The present invention relates to a method for preparing polyesters, in particular from dicarboxylic acids and butanediol, wherein an aluminosilicate is employed, as well as the use of an aluminosilicate in such a pro ^¬ cess.

EP 0698050 Al relates to a method for preparing poly- tetrahydrofurane, wherein tetrahydrofurane (THF) and a zeolite catalyst are used.

DE000019638549A1 discloses the use of zeolite catalysts for the polycondensation of polyethyleneterephthalate. However, this reference does not deal with polyesters based on butanediol (BD) and dicarboxylic acids.

Devroede, J. (2007), Study of the THF formation during TPA-based synthesis of PBT, Eindhoven: Technische Uni- versitat Eindhoven DOI : 10.6100/IR630627 presents the result of trials of polybutyleneterephthalat (PBT) in which aluminosilicates served as co-catalysts to tet- rabutyl orthotitanate in the esterification of tereph- thalic acid (TPA) and transesterification of Dimethyl- terephthalate (DMT) with BD . Whereas aluminosilicates showed an efficient catalytic effect and THF reduction in the transesterification, not any catalytic activity but generation of a large THF quantity was found in the esterification reaction. The synthesis of polyesters using dicarboxylic acids and BD in the presence of a catalyst usually takes place in the process steps of esterification, prepoly- condensation, polycondensation, followed by polymer melt discharge and granulation.

During esterification, dicarboxylic acids and BD form esters in the presence of a catalyst. These pre- products are converted to the final polymer in the sub ^¬ sequent process steps as mentioned above.

During the whole process, but especially in the esteri- fication reaction, the formation of THF from BD occurs as a side reaction. This reaction is acid-catalyzed and is therefore effective as a parallel reaction to the esterification, particularly in the initial phase of the esterification, in which there is still a large amount of free dicarboxylic acid. The THF formation in ^¬ creases the consumption of BD in the polyester production and leads to additional expenses for separation form the condensation water, processing to a marketable product or disposal.

The technical problem underlying the present invention is to provide a process for preparing polyesters using dicarboxylic acids and butanediol which avoids the above draw backs. This is achieved by the sub ect-matter of the independ ^¬ ent claims. Preferred embodiments are defined in the dependent claims. The process of the present invention for preparing pol ^¬ yesters comprises reacting a dicarboxylic acid with bu ^¬ tanediol in the presence of a catalyst, wherein in said process an aluminosilicate is present. By the reaction of the dicarboxylic acid and butanediol in the presence of the catalyst, polyesters are formed. By using an aluminosilicate in said process for preparing the poly ^¬ ester, the formation of THF from butanediol and the therewith associated drawbacks can be reduced. The alu ^¬ minosilicate can be present in the reaction for prepar- ing the polyester from the initial stage on.

In one embodiment, the process for preparing the poly ^¬ ester comprises the steps of esterification, prepoly- condensation and polycondensation, which can be subse- quently followed by polymer melt discharge and granula ^¬ tion. These are usual steps in the preparation of poly ^¬ esters from dicarboxylic acids and butanediol so that the skilled person knows the material and parameters for carrying out these steps. The aluminosilicate can be present in particular in the step of esterification, for example at the starting point of the esterification step .

As mentioned out above, in the process for preparing the polyester, an aluminosilicate is present. Alumino- silicates are compounds having varying amounts of AI ₂ O ₃ and SiC> ₂ . Silicon is surrounded by four oxygen atoms in the form of a tetrahedron, whereas Al is provided in the form of an octahedron. Aluminosilicates are com- pounds in which Al also occupies Si sites and is coor ^¬ dinated by four oxygen atoms. An example of aluminosil ^¬ icates are zeolites, wherein in an embodiment the alu ^¬ minosilicate can be a 4A-type zeolite. The aluminosilicate can have the formula Na ₂ <0 · AI ₂ O ₃ · 2 Si0 ₂ · n H ₂ 0.

In a further embodiment, the aluminosilicate can have a pore size of about 4 A and/or the aluminosilicate can have a particle size d ₅₀ of 0.1 to 0.5 pm. The pore size and the particle size are usual parameters in the field of aluminosilicates/zeolites so that they can be determined in the usual way in this technical field. In an embodiment, the aluminosilicate can be a synthet ^¬ ic and/or crystalline aluminosilicate.

The aluminosilicate can be provided in the form of a powder, and in particular it can be added to the reac- tion mixture for preparing the polyester in the form of a powder .

With the above described aluminosilicates, the amount of THF can be reduced effectively, which is formed in the process for preparing the polyester from a dicarboxylic acid and butanediol.

As mentioned above, in the process for preparing the polyester a catalyst is employed for catalyzing the re ^¬ action between the dicarboxylic acid and the butanedi ^¬ ol. In particular, the catalyst can be a tetrabutyl or- thot itanate . In the above process for preparing the polyester, a di ^¬ carboxylic acid is employed. It is possible to use one dicarboxylic acid or a mixture of two or more carbox- ylic acids being different from each other. In an embodiment, the dicarboxylic acid can be an aro ^¬ matic dicarboxylic acid with one or two aromatic rings, in particular terephthalic acid, isophthalic acid, and naphthalene dicarboxylic acids. The dicarboxylic acid can also be an aliphatic dicarboxylic acid having 2 to 16 C-atoms, in particular succinic acid, glutaric acid, adipic acid, and suberic acid.

The amount of the aluminosilicate can be 100 to 1 000 ppm referred to the final polymer product.

The relation of the catalyst concentration to the alu ^¬ minosilicate concentration can be 2 : 1 to 1 : 40 ex ^¬ pressed in ppm. As mentioned above, the process for preparing the poly ^¬ ester from the reaction of dicarboxylic acid with bu- tanediol can be carried out in the steps of esterifica- tion, prepolycondensation and polycondensation .

In an embodiment, the esterification can be carried out by applying at least one of the following parameters: molar ratio of dicarboxylic acid to butanediol is 0.5 to 1.5;

esterification temperature is 165 °C to 260 °C but below the thermal impairment of the dicarboxylic acid; pressure is 1200 mbar to 200 mbar;

catalyst concentration is 25 ppm - 200 ppm referring to the final product; and

the concentration of the aluminosilicate is 100 -

1000 ppm referring to the final product.

The esterification can be carried out in particular by applying all of the above parameters.

In an embodiment, the prepolycondensation can be carried out by applying at least one of the following pa ^¬ rameters :

temperature is 230 °C - 260 °C, but below of the thermal impairment of the dicarboxylic acid; and

pressure is 600 mbar - 20 mbar.

In particular, the prepolycondensation can be carried out by applying the above parameters together. In one embodiment, the polycondensation can be carried out by applying at least one of the following parame ^¬ ters :

temperatures of 235 °C to 265 °C but below of the thermal impairment of the dicarboxylic acid; and

pressure can be < 1 mbar .

In particular, the polycondensation can be carried out by applying the above parameters together.

The present invention relates also to the use of the aluminosilicate in a process for preparing polyesters comprising reacting a dicarboxylic acid with butanediol in the presence of a catalyst, wherein the process and the compounds used in the process are as defined above.

With the present invention, in particular with the above described embodiments, several advantages can be achieved. The use of the aluminosilicate, in particular as defined above, reduces the formation of THF to a great extent without influence of the catalyst and its activity to esterification, prepolycondensation and polycondensation . Thereby, a substantially lower molar ration of dicarboxylic acid to THF formation and BD de- pletion can be used without fear that the polycondensa- tion process is prematurely terminated due to THF for ^¬ mation and BD depletion and the target viscosity of the polymer is not reached. The esterification process can be controlled in such a way that from the beginning a high number of dicarboxylic acid BD monoesters and oli- gomeric monoesters are formed which exhibit a substan ^¬ tially higher reactivity in the polyester chain build ^¬ ing and lead to a considerable process acceleration. In contrast, the dicarboxylic acid-BD-diesters or oligo- meric diesters formed with a higher BD excess need many trans-esterification steps to remove the redundant BD .

Furthermore, the BD consumption can be optimized for polybutylene terephthalate (PBT) , polybutylene adipate terephthalate (PBAT) and polybutylene succinate (PBS. The quantities of by-products is lowered, the process can be carried out faster and there is a high plant flexibility . The present invention will be illustrated by the fol ^¬ lowing example and comparative example, but it is to be noted that the example and comparative example shall not be construed to limit the invention thereto. Examples

Example 1

1265.5 g TPA ( terephthalic acid) and 1352.8 g adipic acid, 266.1 g butanediol, 1.29 g catalyst Tyzor TnBT (Dorf Ketal), a commercially available tetrabutyl or- thotitanate catalyst (50 ppm Ti related to the final product )—1.82 g aluminosilicate powder of a particle size d ₅ o of 0.1 to 0.5 pm (500 ppm related to the final product) prepared for example from KOSTROLITH ^® 4AP-TR having the formula Na ₂ 0 · A1 ₂ 0 ₃ · 2 Si0 ₂ · n H ₂ 0, a 4A- type zeolite with a pore size of 4 A , were treated 55 minutes in an esterification stage, whereas the tar ^¬ get pressure of 600 mbar was achieved 10 minutes after start. The esterification reaction, the split off of water, started at a temperature of 173 °C (first drop of distillate) and increased until the end of esterifi- cation to 220 °C. During esterification, a distillate in a quantity of

727.9 g containing THF in a quantity of 83.7 g was ob ^¬ tained .

The final product is calculated from the monomers con- sidering the theoretical polymer structure and statis ^¬ tical monomer distribution. Example 1 based on 3645 g final product.

Comparative Example 1

The same procedure as in Example 1 was carried out with the exception that no aluminosilicate was used.

The quantity of the distillate was 996 g, containing THF in a quantity of 278.2 g.

Comparing the THF quantity of Example 1 with that of Comparison Example 1, it is shown that by using an alu ^¬ minosilicate in the process of preparing a polyester from dicarboxylic acids and butanediol, the amount of formed THF can be significantly reduced.