Abstract: A potentially elastic composite filament, preferably a core-sheath composite filament composed of a polyurethane as the core and a polyester as the sheath, wherein the ratio of a polyurethane component having allophanate cross-linkages to a linear polyester component readily soluble in water or an aqueous alkali solution ranges from 1/1 to 90/1 and the polyester component is exposed on the surface of the filament in its cross-section. Although the core component has a high tensile strength and a large elongation at break, these properties are controlled by the sheath component. Therefore, this filament can be formed into a textile structure with good workability similar to that of ordinary synthetic fibers, and the obtained structure develops characteristics as an elastic polyurethane filament when treated with water or alkali.

Abstract: There is disclosed a copolyester which comprises at least four compositions of: (A) terephthalic acid, and/or its ester forming derivative (terephthalic acid component) and isophthalic acid and/or its ester forming derivative (isophthalic acid component) with a molar ratio of 30/70 to 70/30; (b) 5 to 15 mole % of a dicarboxylic acid having sulfonate group and/or its ester forming derivative based on the all dicarboxylic acid components; (C) an alicyclic dicarboxylic acid and/or its ester forming derivative; and (D) 50 mole % of ethylene glycol based on the all glycol components; and a polyester film which comprises a subbing layer containing the copolyester provided on at least one side of a polyester film base.

Abstract: A process for producing a linear polyester comprising a main structural unit of an aromatic dicarboxylic acid ester of an alkylene glycol, particularly, comprising a polyethylene terephthalate or polytetramethylene terephthalate unit, is provided. In the process of the invention, the polycondensation reaction is conducted in the presence of a niobic acid catalyst that is obtained by heat-treating a hydrated niobium compound at the temperature range between 80.degree. C. and 400.degree. C. The process of the invention is also effecitvely applied to the manufacture of an elastomeric polyester comprising a polyalkylene terephthalate as a hard segment and a polyalkylene glycol as a soft segment.