Method for preparing a polyolefin composition

Abstract

The present disclosure relates to a method for preparing a particulate polyolefin composition comprising virgin polyolefin and recycled polyolefin material, the method being carried out by a two-stage cascading extrusion process, comprising: feeding the recycled polyolefin material to a first-stage extruder and feeding the virgin polyolefin to a second-stage extruder device. By detecting and calculating data related to the flow rate of the measured feed virgin material and the production rate of the final composition, the flow rate of the virgin polyolefin supplied to the second-stage extruder can be adjusted in order to meet a preset composition target. The method can obtain a final product having a reproducible composition with reduced energy requirements.

Description

Technical Field

[0001] The present disclosure relates to a method for preparing a polyolefin composition containing recycled polyolefin and virgin polyolefin via melt blending. Background Art

[0002] Polyolefins are increasingly being consumed in large quantities for many applications, including packaging for food and other goods, fibers, automotive parts, and a wide variety of manufactured products. However, the said large-scale use of polyolefins is raising concerns about the environmental impact of the waste generated after first use.

[0003] In fact, currently a large amount of waste plastic materials come from the differential recycling of municipal plastic waste (post-consumer resins) mainly composed of flexible packaging (cast film, blown film, and BOPP film), rigid packaging, blown bottles, and injection-molded containers. Typically, a major recycled polyolefin stream for reshaping can be obtained through a step of sorting from other polymers such as PVC, PET, or PS.

[0004] However, the multi-component nature of the recycled materials usually results in low mechanical and optical properties. Therefore, the recycled polyolefin stream is partially used to replace the virgin polymer in polyolefin formulations.

[0005] In order to obtain polymer products based on recycled polymers with a high level of properties, it is common practice to blend the recycled materials with virgin polymers using a compound extrusion production line.

[0006] A common method is to use both recycled materials and virgin materials in the form of pellets. In some cases, the virgin material in pellet form is added to the recycled material in sheet form during the compounding and filtration steps.

[0007] The production process of melt blending virgin materials and recycled materials both in pellet form requires high production costs and is energy-inefficient because both the recycled materials and the virgin materials must be processed by melt extrusion and separately formed into pellets using separate processes, and finally melt-blended to produce the final composition.

[0008] It is known that the melt blending of recycled sheets and virgin polymers can be carried out in a single extrusion step. However, this process has some inherent weaknesses and in several cases cannot ensure the correctness of the final formulation.

[0009] The inefficiency of a single extrusion step is particularly related to the melt filtration step that all materials will have to undergo, so a melt filter with an overdesigned margin is required to handle the cumulative polymer stream. In addition, since most common melt filters used in recycled material compounding involve some material being purged as waste during the melt filter cleaning process, this will result in the original polymer (uncontaminated) also being purged as waste.

[0010] Finally, in the very common case where the recycled material is not in a free-flowing form or has a high moisture content or has a high content of contaminants, it is difficult to ensure a stable and reliable feed of all components. In fact, a portion of the recycled material (moisture and / or contaminants) will separate at a non-constant rate or will not be homogeneously distributed with the original pellets in the feed hopper, resulting in feed instability and inconsistency. For these reasons, it will be impossible to ensure the compositional consistency of different polymer additives and reinforcements as part of the final formulation.

[0011] Some compounding processes are based on a two-stage extrusion process, where the recycled material is processed in a first-stage extruder (including steps of compaction, moisture removal, and contaminant removal), and the molten stream is fed into a second-stage extrusion line where the virgin polymer (usually in pellet form) and reinforcements can also be added.

[0012] JP2019 - 65092 discloses metering the polymer stream from the first-stage extruder to the second-stage extruder by means of a melt pump, in particular a gear pump. By the gear pump rotational speed and its volumetric displacement, the polymer stream processed in the extruder can be estimated, and based on this estimate, an appropriate amount of peroxide as a molecular weight regulator, other polymers, additives, and reinforcements can be fed into the second extrusion stage in order to achieve the target product formulation.

[0013] However, when using a recycled polyolefin stream (especially in the form of flakes) as the feed for the first extruder, this method is no longer reliable. This is because the calculation results are prone to final composition errors since the gear pump volumetric efficiency and polymer melt density may vary according to the composition of the recycled flakes. Therefore, the final polyolefin composition may have a balance between the virgin / recycled parts that is different from the target balance.

[0014] In addition, the gear pump is an expensive piece of equipment that should potentially be replaced by a cheaper alternative.

[0015] Now, it has unexpectedly been found a method capable of producing a polyolefin composition consisting of recycled polymer, virgin polymer, and optionally other components, thus ensuring an accurate product formulation and reducing energy requirements. Summary of the Invention

[0016] Accordingly, an object of the present disclosure is to provide a method for preparing a granular polyolefin composition comprising virgin polyolefin and recycled polyolefin material having a preset relative content, the method being carried out in a two-stage cascade extrusion process, the method comprising the following steps:

[0017] (i) Supplying the recycled polyolefin material to a first-stage extruder and forming a molten recycled polyolefin material;

[0018] (ii) Feeding the molten recycled polyolefin material stream from the first extruder into a feed inlet of a second-stage extruder located after the feed point of the virgin polyolefin;

[0019] (iii) Feeding the virgin polyolefin at a certain flow rate to the feed point of the second-stage extruder device and extruding the polyolefin composition in granular form;

[0020] (iv) Measuring the flow rate of the virgin polyolefin supplied to the second-stage extruder device and measuring the flow rate of the granules of the final polyolefin composition obtained from the second extruder device;

[0021] The method is characterized in that a calculation unit operation device receives data on the measured flow rate of the virgin polymer and the measured flow rate of the granular final polymer composition, and adjusts the flow rate of the virgin polyolefin supplied to the second-stage extruder in response to the difference between the measured flow rate of the virgin polyolefin and the measured flow rate of the granular final polymer composition, so as to produce a final granular polyolefin composition having the preset relative content of virgin polyolefin and recycled polyolefin material. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematically shows a suitable setting for a two-stage cascade extrusion process according to the present disclosure.

[0023] Figure 2 Schematically shows a suitable setting for a two-stage cascade extrusion process according to the present disclosure, which further includes metering peroxide to the first-stage extruder.

[0024] DETAILED DESCRIPTION OF THE INVENTION

[0025] Although multiple embodiments are disclosed, from the following detailed description, other embodiments will become apparent to those skilled in the art. As will be apparent, certain embodiments disclosed herein can be modified in various obvious aspects without departing from the spirit and scope of the claims presented herein. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.

[0026] The prepared polyolefin composition comprises a virgin polyolefin, a recycled polyolefin material, and optionally one or more additives and / or reinforcing agents.

[0027] According to the present disclosure, the term "virgin" defines a newly produced polyolefin that has not been recycled and is prior to its first use. The virgin polyolefin can be derived from the polymerization of olefins such as ethylene, propylene, butene-1, hexene-1, and octene-1, and mixtures thereof.

[0028] Specific examples of olefin polymers are: high-density ethylene polymers (HDPE, having a density higher than 0.940 g / cc), including ethylene homopolymers, and copolymers of ethylene with α-olefins having 3 to 12 carbon atoms; linear low-density polyethylene (LLDPE, having a density lower than 0.940 g / cc), and very low and ultra-low density (VLDPE and ULDPE, having a density lower than 0.920 g / cc to 0.880 g / cc), which are composed of copolymers of ethylene with one or more α-olefins having 3 to 12 carbon atoms and having a molar content of ethylene-derived units higher than 80%; isotactic polypropylene, and crystalline copolymers of propylene with ethylene and / or other α-olefins, having a propylene-derived unit content higher than 85% by weight; copolymers of propylene and 1-butene, having a 1-butene-derived unit content between 1% and 40% by weight; heterophasic copolymers, including a crystalline polypropylene matrix and an amorphous phase comprising a copolymer of propylene with ethylene and / or other α-olefins.

[0029] The above polyolefins can be obtained by polymerizing the corresponding monomers in the presence of any type of polymerization catalyst (such as single-site or heterogeneous ZN catalysts) and using platform technologies known in the art (such as liquid-phase polymerization, gas-phase polymerization, and mixed liquid / gas-phase polymerization).

[0030] According to the present disclosure, the terms "recycled polyolefin material" and "recycled" denote materials recovered from post-consumer waste (PCW) or post-industrial waste (PIW) that contain a fraction made of polyolefin.

[0031] Preferably, the recycled polyolefin material is produced by sorting PCW or PIW aimed at selecting the polyolefin fraction.

[0032] Depending on requirements, the sorting of the polyolefin fraction can be enhanced in order to obtain as pure a polypropylene or polyethylene fraction as possible. Preferably, the recycled polyolefin material comprises a mixture of polyethylene (PE) and polypropylene (PP) polymers in a weight ratio of from 99:1 to 1:99. When there is more PP in the mixture, its weight ratio to PE is preferably higher than 80 / 20, more preferably higher than 90 / 10, and especially from 95 / 5 to 99:1. When there is more PE in the mixture, its weight ratio to PP is preferably higher than 80 / 20, more preferably higher than 90 / 10, and especially from 95 / 5 to 99:1. The polyethylene (PE) fraction can contain one or more of high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE). The polypropylene fraction (PP) can be a propylene homopolymer or a copolymer of propylene with a lower amount of ethylene and / or butene. Additionally, the feedstock can include other polyolefins such as polybutene. In a specific embodiment, the feedstock can also include a polymer mixture that incorporates other materials such as polystyrene (PS), ethyl-vinyl acetate copolymer (EVA), ethyl-vinyl alcohol copolymer (EVOH), polyvinyl chloride (PVC), or a mixture thereof. In a preferred embodiment, the recycled polyolefin material feedstock consists of a mixture of polyethylene and polypropylene that is greater than 80% wt, and preferably greater than 90% wt.

[0033] The final composition is preferably defined by a formulation that determines the nature of the virgin and recycled polyolefin fractions (polymer fractions) employed, the nature of any additives and / or reinforcements optionally present, their quantity, their amounts, and their respective proportions. The compositions of the polyolefin compositions prepared can differ significantly from one another. Preferably, the polymer fraction of the final polyolefin composition comprises a majority of polyolefins. Preferably, the polyolefin portion in the polymer fraction of the final polyolefin composition is from 80 wt.% to 99.98 wt.%, preferably from 95 wt.% to 99.95 wt.%, and especially from 98 wt.% to 99.9 wt.%.

[0034] Generally, the amount of the polymer fraction in the final composition is higher than 50% wt, preferably higher than 60% wt, and more preferably higher than 70% wt, with the remainder being non-polymer fractions such as additives, fillers, and / or reinforcements.

[0035] The present disclosure relates to the preparation of the above granular polyolefin compositions in an accurate and constant component ratio and in an economical and reliable manner by using a two-stage cascade extrusion process.

[0036] According to the present disclosure, the two-stage cascade extrusion process is a polymer processing process that is carried out by operating two extruders in series, where the extrudate from the first extruder is fed to the second extruder.

[0037] According to the present disclosure, the term "granular polyolefin composition" refers to a polyolefin composition obtained in the form of granules at the end of a two-stage extrusion process.

[0038] Suitable extruder devices for the method of the present disclosure are extruders or continuous mixers. These extruders or mixers can be single-stage or two-stage machines that melt and homogenize the polyethylene composition. Examples of extruders are pin barrel extruders, planetary extruders, or co-rotating disk processors. Other possibilities are combinations of mixers with discharge screws and / or gear pumps. Preferred extruders are screw extruders, and in particular extruders configured as twin-screw machines. Particularly preferred are twin-screw extruders and continuous mixers with discharge elements, and in particular continuous mixers with counter-rotating and intersecting twin screws, or the extruder device comprises at least one co-rotating twin-screw extruder. Machines of this type are conventional in the plastics industry and are manufactured, for example, by Coperion GmbH, Stuttgart, Germany; KraussMaffei Berstorff GmbH, Hannover, Germany; The Japan Steel Works LTD., Tokyo, Japan; Farrel Corporation, Ansonia, USA; or Kobe Steel, Ltd., Kobe, Japan. Suitable extruder devices are generally also equipped with units for pelletizing the melt, such as underwater pelletizers.

[0039] In stage (i), the recycled polyolefin material can be fed in the form of shredded flakes or other free- or non-free-flowing forms, such as, for example, fluff, film rolls, or other low bulk density forms.

[0040] Before feeding the shredded recycled polyolefin material into the extruder, it can optionally be compacted and preheated, preferably in a dedicated compactor with a forced feeding function.

[0041] The recycled polyolefin material is then melted in a first extrusion stage, in which degassing, moisture removal, and removal of contaminants are also carried out by means of melt filtration equipment.

[0042] Depending on the amount and particle size of the solid impurities, several designs of melt filtration units can be applied.

[0043] It is preferred to use a self-cleaning melt filter that can operate for several days without manual intervention instead of a filter element.

[0044] A preferred design of the melt filter is based on a circular perforated plate as the melt filter element in which solid contaminants accumulate, and the holes are produced by laser or by machining or by other suitable techniques. The accumulation of impurities can increase the pressure difference of the melt filter. To perform continuous cleaning of the filter element, a rotary scraper removes the accumulated impurities and guides them to a discharge port, which is opened for a short time to remove process contaminated materials.

[0045] This cleaning cycle can be repeated several times (up to several days of operating time) without manual intervention and without stopping production during the time required to replace the filter element.

[0046] Another option for a continuous melt filter is based on the application of a continuous filtering metal belt through which the polymer stream passes. The accumulation of impurities on the metal filter causes an increase in pressure. Therefore, the blocked filter belt section is pushed out of the polymer channel area and then a clean section is automatically inserted.

[0047] Another series of continuous melt filters that may be installed is the so-called backflush continuous screen changer. When contaminants accumulate on the screen pack, a pressure set point or timer automatically initiates a backflush operation to lift and discharge the impurities from the surface of the screen pack before the screen pack is put back into use.

[0048] Multiple screen pockets are used to achieve continuous melt filtration. During backflushing or screen replacement of each pocket separately, part of the available filter area remains online. Each screen can be self-cleaned sequentially as needed depending on the level of contamination and line pressure until the backflush process can no longer effectively remove the embedded contaminants, at which time the screen pack needs to be replaced.

[0049] In stage (ii), the recycled molten polyolefin stream is then fed into a second stage extruder, which is preferably a twin screw extruder.

[0050] In a preferred aspect of the present disclosure, the flow rate of the recycled molten polyolefin stream is unmeasured.

[0051] In a preferred aspect of the present disclosure, the entry point of the recycled molten polyolefin stream in the second extruder stage is located at the feed point of the virgin polyolefin, preferably after the hopper.

[0052] In stage (iii), the virgin polyolefin can be fed in any form, such as flakes, powder or granules. Preferably, it is fed in powder or granular form. Most preferably, it is fed in powder form. The term "powder form" according to the present disclosure means polymer particles whose particle size and particle size distribution are directly derived from the polymerization process and have not been pelletized.

[0053] The polyolefin composition of the present disclosure may further comprise one or more additives and / or reinforcing agents commonly used in the art. As needed, the additives and / or reinforcing agents can be fed into the first extruder or the second extruder or both. For example, a corrosion-resistant additive can be added to the first extruder to prevent corrosion problems caused by chemicals released from the recycled polyolefin material. For example, other additives (such as stabilizers, antioxidants, etc.) can be fed into the second extruder.

[0054] Before the entry point of the melt stream in the first-stage extruder, preferably the virgin polymer and optionally the additives are subjected to melting and mixing in the first section of the second-stage extruder.

[0055] The two melt streams are homogenized in the second-stage extruder, and optionally a reinforcing agent is further added and incorporated, and then they are formed into pellets by methods known in the art as underwater pelletizing, water-ring pelletizing, strand pelletizing.

[0056] In step (iv) of the method of the present disclosure, the virgin polyolefin and optionally one or more additives and / or reinforcing agents supplied are fed to a metering device before being preferably transferred by gravity to the second extruder device for melting and further mixing.

[0057] Preferably, for example according to a loss-in-weight feeder or a mass flow meter, the virgin polyolefin and optionally one or more additives and / or reinforcing agents supplied are fed by using a dedicated continuous metering device associated with the mixing device.

[0058] According to a preferred embodiment of the present disclosure, by adjusting the flow rates of the virgin polymer and the additives and / or reinforcing agents according to the actually metered amount of the granular final polyolefin composition, it is ensured that the relative proportions between the recycled polyolefin, the virgin polyolefin, and the possible additives and / or reinforcing agents in the final polyolefin composition are consistent with a preset formulation composition. In particular, the calculation unit accurately determines the difference between the actually produced amount and the metered amount of the final polyolefin composition, and the metered flow rates of the virgin polyolefin and the possible additives and / or reinforcing agents. This difference exactly corresponds to the un-metered flow rate of the recycled polyolefin. Based on the calculated flow rate of this recycled polyolefin, the calculation unit adjusts the flow rates of the virgin polyolefin powder feeding device and the additives and / or reinforcing agent feeding device via a suitable controller so as to meet the set points for the virgin polymer and the additives and / or reinforcing agents, which are predetermined based on the preset composition parameters of the final polyolefin composition.

[0059] The accuracy of the metering device is much higher than that achievable by using a melt gear pump because it measures the true polymer particle flow produced and is not affected by pumping efficiency or different melt densities.

[0060] Furthermore, by using the method of the present disclosure, high efficiency is achieved since the recycled material and the virgin polymer material can be subjected to one melting and pelletizing stage.

[0061] The melt flow rate of the polyolefin granules of the final composition is preferably measured on the dry polyolefin granules, i.e., preferably downstream of the commonly used underwater pelletizer and centrifugal dryer. In this embodiment, the flow rate of one or more optionally supplied additives and reinforcing agents is adjusted based on the actual amount of polyolefin granules produced in the second extruder device, and preferably different control characteristics and equipment are used to control the flow rate of the optionally supplied additives and reinforcing agents and to control the feeding of the polyolefin powder.

[0062] The melt flow rate of the polyolefin granules produced in the second extruder device is measured continuously or discontinuously to determine the total production rate. Preferably, the granule flow rate is measured by a granule flowmeter. Suitable granule flowmeters can employ impact plates, measuring troughs, or Coriolis measurement techniques. Such solid flowmeters are commercially available, for example, from Schenck Process, Whitewater, WI, USA or Coperion K-Tron, Gelnhausen, Germany.

[0063] Alternative devices and methods can be applied, such as a weighted movable belt. An example of a discontinuous measurement of the granule flow rate is obtained by measuring the weight increase in the granule collection bin and calculating the granule flow rate based thereon.

[0064] The granule flowmeter is preferably equipped with a controller. As already disclosed, the controller operated by the computing unit can adjust the speed of the feeding devices for supplying the virgin polyolefin and the additives and / or reinforcing agents to the extruder according to the information on the amounts of the virgin polyolefin and the additives and / or reinforcing agents required to be supplied in the preset formulation.

[0065] The same feeding, metering, and control system is operated by the same computing unit, and if used to change the melt flow rate of the recycled polyolefin, the computing unit can also adjust the feeding rate of the peroxide (or an equivalent viscosity reducer) to the first extruder.

[0066] When in use, the peroxide feeding device preferably supplies the peroxide to the feeding point of the extruder for feeding the recycled polyolefin.

[0067] In this embodiment, the melt flow rate measuring device is optionally associated with the outlet of the first stage extruder. Preferably, the device is capable of measuring the melt flow rate online, and the resulting value constitutes an input to the computing unit. Based on the difference between a preset melt flow rate value of the recycled polyolefin material and the measured melt flow rate value, the computing unit, via a suitable controller, operates the peroxide feeding device by adjusting the feeding rate so as to make the measured melt flow rate value consistent with the preset melt flow rate value.

[0068] The computing control unit can be any computing device capable of performing the determination functions of the present disclosure. The computing unit can be a programmable computing controller (PLC) suitable for controlling manufacturing processes such as assembly lines, machines, robotic devices, or any activity requiring high reliability, ease of programming, and process fault diagnosis.

[0069] In a preferred embodiment, by operating in cooperation with a local controller, the local controller sends the data received from the metering device to the computing unit and directly controls the feeding device based on the output received from the computing unit.

[0070] Referring Figure 1 , the recycled polyolefin is supplied to the hopper (2) of the first stage extruder (1), which is provided with a vacuum degassing and melt filter section (4) for moisture removal (3). The molten material flow is supplied via a pipeline (5) to the inlet point (6) of the second stage extruder (7). The virgin polymers in the silos (8, 9) are fed via a pipeline (10) to the second stage extruder (7), and this pipeline receives the polymer through a feeding device (11) equipped with a loss-in-weight metering device (12) or through a feeding device (14) equipped with a mass flow meter metering device (13). Additives and reinforcements can be supplied via feeding devices (15, 17) provided with metering devices (16, 18). The second stage extruder (7) is associated with a slurry underwater granulator (19), which is supplied with water via a pipeline (20), a water circulation pump (21), and a water tank (22). The pellets coming out of the granulator are conveyed via a pipeline (23) to a rotary dryer (24). Then the dried pellets are fed via a pipeline (25) to a pellet metering device (26) and further fed to a storage container (not shown).

[0071] The calculation unit (27) receives input data (28) from the granule metering device (26) and from the metering devices (12, 14, 16, 18) of the feeding devices (11, 13, 15, 17) via the controllers (33, 34), and sends output data (29, 30, 31, 32) via the controllers (33, 34) to the feeding devices (11, 13, 15, 17) to adjust the feeding rate, and sends the output data to the first-stage extruder motor (1) and the second-stage extruder motor (2) to adjust the total flow rate according to the metered total flow rate.

[0072] Reference Figure 2 , the on-line MFR measuring device (35) positioned on the pipeline (36) provides an input (37) to the calculation unit (27), which sends an output (38) to the controller (39) to adjust the speed at which the feeding device (40) equipped with the metering device 41 supplies peroxide to the hopper (2) via the pipeline 42.

Claims

1. A method for preparing a granular polyolefin composition, comprising virgin polyolefin and recycled polyolefin material having a preset relative content, the method being carried out by a two-stage cascade extrusion process, including the following steps: (i) Supplying the recycled polyolefin material to a first-stage extruder and forming a molten recycled polyolefin material; (ii) Feeding the molten recycled polyolefin material from the first-stage extruder into a feed inlet of a second-stage extruder after a feed point of the virgin polyolefin; (iii) Feeding the virgin polyolefin at a certain flow rate to the feed point of the second-stage extruder and extruding the polyolefin composition in granular form; (iv) Measuring the flow rate of the virgin polyolefin supplied to the second-stage extruder and measuring the flow rate of the granules of the final polyolefin composition obtained from the second-stage extruder; The method is characterized in that a calculation unit operation device receives data on the measured flow rate of the virgin polyolefin and the measured flow rate of the granules of the final polyolefin composition, and adjusts the flow rate of the virgin polyolefin supplied to the second-stage extruder in response to the difference between the measured flow rate of the virgin polyolefin and the measured flow rate of the granules of the final polyolefin composition, so as to produce granules of the final polyolefin composition having the preset relative content of the virgin polyolefin and the recycled polyolefin material.

2. The method according to claim 1, wherein the virgin polyolefin is derived from the polymerization of olefins.

3. The method according to claim 1, wherein the virgin polyolefin is derived from the polymerization of olefins selected from ethylene, propylene, butene-1, hexene-1, octene-1 and mixtures thereof.

4. The method according to claim 1, wherein the recycled polyolefin material comprises a mixture of polyethylene (PE) and polypropylene (PP) polymers in a weight ratio of 99:1 to 1:

99.

5. The method according to claim 1, wherein the polyolefin composition further comprises additives and optionally reinforcing agents.

6. The method according to claim 1, wherein the polyolefin composition comprises a polyolefin portion accounting for 80 wt.% to 99.98 wt.% of the total polyolefin composition.

7. The method according to claim 1, wherein the recycled polyolefin material in the form of flakes is compacted and pre-heated before being fed into the first-stage extruder.

8. The method according to claim 1, wherein In the first-stage extruder stage, the molten recycled polyolefin material undergoes degassing, moisture removal and contaminant removal through a melt filtration device.

9. The method according to claim 1, wherein the entry point of the molten recycled polyolefin material in the second-stage extruder stage is located after the feed point of the virgin polyolefin.

10. The method according to claim 1, wherein the virgin polyolefin is fed in powder or granular form.

11. The method according to claim 1, wherein the virgin polyolefin is fed in powder form.

12. The method according to claim 1, wherein in step (iv), the virgin polyolefin and optionally one or more additives and / or reinforcing agents supplied are fed by using a continuous metering device associated with the mixing device.

13. The method according to claim 12, wherein the metering device is a loss-in-weight feeder or a mass flow meter.

14. The method according to claim 1, wherein the flow rates of the virgin polyolefin and the additives and / or reinforcing agents are adjusted according to the actually metered amounts of the particles of the final polyolefin composition, so that the relative proportions between the recycled polyolefin, the virgin polyolefin, and the optional additives and / or reinforcing agents in the final polyolefin composition are consistent with a pre-set formulation composition.

15. The method according to claim 14, wherein the calculation unit determines the difference between the actually produced amount and the metered amount of the final polyolefin composition, and the metered flow rates of the virgin polyolefin and the optional additives and / or reinforcing agents.

16. The method according to claim 15, wherein the calculation unit adjusts the flow rates of the virgin polyolefin feeding device and the additives and / or reinforcing agents feeding device via a suitable controller so as to meet the predetermined set points for the virgin polyolefin and the additives and / or reinforcing agents.

17. The method according to claim 1, wherein the flow rate of the particles of the final polyolefin composition is measured via a particle flow meter on the dry polyolefin particles.

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