Process for recovering polyolefins

Abstract

The invention relates to a method for recovering polyolefin material mixed with contaminants, comprising the following method steps: (I) sorting the material, (II) comminuting the material to form flakes, (III) washing the flakes, (IV) sorting the flakes, (V) a cleaning step, (VI) melting the flakes, and (VII) extruding a melt in an extruder (8). A cleaning step (V) is carried out such that the melt (c) is contacted or mixed with the entrainer (d) and the contaminants are separated together with the entrainer as a load entrainer (e) from the melt.

Description

Methods for recycling polyolefins Technical Field

[0001] This invention relates to a method for recycling polyolefin materials containing contaminants. Background Art Existing Technology

[0002] Products made from polyolefin plastic materials, particularly polyethylene and polypropylene, are susceptible to contamination by external influences (e.g., the contents in the case of packaging) throughout their lifespan. Contaminants can penetrate deep into the material. However, printing inks on packaging, external or internal lubricants, additives (such as antioxidants), anti-blocking additives, or many other added components can pose problems for reuse as recycled materials, whether directly as is or in their degraded form. All low-molar components added to the polymer matrix during the product's original use or absorbed through environmental contact can, in principle, be re-released during subsequent use as recycled materials, potentially leading to sensory impairment and toxicological risks, such as when used in consumer packaging applications. Depending on the application and concentration, components with a molecular size of up to 1000 g / mol are considered potentially hazardous.

[0003] Known methods for separating low molecular weight impurities involve rinsing polyolefin flakes with heated ambient air at elevated temperatures before extrusion and melt formation, or rinsing or subjecting finely powdered polyolefins to vacuum and temperature to remove volatile substances. These methods are limited by the low melting point of polyolefins and the corresponding agglomeration of the material above that point. Agglomerated material can no longer be adequately drawn into the extruder.

[0004] Similar methods are also known in which, instead of a vacuum, a fluid (particularly nitrogen, CO2, or vapor) is drawn through the material to be cleaned before the extruder.

[0005] Methods using degassing screws are also known, which degas the melt directly in the extruder or in a downstream melting reactor under vacuum, thereby degassing volatile components very rapidly. These methods are limited by the effective surface area available for degassing and the time available for degassing.

[0006] Another known method is to decontaminate polyolefins in the form of films, threads, strands, or granules after extrusion, under vacuum and elevated temperatures, or by allowing a medium to flow through at elevated temperatures (air, steam, nitrogen, CO2). These methods are limited by melting point, as films, threads, strands, or granules cannot be industrially processed or bonded above their melting point.

[0007] Known methods utilize solvent recovery, which clean polymers by swelling and extrusion or by targeted dissolution and precipitation. Polymers in a swollen state can exhibit much higher migration properties and therefore possess detergency properties. Summary of the Invention

[0008] The shortcomings of the prior art described herein give rise to the task of proposing a decontamination method that can remove contaminants from polyolefins to be recycled, contaminants that cannot be removed by known methods or can only be removed by difficult means, and achieves the removal of at least 90% of the critical contaminants as required by the European Food Safety Authority (EFSA).

[0009] In the method for recovering polyolefin materials mixed with contaminants, the objective is achieved by the features listed in the characterization section of claim 1. The dependent claims relate to the development and / or advantageous alternative embodiments.

[0010] Preferably, the invention is characterized by performing a cleaning step (V) in which the melt comes into contact with or mixes with an entrainer, and contaminants are separated from the melt along with the entrainer as a loaded entrainer. The entrainer can function in the molten phase of the polymer. This allows contaminants to be removed along with the entrainer, contaminants that would otherwise be difficult or impossible to separate from the polyolefin material. This specifically includes model contaminants such as toluene, chlorobenzene, chloroform, methyl salicylate, phenylcyclohexane, benzophenone, methyl stearate, limonene, butyl salicylate, methyl palmitate, diethylhexyl phthalate, and tris(2-ethylhexyl) trimellitate, which are used to evaluate the cleaning efficiency of the recycling process in so-called challenge tests. The entrainer can reduce the contaminant concentration by at least 90% of the initial concentration. This means that, depending on the initial contamination, EFSA requirements for critical contaminants can generally be met. Due to the particularly large molar mass of tris(2-ethylhexyl) trimellitate (547 g / mol), prior art separation methods can only remove up to 30% from polyolefin materials.

[0011] The steps (I) through (VII) are preferably performed in ascending order. It is also conceivable that these steps may be performed in a different order.

[0012] Essentially, entrainers are additives used in various separation methods to enable the separation of individual substances from a mixture of materials. The entrainer medium, which comes into contact with the melt, is an intentionally added substance that accelerates the transfer of contaminants during polymer degassing or extrusion, or even makes this possible from the outset, leaving primarily the polymer behind during degassing and extrusion itself. These specific contaminants are difficult or impossible to remove using conventional decontamination methods because their molecular weight is typically too large to be extracted at the melting temperature of polyolefins, which is approximately 120°C to 320°C, or because they are not in a gaseous state. Temperatures above 320°C cause the polymer to degrade too quickly; it will burn. Temperatures below 120°C result in insufficient polymer flowability.

[0013] Polyolefin polymers can also exist in forms that prevent or make them difficult to clean, while melt entrainers can facilitate or make cleaning possible.

[0014] In a particularly preferred embodiment of the invention, the polyolefin material is insoluble in the entrainer, and accordingly, a material phase and an entrainer phase (layered or segmented) are present during the cleaning step (V). This allows the entrainer to be easily separated from the melt by degassing in the extruder, and during degassing or pressing, it carries most of the contaminants from the melt into the gas phase. In contrast, cleaning agents that dissolve polyolefin materials must use complex methods (such as thin-film methods) to separate them from the target polymer.

[0015] Advantageously, the entrainer is added to the material in or after the feed zone of the extruder. If the entrainer is introduced into the feed zone, the temperature must be kept low so that it does not evaporate before contacting the melt. If the entrainer is fed into the extruder after the feed zone, a pump must be used to feed it because high pressure exists in that zone of the extruder.

[0016] Preferably, the invention is characterized in that the extruder is operated or constructed such that the volume of the extruder is larger than, at least in certain regions, the volume of the melt contained within the extruder and mixed with the entrainer, and the resulting free volume is provided with a vacuum in order to separate contaminants and entrained media from the melt. The free volume can be achieved, for example, by varying the extruder's feed speed and through volume, or by varying the speed of the melt pump.

[0017] The extruder volume (whether used in conjunction with or independently) can preferably be smaller than the melt containing the entrainer, at least in certain areas, in order to achieve melt extrusion and separation of the entrainer. A pressing process can also be connected after the extruder.

[0018] In another preferred embodiment of the invention, the entrainer is added to the material in a static or dynamic mixer located upstream or downstream of the extruder by mixing the flakes or melt with the entrainer. This allows the polyolefin material in the mixer to swell, thereby improving decontamination. In addition to mixing in the extruder, or instead of mixing in the extruder, the mixing of the polyolefin material with the entrainer can be carried out in the mixer.

[0019] In another preferred embodiment of the invention, the surface of the melt loaded with contaminants is enlarged by an entrainer, preferably by the entrainer forming pores in the melt.

[0020] As described below, a larger surface area improves the separation of contaminants under vacuum or by extrusion.

[0021] Generally, an open melt structure is advantageous. Pores and cavities in the melt increase the surface area exposed to vacuum during degassing. The larger the melt surface area during degassing, the more effective it is. Entrainers that expand the melt surface area are beneficial for degassing.

[0022] Because the structure is open to vacuum, entrained materials and contaminants in these pores are more readily accessible to vacuum, or, after the entrained materials have separated from the pores, residual contaminants in the melt are more easily removed. The bursting of individual or all pores due to pressure differentials can also be specifically utilized.

[0023] Introducing pores and cavities into the melt also facilitates the removal of entrainers and contaminants during the pressing process, similar to a sponge. Here, surface area is also important for contaminant removal during extrusion. During extrusion, the larger the surface area, the better.

[0024] It has been proven useful that polyolefin materials swell with entrainers. Melt-swelling entrainers (such as hexane or heptane) in contact with the polyolefin melt promote the migration of contaminants to the surface because the swollen polymer releases more contaminants during degassing than it does without swelling. Due to the greater molecular distance of the swollen polymer, the entrainer can carry away even larger molecules that would otherwise not be released. Entrainers can have both swelling and pore-forming effects on the polymer melt.

[0025] Advantageously, the swollen or enlarged polyolefin material is extruded in order to remove any remaining load-carrying agent from the material.

[0026] Preferably, the invention is further characterized in that the separated contaminant-loaded entrainer is cleaned and returned to the extruder and / or mixer as treated entrainer. This minimizes the consumption of entrainer, thereby making the decontamination process efficient and cost-effective.

[0027] In a preferred embodiment of the invention, the entrainer is heptane or hexane, and it contacts the melt in an amount of 3 to 9 times, and preferably 3 to 7 times, the weight of the melt, causing the polyolefin material to swell. In this exemplary embodiment, the swelling of the melt further improves decontamination.

[0028] In another preferred embodiment, the entrainer is polar and, in particular, water, and the polar entrainer transports contaminants from the melt to the surface of the melt during phase separation. Due to the lack of compatibility, water always carries the contaminants toward the surface of the melt. It evaporates on the surface in the vacuum zone of the extruder or mixer. Water-soluble contaminants (e.g., formaldehyde) and water-insoluble contaminants (mineral oil) are washed out of the polymer during phase separation and carried into the gas space during evaporation. This is true even if the evaporation temperature of the contaminants has not yet been reached under the existing pressure conditions.

[0029] In another preferred embodiment of the invention, the entrainer is dry ice, in which the melt acquires a porous or foamed structure, and the sublimation of the dry ice carries contaminants into the gas phase. In this exemplary embodiment, the sublimation of dry ice is used to transfer contaminants into the gas phase, which can then be removed by degassing alone or additionally in support of the pressing process.

[0030] Advantageously, the cleaned melt is granulated during the granulation process. Due to the use of melt entrainers, the recovered granules have a very good quality comparable to the original granules. Attached Figure Description

[0031] Further advantages and features will become apparent from the following description of embodiments of the invention with reference to the schematic drawings. In the drawings, in schematic diagrams not drawn to scale: Figure 1: shows a flowchart of a method for recycling polyolefins according to the invention in a first embodiment, and Figure 2: shows a flowchart of a method for recycling polyolefins according to the invention in a second embodiment. Detailed Implementation

[0032] Figure 1 shows a flowchart of a method for recycling polyolefin materials according to the present invention. In particular, it is important to contact the melt with an entrainer in which the polyolefin material is insoluble. Therefore, the entrainer is a melt entrainer and can also be considered an extractant.

[0033] Melt entrainers are intentionally added substances that accelerate the transfer of contaminants during polymer degassing or polymer compression, or even make this possible from the outset, and primarily leave the polymer behind during degassing. Entrainers can prevent or at least reduce long residence times at high temperatures, which, according to the prior art, are necessary and promote the aggregation of polyolefins.

[0034] Entrainers enable the extraction of specific contaminants along with the entrainer itself. These contaminants are too large to be extracted at low melting temperatures or are not in a gaseous state. Even if polyolefins are in a form that prevents or hinders decontamination, entrainers can make decontamination possible.

[0035] The first rotary feeder 2 conveys washed or pre-cleaned and sorted flakes a from the flake template 1 to the flake buffer 3. In the flake buffer, a first vacuum pump 4 draws volatile pollutants into the exhaust gas treatment system 18. The second rotary feeder 5 conveys the pre-cleaned flakes to a heated pressure vessel 6 equipped with a stirrer. In this vessel, the flakes are brought into contact with an entrainer under pressure, producing a mixture b of flakes and entrainer. In the pressure vessel, the mixture is converted into a polymer sponge c, with the entrainer in the pores.

[0036] The polymer sponge c is fed into the extruder 8 under overpressure in a closed system via a conveying unit (e.g., transfer pump 7). At the inlet of the extruder 8, the polymer sponge c can be mixed with an additive package h, which may specifically contain fresh, unused stabilizers and pigments for color compensation. Alternatively, the additive package h can be added in the subsequent melt phase (Figure 2). The additive package can also be added separately in pellet processing machines (e.g., in extrusion equipment for films, tubes, or bottles).

[0037] Figure 2 shows a flowchart of a second embodiment of the method having a second extruder 20. In this embodiment, an additive package h is added to the melt phase of the second extruder 20. According to this embodiment, cleaning is first performed in the first extruder 8, and a second extrusion step (second extruder 20) is provided for adding the additive.

[0038] Depending on the entrainer and polymer variant, the entrainer can cause structural changes in the melt, particularly pore formation, domain formation, bubble formation, layer formation, sponge formation, or it can be uniformly dissolved in the melt.

[0039] Stabilizers may degrade too much in the polymer or be added in too little, or the wrong stabilizer may be present in the recycled material. Add new, suitable, adjusted, and still active stabilizers to the stabilizer package. "New" here means they have not yet reacted with oxygen or free radicals. "Suitable" here means they are not immediately vacuumed again during degassing and are suitable for the application and further recycling steps. "Adjusted" here refers to insufficient quantities, as new products typically require very little stabilizer, but during recycling, a larger quantity is usually needed due to higher temperatures and longer residence times.

[0040] The color of reconstituted granules is usually different from that of new materials; a color correction package may be included in the additive package alongside the stabilizer package.

[0041] In extruder 8, the sheet is melted and the melt undergoes vacuum degassing. During this process, the volume of extruder 8 is intentionally maintained in certain regions larger than the volume required for the melt containing entrainers. The free volume can be achieved, for example, by varying the extruder's feed speed and through volume, or by varying the speed of the melt pump. The free volume is subjected to vacuum, which separates the entrainers and contaminants from the melt. The cleaned melt g is then passed through filter 9 to remove solid contaminants. In the granulation process 10, the melt is converted into granules or pellets.

[0042] Entrainers can also be used to swell the melt, which improves the release of contaminants. Whether used as an adjunct or alone, the extruder volume can be smaller than the melt mixed with the entrainer, at least in some areas, to extrude the melt and separate the loaded entrainer, or an additional pressing process can be added after the extruder.

[0043] During the separation of the entrainer, the swollen polyolefin is extruded to remove any remaining entrainer.

[0044] The entrainer circulates in a separate loop to maximize its reuse and remove contaminants as completely as possible. The loaded entrainer e is fed into processing unit 14. For processing, membrane filtration, semi-permeable membranes, selective precipitation, or chromatographic separation can be used in addition to a condenser column. Other processing methods are also conceivable. The residue k separated from the entrainer, or the separated contaminants, are disposed of. In entrainer template 15, the treated entrainer is mixed with fresh entrainer d to compensate for entrainer loss. The treated entrainer f is returned to pressure vessel 6 using pump 16, and overpressure is established in the pressure vessel.

[0045] The resulting particles are collected in a particle container 11, where a vacuum is maintained by a second vacuum pump 17. This allows for residual degassing and subsequent drying. The extracted gas is fed into an exhaust gas treatment system 18.

[0046] The particles are fed into the particle cooler 13 via a third rotary feeder 12. The particles are cooled by flushing and cooling air blown into the particle cooler 13, thereby removing any remaining contaminants and also feeding it as load exhaust gas h into the exhaust gas treatment system 18. The finally cleaned particles m can be removed from the particle cooler. The three degassing streams collected in the exhaust gas treatment system 18 are cleaned within the system, and the cleaned exhaust gas j can be extracted.

[0047] According to the following exemplary embodiments, various types of entrainers have proven to be effective: Example 1 Melt swelling entrainers (such as hexane or heptane) in polyolefins promote the migration of contaminants to the surface because swollen polymers release more contaminants during degassing than when they are not swollen. Because the molecular distances in swollen polymers are greater, the entrainer can remove even larger molecules that would not otherwise be released. According to a first embodiment, 300% to 700% by weight of heptane is added to a melt volume of bottle-grade HDPE.

[0048] Example 2 This involves adding an entrainer (such as polar water) that is incompatible with the melt to a non-polar polymer (such as polypropylene). Once there are no mixing and shearing elements to repeatedly agitate the water into the material, the polar water tends to separate phase and accumulate on the melt surface. Due to the lack of compatibility, the water seeks out the surface of the melt and carries contaminants from the polypropylene (PP). These evaporate on the surface in the vacuum zone of the extruder or mixer. Water-soluble contaminants (such as formaldehyde) and contaminants that are not readily soluble in water (mineral oil) are washed out of the polymer during phase separation and carried into the gas space during evaporation. This is true even if they do not evaporate at that temperature. An example of an entrainer variant is the addition and dispersion of 1% to 3% by weight of water vapor into the melt volume of the PP melt.

[0049] Example 3 Entrainers compatible with the melt (such as hexane or heptane) dissolve contaminants (such as butyric acid in HDPE). Because of their low solubility upon cooling, the entrainers are repelled by the polymer matrix and thus reach the surface very quickly, where they are separated as a separate phase. Melt-compatible entrainers typically also have a swelling effect. Therefore, it is generally advantageous to extrude the supercooled melt to separate more of the entrained medium. According to a third embodiment, heptane is contacted with the HDPE melt at 300% to 700% by weight of the melt amount.

[0050] Example 4 An entrainer is used to intentionally achieve porous or foamed melt consistency and surface, allowing contaminants to immediately enter the gas phase and be extracted via vacuum. According to a fourth embodiment, 0.5% to 5% by weight of dry ice is added to the melt volume of the PP melt. During this process, the dry ice sublimates and carries away the contaminants.

[0051] Example 5 Using an entrainer intentionally designed to alter the viscosity of the polyolefin allows for a thinner interface to be formed in the degassed extruder, and because of the lower viscosity of the polyolefin, contaminants can exit the polyolefin more quickly. According to the fifth embodiment, 0.3% to 3% by weight of heptane is used to significantly reduce the viscosity of the LLDPE melt.

[0052] Example 6 Washed bottle-grade HDPE flakes (MFI: 0.02 g / 10 min - 10 g / 10 min; 190 °C; -2.16 kg; DIN ISO 1133) are melted at 250 °C in a degassed extruder. Volatile contaminants are separated by vacuum; solid contaminants are separated by melt filtration. Instead of underwater granulation, the melt is swollen in a mixer with heptane (at a ratio of 1 part by weight of HDPE to 7 parts by weight of heptane) at 130 °C, and the swollen HDPE is pumped into a cooling container by a melt pump. Due to the supercooling of the discharged stream, some of the heptane is separated from the dissolved contaminants and can be removed in the free volume of the cooling container. Furthermore, heptane and its contained contaminants are removed from the swollen HDPE by extrusion.

[0053] List of reference numerals : .

Claims

1. A method for recycling polyolefin materials containing contaminants, the method comprising the following steps: (I) material sorting, (II) pulverizing the material into flakes, (III) washing the flakes, (IV) flake sorting, (V) a cleaning step, (VI) melting the flakes, and (VII) extruding the melt in an extruder (8), characterized in that, The cleaning step (V) is performed such that the melt (c) comes into contact with or mixes with the entrainer (d), and the contaminant, together with the entrainer, is separated from the melt as a loaded entrainer (e).

2. The method according to claim 1, characterized in that, The polyolefin material (a) is insoluble in the entrainer (d), and correspondingly, a material phase and an entrainer phase are present during the cleaning step (V).

3. The method according to claim 1 or 2, characterized in that, The entrainer (d) is added to the material (a) in the feed zone of the extruder (8) or after the feed zone.

4. The method according to any of the preceding claims, characterized in that, The extruder (8) is operated or constructed in such a way that the volume of the extruder (8) is greater than, at least in some regions, the volume of the melt (c) contained in the extruder (8) mixed with the entrainer, and the resulting free volume is provided with a vacuum in order to separate the contaminant and the entrainer (e) from the melt (g).

5. The method according to any of the preceding claims, characterized in that, The extruder (8) is operated or constructed in such a way that the volume of the extruder (8) is smaller than, at least in some regions, the volume of the melt (c) contained in the extruder (8) and mixed with the entrainer, so as to achieve the extrusion of the melt and the separation of the loaded entrainer.

6. The method according to claim 1 or 2, characterized in that, The entrainer (d) is added to the material (a) in a static or dynamic mixer (6) upstream or downstream of the extruder (8) by mixing the sheet (a) or the melt with the entrainer (d).

7. The method according to any one of the preceding claims, characterized in that, The surface area of ​​the melt is increased by the entrainer (d), preferably by the entrainer (d) forming pores in the melt.

8. The method according to any of the preceding claims, characterized in that, The polyolefin material (a) is swollen by the entrainer (d).

9. The method according to claim 7 or 8, characterized in that, The polyolefin material (c) containing a swollen or expanded surface is extruded to remove residual load-carrying agent (e) from the material.

10. The method according to any of the preceding claims, characterized in that, The separated entrainer (e) loaded with contaminants is cleaned and returned to the extruder and / or mixer as treated entrainer (f).

11. The method according to any of the preceding claims, characterized in that, The entrainer (d) is heptane or hexane, and is in contact with the melt (c) in an amount of 3 to 9 times, and preferably 3 to 7 times, the weight of the melt, and the polyolefin material (a) swells.

12. The method according to any one of claims 1 to 10, characterized in that, The entrainer (d) is polar and, in particular, water, and the polar entrainer transports contaminants from the melt (c) to the surface of the melt during phase separation.

13. The method according to any one of claims 1 to 10, characterized in that, The entrainer (d) is dry ice, and in its application, the melt (c) is given a porous or foamed structure, and the sublimated dry ice carries the contaminants into the gas phase.

14. The method according to any of the preceding claims, characterized in that, The cleaned melt (g) is granulated in a granulation apparatus (10).

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