Polyamide (PA), nylon, is a significant engineering material and important in textiles, films and casting. It has good strength and stiffness, and is resistant to chemicals and weathering. Something that sets nylon apart is its slippery surface: it has low coefficient of friction. Also referred to as self-lubricating, it helps with moving and sliding parts, and also to reduce squeak and rattle, such as in transport applications.

They may be opaque and brightly coloured, or transparent, and relatively hard to very flexible and tough. There are several types, which are distinguished with numbers. These numbers (such as 6 and 12) indicate the carbon (C) atoms of the repeating monomer they are made up of. A double number (such as 6,6 or 6,12) indicates that two monomers were used and the atom count of each. The proportion of carbon atoms determines the unique characteristics of each type. As the number of carbon atoms rises, moisture absorption is reduced – nylon has a tendency to take-up moisture – which results in improved dimensional stability, endurance and electrical properties, but lower toughness and less formability. Higher numbers are typically more expensive and less widely available.

PA6 and PA66 are the most common and used in a broad range of formats, including fibres, film, injection moulding, casting, extrusion and composites. The major limitation of polyamides is water absorption, which affects dimensional stability – over time absorbed moisture leads to creep (change in length and shape). Therefore, they are used mainly for technical and mechanical parts in automotive, industrial and consumer products, where dimensional tolerances are not critical. The amount of water absorption varies, with PA6 being the most vulnerable, closely followed by PA6,6. This means a big swing in mechanical properties, from dry to conditioned. PA11 has the lowest rate of absorption, which means it has longer term dimensional stability, even in humid or damp environments.

PA11 and PA12, as well as PA6 and PA66, are available from bio-based and potentially renewable ingredients. Castor oil provides the raw material for the monomer, which is polymerised into nylon.

Polyamides that are fully or partially bio-based (bio-PA) offer an alternative from the same polymer class and thus comparable material and processing properties. The use of biomass in production reduces dependence on fossil resources as well as greenhouse gas (GHG) emissions, because plant-based raw materials absorb CO2 from the atmosphere.

Semi-aromatic polyamides – such as polyphthalamide (PPA) and polyarylamide (PARA) – have exceptional engineering properties. The other nylons are aliphatic (polymer consisting of straight or branched chains of atoms). There are several advantages to introducing aromatic structures (closed rings of atoms), such as enhanced strength, stiffness, dimensional stability and increased resistance to heat and chemicals. The disadvantage is that it takes more energy to process and convert. High performance polyamides, such as these, compete with die cast alloys, and have been used as a direct replacement in many cases, such as in automotive, medical and electrical applications.