Patent ID: 7759450

Claim:
A method for producing the polytrimethylene terephthalate resin, which comprises: (1) providing a crude trimethylene terephthalate resin in a molten form, said crude trimethylene terephthalate resin comprising: 60 to 100 mole % of (a) trimethylene terephthalate recurring units, and 0 to 40 mole % of (b) at least one monomer unit selected from the group consisting of monomer units obtained from comonomers which are other than the monomers used for forming said trimethylene terephthalate recurring units and which are copolymerizable with at least one of the monomers used for forming said trimethylene terephthalate recurring units, the total molar amount of (a) monomer units and (b) monomer units being 100 mole %, said crude trimethylene terephthalate resin further comprising a cyclic dimer represented by the following formula (1): said crude trimethylene terephthalate resin having an intrinsic viscosity [η] of from 0.4 to 1.5 dl/g and a cyclic dimer formation index (E) of less than 0.066, said cyclic dimer formation index (E) being defined by the following formula (3): E=W/M (3) wherein M represents the terminal hydroxyl group content of said crude trimethylene terephthalate resin in terms of mole % based on the total molar amount of the trimethylene terephthalate unit, and W represents the re-formation rate of the cyclic dimer in terms of an increase (as expressed by absolute percentage value) in the cyclic dimer content (% by weight), per minute, of the crude trimethylene terephthalate resin as measured at 260° C. in nitrogen gas atmosphere with respect to a sample of the crude polytrimethylene terephthalate resin in a molten form, wherein the molten sample is obtained by melting a cyclic dimer-reduced sample of the crude polytrimethylene terephthalate resin which cyclic dimer-reduced sample has a cyclic dimer content reduced to 0.1% by weight or less; and (2) removing, from said crude polytrimethylene terephthalate resin in a molten form, 0.5% by weight or more, based on the weight of said crude polytrimethylene terephthalate resin, of said cyclic dimer, by volatilization under reduced pressure, wherein said crude trimethylene terephthalate resin is produced by a polycondensation reaction performed in the presence of a catalyst selected from the group consisting of: tin butyrate and tin 2-ethylhexanoate, and wherein the removal of said cyclic dimer is performed by at least one method selected from the group consisting of a method using a guide-wetting fall polymerizer and a method using a thin film evaporator.