Process for the preparation of solid plastic particles from a polycondensate by means of a multi-rotation system

Description

[0001] The invention relates to a method for processing solid plastic particles from a polycondensate by means of a multi-rotation system.

[0002] A fundamental problem with processing polycondensates, particularly hydrolyzable plastics such as PET, in an extrusion process is that a specific residence time and a specific heat input per unit time are required to obtain a homogeneous plastic melt that can be further processed. However, this heat input during the residence time causes the hydrolytic degradation of the plastic if it contains moisture. However, particularly in recycling processes, completely drying the solid before introducing it into the extrusion process would be uneconomical, so PET recycling material is always considered moist. Therefore, solid plastic containing residual moisture is drawn into an extruder, melted, and degassed to remove water as condensate and thereby stop the hydrolytic degradation or even initiate a viscosity-increasing reverse reaction.

[0003] A significant improvement in this context is the multi-rotation system described in WO 2003 033 240 A1. It contains an extruder screw that includes a so-called polyrotation unit between a feed and metering zone for drawing in and melting the plastic and a discharge zone. This unit has a significantly larger diameter than the other zones and also features several rotating satellite screws. The multi-rotation system achieves a significant increase in degassing performance compared to single- and twin-screw systems. Consequently, the residence time of the plastic melt in the polyrotation unit can be kept very short. The publication DE 10 2005 051341 discloses a PET processing process using a multi-screw system.

[0004] The problem remains that at higher moisture contents, extensive hydrolytic degradation has already begun in the metering zone, which often cannot be compensated for in the subsequent polyrotation unit. In any case, the potential of the polyrotation unit to increase intrinsic viscosity can only be used in the overall process to completely or partially eliminate the previous damage, without achieving any improvement beyond the initial properties of the processed plastic.

[0005] To reduce the residence time of the plastic in the metering zone, the screw would have to rotate faster, which in turn would exert more shear and increase the heat input per unit time. This, in turn, promotes the chemical degradation process and further damages the plastic through shearing. While it would theoretically be conceivable to keep the screw speed low and the metering zone short, the external heating power in the extruder section would then have to be significantly increased to melt the plastic at all, which could even lead to burns on the edges of the plastic. The only known solution to the described dilemma is more intensive inline pre-drying of the drawn-in solid before feeding it into the extruder, with correspondingly disadvantageous expenditure of time and money.

[0006] The object of the invention is therefore to provide a method for processing solid plastic particles from a polycondensate by means of a multi-rotation system, with which a reduction of the intrinsic viscosity in the processing process is slowed down or avoided or with which the intrinsic viscosity is even increased.

[0007] The solution according to the present invention consists in a method having the features of claim 1. To carry out the method, the further developed multi-rotation system according to claim 8 is proposed.

[0008] Surprisingly, it was discovered that the invention achieves a significant improvement in the described problem by overcoming the conventional notions of the expert regarding the metering process in the extruder. According to common expert opinion, for example, the pressure in the extruder is an important factor that influences the melting behavior. Furthermore, the aim has always been to transfer only a completely melted and homogenized plastic melt to the next processing stage.

[0009] The invention takes a significantly different approach here. A key feature of the process according to the invention is that the plastic melt still contains clearly visible portions of unmelted plastic particles at the transition from the feed and metering zones. The solids content is at least 5%, preferably even more than 10%. The upper limit should be set at 40% to 50% solids. Because the invention tolerates the plastic not being completely melted and homogenized before degassing, the heat input in the metering zone can be reduced, for example, by reducing external heating, slowing the screw rotation, shortening the first extruder section, and / or internal cooling of the extruder screw.

[0010] Another advantageous feature is that the melting of the still-solid plastic particles is primarily achieved through shock heating in the second extruder section, just before the barrel openings to which the vacuum extraction system is connected. The shock heating is achieved by passing the plastic melt, including the still-solid particles, over the drive shafts of the satellite screws. These screws mesh with a toothing in the barrel bore. As the solid particles are guided through the toothing, high local friction and crushing are created, which not only plasticizes the remaining solid particles very quickly, but also additionally heats the already melted mass fractions in the surrounding area.Since the gears do not cover the entire circumference of the polyrotation unit, the entire volume of plastic melt is not conveyed through the gears, but rather flows form past them via bypasses. However, the effect of the local shock heating also extends into the adjacent areas of the gears.

[0011] The extent of shock heating can be influenced by determining the length of the drive pinions, particularly in relation to the overall length of the polyrotation unit or to the degassing zone as the process-relevant part thereof. According to the invention, an MRS extruder is therefore preferably used in which the torque to be transmitted to the satellite screw only forms the lower limit for the pinion length. Otherwise, however, the pinion length can be selected to be significantly longer than necessary from a strength perspective in order to achieve and enhance the effects described above. Length ratios of 1:40 to 1:6 have proven particularly suitable, with the pinion length being set in relation to the length of the degassing zone immediately following the drive.

[0012] The shock heating takes place immediately before the plastic melt enters the vacuum degassing zone. This ensures that the residence time of the significantly heated and now fully molten plastic, which still contains moisture, until it enters the degassing zone is negligibly short, reducing the exposure time of the moisture to the plastic melt to a fraction of its original value.

[0013] Finally, the invention overcomes the notion that the melting and homogenization of plastic in the extruder must always take place under high pressure. In fact, in the process according to the invention, high pressure only exists in the area of the transition cone between the first and second extruder sections. This is closely followed by the toothed drive area, and adjacent to this, in turn, is the vacuum influence zone. This means that the relatively high back pressure still present at the transition cone is already completely dissipated after a short axial path along the extruder screw, which accounts for significantly less than half, in particular less than 20%, of the length of the polyrotation unit.Already in the area of the drive gearing of the satellite screws, where the shock heating takes place, the pressure in the plastic melt is almost completely dissipated; at least, it is already reduced to such a residual pressure that it no longer has any significance for the plasticizing behavior. In this sense, the process according to the invention achieves a "pressureless plasticization" of the solid particles initially entrained in the plastic melt in the area of the gearing of the satellite screws and the subsequent lengthwise regions until they enter the vacuum window.

[0014] In short, a subcooled plastic melt is essentially created in the first extruder section because not all of the volume fractions have yet been heated above the melting temperature of the processed plastic. The subcooled plastic melt is only reheated shortly before entering the vacuum zone, until the remaining particles melt, releasing the stored residual moisture. The water evaporating from the remaining particles is then immediately sucked away in the vacuum zone before it can even exert its hydrolyzing effect.

[0015] This achieves the following essential effects, which, when the process according to the invention is carried out, significantly reduce the hydrolytic degradation of the plastic melt as well as the damage to the plastic melt caused by shearing during processing: To the extent that water is released during melting in the first extruder section, its damaging effect can only be exerted at a low temperature because the temperature there is deliberately kept at the threshold of the melting temperature. This at least slows down hydrolysis. To reduce heat input, the screw speed in the first extruder section can be kept low; this also reduces the adverse effect of shear. Some of the moisture contained in the plastic particles is not released at all in the first extruder section, but is transported to the next section via the remaining solids. There, release and extraction occur almost simultaneously. The plastic melt is certainly also subjected to strong shearing as it passes through the drive gearing.However, because the vacuum immediately follows, removing water as condensate, and because the temperature is sufficiently high, the polycondensation reaction can begin, leading to molecular chain elongation and repairing the damage.

[0016] In order for the process according to the invention to be carried out as described and to achieve the advantageous effects, there is one control variable in particular that must be specifically monitored and, if necessary, adjusted. This is the gap width at the transition cone and the associated back pressure. If the gap is too narrow, the back pressure increases to such an extent that the conveying capacity of the extruder screw in the first extruder section is insufficient to transfer a constant volume flow to the second extruder section. In this case, the residence time in the first extruder section would increase significantly, which is precisely what is to be avoided.

[0017] A gap that is too wide, on the other hand, increases the flow velocity in the first extruder section. This would result in excessive solids being washed into the following section, which could overload the drive of the satellite screws there and lead to blockages or even damage to the gear teeth.

[0018] The aim of the process according to the invention is, on the one hand, to transfer as much solids as possible so that the moisture contained therein is transported, virtually encapsulated, to the next section and only released very late, close to the extraction point. On the other hand, the solids content should be kept low enough to prevent the pinions from becoming blocked or even allowing unmelted particles to pass through and exit the multi-rotation system on the discharge side.

[0019] The appropriate gap width of the cone gap can be specified structurally depending on the expected viscosity of the plastic melt at the transition cone or can be permanently set before the process is carried out.

[0020] In a multi-rotation system preferred for carrying out the method with the features of claim 8, the gap width can be adjusted by means of an axial displacement of the extruder screw relative to the housing during the process.

[0021] For this purpose, an active control unit can be provided, which, depending on the pressure measured by a pressure sensor upstream of the transition cone, controls an actuator such as a hydraulic cylinder and moves the extruder screw. At high pressure, the extruder screw is pushed slightly forward in the direction of flow, widening the gap. If the pressure drops too far, the opposite movement is forced.

[0022] In practice, the pressure at the transition cone of a multi-rotation system varies greatly, reaching values from 20 bar to 150 bar. During normal operation, the pressure is preferably between 40 bar and 60 bar.

[0023] Based on the example of a multi-rotation system with a diameter of 130 mm for the intake screw and a rotor diameter of 225 mm for the polyrotation unit, the gap width is typically 5 to 10 mm, for example, with an additional adjustment path provided on both sides to be able to react to dynamically changing operating conditions.

[0024] A simple but effective measure is to support the extruder screw on the barrel using at least one spring element, in particular a disc spring. The spring element is subjected to tensile stress because, in a multi-rotation system, the extruder screw is always braced towards the inlet due to the pressure at the tip of the metering zone acting on the cone. This means that the extruder screw is not pushed towards the inlet, as is usual in single-screw extruders, but towards the discharge. It should also be noted that the spring element can only be arranged outside the parts that carry the plastic melt and therefore cannot be positioned on the discharge side. Instead, the spring element must be positioned near the drive for the rotation of the extruder screw and essentially hold it to the barrel, exerting tensile stress. The spring element is located between a stationary and a co-rotating part.The stationary part is connected to the gear via a thread, allowing the entire screw assembly to be moved axially. If the back pressure at the transition cone between extruder sections one and two increases too much, the extruder screw moves axially forward, widening the back pressure gap. Conversely, a decrease in back pressure, due to the spring force, leads to a narrowing of the gap. This creates a balance between the spring force and the propulsive force caused by the back pressure at the transition cone. Due to the high masses and the viscosity of the plastic melt, a spring-damper system is formed that requires no additional damping elements and is sufficiently inert to prevent vibrations.

[0025] The invention is explained in more detail below with reference to the drawings. The figures show in detail: Fig. 1 shows a section of a multi-rotation system; and Fig. 2 shows an extruder screw in side view and a pressure and temperature curve along its length.

[0026] In Figure 1 A section of a known multi-rotation system 100 is shown. An extruder screw is arranged in a housing recess 51 in a housing 50, which is divided into various zones. A polyrotation unit 20 is arranged between a metering zone 12, which serves to homogenize the previously drawn-in and at least partially melted plastic particles, and a discharge zone 30, in which the fully treated plastic melt is discharged. This unit has the following essential features: At the transition from the metering zone 12, a transition cone 21 is formed; a conical gap 52 forms towards the housing 50. This is followed by a drive zone in which pinions 23 of satellite screws 26 run in a rotating ring 24 with internal gearing 24 connected to the housing. Passages 25 are located between the pinions 23. The satellite screws 26 rotate within themselves, while the entire extruder screw rotates, and thus also the rotor in which they are mounted. They extend over the majority of the length of the polyrotation unit 20 and are guided past housing windows 54, to which a vacuum is applied. The satellite screws 26 are mounted with their front tips in a bearing carrier 27, which again features a cone to return from the expanded diameter of the polyrotation unit 20 to the smaller diameter of the discharge zone 30. Accordingly, a further conical gap 53 is formed there.

[0027] The structural design of the multi-rotation system 100 is known in this respect, but differs according to the invention in that the width of the cone gap 52 can be adjusted by an axial displacement of the entire extruder screw relative to the housing 50 in order to use the gap width specifically for pressure control and thereby to influence the proportion of solid components that have not yet melted and are flushed out via the transition cone 21.

[0028] For an understanding of the method according to the invention, Figure 2 the qualitative course of the pressure p and the temperature T versus the axial extension of the extruder screw 101 with its various sections 1, 2, 3 is shown.

[0029] In a feed and metering extruder section 1, solid material is first drawn into a feed zone 11. Pressure is built up in a compression zone 13. In the subsequent metering zone 12, the drawn-in plastic is at least partially melted and homogenized. According to the invention, however, only a portion of the solid material is melted and homogenized, while another portion of 5% to 50%, in particular 10% to 40%, remains as solid in the plastic melt.

[0030] In the temperature curve of Figure 2An average melt temperature is shown, i.e., approximately the average of the respective temperatures of portions of the molten plastic that are in direct contact with the extruder screw and those that are in contact with the inner wall of the housing. However, according to the invention, this still contains solid mass portions with a correspondingly lower core temperature, so that the average melt temperature of the processed plastic in the feed and metering extruder section 1 is below a melting temperature Ts.

[0031] The process is particularly advantageous for the processing of polyester. The melting temperature, depending on the degree of crystallization, is between 235°C and 260°C.

[0032] In order to achieve such a subcooled plastic melt, the extruder screw 101 is cooled at least in the feed and metering extruder section 1. Oil with a feed temperature between 90° and 130°C is used as the heat transfer medium. At the same time, the Figure 2The housing wall (not shown) is heated, for example, to 280°C. Simultaneous heating and cooling in the same section 1 is not a contradiction. The internal cooling serves to dissipate the partial heat generated by the rotation of the extruder screw 101, which is usually higher at this point than required for the process. This is because the screw speed must be matched to the speed required in the multi-screw extruder section 2 and therefore cannot be reduced for the extruder section 1. The heating on the housing, on the other hand, serves to generate a lubricating film of molten plastic, regardless of the proportion of solids in the conveyed plastic melt.

[0033] Due to the heat input from the rotation of the extruder screw 101, the temperature rises slightly during the transition to the multi-screw extruder section 2, although the average temperature of the conveyed plastic volume preferably remains slightly below the melting temperature Ts. Only in the drive zone, i.e., when passing through the area of the drive pinions 23, does the temperature rise sharply, rising significantly above the plastic melting temperature TS. Thus, the plastic is only completely melted precisely there and brought to a temperature level where moisture and contaminants can be extracted by means of the applied vacuum, and the intrinsic viscosity can be increased by promoting the polycondensation reaction.

[0034] The further temperature profile in the discharge extruder section 3, behind the multi-screw extruder section 2, is no longer important for the quality of processing, but is constantly above the melting temperature TS .

[0035] In addition, Figure 2 The pressure curve of the plastic melt in the extruder is plotted over the length of the extruder screw 101. Shown is an example of an extruder screw 101 in which the feed zone 11 is not grooved, resulting in a pressure that gradually increases from there to the transition cone 21.

[0036] Behind the transition cone 21, there are no longer any conveying elements on the extruder screw 101, resulting in an immediate pressure drop. The pressure drops to a vacuum level of almost zero at the satellite screws 26. In the drive zone immediately upstream of the transition cone 21, with the pinions 23, there is therefore no longer any significant pressure, so that the shock heating of the plastic mass that takes place there and causes the plasticization of the remaining solid components takes place virtually pressureless.

Claims

1. Method for processing solid plastics particles from a polycondensate by means of a multi-rotation system (100), having at least the following steps: a) drawing in of the plastics particles and at least partial melting of the plastics particles in a first extruder section (1) with at least one extruder screw (101) rotating in a housing recess (51) of a housing (50); b) transfer of the at least partially melted plastics composition into a second extruder section (2), which is designed as a multi-screw extruder section (2) with a poly-rotation unit (20) and a plurality of satellite screws (26) rotating therein, the diameter of the poly-rotation unit (20) being increased compared with the screw diameter of the first extruder section (1) and a transition cone (21) being formed between the extruder sections (1, 2) and a conical gap (52) being formed in relation to the housing (50); c) forwarding of the plastics composition which has been completely melted in the drive zone (23, 24, 25) through a degassing zone in which a vacuum is applied; d) removal of volatile constituents from the plastics melt in the degassing zone; e) transfer of the plastics melt into a discharge extruder section (3), characterized in that, - in step b), the partially melted plastics composition as supercooled plastics melt contains between 5% by volume and 50% by volume of non-melted and non-dehumidified plastics particles, - in that the remaining plastics particles are subjected to pressureless plasticization by at least parts of the plastics composition being passed through a drive zone (23, 24) which is located downstream of the transition cone (21) in a flow direction and which has exposed drive pinions (23) of the satellite screws (26), and - in that, very shortly before entering the vacuum zone, the supercooled plastics melt is reheated to such an extent that the remaining plastics particles melt and thereby release the stored residual moisture.

2. Method according to Claim 1, characterized in that the solid plastics particles are melted by shock heating when they are passed through the drive zone (23, 24, 25).

3. Method according to Claim 1 or 2, characterized in that the partially melted plastics composition contains between 10% by volume and 40% by volume of non-melted residual particles when it is transferred from the first to the second extruder section (1, 2).

4. Method according to one of Claims 1 to 3, characterized in that the feed and metering zone (10, 12) of the extruder screw (10) is temperature-controlled by a fluid flowing in the interior thereof which has an supply temperature that lies between the glass transition temperature and the melting temperature TS of the plastic of which the plastics particles are composed.

5. Method according to one of Claims 1 to 4, characterized in that the filling level in the multi-screw extruder section (2) is less than 100%.

6. Method according to one of Claims 1 to 5, characterized characterized in that the width of a conical gap (52) between the transition cone (21) of the extruder screw (101) and the housing recess (51) is adjusted via an axial displacement of the extruder screw (101) in relation to the housing (50).

7. Method according to Claim 6, characterized in that the width of the conical gap (52) is adjusted as a function of the pressure at the end of the metering zone (12) of the first extruder section (1), a high pressure leading to an opening of the conical gap (52) and a low pressure leading to a narrowing of the conical gap (52).

8. Method according to Claim 6 or 7, characterized in that the axially displaceably arranged extruder screw is supported on an upstream spring element on the housing and the extruder screw is damped by the viscosity of the amelt in which it is mounted.

9. Multi-rotation system (100) for carrying out the method according to at least one of the preceding claims, having at least one housing (50) with a housing recess (51) which has at least one housing opening (54) in a a degassing zone in which a vacuum is applied, and with an extruder screw (101) which can be rotated in the housing recess (51), the multi-rotation system (100) comprising at least: - a first extruder section (1) with at least one feed zone (11) metering zone (12) on the extruder screw (101); - a second extruder section (2), which is designed as a multi-screw extruder section (2) with a poly-rotation unit (20) and a plurality of satellite screws (26) rotating therein, the diameter of the poly-rotation unit (20) being increased compared with the screw diameter in the first extruder section (1); - a transition cone (21) which is formed between the extruder sections (1, 2) on the extruder screw (101), a conical gap (52) being formed between the transition cone (21) and the housing recess (51); - a drive zone (23, 24) which is located downstream of the transition cone (21) in the flow direction and which has exposed drive pinions (23) of the satellite screws (26); - a discharge extruder section (3), characterized in that the conical gap (52) can be adjusted by means of axial displacement of the extruder screw (101) in relation to the housing (50).

10. Multi-rotation system (100) according to Claim 9, characterized in that the ratio of the length of the pinions (23) of the satellite screws (25) to the axial extent of the degassing zone is 1:40 to 1:6.

11. Multi-rotation system (100) according to Claim 9 or 10, characterized by: - at least one pressure sensor which is arranged upstream of the transition cone (21) in the metering zone (12) ; - an adjusting device by means of which the extruder screw (101) can be displaced axially in relation to the housing (50); - a control unit which is connected to the pressure sensor and the actuating device.

12. Multi-rotation system (100) according to one of the preceding claims, characterized in that provided upstream of the feed zone (11) is a spring element via which the extruder screw (101) is supported on the housing (50).

13. Multi-rotation system (100) according to one of the preceding claims, characterized in that the extruder screw (101) can be temperature-controlled at least in the first extruder section (1) by a fluid flowing in an inner flow channel.

14. Multi-rotation system (100) according to one of the preceding claims, characterized in that the housing (50) can be temperature-controlled at least in the first and second extruder sections (1).

15. Multi-rotation system (100) according to one of the preceding claims, characterized in that a discharge zone (30) of the extruder screw (101) has a diameter that is reduced compared with the poly-rotation unit (20).