Polyamides and their special features
Among the so-called engineering plastics polyamides (PA) are certainly the best known and most widely used plastics. At the same time, they are among the oldest thermoplastics. In the 1930s, nylon (PA6.6) was developed independently in the USA and Perlon (PA6) in Germany, initially as a synthetic fibre and later as a raw material for injection moulding and extrusion.
The number after the abbreviation used to designate polyiamides describes the number of carbon atoms in the basic molecular units or the position of the CO group in the carbon chain. It should be noted that polyamides can be polymerised either from a single basic unit, the amino acid (e.g. PA 6, PA 12), or from two units, the diamines and dicarboxylic acids (e.g. PA 6.6, PA 4.6, PA 6.10, PA 6.12). A third variant is the aromatic polyamides, which are not usually labelled with a number after the abbreviation.
PA 6, PA 12 and PA 6.6 as well as the more temperature-resistant PA 4.6, which can be modified with various fillers and reinforcing materials, are particularly important for technical applications. There are also other PA homo- and co-polymers (PA 11, PA 6.9, PA 6.10, PA 6.12, PA 6.6/6, PA 6/12, etc.), including transparent amorphous types, which are of little technical relevance.
The so-called amide groups, a combination of carbon and oxygen atoms with nitrogen and hydrogen atoms (-NH-CO-), are characteristic of all PAs and also give them their name. The amide groups form strong bridges to neighbouring molecular chains and are therefore responsible for the mechanical and thermal properties of polyamides. The lower the proportion of amide groups in relation to the second important building block, the methylene groups (-CH2-), the poorer the mechanical properties, but also the lower the water absorption (see e.g. PA 12).
PA are predominantly semi-crystalline thermoplastics whose degree of crystallisation can be strongly influenced by the cooling rate of the melt. The higher the degree of crystallisation, the higher the strength, hardness and wear resistance and the lower the moisture absorption. In principle, however, the property profile of PA can be described as balanced. PAs have good strength and hardness, are tough and wear-resistant at the same time. They offer good damping properties and resistance to oils, greases and fuels. A disadvantage is the relatively high water absorption, which causes a change in volume and at the same time significantly influences the physical properties.
PA 6 and PA 12 have the special feature that they can also be moulded. For this purpose, the caprolactam (PA 6) or lauric lactam (PA 12) base units are polymerised into molten PA 6 or PA 12, which can be cast without pressure with the aid of catalysts and quickly polymerises in a suitable mould. In this way, semi-finished products or components of simple geometry can be produced, even in sizes that are unusual for plastics. The advantages of the so-called cast polyamides (PA 6 G or PA 12 G) are the higher molecular weight, which further improves rigidity and toughness, and the lower stress content.