Polyester means many organic salts. This fiber is a man-made one and can be found as stable or filament fiber. Polyester main polymer is polyethylene terephthalate. This fiber has medium weight with density of 1.39 g/cm3, both filament and stable polyester are fine, regular, translucent, crimped or textured. The polyester fiber diameter ranges from 12 µm to 25 µm, their length as a filament limited by the size of yarn package on to which it is wound and as a stable fiber, it depends on the machinery used for spinning. Length to breadth ratio of polyester usually exceeds 2000:1that ensures even stable fibers can be satisfactorily spun into yarn. The microscopic appearance of polyester is very regular and featureless and its cross-section is like nylon fiber.
Polyester filaments and stable fibers are strong to very strong due to their extremely crystalline polymer system resulting in the very good tenacity. Their tenacity remains unaltered when wet because of the completely hydrophobic and extremely crystalline polyester polymer system that resists the entry of water molecules to any significant extent. This crystalline system prevents the polyester from yielding readily when the filament of stable fiber is bent or flexed. This also explains the wrinkle-resistance of polyester textile materials.