MODIFIED POLYESTER AND ITS FIBER

PURPOSE:To produce a modified polyester which can give a fiber excelling in dyeability with a cationic dye or a disperse dye, having a not excessively high rate of alkali weight loss and being capable of an alkali weight loss processing for improving the hand by copolymerizing a poly(ethylene terephthalate) with a specified amount of a specified sulfonate-containing compound. CONSTITUTION:A poly(ethylene terephthalate) is copolymerized with 0.1-10mol%, based on the total glycol component, sulfonate-containing compound represented by the formula (wherein R1 is ethylene or isopropylene; X is C(CH3)2; Y and Z are each SO3M or H; M is a metallic atom or the like; and (m) and (n) are each 1-60) to obtain a modified polyester. As the catalyst for the transesterification step, 0.01-0.1mol% calcium compound or the like is used, and as the catalyst for the polycondensation step, 0.003-0.1mol antimony compound or the like is used. Additives such as stabilizers, lustering agents, colorants, etc., may be added, if necessary. When a fiber is produced from the modified polyester, arbitrary spinning conditions can be adopted.