PROCESS FOR PURIFYING AN ALIPHATIC-AROMATIC POLYESTER

Abstract

The present invention relates to a continuous process for purifying an aliphatic-aromatic polyester constructed from aliphatic dicarboxylic acids, aromatic dicarboxylic acids and aliphatic diols in a degassing apparatus, wherein the crude polyester is degassed for 3 to 30 minutes at a pressure of 0.01 to 5 mbar in the presence of 1% to 7% by weight, based on the total weight of the crude polyester, of an entraining agent.

Claims

1.-11. (canceled)

12. A continuous process for purifying polybutylene adipate coterephthalate in a degassing apparatus, wherein the crude polyester is degassed with an average residence time of 3 to 30 minutes and at a pressure of 0.01 to 5 mbar in the presence of 1% to 7% by weight, based on the total weight of the crude polyester, of an entraining agent.

13. The process according to claim 12, wherein the entraining agent is introduced into the gas space of the degassing apparatus.

14. The process according to claim 12, wherein the entraining agent is selected from the group consisting of: additionally introduced water, ethanol, nitrogen, carbon dioxide, acetone and cyclic propylene carbonate.

15. The process according to claim 12, wherein the degassing apparatus is a thin film evaporator.

16. The process according to claim 12, wherein the degassing apparatus has an internal temperature of 180° C. to 260° C.

17. The process according to claim 12, wherein the crude polyester in the degassing apparatus has a film thickness of less than 2 mm.

18. The process according to claim 12, wherein the crude polyester has an MVR according to DIN EN 1133-1 of 01.03.2012 (190° C., 2.16 kg) of 0.1 to 50 cm.sup.3/10 min.

19. The process according to claim 12, wherein the purification process succeeds the polycondensation stage and the chain extension of the crude aliphatic-aromatic polyester and precedes the isolation of the final pure polyester.

20. Polybutylene adipate coterephthalate obtained by a process according to claim 12.

21. Polybutylene adipate coterephthalate wherein the content by weight of the three cyclic ester dimers is determined by LC-HRMS and from 500 to 1000 ppm, based on the total weight of the polybutylene adipate coterephthalate.

22. Polybutylene adipate coterephthalate wherein the ratio of the three cyclic ester dimers vs. the four cyclic ester trimers is determined by LC-HRMS and lower than 1, based on the total weight of the polybutylene adipate coterephthalate.

Description

PERFORMING THE DEGASSING IN THE MELT

Example 1

[0126] The polyester granulate of polyester i-1 was continuously melted with the aid of a kneader, the melting temperature at the outlet of the kneader being about 200° C.-220° C. The necessary melting energy was supplied to the material via the heater bands of the cylinder and via shearing (rotational speed of the kneader). Alternatively, it was also possible for the polyester to be melted using a single-screw or twin-screw extruder in order to be then supplied from above (laterally) to the vertical thin film evaporator via a heated pipe using a melt pump. In the experiments, the throughput was between 15 and 40 kg/h.

[0127] In the thin film evaporator (type Filmtruder), the melt was drawn out to form a thin film having a thickness of about 1 mm with the aid of a rotor and the appropriate blade geometries and transported downward. The outer shell of the evaporator was heated (200-280° C.) and the residence time in the thin film evaporator was determined/varied via the rotor speed.

[0128] At the same time, it was possible to supply an entraining agent from below in countercurrent and, for better degassing, a vacuum was applied. Once the melt had arrived in the lower part of the thin film evaporator, it was continuously discharged with the aid of a melt pump and drawn out to form strands via a die plate. These were cooled in a water bath and supplied to a granulator in order to then obtain granulate again. As an alternative to strand granulation, it was also possible to use underwater granulation after the melt discharge pump.

[0129] The oligomers were removed under vacuum in the upper region of the thin film evaporator and subsequently condensed.

TABLE-US-00001 TABLE 1 Polyester Example Unit i-1 V1 1 Temperature (kneader) ° C. 200 200 Temperature (thin film ° C. 222 229 evaporator) Temperature (steam) ° C. 100 Throughput kg/h 20 20 Average residence time min 15 15 Film thickness mm 1.3 1.3 Vacuum mbar 0.5 3.1 Stripping medium — H.sub.2O (steam) Amount — 2% by weight Speed of rotation rpm 221 293 Cyclic dimer proportions ppm 1431 1350 897 (total) Cyclic trimer proportions ppm 1342 1346 1214 (total) Cyclic 2AA 2BDO* ppm 784 750 454 Cyclic AA TPA 2BDO** ppm 519 475 335 Cyclic 2TPA 2BDO*** ppm 128 125 108 Cyclic 3AA 3BDO.sup.# ppm 262 258 207 Cyclic 2AA TPA 3BDO.sup.## ppm 592 591 527 Cyclic AA 2TPA 3BDO.sup.### ppm 363 367 349 Cyclic 3TPA 3BDO.sup.#### ppm 125 130 131 Cyclic oligomers Total ppm 4150 3930 3440 THF Start ppm — 350 350 End ppm — 180 70 *Cyclic 2AA 2BDO is the cyclic ester dimer containing two adipic acid units and two 1,4-butanediol units **Cyclic AA TPA 2BDO is the cyclic ester dimer containing adipic acid, terephthalic acid and two 1,4-butanediol units ***Cyclic 2TPA 2BDO is the cyclic ester dimer containing two terephthalic acid units and two 1,4-butanediol units
#Cyclic 3AA 3BDO is the cyclic ester dimer containing three adipic acid units and three 1,4-butanediol units
##Cyclic 2AA TPA 3BDO is the cyclic ester dimer containing 2 adipic acid units, terephthalic acid and three 1,4-butanediol units
###Cyclic AA 2TPA 3BDO is the cyclic ester dimer containing adipic acid, two terephthalic acid units and three 1,4-butanediol units
####Cyclic 3TPA 3BDO is the cyclic ester dimer containing three terephthalic acid units and three 1,4-butanediol units

Example 2

[0130] Example 2 was carried out similar to example 1 but polyester i-3 was used instead of polyester i-1.

[0131] The results are collected in table 2.

TABLE-US-00002 TABLE 2 Polyester Example i-3 2 Stripping medium H.sub.2O (steam) Amount 2% by weight Speed of rotation rpm 293 Cyclic monomer and di- ppm 1562 865 mer proportions (total) Cyclic trimer proportions ppm 853 789 (total) Cyclic SeA BDO ppm 414 114 Cyclic 2SeA 2BDO ppm 397 284 Cyclic SeA TPA 2BDO ppm 628 366 Cyclic 2TPA 2BDO ppm 123 101 Cyclic 3SeA 3BDO.sup.# ppm 126 109 Cyclic 2SeA TPA 3BDO.sup.## ppm 374 332 Cyclic SeA 2TPA 3BDO.sup.### ppm 217 211 Cyclic 3TPA 3BDO.sup.#### ppm 136 137 Cyclic oligomers Total ppm 4570 3820 THF Start ppm — 380 End ppm — 90

Example 3

[0132] The degassing was performed in a thin film evaporator (Sambay). The thin film evaporator had the following general characteristics: The evaporator surface was a tube having a mechanical stirring means in the middle. The melt was passed on to the vertical evaporator surface from above. The thin film evaporator generated a thin film (melt film) on the inner wall of a heated outer shell by mechanical means using a rotor. This resulted in continuous surface replacement, thus ensuring good mass transfer and therefore a high degassing performance. The degassing performance was achieved by addition of a stripping agent (for example water or steam) which was supplied in countercurrent. Also required for good degassing performance was a vacuum of not more than 5 mbar.

[0133] The shape and configuration of the individual rotor blades allowed transport of the viscous product to the discharge section of the processor. The film thickness and the melt conveying was likewise dependent on the geometry of the rotor blades. The large free gas volume allowed a high evaporative concentration ratio in one stage without the risk of product entrainment into the condensation system.

[0134] Example 3 delivered similar results than Example 1: the cyclic ester dimers have been reduced by 32%. The employed Sambay evaporator as described hereinabove, the outer shell being made of glass rather than metal.

[0135] The polyester i-2 was filled into the apparatus as a melt from above and to the side of the vertical glass surface/evaporator surface and melted. After melting the polyester was introduced into the apparatus and spread into a thin film by a stirring means. The stripping agent was introduced in countercurrent from below and a vacuum was then applied. Using the temperature, vacuum and stripping medium reported in table 1 the cyclic oligomers and further byproducts such as for example THF were withdrawn overhead and condensed in the cooler. After the experiment the polyester and the discharged substances were analyzed by LC-HRMS.