METHOD AND DEVICE FOR CLEANING A POLYMER MELT

Description

[0001] The invention relates to a method for cleaning a polymer melt according to the preamble of claim 1 and to a device for cleaning a polymer melt according to the preamble of claim 10.

[0002] When processing thermoplastics, it is well known that during an extrusion process, for example, for the production of strand-like products, the polymer melt must be largely purified of foreign substances. Some thermoplastics contain certain amounts of monomers, oligomers, solvents, and other volatile components as residues from the manufacturing process. Other polymers, such as polyamide, are very hygroscopic, meaning they have a strong tendency to absorb moisture. The absorbed moisture can be trapped or absorbed within the polymer. Furthermore, during melting, granular polymers incorporate high amounts of air into the melt. These components are referred to here as volatile foreign substances.

[0003] In addition to volatile foreign substances, contamination of the polymer melt also occurs, referred to here as solid foreign substances. Such solid foreign substances are impurities that are contained, for example, in the granulate or originate from abrasion of machine parts. Furthermore, decomposition products also occur within the melt, which can lead to unwanted thread breakage, for example, during fiber production.

[0004] In the prior art, it is now common practice to purify the polymer melt in several process steps. To free the polymer melt of volatile foreign substances, it is common to degas the polymer melt. This involves bringing the surface of the polymer melt into contact with a vacuum so that the volatile constituents can be released from the melt. This process is preferably carried out directly during extrusion of the polymer melt by an extruder, as disclosed, for example, in EP 1 400 337 B1. A degassing zone is formed on a screw section of a screw extruder, which creates a vacuum in the extruder section.

[0005] To remove solid foreign matter, the polymer melt is filtered using filter elements. This process step is typically carried out using filter devices, such as those known from DE 199 12 433 A1. The polymer melt is passed through a filter element, which filters out the solid foreign matter from the polymer melt stream depending on the filter fineness.

[0006] The prior art includes many variants of such processes and devices for degassing the polymer melt, as well as many variants of such processes and devices for filtering the polymer melt. These require more or less complex or additional equipment to enable the purification of the polymer melt in multiple process steps. This requires correspondingly long residence times, which increase the risk of the formation of decomposition products.

[0007] Various purification options are known from the prior art. For example, DD 231 029 A1 discloses a method and device for compounding thermoplastics with a filter and degassing. Further examples are EP 0 705 677 A1, WO 2005 / 087477 A1, CN 107 053 635 A, EP 2 439 044 A1, EP 2 008 784 A1, and AT 511 574 A1.

[0008] It is an object of the invention to provide a method and a device for cleaning a polymer melt, with which the polymer melt can be cleaned with the shortest possible residence times.

[0009] A further aim of the invention is to be able to carry out the cleaning of the polymer melt with little equipment expenditure.

[0010] The object is achieved according to the invention by a method having the features of claim 1 and by a device having the features of claim 9.

[0011] Advantageous further developments of the invention are defined by the features and combinations of features of the respective subclaims.

[0012] The invention departs from the traditional division and sequence of degassing and filtration for purifying the polymer melt. It was discovered that the success of degassing does not depend on the presence of foreign particles in the polymer melt. On the contrary, the homogeneity of the polymer melt promotes the escape of volatile foreign substances. The polymer melt is forced through a filter element into a vacuum chamber. The solid foreign substances are absorbed by the filter element, and the volatile foreign substances are collected in the vacuum chamber. This allows both process steps for purifying the polymer melt to be combined into a single process step.

[0013] The device according to the invention comprises only one filter element and the vacuum of a vacuum chamber. This allows the purification of a polymer melt to be carried out in a single process step with minimal equipment expenditure. The polymer melt is forced through the filter element into the vacuum chamber of the vacuum chamber.

[0014] To achieve the greatest possible degassing efficiency, the process variant in which the polymer melt is guided within the vacuum chamber to an outlet surface of the filter element is particularly advantageous. Depending on the shape of the filter element, this allows for relatively large melt surfaces within the vacuum chamber, which promote the escape of volatile contaminants. Furthermore, the flow through the filter element continuously renews the melt surface at the outlet surface of the filter element.

[0015] The volatile impurities escaping from the polymer melt are continuously removed from the vacuum chamber through a vent. This ensures material separation, allowing the volatile impurities to be separately processed and, if necessary, utilized.

[0016] To allow volatile contaminants to escape from the polymer melt within the vacuum chamber, the vacuum is set to a negative pressure in the range of 0.5 mbar to 50 mbar. The negative pressure within the vacuum chamber is selected depending on the thickness of the polymer melt at the exit surface of the filter element. This creates a relatively high vacuum to achieve intensive degassing even with relatively thick melt layers.

[0017] According to an advantageous development of the invention, the purified polymer melt is collected in a sump of the vacuum chamber formed below the filter element and discharged through a melt outlet. A continuous melt flow can be achieved, allowing the purification of the polymer melt to be easily integrated into an extrusion process.

[0018] The polymer melt is fed through a melt inlet into a pressure chamber defined by the filter element at an overpressure. This allows the melt flow rate at the filter element to be adjusted depending on the overpressure.

[0019] Depending on the degree of contamination by solid foreign matter, the polymer melt is filtered with a filter element with a filter fineness ranging from 40 µm to 1,000 µm. For example, in the production of fine filaments in a melt spinning process, a relatively high filter fineness is required to prevent filament breakage.

[0020] Depending on the fineness of the filter element, the polymer melt is introduced into the pressure chamber with an overpressure in the range of 10 bar to 100 bar.

[0021] To ensure sufficient flowability of the polymer melt, depending on the polymer type, the polymer melt is heated to a temperature before purification. Preferably, the polymer melt is heated to a temperature above 200°C.

[0022] To ensure that a sufficient melt surface can be formed within the vacuum chamber after filtration, the device according to the invention provides that the filter element is formed at least from a hollow cylindrical filter candle that extends into the vacuum chamber. The outer sides of the filter candle are utilized to achieve the largest possible external surface for guiding the melt after filtration. Thus, the filter elements are designed with a filter surface on the inlet side that is smaller than the outlet surface, where the melt is guided for degassing purposes.

[0023] To generate the vacuum and to remove the volatile components, the further development of the device according to the invention is provided, in which the vacuum chamber is connected to a collecting container and a vacuum pump via a degassing opening.

[0024] The vacuum pump is designed in such a way that the vacuum in the vacuum chamber can be generated to a negative pressure in the range of 0.5 mbar to 50 mbar.

[0025] The purified melt is preferably collected in a bottom region of the vacuum chamber. According to an advantageous development of the invention, a sump is formed for this purpose, extending below the filter element and connected to a melt outlet in a chamber housing.

[0026] The chamber housing also features a melt inlet that opens into a pressure chamber defined by the filter element. This allows a melt stream of the polymer melt to be introduced directly into the pressure chamber and directed to the filter element.

[0027] Depending on the desired purity of the polymer melt, the filter element has a filter fineness in the range of 40 µm to 1,000 µm. The filter fineness depends on the manufacturing process and the specific product being manufactured.

[0028] To accommodate the polymer melt, the melt inlet can be advantageously connected directly to a pressure source, through which the polymer melt can be introduced into the pressure chamber at an overpressure in the range of 10 bar to 100 bar. An extruder or a pump can be used as the pressure source.

[0029] A heating device is installed in the chamber housing to control the temperature of the polymer melt. This allows the polymer melt to be maintained at a predetermined temperature before and after cleaning.

[0030] The method and device according to the invention are suitable for all extrusion processes in which a polymer melt is processed. In particular, however, polymer melts from a recycling process can also be purified.

[0031] The method according to the invention for cleaning a polymer melt is described in more detail below using some embodiments with reference to the attached figures.

[0032] They represent: Fig. 1 schematically shows a cross-sectional view of a first embodiment of the device according to the invention for cleaning a polymer melt. Fig. 2 schematically shows a cross-sectional view of a further embodiment of the device according to the invention for cleaning a polymer melt. Fig. 3 schematically shows a further embodiment of a device for cleaning a polymer melt, not according to the invention. Fig. 4 schematically shows a further embodiment of a device for cleaning a polymer melt, not according to the invention.

[0033] In the Fig. 1 A first embodiment of the device according to the invention for purifying a polymer melt is shown schematically in a cross-sectional view. The embodiment has a chamber housing 1, which in this example is formed by a cylindrical pot 1.1 and a lid 1.2. The pot 1.1 contains a vacuum chamber 2. Within the vacuum chamber 2, a filter element 4 is held centrally on the pot 1.1. The filter element 4 is designed as a filter candle 5 and projects into the vacuum chamber 2. The filter candle 5 is hollow-cylindrical and has a pressure chamber 6 inside, which is connected to a melt inlet 7 in the chamber housing 1. The melt inlet 7 is formed here on the lid 1.2. The filter candle 5 is closed at one free end and has a filter material 5.1 on the inside and a support wall 5.2 on the outer circumference. The support wall 5.2 has a plurality of outlet openings 5.3 and forms an outer exit surface for guiding the filtered polymer melt within the vacuum chamber 2.

[0034] Below the filter element 4, a sump 9 is formed in the bottom of the pot 1.1 in the vacuum chamber 2. The sump 9 is connected to a melt outlet 8 in the chamber housing 1. The melt outlet 8 is arranged centrally in the bottom of the pot 1.1, with the sump 9 extending in a funnel shape above the melt outlet 8.

[0035] The vacuum chamber 2 is connected to a collecting container 11 and a vacuum pump 12 via a degassing opening 3 in the chamber housing 1. In this embodiment, the degassing opening 3 is formed in the pot 1.1 of the chamber housing 1.

[0036] A heating device 10 is arranged on the periphery of the chamber housing 1. The heating device 10 preferably comprises electrical heating means, which are not shown here.

[0037] During operation, the melt inlet 7 on the chamber housing 1 is connected to a pressure source 13. In this embodiment, the pressure source 13 is formed by a pump. The pump 13 allows the polymer melt to be continuously fed to the melt inlet 7 as a melt stream.

[0038] A discharge pump 14 is connected to the melt outlet 8. The discharge pump 14 discharges the purified polymer melt with a continuous discharge stream.

[0039] During operation, a vacuum with a negative pressure in the range of 0.5 mbar to 50 mbar is first set in the vacuum chamber 2 via the vacuum pump 12. The level of the vacuum in the vacuum chamber 2 depends on the process and the filter elements 4 used.

[0040] A polymer melt is fed to the filter element 4 via the melt inlet 7 via the pressure source 13. The polymer melt is fed at an overpressure in the range of 10 bar to 100 bar. The pressure chamber 6 within the filter candle 5 fills, so that the polymer melt is forced through the filter material 5.1 for filtration. The solid foreign matter is absorbed by the filter material 5.1 depending on the filter fineness. The polymer melt then penetrates the outlet openings 5.3 in the support wall 5.2. The polymer melt adheres to the outlet surface of the support wall 5.2 of the filter candle 5 and, due to gravity, slides within the vacuum chamber towards the sump 9. The polymer melt guided along the outlet surface of the support wall 5.2 forms a jacket-shaped melt surface within the vacuum chamber 2, which is directly exposed to the vacuum.The volatile foreign substances are removed from the polymer melt and collected within the vacuum chamber 2. The volatile foreign substances are removed via the degassing opening 3 and collected in the collection container 11. Within the collection container 11, the volatile foreign substances can be processed and separated, for example, into gaseous and liquid media.

[0041] The purified polymer melt accumulating in the sump 9 is discharged via the melt outlet 8. To generate a continuous melt flow, the discharge pump 14 is connected to the melt outlet 8, allowing a continuous melt flow. A fill level sensor can be arranged within the vacuum chamber 2, which interacts with a pump control system to ensure a substantially constant melt flow. The pump control system can act on the pressure source or the discharge pump, or both.

[0042] For temperature control, the heating device 10 is provided on the periphery of the chamber housing 1. The heating device 10, which preferably comprises electrical heating means, ensures a melt temperature above 200°C. Temperatures of up to 330°C may be required to maintain the temperature of the polymer melt. These temperatures depend essentially on the polymer type of the polymer melt to be purified.

[0043] The process and apparatus according to the invention are fundamentally suitable for all thermoplastic melts. Combined filtration and degassing allow purification of the polymer melt with relatively short residence times.

[0044] In order to enable a higher melt throughput, such as can be generated by extruders, Fig. 2 A further embodiment of the device according to the invention is shown schematically in a cross-sectional view. The embodiment is essentially identical to the aforementioned embodiment according to Fig. 1 , so that at this point only the differences are explained and otherwise reference is made to the above description.

[0045] In the embodiment according to Fig. 2 Several filter elements 4 are provided, which protrude into a vacuum chamber 2. In this exemplary embodiment, three filter candles 5 are arranged parallel to one another and connected to the melt inlet 7 and the pressure chamber 6. The number of filter elements 4 is exemplary. In principle, a large number of filter elements in the range of 100 to 150 can be used to perform combined filtration and degassing to purify the polymer melt.

[0046] In the embodiment according to Fig. 2 Each filter candle 5 is constructed identically and according to the embodiment according to Fig. 1 In this respect, reference is made to the above description.

[0047] In this embodiment, the degassing opening 3 is formed in the upper region of the vacuum chamber 2 in the pot 1.1 of the chamber housing 1. A collecting container 11 and a vacuum pump 12 are connected to the degassing opening 2.

[0048] Below the free ends of the filter elements 4, a sump 9 is formed in the vacuum chamber 2. The sump 9 is connected to a melt outlet 8.

[0049] In the Fig. 2 In the illustrated embodiment, a pressure source 13 in the form of an extruder is provided. An outlet side of the extruder is connected directly to the melt inlet 7 of the cleaning device.

[0050] The function of the Fig. 2 The embodiment shown is identical to the embodiment according to Fig. 1 , so no further explanation is given here.

[0051] When designing the filter elements 4, the outlet surfaces provided by the filter elements are always larger than the filter surface for filtering the polymer melt. Thus, the polymer melt is pressed from the inside to the outside at the filter candles 5, with the outlet surfaces at the periphery of the filter candles 5 creating the melt surfaces for degassing. In principle, filter elements other than filter candles are also suitable for combined filtration and degassing. Fig. 3 For this purpose, a further embodiment not belonging to the claimed invention is shown, in which the filter elements are formed by filter discs.

[0052] In Fig. 3 An embodiment of a device not according to the invention is shown schematically in a cross-sectional view. Fig. 3 The embodiment shown comprises a chamber housing 1 in which a vacuum chamber 2 is formed. The vacuum chamber 2 is connected to a collecting container 11 and a vacuum pump 12 via a degassing opening 3.

[0053] A plurality of filter elements 4 are arranged within the vacuum chamber 2. In this exemplary embodiment, the filter elements 4 are formed by filter discs 15 held on the circumference of a mandrel 16. The mandrel 16 forms a melt channel 16.1 inside, which is connected to a pressure chamber 6 of the filter discs 15 via several distribution openings 16.2. The mandrel 16 penetrates the chamber housing 1 to the outside and forms a melt inlet 7 at one open end.

[0054] Within the vacuum chamber 2, a sump 9 is formed below the vertically aligned filter discs. The sump 9 is funnel-shaped and connected to a lower melt outlet 8 in the chamber housing 1. In this embodiment, the melt outlet 8 and the degassing opening 3 are located opposite each other in the vacuum chamber 2.

[0055] During operation, the melt inlet 7 can be connected directly to an extruder as a pressure source 13. In this respect, the polymer melt is fed via the melt channel 16.1 and the distribution opening 16.2 into the pressure chambers 6 of the filter disks 15. The filter disks 15 have a filter material to filter the solid foreign matter from the polymer melt. The filter material can have a filter fineness in the range of 40 µm to 1,000 µm. After the polymer melt passes through the filter material on the filter disk 15, the polymer melt reaches the outlet surfaces of the filter disks 15, whereby a melt surface of the polymer melt forms on each of the filter disks 15 within the vacuum chamber. The polymer melt then reaches the sump 9 in the floor of the vacuum chamber 2 due to its gravity. During this time, the degassing of the polymer melt takes place.The escaping volatile foreign substances are fed to the collection container 11 via the vent 3. The vacuum set by the vacuum pump 12 ranges from 0.5 mbar to 50 mbar. The purified polymer melt is conveyed out of the vacuum chamber 2 via the melt outlet by the discharge pump 14 in a continuous melt stream. At higher flow rates, the polymer melt could alternatively be conveyed through a discharge extruder.

[0056] In the Fig. 3 In the illustrated embodiment, the heating device 10 for controlling the temperature of the chamber housing 1 was not shown in detail. Likewise, the number of filter discs shown is exemplary. In principle, a larger number of filter discs can also be used for combined filtration and degassing of the polymer melt.

[0057] To improve the melt surface formation, Fig. 4 an alternative embodiment of the device according to the invention is shown. The embodiment according to Fig. 4 is essentially identical to the embodiment according to Fig. 3 , so that only the differences are explained below and otherwise reference is made to the above description of the Fig. 3 is taken.

[0058] In the Fig. 4 In the illustrated embodiment, a drive housing 21 is arranged next to the chamber housing 1. The drive housing 21 is penetrated by the mandrel 16, wherein the mandrel 16 holds a plurality of filter disks 15 within a vacuum chamber 2 at a closed end, and wherein a free end of the mandrel 16 outside the drive housing 21 forms a melt inlet 7. Within the drive housing 21, the mandrel 16 is rotatably mounted and coupled to a drive shaft 20 via a gear pair 18. The drive shaft 20 is connected to a motor 19. The motor 19 can thus set the mandrel 16 in a rotary motion, so that the filter disks 15 held within the vacuum chamber 2 rotate at a predetermined peripheral speed. Thus, a melt layer forming on the exit surfaces of the filter disks 15 is moved, which promotes a renewal of the melt surface.

[0059] In the embodiment according to Fig. 4 A discharge extruder 22 is connected to the melt outlet 8 of the chamber housing 1. The discharge extruder 22 is shown in a horizontal arrangement to continuously convey the purified polymer melt. In principle, the discharge extruder 22 could also be arranged vertically to generate a continuous melt flow of the purified polymer melt.

[0060] The function for cleaning the polymer melt by combined filtration and degassing is in the embodiment according to Fig. 4 identical to the embodiment according to Fig. 3 In this respect, the above description is Fig. 3 Reference is made.

[0061] In the Fig. 1 bis 4 In the illustrated embodiments of the device according to the invention for purifying the polymer melt, the arrangement and selection of the filter elements are exemplary. Thus, the filter candles shown can be Fig. 1 and 2 They can also be held in the vacuum chamber in a different installation position. Vertical, horizontal, or angled installation positions are possible. Furthermore, the filter elements are preferably designed to be replaceable to remove solid foreign matter from the filter elements.

[0062] Likewise, extruders or pumps as well as discharge pumps and discharge extruders on the outlet side can be used as pressure media in the illustrated embodiments according to Fig. 1 bis Fig. 4 can be combined as desired.

Claims

1. Method for cleaning a polymer melt, in which volatile foreign substances and solid foreign substances are removed from the polymer melt, wherein the polymer melt is pressed through a filter element into a vacuum of a vacuum chamber, wherein the solid foreign substances are taken up by the filter element and wherein the volatile foreign substances are collected in the vacuum chamber, and wherein the polymer melt is guided within the vacuum chamber on an outlet face of the filter element, characterized in that the filter element is formed at least from a hollow-cylindrical filter candle projecting into the vacuum chamber.

2. Method according to Claim 1, characterized in that the volatile foreign substances are discharged from the vacuum chamber through a degassing opening.

3. Method according to either of Claims 1 and 2, characterized in that the vacuum in the vacuum chamber is adjusted to a negative pressure in the range of 0.5 mbar to 50 mbar.

4. Method according to one of Claims 1 to 3, characterized in that the polymer melt is collected in a vacuum chamber sump formed beneath the filter element and is discharged through a melt outlet.

5. Method according to one of Claims 1 to 4, characterized in that the polymer melt is introduced, with an excess pressure, via a melt inlet into a pressure space delimited by the filter element.

6. Method according to Claim 5, characterized in that the polymer melt is filtered with a filter fineness of the filter element in the range of 40 µm to 1000 µm.

7. Method according to Claim 6, characterized in that the polymer melt is introduced into the pressure space with an excess pressure in the range of 10 bar to 100 bar.

8. Method according to one of Claims 1 to 7, characterized in that the polymer melt is heated to a temperature above 200°C before it is cleaned.

9. Device for cleaning a polymer melt, with which volatile foreign substances and solid foreign substances can be removed from the polymer melt, comprising a filter element (4) and a vacuum of a vacuum chamber (2), wherein the polymer melt can be guided through the filter element into the vacuum of the vacuum chamber (2), wherein the filter element (4) binds the solid foreign substances and wherein the vacuum chamber (2) receives the volatile foreign substances, characterized in that the filter element (4) is formed at least from a hollow-cylindrical filter candle (5) projecting into the vacuum chamber (2).

10. Device according to Claim 9, characterized in that the vacuum chamber (2) is connected via a degassing opening (3) to a collecting container (11) and a vacuum pump (12).

11. Device according to Claim 10, characterized in that the vacuum in the vacuum chamber (2) can be generated by the vacuum pump (12) at a negative pressure in the range of 0.5 mbar to 50 mbar.

12. Device according to one of Claims 8 to 11, characterized in that a sump (9) is formed beneath the filter element (4) in a bottom region of the vacuum chamber (2) and is connected to a melt outlet (8) in a chamber housing (1).

13. Device according to Claim 12, characterized in that the chamber housing (1) has a melt inlet (7) which opens into a pressure space (6) delimited by the filterelement (4).

14. Device according to one of Claims 9 to 13, characterized in that the filter element (4) has a filter fineness in the range of 40 µm to 1000 µm.

15. Device according to Claim 13 or 14, characterized in that the melt inlet (7) can be connected to a pressure source (13) by means of which the polymer melt can be introduced into the pressure space (6) with an excess pressure in the range of 10 bar to 100 bar.

16. Device according to one of Claims 9 to 15, characterized in that a heater (10) for controlling the temperature of the chamber housing (1) is provided.