| 摘要 |
PROBLEM TO BE SOLVED: To produce a copolyester which is excellent in gas barrier properties, resistances to heat and impact, color tone, clarity, etc., and has a low cyclic oligomer content by adding specified amts. of isophthalic acid and ethylene glycol to a polyethylene terephthalate oligomer and subjecting the resultant mixture to esterification and then to polycondensation. SOLUTION: Isophthalic acid in a molar ratio of isophthalic acid component to terephthalic acid component of (95/5)-(15/85), ethylene glycol, and phosphoric acid or its ester in an amt. of 1×10<-4> -200×10<-4> mol per mol of the acid component are added to a polyethylene terephthalate oligomer having a number average mol.wt. of 2,000 or lower. The resultant mixture is subjected to esterification for at least 0.5 hr and then to polycondensation in the presence of a polycondensation catalyst under such conditions that the temp. X ( deg.C) and degree of vacuum Y (hPa) in the polycondensation satisfy the relation expressed by formulas I and II, until the number average mol.wt. reaches 10,000 or higher.
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