CH721255A2 - Process for recycling polyolefins - Google Patents

Abstract

The invention relates to a process for recycling contaminated polyolefin material, comprising the following process steps: (I) material sorting, (II) comminuting the material into flakes, (III) washing the flakes, (IV) flake sorting, (V) cleaning step, (VI) melting the flakes, and (VII) extruding the melt in an extruder (8). The cleaning step (V) is carried out by contacting or mixing the melt (c) with an entraining agent (d), and separating the contaminants from the melt together with the entraining agent as loaded entraining agent (e).

Description

Field of the invention

[0001] The invention relates to a process for recycling polyolefin material containing contamination.

State of the art

[0002] Products made of polyolefin plastics, particularly polyethylene and polypropylene, are contaminated during their life cycle by external influences (e.g., packaging contents). The contaminants penetrate deep into the material. However, printing inks on the packaging, external or internal lubricants, additives such as antioxidants, antiblocking additives, or many other added components can also pose a problem for reuse as recycled material, either directly or in their degraded form. In principle, any low-molecular components added to the polymer matrix during the product's original use, or which it has absorbed through contact with the environment, can be released as recyclate during subsequent use, which can lead to sensory impairments and toxicological risks, for example, when used in packaging applications close to the end user. Depending on the application and the contents, components with a molecular size of up to 1000 g/mol are considered potentially critical.

[0003] For the removal of low-molecular-weight impurities, processes are known in which polyolefin flakes are flushed with heated ambient air at elevated temperatures prior to extrusion and melt formation, or in which polyolefin as a fine powder is either flushed or subjected to vacuum and heat to remove volatile substances. These processes are limited by the low melting point of polyolefins and the resulting clumping of the material when the melting point is exceeded. Clumped material can no longer be adequately drawn into the extruder.

[0004] Analogous processes are also known in which, instead of a vacuum, a fluid, in particular nitrogen, CO2 or steam, is drawn through the material for purification in front of the extruder.

[0005] Also known are processes using a degassing screw, which degasses the melt directly in the extruder or in a downstream melt reactor using a vacuum, thus rapidly degassing volatile components. These processes are limited by the effective surface area that can be used for degassing and the time available for degassing.

[0006] Also known are processes that decontaminate polyolefins in the form of a film, thread, strand, or granulate after the extruder under vacuum and elevated temperature or with a flowing medium (air, steam, nitrogen, CO2) at elevated temperature. These processes are limited by the melting point, since a film, thread, strand, or granulate cannot be industrially processed or bonded above the melting point.

[0007] Recycling processes using solvents are known that purify the polymer by swelling and pressing, or by targeted dissolution and precipitation of polymers. Polymers in the swollen state can have much higher migration properties and thus decontamination properties.

Object of the invention

[0008] The disadvantages of the described prior art give rise to the task of proposing a decontamination process by which contaminations can be removed from the polyolefin to be recycled which are not removable or can only be removed with difficulty by the known processes and which achieve the removal of the key contaminants of at least 90% as specified by the European Food Safety Authority (EFSA).

Description

[0009] The stated object is achieved in a process for recycling contaminated polyolefin material by the features set forth in the characterizing portion of patent claim 1. Further developments and/or advantageous embodiments are the subject of the dependent patent claims.

[0010] The invention is preferably characterized in that the purification step (V) is carried out by contacting or mixing the melt with an entraining agent, and the contaminants are separated from the melt together with the entraining agent as a loaded entraining agent. The entraining agent can act in the melt phase of the polymer. This allows contaminants that would otherwise be difficult or impossible to separate from the polyolefin material to be removed together with the entraining agent. This includes, in particular, the model contaminants 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 assess the purification efficiency of recycling processes in so-called challenge tests. The entraining agent can reduce the contaminants by at least 90% of the initial concentration. This generally meets EFSA's requirements regarding key contaminants, depending on the initial contamination. Since tris(2-ethylhexyl) trimellitate is particularly large with a molar mass of 547 g/mol, up to 30% of it can be removed from the polyolefin material using state-of-the-art separation processes.

[0011] The sequence of process steps (I) to (VII) is preferably carried out in ascending order. It is also conceivable that the process steps are carried out in a different order.

[0012] Essentially, entraining agents are additives used in various separation processes that enable the separation of individual substances from mixtures of substances. Entraining agents that are brought into contact with the melt are deliberately added substances that accelerate the transfer of contamination during polymer degassing or extrusion of the polymer, or even make this possible in the first place, and that predominantly leave the polymer during degassing and extrusion. These specific contaminants are difficult, if not impossible, to remove using conventional decontamination processes, as their molecular weights are typically too high to be removed at polyolefin melt temperatures of approximately 120 to 320°C, or are not present in gaseous form. Temperatures above 320°C degrade the polymer far too quickly; it literally burns. At temperatures below 120°C, the polymer is not fluid enough.

[0013] The polyolefin polymer may also be in a form which prevents or complicates decontamination and is facilitated or made possible by the melt entraining agent.

[0014] In a particularly preferred embodiment of the invention, the polyolefin material is undissolved in the entraining agent, and accordingly, a material phase and an entraining agent phase (layered or in domains) are present during the purification step (V). This allows the entraining agent to be separated from the melt again with little effort by devolatilization in the extruder, and during devolatilization or pressing, it entrains a large portion of the contaminants from the melt into the gas phase. In contrast, purification agents that dissolve the polyolefin material must be separated from the target polymer again using complex processes, such as a membrane process.

[0015] The entraining agent is advantageously added to the material in the feed zone of the extruder or downstream of the feed zone. If the entraining agent is added in the feed zone, the temperature must be kept low to prevent it from evaporating before it can come into contact with the melt. If the entraining agent is added to the extruder downstream of the feed zone, it must be fed in using a pump, since high pressure prevails in this zone of an extruder.

[0016] The invention is preferably characterized in that the extruder is operated or constructed in such a way that the volume of the extruder is, at least in some areas, larger than the volume of the melt containing the entraining agent, and that the free volume thus obtained is subjected to a negative pressure in order to separate the contaminants and the entraining agent from the melt. The free volume can be achieved, for example, by varying the conveying speeds and passage volumes of an extruder or by varying the speeds of melt pumps.

[0017] In conjunction with or on its own, the extruder volume can preferably be smaller, at least in some areas, than the melt containing the entraining agent, in order to extrude the melt and separate the loaded entraining agent. A pressing process can also be additionally connected downstream of the extruder.

[0018] In a further preferred embodiment of the invention, the entraining agent is added to the material in a static or dynamic mixer upstream or downstream of the extruder by mixing the flakes or melt with the entraining agent. This allows the polyolefin material to swell in the mixer, which can improve decontamination. The polyolefin material can be mixed with the entraining agent in a mixer in addition to mixing in an extruder or instead of mixing in an extruder.

[0019] In a further preferred embodiment of the invention, the melt surface loaded with contaminants is enlarged by the entraining agent, preferably by the entraining agent forming pores in the melt.

[0020] Separation of the contaminants under vacuum or pressing is improved by the larger surface area, as explained below.

[0021] In general, it is advantageous to have an open melt structure. Pores and cavities in the melt increase the surface area to the vacuum created during degassing. The larger the surface area of the melt during degassing, the more effective it is. Entraining agents, which increase the surface area of the melt, support degassing.

[0022] The entraining agent and the contaminants in these pores are thus more easily accessible to the vacuum, and any remaining contaminants in the melt, once the entraining agent has been separated from the pores, are easier to remove due to the structure being open to the vacuum. The bursting of individual or all pores due to a pressure difference can also be specifically exploited.

[0023] Entraining agents, which introduce pores and cavities into a melt, also assist in the removal of entraining agents and contaminants during the pressing process, similar to a sponge. Here, too, surface area is important for the removal of contaminants during pressing. The larger the surface area during pressing, the better.

[0024] It has proven useful for the polyolefin material to be swollen by the entraining agent. Melt-swelling entraining agents such as hexane or heptane, which are brought into contact with the polyolefin melt, promote the migration of contaminants to the surface, since the swollen polymer releases more contaminants during degassing than without swelling. Even larger molecules, which would otherwise not be released, can be transported away by the entraining agent due to the larger molecular spacing of the swollen polymer. The entraining agent can have both a swelling and a pore-forming effect on the polymer melt.

[0025] The swollen polyolefin material or the polyolefin material having an enlarged surface area is extruded to remove any residual loaded entraining agent from the material.

[0026] The invention is also preferably characterized in that the separated contaminant-laden entraining agent is purified and returned to the extruder and/or mixer as treated entraining agent. This minimizes entraining agent consumption, making the decontamination process efficient and cost-effective.

[0027] In a preferred embodiment of the invention, the entraining agent is heptane or hexane and is brought into contact with 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 embodiment, decontamination is further improved by the swelling of the melt.

[0028] In a further preferred embodiment, the entraining agent is polar, and in particular water, and the polar entraining agent transports contaminants from the melt to the melt surface during phase separation. Due to the lack of compatibility, the water always strives for the surface of the melt and entrains contaminants. At the surface, it evaporates in the vacuum zones of the extruder or mixer. Contaminants that are soluble in water (e.g., formaldehyde) and those that are not readily soluble in water (mineral oil) are flushed out of the polymer during phase separation and entrained into the gas space during evaporation. This occurs even if the evaporation temperature of the contaminants has not yet been reached under the prevailing pressure conditions.

[0029] In a further preferred embodiment of the invention, the entraining agent is dry ice, the application of which gives the melt a porous or foamed structure, and the sublimating dry ice entrains the contaminants into the gas phase. In this embodiment, the sublimation of the dry ice is utilized to transfer contaminants into the gas phase, and they are expelled either by degassing alone or with the additional support of a pressing process.

[0030] It proves advantageous if the purified melt is granulated in a granulation process. Due to the use of a melt entraining agent, the recycled granulate has a very high quality, comparable to that of virgin granulate.

[0031] Further advantages and features will become apparent from the following description of an embodiment of the invention with reference to the schematic representations. These are not drawn to scale: Figure 1: a flow diagram of a process according to the invention for recycling polyolefins in a first embodiment; and Figure 2: a flow diagram of the process according to the invention for recycling polyolefins in a second embodiment.

[0032] Figure 1 shows a flow diagram of the inventive process for recycling polyolefin material. Of particular importance is the contacting of the melt with an entraining agent, whereby the polyolefin material is not dissolved in the entraining agent. The entraining agent is therefore a melt entraining agent and can also be regarded as a type of extraction agent.

[0033] Melt entrainers are deliberately added substances that accelerate the transfer of contamination during polymer devolatilization or polymer compression, or even make this possible in the first place, and that predominantly leave the polymer during devolatilization. The long residence time at high temperatures required by the state of the art, which promotes the cluster formation of polyolefins, can be prevented or at least reduced by the entrainers.

[0034] The entraining agent allows specific contaminants whose molecular weights are too high to be removed at low melt temperatures or are not present in gaseous form to be removed together with the entraining agent. Even if the polyolefin is in a form that prevents or complicates decontamination, the entraining agent can facilitate decontamination.

[0035] A first rotary valve 2 conveys the washed, pre-cleaned, and sorted flakes a from a flake reservoir 1 into a flake buffer 3. In the flake buffer, volatile contaminants are sucked into an exhaust air treatment system 18 using a first vacuum pump 4. A second rotary valve 5 conveys the pre-cleaned flakes into a heated pressure vessel 6 equipped with a stirrer. In this vessel, the flakes are brought into contact with an entraining agent under pressure, and a mixture b of flakes and entraining agent is produced. The mixture is converted in the pressure vessel into a polymer sponge c with the entraining agent in the pores.

[0036] Via a conveying unit, for example, a transfer pump 7, the polymer sponge c is fed under overpressure into the extruder 8 in a closed system. At the inlet of the extruder 8, the polymer sponge c can be mixed with an additive package h, which may in particular contain fresh, unused stabilizers and colors for color compensation. Alternatively, the additive package h can be added in a subsequent melting phase (Figure 2). Separate addition of the additive package to a granulate processing machine, e.g., in an extrusion plant for films, pipes, or bottles, is also possible.

[0037] Figure 2 shows a flow diagram of a second embodiment of the process with a second extruder 20. In this embodiment, the additive package h is added to the melt phase of the second extruder 20. According to this embodiment, purification takes place first in the first extruder 8, and a second extrusion step (second extruder 20) is provided for additive addition.

[0038] Depending on the entraining agent and polymer variant, the entraining agent can cause structural changes in the melt, in particular pore formation, domain formation, bubble formation, layer formation, sponge formation or be present homogeneously as dissolved in the melt.

[0039] Stabilizers may be too degraded in the polymer or dosed too low, or the wrong stabilizers may be present in the recycled material. New, suitable, adapted, and still active stabilizers are added to the stabilizer package. "New" in the sense of not yet reacting with oxygen or radicals. "Suitable" in the sense that they are not immediately removed by the vacuum in the degassing and are suitable for use and further recycling steps. "Adapted" in the sense of missing quantity, since virgin material often requires very little stabilizer, but larger quantities are often needed in recycling due to the higher temperatures and residence times.

[0040] The color of the regranulate is usually not identical to the new product; a color correction package may be included in the additive package in addition to the stabilizer package.

[0041] In the extruder 8, the flakes are melted, and the melt is degassed under vacuum. The volume of the extruder 8 is deliberately kept larger in some areas than the volume required by the melt with the entraining agent. The free volume can be achieved, for example, by varying the conveying speeds and passage volumes of an extruder or by varying the speeds of melt pumps. The free volume is subjected to a negative pressure, whereby the entraining agent, along with the contaminants, is separated from the melt. The purified melt g is passed through a filter 9 to remove solid contaminants. The melt is converted into granules or pellets in a granulation unit 10.

[0042] The entraining agent can also be melt-swelling, which improves the release of contaminants. In conjunction with or on its own, the extruder volume can be smaller, at least in some areas, than the melt containing the entraining agent, in order to extrude the melt and separate the loaded entraining agent, or a pressing process can be additionally connected downstream of the extruder.

[0043] The swollen polyolefin is pressed out during separation of the entraining agent in order to remove residual entraining agent.

[0044] The entraining agent is conducted in a separate circuit in order to reuse as much entraining agent as possible and to remove as much contaminant as possible. The loaded entraining agent e is fed to a treatment unit 14. In addition to a condensation column, membrane filtration, a semipermeable membrane, selective precipitation, or chromatographic separation can be used for treatment. Other treatment methods are also conceivable. The residues k or separated contaminants from the entraining agent are disposed of. In the entraining agent reservoir 15, the treated entraining agent is mixed with fresh entraining agent d to compensate for entraining agent losses. A pump 16 returns the treated entraining agent f to the pressure vessel 6, and an overpressure is built up in the pressure vessel.

[0045] The produced granulate is collected in a granulate container 11, in which a negative pressure is maintained by a second vacuum pump 17. This allows for residual degassing and subsequent drying. The extracted gas is fed to the exhaust air treatment system 18.

[0046] The granules are fed to a granule cooler 13 via a third rotary valve 12. The granules are cooled by purging and cooling air blown into the granule cooler 13, thereby removing residual contaminants and also feeding them to the exhaust air treatment 18 as contaminated exhaust air h. The finally purified granules m can be removed from the granule cooler. The three degassing streams collected in the exhaust air treatment 18 are purified in the exhaust air treatment 18, and purified exhaust air j can be removed.

[0047] Various types of entraining agents have proven effective according to the following examples:

1. Example:

[0048] Melt-swelling entrainers such as hexane or heptane in polyolefins promote the migration of contaminants to the surface, as the swollen polymer releases more contaminants during devolatilization than without swelling. Even larger molecules that would otherwise not be released can be removed by the entrainer due to the larger molecular spacing in the swollen polymer. According to the first example, 300 to 700 wt.% heptane is added to the melt of a bottle-grade HDPE.

2. Example:

[0049] Melt-incompatible entraining agents, such as polar water, are added to a non-polar polymer such as polypropylene. The polar water strives for phase separation and collects on the melt surface as soon as no mixing and shearing elements repeatedly stir the water into the material. Due to the lack of compatibility, the water strives for the surface of the melt and entrains contaminants from the polypropylene (PP). At the surface, it evaporates in the vacuum zones of the extruder or mixer. Water-soluble contaminants (e.g., formaldehyde) and those that are not readily soluble in water (mineral oil) are flushed out of the polymer during phase separation and entrained into the gas space during evaporation, even if they would not evaporate at that temperature. An example of an entraining variant is the addition and dispersing of 1 to 3 wt.% water vapor in the melt quantity of a PP melt.

3. Example:

[0050] Melt-compatible entrainers, such as hexane or heptane, dissolve contaminants such as butyric acid in HDPE. Due to its lower solubility, the entrainer is repelled by the polymer matrix upon cooling and thus quickly reaches the surface, where it is separated as a separate phase. Melt-compatible entrainers are generally also swellable. Therefore, it is usually advisable to press out the supercooled melt to separate even more entrainer. According to the third example, 300 to 700 wt% of the melt volume of heptane is brought into contact with an HDPE melt.

4. Example:

[0051] Entraining agents are used, deliberately designed to achieve a porous or foamed melt consistency and surface, so that contaminants immediately enter the gas phase and can be removed via a vacuum. According to Example 4, 0.5 to 5 wt% of dry ice is added to the melt volume of a PP melt. The dry ice sublimates and draws the contaminants along with it.

5. Example:

[0052] Entraining agents are used, which are deliberately used to change the viscosity of the polyolefin so that the polyolefin forms thinner interfaces in the vented extruder and contaminants can leave the polyolefin more quickly due to the lower viscosity of the polyolefin. According to Example 5, 0.3 to 3 wt.% of heptane of the melt is used to significantly reduce the viscosity of an LLDPE melt.

6. Example:

[0053] Washed bottle-grade HDPE flakes (MFI: 0.02 - 10 g/10 min; 190 °C; 2.16 kg; DIN ISO 1133) are melted in a vented extruder at 250 °C. Volatile contaminants are removed by vacuum, and solid contaminants by melt filtration. Instead of underwater pelletizing, the melt is swollen in a mixer with the entraining agent heptane (in a ratio of 1 part by weight HDPE to 7 parts by weight heptane) at 130 °C, and the swollen HDPE is conveyed by a melt pump into a cooling tank. Due to the supercooling of the discharged strand, part of the entraining agent heptane separates with the dissolved contaminants and can be removed in the free volume of the cooling tank. By pressing, heptane and the contaminants it contains are removed from the swollen HDPE.

Legend:

[0054] 1 Flake feeder 2 First rotary valve 3 Flake buffer 4 First vacuum pump 5 Second rotary valve 6 Pressure vessel 7 Transfer pump 8 Extruder 9 Filter 10 Granulation 11 Granule container 12 Third rotary valve 13 Granule cooler 14 Preparation of the entrainer 15 Entrainer feeder 16 Pump 17 Second vacuum pump 18 Exhaust air treatment 20 Second extruder a Pre-cleaned flakes, polyolefin material b Mixture of flakes and entrainer c Polymer sponge with liquid solvent in the pores, d Fresh entrainer e Loaded entrainer f Prepared entrainer g Purified melt with residual entrainer h Additives, fresh stabilizer package j Purified exhaust air k Residues l Rinsing and cooling air m Purified granules

Claims (14)

1. A process for recycling contaminated polyolefin material, comprising the following process steps: (I) material sorting, (II) comminuting the material into flakes, (III) washing the flakes, (IV) flake sorting, (V) cleaning step, (VI) melting the flakes, and (VII) extruding the melt in an extruder (8), characterized in that the cleaning step (V) is carried out by contacting or mixing the melt (c) with an entraining agent (d), and separating the contaminants from the melt together with the entraining agent as a loaded entraining agent (e). 2. Process according to claim 1, characterized in that the polyolefin material (a) is undissolved in the entraining agent (d) and accordingly a material phase and an entraining agent phase are present during the purification step (V). 3. Process according to claim 1 or 2, characterized in that the entraining agent (d) is added to the material (a) in the feed zone of the extruder (8) or after the feed zone. 4. Method according to one 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 at least in regions greater than the volume of the melt (c) containing the entraining agent and mixed with the entraining agent, and the free volume thus obtained is provided with a negative pressure in order to separate the contaminants and the entraining agent (e) from the melt (g). 5. Method according to one 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 at least in regions smaller than the volume of the melt (c) containing the entraining agent, in order to realize extrusion of the melt and separation of the loaded entraining agent. 6. The method according to claim 1 or 2, characterized in that the entraining agent (d) is added to the material (a) in a static or dynamic mixer (6) upstream or downstream of the extruder (8) by mixing the flakes (a) or the melt with the entraining agent (d). 7. Process according to one of the preceding claims, characterized in that the melt surface is enlarged by the entraining agent (d), preferably by the entraining agent (d) forming pores in the melt. 8. Process according to one of the preceding claims, characterized in that the polyolefin material (a) is swollen by the entraining agent (d). 9. The method according to claim 7 or 8, characterized in that the swollen or enlarged surface polyolefin material (c) is pressed out in order to remove residual loaded entraining agent (e) from the material. 10. Process according to one of the preceding claims, characterized in that the separated entraining agent (e) loaded with contaminants is purified and returned to the extruder and/or the mixer as treated entraining agent (f). 11. Process according to one of the preceding claims, characterized in that the entraining agent (d) is heptane or hexane and is brought into contact with the melt (c) in an amount which is 3 to 9 times and preferably 3 to 7 times the weight of the melt, and in that the polyolefin material (a) swells. 12. The method according to any one of claims 1 to 10, characterized in that the entraining agent (d) is polar and in particular is water and the polar entraining agent transports contaminants from the melt (c) to the melt surface during phase separation. 13. A method according to any one of claims 1 to 10, characterized in that the entraining agent (d) is dry ice, the application of which gives the melt (c) a porous or foamed structure and the sublimating dry ice entrains the contaminants into the gas phase. 14. Method according to one of the preceding claims, characterized in that the purified melt (g) is granulated in a granulation (10).