METHOD FOR THE PRODUCTION OF POLYMERS IN WHICH FILLERS ARE INCORPORATED AND HOMOGENETICALLY DISTRIBUTED

Description

[0001] The invention relates to a process for producing polymers in which fillers are incorporated and homogeneously distributed, having the features of the preamble of claim 1.

[0002] It is already known that the properties of plastics such as appearance, feel, conductivity, strength, etc. can be influenced by introducing fillers into a corresponding plastic melt. It can be observed that as the particle diameter of the filler decreases, the ratio of the filler's surface area to volume increases. Since this ratio, or the surface area of the filler, significantly determines the properties of the plastics in a number of applications, it is often necessary to use the finest possible particles. Care must be taken to prevent the particles from clumping, as clumping would reduce the aforementioned ratio or the active surface area. Often, efforts are made to use so-called nanoparticles with diameters of less than 100 nm.Due to the increase in the surface-to-volume ratio, it becomes increasingly difficult to prevent agglomeration of the fillers with decreasing particle size, so that classic methods, such as those known from DE 10 2009 013 418 A1, according to which nanoparticles in the form of powder are mixed into the melt, prove to be unsuitable in practice.

[0003] However, it is also known to introduce suspensions containing fillers into a melt and, after mixing the suspension with the melt, to remove the liquid component of the suspension from the melt.

[0004] WO 2011 / 060839 A1 discloses a process for producing polymers containing finely distributed fillers. Polymer feed material is fed into a twin-screw extruder via the hopper, where it is conveyed and melted under pressure. After pressure buildup, a filler suspension is injected into the melt in the conveying and mixing zone, with the melt pressure being higher than the vapor pressure. After homogeneous mixing of the polymer and filler suspension, the mixture is degassed at the end of a conveying and mixing section in a degassing device, where the solvent evaporates.

[0005] However, the process is not suitable for medium to high-viscosity polymers, as the melt's high viscosity compared to the carrier liquid prevents homogeneous mixing of the polymer with the filler. Despite the dispersion of the fillers in a liquid solvent, a high degree of mixing and distribution can only be achieved by using special mechanical stirring and mixing devices, i.e., devices with high drive power and pressure.

[0006] In the process described in DE 2 409 541 A, fillers are incorporated into a polymeric, sticky base substance by finely grinding the fillers and mixing them into the base substance in a ratio of 1:7. The mixing is carried out using mechanical mixing devices such as stirrers and kneading machines.

[0007] DE 10 2005 025 975 A1 describes the production of high-molecular-weight polymers by solid-phase condensation. This process cannot be carried out in a continuous extruder. A connection to the incorporation of extremely fine fillers is not described.

[0008] GB 1 373 155 A describes the targeted depolymerization of rubber and plastic waste in a multi-screw extruder, with the extruder screws arranged side by side in a single plane. Here, too, there is no connection to the incorporation of extremely fine fillers.

[0009] Further prior art processes are described in EP 0 477 634 A2, US 2002 / 077443 A1 and in "Direct process for the preparation of nanosuspensions and their dosing into thermoplastic matrices for the production of nanocomposites" by Mikonsaari Irma (XP055163097).

[0010] The invention is based on the object of specifying a process for producing a filled polymer which is filled with very finely distributed fillers with particle sizes of less than 10 µm, such as nanoparticles, by which process the fillers can be mixed quickly, with low energy consumption and particularly uniformly into a medium to high viscosity polymer melt without causing damage to the polymer.

[0011] To solve the problem, the method with the features of claim 1 is proposed.

[0012] While traditionally, care is taken during further processing to avoid adversely altering a polymer once it has been produced, the invention takes precisely this step: a polymer that can be split into monomers by a reverse reaction is used to accommodate the fillers, and the carrier fluid used for the fillers is precisely the low-molecular-weight cleavage product that is split off during the formation reaction. Thus, according to the invention, the reverse reaction is deliberately initiated, so that depolymerization occurs before and / or during the mixing of the fillers and the homogenization of the suspension, which reduces the viscosity of the polymer.

[0013] For the invention, all polycondensates can be used as polymers in which the reverse reaction to polycondensation can be initiated under the conditions adjustable during melt extrusion, e.g., with regard to temperature and pressure, as soon as the cleavage product is added as a carrier liquid. Polyesters, polyamides, and polycarbonates are thus considered as plastics, and water or monohydric or polyhydric alcohols can be used as the carrier liquid.

[0014] Most polycondensates important in plastics technology release water during synthesis, so that subsequent addition of water initiates hydrolysis as a reverse reaction. Accordingly, the invention is described below with water as the carrier liquid for the suspension, although other suitable carrier liquids are also encompassed by the invention.

[0015] When the carrier liquid is injected into the previously melted polymer, regardless of whether the carrier liquid is already injected as a suspension with the fillers or initially in pure form, hydrolysis or other reverse reaction immediately begins under the influence of pressure and temperature in the twin-screw extruder. Long polymer chains are broken, and the viscosity drops dramatically. As a result, the viscosity differences between the injected suspension and the now low-viscosity polymer melt become blurred. At this stage, a homogeneous distribution can be achieved very quickly and easily, with only minimal mechanical drive power required for stirring and mixing.On the one hand, the ultimately crucial homogeneous distribution of the fillers in the polymer must be achieved, but a homogeneous distribution of the remaining carrier liquid must also be achieved so that local demixing does not occur during the subsequent removal of the carrier liquid.

[0016] Because the melt has a significantly lower viscosity during this process phase, the fillers can be easily mixed into the melt, quickly forming a homogeneous mixture of the fillers, such as nanoparticles, with the melt. The fillers remain suspended until they come into contact with the melt, meaning there is no prior evaporation of the aqueous carrier medium. The fillers do not agglomerate during the injection and mixing process.

[0017] The term "melt" in the context of the present invention refers not only to the remaining polymeric components in molten form, but also to the possibly increasingly split polymer molecules and monomeric components contained in the mixture as a result of the reverse reaction already carried out.

[0018] The removal of the carrier liquid in the next step significantly increases the viscosity of the polymer melt. This is because, for example, removing water, which forms the carrier liquid for the fillers, from the mixture of polymers, monomers, fillers, and carrier liquid, results in much more than just a mechanical separation of the carrier liquid. Removing the water while simultaneously applying pressure and temperature triggers a new polycondensation reaction, which reconnects molecular chains or extends them with monomers, thus increasing the viscosity.

[0019] The basic idea of the invention is therefore to use a liquid not only as a temporary carrier for the fillers, but also to temporarily chemically modify the polymer into which the fillers are to be mixed. This makes fine mixing easy to achieve temporarily, and at the end of the process, the polymer emerges with the same properties as before, or even with a viscosity adapted and thus optimized for the specific application.

[0020] To carry out the process, polymer input material is fed into a twin-screw extruder via an inlet hopper and then melted in the twin-screw extruder. Subsequently, pressure is built up in the melt in the twin-screw extruder via a conveying and mixing section. After the pressure has been built up, a suspension consisting of the fillers and a carrier liquid is injected into the melt in an injection chamber located in the initial area of the conveying and mixing section. The pressure in the twin-screw extruder or in the injection chamber is set or regulated, depending on the polymer input material and suspension, in the range of 50 bar, preferably 25 bar, and in particular to approximately 5 bar above or below the phase transition limit of the carrier liquid.This homogeneously mixes the suspension with the polymer feed material, reducing the melt viscosity. The homogeneous polymer mixture is transferred to a vented extruder at the end of the conveying and mixing section. In the vented extruder, a vacuum is applied to evaporate and remove the carrier liquid from the polymer mixture, increasing the melt viscosity. The fillers remain homogeneously distributed throughout the polymer mixture, creating a polymer melt containing extremely finely distributed fillers, which is then discharged from the vented extruder.

[0021] It is advantageous if a start-up process is initiated via a control device in which initially only carrier liquid is injected into the conveying and mixing section, so that the polymer input material becomes less viscous. After the control device subsequently switches from carrier liquid injection to suspension injection, the suspension can be immediately mixed into the low-viscosity melt. This ensures that, contrary to the usual processing and treatment methods for polyester, for example, the latter becomes low-viscosity through water injection. When switching from water injection to the aqueous suspension, the filler particles can be mixed very effectively into the low-viscosity melt immediately and without great expenditure of force.The aqueous component of the suspension ensures that the subsequently added and freshly melted polymer material also becomes lower in viscosity through hydrolysis, which is desired here but usually avoided wherever possible, resulting in the advantage of easy and good incorporation of the filler particles into the polymer melt.

[0022] It has proven effective to regulate the pressure in the injection chamber between 20 bar and 200 bar, whereby a pressure sensor in the injection chamber transmits the actual pressure values to the control device, which sets the target pressure by influencing an adjustable melt throttle, which is arranged between the twin-screw extruder and the venting extruder, and / or a suspension pump.

[0023] The advantage is that, for a given flow rate and flow rate, the control device regulates the evacuation performance in the venting extruder by adjusting the negative pressure, whereby the viscosity of the discharged melt can be adjusted.

[0024] A particular advantage arises when the time between suspension injection and the start of evacuation is more than 1 second and less than 30 seconds.

[0025] The reversal of viscosity from low to high viscosity by removing water is subject to time limits depending on the material. If the time between the injection of water or the aqueous suspension and the degassing process is too long, the viscosity can no longer be reversed to the desired extent, so the time window should be greater than 1 second and less than 30 seconds.

[0026] It is important that at least 90%, preferably at least 99%, of the carrier liquid is removed from the melt within 1 to 10 seconds during the evacuation process. This rapid removal of the carrier liquid in one step is necessary to ensure that the viscosity reversal from low to high viscosity can be carried out without damaging the melt. Therefore, the time window for mixing the aqueous suspension into the melt and completing the subsequent evacuation must not exceed approximately 30 seconds, as otherwise, depending on the polymer used, damage to the polymer may occur.

[0027] In order to achieve such rapid degassing of the melt, it is advisable to set the vacuum on the venting extruder between 40 mbar and 0.01 mbar.

[0028] It has proven useful that the amount of suspension added is between 0.02 and 25 volume percent of the polymer amount.

[0029] In this way, the hydrolysis deliberately induced according to the invention, which is usually to be avoided, can be controlled in such a way that the desired viscosity of the polymer melt containing filler particles can be obtained in the end.

[0030] To optimally maintain or better regulate the pressure in the twin-screw extruder, it is recommended that an inert gas be metered into the injection chamber. This can be carbon dioxide, nitrogen, or even a noble gas. Carbon dioxide, for example, has a positive effect on the homogenization process, as carbon dioxide dissolves in large quantities both in water as a carrier liquid and in the polymer melt. Like water, it further reduces the viscosity of the melt and significantly influences the phase transition of the water from liquid to gas.

[0031] In terms of the device, the object is achieved in particular by a cascade circuit of a twin-screw extruder for melting polymer starting material and for mixing it with a suspension using a single-screw vented extruder, wherein the cascade circuit is assigned a control device which controls both the start-up of the cascade circuit and the actual production of the polymer containing finely distributed fillers.

[0032] By cascading a twin-screw extruder for melting and mixing, which can be constructed cost-effectively without a vacuum connection and operated economically without a vacuum, and a vented extruder, two separately driven extruders are used, each of which can be regulated to the optimum speed and power consumption, so that optimal conditions for melting the input material, introducing the suspension and mixing in the vented extruder can be set to the optimum properties for removing the carrier liquid.

[0033] It is advantageous that the vented extruder has a multi-screw extruder part arranged in an evacuation drum, which essentially consists of a rotatably driven guide body which accommodates the plurality of screws in cylindrical recesses, wherein the cylindrical recesses of the guide body enclose the screws in their respective longitudinal extent greater than 180° and less than 360°, and the opening slots thus created are aligned with the wall of the evacuation drum, and wherein the screws are rotationally driven in the guide body via toothed gears.

[0034] The use of a vented extruder, as essentially known from EP 1 434 680 B1, ensures that the melt undergoes a large surface exchange during transport through the multi-screw extruder section, whereby the vacuum can come into contact with the melt over a large area, so that water contained in the melt can evaporate very quickly and be effectively removed from the melt.

[0035] It is advantageous to assign at least one permanently open injection nozzle to the twin-screw extruder in the area of the injection chamber and to connect the injection nozzle(s) to a carrier liquid supply and / or to a suspension pump, wherein the suspension pump is coupled to a suspension silo having an agitator.

[0036] To prevent the filler particles from precipitating or agglomerating in the suspension, the initial use of an agitator is recommended. The use of an open injection nozzle, which presents no further obstacle to the suspension that could lead to agglomeration of the filler particles, also contributes to ensuring that fillers with particle sizes smaller than 10 µm, preferably nanoparticles with particle sizes in the range of 200 nm to 300 nm, and not agglomerates of filler particles, actually enter the melt. This allows for particularly rapid mixing of the filler particles and a good, homogeneous mixing of the melt with the filler particles.

[0037] It is also advantageous to thermally insulate at least one injection nozzle from the injection chamber and / or to control its temperature separately. This ensures that the suspension's carrier liquid, e.g., water, does not begin to evaporate before the suspension reaches the interior of the twin-screw extruder.

[0038] The invention is explained in more detail with reference to a drawing.

[0039] Figure 1 shows a twin-screw extruder 1 and a vented extruder 2, both connected in cascade. The twin-screw extruder has an inlet hopper 3 through which polymer input material can be gravimetrically fed into the twin-screw extruder. In the twin-screw extruder 1, the polymer input material is plasticized and transported via a conveying and mixing section 4 to the outlet 5' of the twin-screw extruder 1. The outlet 5' of the twin-screw extruder 1 merges directly into the inlet 6 of the vented extruder 2 via a throttle 5.

[0040] In the area of the conveying and mixing section 4, the twin-screw extruder 1 has an injection chamber 7 with an injection nozzle 7', which can be fed by a water supply 8 or a suspension pump 9, whereby the suspension pump 9 can draw the suspension from a suspension silo 10. The suspension silo 10 has agitators 11, via which the filler particles remain as evenly distributed as possible in the suspension without agglomerating the filler particles in the suspension.

[0041] The devolatilizing extruder 2 has a vacuum connection 12, ie in this case only one devolatilizing shaft, via which the melt conveyed from the twin-screw extruder 1 into the devolatilizing extruder 2 can be devolatilized in the area of an evacuation drum 13.

[0042] The melt is fed to a discharge tool 15 via a discharge 14, e.g. a screw or pump.

[0043] A control device 16 is coupled to the drive 17 of the twin-screw extruder 1 and to the drive 18 of the venting extruder 2. This allows the plasticizing and mixing processes, as well as the venting process, to be controlled separately and optimally. The control device 16 also controls the intake of the inlet hopper 3, the suspension pump 9, the agitator 11, the water supply 8, the adjustable conveying and shearing elements 19 of the venting extruder 2, the vacuum connection 12, and the discharge 14 (not shown in detail). Other sensors not shown, such as the pressure sensor 20 or pressure gauges at the vacuum connection, temperature sensors, tachometers, etc., also send signals to the control device 16, which controls both start-up and production via the cascade circuit.

[0044] In the twin-screw extruder 1, the polymer input material is first plasticized and transported via the conveying and mixing section 4 to the outlet 5. The control device 16 controls the drive 17 with regard to speed and power, as well as the throttle 5 with regard to the pressure buildup in the conveying and mixing section 4, depending on the signals from a pressure sensor 21.

[0045] Inert gas can be fed into the injection chamber 7 under a correspondingly adjustable pressure via a gas connection 22, which can also be controlled by the control device 16, in order to be able to influence the viscosity of the melt even better.

[0046] For the start-up process, the water supply 8 is first switched on via the control device 16, while the suspension pump 9 is not yet driven. Once a corresponding pressure has built up on the conveyor line, which can be determined by the pressure sensor 21, and the polymer melt has a desired low viscosity due to the water supply, the water supply 8 is switched off via the control device 16 and the suspension pump 9 is switched on so that the suspension pump 9 can then convey the suspension from the suspension silo 10 into the twin-screw extruder 1. Here, the suspension is mixed into the polymer melt as optimally as possible while controlling the speed and power of the drive 17. After the mixed polymer melt has left the twin-screw extruder 1, the melt is fed into the evacuation drum 13 of the vented extruder 2 via conveying and shearing elements 19.The conveying and shearing elements 19 and the signals from the pressure sensor 20 serve to meter the melt before it enters the evacuation drum 13 so that an optimal amount of melt is present in the evacuation drum, which is circulated and transported via screws. The melt offers the largest possible and constantly renewed surface to the vacuum, so that the degassing of the melt can occur very quickly and effectively. As a result, although shear forces are also introduced into the melt via the evacuation drum, the chain length of the polymer molecules becomes longer again. The polymer melt becomes more viscous. By adjusting the vacuum using the control device 16, the desired viscosity of the polymer melt can be set at a preset flow rate and flow rate.

[0047] At the end of the evacuation drum, the melt is fed to the discharge tool 15 via the discharge 14. List of reference symbols

[0048] 1 Twin-screw extruder 2 Degassing extruder 3 Inlet hopper 4 Conveying and mixing section 5 Throttle 5 Output 6 Input 7 Injection chamber 7 Injection nozzle 8 Water supply 9 Suspension pump 10 Suspension silo 11 Agitator 12 Vacuum connection 13 Evacuation drum 14 Discharge 15 Discharge tool 16 Control device 17 Drive 18 Drive 19 Conveying and shearing elements 20 Pressure detection 21 Pressure sensor 22 Gas connection

Claims

1. Process for the production of polymers in which there are fillers incorporated and homogeneously distributed, where: - the particle sizes of the fillers are below 10 µm; - a polymer starting material is input into a twin-screw extruder (1) and is melted there to give a melt, and - in a conveying and mixing section (4), a suspension, which consists of the fillers and of a carrier liquid, is injected into the melt, characterized - in that the melt viscosity is reduced by injection of the carrier liquid in the conveying and mixing section (4) in that a cleavable polycondensate is used as polymer and low-molecular-weight cleavage product arising during the polycondensation is used as carrier liquid, and therefore the molten polymer is at least to some extent depolymerized within the conveying and mixing section (4); - in that the mixture, which consists of the melt whose viscosity is reduced by cleavage, of the remainder of the carrier liquid and of the fillers, is homogenized, and - in that, after the homogenization, the viscosity of the melt is finally in turn increased by use of a devolatilizing extruder (2) in which, for the viscosity increase, a polycondensation is carried out, in that a vacuum is applied and the cleavage product is removed from the devolatilizing extruder (2) by means of the vacuum.

2. Process according to Claim 1, characterized in that, before the suspension is injected, carrier liquid is injected into the melt in order to induce the cleavage.

3. Process according to Claim 1 or 2, characterized - in that a hydrolysable polycondensate is used as polymer and water is used as carrier liquid, and therefore the melt is hydrolyzed within the conveying and mixing section (4); - in that the mixture of the melt whose viscosity is reduced by hydrolysis, of the remainder of the water and of the fillers is homogenized, and - in that, after the homogenization, the viscosity of the melt is finally in turn increased by use of a devolatilizing extruder (2) in which, for the viscosity increase, a polycondensation is carried out, in that a vacuum is applied and the water is removed from the devolatilizing extruder (2) by means of the vacuum.

4. Process according to Claim 3, characterized in that the polymer is polyester.

5. Process according to any of Claims 1 to 4, characterized in that a polycondensate produced with elimination of a monohydric or polyhydric alcohol is used as polymer, and a monohydric or polyhydric alcohol is used as carrier liquid.

6. Process according to any of Claims 1 to 5, characterized - in that a control device (16) controls the individual process steps and initiates a start-up procedure in which initially only carrier liquid is injected into the conveying and mixing section (4) to reduce the viscosity of the polymer starting material, and - in that after subsequent switchover by the control device (16) from carrier-liquid injection to suspension injection the suspension is incorporated into the low-viscosity melt.

7. Process according to any of Claims 1 to 6, characterized in that the pressure in an injection chamber (7) of the twin-screw extruder (1) is between 25 bar and 50 bar.

8. Process according to Claim 7, characterized in that the pressure in the injection chamber (7) is adjusted or controlled to 5 bar above to below the phase-transition boundary of the carrier liquid.

9. Process according to Claim 7 or 8, characterized in that the pressure in the injection chamber (7) is controlled to between 20 bar and 200 bar, where a pressure sensor (21) in the injection chamber (7) provides the actual pressure values to the control device (16), which sets the required pressure by influencing an adjustable melt-flow restrictor (5) arranged between the twin-screw extruder (1) and the devolatilizing extruder (2), and / or by influencing a suspension pump (9).

10. Process according to any of Claims 7 to 9, characterized in that an inert gas is added in the region of the injection chamber in order to maintain or improve control of the pressure.

11. Process according to any of Claims 1 to 10, characterized in that the control device (16) controls the evacuation rate in the devolatilizing extruder (2) by adjusting the reduced pressure for a specified flow rate and flow velocity, thus permitting adjustment of the viscosity of the discharged melt.