DD246772B1 - METHOD FOR PRODUCING POLYBUTYLENEPEPHTHALATE - Google Patents

Description

Field of application of the invention

The invention relates to a process for the preparation of polybutylene terephthalate based on dimethyl terephthalate and 1,4-butanediol.

Characteristic of the known technical solutions

Polybutylene terephthalate is currently produced primarily by transesterification of dimethyl terephthalate with a molar excess of 1,4-butanediol and polycondensation of the resulting transesterification product.

At all stages of the process, vapor mixtures are formed in which, above all, low molecular weight esters of terephthalic acid are present in addition to water, butane-1,4-diol, methanol and tetrahydrofuran.

The return of suitable for the chain construction of polybutylene terephthalate esters of terephthalic acid and 1,4-butanediol by attached columns in the polycondensation process, as for pre- and polycondensation processes for polyethylene terephthalate synthesis in DE-OS 3025574 in conjunction with an installed cyclone or spin separator is unsatisfactory especially for a discontinuous process with low material flow.

The process variant described in DD-PS 131753 for polyethylene terephthalate, according to which the oligomers contained in the resulting pre- and polycondensation condensate selectively deposited at 75-18O ⁰ C and recycled together with the distillatively worked up Dioldestiflat in the production process, is expensive in terms of apparatus and with additional Energy expenditure and Diolverlusten connected in the distillation.

According to DD-WP 210916, the dicarboxylic acid-containing components cleaved in the pre-polycondensation are recycled to the raw material batch or the reaction product in the esterification stage after their condensation without prior distillative workup, the water contained serving to improve the flowability of the terephthalic acid / ethylene glycol paste and the esterification is split off immediately. For the polybutylene terephthalate based on dimethyl terephthalate this variant is not applicable, since in particular the relatively high water content reduces the transesterification rate of dimethyl terephthalate with 1,4-butanediol and the catalytic activity of the titanium contact also required for the polycondensation by hydrolysis.

DD-WP 152332 describes a process for the trouble-free distillation of butane-1,4-diol-containing vapor mixtures to pure butane-1,4-diol for reuse for the synthesis of polybutylene terephthalate based on dimethyl terephthalate. The fully condensed vapors are treated with alkalis to convert the low molecular weight solid components contained therein, especially the dimethyl terephthalate, in combination, which do not evaporate in the subsequent multi-stage distillation. However, this method requires a considerable energy and equipment expense. A further disadvantage results from the deposition of the terephthalate components, which can no longer be attributed to the synthesis process.

Object of the invention

The aim of the invention is. Polybutylene terephthalate energy and material economics with good space-time yield, quality and a low environmental impact manufacture.

Explanation of the essence of the invention

The object of the invention is to prepare the Polykondensationsbrüdengemisch obtained in the Polybutylenterephthalatsynthese with the majority of the split-off 1,4-butanediol without total condensation and downstream distillation and due to the transesterification process.

According to the invention the object is achieved in that the obtained during the polycondensation vapor mixture is cooled to 30-40 ⁰ C and partially condensed at a pressure of 70-250Pa, wherein the components of water, methanol and tetrahydrofuran are largely distilled off. The vapor condensate thus obtained contains at least 96% by mass of 1,4-butanediol, up to 0.7% by mass of methanol, up to 0.7% by mass of tetrahydrofuran, up to 0.7% by mass of water and up to to 3.5% by weight of a mixture of low molecular weight hydroxybutyl terephthalates and dimethyl terephthalate. This mixture is the transesterification stage, in which usually a transesterification of dimethyl terephthalate with a molar excess of 1,4-butanediol pure occurs. In this case, up to 40 parts of the treated, partially condensed vapor mixture are added instead of pure 1,4-butanediol.

It has been shown that there are no disadvantages with respect to the quality of polybutylene terephthalate, the space-time yield and increased by-product formation. The advantages of this method result from the use of the heating energy required for the polycondensation process for the distillation of the vapor mixture. At the same time, the resulting hydroxybutyl terephthalate / dimethyl terephthalate mixture are completely recycled to the synthesis process and the 1,4-butanediol losses avoided, which occur in an external energy-intensive Brüdendestillation.

Embodiment Example 1

194 kg of dimethyl terephthalate, 117 kg of 1,4-butanediol and 70 g of tetraisopropyl titanate were reacted under normal pressure and 175-180 ° C. with stirring in a stainless steel reactor until 63 kg of methanol had been split off. The resulting transesterification product was heated to 250 ⁰ C and the same pressure is lowered to 150Pa. The vapors mixture of 1,4-butanediol, methanol, tetrahydrofuran, water and hydroxybutyl terephthalate oligomers and dimethyl terephthalate removed during the expiring polycondensation is cooled to 35 ° C. in a heat exchanger, partially condensed and collected in a template also charged with the reaction pressure , In the vapor phase, the major amounts of by-products remain on methanol, tetrahydrofuran and water. The partially condensed polycondensation mixture is composed as follows:

23.5 kg butanediol-1.4 0.002 kg methanol 0.002 kg tetrahydrofuran 0.011 kg water 0.524 kg Mixture of hydroxybutyl terephthalate and dimethyl terephthalate

This vapor condensate is reacted with 93.5 kg of 1,4-butanediol 1,4,194 kg of dimethyl terephthalate and 70 g of tetraisopropyl titanate as described above.

Despite this addition of 20 parts of the vapor mixture according to the invention instead of pure 1,4-butanediol, a polybutylene terephthalate is obtained, which does not differ in terms of quality and by-product formation using 1,4-butanediol. Furthermore, a good space-time yield is achieved.

Example 2

29.1 kg of dimethyl terephthalate, 21.5 kg of 1,4-butanediol and 10.5 g of tetraisopropyl titanate are reacted under normal pressure and 175-180 ⁰ C with stirring until 9,4kg of methanol were eliminated.

After raising the product temperature to 245-25O ⁰ C while lowering the pressure to 150Pa, the resulting transesterification product is polycondensed within 120 minutes. The separated off vapors are cooled in a heat exchanger to 35 ⁰ C and partially condensed. The vapor condensate is collected in an also charged with reaction pressure template. Essential parts of the low boilers (tetrahydrofuran, methanol, water) remain in the vapor phase. This 8kg of vapor of the following composition are condensed:

7.72 kg of 1,4-butanediol

0.032 kg of methanol

0.024 kg of tetrahydrofuran

0.024 kg of water

0.02 kg mixture of hydroxybutyl naphthalate and dimethyl terephthalate.

This vapor condensate is reacted with 12 kg of 1,4-butanediol dimethyl terephthalate and 10.5 g of tetraisopropyl titanate as described above.

The polybutylene terephthalate obtained with good space-time yield corresponds in terms of quality and by-product formation to those produced exclusively using butane-1,4-diol.

Claims (2)

Claim: 1. A process for the preparation of polybutyl terephthalate based on dimethyl terephthalate and 1,4-butanediol in a multi-stage process with the process steps transesterification and polycondensation with recycling of processed products in the transesterification process, characterized in that the polycondensation mixture cooled to 30-4O ⁰ C and at a pressure is partially condensed from 70 to 250 Pa and the resulting vapor condensate is used up to 40 parts instead of pure 1,4-butanediol. 2. The method according to claim 1, characterized in that the reused vapor condensate of at least 96% butane-1,4-diol, up to 0.7 wt .-% methanol, up to 0.7 wt .-% tetrahydrofuran, up to 0.7 % By weight of water and up to 3.5% by weight of a mixture of low molecular weight hydroxybutyl terephthalates and dimethyl terephthalate.