CN121969474A - Process for recovering polyolefin - Google Patents

Abstract

This invention relates to a method for recovering polyolefin materials mixed with contaminants, the method comprising the following steps: (I) material sorting, (II) pulverizing the material to form 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). The cleaning step (V) is performed such that the melt (c) comes into contact with or mixes with an entrainer (d), and the contaminants, together with the entrainer, are separated from the melt as a loaded entrainer (e).

Description

Process for recovering polyolefin

Technical Field

The present invention relates to a process for recovering polyolefin materials containing contaminants.

Prior Art

Products made of polyolefin plastic materials, in particular polyethylene and polypropylene, are subject to contamination by external influences (e.g. the contents in the case of packaging) during their life cycle. Contaminants can penetrate deeply into the material. However, printing inks, external or internal lubricants, additives (such as antioxidants), antiblocking additives or many other added components on packaging can pose problems for reuse as recycled materials, whether directly or in their degraded form. All low molar components added to the polymer matrix during the original use of the product or absorbed by environmental contact can in principle be released again during subsequent use as recycled material, which can lead to organoleptic damage and toxicological risks, for example when used in consumer packaging applications. Components having a molecular size of up to 1000g/mol are considered potentially dangerous depending on the application and content.

Methods for separating low molecular weight impurities are known in which the polyolefin flakes are either flushed with heated ambient air at elevated temperature or the polyolefin in the form of fine powder is flushed or subjected to vacuum and temperature to remove volatile substances prior to extrusion and melt formation. These processes are limited by the low melting point of the polyolefin and the corresponding agglomeration of the material above the melting point. Agglomerated material can no longer be sufficiently drawn into the extruder.

Similar methods are also known, in which instead of vacuum a fluid (in particular nitrogen, CO2 or steam) is pumped through the material to be cleaned before the extruder.

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

It is also known to decontaminate polyolefins after the extruder in the form of films, strands or particles under vacuum and at elevated temperature or with the medium flowing through (air, steam, nitrogen, CO 2) at elevated temperature. These methods are limited by the melting point, as the film, strand or particle cannot be industrially processed or glued above its melting point.

Recovery methods using solvents are known, which clean the polymer by swelling and pressing out or by targeted dissolution and precipitation of the polymer. The polymer in the swollen state can have much higher migration properties and thus soil release properties.

Disclosure of Invention

The described drawbacks of the prior art have led to the task of proposing a decontamination process by which contaminants can be removed from the polyolefin to be recovered, which cannot be removed by known methods or can only be removed in a difficult manner, and which achieves a removal of at least 90% of the critical contaminants specified by the European Food Safety Agency (EFSA).

In a process for recycling polyolefin materials mixed with contaminants, said object is achieved by the features listed in the characterising part of claim 1. The dependent claims relate to developments and/or advantageous alternative embodiments.

Preferably, the invention is characterized in that the cleaning step (V) is performed such that the melt is contacted or mixed with the entrainer and the contaminants are separated from the melt together with the entrainer as a loaded entrainer. The entrainer may act in the melt phase of the polymer. This allows contaminants to be removed with the entrainer that would otherwise be difficult or impossible to separate from the polyolefin material. This includes in particular the model contaminants toluene, chlorobenzene, chloroform, methyl salicylate, phenylcyclohexane, benzophenone, methyl stearate, limonene, butyl salicylate, methyl palmitate, diethyl hexyl phthalate and tris (2-ethylhexyl) trimellitate, which are used to evaluate the cleaning efficiency of the recovery process in so-called challenge tests. The entrainer can reduce the contaminant by at least 90% of the initial concentration. This means that depending on the initial contamination, the requirements of the EFSA with respect to critical contaminants can generally be met. Since tris (2-ethylhexyl) trimellitate has a particularly large molar mass of 547g/mol, only up to 30% can be removed from polyolefin materials using the separation processes of the prior art.

The sequence of process steps (I) to (VII) is preferably carried out in ascending order. It is also conceivable to perform the method steps in a different order.

Basically, entrainers are additives used in various separation methods, which enable the separation of individual substances from a substance mixture. The entrainment medium in contact with the melt is an intentionally added substance which accelerates the transfer of contaminants during degassing of the polymer or extrusion of the polymer or even makes this possible at the beginning and leaves the polymer mainly during degassing and extrusion itself. These particular contaminants are difficult or impossible to remove using conventional decontamination methods because their molecular weight is typically too great to withdraw at a melting temperature of the polyolefin of about 120 ℃ to 320 ℃, or they are not present in gaseous form. Temperatures above 320 ℃ degrade the polymer too quickly and they burn. At temperatures below 120 ℃, the flowability of the polymer is insufficient.

The polyolefin polymer may also be present in a form that prevents or makes decontamination difficult, while decontamination may be facilitated or made possible by the melt entraining agent.

In a particularly preferred embodiment of the invention, the polyolefin material is insoluble in the entrainer and accordingly there is a material phase and an entrainer phase (layering or domain division) during the cleaning step (V). This allows the entrainer to be easily separated from the melt by degassing in the extruder and it brings most of the contaminants from the melt into the gas phase during degassing or pressing. In contrast, cleaners that dissolve polyolefin materials must use complex processes (such as thin film processes) to separate from the target polymer.

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

Preferably, the invention is characterized in that the extruder is operated or configured in such a way that the volume of the extruder is, at least in certain areas, greater than the volume of the melt contained in the extruder that is mixed with the entrainer, and that the free volume thus obtained is provided with a vacuum in order to separate contaminants and the entraining medium from the melt. The free volume can be achieved, for example, by different conveying speeds of the extruder and by the volume or by different speeds of the melt pump.

The extruder volume (whether used in a concomitant manner or separately) may preferably be smaller than the entrainer-containing melt in at least some areas in order to achieve extrusion of the melt and separation of the supported entrainer. The extruder may be followed by an additional pressing process.

In another preferred embodiment of the invention, the entrainer is added to the material by mixing the flakes or melt with the entrainer in a static or dynamic mixer located upstream or downstream of the extruder. This allows the polyolefin material in the mixer to swell, which may improve soil release. In addition to or instead of mixing in the extruder, the mixing of the polyolefin material with the entrainer may be performed in a mixer.

In another preferred embodiment of the invention, the surface of the melt loaded with the contaminant is enlarged by the entrainer, preferably by the entrainer forming holes in the melt.

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

In general, it is advantageous to have an open melt structure. The holes and cavities in the melt increase the surface exposed to vacuum during degassing. The larger the surface of the melt during degassing, the more efficient. The entrainer, which enlarges the surface of the melt, facilitates degassing.

Because the structure is open to vacuum, the entrainer and contaminants in the holes are thus more accessible to vacuum, or the remaining contaminants in the melt are more easily removed after the entrainer has been separated from the holes. Bursting of individual or all holes due to pressure differences can also be used in a targeted manner.

The inclusion of the holes and cavities into the melt also facilitates the removal of entraining agent and contaminants during the compaction process, similar to a sponge. Here, the surface is also important for removing contaminants during extrusion. The larger the surface the better during extrusion.

It has proven useful to swell the polyolefin material with the entrainer. Melt swelling entraining agents (such as hexane or heptane) in contact with the polyolefin melt promote migration of contaminants to the surface because the swollen polymer releases more contaminants during degassing than it does without swelling. Because of the greater molecular distance of the swollen polymer, the entrainer can transport away even larger molecules that would not otherwise be released. The entrainer may have both a swelling and pore-forming effect on the polymer melt.

Advantageously, the swollen or enlarged surface polyolefin material is pressed out to remove any remaining supported entrainer from the material.

Preferably, the invention is further characterized in that the separated contaminant-laden 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.

In a preferred embodiment of the invention, the entrainer is heptane or hexane and is contacted with the melt in an amount of 3 to 9 times and preferably 3 to 7 times the weight of the melt and the polyolefin material swells. In this exemplary embodiment, the decontamination is further improved by the swelling of the melt.

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 contaminants towards the surface of the melt. Which evaporates at 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. Even though the evaporation temperature of the contaminants has not been reached under the existing pressure conditions.

In another preferred embodiment of the invention, the entraining agent is dry ice, in the application of which the melt acquires a porous or foamed structure and sublimes the dry ice to carry the contaminant into the gas phase. In this exemplary embodiment, sublimation of dry ice is used to transfer contaminants into the gas phase, which can then be driven off by degassing alone or in addition with the support of the pressing process.

Advantageously, the cleaned melt is granulated during the granulation process. The recycled particles have a very good quality comparable to the original particles due to the use of melt entraining agent.

Drawings

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, which are not drawn to scale:

FIG. 1 is a flow chart showing a process for recovering polyolefin according to the present invention in a first embodiment, and

Fig. 2 shows a flow chart of a process for recycling polyolefin according to the invention in a second embodiment.

Detailed Description

Fig. 1 shows a flow chart of a method for recycling polyolefin material according to the invention. In particular, it is important to contact the melt with an entrainer in which the polyolefin material is insoluble. Thus, the entrainer is a melt entrainer and may also be considered an extractant.

Melt entrainers are deliberately added substances which accelerate the transfer of contaminants during degassing or pressing of the polymer or even make this possible initially and leave the polymer predominantly during degassing. The long residence time at high temperatures, which is necessary and which promotes the aggregation of the polyolefin according to the prior art, can be prevented or at least reduced by the entrainer.

The entrainer enables the extraction of specific contaminants together with the entrainer, which contaminants have a molecular weight too high to be extracted at low melting temperatures or are not present in gaseous form. Even if the polyolefin is in a form that prevents or hinders soil removal, soil removal may be made possible by the entraining agent.

The first rotary feeder 2 conveys washed or pre-cleaned and sorted sheets a from the sheet template 1 into the sheet buffer 3. In the flake buffer, the volatile contaminants are drawn into the exhaust treatment system 18 using the first vacuum pump 4. The second rotary feeder 5 conveys the pre-cleaned flakes into a heated pressure vessel 6 with an agitator. In this vessel, the flakes are contacted with an entrainer under pressure and a mixture b of flakes and entrainer is produced. In the pressure vessel, the mixture is converted into a polymer sponge c with entrainer in the pores.

The polymer sponge c is fed in a closed system under overpressure via a conveying unit, for example a transfer pump 7, to an extruder 8. At the inlet of the extruder 8, the polymer sponge c can be mixed with an additive package h, which can contain in particular fresh, unused stabilizer and pigment for color compensation. Alternatively, the additive package h may be added in a subsequent melt phase (fig. 2). The additive packages can also be added separately in the processing machine of the granules, for example in the extrusion equipment of films, tubes or bottles.

Fig. 2 shows a flow chart of a second embodiment of a method with a second extruder 20. In this embodiment, 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.

Depending on the entraining agent and the polymer variant, the entraining agent may cause structural changes in the melt, in particular pore formation, domain formation, bubble formation, layer formation, sponge formation, or may be homogeneously dissolved in the melt.

The stabilizer may be added in an amount that degrades too much or too little in the polymer, or there may be an erroneous stabilizer in the recycled material. New, suitable, tuned and still active stabilizers are added to the stabilizer package. "New" means here that they have not reacted with oxygen or free radicals. By "suitable" is meant herein that they are not immediately re-evacuated by vacuum during degassing and are suitable for application and further recovery steps. "conditioned" refers herein to insufficient amounts because new products typically require very little stabilizer, but during recovery, greater amounts of stabilizer are typically required due to higher temperatures and longer residence times.

The color of the reconstituted pellets is typically different from the new material, and a color correction package may be included in the additive package alongside the stabilizer package.

In the extruder 8, the flakes are melted and the melt is subjected to vacuum degassing. In this process, the volume of the extruder 8 is deliberately kept in certain areas greater than the volume required for the melt with entrainer. The free volume can be achieved, for example, by different conveying speeds of the extruder and by the volume or by different speeds of the melt pump. The free volume is subjected to a vacuum, which separates the entrainer from the melt along with the contaminants. The cleaned melt g is passed through a filter 9 to remove solid contaminants. In the granulation process 10, the melt is converted into granules or pellets.

The entrainer may also be such that it swells the melt, which improves the release of contaminants. Whether used concomitantly or alone, the extruder volume may be smaller than the melt mixed with the entrainer, at least in some areas, in order to press the melt out and separate the loaded entrainer, or an additional pressing process may be added after the extruder.

The swollen polyolefin is extruded during the separation of the entrainer to remove any remaining entrainer.

The entrainer is circulated in a separate circuit in order to reuse as much of the entrainer as possible and to remove contaminants as completely as possible. The load entrainer e is fed to the processing unit 14. For the treatment, membrane filtration, semipermeable membranes, selective precipitation or chromatographic separation can be used in addition to the condensation column. Other processes can also be envisaged. The residue k separated off from the entrainer or the separated contaminants are disposed of. In the entrainer template 15, the treated entrainer is mixed with fresh entrainer d to compensate for entrainer losses. The treated entrainer f is returned to the pressure vessel 6 by using the pump 16 and an overpressure is established in the pressure vessel.

The produced particles are collected in a particle container 11, in which a vacuum is maintained by a second vacuum pump 17. This allows residual degassing and subsequent drying. The extracted gas is fed to an exhaust treatment system 18.

The granules are fed to a granule cooler 13 via a third rotary feeder 12. The particles are cooled by flushing and cooling air blown into the particle cooler 13, whereby any remaining pollutants are removed and also fed as loaded exhaust gas h to the exhaust gas treatment system 18. The finally cleaned particles m can be removed from the particle cooler. The three deaeration streams collected in exhaust treatment system 18 are cleaned in exhaust treatment system 18 and cleaned exhaust gas j may be withdrawn.

Various types of entrainers have proven effective according to the following exemplary embodiments:

example 1:

melt swelling entraining agents (such as hexane or heptane) in the polyolefin promote migration of contaminants to the surface because the swollen polymer releases more contaminants during degassing than if not swollen. Because of the greater molecular distance in the swollen polymer, the entrainer can remove even larger molecules that would not otherwise be released. According to a first embodiment, 300 to 700 wt% heptane is added to a melt amount of bottle grade HDPE.

Example 2:

An entrainer that is not compatible with the melt, such as polar water, is added to the non-polar polymer, such as polypropylene. Once the water is repeatedly stirred into the material without the mixing and shearing elements, the polar water tends to phase separate and accumulate at the melt surface. Due to the lack of compatibility, water looks for the surface of the melt and carries contaminants from the polypropylene (PP). Which evaporates at the surface in the vacuum zone of the extruder or mixer. Water-soluble contaminants, such as formaldehyde, as well as 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. Even though they do not evaporate at this temperature. An example of an entrainer variant is the addition and dispersion of 1 to 3 wt.% of water vapour into the melt quantity of the PP melt.

Example 3:

Melt compatible entrainers such as hexane or heptane dissolve contaminants such as butyric acid in HDPE. The entrainer, due to its low solubility on cooling, is repelled by the polymer matrix and therefore reaches the surface very quickly, where it is separated as a separate phase. Melt compatible entrainers also typically have a swelling effect. It is therefore often advantageous to press out the supercooled melt in order to separate more entrainment medium. According to a third embodiment, heptane is contacted with the HDPE melt in an amount of 300 wt% to 700 wt% of the melt.

Example 4:

Entraining agents are used which are intended to achieve a porous or foamed melt consistency and surface so that the contaminants immediately enter the gas phase and can be extracted via vacuum. According to a fourth embodiment, 0.5 to 5 wt.% dry ice is added to the melt quantity of the PP melt. During this process, the dry ice sublimates and with it the contaminants will be drawn off.

Example 5:

The use of an entrainer, which is intended to change the viscosity of the polyolefin, allows the polyolefin to form a thinner interface in the degassing extruder and contaminants to leave the polyolefin faster due to the lower viscosity of the polyolefin. According to a fifth embodiment, 0.3 to 3wt% heptane is used in the melt amount to significantly reduce the viscosity of the LLDPE melt.

Example 6:

The washed bottle grade HDPE flakes (MFI: 0.02g/10min-10g/10min;190 ℃ C.; -2.16kg;DIN ISO 1133) are melted in a degassing extruder at 250 ℃. Volatile contaminants are separated by vacuum, and solid contaminants are separated by melt filtration. Instead of underwater pelletization, the melt was swollen in a mixer with entrainer heptane (at a ratio of 1 part by weight HDPE to 7 parts by weight heptane) at 130 ℃, and the swollen HDPE was pumped into a cooling vessel by a melt pump. Due to the supercooling of the exiting strands, some of the entrainer heptane separates from the dissolved contaminants and can be removed in the free volume of the cooling vessel. In addition, heptane and its contained contaminants are removed from the swollen HDPE by extrusion.

List of reference numerals:

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Claims (14)

1. A process for recycling a contaminant-containing polyolefin material, the process comprising the process steps of:

(I) The materials are sorted out and the materials are separated,

(II) comminuting the material into flakes,

(III) washing the flakes,

(IV) sorting the flakes,

(V) a cleaning step of cleaning the substrate,

(VI) melting the flakes, and

(VII) extruding the melt in an extruder (8),

It is characterized in that the method comprises the steps of,

The cleaning step (V) is performed such that the melt (c) is contacted or mixed with an entrainer (d) and the contaminants are separated from the melt together with the entrainer as a supported entrainer (e).

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

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

4. The method according to any of the preceding claims, characterized in that the extruder (8) is operated or configured in such a way that the volume of the extruder (8) is, at least in certain areas, larger than the volume of the melt (c) contained in the extruder (8) mixed with entrainer, and that the free volume thus obtained is provided with a vacuum in order to separate the contaminants 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 configured in such a way that the volume of the extruder (8) is at least in some areas smaller than the volume of the melt (c) mixed with entrainer contained in the extruder (8) in order to achieve the extrusion of the melt and the separation of the supported entrainer.

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

7. The method according to any 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. A method according to any preceding claim, wherein 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) comprising a swollen or enlarged surface is pressed out to remove residual supported entrainer (e) from the material.

10. A method according to any preceding claim, 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. A method according to any preceding claim, characterized in that the entrainer (d) is heptane or hexane and is contacted 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 that the polar entrainer conveys contaminants from the melt (c) to the surface of the melt during phase separation.

13. A method according to any one of claims 1 to 10, wherein the entraining agent (d) is dry ice, in the application of which the melt (c) is given a porous or foamed structure and sublimed dry ice carries the contaminants into the gas phase.

14. A method according to any preceding claim, characterized in that the cleaned melt (g) is granulated in a granulating device (10).