Polyester sewing thread, coloured scanning electron micrograph (SEM). Polyester is a synthetic man-made polymer from the type commonly referred to as polythene terephthalate. It is comprised of a mixture of ethylene glycol and terephthalic acid. Unlike cotton, its individual strands are uniform in shape. Polyester is useful when used in sewing clothes as it has a certain amount of stretch to it. Magnification: x60 when printed at 10cm wide