Abstract: A polyester fiber having excellent thermal dimensional stability and chemical stability as well as high tenacity, which is a drawn yarn produced by melt-spinning a polyester comprising predominantly polyethylene terephthalate, solidification the spun yarn and then drawing the yarn under specified conditions and has specified properties such as intrinsic viscosity, diethylene glycol content, carboxyl group content, birefringence, yarn tenacity and difference of average birefringence between surface and center of monofilament, and further low dry heat shrink and work loss when heat-treated, said polyester fiber being useful as a reinforcement for rubber goods, specifically as a tire cord, and processes for the production of the polyester fiber.

Abstract: A high strength polyester fibrous product reduced in heat build-up and improved in durability and useful for reinforcing rubber materials, which can be prepared by twisting drawn yarns obtained by (1) melt spinning a polymeric composition comprising polyethylene terephthalate as the main polymer component and at least one methylene group-containing polyester in such an amount that the proportion of the total number of methylene groups to the total number of terephthalic acid residues in all the polymer components is more than 2.0 and less than 2.