Method for the manufacture of high quality polycarbonate

Abstract

The present invention relates to a method for the manufacture of polycarbonate comprising the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of one or more of a transesterification catalyst, thereby forming a stream of molten polycarbonate, i') combining said stream of molten polycarbonate obtained in step i) and from 0 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) or step to a melt mixing device, iii) combining, in said melt mixing device, said stream of molten polycarbonate and from 0 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) to a melt mixing device, iii) combining, in said melt mixing device, said stream of molten polycarbonate and from 0 5000 ppm of water based on the weight of the polycarbonate, wherein the total amount of water to be combined with the stream of molten from 5 5000 ppm based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

Description

[0001] METHOD FOR THE MANUFACTURE OF HIGH QUALITY POLYCARBONATE

[0002] The present invention relates to a method for the manufacture polycarbonate using a melt transesterification process.

[0003] Polycarbonate is a well-known material and generally exhibits good mechanical and optical properties. Typical applications include optical media carriers, glazing, extruded sheets, lenses and water bottles. Polycarbonates are generally manufactured using two different technologies. In a first technology, known as the interfacial technology or interfacial process, phosgene is reacted with one or more bisphenols such as bisphenol A (BPA) in a liquid phase. Another well-known technology for the manufacture of polycarbonate is the so-called melt technology, sometimes also referred to as melt transesterification or melt polycondensation technology. In the melt technology, or melt process, a dihydroxy compound, typically a bisphenol, more typically BPA, is reacted with a carbonate, typically a diaryl carbonate, more typically diphenyl carbonate (DPC), in the melt phase. A transesterification catalyst is generally used to reach the desired molecular weight and to advance the polycondensation reaction.

[0004] In the interfacial process the polycarbonate is dissolved in a solvent such as methylene chloride, chlorobenzene, or a mixture of both and the process includes several purification steps prior to the polycarbonate being isolated and provided in powder or pellet form. This means that typically interfacial polycarbonate is substantially free of catalyst or catalyst residues and has a low amount of other impurities.

[0005] In the melt process the polycarbonate is directly obtained from a final reactor and it is not possible, or at least not economically feasible, to purify the polycarbonate. This means that any contaminant that was contained in the raw materials or was generated during the polymerisation process, and further the catalyst or catalyst residues remain present in the obtained polycarbonate. Apart from that, a polycarbonate obtained by the melt transesterification process is also known to be structurally different from interfacial polycarbonate. First of all, melt polycarbonate typically has a minimum amount of branching caused by Fries and / or Kolbe-Schmidt rearrangement mechanisms, which branching is generally absent in interfacial polycarbonate. Secondly, melt polycarbonate typically has a much higher number of phenolic hydroxy end groups while polycarbonate obtained by the interfacial process is typically end-capped and has at most 150 ppm, preferably at most 50 ppm, more preferably at most 10 ppm of phenol hydroxyl end- g roups.

[0006] As a consequence of the foregoing it is often found that melt polycarbonate shows less favourable initial color and / or color stability properties in comparison with interfacial polycarbonate.

[0007] Improving the heat stability and color stability of melt polycarbonate is addressed in US 5,514,767 which discloses a method for the manufacturing a polycarbonate composition comprising performing a hot-melt polycondensation of an aromatic dihydroxy compound and a dicarbonate in the presence of a basic catalyst, adding (B) 0.1 - 10 ppm of a sulfur- containing acid compound with a pKa value less than 3 or a derivative thereof, and (C) 5 - 1000 ppm of water for the polycarbonate while the reaction product polycarbonate (A) is still in a molten state, and kneading the material.

[0008] There is however a need to further improve the quality of melt polycarbonate and accordingly it is an object of the present invention to provide a melt transesterification process that results in a polycarbonate having improved color properties. There is also a need for a polycarbonate which contains a low amount of the starting material dihydroxy compound, such as bisphenol, for example bisphenol A (BPA).

[0009] The foregoing objects are met, at least in part, in accordance with the invention which is directed at a method for the manufacture of polycarbonate comprising the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of one or more of a transesterification catalyst, thereby forming a stream of molten polycarbonate; ii) feeding the stream of molten polycarbonate obtained in step i) to a melt mixing device; iii) combining, in said melt mixing device, said stream of molten polycarbonate and from 5 - 5000 ppm of water based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter. The invention further provides a method for the manufacture of polycarbonate comprising the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of a transesterification catalyst, thereby forming a stream of molten polycarbonate, i') combining said stream of molten polycarbonate obtained in step i) and from 0 - 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) or step i’) to a melt mixing device and iii) combining, in said melt mixing device, said stream of molten polycarbonate of step ii) and from 0 - 5000 ppm of water based on the weight of the polycarbonate, wherein the total amount of water to be combined with the stream of molten polycarbonate in steps I’) and iii) is from 5 - 5000 ppm based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

[0010] It was surprisingly found that the polycarbonate obtained by the method according to the invention has a combination of improved color properties and a relatively low amount of dihydroxy compound.

[0011] It was found that the addition of water results in a polycarbonate having improved color properties. It was, however, observed that the addition of water also results in a higher amount of dihydroxy compound in the polycarbonate obtained. According to the invention, subjecting the stream of molten polycarbonate to a vacuum after the addition of water, prior to passing the stream to a melt filter, was found to reduce this increase in the amount of dihydroxy compound in the polycarbonate while maintaining improved color properties.

[0012] The method according to the invention involves combining the stream of molten polycarbonate formed by melt reaction with water. It will thus be appreciated that the method according to the invention does not comprise an intermediate step of solidifying the molten polycarbonate (e.g. pelletizing the molten polycarbonate) between the step of forming the stream of molten polycarbonate by melt reaction and the step of combing said stream with water. It will further be appreciated that the method follows the steps in a sequential order.

[0013] It is noted that US10106648 and LIS2017 / 015784 disclose a melt polymerization process comprising melt polymerizing a carbonate compound and a dihydroxy compound in the presence of a catalyst composition to form a polycarbonate. These documents do not disclose use of vacuum after adding water to a stream of molten polycarbonate.

[0014] Water addition

[0015] Although the exact mechanism for it is unknown, the present inventors have surprisingly found that, if a small amount of water is added and mixed with the polycarbonate melt coming from the final reactor, the color properties (in particular the yellowing expressed by the parameter b*) are improved.

[0016] In accordance with the invention therefore water is added to a stream of molten polycarbonate after it has reached a desired molecular weight during the polymerisation, i.e. after the reacting of the dihydroxy compound with the carbonate and the formation of the polycarbonate polymer.

[0017] In order for the water to have the desired effect on the color properties it is important that the water is thoroughly mixed with the molten polycarbonate. The mixing is therefore preferably carried out in a static mixer or a melt mixing device such as in particular an extruder.

[0018] For the purpose of the present invention the water needs to be as pure as possible. In particular it should be neutral in pH, meaning a pH from 6.0 - 8.0, preferably from 6.5 - 7.5, more preferably from 6.9 - 7.1. It is preferred that the water has a total dissolved solids content (TDS) of at most 3.0 ppm at 25 °C. The water may have one or more of an iron ion content of at most 0.05 ppm, a copper ion content of at most 0.01 ppm, a carbon dioxide content of at most 2.0 ppm, a silica content of at most 0.1 ppm. The water may have a bicarbonate content of at most 1.0 ppm. It is preferred that the water has an electrical conductivity of at most 5.0 pS / cm at 25°C. More preferably, the electrical conductivity of the water is at most 3.0 pS / cm, more preferably at most 1.0 pS / cm, more preferably at most 0.5 pS / cm, even more preferably 0.05 pS / cm, at 25°C. Measurement methods for measuring the electrical conductivity of water are known to the skilled person. Thus, the electrical conductivity of water is measured using a conductivity meter by determining its resistance between two flat or cylindrical electrodes which are separated by a fixed distance. An alternating voltage is usually applied in order to avoid electrolysis. The conductivity is determined in accordance with ISO 7888:1985.

[0019] The amount of water to be added and mixed is 5 - 5000 ppm based on the weight of the polycarbonate. The present inventors found that if the amount of water is too high this may result in an undesired deterioration of the polycarbonate, in particular an undesired drop in molecular weight. It is also for this reason that the present inventors have found that it is important that the water addition is done in-line prior to the polycarbonate being converted into flakes, powder or pellets. A too low amount will generally not have the desired effect or at least show only a very small improvement in the color properties of the polycarbonate.

[0020] It is preferred that the amount of water is in the range of 10 - 4000 ppm, 30 - 3000 ppm, 50 - 2500 ppm, 75 - 1000 ppm, 100 - 800 ppm or 150 - 500 ppm, with a range of from 200-400 ppm being most preferred, wherein the amount in ppm is based on the weight of the polycarbonate. In a specific embodiment, the amount of water is in the range of 5 - 800 ppm, wherein the amount in ppm is based on the weight of the polycarbonate.

[0021] For the avoidance of doubt it is to be understood that the polycarbonate stream to which the water is added is substantially free of water, meaning that the amount of water in the polycarbonate stream prior to treatment is at most 10 ppm, preferably at most 1 ppm.

[0022] Preferably, the method according to the invention is performed such that loss of molecular weight of the molten polycarbonate by the addition of water in the melt mixing device is limited. This is achieved e.g. by suitably selecting the amount of water to be combined with the stream of molten polycarbonate in the melt mixing device in step i') or iii). Preferably, the molten polycarbonate formed by the melt reaction has a first weight average molecular weight as determined using GPC on the basis of polystyrene standards and the molten polycarbonate after the combination with 5-5000 ppm of water has a second weight average molecular weight as determined using GPC on the basis of polystyrene standards, wherein the second weight average molecular weight is at least 90%, preferably at least 95%, more preferably at least 99% of the first weight average molecular weight.

[0023] Siloxane compound

[0024] A siloxane compound may be combined with the stream of molten polycarbonate. The addition of a siloxane compound was found to also reduce the increase in the amount of dihydroxy compound in the polycarbonate that was related to the addition of water. The amount of the siloxane compound may be 50 to 10000 ppm, e.g. 50 to 5000 ppm or 100 to 1000 ppm, based on the weight of the polycarbonate.

[0025] The siloxane compound may be represented by a general formula (I): wherein each occurrence of R1 may be the same or different and is selected from hydrogen, methyl, ethyl, phenyl and vinyl, each occurrence of R2 may be the same or different and is selected from hydrogen, methyl, ethyl, phenyl and vinyl, each of R3 and R6 is selected from hydrogen, hydroxyl, vinyl, methyl, methoxy and ethoxy, each of R4, R5, R7 and R8 is selected from methyl, methoxy, ethyl, ethoxy and phenyl and x is an integer between 1 and 500. Preferably, x is an integer between 2 and 400, 3 and 300, 4 and 200, 5 and 100 or 10 and 50. A higher value of x leads to the siloxane compound having a lower volatility so that the losses in the vent port of a melt mixing device such as an extruder is reduced. A lower value of x leads to the siloxane compound having a lower viscosity so that it mixes well with the polycarbonate. These preferred ranges of x provide a good balance of the volatility and viscosity for use in the present invention.

[0026] Preferably, each of R4, R5, R7 and R8 is selected from methyl and phenyl.

[0027] Preferably, at least one of R3 and R6 is methyl. In some embodiments, each of R3 and R6 is methyl. In some embodiments, one of R3 and R6 is methyl and the other one of R3 and R6 is hydrogen, hydroxyl or vinyl. In some embodiments, each of R3 and R6 is selected from the group consisting of hydrogen, hydroxyl and vinyl.

[0028] Preferably, at least one of R4, R5, R7 and R8 is methyl. More preferably, at least two, at least three of R4, R5, R7 and R8 are methyl. Most preferably, each of R4, R5, R7 and R8 is methyl.

[0029] In some preferred embodiments, each of R3, R6, R4, R5, R7 and R8 is methyl.

[0030] In some preferred embodiments, the siloxane compound (I) comprises or is a compound represented by: wherein each occurrence of R1 may be the same or different and is selected from hydrogen and phenyl and y is an integer and z is an integer and y+z=x. y / (y+z) may be 0.0 (i.e. y is 0) to 1.0 (i.e. z is 0). y / (y+z) may be at least 0.1 to 0.9, for example 0.4 to 0.6. y / (y+z) may be at most 0.1, at most 0.05 or at most 0.01. y / (y+z) may be at least 0.9, at least 0.95 or at least 0.99. In some preferred embodiments, the siloxane compound (I) comprises or is a compound represented by:

[0031] In some preferred embodiments, the siloxane compound (I) is comprises or is a compound represented by: wherein y is an integer and z is an integer and y+z=x. This has an advantage that the polycarbonate obtained has good haze properties. Preferably, y / (y+z) is at least 0.1 , for example 0.4 to 0.6, and may be at least 0.9, at least 0.95 or at least 0.99. In some embodiments, z is 0.

[0032] In some preferred embodiments, the siloxane compound (I) is represented by:

[0033] wherein y is an integer and z is an integer and y+z=x. This has an advantage that the polycarbonate obtained has good haze properties and the reduction in the amount of bisphenol in the polycarbonate is particularly large. Preferably, y / (y+z) is at least 0.1 , for example 0.4 to 0.6, and may be at least 0.9, at least 0.95 or at least 0.99. In some embodiments, z is 0.

[0034] In some preferred embodiments, the siloxane compound (I) is represented by: wherein y is an integer and z is an integer and y+z=x. This has an advantage that the reduction in the amount of bisphenol in the polycarbonate is particularly large. Preferably, y / (y+z) is at least 0.1 , for example 0.4 to 0.6, and may be at least 0.9, at least 0.95 or at least 0.99. In some embodiments, z is 0. The siloxane compound used according to the invention may consist of one type of the siloxane compound represented by formula (I) or a mixture of different types of the siloxane compound represented by formula (I) such as a mixture of the siloxane compounds represented by formula (ll)-(VI).

[0035] Preferably, the siloxane compound comprises or is a compound selected from the group consisting of phenyl methyl siloxane, methyl hydrogen siloxane, methyl methyl siloxane and phenyl hydrogen siloxane and combinations thereof, most preferably methyl hydrogen siloxane.

[0036] While the present inventors have found the foregoing siloxane compounds, having predominantly R2Si-O2 repeating units, to be advantageous, other siloxane compounds having any combination of R2Si-O2 repeating units, RaSi-O repeating units and / or RSi- O3 repeating units and / or Si-04 repeating units may also be applied (the “R” groups corresponding to any of R1 - R8 as described above).

[0037] Raw materials

[0038] Commercial melt polycarbonate is typically manufactured on the basis of a dihydroxy compound with a carbonate. The dihydroxy compound may e.g. be a dihydroxy compound having a triphenylamine structure, for example described in EP0610912, but typically is a bisphenol. The carbonate may be a diaryl carbonate or a dialkyl carbonate, typically a diaryl carbonate.

[0039] In particular the dihydroxy compound is BPA and the carbonate is DPC. The present invention however is not limited to these starting materials and other bisphenols or carbonates may be used either as such or in combination. For example the present invention also relates to a process wherein the dihydroxy compound comprises BPA and another type of bisphenol. The carbonate is preferably DPC but so-called activated carbonates like for example the ester substituted diaryl carbonates disclosed in US 2008 / 0004417 may also be used. Preferably the polycarbonate manufactured in accordance with the present invention is obtained by reacting BPA and DPC as the only monomers.

[0040] Thus, preferably, the dihydroxy compound comprises or consists of a bisphenol and / or the carbonate comprises or consists of a diaryl carbonate. More preferably, the dihydroxy compound comprises or consists of bisphenol A (BPA) and / or the carbonate comprises or consists of diphenyl carbonate (DPC). Most preferably, the dihydroxy compound consists of bisphenol A (BPA) and the carbonate consists of diphenyl carbonate (DPC).

[0041] The melt transesterification process typically does not comprise a step of purification of the obtained polycarbonate similar to a purification step as carried out in the interfacial process. The purity of the raw materials therefore influences the level of quality of the obtained polycarbonate.

[0042] An advantage of the present invention is that it allows the use of a somewhat lower quality BPA and / or DPC while a polycarbonate is still obtained with an acceptable color tone and bisphenol amount.

[0043] It is preferred that the bisphenol, in particular BPA, has a purity of at least 99.3 wt.%, preferably at least 99.5 wt% or at least 99.9 wt%. In some embodiments, the bisphenol, in particular BPA, has a purity of at most 99.7 wt%.

[0044] Also the diaryl carbonate, in particular DPC, has a purity of at least 98.5 wt.%, preferably at least 98.6 wt% or at least 99.0 wt%. In some embodiments, the carbonate, in particular DPC, has a purity of at most 98.7 wt%.

[0045] Catalyst

[0046] The catalyst used in the process according to the invention is not limited and any catalyst commonly known for transesterification reactions to manufacture polycarbonate can be used. Thus, the catalyst may be a so-called alpha catalyst, a so-called beta-catalyst or a combination of both.

[0047] Suitable examples of alpha-catalyst and beta-catalyst are described in p.15, 1.1 to 28 and p.13, 1.11 to p.14, 1.34 of W02020074983A1 , respectively, which description is incorporated herein by reference.

[0048] Catalyst quencher

[0049] Alpha catalysts are transesterification catalysts that are typically more thermally stable than beta catalysts and therefore typically nearly all of the alpha catalyst (e.g., greater than 80 wt.%, specifically greater than 90 wt.%) survives the polymerisation process. As such, this catalyst is available to catalyse additional and generally undesired reactions downstream of the polymerisation process, such as in the extruder or even in any downstream processing step such as for example injection moulding. Typically a catalyst deactivator, or quencher, or quenching agent, is therefore added at a desired stage in the polymerisation process.

[0050] Preferably, the process comprises combining said stream of molten polycarbonate with a quenching agent for deactivating the transesterification catalyst at least in part, wherein said combining with the quenching agent is performed upstream of said combining with water.

[0051] The quencher preferably comprises a sulfonic acid ester such as an alkyl sulfonic ester of the formula R8SC>3R9wherein R8is hydrogen, C1-C12 alkyl, Ce-Cis aryl, or C7-C19 alkylaryl, and R9is C1-C12 alkyl, Ce-Cis aryl, or C7-C19 alkyl aryl. Examples of alkyl sulfonic esters include benzenesulfonate, p-toluenesulfonate, methylbenzene sulfonate, ethylbenzene sulfonate, n-butyl benzenesulfonate, octyl benzenesulfonate and phenyl benzenesulfonate, methyl p-toluenesulfonate, ethyl p-toluenesulfonate, n-butyl p- toluene sulfonate, octyl p-toluenesulfonate and phenyl p- toluenesulfonate. The sulfonic acid ester can comprise alkyl tosylates such as n-butyl tosylate.

[0052] The quencher can be added in an amount of from 1-10 ppm, based on the total weight of the polycarbonate. The exact amount of quencher depends on the amount of alpha catalyst that is added during the process and should be sufficient to deactivate the remaining catalyst for at least 90%. To that extent the amount of quencher that is added corresponds to 0.1 to 75 times, specifically, 0.5 to 30 times, more specifically 1 to 20 or even more specifically 1.5 to 10 per the neutralization equivalent of the catalyst used. The quencher may be part of a quencher composition further comprising a liquid carrier such as a solvent, or a solid carrier such as a polymer, preferably polycarbonate.

[0053] A preferred quencher is butyl tosylate.

[0054] Process / plant

[0055] Melt transesterification reaction The present invention is not limited with respect to how the melt transesterification reaction is carried out and how the plant to carry out the polymerisation is configured. Typically the process involves a monomer mixing stage, an oligomerisation stage and a polymerisation stage. Over the course of the polymerisation the temperature is generally increased and the pressure generally decreased while the molecular weight of the polycarbonate increases.

[0056] An increase in temperature is required in view of the increasing viscosity of the polymer being formed and the decreasing pressure is required in order to advance the polymerisation reaction and to effectively remove the by-product of the condensation reaction, typically phenol.

[0057] The plant for the manufacture of polycarbonate may be part of an integrated site and the BPA and DPC and / or other raw materials may come directly from other plants or units on-site and producing the said monomers either in solid or in molten form. The invention is however not limited to such an embodiment and raw materials such as BPA and DPC may also be obtained from external sources and added to the equipment in the monomer mixing stage using appropriate feeding equipment and upon application of any optional pre-treatment such as melting, filtering, purification, solvent removal etcetera. For example, BPA and DPC may be provided in tanks or bulk containers and fed directly or indirectly to the monomer mixing unit(s).

[0058] In the monomer mixing stage the raw materials, typically BPA and DPC, are mixed in a molten state. Optionally an amount of beta catalyst may be added in this stage. The monomer mixing stage is preferably carried out at a temperature of from 100 to 250°C, specifically from 150 to 200°C, more specifically from 160 to 185°C. The pressure in the monomer mixing stage is preferably substantially atmospheric such as from 900 to 1100 mbar.

[0059] The alpha catalyst is preferably added downstream of the monomer mixing device and can be added for example upstream of and / or directly to the one or more oligomerisation and / or polymerisation reactors. The oligomerisation stage is preferably carried out in two steps wherein in a first step the temperature is from 230 to 260 °C and the pressure is from 140 to 250 mbar, and wherein in a second step the temperature is higher than in the first step and from 260 to 290 °C and the pressure is from 10 to 50 mbar. The present invention is however not limited to two steps and any number of between 1-6, such as 2, 3, 4 or 5 oligomerisation steps may be used.

[0060] The weight average molecular weight of the oligomer resulting from the oligomerisation stage is preferably at most 20000, preferably from 8000 to 12000 Daltons, determined on the basis of polystyrene standards.

[0061] The polymerisation stage is preferably carried out in two steps wherein in a first step the temperature is from 280 to 315 °C and the pressure is from 1 to 5 mbar and wherein in a second step the temperature is from 280°C to 315 °C and the pressure is from 0.3 to 5.0 mbar. The present invention is however not limited to two steps and any number of between 1-6, such as 2, 3, 4 or 5 polymerisation steps may be used.

[0062] Combining molten polycarbonate with water and siloxane compound

[0063] The method according to the invention involves combining water with stream of molten polycarbonate before (step i')) and / or after (step iii)) feeding the stream of molten polycarbonate to a melt mixing device. Thus, some embodiments of the invention do not comprise step i') and some embodiments of the invention do not comprise step iii). In embodiments not comprising step iii), step iv) is performed on the stream obtained in step ii). In embodiments comprising both steps i') and iii), the amount of water combined in step i') and the amount of water combined in step iii) may be in selected in any ratio, e.g. 1 :100 to 100:1.

[0064] In some embodiments, the method for the manufacture of polycarbonate comprises the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of one or more of a transesterification catalyst, thereby forming a stream of molten polycarbonate; ii) feeding the stream of molten polycarbonate obtained in step i) to a melt mixing device; iii) combining, in said melt mixing device, said stream of molten polycarbonate and from

[0065] 5 - 5000 ppm of water based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

[0066] In some embodiments, the method for the manufacture of polycarbonate comprises the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of one or more of a transesterification catalyst, thereby forming a stream of molten polycarbonate; i') combining said stream of molten polycarbonate obtained in step i) and from 5 - 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) to a melt mixing device; iv) subjecting the stream obtained in step ii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

[0067] In some embodiments, the method for the manufacture of polycarbonate comprises the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of one or more of a transesterification catalyst, thereby forming a stream of molten polycarbonate; i') combining said stream of molten polycarbonate obtained in step i) and from 1 - 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) or step i’) to a melt mixing device and iii) combining, in said melt mixing device, said stream of molten polycarbonate of step ii) and from 1 - 5000 ppm of water based on the weight of the polycarbonate, wherein the total amount of water to be combined with the stream of molten polycarbonate in steps i’) and iii) is 5-5000 ppm based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

[0068] When the method according to the invention comprises step i’), it may be performed by adding the water to a transfer line or static mixer between the final reactor for obtaining the stream of molten polycarbonate and the melt mixing device.

[0069] This is explained below in more detail.

[0070] After the final polymerisation reactor the molten polycarbonate, having substantially the same temperature as that of the final reactor, can be transported to a unit wherein water, optional siloxane compound and further optional components can be added, e.g. a static mixer and / or a melt mixing device, preferably an extruder.

[0071] The water may be added in the melt mixing device, e.g. at a feed section of the melt mixing device, corresponding to the section where the melt coming from the final polymerisation reactor is fed to the extruder. However, the water may also be added after the final polymerisation reactor and before the melt mixing device such as in the transfer piping connecting the final polymerisation reactor and the feed section of the melt mixing device. The water may be added at a single location or at different locations, such as at different sections of the extruder or at the feed section of the melt mixing device and the transfer piping.

[0072] The water is, preferably, not added in the final polymerisation reactor. When a quencher is added to the polymer melt it is preferred that the water is added after the polycarbonate is at least partially quenched. Accordingly it is preferred that the water is added downstream of the addition of a catalyst quencher. The present inventors have found that this configuration is preferred for maintaining the molecular weight at a desired level.

[0073] A siloxane compound may be added in the melt mixing device before, at the same time and / or after the addition of water. In addition to or alternatively to adding water and the siloxane compound to the melt mixing device, water and / or the siloxane compound may be added to a static mixer. For example, water and / or the siloxane compound may be added to a static mixer which receives the molten polycarbonate from the final reactor and then the mixture from the static mixer can be transported to an extruder. In another example, water (or siloxane compound) can be added to a transport line between the final reactor and a first static mixer and the mixture from the first static mixer may be transported to a second static mixer to which the siloxane compound (or water) is added.

[0074] Additives may optionally be added e.g. to the melt mixing device and / or the static mixer. Suitable examples of the optional additives include one or more of an impact modifier, flow modifier, filler, reinforcing agent (e.g., glass fibers or talc), antioxidant, heat stabilizer, light stabilizer, UV light stabilizer and / or UV absorbing additive, plasticizer, lubricant, release agent, in particular glycerol monostearate, pentaerythritol tetra stearate, glycerol tristearate, stearyl stearate, antistatic agent, antifog agent, antimicrobial agent, colorant (e.g., a dye or pigment), flame retardant either or not combined with an anti-drip agent such as polytetrafluoroethylene (PTFE) or PTFE- encapsulated styrene-acrylonitrile copolymer. The present invention is not limited in terms of the type and amount of additives and an embodiment is possible wherein none of these additives exemplified above is added.

[0075] In some embodiments, the method according to the invention comprises combining the stream of molten polycarbonate with tris(2,4-di-tert.-butylphenyl) phosphite (commercially available e.g. as Irgafos® 168 from BASF). The amount of tris(2,4-di-tert.- butylphenyl) phosphite with respect to the amount of the polycarbonate may e.g. be 10 to 500 ppm, for example 10 to 100 ppm or 100 to 500 ppm. It was observed that the effect of the siloxane compound on the decrease in the amount of dihydroxy compound such as bisphenol in the polycarbonate is greater when a higher amount of tris(2,4-di- tert.-butylphenyl) phosphite is combined with the stream of molten polycarbonate.

[0076] The stream of molten polycarbonate is subjected to a vacuum after the stream is combined with said water. This may be performed by a vacuum means provided in the melt mixing device. This reduces the amount of water in the stream of molten polycarbonate supplied from the melt mixing device. The vacuum is in the range of 1 - 800 mbar, for example 100 - 700 mbar or 150 - 500 mbar, such as from 200 - 400mbar

[0077] In accordance with the invention the stream of polycarbonate that is introduced in the melt filter is substantially free and preferably free of water.

[0078] Melt filter

[0079] The stream of molten polycarbonate is passed through a melt filter after the stream has been subjected to vacuum. A melt filter has the function of removing any particulate matter or gels from the stream.

[0080] Additionally, the stream of molten polycarbonate may be passed through a melt filter before the stream is combined with said water.

[0081] The melt filter may have a pore size of for example 2.5 - 60 micrometer.

[0082] Finally the stream of molten polycarbonate is passed through a die into one or more strands which are then cooled and cut into pellets.

[0083] Figure 1 is a schematic and non-limiting example of a plant to manufacture polycarbonate in accordance with the invention.

[0084] BPA and DPC are added as streams A and B1 respectively to monomer mixing device 10. The DPC to BPA ratio in the monomer mixing device is kept fixed. A beta catalyst is added to monomer mixing device 10 via stream C. Monomer mixing device 10 is equipped with a suitable stirrer so as to guarantee a homogeneous mixture in the device. Monomer mixing device 10 is typically maintained at a temperature of from 160 to 185°C and at substantially atmospheric pressure. The stream exiting monomer mixing device 10 is fed to a first oligomerisation reactor 20. For reasons of process flexibility an additional amount of DPC may be added as stream B2. An alpha catalyst is added as a stream D. This monomer mixture is then fed to oligomerisation reactor 20 of the oligomerisation stage. Oligomerisation reactor 20 operates at a temperature of from 230 to 260°C and a pressure of from 140 to 250 millibar. An overhead stream comprising phenol byproduct and optionally monomers or other low molecular weight reaction products is removed via stream 70 and fed to column 50, which separates the phenol from the stream. The phenol is then removed via top stream E for further purification and / or use, while the bottom stream is fed back to reactor 20 as stream 71 . In an alternative embodiment the stream 70 is purified elsewhere and there is no recycle of material to the oligomerisation reactor(s) 20,21.

[0085] The mixture exiting reactor 20 is fed to a second oligomerisation reactor 21 for further reaction. Second oligomerisation reactor 21 operates at temperature of from 260 to 290°C and a pressure of from 10 to 50 millibar. Phenol byproduct is removed from second reactor 21 as a stream E.

[0086] Oligomerisation reactors 20 and 21 constitute the oligomerisation stage, resulting in a stream of polycarbonate oligomer which is fed to first polymerisation reactor 30 and then to second polymerisation reactor 31. Reactor 30 operates at a temperature of from 280 to 315°C and a pressure of 1 to 5 millibar. The stream from the first polymerisation reactor 30 is then fed to a second polymerisation reactor 31 that operates at temperature of from 280 to 315°C and a pressure of from 0.3 to 5.0 millibar. The temperature in reactor 31 is generally higher than in reactor 30 and the pressure in reactor 31 is generally lower than the pressure in reactor 30. Similar to the oligomerisation stage phenol byproduct is removed from the reactors 30 and 31. Polymerisation reactors 30 and 31 together constitute the polymerisation stage.

[0087] The polymer exiting second polymerisation reactor 31 is fed to extruder 40.

[0088] In accordance with the invention an amount of water is added via feed 16a at the feed section of extruder 40 . Alternatively the water may also be injected into the melt stream coming from reactor 31 in the transfer piping and close to the feed section of the extruder, indicated with feed 16b. An amount of an optional siloxane compound and other optional additives may be added, indicated with I. The siloxane compound may alternatively be added at the feed section of the extruder indicated with feed 16a.

[0089] Catalyst quencher is also added to the extruder in order to deactivate the catalyst in the molten polycarbonate. For the avoidance of doubt it is noted that the position for addition of catalyst quencher is not limited and may coincide e.g. with feed 16a or 16b or located at other positions either before or after any of feed 16a and / or 16b.

[0090] The extruder 40 is provided with a vacuum means 40c by which the melt stream to which water has been added is subjected to vacuum.

[0091] Water is removed by vacuum before subjecting the stream of molten polycarbonate to the melt filter 60. This reduces any negative effect caused by the presence of water in the melt being passed through the melt filter 60. A further decrease in the amount of dihydroxy compound in the polycarbonate is therefore achieved.

[0092] The stream extruded from the extruder 40 is subjected to a vacuum and then passed through a melt filter 60 and then extruded to strands, cooled, and cut to pellets indicated with J.

[0093] It is noted that while Figure 1 illustrates polymerisation reactors 30 and 31 to be horizontal polymerisation units, these reactors may likewise each independently be vertical reactors, such as for example wire wetting fall polymerisation units.

[0094] The process indicated in Figure 1 is shown as a single production line. It is however possible that at any point during the process the line is split into two or more parallel lines wherein each line operates at the same or different conditions including monomer mixture composition, temperature, pressure residence time etc. By way of example the stream exiting oligomerisation reactor 21 may be split into two or more different streams after which each stream is polymerised in one or more polymerisation reactors using, by way of example, different conditions resulting in the parallel manufacture of different grades of polycarbonate. Another possibility is to split the stream exiting the final polymerisation reactor 31 and then to feed the polycarbonate stream to different extruders. The parallel operation of (parts of the) production lines as shown in Figure 1 is known to a skilled person.

[0095] Apart from the specific configuration shown in Figure 1 the polycarbonate may be manufactured under one or more of the following preferred conditions.

[0096] It is preferred that the monomer mixing stage comprises addition of a beta catalyst wherein the beta catalyst is a quaternary ammonium or quaternary phosphonium compound or a mixture thereof.

[0097] It is preferred that the oligomerisation stage consists of preparing a polycarbonate oligomer in two oligomerisation reactors and wherein the polymerisation stage consists of preparing the polycarbonate in two polymerisation reactors.

[0098] In a preferred embodiment of method of the invention the bisphenol is BPA, the diaryl carbonate is DPC, a beta catalyst is added in the monomer mixing stage and an alpha catalyst is added prior to feeding the monomer mixture prepared in the monomer mixing device to the first oligomerisation reactor. In a preferred embodiment the bisphenol is BPA, the diaryl carbonate is DPC, the alpha catalyst is NaKHPC>4 and the beta catalyst is tetra-butyl phosphonium acetate.

[0099] The method of the invention is preferably a continuous method.

[0100] Polycarbonate

[0101] The invention further relates to a polycarbonate obtainable by or obtained by the method according to the invention.

[0102] The polycarbonate manufactured in accordance with the invention has an improved color compared to an otherwise identical polycarbonate manufactured under otherwise identical conditions yet without the addition of water to the melt mixing device.

[0103] The polycarbonate may have a weight average molecular weight from e.g. 30,000 to 70,000 g / mol, for example 30,000 to 50,000 g / mol or 50,000 to 70,000 g / mol, as determined using GPC on the basis of polystyrene standards. Preferably, the polycarbonate has an endcap level of at least 60%, more preferably from 65 to 95%, even more preferably 70 to 95%, wherein the endcap level is defined as the percentage of polycarbonate chain ends which are not hydroxyl groups. Thus a polycarbonate having and endcap level of 60% means that the polycarbonate has 40% of chain ends that are phenolic OH end groups, usually resulting from the bisphenol A monomer. The other 60% of end groups do not contain a OH end group and may be phenolic (usually originating from the diphenylcarbonate) or correspond to the end capping agent molecule(s). The amount of chain ends that are end-capped with the endcapping agent is preferably at least 40% on the basis of the total amount of end-groups.

[0104] The endcap level is calculated with the following formula ppmOH X Mn\

[0105] %EC = 100 - -

[0106] \ 340000 ) wherein %EC is the endcap level, ppmOH is the amount of hydroxyl end groups in parts per million by weight and Mn is the number average molecular weight of the polycarbonate based on polycarbonate standards.

[0107] Thus, the endcap level is defined as the mole percentage of end-groups of the polycarbonate that is not a hydroxyl group and can be calculated from the amount of terminal OH groups in the polycarbonate and the number average molecular weight (Mn).

[0108] The polycarbonate may have a degree of branching resulting from Fries (or Kolbe- Schmidt) rearrangement reactions of from 200 - 4000 ppm, for example 500 to 2000 ppm. The degree of branching is commonly referred to as amount of Fries branching. Methods for determining the amount of Fries branching are known to the skilled person and generally include the methanolysis of the polycarbonate followed by HPLC chromatography to identify the total amount of Fries structures. In addition, NMR techniques can be used to determine the type and amount of the branching structures, such as the respective amounts of linear and branched Fries structures.

[0109] The polycarbonate may have a Melt Volume-Flow Rate determined according to ISO1133-1 :2022 at 1.2 kg and 300 °C of e.g. 1.0 to 100.0 cm3 / 10 min, preferably 2.0 to 50.0 cm3 / 10 min, more preferably 3.0 to 30.0 cm3 / 10 min. For example, the polycarbonate may have a Melt Volume-Flow Rate determined according to ISO1133- 1 :2022 at 1.2 kg and 300 °C of 1.0 to 7.5 cm3 / 10 min, 7.5 to 12.5 cm3 / 10 min, or 12.5 to 30.0 cm3 / 10 min.

[0110] Preferably, the polycarbonate has a b* value of at most 7.0, preferably at most 6.5, more preferably at most 6.0, preferably at most 5.5, more preferably at most 4.5, as determined by according to Cl ELAB (ASTM D6290-05) and ASTM E313 using a 45 / 0 geometry, light source D65 and a 10° viewing angle with a 32 mm measurement area.

[0111] It will be appreciated that some amounts of residual materials used during the method for the manufacture for the polycarbonate may remain in the polycarbonate obtained by the method. Accordingly, term “polycarbonate” as used herein is understood to mean not only the polycarbonate molecules but also the residual materials.

[0112] Preferably, the amount of residual dihydroxy compound in the polycarbonate is at most 100 ppm, at most 80 ppm, at most 70 ppm, at most 60 ppm, at most 50 ppm, at most 40 ppm, at most 30 ppm, at most 20 ppm or at most 10 ppm based on the weight of the polycarbonate.

[0113] In some embodiments, the dihydroxy compound is bisphenol A and the amount of residual bisphenol A in the polycarbonate is at most 100 ppm, at most 80 ppm, at most 70 ppm, at most 60 ppm, at most 50 ppm, at most 40 ppm, at most 30 ppm, at most 20 ppm or at most 10 ppm based on the weight of the polycarbonate.

[0114] The amount of residual dihydroxy compound in the polycarbonate is determined by the following method.

[0115] Sample preparation:

[0116] Dissolve 0.5 gram of polycarbonate in 5 ml Dichloromethane (DCM) and precipitate the polycarbonate with 10 ml Methanol. Filter the solution into a LC sampler vial.

[0117] Analytical Technique:

[0118] Liquid Chromatography-Diode Array Detector (HPLC-DAD).

[0119] Column: Zorbax Eclipse XDB-C18 4.6 x 75 mm 3.5 urn.

[0120] Column Temperature: 35°C. Wavelengths: 280nm (BPA).

[0121] The present invention further provides a polycarbonate composition comprising the polycarbonate and additives.

[0122] Suitable examples of the additives include one or more of an impact modifier, flow modifier, filler, reinforcing agent (e.g., glass fibers or talc), antioxidant (primary antioxidant and / or secondary antioxidant), heat stabilizer, light stabilizer, UV light stabilizer and / or UV absorbing additive, plasticizer, lubricant, release agent, in particular glycerol monostearate, pentaerythritol tetra stearate, glycerol tristearate, stearyl stearate, antistatic agent, antifog agent, antimicrobial agent, colorant (e.g., a dye or pigment), flame retardant either or not combined with an anti-drip agent such as polytetrafluoroethylene (PTFE) or PTFE-encapsulated styrene-acrylonitrile copolymer. The present invention is not limited in terms of the type and amount of additives.

[0123] In some embodiments, the polycarbonate composition comprises at least one of a primary antioxidant and a secondary antioxidant.

[0124] In some embodiments, the polycarbonate composition consists of the polycarbonate according to the invention and at least one of a primary antioxidant and a secondary antioxidant.

[0125] In some embodiments, the polycarbonate composition comprises tris(2,4-di-tert.- butylphenyl) phosphite (commercially available e.g. as Irgafos® 168 from BASF). The amount of tris(2,4-di-tert.-butylphenyl) phosphite with respect to the amount of the polycarbonate may e.g. be 10 to 800 ppm, for example 10 to 100 ppm or 100 to 800 ppm.

[0126] In some embodiments, the polycarbonate composition comprises a siloxane compound e.g. in an amount of 50 to 1000 ppm and tris(2,4-di-tert.-butylphenyl) phosphite (commercially available e.g. as Irgafos® 168 from BASF), wherein the amount of tris(2,4- di-tert.-butylphenyl) phosphite with respect to the amount of the polycarbonate is 10 to 800 ppm, for example 10 to 100 ppm or 100 to 800 ppm. It was observed that the effect of the siloxane compound on the decrease in the amount of bisphenol in the polycarbonate is greater when the polycarbonate composition comprises a higher amount of tris(2,4-di-tert.-butylphenyl) phosphite.

[0127] The present invention further provides a thermoplastic composition comprising the polycarbonate according to the invention or the polycarbonate composition according to the invention and at least one further polymer, preferably selected from the group consisting of polycarbonate - polyorganosiloxane copolymers, polycarbonate-polyester copolymers, polyesters, polyolefins, acrylonitrile / butadiene / styrene copolymer, methyl methacrylate / butadiene / styrene copolymer, styrene / butadiene / styrene copolymer (SBS), styrene / ethylene-butylene / styrene copolymer (SEBS), styrene / ethylenepropylene / styrene copolymer (SEPS) styrene / acrylonitrile copolymer (SAN), acrylonitrile / styrene / acrylonitrile copolymer (ASA), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), unsaturated polyester (LIPES), polyamide (PA), thermoplastic urethane (TPU), polystyrene (PS), high impact polystyrene (HIPS), polyvinyl chloride (PVC). The amount of the polycarbonate may e.g. be at 5 to 95 wt%, 50 to 95 wt% or 55 to 95 wt% with respect to the thermoplastic composition.

[0128] The thermoplastic composition according to the invention may be made e.g. by meltmixing the polycarbonate according to the invention or the polycarbonate composition according to the invention and the at least one further polymer.

[0129] The present invention further provides a molded article comprising or consisting of the polycarbonate according to the invention, the polycarbonate composition according to the invention or the thermoplastic composition according to the invention.

[0130] The present invention further provides a method for the manufacture of a molded article comprising molding the polycarbonate according to the invention, the polycarbonate composition according to the invention or the thermoplastic composition according to the invention.

[0131] The present invention will now be further elucidated on the basis of the following nonlimiting examples. Polycarbonate was continuously produced using an apparatus as schematically shown in Figure 1.

[0132] BPA and DPC were introduced in a monomer mixing device which was kept at a temperature of 170 °C at a pressure of about 1050 mbar. 50 micromoles of tetra butyl phophonium acetate (TBPA) per mole of BPA was also added as a beta catalyst.

[0133] The monomer mix was then introduced in the first oligomerisation reactor operating at a temperature of 257°C and a pressure of 180 mbar. The initial DPC / BPA ratio (molar ratio) was adjusted with additional DPC to 1.040 and an amount of NaKHPC>4 as alpha catalyst was added. The carbonate oligomer formed in the first oligomerisation reactor was fed to the second oligomerisation reactor operating at a temperature of 280 °C and a pressure of 37 mbar.

[0134] The so formed carbonate oligomer was then introduced to a first and second polymerisation reactor operating at 300 and 302 °C, respectively. The pressure was selected to accommodate the formation of the desired molecular weight.

[0135] The residence time in the oligomerisation section was 1.8 hours (1.1. hour in the first oligomerisation reactor, 0.7 hours in the second polymerisation reactor) and the residence time in the polymerisation section was 1.0 hours (0.5 hour in each polymerisation reactor).

[0136] After polymerisation the polymer was fed to an extruder where a catalyst quencher (2 ppm of butyl tosylate) was added to deactivate the catalyst. Water (Conductivity <5uS / cm, Bicarbonates <1 ppm, pH 6-8) was also fed to the extruder in amounts indicated in Table 1.

[0137] The stream was subjected to a pressure as indicated in Table 1 (300 mbar being a vacuum) in the extruder and was subsequently passed through a melt filter and then extruded to strands and cut to pellets. b* pellet color was measured in the following manner. Color measurements were done by using a BYK Gardner ColorView 9000, measuring L*, a*, b* values (CIE), Yellowness Index (Yl), and Whiteness Index 9WI) using a 45 / 0 geometry, light source D65 and a 10° viewing angle with a 32 mm measurement area. The color measurement is done according to Cl ELAB (ASTM D6290-05) and ASTM E313. The b* pellet color values are disclosed in the Examples below.

[0138] The amount of the residual BPA was measured by the following method.

[0139] Sample preparation:

[0140] Dissolve 0.5 gram of polycarbonate in 5 ml Dichloromethane (DCM) and precipitate the polycarbonate with 10 ml Methanol. Filter the solution into a LC sampler vial.

[0141] Analytical Technique:

[0142] Liquid Chromatography-Diode Array Detector (HPLC-DAD).

[0143] Column: Zorbax Eclipse XDB-C18 4.6 x 75 mm 3.5 urn.

[0144] Column Temperature: 35°C. Wavelengths: 280nm (BPA).

[0145] MW was determined using GPC on the basis of polystyrene standards.

[0146] The endcap level %EC was determined as

[0147] (ppmOH X Mn\ %EC = 100 - -

[0148] \ 340000 / wherein %EC is the endcap level, ppmOH is the amount of hydroxyl end groups in parts per million by weight and Mn is the number average molecular weight of the polycarbonate based on polycarbonate standards.

[0149] The degree of Fries branching was determined using an Agilent HOOseries HPLC equipped with a MWD G1365B detector. The column is an Agilent Zorbax Eclipse XDB- C18 4.6 x 75 mm. The injection volume is 50ml. The oven temperature is 35°C and the wavelength to acquire data is 320.16 nm. For sample preparation 0.3g of sample is dissolved in 7.5 ml of a solvents mixture based on 5 ml of tetrahydrofuran and 2.5 ml of a 10 % of potassium hydroxide solution in methanol, after heating this sample at 40°C during 20 min, 1.4 ml of acetic acid is added. As explained the amount of Fries branching is the total of branching structures resulting from both Fries rearrangement and Kolbe- Schmidt type or rearrangement reactions. The Fries branching is expressed as ppm by weight. Table 1 : Effect of the addition of water and of applying vacuum in the extruder

[0150] In the context of the present invention, the term "ambient pressure” means a pressure of the surrounding air, when no vacuum is applied. This may be atmospheric pressure, which is around 1013 mbar.

[0151] It can be understood that the addition of water under ambient pressure results in improved color properties but increase in the amount of residual BPA (CE1 compared with CEx 2, CEx 3 and CEx 4).

[0152] The addition of water followed by vacuum also results in improved color properties and increase in the amount of residual BPA (CEx 5 compared with Ex 1, Ex 2 and Ex 3). However, at the same amount of added water, the color properties are better and the amount of residual BPA is smaller when vacuum is used (CEx 2-CEx 4 compared with Ex 1-Ex 3).

[0153] Accordingly, the addition of water followed by vacuum according to the invention results in a combination of good color properties and a relatively low amount of dihydroxy compound.

Claims

C L A I M S1. Method for the manufacture of polycarbonate comprising the steps of i) melt reacting a dihydroxy compound and a carbonate, optionally in the presence of a transesterification catalyst, thereby forming a stream of molten polycarbonate, i') combining said stream of molten polycarbonate obtained in step i) and from 0 - 5000 ppm of water based on the weight of the polycarbonate, ii) feeding the stream of molten polycarbonate obtained in step i) or step i’) to a melt mixing device, iii) combining, in said melt mixing device, said stream of molten polycarbonate and from 0 - 5000 ppm of water based on the weight of the polycarbonate, wherein the total amount of water to be combined with the stream of molten polycarbonate in steps I’) and iii) is from 5 - 5000 ppm based on the weight of the polycarbonate, iv) subjecting the stream obtained in step iii) at least partially to a vacuum in the range from 1 - 800 mbar in order to remove the water from the stream of molten polycarbonate at least in part, and v) passing the stream obtained in step iv) through a melt filter.

2. The method of claim 1 wherein the vacuum is in the range from 100-700 mbar.

3. The method of claim 1 or 2 wherein the melt mixing device is an extruder.

4. The method of any one or more of claims 1-3 wherein the melt filter has a pore size of from 2.5 - 60 micrometer.

5. The method of any one or more of claims 1-4 further comprising combining said stream of molten polycarbonate with a quenching agent for deactivating the transesterification catalyst at least in part, wherein said combining with the quenching agent is performed upstream of said combining of the stream of molten polycarbonate with water.

6. The method of any one or more of claims 1 -5 further comprising, downstream of said melt filter, cooling the stream of molten polycarbonate and cutting the cooled polycarbonate into pellets.

7. The method of any one or more of claims 1-6 wherein the dihydroxy compound comprises or consists of a bisphenol and / or the carbonate comprises or consists of a diaryl carbonate, preferably the dihydroxy compound comprises or consists of bisphenol A (BPA) and / or the carbonate comprise or consist of diphenyl carbonate (DPC), more preferably the dihydroxy compound consists of bisphenol A (BPA) and the carbonate consists of diphenyl carbonate (DPC).

8. The method of any one or more of claims 1-7 wherein said stream of molten polycarbonate is combined with from 50 to 10000 ppm of a siloxane compound, based on the weight of the polycarbonate, upstream of said melt filter.

9. The method of claim 8 wherein the siloxane compound is represented by a general structure (I):whereineach occurrence of R1 may be the same or different and is selected from hydrogen, methyl, ethyl, phenyl and vinyl, each occurrence of R2 may be the same or different and is selected from hydrogen, methyl, ethyl, phenyl and vinyl, each of R3 and R6 is selected from hydrogen, hydroxyl, vinyl, methyl, methoxy and ethoxy, each of R4, R5, R7 and R8 is selected from methyl, methoxy, ethyl, ethoxy and phenyl and x is an integer between 1 and 500.

10. The method of claim 9 wherein the siloxane compound of formula (I) comprises or is a compound represented by:wherein each occurrence of R1 may be the same or different and is selected from hydrogen and phenyl and y is an integer and z is an integer and y+z=x.

11. The method of claim 9 or 10 wherein the siloxane compound of formula (I) comprises or is a compound represented by:(V) orwherein y / (y+z) is at least 0.1, for example 0.4 to 0.6, or at least 0.9, at least 0.95 or at least 0.99, or z is 0.

12. The method of any one or more of claims 8-11 wherein the siloxane compound comprises or is a compound selected from the group consisting of phenyl methyl siloxane, methyl hydrogen siloxane, methyl methyl siloxane and phenyl hydrogen siloxane and combinations thereof, preferably the siloxane compound comprises or is methyl hydrogen siloxane.

13. The method of any one or more of claims 1-12 wherein the amount of water is 5-800 ppm, based on the weight of the polycarbonate.

14. The method of any one or more of claims 1-13 wherein the water has an electrical conductivity of at most 5 pS / cm at 25°C measured in accordance with ISO 7888:1985.

15. Polycarbonate obtainable by or obtained by the method of any one or more of claims 1-13, wherein the amount of residual dihydroxy compound or residual bisphenol A in the polycarbonate is at most 100 ppm, at most 80 ppm, at most 70 ppm, at most 60 ppm, at most 50 ppm, at most 40 ppm, at most 30 ppm, at most 20 ppm or at most 10 ppm based on the weight of the polycarbonate.

16. Method for the manufacture of a molded article comprising molding the polycarbonate according to claim 14.

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