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Material hierarchy: Polymer - Engineering Thermoplastic
An important polymer family that forms characteristic amide linkages (CO-NH) during polymerization is the polyamides (PA). The most important members of the PA family are nylons, of which the two principal grades are nylon-6 and nylon 6,6 (the numbers are codes that indicate the number of carbon atoms in the monomer).
A second group of polyamides is the aramids (aromatic polyamides) of which Kevlar (DuPont trade name) is gaining in importance as a fiber in reinforced plastics. The reason for the interest in Kevlar is that its strength is the same as steel at 20% of the weight.