, and nylon production made a substantial contribution to the war effort as a synthetic silk substitute in military supplies. It was even used as high-grade paper for US currency and made a cameo appearance as the tornado in the 1939 classic film The Wizard of Oz.

Despite the bounding success of his new wonder material, Carothers was a troubled man. Thought to be a manic depressive, he was deeply affected by his sister’s death in 1937. A fellow researcher reported seeing Carothers with cyanide, and that he could name all the famous chemists who had taken their own lives. Carothers married Helen Sweetman, another employee at DuPont, and they had a daughter, but tragically Carothers added his name to his list of heroes by committing suicide by cyanide before her birth.

A synthetic family
Today we refer to DuPont’s patented nylon specifically as nylon-6,6. There now exists a family of synthetic polymers, collectively referred to in general as nylon or polyamide. Two types exist, depending on the synthesis method. Nylons with a double suffix, such as nylon-6,6, nylon-6,9, nylon-6,12 and nylon-4,6 are made in a condensation reaction between a diamine with amine (-NH2) groups on each end and a dicarboxylic acid with (-COOH) groups on each end. The first number in the suffix refers to the number of carbons in the diamine, and the second to the number of carbons in the dicarboxylic acid. The product of this reaction is formed into monomers of intermediate molecular weight and then reacted to produce long polymer chains.

The second type is made by ring opening polymerisation, which involves opening a lactam ring containing both amine and acid groups. The single suffix refers to the number of carbon atoms in the lactam monomer, for example nylon-6, nylon-11 and nylon-12. This synthesis route was developed by companies such as BASF to compete with DuPont’s patented nylon-6,6.

From fabrics to upholstery and instrument strings to automotive parts, nylon is one of the most widely used polymers today. Its versatility is a result of its range of favourable properties. Nylon is a thermoplastic, so above its melting temperature it behaves like a viscous fluid. In this state, the chains are randomly coiled, but upon extrusion in an industrial spinneret, the polymer chains align due to viscous flow. Nylon’s (-CO-NH-) groups are highly polar, so readily form hydrogen bonds between adjacent strands. This effect, coupled with a regular and symmetrical backbone, makes nylon highly crystalline, which is excellent for strong fibres. Cold-drawing these after extrusion further aligns the chains and this enhances the tensile strength.

These strong, durable fibres lend to nylon’s applications in seatbelts, tyre cord, hoses, conveyer belts, sports racket strings, ropes and nets, sleeping bags, tarpaulin, tents, thread, fishing line, dental floss and sutures for surgery. Fabrics made from nylon fibre are lightweight and warm.

Bulk nylon is a semi-crystalline polymer. The amorphous regions promote elasticity, whereas the lamellar crystals provide properties such as strength, rigidity, wear resistance, chemical resistance and thermal resistance. Furthermore, as with most plastics, nylon provides good electrical insulation and this, coupled with its corrosion resistance and toughness, makes it a good choice for high load electrical components such as switch housings, circuit breakers, fuses, cable ties and power tool housings.

A high tensile strength makes nylon an excellent material for synthetic musical instrument strings. Until its discovery, good quality guitar strings were made from catgut. Nylon strings were debuted on stage in New York in January 1944 by classical guitar player Olga Coelho. Andrés Segovia, another classical guitarist, collaborated with stringmaker Albert Augustine and DuPont to improve the strings, which initially produced a faint metallic timbre. Three years of development led to the production of an impressive range of better nylon strings, suitable for both lowand high-pitched playing.

Another notable property of nylons is their high melting temperatures (220°C for nylon-6 and 265°C for nylon-6,6). This, coupled with toughness and low gas permeability, makes nylon suitable for use in food wrapping, even for hightemperature packaging applications such as boil-in-the-bag food packaging. Heat-stabilised nylon systems allow sustained performance at temperatures up to 185°C.

Nylon matrix composites that incorporate reinforcing fibres such as glass or carbon have increased density, structural strength, impact strength and rigidity. Favourable mechanical properties mean such composites can replace metal in lowstress mechanical parts. For example, in a car engine, the inlet manifolds that supply the fuel/air mixture to the cylinders can be made from nylon composites, which are tough, corrosion resistant, lighter and cheaper than aluminium, and even offer improved airflow due to a smoother internal bore. Furthermore, although nylon boasts useful self-lubricating properties for gears and bearings, it can be filled with molybdenum sulphide to further enhance lubricity.

Weighing up the risks
A major disadvantage of nylon is its hygroscopicity. This is the absorption of atmospheric water over time, which decreases favourable properties such as electrical resistance, dimensional stability, strength and stiffness. Water molecules produce polar bonds with the amide groups on the nylon backbone and fit between molecules, causing small displacements and overall matrix swelling. This process increases the molecular mobility of nylon chains, since greater space lowers the secondary forces to allow easier translational motion. The absorption continues until an equilibrium point, and uptake depends on temperature, crystallinity and component thickness. Preconditioning of the component can combat moisture absorption in-situ, and the absorption is reversible, so properties are recovered upon drying. Fillers and reinforcements reduce the effects of absorption, since there is a smaller volume of nylon in the matrix. Furthermore, this absorption can favourably increase impact resistance and flexibility. Nylon-6,12 has half the moisture absorbency of nylon-6,6, therefore the properties of nylon-6,12 can be expected to be more consistent and independent of ambient humidity levels.

In a world that is becoming ever more sensitive to sustainability, it is a huge advantage that nylon, derived from petroleum materials, is fully recyclable. There are two main recycling processes for nylon – mechanical and chemical. In mechanical recycling, the waste nylon is cleaned, cut, remelted and spun into a yarn. The properties and performance of the regenerated material are not degraded upon recycling, unless contaminants are present. In chemical recycling, the waste nylon is again cleaned and cut, then depolymerised to the base molecule, chemically repolymerised, and finally reprocessed into textiles.

From underwear drawers to driving and from Kansas to concert halls, the troubled genius responsible for nylon’s conception could not possibly have foreseen the diverse impact of his precious creation.