Process for producing polycarbonate

Abstract

This invention relates to a method for producing polycarbonate, comprising the steps of: a) providing a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate based on the weight of the solution; b) concentrating and heating the solution of step a) to form a concentrated polycarbonate melt containing solvent, wherein the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of solvent based on the weight of the concentrated polycarbonate melt; and c) further concentrating the concentrated polycarbonate melt of step b) to polycarbonate containing up to 500 ppm of organic solvent using one or more mechanical refiners, wherein the concentrated polycarbonate melt is maintained in a molten state under vacuum conditions at a pressure of up to 500 mbar, and wherein the mechanical refiner continuously or intermittently allows for renewal of the surface area of ​​the polycarbonate melt exposed to vacuum.

Description

Technical Field

[0001] This invention relates to a method for producing polycarbonate. Background Technology

[0002] Polycarbonate is a widely used thermoplastic material, favored for its excellent transparency and physical toughness. A preferred method for producing polycarbonate is often referred to as the "interface method," which involves reacting phosgene and bisphenol A in a two-phase system containing an aqueous phase and a non-aqueous phase of a solution of polycarbonate in an organic solvent. The aqueous phase is then typically removed, and the non-aqueous phase is purified to remove residual salts, catalysts, and other impurities. After purification, the organic solvent is removed, and the polycarbonate is separated from the non-aqueous phase.

[0003] Several methods are known for separation. The residual organic solvent content in the final product should be as low as possible, as they can interfere with polycarbonate. Separation yields polycarbonate powder, which can be mixed with any other components and extruded into granules. Separation can also directly yield granules.

[0004] US10435507 discloses a method for obtaining aromatic polycarbonate with low residual organic solvent content. A polymer solution containing 65 wt% polycarbonate, 33.5 wt% chlorobenzene, and 1.5 wt% dichloromethane is heated in a vertical stripper and then fed into a separation vessel. The residual chlorobenzene content is 5000 ppm, and the residual dichloromethane content is 50 ppm. The concentrated polymer solution is supplied to a static mixer, to which nitrogen gas is added up to 0.1 wt%. The nitrogen-containing polymer melt is further stripped in a foam stripper consisting of a separation vessel located directly above a stripper extruder. The residual chlorobenzene content downstream of the foam stripper is 20 ppm. The stripper extruder is equipped with three stripper zones. The residual chlorobenzene content is 2 ppm. Dichloromethane is no longer detectable (<0.1 ppm).

[0005] It is also desirable that the content of degradation products in the obtained polycarbonate be as low as possible. Summary of the Invention

[0006] The purpose of this invention is to provide a method for producing polycarbonate with low organic solvent residue content and low degradation product content.

[0007] Therefore, the present invention provides a method for producing polycarbonate, comprising the following steps:

[0008] a) Provide a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution.

[0009] b) Concentrate and heat the solution from step a) to form a concentrated polycarbonate melt containing an organic solvent, wherein, based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of organic solvent, and

[0010] c) Using one or more mechanical refiners, the concentrated polycarbonate melt of step b) is further concentrated to a polycarbonate containing up to 500 ppm of organic solvent, wherein within one or more mechanical refiners, the concentrated polycarbonate melt is kept in a molten state under vacuum conditions at a pressure of up to 500 mbar, and wherein the mechanical refiner continuously or intermittently allows for renewal of the surface area of ​​the polycarbonate melt exposed to the vacuum.

[0011] The method of this invention uses a mechanical refiner to prepare polycarbonate with a low organic solvent content. The mechanical refiner used in the method according to the invention continuously generates free surface area for volatile components to diffuse from the liquid polymer phase to the polymer-gas interface and into the continuous gas phase. This is carried out under vacuum to strip the volatiles and reduce the solvent concentration.

[0012] According to the invention, a solution containing 5-40% by weight of polycarbonate is concentrated to a solvent concentration exceeding 500 ppm and at most 5% by weight (step b); subsequently, the concentrated polycarbonate is further concentrated using a mechanical refiner to a solvent concentration of at most 500 ppm (step c). The mechanical refiner has a relatively long residence time, thus applying lower shear stress to the processed material and avoiding a significant increase in local processing temperature. Consequently, compared to, for example, a degassed extruder, the mechanical refiner generates less heat on the processed material, resulting in less product degradation. Therefore, due to the use of a mechanical refiner in this concentration step, the method of the invention advantageously provides polycarbonate with low organic solvent residue and low degradation product content.

[0013] Step a)

[0014] Step a) includes providing a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution. The solution may contain 60-95% by weight of the organic solvent, based on the weight of the solution. This concentration range of polycarbonate in the solution results in a relatively low viscosity, making it easier to process in subsequent concentration steps.

[0015] This solution can be obtained through the interfacial polycarbonate method, which is well known in itself. In some preferred embodiments, the solution to be used in subsequent step b) is a solution directly from the polymerization reactor of the interfacial polycarbonate method.

[0016] Preferably, the polycarbonate solution contains 15-30% by weight of polycarbonate, based on the weight of the solution. The solution may contain 70-85% by weight of organic solvent.

[0017] Preferably, the organic solvent comprises 70% to 90% by weight of dichloromethane.

[0018] Preferably, the organic solvent is dichloromethane, chlorobenzene, or a mixture of chlorobenzene and dichloromethane, with dichloromethane being the most preferred organic solvent.

[0019] Step b)

[0020] Step b) involves concentrating and heating the solution from step a) to form a concentrated polycarbonate melt containing an organic solvent. Based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of solvent.

[0021] Preferably, step b) is performed by subjecting the solution from step a) to at least two devolatilization stages in series. Each devolatilization stage includes heating the solution via a heat exchanger, followed by feeding the heated solution into a degassing vessel. In the degassing vessel, at least a portion of the solvent is separated from the solution by evaporation. The number of devolatilization stages in series in step b) can be, for example, 2, 3, 4, 5, or 6, preferably 2 or 3.

[0022] The solution can be supplied to the heat exchanger from the top to flow downwards, or from the bottom to flow upwards.

[0023] Preferably, the solution is supplied to the heat exchanger at the bottom. This arrangement provides an additional degree of freedom, represented by orifices or control valves for regulating the pressure between the outlet of the heat exchanger and the inlet of the degassing vessel.

[0024] Therefore, in some preferred embodiments, at least one heat exchanger is supplied with a solution at the bottom of the heat exchanger, and the solution is heated within the heat exchanger by flowing upward from the bottom to the top. In a specific embodiment, each heat exchanger is supplied with a solution at the bottom of the heat exchanger, and the solution is heated within the heat exchanger by flowing upward from the bottom to the top.

[0025] In other embodiments, at least one heat exchanger is supplied with a solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top to the bottom of the heat exchanger. More preferably, each heat exchanger is supplied with a solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top to the bottom of the heat exchanger.

[0026] A concentration step via such a devolatilization stage (resulting in a concentrated polycarbonate melt with more than 500 ppm and at most 5% by weight of solvent) is advantageous because a subsequent mechanical refiner can effectively reduce the solvent volume to the desired level. Reducing the solvent to 500 ppm or less via a devolatilization stage requires an undesirable amount of energy and significant complexity when dealing with high viscosity involved.

[0027] The concentration step of the solution in step a) is performed by selecting the pressure and temperature for each stage of the devolatification process to achieve the desired solvent concentration. According to the invention, when the organic solvent contains a relatively large amount of dichloromethane, the devolatification should be carried out at a relatively high pressure. For example, excessively low pressures, typically used for the removal of chlorobenzene, may remove too much solvent, increasing the viscosity to such a high level that the solution becomes difficult to flow through pipes and systems.

[0028] Step c)

[0029] Subsequently, in step c), the concentrated polycarbonate melt from step b) is transferred to one or more mechanical refiners to further remove solvent. The concentrated polycarbonate melt obtained by step c) contains up to 500 ppm of organic solvent. In some embodiments, the amount of organic solvent in the polycarbonate obtained from step c) is up to 100 ppm, preferably up to 60 ppm, more preferably up to 50 ppm, and even more preferably up to 10 ppm. In some embodiments, the amount of organic solvent in the polycarbonate obtained from step c) is from 5 ppm to 50 ppm.

[0030] Dichloromethane is more difficult to remove from polycarbonate melts than, for example, chlorobenzene. In particular, the difficulty in removing dichloromethane from polycarbonate melts results in very low concentrations, for example, at most 500 ppm. Therefore, the present invention is particularly advantageous when dichloromethane is a large proportion of the organic solvent.

[0031] Within a mechanical refiner, concentrated polycarbonate melt is maintained in a molten state under vacuum conditions and a pressure of up to 500 mbar. Preferably, the temperature within the mechanical refiner is 250°C to 350°C. Preferably, the pressure is 1 to 500 mbar, for example, 50 to 500 mbar.

[0032] Unlike degassed extruders, the internal volume occupied by polycarbonate melt in mechanical refiners is relatively low. Preferably, the volume occupied by polycarbonate melt in the mechanical refiner is at most 60% of the internal volume of the mechanical refiner, more preferably at most 50%.

[0033] The residence time in a mechanical refiner is typically longer than that in a degassed extruder. Preferably, the residence time in a mechanical refiner is at least 5 minutes, for example, from 10 minutes to 2 hours.

[0034] Preferably, the mechanical refiner is selected from at least one of a disc ring reactor, a cage reactor, a thin-film evaporator or a scraped-film evaporator, a falling-film evaporator, a horizontal twin-shaft polymerizer, a twin-screw kneader, and a wire wet-fall polymerizer. These devices are well known in themselves.

[0035] For example, a disc-ring reactor is described in US7550116, which is incorporated herein by reference. A disc-ring reactor is typically a cylindrical horizontal heated vessel with inlet and outlet connections at opposite ends for the precondensate and the condensate. The disc-ring reactor comprises multiple elements rotating about a horizontal axis that mix the precondensate and create a large surface area for degassing the condensate as a viscous liquefied material adhering to these elements flows down. The design of the discs can be optimized along the reactor's axis to accommodate varying viscosities at each point. A disc-ring reactor is also described in US2005222371, which is incorporated herein by reference. In some preferred embodiments, the mechanical refiner is a disc-ring reactor, and the residence time in the disc-ring reactor is from 10 minutes to 3 hours, for example, from 60 minutes to 3 hours.

[0036] For example, a coil reactor is described in US20020188091, WO2004101140A1, and WO2007128159A1, which are incorporated herein by reference. A coil reactor comprises a rotatable cylindrical basket having cylindrical porous walls and annular disks spaced at intervals around the perimeter of the basket and along its length. The disks can be designed to be optimized along the reactor's axis to accommodate varying viscosities at each point. In some preferred embodiments, the mechanical refiner is a coil reactor, and the residence time in the coil reactor is from 10 minutes to 3 hours, for example, from 60 minutes to 3 hours.

[0037] For example, US10384145 describes thin-film evaporators and scraped-film evaporators, which are incorporated herein by reference. Typically, a thin-film evaporator includes a vertical or horizontal drum, a supply line for supplying the product to be evaporated, a heating jacket arranged around the periphery of the drum, a discharge line for discharging any remaining residue, and a discharge line for discharging the evaporation section of the product. The general purpose of a thin-film evaporator is to evaporate a volatile fluid from a less volatile fluid. Evaporation occurs through contact between the product and the heated walls of the drum. To improve evaporation efficiency, the drum is equipped with an agitator. Agitation can be achieved in various ways. One well-known type is a fixed-gap agitator, in which a thin gap is formed between the agitator and the inner wall of the drum, thereby pushing the product into the gap during rotation. Another type is a so-called scraped-film agitator, in which the agitator scrapes the product against the inner wall of the drum, thereby forming a thin product film. In some preferred embodiments, the mechanical refiner is a thin-film evaporator or a scraped-film evaporator, and the residence time in the thin-film evaporator or scraped-film evaporator is at least 5 minutes and less than 10 minutes.

[0038] For example, falling film evaporators are described in US9040639 and WO2003042278A1, which are incorporated herein by reference. The process fluid to be evaporated flows downwards in the form of a continuous film under gravity. The fluid forms a film along the tube wall and travels downwards (falls). In some preferred embodiments, the mechanical refiner is a falling film evaporator, and the residence time in the falling film evaporator is at least 5 minutes and less than 10 minutes.

[0039] For example, horizontal biaxial polymerizers are described in EP0529093B1 and US6846103, which are incorporated herein by reference. A horizontal biaxial polymerizer is a polymerizer having two horizontally rotating shafts equipped with blades, which may be disc, pin, spectacle, or wheel-type. The horizontal biaxial polymerizer preferably includes a distillation column. The distillation column is used to prevent the starting compound from escaping from the system during the removal of byproducts. In some preferred embodiments, the mechanical purifier is a horizontal biaxial polymerizer, and the residence time in the horizontal biaxial polymerizer is 10 to 60 minutes.

[0040] For example, a twin-screw kneader is described in US20090304800A1, which is incorporated herein by reference. Examples of twin-screw kneaders include the KRC kneader from Kurimoto, Ltd. In some preferred embodiments, the mechanical refiner is a twin-screw kneader, and the residence time in the horizontal twin-screw kneader is 10 to 30 minutes.

[0041] For example, wire wet-fall polymerizers are described in US5589564A, CA2168630C, US6277945B1, US6320015B1, US7528213B2, and US9321884B2, which are incorporated herein by reference. In a wire wet-fall polymerizer, molten prepolymer is allowed to fall along and contact the surface of a guide (e.g., a wire), thereby achieving polymerization of the molten prepolymer to produce the desired polymer. In some preferred embodiments, the mechanical refiner is a wire wet-fall polymerizer, and the residence time in the wire wet-fall polymerizer is 10 to 60 minutes.

[0042] Preferably, an inert gas other than the solvent is added to the mechanical cleaner. Examples of inert gases include nitrogen, argon, carbon dioxide, water, methane, helium, and combinations thereof.

[0043] Step d)

[0044] The method according to the invention may further include the step of feeding the polycarbonate melt obtained by step c) into an extruder to obtain pellets.

[0045] Preferably, the volume occupied by the polycarbonate melt in the extruder is greater than 60% of the internal volume of the extruder, more preferably at least 70% or at least 80%.

[0046] Preferably, the residence time in the extruder is less than 5 minutes, for example, 10 seconds to 3 minutes or 20 seconds to 1 minute.

[0047] The extruder can be a degassed extruder that includes one or more degassed zones. This further reduces the organic solvent content. Preferably, in a degassed extruder, the amount of organic solvent in the polycarbonate is reduced to at most 10 ppm.

[0048] Operating a degassed extruder allows for the acquisition of the desired organic solvent content. For example, the temperature, pressure, screw speed (RPM), and output rate (kg / h) can be adjusted. The degassed extruder can operate at, for example, 280 to 350°C. To prevent yellowing of the polycarbonate, the temperature in the degassed extruder is preferably set to 280 to 320°C.

[0049] Degassing extruders or devolatile matter extruders are known, for example, in US10435507 and in the book "Der gleichläufige Doppelschneckenextruder" (Co-rotating Twin-Screw Extruder), by Klemens Kohlgrüber, published by Carl Hanser Verlag, ISBN 978-3-446-41252-1, cited in US10435507, pages 193-195.

[0050] In some implementations, the extruder is not a degassed extruder, that is, the extruder does not include a degassed zone.

[0051] In an extruder, concentrated polycarbonate melt can be combined with additives or other components. The composition obtained from the extruder can be cut into pellets.

[0052] Suitable examples of optional additives include one or more of the following: impact modifiers, flow modifiers, fillers, reinforcing agents (e.g., glass fiber or talc), antioxidants, heat stabilizers, light stabilizers, UV stabilizers and / or UV absorbers, plasticizers, lubricants, mold release agents (particularly glyceryl monostearate, pentaerythritol tetrastearate, glyceryl tristearate, stearoyl stearate), antistatic agents, antifogging agents, antimicrobial agents, colorants (e.g., dyes or pigments), and flame retardants (with or without anti-dripping agents such as polytetrafluoroethylene (PTFE) or PTFE-encapsulated styrene-acrylonitrile copolymers). The invention is not limited in the type and amount of additives, and embodiments in which the additives exemplified above are not added may exist.

[0053] Other components that can be added to the extruder may be, for example, at least one other polymer, preferably selected from polycarbonate-polysiloxane copolymers, polycarbonate-polyester copolymers, polyesters, polyolefins, acrylonitrile / butadiene / styrene copolymers, methyl methacrylate / butadiene / styrene copolymers, styrene / butadiene / styrene copolymers (SBS), styrene / ethylene-butene / styrene copolymers (SEBS), styrene / ethylene-propylene / styrene copolymers (SEPS), styrene / acrylonitrile copolymers (SAN), acrylonitrile / styrene / acrylonitrile copolymers (ASA), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), unsaturated polyesters (UPES), polyamides (PA), thermoplastic polyurethanes (TPU), polystyrene (PS), high-impact polystyrene (HIPS), polyvinyl chloride (PVC), polyetherimide, and polysulfone.

[0054] Example

[0055] The present invention will now be described through the following embodiments, but the present invention is not limited thereto.

[0056] Figure 1 An embodiment of a system for implementing the method for producing polycarbonate according to the present invention is shown.

[0057] exist Figure 1The system comprises a series of three devolatilization stages, each stage including a heat exchanger and a degassing vessel. A solution of polycarbonate in an organic solvent is provided, containing 5-40 wt% polycarbonate based on the weight of the solution. This solution is supplied via line 10 to a first heat exchanger 100, and subsequently to a first degassing vessel 110, where a portion of the organic solvent is removed via line 20, thereby concentrating the solution. A first pump 120 is used to supply the solution from the first degassing vessel 110 to a second heat exchanger 200, and subsequently to a second degassing vessel 210, where a portion of the organic solvent is removed via line 20, further concentrating the solution. A second pump 220 is used to supply the solution from the second degassing vessel 210 to a third heat exchanger 300, and subsequently to a third degassing vessel 310, where a portion of the organic solvent is removed via line 20, further concentrating the solution. A concentrated polycarbonate melt containing solvent is obtained, wherein the amount of solvent is at most 5 wt% and greater than 500 ppm based on the weight of the concentrated polycarbonate melt.

[0058] The third pump 320 is used to supply concentrated polycarbonate melt to a mechanical refiner 400, in which the concentration of organic solvent is further reduced to at most 500 ppm.

[0059] A fourth pump 420 supplies concentrated polycarbonate melt to a degassed extruder 500, in which the concentration of organic solvents is further reduced. Additives are supplied to the degassed extruder. This yields polycarbonate composition pellets containing polycarbonate and additives with low organic solvent content.

[0060] simulation

[0061] Table 1-3 shows the simulated results of polycarbonate concentrations in polycarbonate solutions subjected to a series of devolatation stages at different temperatures and pressures (step b of the method of the present invention).

[0062] Modeling was performed using Aspen Plus software, and the phase behavior of the polymer and solvent system was calculated using the Poly-NRTL thermodynamic model.

[0063] Viscosity modeling was performed based on experimental data, in which the viscosity of polycarbonate solutions in up to 80% by weight of dichloromethane was measured. A suitable viscosity model was then incorporated into the Aspen Plus software through interpolation and regression.

[0064] The solubility phase diagram of the polycarbonate-dichloromethane system was integrated into the Aspen Plus simulation. To avoid issues related to solid precipitation from the solvent, the correction temperature (defined as the difference between the actual temperature and the two-phase boundary temperature) was found to be a critical parameter. For a method feasible at any extraction stage, it is important that the correction temperature be at most 10°C. This minimum correction temperature ensures that no solid precipitation occurs, thus allowing for the integrity of the method.

[0065] The evaporation of solvent (dichloromethane in this case) in the polymer solution and the design of the heat exchanger were simulated using HTRI tools (specifically Xchanger Suite, version 9.1). The HTRI unit operations were connected to the ASPEN process using the CAPE-OPEN protocol. This connection facilitated seamless transfer of process conditions and enabled consistent thermodynamic calculations between the HTRI software and Aspen Plus based on the Poly-NRTL model.

[0066] Table 1

[0067]

[0068] Table 2

[0069]

[0070] Table 3

[0071]

[0072] Understandably, the polycarbonate concentration can be increased to 99% by weight, meaning the organic solvent concentration can be reduced to 1% by weight.

[0073] Methods for producing polycarbonate

[0074] This invention relates to a method for producing polycarbonate.

[0075] Polycarbonate is a widely used thermoplastic material, favored for its excellent transparency and physical toughness. A preferred method for producing polycarbonate is often referred to as the "interface method," which involves reacting phosgene and bisphenol A in a two-phase system containing an aqueous phase and a non-aqueous phase of a solution of polycarbonate in an organic solvent. The aqueous phase is then typically removed, and the non-aqueous phase is purified to remove residual salts, catalysts, and other impurities. After purification, the organic solvent is removed, and the polycarbonate is separated from the non-aqueous phase.

[0076] Several methods are known for separation. The residual organic solvent content in the final product should be as low as possible, as they can interfere with polycarbonate. Separation yields polycarbonate powder, which can be mixed with any other components and extruded into granules. Separation can also directly yield granules.

[0077] US10435507 discloses a method for obtaining aromatic polycarbonate with low residual organic solvent content. A polymer solution containing 65 wt% polycarbonate, 33.5 wt% chlorobenzene, and 1.5 wt% dichloromethane is heated in a vertical stripper and then fed into a separation vessel. The residual chlorobenzene content is 5000 ppm, and the residual dichloromethane content is 50 ppm. The concentrated polymer solution is supplied to a static mixer, to which nitrogen gas is added up to 0.1 wt%. The nitrogen-containing polymer melt is further stripped in a foam stripper consisting of a separation vessel located directly above a stripper extruder. The residual chlorobenzene content downstream of the foam stripper is 20 ppm. The stripper extruder is equipped with three stripper zones. The residual chlorobenzene content is 2 ppm. Dichloromethane is no longer detectable (<0.1 ppm).

[0078] US6534619 and US6620906 disclose a multi-step continuous method for evaporating a polymer solution via indirect heat exchange. This method involves using a shell-and-tube heat exchanger, a thin-film evaporator, and a coil evaporator in combination with a downstream separator to concentrate a solution containing 5 to 20 wt% polymer to a solution containing 60 to 75 wt% polymer. The solution is further concentrated in the shell-and-tube heat exchanger and downstream separator to produce a solution containing at least 95 wt% polymer. The solution is then further concentrated in a shell-and-tube heat exchanger with a downstream separator or in an extrusion evaporator with a separator to achieve a solvent and / or other volatile component content of 5 to 500 ppm.

[0079] The purpose of this invention is to provide a method for producing polycarbonate with low organic solvent residue content.

[0080] Therefore, the present invention provides a method for producing polycarbonate, comprising the following steps:

[0081] a) Provide a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution.

[0082] b) Concentrate and heat the solution from step a) to form a concentrated polycarbonate melt containing an organic solvent, wherein, based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of organic solvent, and

[0083] c) Further concentrate the concentrated polycarbonate melt from step b) to a polycarbonate containing up to 500 ppm of organic solvent.

[0084] in

[0085] Step b) is carried out by subjecting the solution to at least two devolatilization stages in series, wherein each devolatilization stage includes heating the solution via a heat exchanger, followed by feeding the heated solution into a degassing vessel, wherein at least a portion of the solvent is separated from the solution by evaporation, and

[0086] Step c) is carried out at least in a degassed extruder.

[0087] The method according to the invention efficiently produces concentrated polycarbonate with a small amount of organic solvent. The concentration range of polycarbonate in the solution of step a) results in a relatively low viscosity, making it easier to process in subsequent concentration steps. The concentration step in step b) via a devolatilization stage (through which the concentrated polycarbonate melt has more than 500 ppm and at most 5% by weight of solvent) is advantageous because the solvent can be efficiently reduced to the desired level in subsequent steps. Reducing the solvent to 500 ppm or less via such a devolatilization stage requires an undesirable large amount of energy.

[0088] Step a)

[0089] Step a) includes providing a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution. The solution may contain 60-95% by weight of the organic solvent, based on the weight of the solution. This concentration range of polycarbonate in the solution results in a relatively low viscosity, making it easier to process in subsequent concentration steps.

[0090] This solution can be obtained through the interfacial polycarbonate method, which is well known in itself. In some preferred embodiments, the solution to be used in subsequent step b) is a solution directly from the polymerization reactor of the interfacial polycarbonate method.

[0091] Preferably, the polycarbonate solution contains 15-30% by weight of polycarbonate, based on the weight of the solution. The solution may contain 70-85% by weight of organic solvent.

[0092] Preferably, the organic solvent comprises 70% to 90% by weight of dichloromethane.

[0093] Preferably, the organic solvent is dichloromethane, chlorobenzene, or a mixture of chlorobenzene and dichloromethane, with dichloromethane being the most preferred organic solvent.

[0094] Step b)

[0095] Step b) involves concentrating and heating the solution from step a) to form a concentrated polycarbonate melt containing an organic solvent. Based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of solvent.

[0096] Step b) is performed by subjecting the solution from step a) to at least two devolatilization stages in series. Each devolatilization stage includes heating the solution via a heat exchanger, followed by feeding the heated solution into a degassing vessel. In the degassing vessel, at least a portion of the solvent is separated from the solution by evaporation. The number of devolatilization stages in series in step b) can be, for example, 2, 3, 4, 5, or 6, preferably 2 or 3.

[0097] The solution can be supplied to the heat exchanger from the top to flow downwards, or from the bottom to flow upwards.

[0098] Preferably, the solution is supplied to the heat exchanger at the bottom. This provides an additional degree of freedom, represented by an orifice or control valve used to regulate the pressure between the outlet of the heat exchanger and the inlet of the degassing vessel.

[0099] Therefore, in some preferred embodiments, at least one heat exchanger is supplied with a solution at the bottom of the heat exchanger, and the solution is heated within the heat exchanger by flowing upward from the bottom to the top. In a specific embodiment, each heat exchanger is supplied with a solution at the bottom of the heat exchanger, and the solution is heated within the heat exchanger by flowing upward from the bottom to the top.

[0100] In other embodiments, at least one heat exchanger is supplied with a solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top to the bottom of the heat exchanger. More preferably, each heat exchanger is supplied with a solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top to the bottom of the heat exchanger.

[0101] A concentration step performed in this type of devolatilization stage (resulting in a concentrated polycarbonate melt with more than 500 ppm and at most 5% by weight of solvent) is advantageous because a subsequent mechanical refiner can effectively reduce the solvent volume to the desired level. Reducing the solvent to 500 ppm or less via such a devolatilization stage requires an undesirable amount of energy.

[0102] The concentration step of the solution in step a) is performed by selecting the pressure and temperature for each stage of the devolatification process to achieve the desired solvent concentration. According to the invention, when the organic solvent contains a relatively large amount of dichloromethane, the devolatification should be carried out at a relatively high pressure. For example, excessively low pressures, typically used for the removal of chlorobenzene, may remove too much solvent, increasing the viscosity to such a high level that the solution becomes difficult to flow through pipes and systems.

[0103] Step c)

[0104] The method according to the invention further includes step c): further concentrating the concentrated polycarbonate melt of step b) to a polycarbonate containing up to 500 ppm of organic solvent. Step c) is carried out at least in a degassed extruder. The degassed extruder includes one or more degassed zones. In some embodiments, the amount of organic solvent in the polycarbonate obtained from step c) is up to 100 ppm, preferably up to 60 ppm, more preferably up to 50 ppm, and even more preferably up to 10 ppm. In some embodiments, the amount of organic solvent in the polycarbonate obtained from step c) is from 5 ppm to 50 ppm.

[0105] In some implementations, the concentrated polycarbonate melt of step b) is transferred from the final degassing container to the degassing extruder.

[0106] In other embodiments, the concentrated polycarbonate melt of step b) is transferred from the final degassing vessel to one or more mechanical refiners before being fed to the degassing extruder. Therefore, in some embodiments, in step c), the concentrated polycarbonate is further concentrated in one or more mechanical refiners before being fed to the degassing extruder, wherein within the one or more mechanical refiners, the concentrated polycarbonate melt is kept in a molten state under vacuum conditions at a pressure of up to 50 kPa (500 mbar), and wherein the mechanical refiners continuously or intermittently allow for renewal of the surface area of ​​the polycarbonate melt exposed to the vacuum.

[0107] Mechanical refiners have relatively long residence times, resulting in relatively low shear stress on the processed material and avoiding significant increases in localized processing temperatures. Therefore, compared to, for example, degassed extruders, mechanical refiners generate less heat on the processed material, leading to less product degradation. Thus, in this embodiment, using a mechanical refiner for this concentration step advantageously provides polycarbonate with low organic solvent residue and low degradation product content.

[0108] The concentrated polycarbonate melt obtained by one or more mechanical refiners contains up to 500 ppm of organic solvent. In some embodiments, the amount of organic solvent in the polycarbonate obtained by one or more mechanical refiners is up to 100 ppm, preferably up to 60 ppm, more preferably up to 50 ppm, and even more preferably up to 10 ppm. In some embodiments, the amount of organic solvent in the polycarbonate obtained by one or more mechanical refiners is from 5 ppm to 50 ppm.

[0109] Dichloromethane is more difficult to remove from polycarbonate melts than, for example, chlorobenzene. In particular, the difficulty in removing dichloromethane from polycarbonate melts results in very low concentrations, for example, at most 500 ppm. Therefore, the present invention is particularly advantageous when dichloromethane is a large proportion of the organic solvent.

[0110] Within a mechanical refiner, concentrated polycarbonate melt is maintained in a molten state under vacuum conditions and a pressure of up to 500 mbar. Preferably, the temperature within the mechanical refiner is 250°C to 350°C. Preferably, the pressure is 1 to 500 mbar, for example, 50 to 500 mbar.

[0111] Unlike degassing extruders, the internal volume occupied by polycarbonate melt in mechanical refiners is relatively low. Preferably, the volume occupied by polycarbonate melt in the mechanical refiner is at most 60% of the internal volume of the mechanical refiner, more preferably at most 50%. The volume occupied by polycarbonate melt in the mechanical refiner can be calculated using the known internal volume of the mechanical refiner and the material feed rate.

[0112] The residence time in a mechanical refiner is typically longer than that in a degassed extruder. Preferably, the residence time in a mechanical refiner is at least 5 minutes, for example, from 10 minutes to 2 hours.

[0113] Preferably, the mechanical refiner is selected from at least one of a disc ring reactor, a cage reactor, a thin-film evaporator or a scraped-film evaporator, a falling-film evaporator, a horizontal twin-shaft polymerizer, a twin-screw kneader, and a wire wet-fall polymerizer. These devices are well known in themselves.

[0114] For example, a disc-ring reactor is described in US7550116, which is incorporated herein by reference. A disc-ring reactor is typically a cylindrical horizontal heated vessel with inlet and outlet connections at opposite ends for the precondensate and the condensate. The disc-ring reactor comprises multiple elements rotating about a horizontal axis that mix the precondensate and create a large surface area for degassing the condensate as a viscous liquefied material adhering to these elements flows down. The design of the discs can be optimized along the reactor's axis to accommodate varying viscosities at each point. A disc-ring reactor is also described in US2005222371, which is incorporated herein by reference. In some preferred embodiments, the mechanical refiner is a disc-ring reactor, and the residence time in the disc-ring reactor is from 10 minutes to 3 hours, for example, from 60 minutes to 3 hours.

[0115] For example, a coil reactor is described in US20020188091, WO2004101140A1, and WO2007128159A1, which are incorporated herein by reference. A coil reactor comprises a rotatable cylindrical basket having cylindrical porous walls and annular disks spaced at intervals around the perimeter of the basket and along its length. The disks can be designed to be optimized along the reactor's axis to accommodate varying viscosities at each point. In some preferred embodiments, the mechanical refiner is a coil reactor, and the residence time in the coil reactor is from 10 minutes to 3 hours, for example, from 60 minutes to 3 hours.

[0116] For example, US10384145 describes thin-film evaporators and scraped-film evaporators, which are incorporated herein by reference. Typically, a thin-film evaporator includes a vertical or horizontal drum, a supply line for supplying the product to be evaporated, a heating jacket arranged around the periphery of the drum, a discharge line for discharging any remaining residue, and a discharge line for discharging the evaporation section of the product. The general purpose of a thin-film evaporator is to evaporate a volatile fluid from a less volatile fluid. Evaporation occurs through contact between the product and the heated walls of the drum. To improve evaporation efficiency, the drum is equipped with an agitator. Agitation can be achieved in various ways. One well-known type is a fixed-gap agitator, in which a thin gap is formed between the agitator and the inner wall of the drum, thereby pushing the product into the gap during rotation. Another type is a so-called scraped-film agitator, in which the agitator scrapes the product against the inner wall of the drum, thereby forming a thin product film. In some preferred embodiments, the mechanical refiner is a thin-film evaporator or a scraped-film evaporator, and the residence time in the thin-film evaporator or scraped-film evaporator is at least 5 minutes and less than 10 minutes.

[0117] For example, falling film evaporators are described in US9040639 and WO2003042278A1, which are incorporated herein by reference. The process fluid to be evaporated flows downwards in the form of a continuous film under gravity. The fluid forms a film along the tube wall and travels downwards (falls). In some preferred embodiments, the mechanical refiner is a falling film evaporator, and the residence time in the falling film evaporator is at least 5 minutes and less than 10 minutes.

[0118] For example, horizontal biaxial polymerizers are described in EP0529093B1 and US6846103, which are incorporated herein by reference. A horizontal biaxial polymerizer is a polymerizer having two horizontally rotating shafts equipped with blades, which may be disc, pin, spectacle, or wheel-type. The horizontal biaxial polymerizer preferably includes a distillation column. The distillation column is used to prevent the starting compound from escaping from the system during the removal of byproducts. In some preferred embodiments, the mechanical purifier is a horizontal biaxial polymerizer, and the residence time in the horizontal biaxial polymerizer is 10 to 60 minutes.

[0119] For example, a twin-screw kneader is described in US20090304800A1, which is incorporated herein by reference. Examples of twin-screw kneaders include the KRC kneader from Kurimoto, Ltd. In some preferred embodiments, the mechanical refiner is a twin-screw kneader, and the residence time in the horizontal twin-screw kneader is 10 to 30 minutes.

[0120] For example, wire wet-fall polymerizers are described in US5589564A, CA2168630C, US6277945B1, US6320015B1, US7528213B2, and US9321884B2, which are incorporated herein by reference. In a wire wet-fall polymerizer, molten prepolymer is allowed to fall along and contact the surface of a guide (e.g., a wire), thereby achieving polymerization of the molten prepolymer to produce the desired polymer. In some preferred embodiments, the mechanical refiner is a wire wet-fall polymerizer, and the residence time in the wire wet-fall polymerizer is 10 to 60 minutes.

[0121] Preferably, an inert gas other than the solvent is added to the mechanical cleaner. Examples of inert gases include nitrogen, argon, carbon dioxide, water, methane, helium, and combinations thereof.

[0122] The concentrated polycarbonate melt from step b) from the final degassing vessel or one or more mechanical refiners is fed into the degassing extruder. Granules are obtained.

[0123] As those skilled in the art know, an extruder is a device comprising an elongated cylindrical tube (cylinder) having an inlet and an outlet, and a screw configured to rotate within the cylinder to convey material from the inlet to the outlet of the cylinder.

[0124] As is known to those skilled in the art, a degassing extruder is an extruder having one or more degassing zones in a barrel, in which low molecular weight substances are extracted by vacuum.

[0125] Preferably, an inert gas other than the solvent is added to the degassed extruder. Examples of inert gases include nitrogen, argon, carbon dioxide, water, methane, helium, and combinations thereof.

[0126] In step c), polycarbonate containing up to 500 ppm of organic solvent is obtained in the barrel of a degassed extruder. The polycarbonate containing up to 500 ppm of organic solvent is then extruded to obtain one or more strands, which are subsequently cooled and cut into pellets.

[0127] Therefore, the method according to the invention preferably includes d) extruding the polycarbonate obtained by step c) from a degassed extruder into one or more strands, followed by cooling and cutting it into pellets.

[0128] A die is located after the outlet of the cylinder. One or more strands emerge from the die, and these strands are subsequently cooled, solidified, and cut into pellets. Components such as a gear pump and / or a melt filter may be present between the cylinder outlet and the die, but no degassing step is performed between the cylinder outlet and the die. For example, the die may be directly after the cylinder outlet; the gear pump may be followed by the die; the gear pump may be followed by the melt filter and then the die; the melt filter may be followed by the gear pump and then the die; or the gear pump may be followed by the melt filter, then another gear pump, and then the die.

[0129] Preferably, the volume occupied by the polycarbonate melt in the degassed extruder is greater than 60% of the internal volume of the extruder, more preferably at least 70% or at least 80%.

[0130] Preferably, the residence time in the degassed extruder is less than 5 minutes, for example, 10 seconds to 3 minutes or 30 seconds to 2 minutes.

[0131] Preferably, in the degassed extruder, the amount of organic solvent in the polycarbonate is reduced to at most 10 ppm.

[0132] Operating a degassed extruder allows for the acquisition of the desired organic solvent content. For example, the temperature, pressure, screw speed (RPM), and output rate (kg / h) can be adjusted. The degassed extruder can operate at, for example, 280 to 350°C. To prevent yellowing of the polycarbonate, the temperature in the degassed extruder is preferably set to 280 to 320°C.

[0133] Degassing extruders or devolatile matter extruders are known, for example, in US10435507 and in the book "Der gleichläufige Doppelschneckenextruder" (Co-rotating Twin-Screw Extruder), by Klemens Kohlgrüber, published by Carl Hanser Verlag, ISBN 978-3-446-41252-1, cited in US10435507, pages 193-195.

[0134] In an extruder, concentrated polycarbonate melt can be combined with additives or other components. The composition obtained from the extruder can be cut into pellets.

[0135] Suitable examples of optional additives include one or more of the following: impact modifiers, flow modifiers, fillers, reinforcing agents (e.g., glass fiber or talc), antioxidants, heat stabilizers, light stabilizers, UV stabilizers and / or UV absorbers, plasticizers, lubricants, mold release agents (particularly glyceryl monostearate, pentaerythritol tetrastearate, glyceryl tristearate, stearoyl stearate), antistatic agents, antifogging agents, antimicrobial agents, colorants (e.g., dyes or pigments), and flame retardants (with or without anti-dripping agents such as polytetrafluoroethylene (PTFE) or PTFE-encapsulated styrene-acrylonitrile copolymers). The invention is not limited in the type and amount of additives, and embodiments in which the additives exemplified above are not added may exist.

[0136] Other components that can be added to the extruder may be, for example, at least one other polymer, preferably selected from polycarbonate-polysiloxane copolymers, polycarbonate-polyester copolymers, polyesters, polyolefins, acrylonitrile / butadiene / styrene copolymers, methyl methacrylate / butadiene / styrene copolymers, styrene / butadiene / styrene copolymers (SBS), styrene / ethylene-butene / styrene copolymers (SEBS), styrene / ethylene-propylene / styrene copolymers (SEPS), styrene / acrylonitrile copolymers (SAN), acrylonitrile / styrene / acrylonitrile copolymers (ASA), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), unsaturated polyesters (UPES), polyamides (PA), thermoplastic polyurethanes (TPU), polystyrene (PS), high-impact polystyrene (HIPS), polyvinyl chloride (PVC), polyetherimide, and polysulfone.

[0137] Materials of the equipment used in the method

[0138] During the concentration step, the polycarbonate solution comes into contact with the inner surfaces of various devices. Studies have found that using a nickel-based alloy containing nickel and chromium on the inner surfaces of the devices used in step b) resulted in less degradation of the polycarbonate.

[0139] Therefore, in some preferred embodiments, at least a portion of the inner surface of at least one heat exchanger and / or at least a portion of the inner surface of at least one degassing vessel used in step b) is made of a nickel-based alloy containing nickel and chromium. Preferably, at least a portion of the inner surface of each heat exchanger and each degassing vessel used in step b) is made of a nickel-based alloy containing nickel and chromium.

[0140] The outlet of the heat exchanger can be the inlet of the degassing container. The outlet of the heat exchanger can also be connected to the inlet of the degassing container, for example, via a connecting pipe. In this case, preferably at least a portion of the inner surface of the connecting pipe connecting the outlet of the heat exchanger and the inlet of the degassing container is made of a nickel-based alloy containing nickel and chromium.

[0141] The outlet of the degassing container can be the inlet of a subsequent heat exchanger. The outlet of the degassing container can also be connected to the inlet of the subsequent heat exchanger, for example, via a connecting pipe. In this case, preferably at least a portion of the inner surface of the connecting pipe connecting the outlet of the heat exchanger and the inlet of the degassing container is made of a nickel-based alloy containing nickel and chromium.

[0142] Preferably, any other surface that comes into contact with the polycarbonate solution during step b) is at least partially made of a nickel-based alloy containing nickel and chromium.

[0143] Preferably, the nickel-based alloy comprises at least 40 wt% (e.g., 40 to 65 wt%) of nickel and at least 12 wt% (e.g., 12 to 25 wt%) of chromium. The nickel-based alloy optionally comprises molybdenum. The nickel-based alloy optionally comprises iron. The nickel-based alloy optionally comprises niobium and / or tantalum. Suitable examples include Inco nickel alloys such as Inco Nickel 600, 617, 625, 690, Nuclear Grade 690, 718, and X-750, and Hastelloy alloys such as C-276 and C-22.

[0144] Preferably, the nickel-based alloy comprises at least 40 wt% nickel (e.g., 40 to 65 wt%), at least 12 wt% chromium (e.g., 12 to 25 wt%), and 1.0 to 20 wt% molybdenum. Suitable examples include Inco nickel alloys such as Inco Nickel 617, 625, and 718, and Hastelloy alloys such as C-276 and C-22.

[0145] Particularly preferred, the nickel-based alloy comprises at least 40 wt% nickel (e.g., 40 to 65 wt%), at least 12 wt% chromium (e.g., 12 to 25 wt%), and 5.0 to 12 wt% molybdenum. Suitable examples include Incor nickel alloys, such as Incor Nickel 617 and 625.

[0146] Particularly preferred, the nickel-based alloy comprises at least 40 wt% nickel (e.g., 40 to 65 wt%), at least 12 wt% chromium (e.g., 12 to 25 wt%), and 5.0 to 12 wt% molybdenum, and also contains niobium and / or tantalum, wherein the total amount of niobium and / or tantalum is 1.0 to 5.0 wt%. Suitable examples include Inco Nickel 625. This results in significantly less degradation of the polycarbonate.

[0147] In some preferred embodiments, the nickel-based alloy comprises at least 40 wt% nickel (e.g., 40 to 65 wt%), at least 12 wt% chromium (e.g., 12 to 25 wt%), and 12 to 20 wt% molybdenum. Suitable examples include Hastelloy C-276 and C-22. Preferably, the nickel-based alloy comprises at least 40 wt% nickel (e.g., 40 to 65 wt%), at least 12 wt% chromium (e.g., 12 to 25 wt%), 12 to 20 wt% molybdenum, and 12 to 17 wt% chromium. Suitable examples include Hastelloy C-276.

[0148] Preferably, the nickel-based alloy is selected from Inco Nickel 600, 617, 625, 690, nuclear grade 690, 718 and X-750, and Hastelloy C-276 and C-22, more preferably from Inco Nickel 617, 625 and 718, and Hastelloy C-276 and C-22, even more preferably from Inco Nickel 617 and 625, and most preferably from Inco Nickel 625.

[0149] The inner surface of the device used in step c) is preferably made of nitrided steel to reduce the degradation of polycarbonate.

[0150] Therefore, it is preferred that at least a portion of the screw and inner surface of the barrel of the degassed extruder used in step c) be made of nitrided steel.

[0151] In an embodiment where a mechanical polisher is used in step c), preferably at least a portion of the inner surface of the mechanical polisher is made of nitrided steel.

[0152] The present invention will now be described through the following embodiments, but the present invention is not limited thereto.

[0153] Figure 1 An embodiment of a system for implementing the method for producing polycarbonate according to the present invention is shown.

[0154] exist Figure 1The system comprises a series of three devolatilization stages, each stage including a heat exchanger and a degassing vessel. A solution of polycarbonate in an organic solvent is provided, containing 5-40 wt% polycarbonate based on the weight of the solution. This solution is supplied via line 10 to a first heat exchanger 100, and subsequently to a first degassing vessel 110, where a portion of the organic solvent is removed via line 20, thereby concentrating the solution. A first pump 120 is used to supply the solution from the first degassing vessel 110 to a second heat exchanger 200, and subsequently to a second degassing vessel 210, where a portion of the organic solvent is removed via line 20, further concentrating the solution. A second pump 220 is used to supply the solution from the second degassing vessel 210 to a third heat exchanger 300, and subsequently to a third degassing vessel 310, where a portion of the organic solvent is removed via line 20, further concentrating the solution. A concentrated polycarbonate melt containing solvent is obtained, wherein the amount of solvent is at most 5 wt% and greater than 500 ppm based on the weight of the concentrated polycarbonate melt.

[0155] The third pump 320 is used to supply concentrated polycarbonate melt to a mechanical refiner 400, in which the concentration of organic solvent is further reduced to at most 500 ppm.

[0156] A fourth pump 420 supplies concentrated polycarbonate melt to a degassed extruder 500, in which the concentration of organic solvents is further reduced. Additives are supplied to the degassed extruder. This yields polycarbonate composition pellets containing polycarbonate and additives with low organic solvent content.

[0157] simulation

[0158] Table 1-3 shows the simulated results of polycarbonate concentration in polycarbonate solutions undergoing a series of devolatation stages at different temperatures and pressures (step b of the method of the present invention).

[0159] Modeling was performed using Aspen Plus software, and the phase behavior of the polymer and solvent system was calculated using the Poly-NRTL thermodynamic model.

[0160] Viscosity modeling was performed based on experimental data, in which the viscosity of polycarbonate solutions in up to 80% by weight of dichloromethane was measured. A suitable viscosity model was then incorporated into the Aspen Plus software through interpolation and regression.

[0161] The solubility phase diagram of the polycarbonate-dichloromethane system was integrated into the Aspen Plus simulation. To avoid issues related to solid precipitation from the solvent, the correction temperature (defined as the difference between the actual temperature and the two-phase boundary temperature) was found to be a critical parameter. For a method feasible at any extraction stage, it is important that the correction temperature be at most 10°C. This minimum correction temperature ensures that no solid precipitation occurs, thus allowing for the integrity of the method.

[0162] The evaporation of solvent (dichloromethane in this case) in the polymer solution and the design of the heat exchanger were simulated using HTRI tools (specifically Xchanger Suite, version 9.1). The HTRI unit operations were connected to the ASPEN process using the CAPE-OPEN protocol. This connection facilitated seamless transfer of process conditions and enabled consistent thermodynamic calculations between the HTRI software and Aspen Plus based on the Poly-NRTL model.

[0163] Table 1

[0164]

[0165] Table 2

[0166]

[0167] Table 3

[0168]

[0169] Understandably, the polycarbonate concentration can be increased to 99% by weight, meaning the organic solvent concentration can be reduced to 1% by weight.

[0170] Polycarbonate melts with various levels of chloromethane concentration were supplied to the degassed extruder under the conditions shown in Table 4.

[0171] Table 4

[0172]

[0173] Understandably, the desired low organic solvent levels can be achieved by selecting the operating conditions of the degassed extruder.

Claims

1. A method for producing polycarbonate, comprising the following steps: a) Provides a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution. b) Concentrate and heat the solution from step a) to form a concentrated polycarbonate melt containing the solvent, wherein, based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of solvent, and c) Using one or more mechanical refiners, the concentrated polycarbonate melt of step b) is further concentrated to a polycarbonate containing up to 500 ppm of organic solvent, wherein within the one or more mechanical refiners, the concentrated polycarbonate melt is kept in a molten state under vacuum conditions at a pressure of up to 500 mbar, and wherein the mechanical refiner continuously or intermittently allows for renewal of the surface area of ​​the polycarbonate melt exposed to the vacuum.

2. The method according to claim 1, wherein the amount of organic solvent in the polycarbonate obtained from step c) is at most 100 ppm, preferably at most 60 ppm, more preferably at most 50 ppm, and even more preferably at most 10 ppm.

3. The method according to any one or more of claims 1-2, wherein the organic solvent comprises 70% to 90% by weight of dichloromethane.

4. The method according to any one or more of claims 1-3, wherein the organic solvent is dichloromethane, chlorobenzene, or a mixture of chlorobenzene and dichloromethane, preferably dichloromethane.

5. The method according to any one or more of claims 1-4, wherein the polycarbonate solution in step a) comprises 15-30% by weight of polycarbonate.

6. The method according to any one of claims 1-5, wherein the temperature within the mechanical refiner is 250°C to 350°C and / or the pressure is 1 to 500 mbar.

7. The method according to any one or more of claims 1-6, wherein the mechanical refiner is selected from at least one of a disc ring reactor, a cage reactor, a thin film evaporator or a scraped film evaporator, a falling film evaporator, a horizontal twin-shaft polymerizer, a twin-screw kneader, and a wire wet falling polymerizer.

8. The method according to any one or more of claims 1-7, wherein an inert gas other than the solvent is added to the mechanical cleaner, preferably wherein the inert gas is selected from nitrogen, argon, carbon dioxide, water, methane, helium and combinations thereof.

9. The method according to any one or more of claims 1-8, wherein the volume occupied by the polycarbonate melt in the mechanical refiner is at most 60%, preferably at most 50%, of the internal volume of the mechanical refiner, and / or wherein the residence time in the mechanical refiner is at least 5 minutes, for example, from 10 minutes to 2 hours.

10. The method according to any one or more of claims 1-9, wherein the mechanical refiner includes a mixing device that continuously mixes the polycarbonate melt, thereby continuously renewing the surface area of ​​the polycarbonate melt exposed to vacuum.

11. The method according to any one or more of claims 1-10, wherein at least two tandem devolvement stages are used to concentrate the polycarbonate solution, wherein each devolvement stage comprises heating the solution via a heat exchanger, followed by degassing the heated solution in a degassing vessel, wherein a portion of the solvent is separated from the solution by evaporation.

12. The method of claim 11, wherein at least one of the heat exchangers is supplied with the solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top of the heat exchanger to the bottom.

13. The method according to any one or more of claims 1-12, wherein the polycarbonate melt obtained from step c) is fed into an extruder, optionally combined with additives or other components, and cut into pellets.

14. The method of claim 13, wherein the extruder includes one or more degassing zones, and wherein the amount of organic solvent in the polycarbonate is reduced to at most 10 ppm.

15. The method of claim 13, wherein the extruder does not include a degassing zone, and wherein the amount of organic solvent in the polycarbonate obtained from step c) is at most 10 ppm.

16. A method for producing polycarbonate, comprising the following steps: a) Provides a solution of polycarbonate in an organic solvent, wherein the solution contains 5-40% by weight of polycarbonate, based on the weight of the solution. b) Concentrate and heat the solution from step a) to form a concentrated polycarbonate melt containing the solvent, wherein, based on the weight of the concentrated polycarbonate melt, the concentrated polycarbonate melt contains up to 5% by weight and more than 500 ppm of solvent, and c) Further concentrate the concentrated polycarbonate from step b) to a polycarbonate containing up to 500 ppm of organic solvent. in Step b) is carried out by subjecting the solution to at least two devolatilization stages in series, wherein each devolatilization stage includes heating the solution via a heat exchanger, followed by feeding the heated solution into a degassing vessel, wherein at least a portion of the solvent is separated from the solution by evaporation, and Step c) is carried out at least in a degassed extruder.

17. The method according to claim 1, wherein the amount of organic solvent in the polycarbonate obtained from step c) is at most 100 ppm, preferably at most 60 ppm, more preferably at most 50 ppm, and even more preferably at most 10 ppm.

18. The method according to any one or more of claims 1-2, wherein the organic solvent comprises 70% to 90% by weight of dichloromethane.

19. The method according to any one or more of claims 1-3, wherein the organic solvent is dichloromethane, chlorobenzene, or a mixture of chlorobenzene and dichloromethane, preferably dichloromethane.

20. The method according to any one or more of claims 1-4, wherein the polycarbonate solution comprises 15-30% by weight of polycarbonate.

21. The method according to any one or more of claims 1-5, wherein in step c), before feeding the concentrated polycarbonate to the degassed extruder, the concentrated polycarbonate is further concentrated in one or more mechanical refiners, wherein in the one or more mechanical refiners, the concentrated polycarbonate melt is maintained in a molten state under vacuum conditions at a pressure of up to 50 kPa (500 mbar), and wherein the mechanical refiner continuously or intermittently allows for renewal of the surface area of ​​the polycarbonate melt exposed to the vacuum.

22. The method of claim 6, wherein the temperature within the mechanical refiner is 250°C to 350°C and / or the pressure is 0.1 to 50 kPa (1 to 500 mbar).

23. The method according to any one or more of claims 6-7, wherein the mechanical refiner is selected from at least one of a disc ring reactor, a cage reactor, a thin film evaporator or a scraped film evaporator, a falling film evaporator, a horizontal twin-shaft polymerizer, a twin-screw kneader, and a wire wet falling polymerizer.

24. The method according to any one or more of claims 6-8, wherein the volume occupied by the polycarbonate melt in the mechanical refiner is at most 60%, preferably at most 50%, of the internal volume of the mechanical refiner.

25. The method according to any one or more of claims 6-9, wherein the residence time in the mechanical refiner is at least 5 minutes, for example, from 10 minutes to 2 hours.

26. The method according to any one or more of claims 6-10, wherein the mechanical refiner includes a mixing device that continuously mixes the polycarbonate melt, thereby continuously renewing the surface area of ​​the polycarbonate melt exposed to vacuum.

27. The method according to any one or more of claims 1-11, wherein an inert gas other than the solvent is added to the degassed extruder and / or the mechanical refiner, preferably wherein the inert gas is selected from nitrogen, argon, carbon dioxide, water, methane, helium, and combinations thereof.

28. The method according to any one or more of claims 1-12, wherein at least one of the heat exchangers is supplied with the solution at the top of the heat exchanger, and the solution is heated in the heat exchanger by flowing downward from the top of the heat exchanger to the bottom.

29. The method according to any one or more of claims 1-13, wherein at least a portion of the inner surface of at least one of the heat exchangers used in step b) and / or at least a portion of the inner surface of at least one of the degassing vessels is made of a nickel-based alloy comprising nickel and chromium.

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