Disclosed are polyester multi-filamentary yarn useful as a reinforcement for tires and a dipped cord formed therefrom. The polyester multi-filamentary yarn comprises at least 90 mol % of polyethylene terephthalate and has an intrinsic viscosity of 0.70.about.1.2 and a tenacity of 5.5.about.8.5 g/d with an intermediate elongation difference (E1-E0) between intermediate elongations E0 and E1 amounting to 6% or greater. The polyester dipped cord is produced by subjecting at least two strands of polyester multi-filamentary yarn to first twisting and second twisting, the polyester multi-filamentary yarn comprising 90 mol % of polyethylene terephthalate; forming the strands into a fabric; and treating the fabric with blocked isocyanate and resorcinol formaldehyde latex (RFL), wherein the cord satisfies the following characteristics: a) a tenacity of 5.0 g/d or greater, b) a dimensional stability index (E.sub.4.5 +SR) of less than 7.0%, c) a breaking elongation of 9% or greater, and d) an intermediate elongation difference (E1-E0) of 3% or less. Showing a harmony of high elastic modulus and low shrinkage, the filamentary yarn and dipped cords are superior in dimensional stability and fatigue resistance, so they can be used as reinforcements for rubber composites such as tires.
A polyester fiber containing polyethylene terephthalate at 90 mol % or higher of a whole repeating unit in a molecular chain thereof, the fiber having an intrinsic viscosity (IV) of 0.85 dl/g or higher and simultaneously meeting the following characteristics: (a) strength.gtoreq.6.0 cN/dtex; (b) strength.times.(breaking elongation).sup.0.5.ltoreq.26.0 cN/dtex. %.sup.0.5; (c) monofilament linear density.ltoreq.5.0 dtex; and (d) main dispersion peak temperature of loss tangent (tan .delta.) in the measurement of dynamic viscoelasticity at 110 Hz.ltoreq.147.0.degree. C., and a polyester dipped cord, which is obtainable by twisting one or more than one base yarn together into a pretwisted yarn, where the base yarn is made of a polyester fiber twisting two or more pretwisted yarns together into a greige cord; and subjecting the greige cord to dip treatment to give a dipped cord simultaneously meeting the following characteristics: (a) tenacity conversion efficiency in the dip treatment (dipped cord tenacity/greige cord tenacity).gtoreq.96%; and (b) elongation at a specific load+dry heat shrinkage.ltoreq.7.5%.
