Process for preparing polybutylene terephthalate from pet scrap

Process for producing polybutylene terephthalate from polyethylene terephthalate scrap, said polybutylene terephthalate having less than 1.0 wt % units of ethylene glycol and an intrinsic viscosity of at least 0.90 dl/g, determined on a solution 0.5 g polymer in 100 ml of a mixture of phenol and 1,2-dichlorobenzene (3:2 parts by weight) at 25.degree. C. characterized in that

• a) shredded polyethylene terephthalate scrap (PET) is subjected to depolymerization with 1,4-butanediol (1,4-BD) in a ratio of 0.2 to 1.0 mol 1,4-BD per mol PET in the presence of a catalyst corresponding to 10 to 100 ppm titanium or tin, based on PET, at a temperature in the range of 200.degree. to 250.degree. C. at a pressure of at least 1.0 bar,
• b) the resulting mixture from step a) is subjected to ester exchange with a continuous feed of additional 1,4-butanediol in a ratio of 1.0 to 3.0 mol 1,4-BD per mol PET and additional catalyst corresponding to 50 to 250 ppm titanium or tin, based on PET, at a temperature in the range of 190.degree. to 250.degree. C., a pressure in the range of 0.1 to 1.0 bar and a residence time in the range of 90 to 180 minutes while at the same time distilling off ethylene glycol and tetrahydrofuran as formed,
• c) the resulting ester exchange mixture from step b) is precondensed with a continuous reduction in pressure to a value in the range of 100 to 10 mbar while at the same time raising the temperature to the range of 230.degree. to 260.degree. C. with a residence time in the range of 45 to 90 minutes while at the same time distilling off excess 1,4-butanediol and reaction water,
• d) the precondensation mixture from step c) is polycondensed after adding additional catalysts corresponding to 10 to 100 ppm titanium or tin, based on PET, at a temperature in the range of 230.degree. to 260.degree. C. and a maximum pressure of 1.5 mbar.