A new polycarbonate recycling method by a novel combination of physical and chemical recycling steps

Abstract

Described is a process for the recycling of polycarbonate from particulate polycarbonate waste comprising a) providing the particulate polycarbonate waste, b) solvent-based purification of the particulate polycarbonate waste, c) partially depolymerizing of the purified polycarbonate, d) repolymerizing of the partially depolymerized polycarbonate. The process of the invention is an improved and adjustable process for the recycling of polycarbonates in high quality and purity, which is applicable on a technical and commercial scale at a relatively low energy demand.

Description

[0001] MEISSNER BOLTE

[0002] Postfach 860624 81633 Munchen

[0003] EPC Engineering & Technologies GmbH December 16, 2024

[0004] Dr.-Bonnet-Weg 1 M / EPC-101-PC

[0005] 99310 Arnstadt

[0006] Deutschland

[0007] A NEW POLYCARBONATE RECYCLING METHOD BY A NOVEL COMBINATION OF PHYSICAL AND CHEMICAL RECYCLING STEPS

[0008] The present invention relates to a process for recycling of polycarbonate from polycarbonate waste to obtain recycled polycarbonate and a plant for the preparation of recycled polycarbonate from polycarbonate waste according to said process.

[0009] Polycarbonate (PC) is a thermoplastic polymer with a broad approach for several modern polymer applications. Since PC is showing outstanding physical properties such as thermostability, impact strength, or optical characteristics, the worldwide demand of PC production has significantly increased during the past decades and forecasted to further increase.

[0010] Recycling of thermoplastic polymers is classified in the fields of physical and chemical recycling. Physical recycling describes, according to current documents from the DIN Institute, a process in which the chemical molecular structure is not changed. Chemical recycling, on the other hand, changes the molecular structure. Physical recycling is often used as a synonym for mechanical recycling, though among other things solvent-based recycling is a physical recycling process as well.

[0011] On a technical and commercial scale, only the mechanical recycling of polycarbonate is currently available. In such processes, polycarbonate is melted and then solidified again. MEISSNER BOLTE M / EPC-101-PC

[0012] 2

[0013] Solvent based recycling processes where polymers like polycarbonate are dissolved and subsequently precipitated again are technically known. Various chemical recycling processes are technically known as well, however both, solvent based and chemical recycling of those polymers, are not available on a commercial scale. Each recycling process comes along with advantages and disadvantages. In the following common processes and their disadvantages are described.

[0014] In chemical recycling, polycarbonate is de-polymerized by the use of various chemicals. Depending on which chemicals are used, different basic (petro-) chemicals, basic polymer molecules, (petro-) chemical intermediates, etc. are produced that can be used in the chemical industry. Original educts of polycarbonate (monomers) can be produced as well.

[0015] Chemical recycling of polymers is not yet available on a technical scale. Chemical depolymerization of the polymer has a high energy demand, e.g. because of high process pressure and / or temperatures required. Although these processes promise high product purity, where it is unclear whether they can handle different polycarbonate waste streams. Therefore, the effort of pre-sorting is estimated to be significantly high.

[0016] Thus, the main disadvantages of chemical recycling are:

[0017] • It is not possible to recycle PC blends or compounds. Hence, increased upstream sorting efforts are necessary, which, however, does not resolve the blend materials issue as such.

[0018] • It can be assumed that this type of recycling process consumes a lot of energy due to high temperatures required.

[0019] Mechanical recycling describes recycling-processes of polymers, where the particular polymer is melted and solidified again and e.g. granulated for further processing like injection molding, melt spinning, bottle blowing, etc. In technical terms, the polymer is melted in an extruder and subsequently processed via the process steps mentioned above. If necessary, the polymer is degassed, and additives are added.

[0020] At present, mechanical recycling of polycarbonate is only possible for waste streams with a high degree of polymer purity and precisely defined properties. The MEISSNER BOLTE M / EPC-101-PC

[0021] 3 application of polycarbonate is defined by its melt flow index (MFI or MFR or MVR), which basically describes the different average molecular weights or various long polymer chains. Due to the diversity of applications, there is also a wide variety of polycarbonate products as potential feedstock waste. Therefore, mechanical recycling is limited in its application. In addition, the polymer is thermally damaged during every cycle of melting and solidification. Thereby the quality of recycled polymer is not suitable for every usage, such as optical lenses.

[0022] Thus, the main disadvantages of mechanical recycling are:

[0023] • The system can only use a) either a mono-fraction of correctly sorted distinct blends or distinct compounds, or b) pure PC waste material of a distinct MFI quality. However, for mechanical recycling the PC quality drops after each a process run and cannot be kept stable.

[0024] • The molecular structure cannot be changed, which means that the MFR of polycarbonate cannot be adjusted, and therefore the recycling product has fluctuating properties depending on the waste material used.

[0025] Solvent-based recycling, a special type of mechanical recycling, describes a process in which a polymer or a compound thereof is (selectively) dissolved in a solvent. Depending on the solvent and material combination, the polymer solution is purified and precipitated again. Precipitation can take place by evaporation of the solvent or other methods. The system may be suitable for some blends and compounds, where the pure PC fraction can be separated from other type of blend polymer component.

[0026] Solvent-based recycling is not yet available on a technical scale. The polymer chain length cannot be influenced by the process. A polycarbonate with sufficiently high quality for optical applications has not yet been produced by solvent-based recycling.

[0027] Thus, the main disadvantages of solvent-based recycling are:

[0028] • The molecular structure of the PC cannot be changed, which means that the MFR of polycarbonate to be recycled cannot be adjusted. Therefore, depending MEISSNER BOLTE M / EPC-101-PC

[0029] 4 on the type of PC waste employed, the recycling product has fluctuating MFRs and hence fluctuating properties

[0030] US 7935736 B2 relates to a solvent-based recycling process for recycling polyesters such as polyethylene terephthalate (PET) or polyester mixtures. DE 102005026451 Al describes a solvent-based recycling process for recycling polystyrene from polystyrene-containing waste. US 10,934, 410 B2 is concerned with a solvent-based recycling process for recycling polyolefin from polyolefin-containing waste.

[0031] The object of the present invention was to overcome the disadvantages of the prior art discussed above. Specifically, the task underlying the present invention was to provide a process for recycling of polycarbonate waste with which recycled polycarbonate having a desired, adjustable and non-fluctuating quality, e.g. with respect to the MFI of the PC, can be obtained, regardless of the type of polycarbonate waste used. It should be also possible to recycle PC waste including PC blends or compounds.

[0032] Moreover, a drop of the quality of the polycarbonate should be avoided. To the contrary, the process should result in recycled polycarbonates of high quality, such as polycarbonates suitable for optical applications. In addition, energy consumption of the process should be relatively low.

[0033] The inventors found that the object could be solved by combining solvent-based recycling with a chemical recycling in a smart approach, wherein the chemical recycling includes partial depolymerization of a purified polycarbonate and repolymerization of the partially depolymerized polycarbonate.

[0034] Accordingly, the present invention relates to a process for the recycling of polycarbonate from particulate polycarbonate waste comprising the following steps: a) providing the particulate polycarbonate waste, b) solvent-based purification of the particulate polycarbonate waste comprising bl) mixing the particulate polycarbonate waste with a solvent to dissolve polycarbonate from the particulate polycarbonate waste to obtain a mixture of a polycarbonate solution and undissolved residue, b2) separating the polycarbonate solution from the undissolved residue in the mixture obtained, b3) removing solvent from the polycarbonate solution to obtain a purified polycarbonate in solid form and / or melt form, MEISSNER BOLTE M / EPC-101-PC

[0035] 5 c) partially depolymerizing of the purified polycarbonate comprising cl) adding water, preferably demineralized water, to the purified polycarbonate to obtain a mixture comprising water and the purified polycarbonate, c2) partially depolymerizing of the mixture comprising water and the purified polycarbonate in a melted state to obtain a partially depolymerized polycarbonate, d) repolymerizing of the partially depolymerized polycarbonate comprising dl) adding a carbonate ester, preferably a diarylcarbonate, to the partially depolymerized polycarbonate to obtain a mixture comprising the partially depolymerized polycarbonate and the carbonate ester, d2) repolymerizing the mixture comprising the partially depolymerized polycarbonate and the carbonate ester at a temperature of at least 200 °C and at a pressure below atmospheric pressure with removal of by-product generated to obtain the recycled polycarbonate.

[0036] The process of the invention can provide a recycled polycarbonate having a desired, adjustable and non-fluctuating quality, e.g. with respect to the MFI of the PC. In particular, by combining a solvent-based purification process with a particular chemical recycling process, the inventive recycling process can use different polycarbonate blends and compounds having different melt flow rates as the raw material to achieve this result.

[0037] In this new approach, the quality or melt flow rates of the recycled polycarbonate can be controlled, so that the quality of the recycled PC can be adjusted according to the requirements. The inventive process can even provide recycled polycarbonate of high quality suitable for optical applications. The inventive process also has a relatively low energy demand.

[0038] The inventive process combines the positive characteristics of mechanical, chemical and solvent-based recycling processes in such a way that the disadvantages of said recycling processes are eliminated.

[0039] The main advantages of the inventive can be summarized as follows:

[0040] In particular, in comparison to mechanical recycling, a variable PC waste stream can be used as a source material and input for the process. The MEISSNER BOLTE M / EPC-101-PC

[0041] 6 solvent based part can dissolve PC out of a compound or blend (a mixture of different polymers or different solid materials).

[0042] The molar mass of the product obtained can be adjusted according to the requirements. This means that the process is flexible for producing recycled PC for different applications.

[0043] The sorting effort for the waste can be reduced by using a suitable solvent. The product quality of the recycled PC (rPC) is higher than with mechanical recycling (upcycled quality polycarbonate).

[0044] The invention is further explained by the accompanying drawing, wherein:

[0045] Fig. 1 is a block flow diagram giving a schematic overview of the inventive process.

[0046] In the following, the invention is described in detail.

[0047] Polycarbonate may be abbreviated as PC in the following. Throughout the application, the units bar(a) and mbar(a) refer to the absolute pressure.

[0048] The recycling process of the invention for the preparation of recycled polycarbonate from polycarbonate waste comprises at least the process steps a) to d) described below in detail. In general, the process of the invention is a multi-stage process including a smart combination of a solvent-based purification and chemical-based recycling including partial depolymerization and a final repolymerization to obtain recycled polycarbonate.

[0049] Process step a) - providing the particulate polycarbonate waste

[0050] The polycarbonate waste as raw material can be of various origin. For instance, the origin may be a relatively pure polycarbonate waste generated during the manufacture of polycarbonate-based articles. The origin may also be a mixed waste such as municipal waste, industrial waste or plastic waste wherein polycarbonate waste may be blended with other waste types.

[0051] Typically, the polycarbonate waste provided is obtained by pre-sorting a waste including polycarbonate waste to separate from various contaminants such as other plastic waste or non-plastic waste. Such pre-sorting of waste is common in waste management and can be carried out by well-known techniques. MEISSNER BOLTE M / EPC-101-PC

[0052] 7

[0053] The polycarbonate waste used as a starting material is a particulate polycarbonate waste which is usually obtained by shredding a polycarbonate waste or pre-sorted polycarbonate waste, respectively. The shredding or crushing can be adjusted to obtain smaller pieces and a suitable particle size of the PC waste, which simplifies dissolution of PC in the following step.

[0054] Accordingly, particulate PC waste suitable for the inventive process may be obtained by shredding a feedstock material such as a pre-sorted and / or compacted PC waste. The PC waste may be also an off spec PC material, e.g. PC granules that are produced in a conventional virgin PC production plant but out of specified / guaranteed product quality. Such off spec PC material may already have the suitable particle size suitable as a particulate PC waste or may require a shredding as a pre-treatment. Such feedstock material is available on the market.

[0055] The particulate polycarbonate waste used as feedstock material for the most part may be mainly consisting of polycarbonate material, though other impurities depending on the waste material may be part of the feedstock.

[0056] The particulate polycarbonate waste generally contains ingredients other than PC such as additives, impurities or contaminants, but it may be also mainly consist of PC. Common PC articles may include additives contained in the PC such as fillers, plasticizers and other additives typically used for the preparation of PC articles, which generally cannot be removed by pre-sorting and / or shredding operation. Impurities or contaminants are materials, which have not been removed by presorting and / or shredding operation, e.g. plastics other than PC such as acrylonitrile butadiene styrene (ABS) or non-plastic impurities. Specifically, when the waste includes composites including PC bonded to other materials such as other plastics, these other materials are usually not removed by pre-sorting and / or shredding operation.

[0057] In principle, with the inventive process every PC holding waste stream or off-spec material even as part of a production plant can be converted into a recycled PC with a quality defined by its MFR for any required application. Waste streams with a high polycarbonate content from various industrial sectors are economically suitable. Specific examples are automotive plastic light fractions, plastic fractions of medical waste or electronic waste, optionally pre-sorted to enrich the polycarbonate content. MEISSNER BOLTE M / EPC-101-PC

[0058] 8

[0059] In a preferred embodiment, the particulate polycarbonate waste provided has a polycarbonate content of 5 to 100 % by weight, preferably 10 to 100 % by weight, more preferably 20 to 100% by weight, such as 10 to 90% by weight, such as 20 to 90% by weight, based on the dry weight of the polycarbonate waste. The balance are additives contained in the PC material or impurities as discussed above.

[0060] In a preferred embodiment, the polycarbonate waste provided is a particulate polycarbonate waste, wherein the mean particle size of the particulate polycarbonate waste is preferably less than 100 mm, more preferably less than 50 mm. In general, the mean particle size is preferably not less than 1 mm. The mean particle size refers to the weight average and can be determined by standard sieve analysis. The shredding which is usually applied to obtain the particulate polycarbonate waste serves to obtain smaller pieces or smaller particle sizes, respectively, to create as much surface area as possible to facilitate later dissolving.

[0061] The particle size therefore directly refers to the time duration of the PC dissolution processing step, because the PC containing waste particles are more efficiently surrounded by the liquid solvent. This is allowing the liquid solvent to better penetrate into the PC containing waste particle surface.

[0062] The particulate PC waste is usually obtained by a pre-treatment step of shredding or crushing a PC waste, which may be a pre-sorted PC waste, a compacted PC waste, an off spec PC waste or a combination thereof.

[0063] The particulate PC waste used as feedstock material, in particular particulate polycarbonate waste particles having a defined particle size (between mm to cm), are generally stored in one or more buffer silos.

[0064] The particulate polycarbonate waste particles stored in the in the one or more buffer silos can then be dosed gravimetrically and / or by a rotary valve, which is installed at a bottom discharge of the silo, and transported, e.g. via another air conveying system, into a feed hopper device, e.g. a vessel on load cells. This allows to feed the feedstock material in a defined amount for the required batch size. By opening a bottom valve, the feedstock material can be gravimetrically transferred into a dissolution vessel.

[0065] Hence, the stored particulate PC waste, in particular shredded polycarbonate waste particles, can be transported from the one or more buffer silos into a dissolution MEISSNER BOLTE M / EPC-101-PC

[0066] 9 vessel used for the solvent-based purification step via rotary valves and a feed hopper device, e.g. a vessel on load cells.

[0067] Process step b) - solvent-based purification of the particulate polycarbonate waste

[0068] The solvent-based purification of the particulate polycarbonate waste of step b) includes as step bl) in which the particulate polycarbonate waste is mixed with a solvent to dissolve polycarbonate from the polycarbonate waste to obtain a mixture of a polycarbonate solution and undissolved residue.

[0069] The solvent may be one solvent compound or a mixture of two or more solvent compounds. The solvent to be used is a solvent, which can dissolve polycarbonate and is preferably a selective solvent. That is, the solvent is preferably selected such that PC has a good solubility in the solvent, whereas ingredients other than PC included in the PC waste such as additives, impurities or contaminants, show a low solubility in the solvent or are preferably insoluble in the solvent. Of course, this also depends on the other ingredients present.

[0070] Solvents for polycarbonate and in particular selective solvents, which show low or no dissolving power for additives, impurities or contaminants such as other plastics than PC, e.g. ABS, are known to the skilled person.

[0071] Examples of suitable solvent or selective solvents to dissolve polycarbonate are chlorinated hydrocarbons, aromatic hydrocarbons, ketones, ethers, such as linear or cyclic ethers, esters, pyrrolidones, amides, carbon sulfides, phosphate esters, phosphoryl compounds, carboxylic acids or a combination thereof. The solvent is preferably selected from the group consisting of ketones, ethers, such as linear or cyclic ethers, cycloalkanes, esters, in particular acetone, methyl ethyl ketone, tetrahydrofuran, dialkyl esters of dicarboxylic acids and fatty acid alkyl esters, e.g. fatty acid methyl esters (FAME), or combinations thereof. The solvents mentioned above, in particular the chlorinated hydrocarbons and the aromatic hydrocarbons, may include one or more substituents. The substituent can be selected e.g. from halogenide, such as chloride or bromide, nitro, alkyl, amino, hydroxy, ether, acyl chloride groups or a combination thereof.

[0072] Examples of further specific solvents are dichloromethane (DCM), chloroform, 1- chloro-l-nitroethane, bromotrichloro methane, ethylene chloride, cresol, xylene, aniline, anisole, benzoyl chloride, trichlorophenol, chlorotoluene, tricresyl MEISSNER BOLTE M / EPC-101-PC

[0073] 10 phosphate, phosphorous oxychloride, cyclopentanone, cyclohexanone, methyl isoprenyl ketone, methyl isobutyl ketone (MIBK), 3-penten-2-one, methacrylic acid, 1,4-dioxane, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), carbon disulfide or combinations thereof.

[0074] The weight ratio of particulate polycarbonate waste to solvent in the mixing step bl) may vary depending on the type of solvent and the ingredients included in the PC waste. Typically, the weight percentage of the particulate polycarbonate waste based on total weight of the mixture of particulate polycarbonate waste and solvent is in the range of from 5 to 50 %, preferably of from 10 to 30 %.

[0075] The mixing of the particulate polycarbonate waste with the solvent according to step bl) is preferably carried out in a dissolution vessel provided with mixing means. The mixing of the particulate polycarbonate waste with the solvent according to step bl) is preferably carried out at a temperature, which is at or below the boiling temperature of the solvent used. It goes without saying that the temperature is above the melting temperature of the solvent used. Moreover, the mixing of the particulate polycarbonate waste with the solvent according to step bl) is carried out at a temperature below the degradation point of the polycarbonate. Hence, the temperature should be between the melting point and at or below the boiling point of used solvent and / or below the degradation point of the polycarbonate. The temperature can be e.g. in the range of 35 to 255°C, preferably 50 to 205 °C. The mixing and heating of the mixture supports dissolution and can reduce the duration for dissolving PC in the solvent and to avoid precipitation of PC due to cooling down of the PC solution.

[0076] The solvent mainly dissolves the polycarbonate polymer. Other polymers like ABS etc. will remain solid and can be removed at a later stage. Furthermore, additives and ingredients included in the PC waste also remains solid. Accordingly, in mixing step bl), PC is dissolved in the solvent and a mixture of a polycarbonate solution and undissolved residue is obtained. The undissolved residues may be polymeric impurities (other plastics such as ABS) or non-polymeric impurities such as additives. The undissolved residues are solid and can be separated from the PC solution in the next step.

[0077] The mixing step bl) is generally carried out in a vessel or dissolution vessel, preferably a stirred dissolution vessel, such as a stirred vessel The mixing step bl) can be carried out under atmospheric pressure and / or under slight overpressure MEISSNER BOLTE M / EPC-101-PC

[0078] 11 and / or under vacuum, e.g. in the range of 300 mbar(a) to 5 bar(a), preferably 1 bar(a) to 3 bar(a).

[0079] The particulate PC waste and the solvent can be added into the dissolution vessel in any suitable order. In general, it is preferred to charge the dissolution vessel with a portion of the solvent in advance, followed by adding simultaneously the particulate PC waste and further solvent into the dissolution vessel to reach the desired weight ratio between particulate PC waste and solvent.

[0080] After completion of the mixing step bl), the complete mixture of polycarbonate solution and undissolved residue is generally transferred from the dissolution vessel to the device for the separation step b). The dissolution vessel is preferably provided with a bottom valve to transfer the whole inventory of the dissolution vessel into the separation unit, preferably gravimetrically. Hence, the mixture of the polycarbonate solution and undissolved residue obtained in step bl) is preferably gravimetrically transferred from the dissolution vessel into the separation unit.

[0081] In step b2) of the solvent-based purification operation, the polycarbonate solution is separated from the undissolved residue in the mixture obtained. The undissolved residue is solid, for instance in form of particles. Any common technique for liquid / solid separation can be used.

[0082] The separation of the polycarbonate solution from the undissolved residues may occur e.g. via filtration and / or decantation. Depending on the size distribution of the undissolved residues, it may be appropriate to effect separation in stages, e.g. a filtration and / or decantation for separating undissolved residues having a larger size and a subsequent special filter process to remove remaining undissolved residues of smaller size such as microparticles.

[0083] In general, separating the polycarbonate solution from the undissolved residue according to step b2) is carried out in a separation unit.

[0084] The mixture comprising the polycarbonate solution and undissolved residue remains in the separation unit. The separation process can be operated under atmospheric pressure and / or under slight overpressure or under vacuum, e.g. in the range of 300 mbar(a) to 5 bar(a), preferably 1 bar(a) to 3 bar(a). During the separation process, undissolved residues are removed from the polycarbonate solution. The temperature of separation unit is kept at a temperature level so that precipitation of MEISSNER BOLTE M / EPC-101-PC

[0085] 12

[0086] PC due to cooling down of the PC solution is avoided. The removal of the undissolved residues from the polycarbonate solution can be effected by a common solid / liquid separation technique carried out in a cascade system comprising a separation unit and optionally a recovery unit and / or at least one filtration system .

[0087] The liquid solution, i.e. the polycarbonate solution, is discharged from the separation unit. This discharge of the polycarbonate solution can be effected via a pump, e.g. a lobe pump, a centrifugal pump or a vortex pump.

[0088] The polycarbonate solution discharged is preferably passing an additional filtration system to separate remaining undissolved residues down to a defined particle size.

[0089] A suitable filtration system may include one filtration stage or more preferably two or more filtration stages. Examples for suitable filtration systems or filtration stages are self-cleaning filters, equipped with mechanical cleaning by scraper-blades, candle-type filters, cartridge-type filters, screen-changer filters or a combination thereof.

[0090] The polycarbonate solution separated from the undissolved residue, preferably after passing the additional filtration system, can be used directly for the subsequent solvent removal step b3), but is preferably transferred into a buffer vessel for temporary storage.

[0091] After discharge of the polycarbonate solution from one or more side outlets of separation unit, the remaining undissolved residues as well as a smaller amount of polycarbonate solution will fall down in the bottom of the separation unit. By opening an outlet, preferably a bottom outlet, or valve of the vessel, the material composed of undissolved residues and remaining PC solution is discharged from the separation unit.

[0092] It is preferred to recover PC solution from the material discharged from the outlet, preferably the bottom outlet. In this regard, in a preferred embodiment an additional recovery unit is arranged downstream the separation unit to recover further PC solution from the material discharged from the outlet. The recovered PC solution can be combined with the PC solution discharged from the one or more outlets of the separation unit. MEISSNER BOLTE M / EPC-101-PC

[0093] 13

[0094] In general, the further PC solution recovered by the recovery unit is combined with the PC solution obtained via the one or more outlets of the separation unit before the combined PC solution stream is passed through the additional filtration system.

[0095] As mentioned before, the PC solution separated from the undissolved residues is preferably collected in a buffer vessel for temporary storage. The buffer vessel is generally operated under atmospheric pressure and / or under slight overpressure and / or under vacuum, e.g. in the range of 300 mbar(a) to 5 bar(a), preferably 1 bar(a) to 3 bar(a).

[0096] The buffer vessel can store several batches of polycarbonate solution obtained by repeating mixing step bl) and separating step b2) with several charges of polycarbonate waste and allows for continuous feed of the PC solution to the solvent removal step b3). Thus, the buffer vessel enables decoupling of upstream processes preferably operated batchwise (steps bl) and b2)) from downstream processes preferably operated continuously (step b3) and subsequent steps c) and d)).

[0097] Hence, in a preferred embodiment of the inventive process, step bl) and step b2) of the solvent-based purification are carried out batch-wise, whereas step b3) of the solvent-based recycling and both the partial depolymerization according to step c) and the repolymerization according to step d) are carried out in a continuous process.

[0098] Hence, in a preferred embodiment, the mixture of the polycarbonate solution and undissolved residue obtained in step bl) is transferred into a separation unit, wherein the undissolved residue is removed from the polycarbonate solution. Thereafter, the separated polycarbonate solution is preferably discharged from at least one outlet at the side portion of the separation unit.

[0099] Moreover, it is preferred that a filtration system is arranged downstream the separation unit, specifically downstream the side outlet thereof, through which the polycarbonate solution discharged from side outlet of the separation units passed to remove remaining undissolved residues, such as microparticles.

[0100] In addition, the polycarbonate solution obtained from step b2) is preferably stored in one or more buffer vessels which enables transition of batchwise operation, MEISSNER BOLTE M / EPC-101-PC

[0101] 14 preferably used for steps bl) and b2), to continuous operation, preferably used for step b3) and following steps c) and d).

[0102] In process step b3), the solvent from the polycarbonate solution is removed to obtain a purified polycarbonate in solid form and / or melt form. The removal of the solvent can be carried out e.g. by evaporating the solvent, by crystallization extraction, by precipitation using a precipitant or a combination thereof. The solvent can be evaporated e.g. by distillation or drying, such as spray drying. The solvent removed in step b3) is suitably re-used in step bl) as the solvent (solvent recycling).

[0103] If additives etc. are incorporated in the PC polymer, it may be preferably or necessary to remove the additives by a separate process step. These additives may be e.g. removed via precipitation out of the solution. Precipitation of PC can be achieved by adding a precipitant. A suitable precipitant is e.g. an anti-solvent for PC, such as water.

[0104] Crystallization extraction can be achieved by cooling the PC solution so that PC crystallizes from the solution. Optionally, partial removal of solvent by evaporation supports crystallization. In both cases, the crystallized PC or precipitated PC, respectively, is separated from the solvent by common techniques.

[0105] In a preferred embodiment, the solvent is removed by evaporation which is suitably carried out in one or more evaporation devices to which the PC solution obtained in step b2) is fed. Any common evaporating device can be used. Examples for a suitable evaporating device are an evaporation vessel, a falling film evaporator, a kettle boiler, a spray dryer, an extruder or a combination thereof.

[0106] The evaporation of the solvent from the PC solution in the one or more evaporation devices can be carried out at atmospheric pressure or under vacuum, preferably under vacuum, more preferably under a vacuum in the range of from 900 to 0.1 mbar(a), wherein the evaporation is preferably effected in stages, preferably in two or more evaporation devices arranged in line. Of course, heating of the PC solution in the evaporation device is usually necessary depending on the type of solvent and the pressure / vacuum applied. MEISSNER BOLTE M / EPC-101-PC

[0107] 15

[0108] One or more evaporating devices may be used. In a preferred embodiment, two or more evaporating devices connected in line are used to successively concentrate the PC solution from evaporation stage to evaporation stage.

[0109] In a preferred embodiment, two or more evaporating devices connected in line are used, wherein the last evaporating device is an extruder, which serves to remove residual solvent from the resulting purified polycarbonate. The extruder can be operated under low (rough) vacuum and / or fine vacuum at a temperature, where the purified polycarbonate is in melt form. The extruder also serves to degas the purified polycarbonate. In an alternative embodiment, two or more evaporating devices connected in line are used, wherein the last evaporating device is a spray dryer, which serves to remove residual solvent from the resulting purified polycarbonate. In this case, the purified polycarbonate is obtained in solid form.

[0110] Each of the one or more evaporation devices can be connected to individual vacuum pumps and / or compression stages. The evaporated solvent vapors are passing a condensation system composed of connected heat exchangers (e.g. tube bundle), or spray condensers, etc., due to vacuum conveying, in which the solvent vapors are condensed. The condensed solvent is suitable collected for recovery in a solvent storage and distribution system. The solvent storage and distribution system is suitably a storage vessel fitted with connecting lines to redistribute the collected solvent. In normal operation, the system is pumping solvent in a loop from the storage vessel outlet back to storage vessel inlet.

[0111] Due to the heat and / or cooling demand of the solvent, e.g. for decreasing the required dissolution time inside the dissolution vessel, the circulating solvent is heated and / or cooled via a heater (electrical or thermal oil) and / or a cooler (cooling water).

[0112] Another feature of the solvent-based purification plant part can be a potential energy recovery system. Thus, the circulating solvent can be used to condensate the incoming solvent vapors inside the condensation system, whilst transferring and reusing the condensation energy from the vapor to the loop circulating solvent of the central solvent storage and distribution system. Through this feature, the energy demand of the plant will decrease significantly. MEISSNER BOLTE M / EPC-101-PC

[0113] 16

[0114] In general, the purified polycarbonate in solid and / or melt form obtained should have a very low or neglectable solvent residue, e.g. less than 10000 ppm, preferably 0 to 1500 ppm solvent in polymer.

[0115] The purified polycarbonate obtained in step b3) is generally in solid form and / or in melt form. The melt form can be obtained at sufficient temperatures during operation. This can be achieved e.g. by using an extruder, in particular by using a device for degassing purposes, such as a degassing extruder, a degassing kneader, a falling-film evaporator, or a horizontal disk ring reactor. The solid form can be for instance a solid mass, e.g. a gel, or a powder. The solid form for instance a solid mass, e.g. a powder, can be obtained in particular by using a spray dryer.

[0116] The purified polycarbonate obtained in step b3), preferably in melt form, is then fed to the device for carrying out the partial depolymerization according to step c).

[0117] Process step c) - partial depolymerization of purified polycarbonate

[0118] In step c) of the inventive process, the purified polycarbonate is partially depolymerized, which comprises addition of water to the purified polycarbonate in step cl) and the partial depolymerization in step c2).

[0119] According to step cl) water, preferably demineralized water, is added to the purified polycarbonate to obtain a mixture comprising water and the purified polycarbonate. The water, preferably demineralized water, may be added to the device where the partial depolymerization is carried out or added into a feeding line where the purified polycarbonate is fed to said device.

[0120] Water, preferably demineralized water, is added to the purified polycarbonate in order to partially hydrolyse the PC polymers in partial depolymerization step c2), in particular in the first step of the partial depolymerization step c2).

[0121] The water proportion is an important factor to control the depolymerization degree and can be adjusted during operation as discussed below. The mixture comprising purified polycarbonate and water, preferably demineralized water, of step cl) may e.g. comprise 0.01 to 100 kg of water per 1 ton of purified polycarbonate, preferably 0.10 to 50 kg of water per 1 ton of purified polycarbonate, more preferably 0.25 to 25 kg of water to 1 ton of purified polycarbonate. MEISSNER BOLTE M / EPC-101-PC

[0122] 17

[0123] Preferably, the mixture comprising purified polycarbonate and water of step cl) further comprises a depolymerization catalyst. Any depolymerization catalyst known by the skilled person to support depolymerization or hydrolysis, respectively, or combinations thereof, can be used.

[0124] The depolymerization catalyst may be e.g. selected from alkali hydroxides, alkaline earth hydroxides, alkali oxides, alkaline earth oxides, quaternary phosphonium salts, or a mixture thereof, wherein the depolymerization catalyst may be added e.g. in form of an alkaline aqueous solution.

[0125] During the partial depolymerizing step c2), the mixture comprising water and the purified polycarbonate is partially depolymerized in a melted state to obtain a partially depolymerized polycarbonate. In general, the depolymerizing step includes hydrolysis of the purified polycarbonate (hydrolysis stage) and degassing of the reaction product (degassing stage).

[0126] The partially depolymerizing of step c2) may be carried out at a temperature of at least 100°C, preferably at least 120°C, more preferably at least 160°C. The temperature is generally not more than 350°C, such as in the range of 100°C to 300°C, preferably 120°C to 250°C. Preferably, the partially depolymerizing of step c2) is carried out at a temperature above the melting point of PC.

[0127] In a preferred embodiment, the partially depolymerizing of step c2) is carried out in stages with one or more stages, in particular one or more hydrolysis stages, at a pressure above 1 bar(a), preferably in the range of from 10 to 300 bar(a), more preferably 100 to 250 bar(a), and one or more stages, in particular one or more degassing stages, at a pressure equal to or below 1 bar(a), preferably 0.5 mbar(a) to 1 bar(a).

[0128] The gaseous by-product of hydrolysis I depolymerization reaction and water vapor are withdrawn, preferably from an upper part of hydrolysis stage.

[0129] The average chain length or melt flow rate (MFR) of the depolymerized PC obtained from step c2) depends on the mean chain length or melt flow rate (MFR) of the polycarbonate waste used as the starting material for the process. Thus, the mean chain length of the depolymerized PC obtained from step c2) may be e.g. in the range of 80% to 20%, preferably 60 to 30%, of the mean chain length of the purified polycarbonate entering the partially depolymerizing step c). Alternatively, MEISSNER BOLTE M / EPC-101-PC

[0130] 18 the MFR of the depolymerized PC obtained from step c2) may be e.g. in the range of 1.5 times to 60 times, preferably 2 times to 50 times, more preferably 2 times to 10 times, the MFR of the purified polycarbonate entering the partially depolymerizing step c).

[0131] A chain length of 1 represents 1 polymer unit.

[0132] For instance, the partially depolymerized polycarbonate obtained from step c2) may have e.g. a mean chain length in the range of 80 to 20. The corresponding mean chain length of the purified polycarbonate entering the partially depolymerizing step c) may have e.g. a mean chain length in the range of approximately 160 to approximately 50.

[0133] Alternatively, the partially depolymerized polycarbonate obtained from step c2) may have e.g. a MFR in the range of 40 to 245 g / 10 min. The corresponding MFR of the purified polycarbonate entering the partially depolymerizing step c) may have e.g. a MFR in the range of approximately 1 to 100 g / 10 min.

[0134] It is usually appropriate to carry out the partially depolymerizing of step c2) to a controlled degree of depolymerization. The degree of depolymerization is preferably controlled by determination of the melt flow rate (MFR) of the polycarbonate melt in step c2). The MFR can be determined offline, at line, online and inline, preferably inline. The MFR is preferably measured via inline process measurement. The data obtained can be used to control the partial depolymerization in real-time.

[0135] The determined MFR can be used to control the partial depolymerization by adjusting, preferably continuously adjusting, the amount of water added in step cl) (reverse control). For instance, if the determined MFR of the polycarbonate melt in step c2) deviates from the targeted MFR, i.e. the targeted depolymerization degree, the reaction conditions, in particular the amount of water added, can be adjusted to adjust the MFR of the polycarbonate melt to the targeted one. This has the advantage that the polycarbonate melt can be continuously adjusted to a targeted MFR or a targeted depolymerization degree, respectively.

[0136] The MFR also termed melt flow index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. The MFR is inversely proportional to the molecular weight or polymerization degree and can thus be used to monitor the depolymerization degree. The MFR can be determined according to MEISSNER BOLTE M / EPC-101-PC

[0137] 19

[0138] DIN EN ISO 1133-1:2022-10, using e.g. melt flow tester Instron-Ceast "MF 20". Dimensions: barrel width: 9.55 mm, capillary die width: 2.0955 mm, capillary die length: 8.00 mm; Parameters: load: 1.2 kg, temperature: 300 °C.

[0139] The depolymerized polycarbonate obtained in hydrolysis stage is then conveyed to the degassing stage in step c2). The volatile impurities and gaseous byproducts are then removed from an upper part of degassing stage by a vacuum unit. The degassing zone, also referred to as the vacuum zone, has the function to effectively release gases, moisture, residual monomers, or by-products that can lead to the formation of voids within the PC melt.

[0140] In a preferred embodiment, during and / or after, preferably after, partial depolymerizing in step c2) impurities are removed by a polymer filter, preferably by a continuous polymer filter, from the PC melt. The impurities to be removed are in particular potential degraded or highly cross-linked polymer chains.

[0141] In a preferred embodiment, the degree of depolymerization is controlled by determination of the melt flow rate (MFR) of the polycarbonate melt in step c2), wherein the MFR is preferably via inline process measurement, and during and / or after, preferably after, depolymerizing in step c2) impurities are removed by a polymer filter, preferably a continuous polymer filter.

[0142] The partial depolymerization of the polycarbonate can be carried out in an extruder or a kneader or in one or more reactors, preferably in an extruder. As mentioned before, it is preferred to carry out in two or more stages, including one or more hydrolysis / depolymerization stages and one or more degassing stages, with different pressures. In general, in a successive stage, the temperature is equal to or preferably higher and the pressure is equal to or preferably lower than in the preceding stage. An extruder can be operated with multiple zones in which the temperature / pressure conditions can be separately adjusted. Accordingly, two or more stages with different pressure / temperature conditions can be implemented in a single extruder.

[0143] In the partial depolymerization step c2), the PC polymer is hydrolyzed with added water, preferably demineralized water, optionally mixed with a depolymerization catalyst (hydrolysis stage of step c2). The long polymer chains are shortened at the molecular level, but not completely decomposed. Due to the water content in combination with the reaction conditions, depolymerization can take place in a MEISSNER BOLTE M / EPC-101-PC

[0144] 20 controlled manner. The polymer chain length, which is irregular in PC waste, is equalized by the adjustable depolymerization. In addition to that, other low volatile impurities within the polymer matrix could be degassed during this partial depolymerization process (degassing stage of step c2)). This new process step of partial depolymerization, compared to a state-of-the-art chemical recycling, also saves energy as the polymer chains are not fully destroyed.

[0145] The partial depolymerization reaction can be illustrated exemplary as follows (R1 = residue of the previous PC unit, R2 = Rest of the next PC unit):

[0146] As can be seen from the above equation, a hydroxyaryl compound such as phenol is released by the reaction. Depending on the pressure and temperature conditions, CO2 is also produced as a result of this reaction, as the OCOOH group on the polycarbonate is not stable enough to continue to exist.

[0147] In general, polycarbonate is commercially available with different molecular weights or polymer chain lengths. This parameter plays an important role in the selection of the polycarbonate depending on the specific application. The manufacturers generally specify the molecular weight or the viscosity / melt flow rate as a characterization of the product. The higher the molecular weight, the lower the MFR (melt flow rate). Since polymers are not individual molecules, the given molecular weight refers to an average related to the molecular weight distribution.

[0148] Compared to a pure physical recycling process such as mechanical recycling and solvent-based recycling, the inventive process including partial depolymerization and repolymerization can change the molecular weight back to that of the virgin products or similar to that. Conventional chemical recycling goes much further, since the polymer molecules are completely broken down into the monomers. This comes along with a huge energy demand of energy. The monomers generated may be used again to recycle polycarbonate, but again significant amounts of energy are consumed. Thus, the inventive process enables reproduction of "fresh" polycarbonate with a considerably lower energy demand as compared to conventional chemical recycling. As compared to mechanical recycling, the MEISSNER BOLTE M / EPC-101-PC

[0149] 21 polycarbonate obtained has a significantly higher quality so that it is even suitable for optical applications.

[0150] Step d) - Repolymerizing of the partially depolymerized polycarbonate

[0151] The repolymerizing step d), in which the partially depolymerized polycarbonate obtained from step c2) is repolymerized, comprises step dl) adding a carbonate ester, preferably a diarylcarbonate, to the partially depolymerized polycarbonate to obtain a mixture comprising the partially depolymerized polycarbonate and the carbonate ester.

[0152] As mentioned before, before step d) it is preferred to filtrate the depolymerized polycarbonate to remove impurities, e.g., potential degraded or highly cross-linked polymer chains. In step dl) a carbonate ester, preferably a diarylcarbonate, is added to the partially depolymerized polycarbonate to obtain a mixture comprising the partially depolymerized polycarbonate and the carbonate ester and in step d2) the mixture comprising the partially depolymerized polycarbonate and the carbonate ester is subjected to re-polymerization.

[0153] The carbonate ester is usually a diarylcarbonate (DAC), such as di-(Ce to Cw-aryl) carbonic acid ester, preferably diphenylcarbonate (DPC).

[0154] The carbonate ester, preferably DAC, in particular DPC, may be added to the device where the repolymerization is carried out or added into a feeding line where the partially depolymerized polycarbonate obtained from step c2) is fed to said device. Optionally, a mixer such as a static mixer may be implemented in the feeding line to support mixing of the partially depolymerized polycarbonate with the carbonate ester.

[0155] The addition of a carbonate ester, in particular a diarylcarbonate, enables generating -COOH end groups via transesterification. As mentioned before, OCOOH group on the polycarbonate are eliminated under generation of CO2 and / or hydroxyaryl during the partial depolymerization reaction.

[0156] A polycondensation catalyst is preferably added to the mixture comprising the partially depolymerized polycarbonate and the carbonate ester to support the repolymerization. One or more polycondensation catalyst may be used. The skilled person is familiar with suitable polycondensation catalysts for this reaction. MEISSNER BOLTE M / EPC-101-PC

[0157] 22

[0158] Examples for a suitable polycondensation catalyst are selected from alkali hydroxides, alkaline earth hydroxides, alkali oxides, alkaline earth oxides, quaternary phosphonium salts, or a mixture thereof. Examples of alkali hydroxides, alkaline earth hydroxides, alkali oxides, and alkaline earth oxides suitable for the present invention are sodium hydroxide, potassium hydroxide and lithium hydroxide. Examples of quaternary phosphonium salts are tetramethylphosphonium hydroxide, tetramethylphosphonium formate, tetramethylphosphonium acetate, tetramethylphosphonium benzoate, tetraethylphosphonium hydroxide, tetraethylphosphonium formate, tetraethylphosphonium acetate, tetraethylphosphonium benzoate, tetrabutylphosphonium hydroxide, tetrabutylphosphonium acetate, tetrabutylphosphonium benzoate, tetraphenylphosphonium hydroxide, tetraphenylphosphonium acetate, tetraphenylphosphonium phenolate, tetrabutylphosphonium acetate, tetramethylphosphonium tetraphenylborohydride, tetraphenylphosphonium bromide, tetraphenylphosphonium tetraphenylboranate, tetra (p-tert-butylphenyl) phosphonium diphenyl phosphate, triphenylbutylphosphonium phenolate, triphenylbutylphosphonium tetraphenylboranate, tetraphenylphosphonium chloride, tetraphenylphosphonium fluoride or a mixture thereof.

[0159] The mixture comprising the partially depolymerized polycarbonate and the carbonate ester, preferably diarylcarbonate, in particular DPC, of step dl) may comprise 1 to 500 kg of carbonate ester per 1 ton partially depolymerized polycarbonate, preferably 3.5 to 350 kg of carbonate ester per 1 ton partially depolymerized polycarbonate.

[0160] In step d2), the mixture comprising the partially depolymerized polycarbonate and the carbonate ester, optionally in the presence of a polycondensation catalyst, is repolymerized at a temperature of at least 200 °C and at a pressure below atmospheric pressure with removal of by-products generated to obtain the recycled polycarbonate.

[0161] The repolymerizing according to step d2) may be carried out e.g. at a temperature in the range of 280 °C to 320 °C, preferably 290 °C to 310°C, and at a pressure of 2 mbar(a) to 0.25 mbar(a), preferably 1.25 mbar(a) to 0.25 mbar(a).

[0162] The repolymerization is a polycondensation and takes place in form of a transesterification reaction with the carbonate ester, preferably diarylcarbonate, MEISSNER BOLTE M / EPC-101-PC

[0163] 23 more preferably diphenylcarbonate. The carbonate ester addition compensates for the loss of CO2 and / or hydroxyaryl caused during the depolymerization reaction.

[0164] The following two equations illustrate the mechanism of transesterification and polycondensation reaction. It is to be noted that the equations are based on monomers and for illustrative purpose only, since as discussed above the partially depolymerized polycarbonate obtained from step c2) which is repolymerized in step d2) are still (shortened) polymers but not monomers. However, the reaction principles equally apply.

[0165] A transesterification reaction is illustrated below with respect to the reaction between monomers bisphenol A (BPA) and diphenylcarbonate (DPC).

[0166] The polycondensation or polymer chain formation continues as already-formed shorter polymer chains react with each other to form longer polymer chains.

[0167] The partially depolymerized polycarbonate is re-polymerized during step d2). Byproducts such as water (demineralized water), arylhydroxide, such as phenol, BPA or DAC, in particular DPC, of the reaction and are removed with the aid of the high vacuum. In general, the by-products comprise arylhydroxide compound, in particular phenol, as the main component (cf. equations above). Trace amounts of water may be included in the volatile by-product. MEISSNER BOLTE M / EPC-101-PC

[0168] 24

[0169] The reaction can be adjusted using various parameters so that a relatively uniform product chain length can be set. In order to repolymerize (rebuild) the chains after partial depolymerization and to regain the desired molecular weight, the repolymerization (polycondensation) step is carried out. To ensure that the reaction goes in the right direction, the reaction conditions in the chemical equilibrium are important. Hence, the reaction is carried out at a pressure below atmospheric pressure. Cleaved byproducts such as arylhydroxide such as phenol and other volatile components are continuously removed from the reaction, thus driving the chemical equilibrium to the polycondensation reaction. The pressure in the reactor can be used to control the chain build-up / target molecular weight.

[0170] In a preferred embodiment, the repolymerizing of step d2) is carried out in a reactor, which is preferably a horizontal agitated reactor, such as a disk-ring reactor, or a cage-type reactor. The reactor is preferably a horizontal agitated reactor, in particular a disk-ring reactor.

[0171] The reactor preferably generates a high surface exchange rate with the highly viscous polycarbonate melt medium in order to ensure the evaporation of highly volatile materials. Moreover, to obtain an excellent quality, the so-called dead space of the reactor where polymer can settle is avoided. The residence time for the polymer melt, therefore, is defined via a constant but adjustable level.

[0172] In a preferred embodiment, the repolymerizing of step d2) includes a final MFR adjustment. In this regard, the MFR of the polycarbonate prepared during repolymerization is determined and depending on the result, the conditions such as temperature, pressure and / or residence time may be adjusted to control the MFR of the recycled polycarbonate to the targeted MFR.

[0173] In a preferred embodiment, during and / or after, preferably after, repolymerizing in step d2) impurities are removed by a polymer filter, preferably by a continuous polymer filter, from the PC melt. The impurities to be removed are in particular potential degraded or highly cross-linked polymer chains.

[0174] It goes without saying that the recycled polycarbonate obtained from the repolymerizing step d) has a higher mean chain length than the partially depolymerized polycarbonate obtained in step c) which is used as a starting material for repolymerizing step d). Analogously, the recycled polycarbonate obtained from MEISSNER BOLTE M / EPC-101-PC

[0175] 25 the repolymerizing step d) has a lower MFR than the partially depolymerized polycarbonate.

[0176] The recycled polycarbonate obtained from step d2) preferably has a chain length of about 60 to 200, preferably about 65 to 150. The weight average molecular weight (Mw) of the recycled polycarbonate obtained from step d2) is preferably in the range of about 16,000 to 38,000.

[0177] Suitable and preferred parameters for the purified polycarbonate obtained in step b3) (purified PC waste), the partially depolymerized polycarbonate obtained in step c2) (partially depolymerized PC waste) and recycled polycarbonate obtained in step d2) (recycled PC) are compiled in the following table. The preferred relation between the mean chain length and MFR, respectively, of the depolymerized PC and the purified polycarbonate have been discussed above.

[0178] After rebuilding the chain by repolymerization, a quality comparable to that of a new product can be produced again. As discussed above, the recycled PC obtained can be prepared with a desired average molecular weight and has a high quality so that it is also suitable for optical applications. Optical applications refer to applications where PC is used in a transparent form. For such applications, a PC of high quality is necessary.

[0179] The recycled polycarbonate is usually in melt form when leaving the device in which the repolymerization is effected. Usually, the subsequent treatments (finishing) are carried out to provide the recycled PC in a suitable form.

[0180] As such finishing, the process may comprise granulation or pelletizing of the recycled polycarbonate obtained from step d2). In the granulation or palletization, the recycled polycarbonate obtained in step d2) is solidified and generally cut into MEISSNER BOLTE M / EPC-101-PC

[0181] 26 chips. The chips are of commercially suitable quality and have a purity, which is suitable for many applications, in particular optical applications.

[0182] The invention is further directed to a plant for recycling polycarbonate from particulate polycarbonate waste, preferably according to a process of the invention as described above, wherein the plant comprises i) a dissolution vessel provided with a mixing means for mixing the particulate polycarbonate waste with a solvent to dissolve polycarbonate from the particulate polycarbonate waste according to step bl), ii) a separation unit for separating the polycarbonate solution from the undissolved residue according to step b2), iii) one or more devices, in particular one or more evaporating devices, for removing solvent from the polycarbonate solution according to step b3), iv) an extruder for partially depolymerizing according to step c2), and v) a horizontal agitated reactor or a cage-type reactor for repolymerizing according to step d2), wherein the horizontal agitated reactor is preferably a disk-ring reactor.

[0183] All indications as well as general and preferred embodiments described above for the inventive process equally apply so that reference is made thereto. In particular, the devices of the inventive plant have been discussed above with respect to the inventive process to which reference is made.

[0184] In a preferred embodiment, the inventive plant further comprises a recovery unit connected downstream to the outlet at the bottom of the separation unit to separate polycarbonate solution from the undissolved residue discharged .

[0185] In a preferred embodiment, the inventive plant further comprises a filtration system connected to the at least one outlet at the side portion of the separation unit, so that the polycarbonate solution discharged from the outlet and optionally polycarbonate solution recovered by the recovery unit can be passed through the filtration system to remove remaining undissolved residues such as microparticles.

[0186] In a preferred embodiment, the extruder for partially depolymerizing is provided with a MFR measurement system for determining the MFR of the polycarbonate melt included, preferably an inline process measurement system. MEISSNER BOLTE M / EPC-101-PC

[0187] TJ

[0188] In a preferred embodiment, the extruder for partially depolymerizing is provided with a polymer filter, preferably a continuous polymer filter, for removing byproducts from the polycarbonate melt. The polymer filter is generally located downstream of the extruder.

[0189] In a preferred embodiment, the inventive plant comprises a buffer vessel for temporary storing the polycarbonate solution obtained in step b2). The buffer vessel is provided with an outlet to feed the polycarbonate solution to the one or more devices for removing the solvent, in particular the one or more evaporation devices, wherein in case where two or more devices for removing the solvent in line are present, the polycarbonate solution is fed to the first of them.

[0190] In a preferred embodiment, the dissolution vessel and the separation unit are configured such that the content of the dissolution vessel can be gravimetrically transferred into the separation unit.

[0191] In a preferred embodiment, two or more evaporating devices in line are used for removing solvent from the polycarbonate solution, wherein the last of the evaporating devices is preferably an extruder.

[0192] In a preferred embodiment, the plant comprises a pelletizing system to solidify the recycled polycarbonate leaving the horizontal agitated reactor or cage-type reactor as a melt and cut the solidified polycarbonate into chips.

[0193] In a preferred embodiment, the plant comprises i) a dissolution vessel provided with a mixing means for mixing the particulate polycarbonate waste with a solvent to dissolve polycarbonate from the particulate polycarbonate waste according to step bl), ii) a separation unit for separating the polycarbonate solution from the undissolved residue according to step b2), iii) one or more devices, in particular one or more evaporating devices, for removing solvent from the polycarbonate solution according to step b3), e.g., an evaporation vessel followed by an extruder to obtain PC melt or an evaporation vessel followed by a spray dryer to obtain PC solid such as PC powder, iv) an extruder for partially depolymerizing according to step c2), in particular for water addition, partially depolymerizing via hydrolysis process and degassing, MEISSNER BOLTE M / EPC-101-PC

[0194] 28 v) optionally a polymer filter, in particular a continuous polymer filter, for removing impurities from the PC melt obtained from step c2), vi) a mixing means, in particular a static mixer, for mixing the PC melt with a carbonate ester, preferably a diarylcarbonate, according to step d l), vii) a horizontal agitated reactor or a cage-type reactor for repolymerizing according to step d2), wherein the horizontal agitated reactor is preferably a disk-ring reactor, viii) a filtration system connected to the at least one outlet at the side portion of the separation unit, so that the polycarbonate solution discharged from the outlet and the polycarbonate solution separated by the recovery unit, which are preferably combined to a single polycarbonate solution stream, can be passed through the filtration system to remove remaining undissolved residues such as microparticles, and ix) the extruder for partially depolymerizing is provided with a MFR measurement system for determining the MFR of the polycarbonate melt included, preferably an inline process measurement system.

[0195] Besides the process steps and devices described before, including a solvent recovery and purification system, common additional process units are usually employed for operation of the recycling plant, e.g. an economizing system for heat (energy) recovery, vacuum generation and utility generation.

[0196] The invention is further illustrated by the accompanying drawing. The drawing is provided for illustration purposes only and are by no way intended to limit the scope of the present invention.

[0197] Fig. 1 is a block flow diagram giving a schematic overview of the inventive process including optional additional features. Providing of particulate PC waste usually includes pre-sorting of (mixed) waste and shredding and / or milling (steps (al), (a2). The particulate PC waste provided is subjected to a solvent-based recycling process of the PC waste including dissolving, purification by separating undissolved residues and solvent removal to obtain a purified PC (steps (bl), (b2), (b3). The solvent removed is recovered for re-use. The purified PC is subjected to a chemical recycling including a partial depolymerization (steps (cl), (c2)) and repolymerization of the partial depolymerized PC (steps (dl), (d2)). Partial depolymerization is controlled via MFR adjustment for which the MFR value of the reaction product is monitored. Re-polymerization of the partial depolymerized PC is a polycondensation reaction where by-products are removed by a vacuum system MEISSNER BOLTE M / EPC-101-PC

[0198] 29 and includes a final MFR adjustment. The PC melt obtained can by subjected to finishing to prepare PC chips of high quality.

Claims

MEISSNER BOLTE M / EPC-101-PC30Claims1. A process for the recycling of polycarbonate from particulate polycarbonate waste comprising the following steps: a) providing the particulate polycarbonate waste, b) solvent-based purification of the particulate polycarbonate waste comprising bl) mixing the particulate polycarbonate waste with a solvent to dissolve polycarbonate from the particulate polycarbonate waste to obtain a mixture of a polycarbonate solution and undissolved residue, b2) separating the polycarbonate solution from the undissolved residue in the mixture obtained, b3) removing solvent from the polycarbonate solution to obtain a purified polycarbonate in solid form and / or melt form, c) partially depolymerizing of the purified polycarbonate comprising cl) adding water, preferably demineralized water, to the purified polycarbonate to obtain a mixture comprising water and the purified polycarbonate, c2) partially depolymerizing of the mixture comprising water and the purified polycarbonate in a melted state to obtain a partially depolymerized polycarbonate, d) repolymerizing of the partially depolymerized polycarbonate comprising dl) adding a carbonate ester, preferably a diarylcarbonate, to the partially depolymerized polycarbonate to obtain a mixture comprising the partially depolymerized polycarbonate and the carbonate ester, d2) repolymerizing the mixture comprising the partially depolymerized polycarbonate and the carbonate ester at a temperature of at least 200 °C and at a pressure below atmospheric pressure with removal of by-product generated to obtain the recycled polycarbonate.

2. The process according to claim 1, wherein the particulate polycarbonate waste provided has a polycarbonate content of 5 to 100 % by weight, preferably 10 to 100 % by weight, more preferably 20 to 100% by weight, such as 10 to 90% byMEISSNER BOLTE M / EPC-101-PC31 weight, such as 20 to 90% by weight, based on the dry weight of the particulate polycarbonate waste, and / or wherein the mean particle size of the particulate polycarbonate waste is less than 100 mm, more preferably less than 50 mm, and preferably not less than 1 mm.

3. The process according to claim 1 or claim 2, wherein the particulate polycarbonate waste provided is obtained by shredding or crushing a polycarbonate waste selected from a pre-sorted waste, a compacted waste, an off spec PC waste or a combination thereof.

4. The process according to any one of the preceding claims, wherein the solvent to dissolve polycarbonate is selected from the group consisting of ketones, ethers, such as linear or cyclic ethers, cycloalkanes, esters or combinations thereof, in particular acetone, methyl ethyl ketone, tetrahydrofuran, dialkyl esters of dicarboxylic acids, fatty acid alkyl esters, dichloromethane (DCM), chloroform, 1-chloro-l-nitroethane, bromotrichloro methane, ethylene chloride, cresol, xylene, aniline, anisole, benzoyl chloride, trichlorophenol, chlorotoluene, tricresyl phosphate, phosphorous oxychloride, cyclopentanone, cyclohexanone, methyl isoprenyl ketone, methyl isobutyl ketone (MIBK), 3-penten-2-one, methacrylic acid, 1,4-dioxane, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), carbon disulfide or combinations thereof.

5. The process according to any one of the preceding claims, wherein mixing the particulate polycarbonate waste with the solvent according to step bl) is carried out at a temperature between the melting temperature and the boiling temperature of the solvent and / or below the degradation point of the polycarbonate, e.g. in the range of 35 to 255 °C, and / or the weight percentage of the particulate polycarbonate waste based on total weight of the mixture of particulate polycarbonate waste and solvent is in the range of from 5 to 50 %, preferably of from 10 to 30 %.

6. The process according to any one of the preceding claims, wherein mixing the particulate polycarbonate waste with the solvent according to step bl) is carried out in a dissolution vessel provided with aMEISSNER BOLTE M / EPC-101-PC32 mixing means, and / or wherein separating the polycarbonate solution from the undissolved residue according to step b2) is carried out in a separation unit.

7. The process according to claim 6, wherein the mixture of the polycarbonate solution and undissolved residue obtained in step bl) is transferred into the separation unit to remove the undissolved residue from the polycarbonate solution, whereafter the separated polycarbonate solution is discharged from the separation unit, wherein the discharged polycarbonate solution is preferably passed through a filtration system to remove remaining undissolved residues, wherein the mixture of the polycarbonate solution and undissolved residue obtained in step bl) is preferably gravimetrically transferred from the dissolution vessel into the separation unit.

8. The process according to claim 6 or claim 7, wherein after discharge of the polycarbonate solution from the separation unit, the undissolved residues and remaining polycarbonate solution is discharged from an outlet of the separation unit and fed to a recovery unit to recover remaining polycarbonate solution from the undissolved residues discharged.

9. The process according to any one of the preceding claims, wherein removing solvent in step b3) is carried out by evaporating the solvent, crystallization extraction, precipitation using a precipitant or a combination thereof, and / or wherein the solvent removed in step b3) is re-used in step bl), and / or wherein the purified polycarbonate in solid form and / or melt form obtained in step b3) is a gel, a melt or a powder.

10. The process according to any one of the preceding claims, wherein the mixture comprising purified polycarbonate and water of step cl) comprises 0.01 to 100 kg of water per 1 ton of purified polycarbonate, preferably 0.10 to 50 kg of water per 1 ton of purified polycarbonate, most preferably 0.25 to 25 kg of water to 1 ton of purified polycarbonate, and / orMEISSNER BOLTE M / EPC-101-PC33 wherein the mixture comprising purified solid polycarbonate and water of step cl) further comprises a depolymerization catalyst.

11. The process according to any one of the preceding claims, wherein partially depolymerizing of step c2) is partially depolymerizing to a controlled degree of depolymerization, wherein the degree of depolymerization is preferably controlled by determination of the melt flow rate (MFR) of the polycarbonate melt in step c2), wherein the MFR is preferably measured via inline process measurement.

12. The process according to any one of the preceding claims, wherein during and / or after, preferably after, depolymerizing in step c2) impurities are removed by a polymer filter, preferably a continuous polymer filter, and / or wherein during and / or after, preferably after, repolymerizing in step d2) impurities are removed by a polymer filter, preferably a continuous polymer filter.

13. The process according to any one of the preceding claims, wherein the mixture comprising the partially depolymerized polycarbonate and the carbonate ester, preferably diarylcarbonate, of step dl) comprises 1 to 500 kg of carbonate ester per 1 ton partially depolymerized polycarbonate, preferably 3.5 to 350 kg of carbonate ester per 1 ton partially depolymerized polycarbonate.

14. The process according to any one of the preceding claims, wherein partially depolymerizing of step c2) is carried out at a temperature of at least 100°C, preferably at least 120°C, more preferably at least 160°C, such as 100°C to 300°C, preferably 120°C to 250°C, and / or wherein repolymerizing of step d2) is carried out at a temperature in the range of 280 °C to 320 °C, preferably 290 °C to 310°C, and at a pressure of 2 mbar(a) to 0.25 mbar(a), preferably 1.25 mbar(a) to 0.25 mbar(a).

15. The process according to any one of the preceding claims, wherein partially depolymerizing of step c2) is carried out in stages with one or more stages at a pressure above 1 bar(a), preferably in the range of from 10 to 300 bar(a), more preferably 100 to 250 bar(a), and one or moreMEISSNER BOLTE M / EPC-101-PC34 stages at a pressure equal to or below 1 bar(a), preferably 0.5 mbar(a) to 1 bar(a).

16. The process according to any one of the preceding claims, wherein the purified polycarbonate entering step c) has a mean chain length of about 50 to 160, and / or wherein the partially depolymerized polycarbonate obtained from step c2) has a mean chain length of 20 to 80, and / or wherein the recycled polycarbonate obtained from step d2) has an mean chain length of about 60 to 200, preferably about 65 to 150, and / or a weight average molecular weight (Mw) in the range of about 16,000 to 38,000.

17. The process according to any one of the preceding claims, wherein partially depolymerizing of step c2) is carried out in an extruder or in one or more reactors, and / or wherein repolymerizing of step d2) is carried out in a horizontal agitated reactor or a cage-type reactor, preferably a horizontal agitated reactor in form of a disk-ring reactor.

18. The process according to any one of the preceding claims, wherein step bl) and step b2) of the solvent-based recycling of the polycarbonate waste is carried out batch-wise and step b3) of the solvent-based recycling and both the partial depolymerization according to step c) and the repolymerization according to step d) are carried out in a continuous process.

19. The process according to any one of the preceding claims, wherein the repolymerizing of step d2) includes a final MFR adjustment, and / or the process comprises granulation or pelletization of the recycled polycarbonate obtained from step d2).

20. A plant for the preparation of recycled polycarbonate from particulate polycarbonate waste according to the process of any one of claims 1 to 19, wherein the plant comprises i) a dissolution vessel provided with a mixing means for mixing the particulate polycarbonate waste with a solvent to dissolveMEISSNER BOLTE M / EPC-101-PC35 polycarbonate from the particulate polycarbonate waste according to step bl), ii) a separation unit for separating the polycarbonate solution from the undissolved residue according to step b2), iii) one or more devices, preferably one or more evaporating devices, for removing solvent from the polycarbonate solution according to step b3), e.g., an evaporation vessel followed by an extruder to obtain PC melt or an evaporation vessel followed by a spray dryer to obtain PC solid such as PC powder, iv) an extruder or kneader for partially depolymerizing according to step c2), in particular for water addition, partially depolymerizing via hydrolysis process and degassing, v) optionally a polymer filter, in particular a continuous polymer filter, for removing impurities from the PC melt obtained from step c2), vi) a mixing means, in particular a static mixer, for mixing the PC melt with a carbonate ester, preferably a diarylcarbonate, according to step dl), and vii) a horizontal agitated reactor or a cage-type reactor for repolymerizing according to step d2), wherein the horizontal agitated reactor is preferably a disk-ring reactor.

21. The plant according to claim 20, further comprising a recovery unit connected downstream to the outlet of the separation unit to recover PC solution from the undissolved residue discharged, and / or a filtration system connected to the at least one outlet at the side portion of the separation unit, so that the polycarbonate solution discharged from the outlet can be passed through the filtration system to remove remaining undissolved residues such as microparticles.

22. The plant according to claim 20 or claim 21, wherein the extruder for partially depolymerizing is provided with a MFR measurement system for determining the MFR of the polycarbonate melt included, preferably an inline process measurement system, and / or the extruder for partially depolymerizing is provided with a polymer filter, preferably a continuous polymer filter, for removing by-MEISSNER BOLTE M / EPC-101-PC36 products from the polycarbonate melt, wherein the polymer filter is preferably located downstream of the extruder.

23. The plant according to any of claims 20 to 22, including at least one or all of the following features: a buffer vessel for temporary storing the polycarbonate solution obtained in step b2), the dissolution vessel and the separation unit are configured such that the content of the dissolution vessel can be gravimetrically transferred into the separation unit, two or more evaporating devices in line are used for removing solvent from the polycarbonate solution, wherein the last evaporating device is an extruder, a pelletizing system to solidify the recycled polycarbonate leaving the horizontal agitated reactor, or disk-ring reactor, or a cage-type reactor as a melt and cut the solidified polycarbonate into chips.

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