Polyamide (nylon-6, nylon-66): properties and applications

Polyamide (PA), commonly known as nylon, is one of the most widely used engineering thermoplastics in modern industry. It is a semicrystalline polymer characterised by the presence of amide groups (–CO–NH–) in the main chain, which impart high mechanical strength, good toughness, wear resistance and chemical stability. Nylon was invented by American chemist Wallace Carothers at DuPont in the 1930s, with industrial production beginning at the end of the decade.

The polyamide family is classified by the number of carbon atoms in the monomer building blocks. Among the many types of nylon, nylon-6 (PA6) and nylon-66 (PA66) occupy the leading positions, accounting for over 80% of global polyamide consumption in engineering applications.

Chemical Structure and Synthesis

Nylon-6 (PA6) is an "AB"-type polyamide produced by ring-opening polymerisation of caprolactam. Its empirical formula is C₆H₁₁NO. The molecular chains of PA6 feature unidirectional orientation of NHCO amide bonds along the chain, resulting in a monoclinic crystal structure.

Nylon-66 (PA66) is an "AABB"-type polyamide synthesised by polycondensation of hexamethylenediamine and adipic acid. Both monomers contain 6 carbon atoms each, which gives the material its name. Unlike PA6, the amide bonds in PA66 alternate along the chain, forming a triclinic crystal lattice. The empirical formula of PA66 is C₁₂H₂₂N₂O₂, and its molar mass is 226 g/mol.

Despite structural differences, both polymers form extensive hydrogen-bonded structures: macromolecules align linearly, and hydrogen bonds between CO and NH groups of adjacent chains are energetically maximised. The most stable crystalline form for both materials is the α-crystal modification.

Physical and Mechanical Properties

PA6 exhibits high mechanical strength: tensile strength of 60–120 MN/m² and flexural strength of 70–100 MN/m². The melting temperature of PA6 ranges from 215–220 °C, with a glass transition temperature of 50–75 °C in the dry state. The material offers excellent toughness and impact strength, even at low temperatures. PA6 can withstand steam sterilisation up to 140 °C. Tensile strength is approximately 700 kgf/cm².

PA66 demonstrates higher strength characteristics. Its melting temperature is 255–264 °C, approximately 40 °C higher than PA6. Tensile strength reaches 75.9–82.9 MPa, or about 770 kgf/cm². The flexural modulus of PA66 is higher (3.03 GPa vs 2.6 GPa for PA6). Glass transition temperature in the dry state is 56–70 °C. The density of PA66 is 1.14 g/ml.

One key difference between the materials is crystallisation kinetics: PA66 always crystallises faster than PA6, and the morphological transition in PA66 occurs at a temperature 20 °C higher.

Moisture Absorption and Its Effects

Polyamides exhibit significant hygroscopicity. PA6 has high moisture absorption — up to 9–10% at saturation. PA66 absorbs slightly less moisture — about 8.5–9% at saturation. At 65% relative humidity and 21 °C, moisture content for both materials is 3.4–4.5%.

Moisture absorption significantly affects material properties. Wetting reduces the glass transition temperature: for PA6 it drops to –22…–32 °C, for PA66 to approximately –15 °C. Under high humidity conditions, PA6 demonstrates better impact toughness and flexural fatigue strength compared to PA66. However, the dimensional stability of PA6 parts suffers more: dimensional error in humid conditions is about 12% for PA6 versus 3% for PA66.

Thermal and Chemical Resistance

PA66 retains its properties over a wide temperature range from –30 °C to +260 °C. PA6 has a lower upper operating temperature limit of 80–105 °C. The thermal decomposition temperature of PA66 is approximately 340 °C, higher than PA6 (>300 °C).

Both polymers exhibit excellent resistance to oils, greases, hydrocarbons and many solvents. PA6 is resistant to dilute acids and alkalis. PA66 is resistant to petroleum products, fuels and many types of oils. Both materials are not resistant to concentrated mineral acids.

Electrical Properties

PA6 has an electrical strength of 20–45 kV/mm, making it suitable for electrical applications. PA66 is also used for insulators and connectors due to its dielectric strength and resistance to high temperatures.

Modification of Polyamides

To improve properties, polyamides undergo various types of modification:

• Glass fibre reinforcement — increases stiffness, strength and heat resistance. The heat deflection temperature of pure PA66 is about 65 °C, but after glass fibre modification it can reach over 200 °C. The tensile strength of glass-filled PA66 can exceed 180 MPa.

• Impact modification — improves toughness and impact resistance.

• Anti-friction additives — reduce the coefficient of friction and increase wear resistance.

• Stabilisers — improve thermal and light stability.

• Nanoclay filling — improves barrier properties and mechanical characteristics.

Applications

PA6 is widely used in the automotive industry (engine components, filter housings, fans, casings); electrical engineering (electrical connectors, insulation materials); mechanical engineering (plain bearings, moderately loaded gears, gearboxes, extruded semi-finished products — rods and sheets); and the textile industry (fibres, threads, fabrics, clothing, carpets, ropes and industrial belting).

PA66 is in demand for highly loaded gears; mechanical parts operating in hot environments; under-bonnet components; cable ties and technical fasteners; bearings, gears, cams and other mechanically loaded parts; insulators and connectors in electrical engineering; and consumer goods (kitchen utensils, sports equipment).

Processing

Both materials are easily processed by standard thermoplastic processing methods, including injection moulding, extrusion and melt forming. Thanks to their versatility and the ability to modify them with various additives, polyamides allow the creation of products with a wide range of properties.

Conclusion

Polyamides (nylon-6 and nylon-66) remain key materials in the automotive, electrical, mechanical engineering and textile industries due to their unique combination of strength, wear resistance, chemical resistance and processability. The choice between PA6 and PA66 is determined by specific requirements for thermal stability, impact toughness, moisture resistance and dimensional accuracy of the product.