Continuous polycondensation based on dimethyl terephthalic acid DMT

1 Continuous polycondensation based on dimethyl terephthalic acid (DMT) 

The initial reaction of the DMT-based process exchanges a methyl ester group of the DMT for 

When an adequate yield of this intermediate has been produced, excess EG is removed by distillation at atmospheric pressure and at a temperature of between 235 and 250 °C. A phosphoms compound, e.g. polyphosphoric acid is added at this stage as a processing stabiliser. Prior to further polymerisation, the manganese catalyst has to be deactivated. Otherwise, the manganese will not only continue to catalyse the polymerisation of PET, but also cause the formation of undesirable side reactions which will increase the yellowness of the final PET and result in poor thermal stability. 

The polymer is produced by polycondensation (PC) reactions in which excess EG is removed. The temperature is raised to 285 - 300 °C at a vacuum of 1 - 2 torr of absolute pressure. Typically, antimony in the form of its trioxide, triglycolate, triacetate or other compounds (also without antimony) is added to catalyse the polycondensation reaction. Excess EG is removed by vacuum resulting in an increase of the molecular weight. 

The molecular weight of the polymer is usually measured as solution viscosity (SV) or intrinsic viscosity (IV). The IV of a typical amorphous polymer is 0.64 (equivalent to an SV of 835). The melted polymer is extruded, cooled, and cut into chips, which are transported to silos before further processing. 

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