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Polyamides and their special features

Among the so-called technical or engineering plastics, polyamides (PA) are certainly the best-known and most widely used plastics. At the same time, they are among the oldest thermoplastics. During the 30 years of the 20th century, Nylon (PA6.6) was developed in the USA and (indepentdently) Perlon (PA6) in Germany, initially as a synthetic fiber, later also as a raw material for injection molding and extrusion.

The number after the abbreviation PA, which is customary for the designation of polyamides, describes the number of carbon atoms of the basic molecule units or the position of the CO group in the carbon chain. It should be noted that polyamides can be polymerized either from a single basic unit, the amino acid (e.g. PA 6, PA 12) or of two units, the diamines and dicarboxylic acids (e.g. PA 6.6, PA 4.6, PA 6.10, PA 6.12). A third variant are the aromatic polyamides, which are usually not combined with a number after the abbreviation.

Of particular importance for technical applications are PA 6, PA 12 and PA 6.6 and the more temperature-resistant PA 4.6, which can be modified with different fillers and reinforcing materials. In addition, there are 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, however, have little relevance for technial applications.

Characteristic and at the same time eponymous for all PA are the so-called amide groups, a combination of carbon- and oxygen- with nitrogen- and hydrogen-atoms (-NH-CO-). The amide groups form strong bridges to neighboring molecular chains and are thus responsible for the mechanical and thermal properties of the polyamides. The lower the proportion of amide groups in relation to the second important building block, the methylene groups (-CH2-), the worse are the mechanical properties, but the lower is the water absorption (see e.g. PA 12).

PAs are mostly semi-crystalline thermoplastics whose degree of crystallization can be strongly influenced by the cooling rate of the melt. With increasing degree of crystallization strength, hardness and wear resistance increase, the the moisture absorption decreases. In principle, however, the property profile of PA can be described as balanced. PA have good strength and hardness, at the same time they are tough and wear resistant. PA provide good damping properties and resistance to oils, greases and fuels. A disadvantage is the relatively large water absorption which causes change in volume and at the same time significantly influences the physical properties.

PA 6 and PA 12 offer the special feature that they can also be used in casting processes. For this purpose, the caprolactam (PA 6) or lauric lactam (PA 12) bases are polymerized to molten PA 6 or PA 12 which - with the aid of catalysts - can be pressureless cast and quickly polymerizes in a suitable mold. In this way, semi-finished products or components with a simple geometry can also be manufactured in sizes that are unusual for plastics. Advantages of the so-called "cast polyamides" (PA 6 G or PA 12 G) are the higher molecular weight which further improves stiffness and toughness, and the lower stress content.

If you would like more information, just click here on the desired material group polyamide 6, polyamide 6.6, polyamide 4.6 or polyamide 12.

Click here for an overview of the polyamides!