Patent ID: 7550552

Claim:
A continuous process for producing polybutylene terephthalate, comprising: continuously subjecting terephthalic acid and 1,4-butanediol to an esterification reaction in the presence of a titanium compound catalyst using one-stage or plural-stage esterification reaction vessels, and then subjecting the resultant reaction product to a polycondensation reaction, said process satisfying the following requirements (A) to (C): (A) a pressure (P) in at least one of said esterification reaction vessels being in the range of 20 to 90 kPa; (B) a concentration of the titanium compound catalyst used in the esterification reaction being not more than 90 ppm, calculated as a concentration (α) by weight of titanium atom, based on the weight of the polybutylene terephthalate finally produced; and (C) the following formulae (I) and (II) being satisfied: β= BM/TM− 0.03×α+0.045 ×P (I) β=4.0 to 7.0 (II) wherein TM is the number of moles of terephthalic acid supplied to the esterification reaction vessels per unit time; BM is the number of moles of whole 1,4-butanediol supplied continuously to the esterification reaction vessels per unit time; α is a concentration (unit: ppm) of the titanium catalyst used in the esterification reaction (calculated a concentration by weight of titanium atom based on the finally produced PBT); and P is a pressure (unit: kPa) in at least one of the esterification reaction vessels, and wherein BM/TM represented by formula (I) is not less than 2.1.