How to process lactide

Abstract

The present invention relates to a method for treating crude lactide comprising the steps of separating a crude lactide stream containing L-lactide, D-lactide, and meso-lactide in one or more steps to form a meso-lactide stream containing at least 50% by weight meso-lactide (based on the total weight of lactide in the stream) and at least one purified lactide stream containing L-lactide and / or D-lactide; oligomerizing the meso-lactide stream to form an oligomer-containing stream containing lactic acid oligomers; and depolymerizing the oligomer-containing stream to form a product stream containing meso-lactide, L-lactide and D-lactide. The present invention also relates to a method for producing polylactide, and to a method for producing a racemic mixture of (S)- and (R)-lactic acid.

Description

[Technical field]

[0001] The present invention relates to a method for treating a crude lactide stream, to a method for producing polylactide, and to a method for producing a racemic mixture of (S)- and (R)-lactic acid. [Background technology]

[0002] Polylactide (also known as poly(lactic acid) or PLA) is a polymer with applications in a variety of areas, from packaging to disposable tableware. Polylactide is a polymer derived from lactic acid. Lactic acid is a chiral molecule and therefore exists as either (S)-lactic acid or (R)-lactic acid. Commercially available polylactides generally have a high proportion of (S)-lactic acid units and are usually obtained by ring-opening polymerization of (mainly) L-lactide, which is a dimer of (S)-lactic acid.

[0003] Lactide is typically synthesized by oligomerizing lactic acid to form lactic acid oligomers via a polycondensation reaction. These oligomers are then depolymerized to form crude lactide containing L-lactide ((S,S)-lactide), D-lactide ((R,R)-lactide), and meso-lactide ((S,R)-lactide). See, for example, U.S. Pat. No. 5,357,035, U.S. Pat. No. 5,521,278, International Publication No. WO 2010 / 105143, and U.S. Patent Application Publication No. US 2014 / 031566. When oligomers are synthesized from (S)-lactic acid, the resulting oligomers will contain mostly (S)-lactic acid units. Depolymerization of such oligomers will result in crude lactide in which L-lactide is the predominant stereoisomer. Under certain conditions, lactide can be converted into different stereoisomers by a racemization process.

[0004] L-lactide, D-lactide and meso-lactide can be fed to the polymerization reactor in various ratios. Because each L-lactide molecule contributes two (S)-lactic units to the polymer, each D-lactide molecule contributes two (R)-lactic units to the polymer, and each meso-lactide molecule contributes one (S)-lactic unit and one (R)-lactic unit to the polymer, the ratio in which the lactides are fed to the polymerization reactor determines the ratio of (S)-lactic units to (R)-lactic units in the polymer. The ratio of (S)-lactic units to (R)-lactic units in the polymer is important because it determines most of the physical properties of the polylactide. A polylactide that contains predominantly (S)-lactic units, e.g., 10% (R)-lactic units, will be less crystalline than a polylactide that contains only (S)-lactic units.

[0005] However, the ratio of (S)- to (R)-lactic units in the polymer is not the only factor that contributes to the physical properties of polylactide. Another important factor is the presence or absence of impurities, such as hydroxyl-containing impurities, in the lactide stream fed to the polymerization reactor. Hydroxyl-containing impurities, if present in the lactide stream being polymerized, will reduce the average molecular weight of the resulting polylactide, as described in U.S. Pat. No. 5,357,035. Impurities can also affect the color of the polylactide, its thermal stability, and its suitability for food contact applications.

[0006] WO 2010 / 105143 describes a process in which meso-lactide is separated from crude lactide and recycled to an oligomerization reactor (for oligomerization of lactic acid via a polycondensation reaction) and / or a depolymerization reactor. The conditions in these reactors are such that a portion of the recycled meso-lactide is converted to L- or D-lactide. There are two reasons for doing so. First, recycling of meso-lactide results in an increase in the non-predominant lactide (D-lactide or L-lactide, depending on whether lactide synthesis began with (S)- or (R)-lactic acid) compared to a process in which meso-lactide is not recycled. This is said to be advantageous because (R)-lactic units can be introduced into the polylactide via D-lactide instead of via meso-lactide. Second, the meso-lactide stream obtained in WO 2010 / 105143 is rich in so-called "intermediate-boiling impurities," which, if present in the lactide stream sent to the polymerization reactor, result in discoloration of the polylactide. By recycling meso-lactide to the oligomerization reactor and / or depolymerization reactor, the impurities are partially diverted away from the polymerization reactor, and an increased amount of D-lactide can be separated and used directly for polymerization in the purified L- and D-lactide stream.

[0007] Thus, in summary, the process described in WO 2010 / 105143 aims to reduce the need to use meso-lactide in the lactide stream sent to the polymerization reactor by recycling and converting it to L- and D-lactide. The first drawback of this process is that the conversion of meso-lactide to L- and D-lactide is carried out under conditions that also result in the conversion of L- and D-lactide to meso-lactide, which limits the overall efficiency of the process. The second drawback of the process is that recycling the meso-lactide stream can contaminate the main lactide synthesis reactor, since impurities will be concentrated in the recycled meso-lactide stream. Moreover, the meso-lactide sent (directly) to the polymerization reactor from the final distillation step may contain some impurities, which will result in discoloration of the product synthesized. A further drawback of this process is that it results in the accumulation of less predominant lactide in the process as it continues to be recycled to the beginning of the process.

[0008] Methods have also been described in the art in which a meso-lactide stream is subjected to racemization. US Patent Application Publication No. US2014 / 031566 describes a method in which meso-lactide is separated from a crude lactide stream to form a meso-lactide stream. The meso-lactide is then subjected to racemization, thereby forming L- and D-lactide. Again, the racemization conditions are such that conversion of L- and D-lactide to meso-lactide occurs. From the racemized mixture, an enriched meso-lactide stream may remain and can be processed in a variety of ways. The stream may be discarded or used in a lower value application. It is said that the "intermediate boiling impurities" present in the enriched meso-lactide stream may be removed by extraction or chemical processing methods. Chemical processing methods include converting the "intermediate boiling impurities" or meso-lactide to either a different chemical species. This different species is said to be more easily separated from the "intermediate boiling impurities" or meso-lactide. No mention is made of what species the "intermediate boiling impurities" or meso-lactide should be converted to in order to facilitate separation.

[0009] International Publication No. WO2010 / 105142 is directed to a method of recovering lactide by subjecting a lactide stream to a catalytic racemization step. It also describes removing meso-lactide from the remaining lactide stream to remove non-predominant lactides and impurities. In a preferred embodiment, this meso-lactide fraction is subjected to a racemization step. Again, the conditions during racemization are such that conversion of L- and D-lactide to meso-lactide occurs, resulting in a loss in yield. Summary of the Invention [Problem to be solved by the invention]

[0010] There is a need in the art for a process that specifically converts meso-lactide to L-lactide and D-lactide (i.e., without converting a significant portion of the L- and D-lactide back to meso-lactide), allows for the containment of acid-containing impurities and their easy removal from the process, and does not involve the accumulation of the non-predominant lactide. The present invention provides such a process. [Means for solving the problem]

[0011] In one aspect, the invention is a method for treating a crude lactide stream, the method comprising: separating a crude lactide stream containing L-lactide, D-lactide, and meso-lactide in one or more steps to form a meso-lactide stream containing at least 50 weight percent meso-lactide (based on the total weight of lactide in said stream) and at least one purified lactide stream containing L-lactide and / or D-lactide; oligomerizing the meso-lactide stream to form an oligomer-containing stream that comprises lactic acid oligomers; and depolymerizing said oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide and D-lactide. The present invention relates to the above method, which comprises the steps of:

[0012] An important feature of the invention is that the oligomerization-depolymerization sequence used in the process according to the invention specifically converts a stream containing at least 50% by weight of meso-lactide (via lactic acid oligomers) to a significant portion of L- and D-lactide. Because the oligomerization is generally carried out in a dedicated oligomerization reactor in the absence of significant amounts of L- and D-lactide, the undesirable conversion of these compounds to meso-lactide is limited. This means that energy is efficiently expended to convert a relatively low-value intermediate (meso-lactide) to a high-value intermediate (L- and D-lactide). In fact, as shown in the examples, conversion of meso-lactide to L- and D-lactide occurs to a high degree. Such high conversions are not achieved if the meso-lactide is recycled and subjected to a racemization process, such as those described in International Publication Nos. WO 2010 / 105143, WO 2010 / 105142, and U.S. Patent Application Publication No. US 2014 / 031566.

[0013] In addition, in the process of the invention, the lactic units are rearranged via intermediate oligomers rather than racemization (although racemization of a certain fraction is not precluded), thus preventing the yield losses that occur when meso-lactide is subjected to a racemization step. Moreover, in the process of the invention, accumulation of the non-predominant lactic acid enantiomer is prevented, while at the same time producing high value intermediates (L- and D-lactide).

[0014] The process according to the invention also makes it possible to convert meso-lactide to L- and D-lactide without contaminating the main lactide synthesis reactor with recycled meso-lactide and / or impurities that are contained in other parts of the reactor, where they do not interfere with the main lactide synthesis. This results in a more stable process, meaning that the content of the main lactide synthesis (e.g., the content of the crude lactide stream) is surprisingly consistent over time.

[0015] In another aspect, the invention relates to a method for producing polylactide, the method comprising obtaining at least one stream according to a method for treating crude lactide according to the invention, feeding at least a portion of the at least one stream, with or without intermediate treatment, directly or indirectly to a polymerization reactor, and polymerizing the at least one stream to form polylactide.

[0016] In yet another aspect, the present invention relates to a method for producing a racemic mixture of lactic acid, the method comprising hydrolyzing at least a portion of a stream comprising a racemic mixture of L-lactide and D-lactide obtained according to the method for treating crude lactide according to the present invention to form a racemic mixture of (S)- and (R)-lactic acid. [Brief description of the drawings]

[0017] [Figure 1] In FIG. 1, a crude lactide stream (1) containing L-lactide, D-lactide, and meso-lactide is sent to a separation unit (2), which may be, for example, a distillation unit or a crystallizer. [Diagram 2] In FIG. 2, a preferred process is illustrated in which the oligomerization of the meso-lactide stream (3) and the depolymerization of the so obtained lactic acid oligomers occur in the same reactor. [Diagram 3] In FIG. 3, a process according to FIG. 2 is illustrated in which L-lactide stream (4a) and stream (12) containing a racemic mixture of L-lactide and D-lactide are combined to form polymerizable stream (15), which is then sent to a polymerization reactor (16) where it is reacted to form polylactide. [Figure 4] In FIG. 4, a process according to FIG. 3 is illustrated where the process is operated in batch mode and meso-lactide is collected in a holding tank prior to oligomerization. [Diagram 5]In FIG. 5 a process according to FIG. 2 is illustrated in which at least a part of the stream (12) comprising a racemic mixture of L- and D-lactide is sent to a hydrolysis reactor (19), where it is hydrolyzed with water to form a racemic mixture of (S)- and (R)-lactic acid (20), which may be (partially) sent, for example, to an oligomerization reactor (in particular a polycondensation reactor) or to a depolymerization reactor used to prepare crude lactide. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Aspects of the invention will be explained in more detail below, and particular advantages of the method, as well as particular embodiments thereof, will become apparent from the further specification.

[0019] Method for treating crude lactide

[0020] As mentioned above, disclosed herein is a method for treating a crude lactide stream, the method comprising: separating a crude lactide stream containing L-lactide, D-lactide, and meso-lactide in one or more steps to form a meso-lactide stream containing at least 50 weight percent meso-lactide (based on the total weight of lactide in said stream) and at least one purified lactide stream containing L-lactide and / or D-lactide; oligomerizing the meso-lactide stream to form an oligomer-containing stream that comprises lactic acid oligomers; and depolymerizing said oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide and D-lactide. The process includes the steps of:

[0021] The crude lactide stream comprises L-lactide, D-lactide and meso-lactide. The crude lactide may comprise 75-95% by weight (based on the total weight of lactide in the stream), preferably 80-95% by weight, of L-lactide. The crude lactide may comprise 0.01-5% by weight (based on the total weight of lactide in the stream) of D-lactide. The crude lactide may comprise 1-25% by weight (based on the total weight of lactide in the stream) of meso-lactide. The amount of meso-lactide in the composition may be determined by HPLC, for example, using a water / acetonitrile mixture as eluent and a UV detector. Lactic acid and lactoyl lactic acid will elute first, followed by L- and D-lactic acid, and then oligomers of lactic acid. The crude lactide stream may contain residual amounts of lactic acid, lactic acid esters and / or water, which can be removed by distillation, crystallization or extraction. These residual amounts, if present, can be removed before or during the separation step, but are preferably removed before the separation step. The crude lactide stream may contain acid-containing impurities, such as lactoyl lactic acid, succinic acid and acetic acid. The acid-containing impurities may be present in the crude lactide stream in an amount such that the crude lactide stream has a free acid content of at least 20 meq / kg, in particular at least 50 meq / kg, and / or not more than 150 meq / kg. As a maximum, the acid-containing impurities may be present in the crude lactide stream in an amount of not more than 250 meq / kg. The free acid content used herein can be determined by titration, for example, titration with sodium methylate or potassium methylate in anhydrous methanol.

[0022] The crude lactide stream can be separated by any suitable separation technique. Preferably, the crude lactide stream is separated by distillation or crystallization (e.g., solvent crystallization or melt crystallization), or any combination of both. These separation techniques separate meso-lactide and at least a portion of the acid-containing impurities, if present, from L- and D-lactide. This results in the formation of a meso-lactide stream containing meso-lactide (and optionally, acid-containing impurities) and at least one purified lactide stream containing L-lactide and / or D-lactide. In a preferred process, at least one distillation step is used to separate the crude lactide stream. When distillation is used, other streams removed from the distillation may be treated as purge streams, e.g., overhead streams containing volatile components (e.g., water, lactic acid, formic acid, and acetic acid).

[0023] The at least one purified lactide stream containing D-lactide and / or L-lactide is relatively pure when compared to crude lactide. In particular, the amount of meso-lactide and the amount of (volatile) acid-containing impurities in the at least one purified lactide stream are less than in the crude lactide stream. For example, the amount of meso-lactide in the at least one purified D- and / or L-lactide stream may be 50% or less, preferably 25% or less, and more preferably 10% or less of the amount of meso-lactide in the crude lactide stream.

[0024] It is preferred that at least one purified lactide stream comprises primarily L-lactide. Thus, one or more of the purified lactide streams may comprise at least 80% by weight L-lactide (based on the total weight of lactide in the stream), preferably at least 85% by weight L-lactide, more preferably at least 90% by weight L-lactide, even more preferably at least 95% by weight L-lactide. In a more preferred embodiment, at least one of the purified lactide streams comprises an amount of L-lactide as defined above (i.e., at least 80% by weight L-lactide, or the preferred amount as defined above, based on the total weight of lactide in the stream), and 5% by weight or less of D-lactide, preferably 3% by weight or less of D-lactide, more preferably 1% by weight or less of D-lactide. In an even more preferred embodiment, at least one of the purified lactide streams contains 5% by weight or less meso-lactide (based on the total weight of lactide in the stream), preferably 4% by weight or less meso-lactide, more preferably 3% by weight or less meso-lactide, and even more preferably 2% by weight or less meso-lactide (preferably in combination with the preferred amounts of L-lactide specified above, and preferably (also) in combination with the preferred amounts of D-lactide specified above).

[0025] The amount of acid-containing impurities in the at least one purified lactide stream is low when compared to the amount of acid-containing impurities in the crude lactide stream. The acid-containing impurities may be present in the at least one purified lactide stream in an amount such that each purified lactide stream has a free acid content of 100 meq / kg or less, particularly 50 meq / kg or less, more particularly 25 meq / kg or less, and even more particularly 10 meq / kg or less.

[0026] The purified lactide stream may be subjected to one or more purification steps to remove residual meso-lactide and / or acid-containing impurities from the at least one purified lactide stream. Suitable purification steps include distillation and crystallization. A crystallization approach (e.g., solvent crystallization or melt crystallization) may be preferred because crystallization can provide a substantially pure (i.e., >99% pure) L- and / or D-lactide stream, preferably a substantially pure L-lactide stream. Typically, lactide products may be obtained with very low free acid values, on the order of <10 meq / kg. This is advantageous because such L- and / or D-lactide streams can be sent to a polymerization reactor to form polylactide using less polymerization catalyst and catalyst deactivator, all of which result in polylactide having high thermal stability and low discoloration.

[0027] The meso-lactide stream contains at least 50% by weight meso-lactide (based on the total weight of lactide in the stream), preferably at least 80% by weight meso-lactide, more preferably at least 85% by weight meso-lactide, and even more preferably at least 90% by weight meso-lactide. As a maximum, the meso-lactide stream may contain 100% by weight meso-lactide (based on the total weight of lactide in the stream). The meso-lactide stream may further contain acid-containing impurities that may be present in the crude lactide stream. These acid-containing impurities may be concentrated in the meso-lactide stream when compared to the amount of acid-containing impurities present in the crude lactide stream. In particular, the amount of acid-containing impurities in the meso-lactide stream (% by weight based on the total weight of the meso-lactide stream) may be 1.1 to 25 times, preferably 2 to 20 times, and more preferably 5 to 15 times, the amount of acid-containing impurities in the crude lactide stream (% by weight based on the total weight of the crude lactide stream). The acid-containing impurities may be present in the meso-lactide stream in an amount such that the meso-lactide stream has a free acid content of from 50 to 500 meq / kg, particularly from 75 to 350 meq / kg, and more particularly from 100 to 250 meq / kg.

[0028] The meso-lactide stream obtained by the separation step is subjected to an oligomerization step. The oligomerization is carried out by ring-opening polymerization of the meso-lactide stream. Since lactic acid oligomers are typically prepared by polycondensation of lactic acid, the method suitable for such ring-opening oligomerization is different from the conventional method used to prepare lactic acid oligomers. The ring-opening oligomerization used in the method according to the present invention is particularly useful because water is not formed as a by-product of the oligomerization reaction. As a result, it is possible to predict and control the degree of polymerization of the oligomers formed surprisingly accurately. As a result, this allows lactic acid oligomers with a low degree of polymerization to be stably obtained, and therefore it is possible to optimize the total time required to convert meso-lactide to L- and D-lactide. Moreover, because water does not need to be removed from the reactor to drive the oligomerization reaction, less energy is required for the oligomerization than is required by conventional processes which rely on the polycondensation of lactic acid and produce water as a by-product.

[0029] The meso-lactide stream obtained by the separation step may be sent directly or indirectly (preferably directly) to an oligomerization step. Prior to being subjected to the oligomerization step, the meso-lactide stream may or may not be purified (preferably not).

[0030] The meso-lactide stream obtained by the separation step may or may not be combined with a further lactide stream in the oligomerization step. The further lactide stream is a stream that contains at least 80% by weight, especially at least 90% by weight, lactide. It is preferred that the further lactide stream contains at least 50% by weight, preferably at least 80% by weight, and more preferably at least 90% by weight, meso-lactide.

[0031] The feed to the oligomerization step preferably consists of at least 60% by weight of a meso-lactide stream obtained directly or indirectly from the separation step, and not more than 40% by weight of a further lactide stream. For reasons of processing efficiency, it may be preferred that the weight percentage of the further lactide stream be not more than 20% by weight of the feed to the oligomerization step, particularly not more than 10% by weight, more particularly not more than 5% by weight, even more preferably not more than 2% by weight, and even more preferably not more than 1% by weight. In one embodiment, no further lactide stream is added to the oligomerization step, and particularly the feed to the oligomerization step consists of a meso-lactide stream obtained from the separation step.

[0032] In one embodiment, all streams provided to the oligomerization step contain at least 50% by weight meso-lactide, preferably at least 60% by weight, more preferably at least 70% by weight, even more preferably at least 80% by weight, and even more preferably at least 90% by weight.

[0033] The stream oligomerized in the oligomerization step contains at least 50% meso-lactide by weight (based on the total weight of lactide in the stream), preferably at least 80% meso-lactide by weight, more preferably at least 85% meso-lactide by weight, and even more preferably at least 90% meso-lactide by weight. As a maximum, the meso-lactide stream may contain 100% meso-lactide by weight (based on the total weight of lactide in the stream).

[0034] It should be noted that the feed provided to the oligomerization step generally has a relatively low lactic acid content. The feed provided to the oligomerization step generally has a lactic acid content of 10% by weight or less, particularly 5% by weight or less, more particularly 2% by weight or less, and often much lower. Some lactic acid may be added to improve processing properties and flowability, but this is not necessary and the amount added may be minimal. This is a substantial difference from the feed provided to the oligomerization step in the method of WO2010 / 105143.

[0035] In some embodiments, the oligomerization of the meso-lactide stream takes place in a dedicated oligomerization reactor. As used herein, a "dedicated oligomerization reactor" is an oligomerization reactor provided with a feed that consists of at least 50% by weight meso-lactide. In particular, the feed provided to a dedicated oligomerization reactor can consist of at least 80% by weight, more particularly at least 90% by weight, meso-lactide.

[0036] Initiators may be used to control the chain length of the lactic acid oligomers and may be selected from initiators known in the art. Suitable initiators include primary monoalcohols (e.g., primary C 3~20 Alkyl alcohols, such as 1-hexanol, 1-decanol, 2-ethyl-1-hexanol and 1-pentadecanol), polyfunctional alcohols (e.g., C 3~20 Alkyl di- and / or tri-ols, such as 1,4-butanediol and 1,6-hexanediol, and hydroxy acids (e.g., C 3~20 Hydroxy acids may be selected from glycolic acid, e.g., mandelic acid and lactic acid. Lactic acid is particularly preferred for the present invention since it allows for 100% lactic acid based oligomers. Depending on the allowable reaction time in the oligomerization reactor, a catalyst may be used to increase the ring-opening oligomerization rate. Suitable catalysts are well established in the art and will be described below.

[0037] The oligomerization can be carried out continuously, semi-continuously, or batchwise. Continuous stirred tank reactors, tube reactors, and pipe reactors are suitable reactor types. The reactors may be used in series. Due to the low flow and low viscosity compared to other parts of the lactide process, the oligomerization is preferably carried out in a stirred tank reactor or a tank reactor equipped with a bottom recirculation pump.

[0038] The oligomerization is preferably carried out in a batch mode, since this allows for depolymerization of lactic acid oligomers in the same reactor, thus minimizing operating costs. To facilitate batchwise operation of the reactor, the meso-lactide stream obtained by separating the crude lactide may be sent directly or indirectly (preferably directly) to a holding tank, where the meso-lactide is recovered and stored for a period of time (e.g., 30 minutes to 12 hours), typically under an inert atmosphere (e.g., under a N2 or Ar atmosphere). Storage in the holding tank should be under conditions such that the meso-lactide does not substantially react or decompose. Depending on the holding time, it may be preferred to store the meso-lactide at a temperature of 80° C. or less, particularly 70° C. or less, more particularly 60° C. or less. It is preferred that the lactide is stored in liquid form. Thus, the temperature during storage is above the melting point of meso-lactide, e.g., above 50° C. Once the desired amount of meso-lactide has been collected in the holding tank, the collected meso-lactide can be oligomerized (in a dedicated oligomerization reactor). By operating the process according to the invention in this manner, greater economies of scale are achieved. By selecting the storage conditions as described above, oligomerization is prevented and increases in free acid levels (due to the production of acids, such as acetic and pyruvic acid) are avoided.

[0039] The oligomerization reaction is preferably carried out at a temperature in the range of 50-220°C, more preferably 50-180°C, even more preferably 80-150°C. The pressure is preferably in the range of 1-10 bar, especially 1-5 bar, more especially 1-2 bar. Operation at subatmospheric pressure is not required. The residence time of the mixture of meso-lactide and lactic acid oligomers in the oligomerization reactor is preferably 100-130°C. The residence time is selected so that the oligomers formed have the desired molecular weight. The lactic acid oligomers formed may have a degree of polymerization of 2-80, preferably 3-50, more preferably 4-30, even more preferably 5-20. These low degrees of polymerization are preferred because lactic acid oligomers with low degrees of polymerization can be more easily depolymerized due to their inherently high concentration of hydroxyl end groups. This allows to reduce the residence time of the oligomer-containing stream in the depolymerization reactor, thus allowing a more efficient use of the reactor used for depolymerization. The degree of polymerization is calculated from the concentration of end groups, as determined by titration, as known by those skilled in the art. As an example, when an initiator containing free acid end groups (e.g., lactic acid) is used as the initiator, the amount of end groups in molar equivalents per kilogram is equal to the average molar amount of oligomer chains per kilogram. The reciprocal of this value indicates the average molecular weight, which, when divided by 72 g / mol, gives the degree of polymerization. The value of free acid is typically achieved by titration with sodium or potassium methylate in anhydrous methanol.

[0040] The oligomerization of meso-lactide is usually carried out in the presence of a catalyst. Suitable catalysts include tin(II) chloride, tin(II) bromide, tin(IV) chloride, tin(IV) bromide, tin(II) oxide, bis(2-ethylhexanoate)tin(II), butyltin tris(2-ethylhexanoate), monobutyltin oxide, dibutyltin dilaurate, tetraphenyltin, lead(II) oxide, zinc stearate, antimony triacetate, antimony(2-ethylhexanoate), bismuth(2-ethylhexanoate), calcium stearate, and magnesium stearate. Metal-containing catalysts such as those described above may be used in an amount of 20 to 2000 ppm (calculated based on the weight of the reaction mixture). A solvent may be used for the oligomerization of meso-lactide. Suitable solvents are benzene, toluene, xylene, and tetrahydrofuran.

[0041] The oligomer-containing stream formed in the oligomerization step may contain at least 80% by weight of lactic acid oligomers (based on the total weight of oligomers and lactide in the stream), preferably at least 85%, more preferably at least 90%, and even more preferably at least 95% by weight. The oligomer-containing stream may still contain a residual amount of meso-lactide. However, the oligomer-containing stream preferably contains no more than 15% by weight meso-lactide, more preferably no more than 10% by weight meso-lactide, and even more preferably no more than 5% by weight meso-lactide.

[0042] The lactic acid oligomers formed in the oligomerization process may still be contaminated with acid-containing impurities. In some embodiments, these acid-containing impurities are removed from the lactic oligomers before they are depolymerized to form the product stream. This is because some acid-containing impurities can be easily removed from the oligomers, but difficult from meso-lactide. Thus, a preferred method for treating a crude lactide stream is also disclosed herein, which method comprises: separating a crude lactide stream containing L-lactide, D-lactide, meso-lactide, and acid-containing impurities in one or more steps to form a meso-lactide stream containing at least 50 weight percent meso-lactide (based on the total weight of lactide in said stream) and at least one purified lactide stream containing L-lactide and D-lactide; oligomerizing said meso-lactide stream to form an oligomer-containing stream comprising lactic acid oligomers and acid-containing impurities; removing acid-containing impurities from the oligomer-containing stream to form; and depolymerizing said purified oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide and D-lactide. The process includes the steps of:

[0043] The oligomerization process results in the formation of an oligomer composition as an intermediate, also referred to herein as the oligomer-containing stream formed in the oligomerization process.

[0044] In one embodiment, the present invention relates to an oligomeric composition comprising: A degree of polymerization of 2 to 80, preferably 3 to 50, more preferably 4 to 30, and even more preferably 5 to 20; an overall ratio of (S)- to (R)-lactic units in the range of 0.25:1 to 4:1, in particular 0.3:1 to 3:1, more in particular 0.5:1 to 2:1, even more in particular 0.7:1 to 1.4:1, even more in particular 0.8:1 to 1.25:1, even more in particular 0.9:1 to 1.1:1, or even 0.95:1 to 1.05:1; The present invention is characterized in that it has

[0045] The oligomer composition preferably contains no more than 15% by weight residual meso-lactide monomer, more preferably no more than 10% by weight, even more preferably no more than 5% by weight. In one embodiment, the oligomer composition preferably contains no more than 15% by weight lactide monomer (meso, L and D combined), more preferably no more than 10% by weight lactide monomer, even more preferably no more than 5% by weight lactide monomer. It is further preferred that the oligomer composition contains at least 80% by weight lactic acid oligomers (based on the total weight of oligomers and lactide in the stream), preferably at least 85% by weight, more preferably at least 90% by weight, even more preferably at least 95% by weight. The preferred embodiments set forth above for the oligomer-containing stream formed in the oligomerization step, whether after an optional purification step or not, also apply here.

[0046] In one embodiment, the oligomer composition contains a non-zero amount of alternating meso-lactide linkages, as can be quantified using methine decoupling proton NMR measurements in CDCl3. In one embodiment, the fraction of the integral of the sss, iss, and ssi tetrads as a function of the integral of the total methine integral is at least 0.05. The fraction of the integral of the iss / ssi tetrads as a function of the integral of the total methine integral can be, for example, at least 0.1, or at least 0.2. In one embodiment, the fraction of the integral of the iss / ssi tetrads as a function of the integral of the total methine integral is 0.9 or less.

[0047] The lactic acid oligomers in the (optionally purified) oligomer-containing stream are depolymerized to form a product stream containing meso-lactide, L-lactide and D-lactide. Methods for depolymerizing lactic acid oligomers are known in the art. These methods include depolymerizing the lactic acid oligomers in the presence of a depolymerization catalyst. The depolymerization catalyst may be a metal catalyst (i.e., a metal-containing catalyst as defined above) that is also used for the oligomerization reaction. Tin(II) bis(2-ethylhexanoate) is often used commercially and is therefore a preferred catalyst for the depolymerization reaction. The depolymerization reaction is preferably carried out at a temperature of 160-260° C., a pressure of 0.5-10.0 kPa (5-100 mbar), and a residence time of 10 minutes to 8 hours. The depolymerization is preferably carried out in the same reactor used to synthesize the lactic acid oligomers, especially when the oligomerization and depolymerization steps are carried out in a batchwise manner. The batchwise operation of the process is preferred because it significantly reduces the investment required to set up equipment.

[0048] In the depolymerization process, the amounts of L-lactide, D-lactide, and meso-lactide produced depend somewhat on the temperature. The ratio of L-lactide to D-lactide is kept constant, but the amount of meso-lactide can be optimized (minimized) by lowering the synthesis temperature. Thus, the depolymerization reactor is preferably carried out at a temperature of 160-260°C, more preferably at a temperature of 160-240°C, even more preferably at a temperature of 160-220°C, and even more preferably at a temperature of 160-200°C.

[0049] In one embodiment, a racemizing agent may be added as needed to increase the amount of L-lactide and D-lactide synthesized during the depolymerization reaction. Suitable racemizing agents include hydroxide salts (e.g., LiOH, NaOH, KOH, Mg(OH), etc., preferably NaOH) and acetate salts (e.g., sodium acetate, potassium acetate). Other suitable racemizing agents are known in the art and include metal salts of alkyl alcohols (i.e., salts of the structure XOR, where X is a metal selected from the group consisting of Li, Na, and K, and R is a substituted or unsubstituted C 1~8 alkyl, e.g., tert-butoxide salt), pyridine (preferably 1,4-lutidine, 2,6-lutidine, 3,5-lutidine, 2,6-di-tert-butylpyridine, or 4-dimethylaminopyridine), and a non-nucleophilic base (e.g., quinuclidine, 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), etc.). The racemization agent may be added in an amount of 500 to 5000 ppm, preferably 750 to 2500 ppm, more preferably 900 to 1100 ppm.

[0050] The product stream comprises meso-lactide, L-lactide and D-lactide. The amount of meso-lactide in the product stream may be from 10 to 60% by weight (based on the total weight of lactide in the stream), particularly from 15 to 50% by weight, more particularly from 25 to 40% by weight. Thus, the product stream may comprise from 40 to 90% by weight, particularly from 50 to 85% by weight, more particularly from 60 to 75% by weight of L-lactide and D-lactide (combined weight based on the total weight of lactide in the stream). It will be appreciated that the product stream will generally contain equal amounts of L- and D-lactide, and thus the particular weight percentages given above may also refer to the weight percentage of rac-lactide in the product stream. The amount of other lactides (i.e., L-lactide and D-lactide) relative to the amount of meso-lactide can be determined by HPLC and can be a ratio of 1 to 6, preferably a ratio of 1.1 to 3, more preferably a ratio of 1.5 to 3. A high ratio is desirable because it indicates efficient conversion of meso-lactide to L- and D-lactide. A low amount of oligomers in the product stream is desirable because it simplifies downstream processing of the product stream (e.g., separation of meso-lactide from L- and D-lactide). Thus, the amount of lactic acid oligomers in the product stream is preferably less than 5 wt.%, more preferably less than 3 wt.%, even more preferably less than 1 wt.% (calculated relative to the total weight of the product stream). The product stream can have a free acid content of 50 to 500 meq / kg, particularly 75 to 350 meq / kg, and more particularly 100 to 250 meq / kg.

[0051] The product stream containing meso-lactide, L-lactide, and D-lactide may be separated in one or more steps to form a stream containing a racemic mixture of L-lactide and D-lactide and a second stream containing meso-lactide. The product stream is preferably separated by distillation or crystallization (e.g., solvent crystallization or melt crystallization). Distillation may be preferred, especially when the amount of racemic lactide is too small to make a crystallization operation economical. Those skilled in the art will recognize that the product stream is either separated in a dedicated distillation process or returned to the distillation upstream, for example, to the distillation section used to separate the crude lactide into a meso-lactide-containing stream and an L- and D-lactide stream. In embodiments where the product stream contains too many color-forming species and distillation becomes too inefficient, crystallization may be preferred. Crystallization may also be preferred when the flow rate of the product stream is small enough that long batch times in crystallization can be tolerated, for example in a static melt crystallization process.

[0052] Accordingly, also disclosed herein is a preferred method for treating a crude lactide stream, the method comprising: separating a crude lactide stream containing L-lactide, D-lactide, and meso-lactide (and optionally, acid-containing impurities) in one or more steps to form a meso-lactide stream containing at least 50 weight percent meso-lactide (based on the total weight of lactide in the stream) (and optionally, acid-containing impurities), and at least one purified lactide stream containing L-lactide and / or D-lactide; oligomerizing said meso-lactide stream to form an oligomer-containing stream comprising lactic acid oligomers (and optionally, acid-containing impurities); Optionally, removing acid-containing impurities from said oligomer-containing stream to form a refined oligomer-containing stream; depolymerizing the (optionally purified) oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide and D-lactide; and separating said product stream in one or more steps to form a stream comprising a racemic mixture of L-lactide and D-lactide and a second stream comprising meso-lactide. The process includes the steps of:

[0053] Additionally or alternatively, it may be desirable to increase the content of D-lactide in the L-lactide and D-lactide streams coming from the distillation section, e.g., in a purified L-lactide and / or D-lactide stream as defined herein. This may be achieved by recycling at least a portion of said product stream to such distillation section, the amount depending on the target ratio of D-lactide, L-lactide, and meso-lactide in the stream exiting the distillation section, e.g., in the purified L-lactide and / or D-lactide stream.

[0054] The stream containing a racemic mixture of L-lactide and D-lactide may be one in which L-lactide and D-lactide make up at least 80% by weight of the stream, preferably at least 85% by weight, more preferably at least 90% by weight, and even more preferably at least 95% by weight. In a preferred embodiment, the stream containing a racemic mixture of L-lactide and D-lactide contains 5% by weight or less meso-lactide (based on the total weight of lactide in the stream), preferably 4% by weight or less meso-lactide, more preferably 3% by weight or less meso-lactide, and even more preferably 2% by weight or less meso-lactide. The acid-containing impurities may be present in the stream containing a racemic mixture of L-lactide and D-lactide in an amount such that the stream has a free acid content of 1 to 50 meq / kg, in particular 1 to 25 meq / kg, more particularly 1 to 10 meq / kg, and even more particularly 1 to 10 meq / kg.

[0055] Residual meso-lactide and / or acid-containing impurities in a stream containing a racemic mixture of L-lactide and D-lactide can be removed by one or more purification steps. Suitable purification methods include distillation and crystallization. A crystallization approach (e.g., solvent crystallization or melt crystallization) may be preferred because a substantially pure (i.e., >99% pure) racemic L- and D-lactide stream can be obtained by crystallization. This is advantageous because such a pure racemic L- and D-stream can be sent to a polymerization reactor to form a nearly colorless polylactide. A combination of distillation and crystallization is also desirable, for example, distillation is used to remove at least a portion of the remaining meso-lactide and thus purify the feed to allow for crystallization of rac-lactide (i.e., a racemic mixture of L-lactide and D-lactide).

[0056] The second stream containing meso-lactide may contain at least 50% by weight meso-lactide (based on the total weight of lactide in the stream), preferably at least 80% by weight meso-lactide, more preferably at least 85% by weight meso-lactide, more preferably at least 90% by weight meso-lactide, and even more preferably at least 95% by weight meso-lactide. In a preferred embodiment, the acid-containing impurities are concentrated in the second stream containing meso-lactide, such that the second stream has a free acid content of from 10 to 500 meq / kg, particularly from 20 to 250 meq / kg, and more particularly from 50 to 100 meq / kg. The total amount of acid-containing impurities in the second stream containing meso-lactide may be less than 4% by weight (based on the total weight of the second stream). Examples of impurities that are typically contained in the second stream are (monomer) lactic acid, acetic acid, and succinic acid.

[0057] At least a portion of the second stream containing meso-lactide may be recycled, directly or indirectly, to an earlier stage of the process. For example, at least a portion of the second stream may be recycled to a separation device for separating crude lactide, to a holding tank (where meso-lactide may be stored until it is oligomerized), and / or to the oligomerization step. If the process is operated in a batch mode, it may be preferred to recycle the second stream to a holding tank. Recycling the second stream may be particularly preferred if the oligomer-containing stream contains acid-containing impurities and these impurities are not removed. Additionally or alternatively, at least a portion of the second stream containing meso-lactide may be purged.

[0058] The remaining L- and / or D-lactide and the remaining acid-containing impurities in the second stream may be removed by one or more purification steps. The remaining L-lactide and / or D-lactide can be removed via distillation or crystallization. The remaining acid-containing impurities may be removed by solvent crystallization or melt crystallization, for example, as demonstrated in European Patent Application No. EP21195962.2. The resulting purified meso-lactide stream may be subjected to polymerization, alone or together with a purified L-lactide and / or D-lactide stream and / or a stream containing a racemic mixture of L-lactide and D-lactide as defined herein. When acid-containing impurities are removed from the oligomer-containing stream or the second stream containing meso-lactide, at least a portion of the second stream containing meso-lactide may be sent to a polymerization reactor. In the polymerization reactor, the meso-lactide is subjected to polymerization conditions described below to form an amorphous (colorless) polylactide, or it can be combined with L- and / or D-lactide to form a more crystalline polylactide.

[0059] For completeness, it is noted that the method according to the invention can be part of a comprehensive process for producing lactide. Thus, in some embodiments, the method according to the invention comprises: forming lactic acid oligomers; depolymerizing the lactic acid oligomers to form a crude lactide stream comprising L-lactide, D-lactide and meso-lactide (and optionally, acid-containing impurities); separating the crude lactide stream in one or more steps to form a meso-lactide stream containing at least 50 weight percent meso-lactide (based on the total weight of lactide in said stream) and at least one purified lactide stream containing L-lactide and / or D-lactide; oligomerizing said meso-lactide stream to form an oligomer-containing stream comprising lactic acid oligomers (and optionally, acid-containing impurities); Optionally, removing acid-containing impurities from said oligomer-containing stream to form a purified oligomer-containing stream; depolymerizing the (optionally purified) oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide and D-lactide; and separating said product stream in one or more steps to form a stream comprising a racemic mixture of L-lactide and D-lactide and a second stream comprising meso-lactide. The process includes the steps of:

[0060] The lactic acid oligomers formed in the first step of this general process may have the same characteristics as those defined above for the lactic acid oligomers of the oligomer-containing stream and are formed in an oligomerization reactor. These lactic acid oligomers are preferably formed by polycondensation of lactic acid. The depolymerization of the lactic acid oligomers formed in this first step may be carried out under the same conditions as those defined above for the depolymerization of the oligomer-containing stream and is generally carried out in a dedicated depolymerization reactor. The preferred embodiments of the other steps of this general process are the same as those defined above.

[0061] Also disclosed herein is a lactide, preferably a racemic mixture of L-lactide and D-lactide, obtained by the method of treating a crude lactide stream according to the present invention.

[0062] Process for Producing Polylactide

[0063] The above-described method for treating crude lactide results in the formation of multiple lactide streams suitable for the synthesis of commercial grade polylactide.Therefore, also disclosed herein is a method for producing polylactide, the method comprising obtaining at least one stream according to the method for treating crude lactide according to the present invention, feeding at least a portion of the at least one stream directly or indirectly to a polymerization reactor, and forming polylactide.

[0064] In some embodiments, the method of producing a polylactide includes: a) obtaining at least one purified stream comprising L-lactide and / or D-lactide according to a method for treating crude lactide according to the invention, providing at least a portion of the at least one purified stream directly or indirectly to a polymerization reactor and forming a polylactide; and / or b) obtaining a stream comprising a racemic mixture of L-lactide and D-lactide according to the method of the present invention, feeding at least a portion of said stream directly or indirectly to a polymerization reactor, and forming a polylactide. The process includes the steps of:

[0065] As mentioned above, the properties of polylactide are largely determined by the ratio of (S)- to (R)-lactic units in the polymer. It is therefore preferred that at least 80% (particularly at least 90%) of the lactic acid monomers in the polylactide are (S)- or (R)-lactic units, preferably (S)-lactic units, and the remainder of the lactic acid monomers in the polylactide are the opposite enantiomer. To obtain polylactide having these amounts of (S)- and (R)-lactic units, the various streams obtained according to the process for treating crude lactide may be combined with other streams obtained according to the process, or with pooled L-lactide, D-lactide and / or meso-lactide. For example, the polylactide can be synthesized from a mixture containing 0-20% by weight, preferably 5-20% by weight, of meso-lactide and 80-100% by weight, preferably 80-95% by weight, of L- and / or D-lactide, preferably L-lactide. Uniquely, the process of the invention makes it possible to produce polylactide using only L-lactide and rac-lactide, without a significant loss in yield with meso-lactide. This is desirable since it is known that the meso-lactide stream typically contains the most color-forming impurities.

[0066] If the amount of meso-lactide in the product is low (e.g., less than 2% by weight), it can be purged and polylactic acid produced from (only) a combination of racemic lactide, L-lactide, and optionally D-lactide. Commercially available PLA products then typically require 2-15% D-lactide in the final mixture for polymerization. The method for producing polylactide according to the present invention includes: obtaining at least one purified stream comprising L-lactide and / or D-lactide according to the method for treating a crude lactide stream according to the invention; according to the method for treating a crude lactide stream according to the invention to obtain a stream comprising a racemic mixture of L-lactide and D-lactide; combining at least a portion of said at least one purified stream containing L-lactide and / or D-lactide with at least a portion of said stream containing a racemic mixture of L-lactide and D-lactide to form a polymerizable stream; and polymerizing the polymerizable stream to form a polylactide, optionally wherein the polymerizable stream contains 1 to 20% by weight D-lactide (based on the total amount of lactide in the stream), preferably 2 to 15% by weight D-lactide. The process includes the steps of:

[0067] Generally, polymerization is carried out by providing lactide to a polymerization reactor where it is subjected to polymerization conditions, usually in the presence of a polymerization catalyst. Suitable polymerization conditions are known in the art. The suitable polymerization conditions may, for example, include reacting lactide at a temperature of from 100 to 225°C, particularly from 120 to 220°C, more particularly from 130 to 210°C. Suitable polymerization catalysts are also known in the art. The catalysts described above for the oligomerization of meso-lactide may be used here as well, and may optionally be used in catalytically effective amounts, for example from 1 to 2000 ppm (calculated on the weight of monomer). The polymerization reaction is usually allowed to continue until a prevailing thermal equilibrium concentration of residual lactide is reached, typically from 3 to 8% by weight at the temperatures described above. Once the desired conversion has been achieved, the polymerization catalyst is often deactivated by the addition of a catalyst deactivator, which stabilizes the polylactide product against catalyzed back-biting and allows for low residual lactide levels of less than 0.5 wt.%, thus imparting thermal stability to the final product and ensuring suitability for melt processing in PLA converters. The resulting polylactide may have an absolute number average molecular weight (Mn) of 20 to 150 kg / mol, preferably 35 to 100 kg / mol, as determined by gel permeation chromatography with light scattering detection.

[0068] In some embodiments, the process for treating crude lactide and the process for producing polylactide are combined to form an integrated process. Such an integrated process for producing polylactide includes: forming lactic acid oligomers; depolymerizing the lactic acid oligomers to form a crude lactide stream comprising L-lactide, D-lactide and meso-lactide (and optionally, acid-containing impurities); separating the crude lactide stream in one or more steps to form a meso-lactide stream that is at least 50 weight percent (based on the total weight of lactide in the stream) (and optionally, acid-containing impurities) and at least one purified lactide stream that contains L-lactide and / or D-lactide; oligomerizing said meso-lactide stream to form an oligomer-containing stream comprising lactic acid oligomers (and optionally, acid-containing impurities); Optionally, removing acid-containing impurities from said oligomer-containing stream to form a purified oligomer-containing stream; depolymerizing the (optionally purified) oligomer-containing stream to form a product stream comprising meso-lactide, L-lactide, and D-lactide; separating the product stream in one or more steps to form a stream comprising a racemic mixture of L-lactide and D-lactide and a second stream comprising meso-lactide; Optionally, purifying said stream containing a racemic mixture of L-lactide and D-lactide; combining L-lactide (e.g., a purified lactide stream comprising L-lactide obtained by separating the crude lactide) with at least a portion of the (purified) stream comprising a racemic mixture of L-lactide and D-lactide to form a polymerizable stream (optionally comprising 2.0 to 15% by weight of D-lactide, calculated on the total weight of the polymerizable stream); and polymerizing said polymerizable stream to form a polylactide. The method may include the steps of:

[0069] The lactic acid oligomers formed in the first step of this integrated process may have the same characteristics as those defined above for the lactic acid oligomers of the oligomer-containing stream and are formed in an oligomerization reactor. These lactic acid oligomers are preferably formed by polycondensation of lactic acid. The depolymerization of the lactic acid oligomers formed in this first step may be carried out under the same conditions as those defined above for the depolymerization of the oligomer-containing stream and is generally carried out in a dedicated depolymerization reactor. The preferred embodiments of the other steps of this integrated process are the same as those defined above.

[0070] Also disclosed herein is a polylactide obtainable by the process for producing polylactide according to the present invention.

[0071] Method for producing lactic acid

[0072] The stream containing the racemic mixture of L-lactide and D-lactide obtained according to the invention may be substantially pure (i.e., as described above, has a free acid content of 10 meg / kg or less). By hydrolyzing this stream, a very pure racemic mixture of lactic acid can be obtained, which is even suitable for food applications.

[0073] Therefore, a process for producing a racemic mixture of lactic acid is also disclosed, which process comprises hydrolyzing at least a portion of a stream containing a racemic mixture of L-lactide and D-lactide obtained according to the process for treating crude lactide according to the invention to form a racemic mixture of (S)- and (R)-lactic acid. Suitable conditions for hydrolyzing lactide are known in the art.

[0074] In some embodiments, a racemic mixture of (S)-lactic acid and (R)-lactic acid is provided directly or indirectly to an oligomerization reactor or a depolymerization reactor.

[0075] Also disclosed herein is a racemic mixture of (S)-lactic acid and (R)-lactic acid obtainable by the process for producing a racemic mixture of (S)-lactic acid and (R)-lactic acid according to the present invention.

[0076] The above disclosed method will be described with reference to Figures 1 to 5, without being limited thereto or by them.

[0077] In Figure 1, a crude lactide stream (1) containing L-lactide, D-lactide, and meso-lactide is sent to a separation unit (2), which may be, for example, a distillation unit or a crystallizer. The crude lactide stream (1) is separated to form a meso-lactide stream (3) containing at least 50% by weight meso-lactide (based on the total weight of lactide in that stream), and at least one purified L- and / or D-lactide stream (4) containing L- and / or D-lactide and optionally a small amount of meso-lactide (e.g., less than 2% by weight, based on the total weight of lactide in stream (4)). Only one purified L- and / or D-lactide stream (4) is shown. This purified L- and / or D-lactide stream (4) may be subjected to a purification step, such as solvent crystallization or melt crystallization, in purification unit (5) to obtain a substantially pure L-lactide stream (4a) and a further stream (4b) containing L- and / or D-lactide. Both streams may be isolated and processed as desired. The meso-lactide stream (3) is sent to an oligomerization reactor (6), resulting in the formation of an oligomer-containing stream (7) containing lactic acid oligomers (and optionally, acid-containing impurities). This oligomer-containing stream (7) may be subjected to one or more optional purification steps. For example, the oligomer-containing stream (7) may be sent to an optional purification unit (8) to remove acid-containing impurities present in the oligomer-containing stream (7). The oligomer-containing stream (7) (or purified oligomer-containing stream (7a)) is sent to a depolymerization reactor (9), which may be the same reactor as oligomerization reactor (6), where the lactic acid oligomers are depolymerized to form a product stream (10) containing meso-lactide, L-lactide, and D-lactide. The product stream (10) can be sent to a further separation unit (11), such as a distillation unit or a crystallizer, to separate the product stream (10) into a stream (12) containing a racemic mixture of L-lactide and D-lactide and a second stream (13) containing meso-lactide.The second stream (13) containing meso-lactide may be (partially) recycled to the oligomerization reactor (6) and / or to the separation unit (2) (as depicted in FIG. 1 ) and / or may be (partially) subjected to a purification step, after which the resulting purified meso-lactide stream may be sent to the polymerization reactor for polymerization along with the L-lactide stream (4a) and / or stream (12) containing a racemic mixture of L- and D-lactide.

[0078] In Figure 2, a preferred process is illustrated in which the oligomerization of the meso-lactide stream (3) and the depolymerization of the lactic acid oligomers so obtained occur in the same reactor. Thus, in Figure 2, the meso-lactide stream (3) is provided to a reactor (14) where it is oligomerized to form lactic acid oligomers. The lactic acid oligomers are then depolymerized in the same reactor (14) to form a product stream (10) containing meso-lactide, L-lactide, and D-lactide. The product stream (10) can be sent to a further separation unit (11), such as a distillation unit or a crystallizer, for separation into a stream (12) containing a racemic mixture of L-lactide and D-lactide and a second stream (13) containing meso-lactide, as in Figure 1. The second stream (13) containing meso-lactide may be recycled to the reactor (14) and / or to the separation apparatus (2) and / or may be (partially) subjected to a purification step, after which the resulting purified meso-lactide stream may be sent to the polymerization reactor for polymerization together with the L-lactide stream (4a) and / or stream (12) containing a racemic mixture of L- and D-lactide.

[0079] In Figure 3, a process according to Figure 2 is illustrated in which L-lactide stream (4a) and stream (12) containing a racemic mixture of L-lactide and D-lactide are combined to form polymerizable stream (15), which is sent to polymerization reactor (16) where it is reacted to form polylactide. The streams may be combined prior to entering polymerization reactor (16) (as illustrated in Figure 3) or may be combined within polymerization reactor (16). The polylactic acid product is removed from polymerization reactor (16) via line (17) and may be processed as desired.

[0080] In Figure 4, a process according to Figure 3 is illustrated, where the process is operated in batch mode and meso-lactide is collected in a holding tank prior to oligomerization. In particular, the meso-lactide stream (3) is collected in a holding tank (18) where it is stored under N2 or Ar at a temperature of 50-60°C for a period of time. For example, the meso-lactide stream can be stored until a desired amount of meso-lactide has been recovered. The recovered meso-lactide stream (3a) is then sent to a reactor (14) where the meso-lactide is subjected to an oligomerization-depolymerization sequence in accordance with the invention to form product stream (10). In some embodiments, reactor (14) can be used as the holding tank (18).

[0081] In Figure 5, a process according to Figure 2 is illustrated, in which at least a portion of the stream (12) comprising a racemic mixture of L-lactide and D-lactide is sent to a hydrolysis reactor (19), where it is hydrolyzed with water to form a racemic mixture of (S)- and (R)-lactic acid (20), which may be (partially) sent, for example, to an oligomerization reactor (in particular a polycondensation reactor) or to a depolymerization reactor used to prepare crude lactide. The racemic mixture of (S)- and (R)-lactic acid (19) can also be isolated and processed, if desired, for example in a food application.

[0082] When amounts, concentrations, dimensions, and other parameters are expressed in the form of ranges, preferred ranges, upper values, lower values, or preferred upper and lower values, it is to be understood that any range obtained by combining any upper value or preferred value with any lower value or preferred value is also specifically disclosed, whether or not the resulting range is expressly stated in the context. Additionally, all percentages referred to herein are to be understood as being percentages by weight, unless otherwise specified.

[0083] All documents mentioned herein are either incorporated by reference in their entirety or, in their place, are incorporated for the purposes of providing the disclosure on which they are specifically relied upon.

[0084] Working Example The following examples illustrate the practice of the invention in certain preferred embodiments and are not intended to be limiting. Other embodiments within the scope of the invention will be apparent to those of ordinary skill in the art.

[0085] Chemicals and Methods

[0086] The meso-lactide used for these tests was obtained from Total Corbion PLA bv and exhibited a free acid content (expressed as lactic acid) of 0.85 and a purity of greater than 90%. To control the degree of polymerization, heat stable lactic acid (HS100) from Corbion was used.

[0087] Example 1: Ring-opening oligomerization

[0088] A four-neck round bottom flask was equipped with a temperature probe, a temperature controller, an overhead stirrer, a Liebig condenser, and a heating mantle, which was cooled with tap water to condense the water produced.

[0089] To initiate the oligomerization, a round-bottom flask equipped with a temperature probe and overhead stirring was charged with meso-lactide and 7.7 wt. % heat stable lactic acid (HS100, Corbion, as received). Subsequently, 600 ppm of tin(II) bis(2-ethylhexanoate) was added, and the mixture was heated with stirring to 180°C and held at this temperature for 4 hours. The reaction was carried out at atmospheric pressure.

[0090] The reaction is stopped by removing the heat source and cooling the mixture to room temperature. The degree of polymerization in the oligomer-containing stream thus obtained was reproducibly obtained in the range of 10 to 12. The total amount of lactide monomers (meso, L-, and D-) in the oligomer-containing stream was typically 2.2 wt%.

[0091] This example demonstrates the concept of producing lactic acid oligomers from meso-lactide with a predictable degree of polymerization.

[0092] Example 2: Synthesis of lactide using tin(II) bis(2-ethylhexanoate) (only)

[0093] The same round-bottom flask used for oligomerization is fitted with a 10 cm Vigreux column and a Liebig condenser set at 96° C. to condense the lactide. The condenser is connected to a graduated cylinder where the lactide is collected. The top of the cylinder is connected to a vacuum pump through a cold trap.

[0094] To initiate lactide synthesis, the oligomer-containing stream (here, the product of Example 1) is gradually heated to 210°C, and once melted, the overhead stirring is started and slowly increased to 100 rpm. At a melt temperature of 210°C, the vacuum is reduced to 5 mbar at a rate of 100 mbar per minute. The moment the vacuum reached 5 mbar was taken as t=0. Infrared heating lights were placed at any cold spots to prevent the setup from clogging with solidified lactide.

[0095] At set time intervals, the amount (volume) of lactide in the cylinder is measured, as well as the melt and top temperatures of the experiment, and the degree of vacuum. After 140 minutes of synthesis, the reaction is stopped by removing the heat source. The lactide contained in the product stream thus obtained is analyzed for stereochemical purity, the amount of meso-lactide, and the sum of L-lactide and D-lactide. Lactide was obtained in 94% yield (based on the starting amount of oligomers) with a stereochemical purity of 55% L-lactide. The lactide contained 55.5% by weight of meso-lactide and 40.6% by weight of L- and D-lactide, as well as some small amounts of other compounds (less than 4% by weight in total), most of which were lactic acid and smaller oligomers.

[0096] This example shows that using the process according to the invention, high conversions to lactide can be obtained, with about 40.6 wt. % of the meso-lactide being converted to rac-lactide.

[0097] Example 3: Synthesis of crude lactide racemized using stannous octoate and sodium hydroxide

[0098] The protocol of Example 1 was repeated, except that 1000 ppm of sodium hydroxide was added to the four-neck round-bottom flask simultaneously with the addition of 600 ppm of stannous octoate. After 4 hours of reaction time at 180° C., the oligomerization reaction was complete, and the crude lactide protocol of Example 2 was repeated. After 140 minutes of synthesis time, crude lactide was again obtained in high, albeit slightly lower, yield (75%), but with a significantly higher proportion of L- and D-lactide (55.4%) and therefore a lower amount of meso-lactide (39%).

[0099] This example shows that the amount of rac-lactide produced can be further increased by the addition of a racemizing agent.

Claims

1. A method for processing a coarse lactide flow, the method being A crude lactide stream containing 75–95% by weight of L-lactide, 0.01–5% by weight of D-lactide, and 1–25% by weight of mesolactide, where all percentages are calculated relative to the total weight of lactide in the stream, is separated in one or more steps to form a mesolactide stream containing at least 50% by weight of mesolactide based on the total weight of lactide in the stream, and at least one purified lactide stream containing L-lactide and / or D-lactide, where the amount of mesolactide in the at least one purified L- and / or D-lactide stream is 50% or less of the amount of mesolactide in the crude lactide stream; and The mesolactide flow is oligomerized to form an oligomer-containing flow that includes lactic acid oligomers; This process includes, The oligomer-containing stream is subjected to a depolymerization step to form a product stream containing mesolactide, L-lactide, and D-lactide. The aforementioned method.

2. The method according to claim 1, further comprising separating the product flow in one or more steps to form a flow containing a racemic mixture of L-lactide and D-lactide and a second flow containing mesolactide.

3. The method according to claim 2, wherein the product stream is separated by distillation, crystallization, or a combination thereof.

4. The method according to claim 2 or 3, further comprising reusing the second flow containing the mesolactide in a step of separating the crude lactide flow and / or a step of oligomerizing the mesolactide flow, wherein the second flow containing the mesolactide is obtained by subjecting the product flow containing mesolactide, L-lactide and D-lactide obtained from the method according to claim 1 to one or more separation steps to form a flow containing a racemic mixture of L-lactide and D-lactide and a second flow containing mesolactide.

5. The method according to any one of claims 1 to 3, wherein the lactic acid oligomer has a degree of polymerization of 2 to 80 when calculated from the concentration of the terminal group determined by titration.

6. The method according to any one of claims 1 to 3, wherein the product stream contains a total amount of L-lactide and D-lactide, based on the total weight of lactide in the stream, in an amount of 40 to 90% by weight.

7. The method according to claim 2 or 3, wherein a stream containing a racemic mixture of L-lactide and D-lactide has a free acid content of less than 25 meq / kg, as determined by titration.

8. The aforementioned coarse lactide flow Forming lactic acid oligomers; and, The lactic acid oligomer is depolymerized to form the crude lactide flow containing L-lactide, D-lactide, and mesolactide. The method according to any one of claims 1 to 3, obtained by the process described above.

9. The method according to any one of claims 1 to 3, wherein the oligomerization is performed in a batch manner, and the method includes a step of storing the mesolactide flow in a holding tank at a temperature of 50 to 80°C in an inert atmosphere for a predetermined time before oligomerization.

10. A method for producing polylactide, the method comprising the steps of obtaining a product flow according to the method of any one of claims 1 to 3, supplying at least a portion of the flow to a polymerization reactor, and forming polylactide.

11. The method described above is a) Obtaining at least one purified stream containing L-lactide and / or D-lactide according to the method of any one of claims 1 to 3, supplying at least a portion of the at least one purified stream to a polymerization reactor, and forming polylactide; and / or b) Obtaining a stream containing a racemic mixture of L-lactide and D-lactide according to the method of any one of claim 2 or 3, supplying at least a portion of the stream to a polymerization reactor, and forming polylactide. The method according to claim 10, including the method described in claim 10.

12. The method described above is Obtain the at least one purified stream containing L-lactide and / or D-lactide according to the method described in any one of claims 1 to 3; Obtain a flow containing a racemic mixture of L-lactide and D-lactide according to the method described in any one of claims 2 or 3; To combine at least a portion of the at least one purified stream containing L-lactide and / or D-lactide with at least a portion of the stream containing a racemic mixture of L-lactide and D-lactide to form a polymerizable stream; and, Polymerizing the polymerizable flow to form polylactide, optionally, wherein the polymerizable flow may contain 1 to 20% by weight of D-lactide based on the total amount of lactide in the flow. The method according to claim 11, including the method described in claim 11.

13. A method for producing a racemic mixture of (S)-lactic acid and (R)-lactic acid, the method comprising hydrolyzing at least a portion of a stream containing a racemic mixture of L-lactide and D-lactide obtained according to the method of any one of claims 2 or 3 to form a racemic mixture of (S)-lactic acid and (R)-lactic acid.

14. The method according to claim 13, wherein the racemic mixture of (S)-lactic acid and (R)-lactic acid is supplied to an oligomerization reactor or a depolymerization reactor.

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