Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-12-19
2003-12-02
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S308600, C528S481000, C528S503000, C526S065000, C526S071000
Reexamination Certificate
active
06657040
ABSTRACT:
The present invention relates to a process for the continuous preparation of polybutylene terephthalate (PBT) from terephthalic acid (TPA) and 1,4-butanediol (BDO).
The preparation of polybutylene terephthalate from dimethyl terephthalate (DMT) and 1,4-butanediol is known from the prior art. A disadvantage of this process is that tetrahydroftiran (THF) formed in small amounts as by-product forms an azeotrope with the methanol liberated during the reaction and therefore can be recovered as a material of value only with great difficulty.
The direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol is made difficult by the formation of relatively large amounts of THF, resulting in loss of 1,4-butanediol required for the reaction. Furthermore, not only THF but also 2,5-dihydrofuran (2,5-DHF) is formed from 1,4-butanediol. The 2,5-dihydrofuran is difficult to separate from THF and therefore contaminates and reduces the quality of the valuable product THF. A further problem in the direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol is that terephthalic acid is not soluble in 1,4-butanediol and goes into solution only during the esterification with 1,4-butanediol. However, to produce high quality polybutylene terephthalate, it is extremely important for this to be free of contaminants such as free acid groups from terephthalic acid. For this reason, the terephthalic acid should be completely esterified and dissolved before the actual polycondensation commences.
Processes concerned with the direct preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol are already known from the prior art. DD-A 269 296 relates to a continuous process for preparing polyalkylene terephthalates. Setting of appropriate reaction parameters in the esterification step of the dicarboxylic acid used with the glycol used is said to decisively favor removal of water from the esterification phase so that esterification products are obtained both with a high degree of conversion and a high mean degree of polymerization. The esterification step is carried out in a reactor cascade in which the temperature is increased and the pressure is reduced from reactor to reactor. The example described relates to the preparation of polyethylene terephthlate from terephthalic acid and ethylene glycol.
EP-A 0 431 977 describes a process for increasing the direct esterification rate of a diacid and 1,4-butanediol to esterification of >95% of the acid groups. The process can be carried out continuously in three reactors. The process described comprises:
a) mixing 1,4-butanediol and diacid in a ratio of at least 2:1,
b) heating the reaction mixture to 180° C.,
c) adding a suitable catalyst and
d) reacting the mixture at atmospheric pressure and a mean temperature in the range from 180 to 245° C. for a maximum of 60 minutes.
In this process, less than 5% of the 1,4-butanediol is said to be cyclized to THF. However, the content of free acid groups in the end product is high.
EP-A 0 046 670 relates to a process for preparing polybutylene terephthalate by direct esterification of terephthalic acid and 1,4-butanediol. The process comprises an esterification step at a temperature of up to 215° C. and atmospheric pressure. After consumption of most of the terephthalic acid, at which stage, however, from 10 to 40% by weight of terephthalic acid is still present, i.e. before the clearing point, the polycondensation stage is carried out at a temperature higher than that in the esterification stage.
DE-A 27 11 331 describes the preparation of polyester oligomers by means of a two-stage esterification. Here, the first esterification stage is carried out at atmospheric pressure and the second esterification stage is carried out at atmospheric pressure or subatmospheric pressure, at a temperature of about 250° C. in both stages. However, the examples describe only the reaction of terephthalic acid with ethylene glycol.
DE-A 35 44 551 relates to the continuous preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol. The preparation is carried out in three stages. The first stage, namely the esterification, is carried out at from 225 to 260° C. and a pressure of from 0.1 to 1 bar. The second stage, the precondensation, is carried out at from 230 to 260° C. and a pressure of from 10 to 200 mbar, and the third stage, the polycondensation, is carried out at from 240 to 265° C. and a pressure of from 0.25 to 25 mbar.
It is an object of the present invention to provide a process for preparing polybutylene terephthalate which is improved compared to the prior art. In particular, formation of THF from the 1,4-butanediol used should be as low as possible and the polybutylene terephthalate obtained should have a very low content of free acid groups.
The achievement of this object starts out from a process for the continuous preparation of polybutylene terephthalate from terephthalic acid and 1,4-butanediol, comprising:
a) direct esterification of terephthalic acid with 1,4-butanediol in a reactor cascade comprising at least two reactors,
b) precondensation of the esterification product obtained in stage a), and
c) polycondensation of the precondensate obtained in stage b).
In the process of the present invention, the temperature decreases along the reactor cascade in stage a).
The polybutylene terephthalate prepared according to the present invention is of excellent quality. It has a low content of acid and alcohol groups. The formation of THF and 2,5-DHF from 1,4-butanediol is low. As a result, only little 1,4-butanediol is lost, so that the yield of polybutylene terephthalate based on 1,4-butanediol is high.
a) Esterification Stage
The stage a) is carried out in a reactor cascade comprising at least two reactors, preferably from two to five reactors, particularly preferably three reactors. The reactors used are generally stirred vessels.
The temperature range for the overall esterification stage is generally from 170 to 250° C., preferably from 180 to 240° C., particularly preferably from 190 to 230° C. According to the present invention, the temperature decreases along the reactor cascade, i.e. the esterification temperature drops from reactor to reactor. The temperature in an esterification reactor is generally from 2 to 30° C. lower than that in the preceding reactor. The temperature preferably drops by from >5 to 30° C. from reactor to reactor. In a preferred embodiment, the reactor cascade comprises three reactors, and the temperature in reactor 1 (T1) is generally from 200 to 250° C., preferably from 210 to 240° C., particularly preferably from 218 to 230° C. The temperature in reactor 2 (T2) is generally from 190 to 230° C., preferably from 200 to 225° C., particularly preferably from 205 to 220° C., and the temperature in reactor 3 (T3) is generally from 170 to 220° C., preferably from 180 to 215° C., particularly preferably from 190 to 210° C., with the temperature dropping from reactor to reactor.
The residence times for the overall esterification stage are generally from 140 to 430 minutes, preferably from 160 to 420 minutes, particularly preferably from 170 to 390 minutes. In the case of a reactor cascade made up of three reactors, the residence time in the first reactor (V1) is generally from 100 to 250 minutes, preferably from 110 to 250 minutes, particularly preferably from 120 to 240 minutes, in the second reactor (V2) generally from 20 to 105 minutes, preferably from 30 to 100 minutes, particularly preferably from 30 to 90 minutes, and in the third reactor (V3) generally from 20 to 75 minutes, preferably from 20 to 70 minutes, particularly preferably from 20 to 60 minutes.
The esterification stage is generally carried out at a reaction pressure of not more than 1 bar. Preference is given to a pressure of <1 bar. The experimental parameters pressure (p) and temperature (T) in the respective reactor particularly preferably lie within a plane defined by
p
(lower limit)=0.348×(
T
/° C.)
2
&mi
Heitz Thomas
Klatt Martin
Neuhaus Ralf
Acquah Samuel A.
BASF - Aktiengesellschaft
Keil & Weinkauf
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