EP1833874A1

EP1833874A1 - Method and device for producing polyester granules and/or shaped parts with a low acetaldehyde content

Info

Publication number: EP1833874A1
Application number: EP05809991A
Authority: EP
Inventors: Kurt Hanimann; Werner Stibal
Original assignee: Uhde Inventa Fischer AG
Current assignee: Uhde Inventa Fischer AG
Priority date: 2004-12-07
Filing date: 2005-12-06
Publication date: 2007-09-19
Also published as: RU2412955C2; CN101072810B; KR20080028835A; WO2006060930A1; RU2007120623A; US8470220B2; KR101250302B1; CN101072810A; US20090302489A1

Abstract

The invention relates to a method for producing polyester granules and/or shaped parts from a melt resulting from a polycondensation (1). The invention provides that, in order to obtain a low acetaldehyde content, the melt, in the discharge area (2) of the polycondensation and at a temperature between 270 and 285 °C, is kept in contact with a gas space under reduced pressure. With regard to the granules, a standard crystallization (5) and drying (6) with air serving as drying gas are sufficient within the scope of the inventive method. The shaped parts can be produced directly from the melt by using, for example, an injection molding machine (9). The inventive device carries out a polycondensation whose at least final stage is provided in the form of a disc reactor inside of which, while avoiding a formation of sediments to the greatest possible extent, the melt is directly conveyed from disc to disc by a combination of rotating discs and by static scrapers mounted on the periphery of the reactor. In the discharge area, the melt is conveyed by static scrapers directly into the discharge unit without the melt accumulating in this location.

Description

DESCRIPTION TITLE

Process and apparatus for the preparation of low-acetaldehyde polyester granules and / or molded articles

TECHNICAL AREA

The present invention relates to a method and an apparatus for producing low-acetaldehyde polyester granules and / or shaped articles from a melt discharged from a polycondensation.

The polyester may be, in particular, polyethylene terephthalate or its modified copolymers having acid-side modification amounts, e.g. of isophthalic acid or on the diol side e.g. of cyclohexadimethanol. The material is particularly suitable as a packaging material in the form of films, films or hollow bodies such. Bottles.

In the preparation of the material as well as its further processing resulting from thermal damage acetaldehyde (hereinafter also: AA) as an undesirable by-product, wherein the AA content in the melt immediately after the Austragsaggregat usually 20 - 100 ppm , AA is very odor and taste intensive and disturbs especially in the food industry, because it diffuses even in a cold state in the food. For the beverage industry, the following limits have been established for the AA content in the preforms: 1-3 ppm for water applications and 4-10 ppm for sweet drinks (CSDs). This means that, at least for these applications, the AA content has to be drastically reduced.

STATE OF THE ART

To reduce the AA content, various methods are already known: A first such method consists in chemically binding the AA by adding additives, so-called AA blockers. However, these additives can lead to yellowing and / or clouding of the material. In addition, the AA blockers have to meet the requirements of the food industry. Their use is therefore limited. It is customary to dose AA blocker for high requirements, such as for water directly during injection molding, in order to compensate for the formation of AA occurring here.

Another method is to evaporate AA at elevated temperatures below the melting point. In general, this is done in combination with a solid phase postcondensation, wherein the degree of polycondensation is increased at a temperature between 190-230 ° and at the same time AA is deposited. To prevent thermo-oxidative damage, this is carried out in a stream of an inert gas such as nitrogen. The granules thus obtained typically have an AA content of less than 1 ppm. However, the treatment is time-consuming, requires large appliances, lasts almost a whole day and consumes a lot of energy. The method is also applicable only to granules, but not if the melt coming from the polycondensation is directly e.g. an injection molding machine for the production of preforms is supplied.

Aftertreatment of the granules with air to remove AA is also possible in principle. Since the temperature must be kept relatively low in order to avoid thermo-oxidative damage, the reduction of AA takes a relatively long time. In addition, can be in this way for a standard produced granules without the mentioned

Solid phase postcondensation, which does not reach the low AA values required above all for food packaging.

From EP 0 727 303 a method for reducing the AA content is known, in which the melt is exposed to a vacuum after discharge from the polycondensation by means of a multi-shaft reactor, so that it comes to a vaporization of the AA. The evaporation can still be supported by an inert gas stream. In EP 0 968 243, therefore, nitrogen is mixed into the melt and the AA released during the subsequent relaxation. Although it is mentioned in EP 0 727 303 that, with sufficient surface renewal rate and under vacuum, extremely low AA contents can be achieved, this method has hitherto not been able to be established as a result of the high expenditure on apparatus.

As the last stage of polycondensation reactors for polyester, disk reactors have been established in the prior art. A schematic representation of such a reactor is shown in FIG. 2. Disk reactors operate on the principle that a horizontal cylindrical apparatus 20 with a diameter between 50 cm and 500 cm by controlling the continuous product entry (product in) and - product out to around one quarter to a maximum of half filled with melt. There is vacuum over the melt (vaper out). From the melt sump 21 shown dotted pull on a longitudinal shaft 22 or a basket-like structure arranged rotating disks, of which only two are designated 23, the melt high and drain them gravimetrically, whereby a large surface of the melt relative to the free space (vacuum ) is generated.

The level gradient forming in the sump 21 ensures inter alia the longitudinal flow of the melt through the reactor. It is usually by static internals such. Weirs, of which only two are designated 24, and controls the arrangement and speed of the rotating discs 23 in the reactor. The sump 21 serves primarily to ensure the uniform loading of the discs 23 with melt. In the last section of the reactor, the so-called Austragsbereich 25, in which the melt at elevated viscosity for

The feed pressure to the discharge unit 26 is usually by arranging the Austragsaggregats at a certain distance from the bottom of the cylindrical apparatus 20 at the lower end of a collection process 27 still elevated. The height of the liquid column can be up to 200 cm here.

Infoige the thermal degradation are formed in principle in this process in the melt continuously by-products, especially acetaldehyde. At the product surface exposed to the vacuum and up to a depth of no more than 3 mm, the acetaldehyde can evaporate out of the melt due to the phase equilibrium, so that a very low AA content is established there in the melt. This is particularly the case on the discs 23, where thin product layers are formed. In contrast, the acetaldeyhyd formed in the thick sump layer 21 can only diffuse to the surface to a very small extent and thereby accumulates in the melt. If one takes the discharge area 25 with the aforementioned collection process 27, in which the entire polymer is no longer in contact with the vacuum in the reactor, the AA content can be up to

Discharge unit 26 in this method already significantly greater than 10 ppm.

PRESENTATION OF THE INVENTION

The object of the invention is to specify how the AA content can be reduced in a simpler and more economical manner. This object is achieved according to the invention by a method as characterized in claim 1. An apparatus for carrying out the method is specified in claim 19.

In the process according to the invention, the melt in the discharge region of a polycondensation is kept in contact with a gas space under reduced pressure at a temperature between 270 ° and 285 ° C.

In the discharge of the polycondensation usually much higher temperatures are driven, namely between 285 - 295 ° C. By the invention according to the lower temperature between 270 - 285 ⁰ C, the level of thermal

Damage to the Poiymermaterials, which is the cause of the formation of AA, kept low. This results in the discharge of polycondensation from the outset less AA than usual in the art. The inventive temperature reduction is possible by skillful leadership of the melt flow in the reactor.

Furthermore, unlike in the prior art, in the discharge region of the polycondensation a contact of the melt is made with a gas space under reduced pressure, in which AA can evaporate. This corresponds to the teaching itself mentioned EP O 727 303, but with the difference that this is done without much additional effort already in the polycondensation.

Due to the low temperature in the discharge region of the polycondensation and the resulting low thermal damage to the polymer material, the existing formation potential for AA is low, so that the AA content in the melt can not rise drastically even during its further processing. An only low AA content therefore also results in the granulate produced from the melt and / or the preforms produced from the melt.

The temperature of the melt in the discharge region of the polycondensation is preferably set to a value between 275 and 280 ^° C.

The pressure in the gas space is preferably kept below 2 mbar, further preferably below 1 mbar.

If the melt with a discharge unit such. a discharge pump is discharged from the discharge of the polycondensation, it is advantageous if the melt is kept up to the discharge unit with the gas space in contact under reduced pressure. It should not arise before the Austragsaggregat the usual swamp there.

The apparatus preferred for carrying out the method according to the invention has, according to claim 19, a novel disk reactor in which, as far as possible, a sump is dispensed with. For this purpose, the melt is transported directly from disk to disk by a combination of rotating disks and static scrapers attached to the circumference of the reactor. In the discharge area, static wipers convey the melt directly into the discharge unit, without any accumulation of melt occurring here as well.

The disk structure of the novel disk reactor is chosen so that no thick layers arise and the replacement of the product is ensured on the disc. The discharge pump is flanged with a rectangular inlet flange directly to the reactor and under the last scrapers and discs of Agitator arranged. The thickness of the melt layers throughout the reactor can be kept below 10 cm, predominantly even below 3 cm. The new arrangement allows the controlled guidance of the melt on the discs as well as in ^■ the longitudinal direction of the reactor, especially at high degrees of polymerization and relatively low temperature.

According to the invention, the melt leaves the polycondensation with an AA content of approximately 0 ppm. In any case, less than 5 ppm of AA content can be achieved. This means a drastic reduction in comparison to the AA content of 20-50 ppm which is customary per se at the outlet of the polycondensation directly after the discharge unit. In the amorphous granulate thereby reduces the AA content to <10 ppm compared to> 50 ppm in the conventional method.

In order for the melt after its discharge from the polycondensation, inter alia, to be used directly for the production of preforms on an injection molding machine, it can be used in polycondensation, e.g. by using a polycondensation (high-viscosity finisher), to an intrinsic viscosity between 0.5 - 0.9 dl / g, in particular between 0.70 - 0.85 di / g, can be adjusted.

If the melt from the polycondensation is discharged into at least one melt line, it is preferred if the melt in this at least one melt line is kept in the same temperature range as stated above for the discharge range of the polycondensation, so that it is not additionally present in the melt line to a thermal damage of the polymer material comes.

Because of the formation potential of AA in the melt, which is not completely avoidable within the scope of the process according to the invention, it is generally favorable for a low AA content in the granules and / or in the preforms if the residence time of the melt in the melt line (s) is up to granulation and / or the device used for the production of the preforms. (eg injection molding machine) is as short as possible. Preferably, the residence time in the melt line should not exceed 3 minutes, in particular 1 minute, until granulation. To to the apparatus for the production of preforms, the melt in the melt line should have a residence time <5 min, preferably <3 min.

In the conventional methods, the residence time of the melt after the polycondensation to granulation or shaping in the or

Melting lines considerably longer and is between 15 and 30 min. In order to come to the short residence times according to the invention, it requires special structural provisions and apparative training, such. the use of diagonal through-flow gear pumps for pressure increase in the melt line (s) and / or the use of melt filters of the type

Screen changer with a population of candle filters instead of commonly used flat screens.

The granulation can take place in a customary manner by pelletizers with a subsequent standard crystallization, wherein the granules are moved, for example with a stirrer or in a fluidized bed and with a drying gas at 150 - 200 ⁰ C dried. Air is preferably used as the drying gas and the drying is carried out at 165-175 ⁰ C.

The granulation can also be carried out by hot stripping with a subsequent direct crystallization by utilizing the intrinsic heat of the granules (latent heat crystallization) and with a drying as indicated above.

In both crystallizations and subsequent drying, the AA content in the granules is reduced from <10 ppm to at least <2 ppm. Taking advantage of all measures described can be obtained according to the inventive method granules with an AA content of 1 ppm.

In the context of the process according to the invention, it is therefore particularly advantageously possible to dispense with the solid-phase postcondensation of the granules customary in the prior art in order to reduce the AA content. As a result, the systems required for such a post-condensation are not needed and it saves time and energy. It may also be possible to dispense with nitrogen as the drying gas, because the mentioned crystallizations are also carried out with air as the drying gas can. By the solid phase postcondensation, the granules moreover receive an irregular viscosity and a high and irregular crystallinity, which is not desirable because of the associated high melting energy. The viscosity increase which can be achieved with the solid-phase postcondensation to values required for the further processing can, as already mentioned, be achieved by suitable treatment of the melt still in the polycondensation. As has surprisingly been found in the granules produced according to the invention results in about the same AA level and about the same formation rate for AA as in granules, which was prepared by the standard method mentioned above, including a solid phase postcondensation.

The aforementioned possibility of using AA blockers to reduce the AA content is, of course, also still available within the scope of the process according to the invention and can be used additionally, in particular in the production of moldings directly from the melt. As a result, AA contents in the moldings <5 ppm can be achieved.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in more detail with reference to embodiments in conjunction with the drawings. Show it:

1 shows a schematiέcher representation of a plant for the production of polyester granules and / or polyester moldings with low acetaldehyde content according to the invention;

Fig. 2 is a schematic representation of a prior art disk reactor; and

Fig. 3 is a particularly suitable for carrying out the inventive method, modified disk reactor.

WAYS FOR CARRYING OUT THE INVENTION

In Fig. 1, 1 denotes a polycondensation reactor (eg DISCAGE® of the Applicant), in which a melt of a polyester material is produced. With 2 is a discharge unit referred to, which simultaneously forms the discharge region of the polycondensation reactor 1 and closes and in which it may be a discharge pump. ^"

3 shows a schematic representation of a novel polycondensation reactor which is preferably used in the context of the invention.

2 is a disk reactor in the form of a horizontally-lying cylindrical apparatus 30 with disks arranged on a longitudinal shaft 32 or a basket-like structure, of which only two are designated by 33. The reactor shown in FIG. In contrast to the reactor of FIG. 2, an unfavorable bottom product which is formed for the evaporation is dispensed with as far as possible. For this purpose, the melt is transported directly from disk to disk 33 by static wipers attached to the circumference of the reactor, two of which are denoted by 34. In the discharge area 35, the melt is conveyed directly into the discharge unit or pump 37 by static wipers 36, so that no accumulation of melt takes place here as well. Also is dispensed to a collection process. The pump 37 is flanged to a rectangular flange 38 directly to the reactor.

In the novel reactor, the continuous melt stream is connected to a gas space under reduced pressure everywhere, right up to the discharge area in a thin layer. It is just worked on the physical tear-off.

In the following, reference is again made to FIG. 1. The discharge unit 2 conveys the polyester melt from the polycondensation reactor 1 under pressure into a melt line 3, which subsequently divides into the strands 3.1 and 3.2. The strand 3.1 is divided again into strands 3.1.1 and 3.1.2.

The strand 3.1.1 leads to a standard granulation 4, for example using a conventional strand pelletizer. The granules formed by this is fed to a standard crystallization 5, in which the granules are moved, for example by an agitator or on a fluidized bed. This is followed by a drying stage 6, in which the granules, for example, in a shaft dryer in the flow of a hot Drying gas is dried. 7 denotes a container or a filling station for the finished granules.

The strand 3.1.2 leads to a granulation 8 by hot deduction and with a direct crystallization by utilizing the intrinsic heat of the granules. The granules produced in this way are then fed to the aforementioned drying stage 6 and, via them, likewise pass into the container or the filling station 7. Alternatively, the granulation can be driven directly into the drying stage 6 by hot-cutting. From there, the granules are then returned to the filling station 7.

The strand 3.2 leads to a tool 9 with which a molded part can be produced directly from the melt. The tool 9 may in particular be an injection molding machine and the molding may be a bottle preform.

In line 3.1, an injection site 10 is shown in dashed lines in conjunction with a static mixer, via which, if necessary, an AA blocker can be injected into the melt. In strand 3.2, a corresponding injection site is provided and denoted by 1 1.

Finally, a filter 12 could also be provided in the melt line 3.

Examples

In the table below, columns B - F are intrinsic values

Viscosity IV, the AA content and for the degree of crystallization, as they can be achieved with a device according to FIG. 1 according to the inventive method for a polyester granules before and after the drying stage or in the molded part. Column A shows comparative values as used in a prior art method including solid phase post-condensation (SSP = Solid State

Polycondensation) are typical instead of drying according to the invention. The polyester material is polyethylene terephthalate in all examples. For the examples according to the invention, a polycondensation reactor according to FIG. 3 was used within a continuous process. The IV values and the AA values were determined according to ASTM. The degree of crystallinity was determined by the density method using the following conventional formula:

wherein the density of 100% crystalline PET with p _c = 1.455 g / cm ³ and the density of amorphous PET with p _a = 1.332 g / cm ^{3 is} set.

The process conditions were as follows:

Example A (Comparative Example)

(a) granulation comprises:

Polycondensation reactor according to FIG. 2 (continuous melt polycondensation): temperature 290 ^° C.

Vacuum 1 mbar

Residence time in the melt line 12 min

(b) Postcrystallization is omitted as a "separate step". Therefore no values (c) After SSP / drying:

Precrystallization 45 min at 200 ⁰ C.

Crystallization 60 min at 209 ⁰ C SSP reactor 12 h at 209 ⁰ C with N ₂ as the drying gas

(d) Molding (spraying after remelting the granules):

Temperature in the tool. 290 ° C residence time in the melt 50 sec

Example B

(a) Granulation comprises: polycondensation reactor (continuous melt polycondensation): temperature 276 ^° C.

Vacuum 0.95 mbar

Residence time in the melt line 1.5 min

(B) crystallization: in a fluidized bed for 60 min at 17O ⁰ C.

(c) Drying: in the shaft dryer for 12 h at 170 ° C with air as the drying gas

(d) Molding (spraying after remelting the granules):

Temperature in the tool 280 ° C

Residence time in the melt 50 sec.

Example C

(a) granulation comprises:

Polycondensation reactor (continuous melt polycondensation): temperature 276 ^° C. Vacuum 0.95 mbar

Residence time in the melt line 45 sec

(b) Crystallization: in the fluidized bed for 15 minutes at 170 ^D C

(c) Drying: in the shaft dryer for 12 h at 170 ⁰ C m as drying gas

(d) molding (spraying after remelting the granules): temperature in the tool 280 ⁰ C.

Residence time in the melt 50 sec

Example D

Addition of 250 ppm of an AA blocker at the injection site 10 in the form of anthranilamides from Colormatrix, Knowley, Meresyde, United Kingdom. Otherwise as example B.

Example E

(a) Cleaning includes:

Polycondensation reactor (continuous melt polycondensation): temperature 276 ° C

Vacuum 0.95 mbar

Residence time in the melt line 4 min

(b), (c) are omitted as separate process steps: No values

(d) molded part (injection directly from the melt)

Temperature in the tool 275 ⁰ C

Residence time in the melt 30 sec Example F

Addition of 250 ppm of an AA blocker at the injection site 11 in the form of anthranilamides from Colormatrix, Knowley, Meresyde, United Condom. Otherwise as example E.

The examples given show that the process according to the invention gives approximately the same values for the intrinsic viscosity IV and the AA content in the finished granules and / or in directly produced moldings, as are typical of corresponding products which have been obtained the technique below

Inclusion in particular a complex solid phase postcondensation are typical. With the additional use of AA-blockers, the AA values can be lowered as much as required, for example, for the production of water bottles. Even after the remelting and further processing of granules produced according to the invention, the AA values remain at a level comparable to the prior art.

The division of the melt line 3 into several strands as shown in Fig. 1 and the parallel application of different granulation and crystallization processes or a direct production of moldings from the melt is merely exemplary. The invention could also be used with only one granulation or only for direct molding production. On the other hand, it would be possible to run other processes in parallel. By further division of the strand 3.2 in particular several tools of the type of tool 9 of Fig. 1 could be operated in parallel.

NAME LIST

1 polycondensation reactor

2. Discharge unit

3 melt line

3.1 partial strand of the melt line

3.1.1 Sub-branch of the melt line

3.1.2 Partial strand of the melt line

3.2 partial strand of the melt line

4 granulation (strand granulator)

5 standard crystallization

6 drying stage (shaft dryer)

7 containers or a filling station for the finished granules

8 granulation by hot stripping and with direct crystallization

9 Tool for molded parts (injection molding machine)

10 injection parts for AA blocker

11 injection parts for AA blocker

12 filters

20 cylindrical apparatus

21 melt sump

22 longitudinal shaft or basket-like structures

23 slices

24 weirs

25 discharge area

26 discharge unit / pump

27 collection process

30 cylindrical apparatus

32 longitudinal shaft or basket-like structures

33 slices

34 static wipers

35 discharge area

36 static wipers

37 Discharge unit / pump

38 flange

Claims

1. A process for the preparation of polyester granules and / or molding with low acetaldehyde content from a melt, which is discharged from a polycondensation, characterized in that the melt in the discharge of the polycondensation at a temperature between 270-285 ⁰ C. a gas space is kept in communication under reduced pressure.

2. The method according to claim 1, characterized in that the melt is maintained in the discharge of the polycondensation at a temperature between 275 - 280 ⁰ C with a gas space under reduced pressure in combination.

3. The method according to claim 1 or 2, characterized in that the pressure in the gas space below 2 mbar, preferably below 1 mbar, is maintained.

4. The method according to any one of claims 1-3 and wherein the melt is discharged with a discharge from the discharge of the polycondensation, characterized in that the melt in the discharge is held up to the discharge with a gas space under reduced pressure in contact.

5. The method according to any one of claims 1 - 4, characterized in that the acetaldehyde content of the melt in the discharge of the polycondensation to a value <5 ppm, in particular <2 ppm, is maintained.

6. The method according to any one of claims 1-5, characterized in that the melt in the discharge of the polycondensation, preferably by using a high-viscosity finisher, to an intrinsic viscosity between 0.5 - 0.9 dl / g, preferably between 0.70 - 0.85 dl / g, is set.

7. The method according to any one of claims 1-6 and wherein the melt from the polycondensation in at least one melt line is discharged, characterized in that the melt in the at least one melt line on a Temperature between 270 - 285 ⁰ G, preferably between 275 - 280 ⁰ C, is maintained.

8. The method according to any one of claims 1-7 and in which the melt is discharged from the polycondensation in a melt line, which leads to a granulation, characterized in that the melt in the melt line to granulation a residence time <4min, preferably <2 min, more preferably <1 min.

9. The method according to any one of claims 1-8, characterized in that the

Granules with a standard crystallization with agitation and with a drying with a drying gas at a temperature between 150 - 200 ⁰ C, preferably between 165 - 175 ⁰ C, further treated.

10. The method according to any one of claims 1-8, characterized in that the granules produced by hot exhaust and with a direct crystallization by utilizing the heat of the granulate and a drying with a drying gas at a temperature between 150 - 200 ⁰ C, preferably between 165 - 175 ⁰ C, is treated.

11. The method according to any one of claims 9 or 10, characterized in that the drying is carried out with air as the drying gas at a temperature <175 ° C.

12. The method according to any one of claims 9 - .11, characterized in that the drying is carried out for 5 - 2O h, preferably for 10 - 15 h.

13. The method according to any one of claims 1-7 and wherein the melt is discharged from the polycondensation in a melt line, which leads to a device for the production of moldings, characterized in that the melt in the melt line up to the device a residence time <5 min, preferably <3 min.

14. The method according to any one of claims 1-13, characterized in that acetaldehyde blocker are added to the melt before granulation and / or the production of the moldings.

15. The method according to any one of claims 8 - 14, characterized in that the melt is filtered in the melt line.

16. The method according to any one of claims 1-15, characterized in that the melt in the discharge region of the polycondensation with a layer thickness of less than 10 cm and predominantly less than 3 cm is held in communication with the gas space under reduced pressure.

17. The method according to any one of claims 1-16, characterized in that the melt is maintained in the entire polycondensation at a temperature between 270-285 ⁰ C with a gas space under reduced pressure in combination.

18. The method according to any one of claims 1-17, characterized in that it is continuous.

19. Apparatus for carrying out the method according to one of claims 1 -

18, characterized in that it comprises a polycondensation, whose at least last stage is designed as a disk reactor in which, while largely avoiding a sump, the melt is transported directly from disk to disk by a combination of rotating disks and mounted on the periphery of the reactor static scrapers and in the discharge area from which by static scraper the melt is conveyed directly into the discharge, without that here an accumulation of melt takes place.