Process for preparing polyester

Abstract

The invention provides a process for preparing a polyester comprising a liquid phase polycondensation step in which a dicarboxylic acid including terephthalic acid or its ester derivative and a diol including ethylene glycol or its ester derivative are subjected to polycondensation reaction in liquid phase and molten state under heating in the presence of a polycondensation catalyst to produce a polyester (a) having an intrinsic viscosity, as measured in o-chlorophenol, of 0.7 to 1.5 dl/g; a hot water treatment step in which the polyester (a) is contacted with hot water of 60.degree. to 130.degree. C. for 30 minutes to 10 hours; a drying step in which the polyester after the hot water treatment step is dried at a temperature of 110.degree. to 150.degree. C. for 30 minutes to 6 hours; and a heat treatment step in which the polyester after the drying treatment step is heated at a temperature of 160.degree. to 200.degree. C. for 1 to 10 hours; wherein the finally obtained polyester has an acetaldehyde content of not more than 10 ppm. The polyester (a) obtained in the step can be precrystallized in a precrystallization step by keeping it in an atmosphere of a heat-up crystallization temperature (T.sub.c1) to 200.degree. C. for 1 minute to 4 hours, prior to the drying step and the heat treatment step. Further, when the precrystallization step is carried out, the polyester obtained in the step can also be subjected to the heat treatment step and then the hot water treatment step. The process of the invention has advantages that a solid phase polycondensation step can be omitted, and the process can provide a polyester having a low acetaldehyde content as well as reduced tendency to increase acetaldehyde during the molding process. Thus the polyester obtained by the process of the invention is particularly suited as a material for preform and beverage bottles formed from the preform. These bottles do not alter the tastes of the contents therein.