What are polyamides?
Polyamide, any polymer (substance composed of long, multiple-unit molecules) in which the repeating units in the molecular chain are linked together by amide groups. Amide groups have the general chemical formula CO-NH. They may be produced by the interaction of an amine (NH2) group and a carboxyl (CO2H) group, or they may be formed by the polymerization of amino acids or amino-acid derivatives (whose molecules contain both amino and carboxyl groups). Broadly defined, the polyamides include proteins and peptides, which are naturally produced polymers consisting of amino-acid repeating units.
The best known manufactured polyamides are often called nylons (the trade name given by the manufacturer, DuPont) and these are aliphatic polyamides. However, other manufactured polyamides are also important and these include an aromatic polyamide, Kevlar and plastics produced from carbamide (urea). The nomenclature for describing the linear, aliphatic polyamides (the nylons) is based on the number of carbon atoms in the repeating unit.
Uses of polyamides
The properties of the polyamides (nylons), which include high strength, abrasion resistance, and resilience, make them very important in the manufacture of clothing and carpets. Although these polyamides account for 95% of the material used in women’s hosiery, this still only accounts for about 5% of the total fibres used to make clothing. Nevertheless this is more than either the polypropenoates (acrylics) or wool but it is substantially less than either cotton or polyesters. The polyamides (nylons) are also used in engineering plastics, for example, in cars, and for making films for food packaging. They are used in films for their good balance between mechanical strength and barrier properties against oxygen, smells and oils.
For use as an engineering plastic, polyamides are often compounded with fillers, pigments, glass fibre and toughening agents to give specific properties to the polymer. However, for either continuous filament or staple fibres, which are melt spun at very high speeds (ca 6 km every minute), there is great emphasis on controlling the polymer chemistry and the way the yarn is produced in order to ensure the production of the high quality material needed for particular purposes. For example, the thread for use in stockings needs to be strong, as well as very fine, so the molecular mass and hence tensile properties of the polymer must be carefully controlled.