Extruders for the viscosity-enhancing preparation of fusible polymers

Abstract

The invention relates to an extruder (100), comprising a housing (10) with an inner housing recess (18), wherein an extruder screw (20) with a spiral extruder screw thread (32, 33, 34) is rotatably mounted. The outer diameter of the extruder screw (20) is subdivided into a diameter starting area, a diameter center area and a diameter end area, wherein the diameter center area (23) has a larger outer diameter than the other diameter areas, and a conical transition is formed between the areas of different diameters, and wherein at least one degassing zone (23.2) is formed in the diameter center area (23), the degassing zone having a housing recess, and at least one suction port (15) extending from the housing recess to the outside of the housing (10). The flow channel formed between the extruder screw shaft core and the inner wall of the inner housing recess (18) is designed as an annular expansion nozzle, wherein the outer diameter of the extruder screw thread (32, 33, 34) is constant and the radial flow channel height increases.

Description

Technical Field

[0001] The invention relates to an extruder for viscosity-enhancing preparation of fusible polymers. Background Art

[0002] In plastics technology, extruders are used to plasticize and process polymers. If the goal is only plasticization, many designs are available as single rotors with one extruder screw or twin rotors with two extruder screws, whereby the special geometry of the extruder screws results in the plastic being sucked in as solid granules at one end and then melted and ejected as a liquid. In many applications, homogenization and degassing are also carried out, e.g. to remove moisture contained in the solids. The disadvantage here is usually the high shear stresses in the extruder leading to a reduction in the molecular chain length in the polymer and thus to a decrease in viscosity. These effects occur especially with high-viscosity polymers. For some applications, however, the viscosity may not drop too much during plasticization in the extruder, since a certain viscosity of the molten polymer is required for subsequent processing. This applies, for example, to the recycling of hydrolyzable polycondensates such as polyester (PET), especially in connection with demanding downstream processing operations such as the production of textile plastic fibers.

[0003] DE 22 37 190 A describes an extruder for processing rubber compounds, which is equipped with an extruder screw. Degassing is improved by increasing the screw diameter in the suction zone, while the flight depth of the extruder screw flight is kept constant compared to the feed zone and discharge zone to ensure a constant feed rate. Applications for polymers other than rubber and selective influencing of viscosity are not mentioned.

[0004] An extruder with a multi-rotation system for plasticizing, in particular for processing polymers with a simultaneous increase in viscosity, is known from EP 1 434 680 B1. This so-called MRS extruder has an extruder screw with a multi-screw extruder section, in which several driven satellite screws are arranged around the main screw. They rotate as a whole with the extruder screw and at the same time they rotate about their own axis. In the multi-screw extruder section, the degree of mixing is high and the surface area is increased, so that the exhaust system provided in this area on the extruder housing is particularly effective. Due to the effective extraction of a large part of the water contained in the polymer, a significant chain elongation can be achieved in the polycondensate, thereby increasing the intrinsic viscosity. Since the impurities are separated at the same time, the MRS extruder is particularly suitable for recycling PET and high-purity PET can be obtained directly from the recycled material in continuous operation and can be used for beverage and food packaging without further downstream processing.

[0005] While the required viscosity for beverage and food packaging can be easily achieved using the familiar MRS extruder, the problem lies in the application process that requires PET with a higher viscosity, and even if the various process parameters on the MRS extruder are changed, the maximum achievable intrinsic viscosity must be set. Therefore, the increase and decrease of viscosity always occur simultaneously, whereby the effect of increasing viscosity still dominates at the beginning, but eventually balances with the opposite effect. Summary of the invention

[0006] It was therefore an object of the present invention to be able to produce polymer melts, in particular PET, whereby high intrinsic viscosities of at least 0.7 ml / g can be achieved.

[0007] This object is achieved by an extruder having the features described in the invention.

[0008] Surprisingly, success came from a concept that combines the individual design features of the MRS extruder with the familiar single rotor for rubber processing and adds another major innovation.

[0009] Firstly, the solution of the invention is based on the monorotor concept, i.e. on an extruder with only one extruder screw. According to the invention, the satellite screws present in known MRS extruders and having the largest share in polymer shearing are eliminated. In order to increase the peripheral speed in the vacuum extraction zone, a partial diameter increase of the extruder screw is provided in the degassing zone. However, the viscosity increase achievable with the extruder according to the invention is mainly due to the fact that the degassing zone is divided into two parts and comprises the following basic features:

[0010] For reference to extruder screw geometry, the diameters are specified as follows:

[0011] -D1 in the feeding / metering area

[0012] -D2 in the discharge area and

[0013] -D2 is in the middle area where the degassing zone is located.

[0014] The outer diameter of the extruder screw, which is defined by the outer edge of the extruder screw flight on the extruder screw shaft, is significantly increased compared to the preceding feed and metering zone and the adjacent discharge zone and is at least 1.2 to 2.0 times the diameter there. It is preferably substantially constant over the length, resulting in a cylindrical housing. This makes it easier to produce the relevant holes in the housing and small axial displacements between the extruder screw and the housing are possible. Only in the transition zone to the part of the extruder screw before and after this, a conical shape is preferred.

[0015] On the other hand, the outer diameter of the extruder screw shaft core varies greatly in two parts of the degassing zone: it is also large in the upstream initial part, resulting in a shallow flight depth between the parallel parts of the extruder screw flight, and is much smaller in the adjacent end part, resulting in a deeper channel.

[0016] At least one suction port of the housing is located at a position where the thread depth is greater. It can extend to a diameter step of the shaft core between the starting and ending parts.

[0017] The melt, which has been plasticized in the first part of the extruder screw, is strongly compressed in the initial region of the degassing zone, where the free volume in the flight formed between the screw flight, the shaft core and the barrel bore is very small.

[0018] However, in the end area of ​​the degassing zone, the volume is much larger and can hardly be filled by the melt fed in. Therefore, at the diameter step of the corrugated core, the melt suddenly expands into the free volume. The melt flow breaks up and leads to a significant increase in the melt surface area, which enables the volatile substances to be extracted from the melt.

[0019] Due to the elimination of the driven satellite screw, the driving power of the extruder according to the present invention is reduced compared with the prior art.

[0020] If a diameter ratio D2>1.5xD1 is selected, a larger area of ​​interaction between melt and vacuum can be ensured in the extruder vacuum chamber formed by the exhaust port.

[0021] Suitably, the screw length in the degassing zone is 2 x D2. This results in the largest possible area that can be degassed via the degassing port, so the area over which the vacuum is effective is only much smaller than in a conventional multi-screw extruder section.

[0022] For example, if the screw pitches in the intake zone and the degassing zone are substantially the same, it is advantageous if at least one further screw flight having substantially the same pitch is provided in the degassing zone of the screw between the screw flights.

[0023] Due to the increased diameter of the end zone of the extruder degassing zone, the screw flights are spaced much further apart than in the feed / metering zone or the screw discharge zone, with the same pitch as in the feed and metering zones. By providing at least one additional flight within the first flight, there are more shear points along the length of the screw in the degassing zone, and the helical lines between the barrels aid in stirring and feeding, thereby further increasing the surface area of ​​the melt in the degassing zone.

[0024] However, it is also possible, for example, for the screw pitch in the feed / metering zone and in the discharge zone to be essentially the same, but for the screw pitch in the degassing zone to be greater than there.

[0025] As a result, the screw flights become tighter in the degassing zone of the extruder. This can also result in more melt stirring and feeding, increasing the melt surface area in contact with the vacuum.

[0026] Advantageously, the flight depth of the screw flights in the degassing zone is at least 10% of the diameter D2 of the screw in the degassing zone. However, it is also advantageous if the surface area of ​​the flights formed between the screw flights in the degassing zone of the screw is at least 1.5 times the surface area of ​​the channels provided between the screw flights in the entry zone.

[0027] Each of these measures ensures that the channels between the screw flights are not completely filled with melt. In addition, the movement of the screw allows the melt to be stirred in the channels. These measures also serve to ensure that a larger melt surface is simultaneously in contact with the main vacuum at the degassing connection, so that the melt can be degassed more effectively.

[0028] It may be advantageous if the extruder has an adjustable throttle or an adjustable retaining ring at the transition from the metering zone to the degassing zone, by means of which the shear gap can be adjusted. On the one hand, this ensures that only suitably plasticized melt can enter the degassing zone. On the other hand, a certain degree of sealing is achieved, which ensures that the negative pressure in the inlet zone cannot be short-circuited.

[0029] In short, the invention is based on the fact that the fracturing, swirling and stirring of the melt in the suction zone is not achieved by mechanical mixing elements as in the prior art, but by the principle of an expansion nozzle, which is achieved in the present invention by a sudden reduction in the core diameter, with this zone having a constant outer thread diameter and a constant housing bore inner diameter.

[0030] The principle of the expansion nozzle in the degassing zone according to the invention requires, in addition to the mechanical influence on the melt, a temperature influence, ie cooling. The cooling that occurs can be used in various ways as an additional effect in the extruder according to the invention.

[0031] While in prior art MRS extruders, internal cooling of the extruder screw shaft in the degassing zone is almost always necessary to compensate for the huge heat input due to mechanical shearing, according to the invention, this can be omitted at least in the end region of the degassing zone. This at least reduces the required cooling power of the entire extruder screw.

[0032] In some cases, the cooling is so severe that the melt may partially freeze. To solve this problem, heating of the end area of ​​the degassing zone can be provided. For this purpose, for example, the housing can be heated with heating tapes.

[0033] On the other hand, since the extruder according to the invention still has a high heat input due to the shear in the feed and metering zones, it is proposed to abandon external temperature control of the extruder screw and instead circulate a fluid for temperature control through the internal channels of the screw, for which only an external pump is provided, but no external heat exchanger. The fluid is introduced into the internal screw bore at the end of the shaft, heated in the feed and metering zones and possibly in the initial region of the degassing zone, and then transfers the heat of the final region to the cooled melt guided in the deep screw flights, possibly in the initial part of the degassing zone, and then transfers the heat of the final part to the cooled melt guided in the deep screw flights. Discharge takes place at the other end of the extruder screw shaft. The return to the pump is external.

[0034] It is advantageous if the screw has temperature control channels, in particular in the degassing zone, for example in the form of peripheral channels or concentric channels, to ensure fast and precise adjustment of the screw surface temperature. Even the helical flight can form channels. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The invention is explained in more detail below with reference to an exemplary embodiment of an extruder shown in the accompanying drawings. In detail the figure shows:

[0036] Figure 1 This is an external perspective view of the extruder;

[0037] Figure 2 is a perspective view of the extruder screw;

[0038] Figure 3 It is a detailed side view of the extruder screw;

[0039] Figure 4 This is a detailed perspective view of the extruder;

[0040] Figure 5 A detailed side view of the extruder, with a partial cutaway view, and

[0041] Figure 6 is a schematic cross-sectional view of an extruder. Specific embodiments

[0042] exist Figure 1 In the figure, an extruder 100 according to the invention is shown from the outside in a perspective view, wherein the end bearings and drive elements are not shown. In particular, a housing 10 with an inner housing recess 18 is visible, in which an extruder screw 20 is rotatably mounted. The housing 10 has an inlet region 11 with a feed opening 12 for solid polymer particles. An intermediate region 14 with an enlarged diameter is connected via a connecting flange 13, which has at least one housing opening 15 extending into the inner housing recess 18. A suction device, in particular a vacuum pump, is connected to the housing opening 15.

[0043] A further connecting flange 16 is connected to the end region 17 of the housing 10, the diameter of which is reduced again and corresponds approximately to the diameter of the initial region 11. At the end of the end region 17, a housing recess 18, which is designed in particular as a cylindrical bore, opens so that the processed polymer melt can be discharged from this point for further processing.

[0044] Figure 2 The extruder screw 20 is shown in a perspective view. The feed zone 21.1 is used to feed the polymer as solid particles. This is followed by a metering zone 21.2. The feed zone 21.1 and the metering zone 21.2 together form the initial diameter region 21 and have a common spiral extruder screw flight 31. The extruder screw 20 has a discharge zone 25 with the same or similar diameter as the feed zone 21.1 and the metering zone 21.2, and also has only one extruder screw flight 35.

[0045] Between them, in the region of the central diameter, there is a degassing zone 23, which is further divided into an initial region 23.1 and an end region 23.2. In the degassing zone 23, the screw shaft core, whose diameter varies along its length, is surrounded by a total of three extruder screw flights 32, 33, 34 that are intertwined.

[0046] exist Figure 3 In the figure, the cross section of the extruder screw 20 which is important to the present invention is shown in an enlarged side view, and the respective outer diameters D1, D2, D3 are also marked. Exemplary dimensions and geometric relationships are as follows:

[0047] In the metering zone 21 . 2 , the extruder screw flight 31 has a relatively small outer diameter D1 of 110 mm.

[0048] - in the discharge zone 25 , the extruder screw flight 35 has an outer diameter D2 which is 0.8 to 1.2 times the outer diameter D1 , i.e. approximately equal to D1 , but may be 20% larger or smaller;

[0049] In the degassing zone 23 , the extruder screw flights 32 , 33 , 34 have a uniform outer diameter D2 which is at least 1.5 times, in particular 2 times, greater than D2 . In the example, D2 = 190 mm.

[0050] Thus, the outer diameters D1, D2 and D3 vary only between the regions, but are constant within the respective regions 21.2, 23, 25. A conical transition zone 22, 24 is formed between them.

[0051] The core diameter is substantially constant in the metering zone 21.2 and in the discharge zone 25. Slight variations in the core diameter and / or the screw pitch are provided, as is common in extrusion technology, in order to achieve homogenization and compaction and / or to locally influence the flow rate.

[0052] Following the transition from the degassing zone 23 to the discharge zone 25 , for example, the core diameter of the discharge zone 25 is reduced compared to the diameter in the further process, so that melt pressure can be built up again in the discharge zone after the melt pressure in the degassing zone has reached almost zero due to the vacuum there.

[0053] It is important for the invention that the core diameter in the degassing zone 23 decreases abruptly at the transition point 23.4. Whereas in the initial part 23.1 of the degassing zone 23 the core diameter is large and the height of the extruder screw flights 32, 33, 34 and therefore the height of the flight 41 formed between them is small, the core diameter in the end part 23.2 is much smaller. In the example given, in the case of the flight 41 in the initial part 23.1 the thread depth is 4 mm, in particular between 10% and 20% of the outer diameter D2. In the case of the thread 42, in the end part 23.2 the thread depth is 32 mm, so that the height of the thread 42 is increased by 3 to 10 times compared to the thread 41 in the initial part 23.1.

[0054] Figure 3 The double dashed lines in the figure are used to indicate the course of the extruder screw flights. In the metering zone 21.2 and the discharge zone 25, there is only one extruder screw flight 31, 35 in each case. In the degassing zone 23, the double dashed lines only indicate the course of the first extruder screw flight 32. It can be clearly seen that these lines pass through two further extruder screw flights 33, 34 in each case. Thus, a total of three extruder screw flights 32, 33, 34 that are intertwined with each other are formed in the degassing zone 23.

[0055] Figure 4 A perspective view of the transition from the metering zone 21.2 to the degassing zone 23 is shown. For this purpose, the housing parts 11 and 13 (see Figure 1 ) is removed so that the conical transition zone 22 can be clearly seen. The extruder screw flight 31 of the metering zone 21.2 extends in front of the transition zone 22. Already in the transition zone 22, the three extruder screw flights of the degassing zone 23 have their starting point, whereby Figure 4 Only the beginning of the extruder screw flights 32, 33 is visible in the figure. The termination of the extruder screw flight 31 before the transition zone 22 and the beginning of the three extruder screw flights 32, 33 and 34 in the transition zone 22 lead to a premature splitting of the melt flow into three partial flows.

[0056] Figure 5The essential parts of an extruder 100 according to the invention are shown in a partially sectional view. Here, the middle region 14 of the housing 10 is shown in section. On the one hand, this shows that the inner wall 19 of the housing recess extends completely straight in cross section, that is, the housing bore is cylindrical, except for the interruption at the suction opening 15. Furthermore, it can be seen that the outer edges of all three extruder screw threads 32, 33, 34 always end very close to the front of the inner wall 19. In the initial region 23.1, a very narrow channel 41 is formed, through which the entire melt flow must be conveyed. Finally, Figure 5 The course of the extruder core diameter is clearly shown, which suddenly decreases at the transition point 23.4 and then remains smaller in the end region 23.2. As a result, a large-volume channel 42 is formed between the inner wall 19, the parallel sections of the extruder screw flights 32, 33, 34 and the extruder core.

[0057] Figure 6 is a highly exaggerated schematic representation of the dimensional relationships on an extruder screw 20, showing the core diameter and the outer diameter measured on the outer edge of the thread. This representation makes it clear in particular how the thread depth changes over the length of the extruder screw 20. At the end of the metering zone 21, the core diameter increases. The outer diameter D1 remains constant. This reduces the thread depth. The conveyed melt is compressed. In the transition zone 22, the flowable volume expands due to the increase in the outer diameter to D2. This is compensated by a further reduction in the channel depth in the tapered transition zone 22. The aim is to convey the melt to the initial region 23.1 in a way that fills the flow channel. The narrow gap there also increases the shear forces.

[0058] In the middle of the degassing zone 23, the corrugation core diameter suddenly decreases significantly, while the outer diameter D2 of the thread remains unchanged. The volume of the flow channel formed there is no longer filled by the melt fed through the initial area 23.1. This leads to a sudden expansion of the previously highly sheared and therefore also highly heated melt. During the expansion process, the volatile substances contained are dissolved particularly well and can be extracted, as shown by the box arrows.

[0059] This is followed by a narrowing of the multiple flow channels in order to collect the melt-free gas again and to convey it uniformly. To this end, the flow channels initially taper slightly towards the transition zone 24. In the transition zone 24, the flight and the corrugation core each have different taper angles, which also leads to a widening of the flow channels. Between the transition zone 24 and the start of the discharge zone 25, a short constant channel depth is provided before the shaft core diameter increases again, so that the channel depth decreases, while the outer diameter D2 of the extruder screw flight remains constant.

Claims

1. An extruder (100) for the viscosity-increasing processing of fusible polymers, comprising at least a housing (10) with an inner housing bore (18), wherein an extruder screw (20) with at least one spiral extruder screw flight (31, 32, 33, 34, 35) is rotatably arranged, The extruder screw (20) is subdivided into a diameter start region (21), a diameter center region and a diameter end region with respect to its outer diameter, wherein: - a diametrical center region has a larger outer diameter than other diametrical regions; - forming a conical transition zone (22, 24) between diameter regions of different diameters; at least one degassing zone (23) is formed in the diametrical region and has a housing recess from which at least one suction opening (15) extends to the outside of the housing (10), It is characterized in that - wherein the extruder screw (20) is formed in the central diameter region in such a way that the flow channel formed between the extruder screw shaft core and the inner wall (19) of the housing recess (18) is designed as an annular expansion nozzle, the outer diameter of at least one extruder screw thread (32, 33, 34) remains unchanged, and the radial flow channel height increases; and - wherein at least one suction opening (15) is arranged in an end region (23.2) at the end of the degassing zone (23).

2. The extruder (100) according to claim 1, characterized in that: The extruder screw (20) is functionally divided into at least a metering zone (21.2), a degassing zone (23) and a discharge zone (25), wherein: - means for compressing and / or homogenizing the polymer melt are formed on the extruder screw in a metering zone (21.2), which, viewed in the flow direction, extends from a diameter start region (21) across a conical transition region (22) to a diameter center region; and The discharge zone (25) is formed completely in the diameter end region.

3. The extruder (100) according to claim 1 or 2, characterized in that: The extruder screw flights (32, 33, 34) have an outer diameter D2 in a diametral center region which corresponds to at least 1.5 times the diameter D1 in a diametral start region (21).

4. The extruder (100) according to claim 1 or 2, characterized in that: The diametrical center region has an initial region (23.1) and an end region (23.2), and the radial thread depth of the thread formed between adjacent portions of at least one extruder screw thread (32, 33, 34) in the initial region (23.1) is less than the radial thread depth in the end region (23.2).

5. The extruder (100) according to claim 1 or 2, characterized in that: The flight depth of the extruder screw flight (32, 33, 34) in the end region (23.2) of the diametral region is at least three times the flight depth in the initial region (23.1).

6. The extruder (100) according to claim 3, characterized in that The flight depth of the at least one extruder screw flight (32, 33, 34) in the initial region (23.1) of the degassing zone (23) is 1% to 5% of the outer diameter D2.

7. The extruder (100) according to claim 3, characterized in that The flight depth of at least one extruder screw flight (32, 33, 34) in the end region (23.2) is at least 10% of the outer diameter D2 in the degassing zone (23).

8. The extruder (100) according to claim 1 or 2, characterized in that: The thread depth of the at least one extruder screw thread (32, 33, 34) in the end region (23.2) is at least 20 mm.

9. The extruder (100) according to claim 3, characterized in that: The length of the degassing zone (23) is at least 2.0 times the outer diameter D2.

10. The extruder (100) according to claim 1 or 2, characterized in that: The extruder screw (20) has a conical transition zone (22, 24) at the transition from the metering zone (21.2) to the initial zone (23.1) and / or at the transition from the end zone (23.2) to the discharge zone (25), in which the extruder screw flight (31, 32, 33, 34, 35) is interrupted.

11. The extruder (100) according to claim 1 or 2, characterized in that: At least in the degassing zone (23), at least two extruder screw flights (32, 33, 34) with the same pitch, which are intertwined with each other, are formed on the extruder screw (20).

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