EP0000918B1 - Process for the preparation of linear, high molecular polyesters - Google Patents

Description

The invention relates to a process for the preparation of high molecular weight linear polyesters which are derived from dicarboxylic acids or their ester-forming derivatives and diols and which consist of at least 50 mol% of polyethylene or polybutylene terephthalate units, by condensation of polyester precondensates with a relative viscosity of at least 1.05 at temperatures from 270 to 340 ° C under reduced pressure.

In the manufacture of high molecular weight linear polyesters, low molecular precondensates with low viscosity are converted into high molecular weight condensates at temperatures of 260 to 300 ° C. under reduced pressure with elimination of diols. At the high temperatures required, however, polyester melts are unstable, which results in an increased carboxyl end group content. In the process known from DE-B 17 45 541, the polyester precondensate is passed through a horizontal device which is divided into chambers, the melt being formed into a thin film in each chamber. The method has the disadvantage that it takes considerable time, e.g. several hours, and the higher-condensed melt formed in the film is continuously returned to the bottom of the lower molecular weight. From DE-A 19 59 455 a method is also known in which the condensing melt is passed over a series of zones arranged one above the other, the condensing mass circulating in each zone and thereby coming into contact with the heating wall alternately and from zone to zone flows under gravity and film formation. The method has the disadvantage that dead spaces form within the individual zones, which lead to backmixing. Furthermore, the method has the disadvantage that the condensation time still takes too much time for particularly sensitive polyesters. The process known from FR-A 15 45 487, in which the condensing melt is passed over a plurality of rotating, inclined surfaces, still requires about 30 minutes for the condensation. It is noteworthy that in all processes the temperature is either kept at the same level or increased with increasing viscosity.

It was therefore the technical task to carry out the condensation in the production of high molecular weight linear polyesters in such a way that a minimum of time is required and the lowest possible carboxyl end group content is achieved even with sensitive polyesters.

This technical problem is solved in a process for the production of high molecular weight linear polyesters, which are derived from dicarboxylic acids or their ester-forming derivatives and diols and which consist of at least 50 mol% of polyethylene or polybutylene terephthalate units, by condensing polyester precondensates with a relative viscosity of at least 1.05, at temperatures of 270 to 340 ° C under reduced pressure, the temperature being lowered during the condensation, characterized in that the polyester precondensate is briefly heated to a temperature of 290 to 340 ° C and then by evaporating the released diol under a pressure of 0.133 to 2.66 mbar, the temperature lowered by 30 to 50 ° C with the proviso that the final temperature is at least 10 ° C above the melting point of the polyester produced.

The new process has the advantage that the condensation takes less time than before. Furthermore, the new process has the advantage that very low carboxyl end group contents are achieved even with sensitive polyesters, such as polybutylene terephthalate.

The new process is remarkable in that the condensation is carried out with a continuously decreasing temperature. With regard to DE-A 19 20 954 and FR-A 15 45 487, it was assumed that short residence times could only be achieved with increasing temperature.

The high molecular weight linear polyesters, like the polyester precondensates, are derived from dicarboxylic acids or their ester-forming derivatives, such as alkyl esters. Aliphatic are preferred. and / or aromatic dicarboxylic acids with a molecular weight <390. In addition to the carboxyl group, particularly preferred dicarboxylic acids have a hydrocarbon structure. Alkanedicarboxylic acids with 5 to carbon atoms or benzene or naphthalenedicarboxylic acids, in particular those derived from benzene, have acquired particular industrial importance. Terephthalic acid is particularly noteworthy. Suitable starting materials are, for example, glutaric acid, adipic acid, sebacic acid, terephthalic acid, isophthalic acid, succinic acid, naphthalene-2,6-dicarboxylic acid, 4,4'-diphenyl-4,4' Diphenoxydicarbonsäure or their C, - to C _4- alkyl esters.

Preferred diols are aliphatic, cycloaliphatic or aromatic diols with a molecular weight <280. Apart from the hydroxyl groups, they preferably have a hydrocarbon structure. Alkanediols, in particular those having 2 to 6 carbon atoms, have gained particular industrial importance. Suitable diols are, for example, ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,5-pentanediol, decamethylene glycol, neopentyl glycol or 1,4- Bis-hydroxymethylcyclohexane. Ethylene glycol and 1,4-butanediol have become particularly important.

The polyesters and also their precondensates consist of at least 50 mol% of polyethylene terephthalate or polybutylene terephthalate units. The rest can also consist of other short-chain polyester units derived from the above-mentioned polyester-forming starting materials. Polyesters which are 70 to 100 mol% of polyethylene or polybutyl are particularly preferred enterephthalate exist. The method according to the invention has attained particular importance for the production of polybutylene terephthalate.

The polyester precondensates are obtained in a manner known per se by reacting dicarboxylic acids or their esters with a diol in the presence of catalysts such as titanium acid esters, antimony, manganese or zinc compounds, e.g. their fatty acid salts, at temperatures from 150 to 260 ° C. The diglycol esters of carboxylic acids obtained in this way are precondensed under reduced pressure at temperatures of 230 to 270 ° C. Such precondensates have a relative viscosity of at least 1.05 (measured as a 0.5 percent by weight solution in a mixture of phenol and o-dichlorobenzene in a weight ratio of 3: at 25 ° C.). As a rule, polyester precondensates are used with a relative viscosity of 1.05 to 1.2. The production of such precondensates is described, for example, in DE-A 25 14 116.

The condensation of the polyester precondensates to high molecular weight polyesters is carried out at temperatures of 270 to 340 ° C under reduced pressure. It is advantageous to maintain pressures from 0.133 to 2.66 mbar. It goes without saying that the diols which split off during the condensation are continuously removed from the reaction mixture.

An essential feature of the invention is that the condensation is first started at a temperature of 290 to 340 ° C. and the temperature is lowered as the condensation proceeds. With the proviso that the final temperature is at least 10 ° C, advantageously 30 ° C above the melting point of the polyester produced. The initial temperature also depends on the type of pre-condensate. In the case of polyethylene terephthalate, starting temperatures of 320 to 340 ° C have proven particularly useful, while temperatures of 290 to 310 ° C have proven particularly favorable in the production of polybutylene terephthalate. The high initial temperature is advantageously reduced by 30 to 50 ° C. during the condensation. The temperature drops continuously. The final temperature depends essentially on the melting point of the polyester produced and should be so high above its melting point that no solidification occurs and further processing is not hindered. As a rule, temperatures around 10 ° C above the melting point have been found to be useful.

The process according to the invention can also be carried out advantageously by continuously lowering the temperature by 30 to 50 ° C. in the course of the condensation by adiabatic operation.

This procedure has the advantage that the optimal reaction temperature is largely independent. Furthermore, the new process has the advantage that the retrofitting of a heat exchanger before the condensation stage of the polyester condensation can be carried out in conventional condensation reactors after an optimal temperature-time profile.

First of all, the low-viscosity polyester precondensate melt is heated by 30 to 50 ° C. above the temperature which the fully condensed polyester should have after the polycondensation before entering the polycondensation zone. The temperature of the polyester precondensate is expediently first increased to the extent that it decreases in the subsequent polycondensation. The polyester precondensate melt is advantageously heated briefly in a heat exchanger, e.g. Tube or plate heat exchangers or in a similar suitable device.

After the polyester precondensate melt thus heated has entered the condensation zone, the condensation starts as a result of transesterification reactions. The heat energy required for the reaction and evaporation of the released diol and any by-products, for example tetrahydrofuran, in the condensation of polyesters containing 1,4-butanediol is taken from the heat content of the melt. As a result, the temperature drops as the condensation reaction proceeds. As a result, the optimal reaction temperature is largely independent. Accordingly, the areas in the polycondensation zone which come into contact with the melt are kept at the temperature which the polyester should have after polycondensation, up to a temperature which is below the final temperature of the polycondensation up to 10 ° C. Towards the end of the polycondensation, the melt then has a temperature which corresponds to the surfaces in contact with the melt or, as a result of the absorption of mechanical energy during the movement of the melt, the temperature of the contact surfaces by 5 to 10 ° C., depending on the design of the device used exceeds.

The condensation is preferably carried out in a thin layer. According to the invention, thin layers are understood to be those with a layer thickness of up to 7 mm.

Polyesters obtainable by the process of the invention are suitable for the production of shaped structures such as threads, foils, injection-molded or extruded parts, and also for coatings.

The process according to the invention is illustrated in the following examples.

Examples (a) Preparation of the polyester precondensate

1,000 g of dimethyl terephthalate and 685 g of 1,4-butanediol were heated to 130 ° C. in a 2 liter round-bottomed flask equipped with a stirrer, nitrogen inlet and packed column. At this temperature, 1.5 g of tetrabutyl orthotitanate was added as the transesterification catalyst with stirring. The distillation of methanol soon began. The temperature was raised to 220 ° C within 2 hours. After this time, 330 g of methanol had been distilled off and the transesterification reaction was complete ends. The packed column was then replaced by a descending condenser and the temperature was raised to 250 ° C. in the course of 15 minutes. The pressure was then steadily and linearly reduced to 13.3 mbar within 40 minutes with rapid stirring. At this pressure the mixture was stirred for a further 5 minutes and the precondensation was then ended by breaking the vacuum with nitrogen.

The precondensate was poured into a bowl under nitrogen, where it quickly solidified. The relative viscosity of this precondensate was 1.13.

(b) condensation

The condensation was carried out in a 250 ml round-bottom flask equipped with a stirrer, cooler and nitrogen inlet, which was heated by a Woodsches metal bath. For post-condensation, 50 g of the pre-condensate were melted under nitrogen at the selected post-condensation temperature. After melting and temperature compensation, the flask was quickly evacuated to a pressure of approximately 0.67 mbar. The stirring speed was adapted to the respective viscosity. After a predetermined time, the post-condensation was interrupted by breaking the vacuum with nitrogen.

The results are shown in the table below.

Comparative Examples 1 to 4 show the relative viscosity as a function of the post-condensation temperature after a reaction time of 15 minutes. According to the prior art, the temperatures were kept constant during the polycondensation time. If the temperature is increased from 255 ° C to 280 ° C, higher relative viscosities are obtained and thus a higher degree of polycondensation. When the temperature is increased further to 290 ° C., the relative viscosity drops again and a yellowish product is obtained.

In contrast, Examples 1 and 2 were carried out by the process according to the invention. For this purpose, the metal bath was preheated to 295 ° C. and the post-condensation reaction was started after the precondensate had melted. In the course of the post-condensation, the temperature of the heating bath was lowered in the steps given in the table. According to the Ver driving a relative viscosity of 1.67 was reached within 12 minutes (Example 2).

Example 3 A. Preparation of a polybutylene terephthalate precondensate

20 kg of dimethyl terephthalate and 13.7 kg of 1,4-butanediol were heated to 130 ° C. in a 40 l stirred kettle equipped with a stirrer, nitrogen inlet and dephlegmator. At this temperature, 30 g of tetrabutyl orthotitanate were added as a transesterification catalyst with stirring. Thereafter, the distillation of methanol started. The temperature was raised to 220 ° C within 2 hours. After this time, 6.6 kg of methanol had been distilled off and the transesterification reaction had ended. The temperature was raised to 250 ° C. within 30 minutes. The pressure was then steadily and linearly reduced to 13.3 mbar within 45 minutes. At this stage, the relative viscosity of the polybutylene terephthalate precondensate thus obtained was 1.12.

B. condensation of the polybutylene terephthalate precondensate according to the inventive method

The polybutylene terephthalate precondensate was pressed through a plate heat exchanger and thereby heated to 285 ° C. and passed into a condensation vessel of 40 l content preheated to 285 ° C. with diphyl steam. This heating process took 10 minutes. The flow of the diphyl vapor was then interrupted and, with rapid stirring, a vacuum of 1.33 mbar was suddenly established in the condensation vessel.

As the polycondensation now takes place, the melting temperature dropped within. 33 minutes exponentially to 250 ° C. From this point on, the temperature was kept at 250 ° C. by diphyl vapor. After a further 21 minutes, the vacuum was released and the melt was discharged under nitrogen pressure. The relative viscosity of the polybutylene terephthalate thus produced was 1.72.

Comparative Example 5

In this experiment, the polybutylene terephthalate precondensate was also pressed through the plate heat exchanger within 10 minutes, but the temperature was kept at 250.degree. The condensation boiler was preheated to 250 ° C. At this temperature, a vacuum of 1.33 mbar was suddenly produced with rapid stirring and condensed under these conditions for 54 minutes. After a condensation time of 54 minutes, the vacuum was released and the melt was discharged. The relative viscosity was only 1.49.

Claims (3)

1. A process for the manufacture of high molecular weight linear polyesters which are derived from dicarboxylic acids or their ester-forming derivatives and from diols, and consist to the extent of at least 50 mole% of polyethylene or polybutylene terephthalate units, by condensing polyester precondensates, having a relative viscosity of at least 1.05, under reduced pressure at from 270 to 340°C, the temperature being lowered as the condensation progresses, characterized in that the polyester precon- densate is heated for a short period at a temperature of from 290 to 340°C and then the temperature is lowered by from 30 to 50°C by vaporizing the liberated diol under a pressure of from 0.133 to 2.66 millibars, with the proviso that the final temperature is at least 10°C above the melting point of the polyester produced.

2. A process as claimed in claim 1, characterized in that the condensation is carried out in a thin layer.

3. A process as claimed in claims 1 and 2, characterized in that the temperature is lowered continuously, during the condensation, by from 30 to 50°C, by carrying out the process adiabatically.