Method for purifying meso-lactide

Abstract

The invention is a process for purifying meso-lactide having a free acid content of at least 30 meq / kg, comprising at least the steps of subjecting a feedstock containing at least 75% by weight meso-lactide to solvent crystallization followed by recovery of the product meso-lactide, wherein the solvent crystallization step comprises the reaction of a ketone of the following formula: R1-C(═O)-R2, where R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, and a ketone of the following formula: R3-O-R4, where R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl. The present invention relates to a process for producing meso-lactide in an aqueous solution containing 70% by weight, based on the total of at least one compound selected from the group of ethers of meso-lactide (selected from p-butyl, n-butyl, sec-butyl, t-butyl and isobutyl), the solvent being present in an amount of 5 to 50% by weight, calculated on the total of the feedstock and solvent, and the solvent crystallization conditions being selected such that at least 15% of the meso-lactide in the feedstock crystallization is in solid form, and wherein the solid meso-lactide recovered from a final step of recovering product meso-lactide following solvent crystallization has a free acid content of 20 meq / kg or less. The process according to the invention provides meso-lactide in high purity and in high yield in an efficient manner, while minimizing the formation of lactic acid and linear lactic acid oligomers. The invention also relates to processes for producing purified meso-lactide and purified L-lactide and / or D-lactide streams, and to a process for producing polylactide which includes a step of purifying meso-lactide.

Description

[Technical Field]

[0001] The present invention relates to a method for purifying meso-lactide. The present invention also relates to a method for producing purified meso-lactide and purified L-lactide and / or D-lactide streams, and to a method for producing polylactide, which includes the step of purifying meso-lactide. [Background technology]

[0002] Lactide, the cyclic dimer of lactic acid, is well known in the art as a starting material for polylactide polymers, also referred to as polylactic acid or PLA. PLA is used in medical applications, such as biodegradable sutures, clamps, bone plates, and biologically active controlled-release devices. Additionally, PLA is biodegradable and can be obtained from renewable resources, making it an attractive polymer for many technical applications, such as packaging. There are three stereochemical forms of lactide: L-lactide, which consists of two L-lactic acid monomers; D-lactide, which consists of two D-lactic acid monomers; and meso-lactide, which consists of an L-lactic acid monomer and a D-lactic acid monomer. An equimolar blend of L-lactide and D-lactide is also referred to as racemic (or rac-) lactide or D,L-lactide.

[0003] The amounts of different monomers affect the properties of the resulting PLA, including its crystallization rate and melting point. Therefore, it is common to adjust the relative amounts of different lactide stereoisomers in PLA. L-lactide and D-lactide are enantiomers of each other and have the same physical properties, such as melting point and boiling point, and therefore the same distillation behavior. Meso-lactide has different physical properties and is more volatile than L-lactide and D-lactide.

[0004] Conventionally, lactide is produced from lactic acid by a process that includes the steps of polymerizing lactic acid to form lactic acid oligomers and depolymerizing the lactic acid oligomers in the presence of a catalyst to form lactide. Lactic acid can be obtained from many sources, for example, by subjecting a hydrocarbon source to a fermentation medium to produce lactic acid, followed by isolating the lactic acid.

[0005] An integrated method for producing PLA starting from lactic acid is described, for example, in Henton (Natural Fibers, Biopolymers, and Biocomposites, Edited by Amar K. Mohanty, Manjusri Misra, Lawrence T. Drzal, ISBN 9780849317415, Published April 8, 2005 by CRC Press, Chapter 16, Polylactic Acid Technology; DE Henton, P. Gruber, J. Lunt, and J. Randall). Lactic acid obtained through fermentation is converted into a prepolymer through condensation polymerization. The prepolymer is then converted into lactide. The lactide is subjected to a first distillation step, specifically designed to remove water and lactic acid monomers. The lactide is then subjected to a second distillation step in which the meso-lactide is separated, resulting in, on the one hand, a meso-lactide-rich stream, and, on the other hand, a lactide stream that is depleted of meso-lactide and thus enriched in L-lactide and / or D-lactide. Portions of the two streams can be combined and provided to a polymerization step in which PLA is formed through ring-opening polymerization.

[0006] The step of separating meso-lactide from L- and / or D-lactide allows for the regulated feeding of meso-lactide to the polymerization reactor, resulting in the desired polymer conditions as set forth above. It also allows the meso-lactide and L- and / or D-lactide to be provided for separate further use. The meso-lactide can also be recycled back to an earlier step in the process, such as a polymerization or depolymerization step.

[0007] As is well known in the art, the presence of contaminants in polymerization processes is often associated with detrimental effects on polymer properties. For many polymers, the presence of contaminants has been shown to result in undesirable yellowing of the polymer. The presence of contaminants can also affect further properties of the polymer, such as its molecular weight. For example, Auras et al. (Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications, 2010) states the following: "The lactide monomer specifications and permitted impurity levels for PLA are defined by the polymerization mechanism and the catalyst applied. PLA is commercially produced by the ROP of lactide in bulk. The tin(II)-catalyzed process allows good control of molecular weight and reaction rate as long as it is carried out in the absence of impurities such as water, metal ions, lactic acid, or other organic acids. Therefore, purification of crude lactide is essential for the industrial production of high molecular weight PLA (Mw>100 kg / mol). In fact, lactide is the ultimate form of lactic acid and its purest dehydrated form."

[0008] In the processes described above, a meso-lactide stream and a meso-lactide-depleted L- and / or D-lactide vapor are obtained through two successive distillation steps. Nevertheless, it may still be preferable to subject either or both streams to a further purification process to remove additional impurities. This is particularly the case for meso-lactide, which may contain significant amounts of volatile components resulting from the second distillation step in which it is separated from the D- and L-lactide. Summary of the Invention [Problem to be solved by the invention]

[0009] Thus, there is a need in the art for a method for purifying meso-lactide. It will be apparent to those skilled in the art that there are stringent yield and efficiency requirements for such a method to operate on a commercial scale. In the context of meso-lactide, an effect that is particularly to be avoided is the formation of lactic acid and linear lactic acid oligomers. On the one hand, meso-lactide has a relatively high reactivity, making the risk of lactic acid and linear lactic acid oligomer formation real. On the other hand, the formation of these compounds not only reduces the yield of meso-lactide, but their presence itself catalyzes the conversion of lactide to lactic acid and linear lactic acid oligomers, thereby worsening the effect. Furthermore, any meso-lactide purification method should not generate contaminants that are difficult to remove and should be capable of producing meso-lactide of sufficient, e.g., polymer-grade, purity in an efficient manner.

[0010] The present invention provides a solution to these problems. [Means for solving the problem]

[0011] The present invention provides a method for purifying meso-lactide having a free acid content of at least 30 meq / kg, comprising the steps of: The method comprises at least one step of subjecting a feedstock containing at least 75% by weight meso-lactide to solvent crystallization and subsequent recovery of the product meso-lactide, wherein the solvent crystallization step comprises subjecting at least one compound selected from the group consisting of ketones of the following formula: R1-C(═O)-R2, where R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, and ethers of the following formula: R3-O-R4, where R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl. In total, at least The solvent is present in an amount of 5 to 50% by weight calculated on the total of the feedstock and the solvent, and the solvent crystallization is carried out in a solvent containing 70% by weight of the feedstock and the solvent. ofconditions are selected such that at least 15% of the meso-lactide in the feedstock crystallizes in solid form; Now, to the above process, wherein the solid meso-lactide recovered from the final step of solvent crystallization and subsequent product meso-lactide recovery has a free acid content of 20 meq / kg or less. [Effects of the Invention]

[0012] It has been discovered that the combination of various features of the present invention results in a process that provides meso-lactide in high purity and yield in an efficient manner, while minimizing the production of lactic acid and linear lactic acid oligomers.

[0013] Note that the possibility of subjecting meso-lactide to solvent crystallization has been described above.

[0014] U.S. Pat. No. 5,053,485 describes the production of polymers of meso-lactide and, optionally, other monomers. In this document, DL-lactide can be crystallized, for example, in toluene and simply separated from the mother liquor, which is primarily composed of meso-lactide and toluene. After evaporating this mother liquor, a liquid consisting of meso-lactide, free acid, and by-products remains. The crude meso-lactide can be recovered from the solvent and recrystallized. Ethanol, cyclohexane, toluene / cyclohexane (1:1), toluene / ethanol (1:1), and methyl ethyl ketone are listed as solvents, with isopropyl alcohol (IPA) being indicated as the preferred solvent. It is noted that if the crystallization process continues for too long, the free acid content of the final meso-lactide will be too high, while the yield will be too low, so crystallization should be allowed to proceed rapidly. No crystallization examples are provided in this document.

[0015] U.S. Pat. No. 5,214,159 is directed to the preparation of meso-lactide, which involves separating meso-lactide from D,L-lactide. It indicates that meso-lactide may be recrystallized for further purification. Preferred solvents are C1-C4 alcohols, preferably isopropanol, or toluene. These documents do not describe the specific solvent crystallization procedures of the present invention.

[0016] U.S. Pat. No. 5,364,086 describes a method for purifying crude lactide, in which the crude lactide is recrystallized using a poor solvent and a good solvent. During crystallization, both solvents are present simultaneously. The ratio of the poor solvent to the good solvent ranges from 1:1 to 5:1. Methyl ethyl ketone is cited as an example of a good solvent. This document does not describe crystallizing a feedstock containing at least 75% meso-lactide by weight with 5 to 50% by weight of a solvent containing at least 70% by weight of a ketone. This document also does not describe the free acid content of lactide or the amount of lactide that crystallizes.

[0017] The method according to the present invention comprises: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight polylactic acid; Crude lactide containing meso-lactide and L-lactide and / or D-lactide wherein either L-lactide or D-lactide is the non-predominant lactide. The present invention finds particular application in the production of purified meso-lactide and purified L- or D-lactide streams from processes comprising the steps of:

[0018] This type of process generally produces crude lactide that includes meso-lactide, as well as L-lactide and / or D-lactide, where either L-lactide or D-lactide is the non-predominant lactide; as described above, the crude lactide is often separated to form a meso-lactide stream and a meso-lactide-depleted stream, which will be referred to herein as a purified L- and / or D-lactide stream. The meso-lactide stream formed in this separation step can be treated by the solvent crystallization process described herein. This can be done directly; no prior crystallization is required.

[0019] Therefore, the present invention also provides a method for producing a method for manufacturing a semiconductor device comprising: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight polylactic acid; Crude lactide containing meso-lactide and L-lactide and / or D-lactide wherein either L-lactide or D-lactide is the non-predominant lactide. c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, where the purification process is a solvent crystallization process described herein; and spirit recovering the produced meso-lactide and purified L-lactide and / or D-lactide streams. No intermediate crystallization step is carried out between separation step c) and the solvent crystallization process carried out in step d). In particular, the meso-lactide stream formed in step c) containing at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg may be processed directly in step d) without any intermediate purification or concentration step.

[0020] The solvent crystallization process described herein is also particularly attractive for incorporation into a method for producing polylactide. Therefore, the present invention also provides a method for producing a polylactide comprising: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight polylactic acid; Crude lactide containing meso-lactide and L-lactide and / or D-lactide wherein either L-lactide or D-lactide is the non-predominant lactide. c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, where the purification process is a method provided herein; and e) subjecting at least a portion of the purified L-lactide and / or D-lactide stream, at least a portion of the purified meso-lactide, or a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide to a polymerization process to form polylactide. Again, no intermediate crystallization step is carried out between separation step c) and the solvent crystallization process carried out in step d). In particular, the meso-lactide stream formed in step c) containing at least 75 wt. % meso-lactide having a free acid content of at least 30 meq / kg may be processed directly in step d) without any intermediate purification or concentration step.

[0021] Further advantages of the invention and its various embodiments will become apparent from the further description. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 shows an optical microscope photograph. DETAILED DESCRIPTION OF THE INVENTION

[0023] The invention will be described in more detail below.

[0024] The reason for performing the solvent crystallization step is the presence of contaminants, particularly acidic contaminants. Therefore, the meso-lactide feedstock provided to the solvent crystallization step has a free acid content of at least 30 meq / kg. The free acid content can be determined by titration in water-free methanol with, for example, sodium methylate or potassium methylate.

[0025] The free acid content depends on the source of meso-lactide and can be much higher. In fact, the process according to the invention is particularly attractive for processing feedstocks with higher free acid contents because it is carried out at relatively low temperatures compared to, for example, melt crystallization or distillation. As noted above, the presence of free acid increases the reactivity of the feedstock, and the low temperature of the process helps minimize side reactions that occur during crystallization. Thus, in some embodiments, the meso-lactide feedstock has a free acid content of at least 50 meq / kg, preferably at least 80 meq / kg, particularly at least 120 meq / kg, or at least 150 meq / kg. The upper limit is not critical; 700 meq / kg may be cited as an upper limit. Generally, the free acid content is lower, for example, 400 meq / kg or less, particularly 300 meq / kg or less, and often 200 meq / kg or less.

[0026] The starting material for the process according to the invention is a feedstock containing at least 75% meso-lactide by weight. The amount of meso-lactide is calculated on the feedstock supplied to the solvent crystallization step. Preferably, the feedstock contains at least 80% meso-lactide by weight, more particularly at least 90% meso-lactide by weight, and in some embodiments at least 95% meso-lactide by weight. The amount of meso-lactide in a lactide composition may be analyzed using HPLC with a water / acetonitrile mixture as eluent and a UV detector, where lactic acid and lactoyl lactic acid elute first, followed by meso-lactide, and, with longer elution times, L- and D-lactide, and then oligomers of lactic acid.

[0027] The feedstock may also contain volatile acids, such as acetic acid, pyruvic acid, succinic acid and anhydrides, typically in amounts significantly lower than 1% by weight. Additional compounds include 2,3-butanediol and (esters of) other hydroxy acids that may be formed during fermentation. These may be present at tens to thousands of ppm. The process according to the invention also reduces the presence of these compounds in the meso-lactide.

[0028] In one embodiment, the meso-lactide-containing feedstock is provided in liquid form, which may be obtained, for example, from a condenser after a distillation process.

[0029] The meso-lactide feedstock contains at least one compound selected from the group of ketones of the formula: R1-C(=O)-R2, where R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, and ethers of the formula: R3-O-R4, where R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl. In total, at least It is subjected to a solvent crystallization process in a solvent containing 70% by weight of the compound.

[0030] Since the use of a ketone or ether as disclosed herein is an important feature of the present invention, the solvent is At least 80% by weight, more particularly at least 90% by weight, even more particularly at least 95% by weight, even more particularly at least 98% by weight of the total consists of at least one compound selected from the group of ketones and ethers as defined above. It is preferable.

[0031] Preferably, the solvent contains less than 5 wt. %, particularly less than 2 wt. %, and more particularly less than 1 wt. % of each of alcohols, esters, and aromatic solvents. The presence of alcohols and esters is disadvantageous because they may react with contaminants present in the lactide or the mixture. The presence of aromatic solvents is disadvantageous in view of the environmental and food contact concerns associated therewith.

[0032] When a ketone is used of the formula R1-C(=O)-R2, R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, with R1 being preferably methyl. It is even more preferred that R1 is methyl and R2 is selected from methyl, ethyl, and isobutyl. It is particularly preferred that both R1 and R2 are methyl. It is particularly preferred that when both R1 and R2 are methyl, the resulting product is dimethyl ketone or 2-propanone (also known as acetone).

[0033] When an ether of the formula R3-O-R4 is used, R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl and isobutyl, and R1 is preferably methyl, ethyl or isopropyl. It is particularly preferred that R3 and R4 are independently selected from methyl, ethyl or isopropyl. Diisopropyl ether may be mentioned as a preferred example.

[0034] The use of ketones has been found to be preferable to the use of ethers.

[0035] It has been found that the use of certain solvents described herein, particularly preferred solvents, results in an efficient crystallization process with good separation of free acid impurities, wherein the resulting meso-lactide can be efficiently separated from the mother liquor. In particular, it has been found that meso-lactide crystal slurries formed using certain solvents, particularly acetone, have attractive filtration characteristics. This means that when crystals are formed in a slurry, for example, during suspension crystallization, they can be efficiently separated from the mother liquor using conventional filtration equipment, such as a belt filter. In addition, it has been found that the crystals can be easily washed with additional solvent, which further reduces the free acid content of the meso-lactide.

[0036] As indicated above, the solvent contains at least one compound selected from the group of ketones and ethers identified herein. , in total The solvent contains at least 70% by weight of a compound selected from the group of ketones and ethers specified herein. The use of a mixed solvent of various compounds is in principle possible, but is not generally considered necessary or desirable. Thus, in one embodiment, the solvent contains a compound selected from the group of ketones and ethers specified herein. , in totalIt comprises at least 70% by weight, preferably at least 80% by weight, more particularly preferably at least 90% by weight, even more particularly preferably at least 95% by weight, and even more particularly preferably at least 98% by weight.

[0037] The meso-lactide is dissolved in a solvent in an amount such that the solution to be subjected to solvent crystallization has a solvent content of 5 to 50% by weight, calculated on the combined feedstock and solvent. It has been found that the solubility of meso-lactide in selected solvents is so good that a limited amount of solvent, i.e., 50% by weight or less, is sufficient to obtain the necessary dissolution of meso-lactide in the solvent. From the standpoint of process efficiency, less solvent is considered preferable. Thus, in some embodiments, 40% by weight or less of solvent is used, particularly 35% by weight or less, and in some embodiments, 30% by weight or less. A value of 5% by weight has been found to be a minimum. Below this value, insufficient solubility is generally obtained to provide adequate dissolution, crystallization, and separation behavior. A solvent content of at least 10% by weight may be preferred.

[0038] The exact amount of solvent will depend on the nature of the solvent and the temperature of the mixture of solvent and meso-lactide: the higher the temperature, the higher the solubility of meso-lactide in the solvent, and consequently the less solvent needed to obtain a solution of meso-lactide in the solvent.

[0039] As will be apparent to those skilled in the art, the presence of significant amounts of water in the system must be avoided because water accelerates the hydrolysis of lactide to lactic acid, and therefore it is preferable to use a water-free solvent or otherwise take the necessary steps to prevent the presence of water.

[0040] Once a solution of meso-lactide in the solvent is obtained, the solution is brought to crystallization conditions. Crystallization can be promoted by a decrease in temperature, by removal of the solvent, or a combination thereof, for example, through evaporation of the solvent. In one embodiment, the temperature of the solution is reduced to a value, for example, below 50°C, particularly below 45°C, more particularly below 40°C, even more particularly below 35°C, and even more particularly below 30°C. From an industrial point of view, operation below 0°C may not be practical. A lower limit may include 5°C, particularly 10°C. As will be apparent to those skilled in the art, temperature affects the crystal yield, with lower temperatures resulting in higher yields. The suitable temperature will depend on the nature and amount of the solvent and the desired crystal yield in the individual crystallization step.

[0041] Depending on the saturation state of the solution, crystallization may be accelerated by the addition of seed crystals.

[0042] The solvent crystallization steps are conducted so that at least 15% of the meso-lactide in the feedstock crystallizes in solid form. The optimum amount will depend on the process and the ability to efficiently separate the impure mother liquor from the solid meso-lactide. Generally, lower crystallization amounts result in larger crystals, improved filtration and washing characteristics, and therefore higher purity. Higher amounts, on the other hand, may be associated with higher overall process efficiency. In one embodiment, at least one, and preferably all, of the solvent crystallization steps are conducted so that at least 20%, particularly at least 25%, and more particularly at least 30% of the meso-lactide in the feedstock crystallizes in solid form. Conversely, crystallization levels above 80% may result in the slurry becoming too viscous for effective stirring and pumping, resulting in crystal wastage and poor liquid-solid separation. Therefore, it may be preferable to operate at or below 80%. The preferred crystallization level in a particular embodiment will also depend on the type and concentration of the solvent and the process configuration.

[0043] As shown above, the process according to the invention is an efficient way to reduce the free acid content of meso-lactide. This can be expressed through a reduction factor, which is defined as the free acid content of the starting material divided by the free acid content of the product crystals. Thus, a reduction factor of 1 means that the free acid content is not reduced. A reduction factor of 2 means that the free acid content has been halved. It has been found that the process according to the invention makes it possible to achieve a reduction factor of at least 2, in many embodiments at least 3, or at least 4, and in some embodiments even higher, in a single crystallization step. Based on the teachings herein, it is within the skill of one in the art to select the nature and amount of solvent and the crystallization temperature so that the desired reduction factor can be achieved for each crystallization step.

[0044] Suitable equipment includes the use of static crystallizers, scraped-wall crystallizers, and stirred vessels. Stirred vessel suspension crystallizations can be cooled by external wall cooling or evaporative cooling, or both. Suitable equipment is known to those skilled in the art and does not require further description herein. Suspension crystallization in stirred vessels is also believed to be preferred in view of the good filterability of the meso-lactide crystals that are formed. The crystals can be recovered via centrifugation, but recovery via filtration is believed to be preferred. The use of a belt filter may be particularly attractive.

[0045] If necessary, the meso-lactide crystals recovered from the solvent can be washed with additional solvent to remove impurities adhering to the crystals. To prevent the meso-lactide from dissolving in the wash solvent, the wash solvent is generally carried out at a relatively low temperature, i.e., at, or generally below, the temperature at which solvent crystallization is carried out.

[0046] In one embodiment, the solvent crystallization is carried out as a suspension crystallization, followed by recovery of the crystals by filtration, preferably on a belt filter, followed by washing of the crystals, preferably also on a belt filter. Another preferred method for the solid-liquid separation step is centrifugal filtration, in which the mother liquor can be efficiently separated from the crystals.

[0047] It has been found that the solvent content of the crystals obtained in the method according to the present invention can be relatively low, for example, less than 1 wt%, particularly less than 0.7 wt%, more particularly less than 0.5 wt%.As a result of the relatively high volatility of the solvent, the solvent content in the crystals can be easily further reduced by evaporating the solvent, for example, by evaporating the solvent under subatmospheric pressure.

[0048] Depending on the amount of meso-lactide that crystallizes in the crystallization step, the mother liquor that is separated from the solid meso-lactide may still contain a significant amount of meso-lactide. The mother liquor can be treated as desired. The mother liquor can be reheated and recycled to dissolve more meso-lactide. The mother liquor can also be subjected to a further crystallization step to recover additional meso-lactide. The mother liquor can also be discharged from the process or hydrolyzed to obtain technical-grade lactic acid. The method for treating the mother liquor will depend not only on the amount of meso-lactide present in the mother liquor, but also on the amount of contaminants. Obviously, if the amount of contaminants in the mother liquor is too high, recycling them back into the crystallization process may result in the addition of contaminants to the system rather than a reduction, and should be avoided.

[0049] One advantage of the solvents used herein is that they are relatively easy to purify, for example, through distillation. Thus, in one embodiment, the method includes the steps of subjecting the used solvent to a distillation step and providing the purified solvent to the meso-lactide crystallization step.

[0050] The meso-lactide crystals recovered from the solvent crystallization process will have a reduced contaminant content compared to the contaminant content of the meso-lactide crystals when they entered the process. Nevertheless, depending on the contaminant content of the meso-lactide when it entered the process and the manner in which the process is carried out, it is possible that the contaminant content may still be unacceptably high. Therefore, it is possible to treat the meso-lactide recovered from the solvent crystallization process with The product is subjected to further processing by solvent crystallization and subsequent recovery of meso-lactide. may be preferred, and the process may be repeated as necessary. Solvent crystallization and subsequent recovery of the product meso-lactide A total of 2 to 10 consecutive steps, particularly 2 to 6 consecutive steps, are carried out.

[0051] The preferences described above for the first solvent crystallization and recovery step apply equally to the further solvent crystallization and recovery steps. These steps may be carried out in the same manner or in different manners. It is preferred to use the same solvent in all crystallization steps to allow for efficient recovery and reuse of the solvent. The crystallization conditions, e.g., the percentage of meso-lactide that crystallizes in each step, can be different.

[0052] In one embodiment, in a series of consecutive crystallizations, solvent recovered from a later crystallization step is recycled to a previous crystallization step, which is possible because the solvent recovered from the later crystallization step has a lower contaminant content than the solvent recovered from the previous crystallization step.

[0053] For high yields in the overall process, additional lactide may be recovered from the waste solvent from the crystallization step, which may be at the expense of overall purity, and the final process will be chosen to provide a good balance between yield and quality.

[0054] The process according to the invention can be carried out as a batch process or as a continuous process.

[0055] In the method according to the present invention, Solvent crystallization and subsequent recovery of the product meso-lactideThe solid meso-lactide recovered from the final step has a free acid content of 20 meq / kg or less. Preferably, the free acid content is 15 meq / kg or less, especially at most 10 meq / kg or less. Depending on the specific use of the meso-lactide, it may be desirable to have the free acid content lower, for example, 5 meq / kg or less, 2 meq / kg or less, or 1 meq / kg or less. The acceptable free acid content will also depend on the amount of meso-lactide that will be applied to further uses.

[0056] As described above, in one embodiment, the meso-lactide purification method of the invention comprises: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight polylactic acid; Crude lactide containing meso-lactide and L-lactide and / or D-lactide wherein either L-lactide or D-lactide is the non-predominant lactide. c) separating meso-lactide from the crude lactide, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream. d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, where the purification process is a solvent crystallization process as set out herein; The method is incorporated into a process for producing a purified meso-lactide and a purified L-lactide and / or D-lactide stream, the process comprising the steps of:

[0057] Steps a), b) and c) are known per se in the art. They hardly need to be explained here. It may be noted that: In step a), low molecular weight poly(lactic acid) is formed by condensation polymerization of lactic acid. The lactic acid is often obtained from biological processes and generally has high optical purity. Depending on the source, it may contain at least 90% L-lactic acid, particularly at least 95% L-lactic acid, more particularly at least 98% by weight. Conversely, it may contain at least 90% D-lactic acid, particularly at least 95% D-lactic acid, more particularly at least 98% by weight. The condensation polymerization can be carried out as known in the art. It generally involves subjecting lactic acid to subatmospheric pressure, e.g., 50-500 mbar, and elevated temperature, e.g., 100-200°C, to remove water and induce polymerization. The resulting average degree of polymerization is generally 5-20.

[0058] Next, the low-molecular-weight polylactic acid obtained in step a) is subjected to a depolymerization step to convert the low-molecular-weight polylactic acid into lactide. Depolymerization is also known in the art. Depolymerization is generally carried out in the presence of a catalyst. Metal-containing catalysts are often used, particularly catalysts based on tin, zinc, aluminum, lead, antimony, lead, calcium, and magnesium, for example, in the form of halide salts or salts of organic acids, such as fatty acids. Tin(II) bis(2-ethylhexanoate) is often used commercially. Typical concentrations of lactide synthesis catalysts are 20 to 2000 ppm. Reaction conditions include a temperature of 160 to 260°C, a pressure of 5 to 100 mbar, and a residence time of 10 minutes to 8 hours.

[0059] The products from the depolymerization step are: Crude lactide containing meso-lactide and L- and / or D-lactide where L- or D-lactide is the predominant lactide and the other is the non-predominant lactide. Which lactide is the predominant lactide will depend on the stereochemistry of the starting lactic acid. Generally, L-lactide will be the predominant lactide. Crude lactide also contains water and lactic acid.

[0060] As will be apparent to those skilled in the art, the crude lactide may be a predominant lactide selected from meso-lactide, L-lactide, and D-lactide, and Optionally, it includes a non-predominant lactide selected from D-lactide and L-lactide, depending on the reaction conditions and previous separation steps, which will be referred to herein as meso-lactide (on the one hand), and L-lactide and / or D-lactide (on the other hand).

[0061] In step c), meso-lactide is separated from the crude lactide in one or more steps, thereby forming a meso-lactide stream containing at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream. This separation can be carried out in one or more steps. Generally, the crude lactide stream is first subjected to a first distillation step, in which water, lactic acid, and other low-boiling impurities are removed from the system. The lactide stream can then be treated as desired in one or more steps. In one embodiment, the lactide fraction is subjected to a further distillation step, thereby forming a meso-lactide stream containing at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream. The stereochemical purity of the purified L-lactide and / or D-lactide stream will generally be such that it consists of at least 90%, particularly at least 94%, more particularly at least 96%, and even more particularly at least 98%, predominant lactide, particularly L-lactide, calculated on the sum of D-lactide and L-lactide.

[0062] Both the distillation process and its alternative separation processes are known in the art and require no further explanation.

[0063] Thus, the process of this embodiment produces a purified meso-lactide and a purified L-lactide and / or D-lactide stream. Both products can be processed as desired. For example, they can be submitted to a polymerization process for the production of polymers containing lactide monomer. They can also be used in the production of high-purity lactic acid, and in the production of coatings, sealants, adhesives, resins and hot melts, or esters.

[0064] As noted above, in one embodiment, the meso-lactide purification process of the invention may be incorporated into a process for producing polylactide. Thus, the invention also provides a method for producing meso-lactide. a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight polylactic acid; Crude lactide containing meso-lactide and L-lactide and / or D-lactide wherein either L-lactide or D-lactide is the non-predominant lactide. c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, where the purification process is a method provided herein; and e) subjecting at least a portion of the purified L-lactide and / or D-lactide stream, at least a portion of the purified meso-lactide, or a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide to a polymerization process to form polylactide. The present invention relates to a method for producing polylactide, comprising the steps of:

[0065] For steps a), b) and c) of this method, see above.

[0066] With regard to the polymerization step e), the production of PLA from lactide monomers is known in the art and does not need to be further described here.

[0067] Generally, polymerization is carried out by providing the monomers to a polymerization reactor, where the polymerization is carried out by bringing them to polymerization conditions, generally in the presence of a polymerization catalyst. Suitable polymerization conditions include temperatures from 100°C to 225°C, particularly from 130°C to 220°C, and more particularly from 170°C to 210°C.

[0068] Suitable catalysts are known in the art. The catalysts described above for the depolymerization process may also be used herein. The catalyst is used in a catalytically effective amount, for example, 1 to 2000 ppm calculated based on the weight of the monomer. Polymerization will be carried out until the desired molecular weight is reached. When the desired molecular weight is reached, the catalyst is often deactivated by adding a catalyst killer to prevent the formation of free acid.

[0069] In step e), at least a portion of the purified L-lactide and / or D-lactide stream, at least a portion of the purified meso-lactide, or a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide is subjected to a polymerization process to form polylactide. Thus, the claims encompass the polymerization of meso-lactide alone, the polymerization of the purified L-lactide and / or D-lactide stream alone, and the polymerization of a combination of meso-lactide and the purified L-lactide and / or D-lactide stream. In the latter case, the amount of meso-lactide will generally be in the range of 2 to 30 weight percent, calculated on the sum of meso-lactide, D-lactide, and L-lactide, depending on the desired properties of the final polymer. Selection of an appropriate amount of meso-lactide is within the skill of an artisan. Depending on the amount of meso-lactide available, it is also conceivable to provide a portion of the meso-lactide to polymerization step a) or to depolymerization step b), in which the meso-lactide will be racemized to L-lactide and D-lactide.

[0070] The amount of meso-lactide will generally range from 1 to 30% by weight, calculated as the sum of meso-lactide, D-lactide, and L-lactide, depending on the properties of the final polymer. Selection of an appropriate amount of meso-lactide is within the skill of the art. Depending on the amount of meso-lactide available, it is also contemplated to provide a portion of the meso-lactide to polymerization step a) or depolymerization step b). In these steps, the meso-lactide will be racemized to L-lactide and D-lactide.

[0071] As will be apparent to those skilled in the art, different embodiments of the present invention can be combined unless they are mutually exclusive. When amounts, concentrations, dimensions, and other parameters are expressed in the form of ranges, preferred ranges, upper and lower limits, or preferred upper and lower limits, it should be understood that any range obtained by combining any upper or preferred value with any lower or preferred limit is also specifically disclosed, regardless of whether the resulting range is expressly stated in the context.

[0072] All documents mentioned herein are incorporated by reference in their entirety or, alternatively, for the disclosure on which they are specifically relied upon.

[0073] The following examples illustrate the practice of the present invention in some of its preferred embodiments. Other embodiments within the scope of the claims will be apparent to those of skill in the art.

[0074] Example 1: Proof of principle

[0075] Meso-lactide raw material having a meso-lactide content of greater than 99% by weight and a free acid content of 34 meq / kg was subjected to solvent crystallization as follows: Solid meso-lactide was combined with acetone as the solvent. Two experiments were performed, using 10% and 20% by weight acetone, respectively. In each experiment, the mixture of solvent and meso-lactide was heated in a double-jacketed, temperature-controlled vessel with a bottom glass filter to a temperature at which all of the meso-lactide dissolved. The temperature was then slowly reduced to just above the pre-estimated crystallization point. Seed crystals were added to initiate crystallization, and the mixture was cooled until a slurry with approximately 50% crystal content was obtained. For the 10% acetone solution, the temperature was 31°C, and for the 20% acetone solution, the mixture was further cooled to 20°C. The crystals were isolated via vacuum filtration. The crystallization results are shown in the table below.

[0076] [Table 1]

[0077] These examples show that even when starting from a relatively low free acid content, the free acid can still be significantly reduced by using solvent crystallization with acetone (note that the data is for unwashed crystals). It also shows that relatively small amounts of solvent can be used. An optical micrograph is shown in Figure 1. It can be seen that plate-like crystals formed and that the crystals were relatively uniform in size and shape. Typical crystal sizes were in the 300 micron range. The filtration properties of the material were excellent, as was the removal of free acid.

[0078] Example 2: Solvent selection, crystallization temperature, solvent in product

[0079] In this example, the starting material was a meso-lactide feedstock obtained from a distillation process in which meso-lactide was separated from other typical components of crude lactide. The meso-lactide feedstock contained greater than 90% meso-lactide by weight. The free acid content varied depending on storage conditions and time.

[0080] Crystallization was carried out as follows: solid meso-lactide and solvent (1 liter total) were added to a 2-liter jacketed glass filter funnel. The mixture was heated to 55°C, at which point the meso-lactide was completely dissolved in the solvent. The mixture was cooled at 20°C / hour to a temperature 2°C above the theoretical crystallization temperature (previously determined). The mixture was then slowly cooled at 2°C / hour. Meso-lactide seed crystals were added in 1°C increments. When the mixture reached a temperature where the seed crystals no longer dissolved, the mixture was held at that temperature for 30 minutes. After 30 minutes, the mixture was further cooled at 2°C / hour to a temperature where approximately 50% of the meso-lactide was crystallized (as predicted by the solubility curve), forming a slurry. Stirring was applied throughout the entire process.

[0081] The slurry was filtered through a glass filter and a sample of the mother liquor was taken to determine the solvent concentration. The crystals from the filter were dried in a vacuum oven.

[0082] The table below shows the crystallization temperature and the temperature at which 50% lactide crystals are obtained for experiments carried out with three different solvents and three different solvent concentrations. The solvent content of the final crystals is also given.

[0083] [Table 2]

[0084] From this data, it can be seen that all experiments provide suitable conditions for the production of slurries with high solids contents, while providing very effective filtration operations. The solvent content of the crystals can be considered as a measure for the filtration properties of the crystals. These values ​​are very low, providing good separation of the crystals and the mother liquor. Moreover, the low amount of solvent in the crystals provides a starting point for enabling very low final amounts of residual solvent after further processing, for example, through (vacuum) drying or distillation.

[0085] Example 3: Solvent Selection and Free Acid Content

[0086] Crystallization experiments were conducted using a meso-lactide feedstock with a meso-lactide content of 91% by weight using the method and apparatus provided in Example 2. The free acid content of the feedstock varied from sample to sample. The table below provides the free acid content of the feedstock, the free acid content of the product, and the reduction factor.

[0087] [Table 3]

[0088] From this table it can be seen that even when the amount of free acid in the starting material is low and the amount of solvent is low, a significant reduction in free acid content can be achieved.

[0089] Example 4: Crystal yield per pass

[0090] In this example, the effect of crystalline content on product yield was investigated. Two experiments were conducted under the conditions of Example 2: one to obtain 50% meso-lactide crystals and one to obtain 75% meso-lactide crystals. Acetone was used as the solvent in an amount of 10%. The table below gives the crystallization temperature and the temperature required to obtain the desired percentage of crystals.

[0091] [Table 4]

[0092] As expected, the temperature required to obtain 75% crystallinity is significantly lower than that required to obtain 50% crystallinity.

[0093] Example 5: Two-stage crystallization

[0094] When the meso-lactide feedstock has a relatively high free acid content, it may be necessary to subject the product from the first crystallization step to a second crystallization step. This is illustrated in this example. The feedstock had a meso-lactide content of greater than 90%. Crystallization was carried out using 20% ​​by weight acetone.

[0095] [Table 5]

[0096] [Table 6]

[0097] This example shows that recrystallization results in a product with reduced free acid content and reduced crystalline solvent content. It also shows that a high purification factor for the free acid can be achieved regardless of the starting amount of free acid.

[0098] Example 6: Crystal recovery via centrifugation

[0099] An experiment was carried out using 20% ​​acetone under the conditions of Example 2, except that the crystals were isolated using centrifugation rather than vacuum filtration. Centrifugation appeared to give crystals with a solvent content of 0.5% by weight, rather than the 1% obtained in Example 2 using the same solvent content. The free acid content of the feed and product is provided in the table below.

[0100] [Table 7]

[0101] Apparently, a low solvent content of the crystals is associated with a low free acid content of the product, which means that the free acid is concentrated in the solvent attached to the crystals, which means that washing the crystals with more solvent will result in a lower free acid content of the product.

[0102] Example 7: Static Crystallizer

[0103] Solvent crystallization was carried out under static conditions in a 1 L Erlenmeyer flask. First, 500 g of meso-lactide was melted and stirred at 55° C., and a total of 100 g of acetone was slowly added, taking care to maintain the temperature above 52° C. The solution was then allowed to statically cool to room temperature overnight under ambient conditions.

[0104] The table below provides feedstock and product properties.

[0105] [Table 8]

[0106] It is found that static solvent crystallization can also provide meso-lactide crystals with reduced free acid content. Moreover, evidence is provided that further purification, optionally by washing steps, is possible. Care should be taken in such further washing steps to limit dissolution of the crystals as much as possible, as this will affect yield.

[0107] Example 8 (comparative): Higher solvent content

[0108] In this example, the effect of using more than 50% by weight of solvent was investigated. 500 g of meso-lactide was melted and stirred at 55°C, and 750 g of acetone was slowly added, taking care to keep the temperature above 52°C. The solvent content, calculated as the sum of meso-lactide and solvent, was 60% by weight. The solution was cooled, and meso-lactide seeds were added at various temperatures. The results are given in the table below.

[0109] [Table 9]

[0110] At each temperature tested, the seeds dissolved, meaning that this amount of solvent did not allow crystallization at an economically attractive temperature.

[0111] Example 9 (Comparative): Isopropanol as Solvent

[0112] As described in Example 1, solid meso-lactide particles were combined with isopropanol as a solvent in an amount of 20% by weight (calculated as the sum of meso-lactide and isopropanol). The mixture of solvent and meso-lactide was heated in a double-jacketed, temperature-controlled vessel with a bottom glass filter to dissolve the meso-lactide and perform solvent crystallization. However, the solubility of meso-lactide in isopropanol was very low, so meso-lactide could not be dissolved in isopropanol at temperatures below the melting point of meso-lactide. Therefore, it was not possible to perform solvent crystallization in the presence of 20% by weight isopropanol. Further investigation of the solubility of meso-lactide in isopropanol revealed that the ability of isopropanol to dissolve meso-lactide was significantly lower than in the case of the solvent according to the present invention. It should also be noted that the presence of residual alcohol solvent limits the maximum achievable molecular weight of polymers produced from lactide. This is because the alcohol can react in the esterification reaction.

[0113] The present invention also provides the following: [Section 1] 1. A method for purifying meso-lactide having a free acid content of at least 30 meq / kg, comprising: at least one step of subjecting a feedstock containing at least 75% by weight meso-lactide to solvent crystallization and subsequent recovery of the product meso-lactide, wherein the solvent crystallization step comprises subjecting at least one compound selected from the group of ketones of the following formula: R1-C(=O)-R2, where R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, and ethers of the following formula: R3-O-R4, where R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl. In total, at least 70% by weight of the solvent, the solvent being present in an amount of 5 to 50% by weight calculated on the total of the feedstock and the solvent, and the solvent crystallization of conditions are selected so that at least 15% of the meso-lactide in the feedstock crystallizes in solid form; wherein the solid meso-lactide recovered from the final step of solvent crystallization and subsequent product meso-lactide recovery has a free acid content of 20 meq / kg or less. The method. [Section 2] Item 1. The method according to Item 1, wherein a total of 2 to 10 consecutive steps of solvent crystallization and subsequent recovery of the product meso-lactide are carried out, particularly 2 to 6 consecutive steps. [Section 3] 3. The method according to claim 1 or 2, wherein the step of solvent crystallization is carried out in a solvent, the solvent consisting of at least 80% by weight, more particularly at least 90% by weight, even more particularly at least 95% by weight, even more particularly at least 98% by weight of at least one compound selected from the group of ketones and ethers specified in claim 1. [Section 4] 4. The method of any one of items 1 to 3, wherein in at least one, and preferably all, of the steps of solvent crystallization and subsequent recovery of the product meso-lactide, a ketone solvent is used in which R1 is methyl and R2 is preferably selected from methyl, ethyl, and isobutyl, and wherein R1 and R2 are more preferably both methyl. [Section 5] 4. The method according to any one of items 1 to 3, wherein an ether solvent in which R3 and R4 are independently selected from methyl, ethyl, or isopropyl is used in at least one, and preferably all, of the steps of solvent crystallization and subsequent recovery of the product meso-lactide, and wherein R3 and R4 are more preferably both isopropyl. [Section 6] Item 6. The method according to any one of Items 1 to 5, wherein acetone is used as a solvent. [Section 7] 7. The method according to any one of items 1 to 6, wherein at least one, and preferably all, of the solvent crystallization steps is carried out so that at least 20%, particularly at least 25%, more particularly at least 30%, and / or up to 80% of the meso-lactide in the feedstock is crystallized in solid form. [Section 8] 8. The method of any one of items 1 to 7, wherein the feedstock comprising meso-lactide has a free acid content of at least 50 meq / kg, preferably at least 80 meq / kg, particularly at least 120 meq / kg, or at least 150 meq / kg, and / or 700 meq / kg or less, particularly 400 meq / kg or less, more particularly 300 meq / kg or less, and often 200 meq / kg or less. [Section 9] 9. The method of any one of items 1 to 8, wherein the feedstock containing meso-lactide contains at least 80% by weight meso-lactide, even more particularly at least 90% by weight meso-lactide, and even more particularly at least 95% by weight meso-lactide. [Section 10] 10. The method according to any one of items 1 to 9, wherein 40% by weight or less, in particular 35% by weight or less, more particularly 30% by weight or less, in particular at least 10% by weight of solvent is used. [Section 11] 11. The method of any one of Items 1 to 10, wherein the solid meso-lactide recovered from the final step of solvent crystallization and subsequent product meso-lactide recovery has a free acid content of 15 meq / kg or less, particularly 10 meq / kg or less, for example, 5 meq / kg or less, 2 meq / kg or less, or 1 meq / kg or less. [Section 12] Item 12. The method according to any one of items 1 to 11, wherein at least one, preferably all, of the solvent crystallization steps is carried out at a temperature of the mixture of solvent and feedstock reduced to less than 40°C, in particular less than 30°C, and / or at least 10°C. [Section 13] 13. The method according to any one of items 1 to 12, wherein at least one, preferably all, of the solvent crystallization steps is carried out under evaporation of the solvent. [Section 14] 14. The method according to any one of items 1 to 13, wherein at least one, preferably all, of the solvent crystallization steps is carried out in a static crystallizer, a scraped-wall crystallizer, or a stirred vessel, with a stirred vessel being preferred. [Section 15] Item 15. The method according to item 14, wherein at least one, preferably all, of the solvent crystallization steps is carried out in a stirred vessel, and wherein the solid lactide is recovered using filtration, for example belt filter or centrifugal filtration, in particular through a belt filter. [Section 16] 16. The method according to any one of items 1 to 15, wherein the solid meso-lactide recovered from the step of solvent crystallization and subsequent product meso-lactide recovery is subjected to a washing step, particularly a washing step using a solvent. [Section 17] 1. A process for producing a purified meso-lactide and purified L-lactide and / or D-lactide stream, comprising: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight poly(lactic acid) to form a crude lactide comprising meso-lactide and L-lactide and / or D-lactide, wherein either L-lactide or D-lactide is the non-predominant lactide; c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, wherein the purification process is a process according to any one of items 1 to 16; and recovering the purified meso-lactide and purified L-lactide and / or D-lactide streams. The method, comprising the steps of: [Section 18] Item 18. The method according to any one of items 1 to 17, wherein the purified meso-lactide and / or purified L-lactide and / or D-lactide stream is converted to high-purity lactic acid by hydrolysis with water. [Section 19] 1. A method for producing polylactide, comprising: a) reacting lactic acid to form low molecular weight polylactic acid; b) depolymerizing the low molecular weight poly(lactic acid) to form a crude lactide comprising meso-lactide and L-lactide and / or D-lactide, wherein either L-lactide or D-lactide is the non-predominant lactide; c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, wherein the purification process is a method according to any one of paragraphs 1 to 14; and e) subjecting at least a portion of the purified L-lactide and / or D-lactide stream, at least a portion of the purified meso-lactide, or a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide to a polymerization process to form polylactide. The method, comprising the steps of: [Section 20] 20. The method of claim 19, wherein a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide is provided to a polymerization step to form polylactide. [Section 21] 21. The method of claim 19 or 20, wherein at least a portion of the purified meso-lactide formed in step d) is provided to step a) or step b).

Claims

1. 1. A method for purifying meso-lactide having a free acid content of at least 30 meq / kg, comprising: at least one step of subjecting a feedstock containing at least 75% by weight meso-lactide to solvent crystallization and subsequent recovery of product meso-lactide, wherein the solvent crystallization step is conducted in a solvent containing a total of at least 70% by weight of at least one compound selected from the group of ketones of the following formula: R1-C(═O)—R2, where R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, and ethers of the following formula: R3-O-R4, where R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, wherein the solvent is present in an amount of 5 to 50% by weight, calculated on the total of the feedstock and solvent, and the conditions for the solvent crystallization are selected such that at least 15% of the meso-lactide in the feedstock crystallizes in solid form; wherein the solid meso-lactide recovered from the final step of solvent crystallization and subsequent product meso-lactide recovery has a free acid content of 20 meq / kg or less. The method.

2. The method of claim 1, wherein a total of 2 to 10 successive steps of solvent crystallization and subsequent recovery of product meso-lactide are carried out.

3. 2. The method of claim 1, wherein the step of solvent crystallization is carried out in a solvent, the solvent consisting of at least 80% by weight of the total of at least one compound selected from the group of ketones and ethers specified in claim 1.

4. The method of claim 1, wherein in at least one of the steps of solvent crystallization and subsequent recovery of product meso-lactide, a ketone solvent is used in which R1 is methyl and R2 is selected from methyl, ethyl, and isobutyl.

5. The method of claim 1, wherein in at least one of the steps of solvent crystallization and subsequent recovery of product meso-lactide, an ether solvent is used, wherein R3 and R4 are independently selected from methyl, ethyl, or isopropyl.

6. 10. The method of claim 1, wherein acetone is used as the solvent.

7. 10. The method of claim 1, wherein at least one of the steps of solvent crystallization is conducted such that at least 20% and not more than 80% of the meso-lactide in the feedstock is crystallized in solid form.

8. 10. The method of claim 1, wherein the feedstock comprising meso-lactide has a free acid content of at least 50 meq / kg and no more than 700 meq / kg.

9. 10. The method of claim 1, wherein the feedstock containing meso-lactide contains at least 80% by weight meso-lactide.

10. 10. The method of claim 1, wherein no more than 40% and at least 10% by weight of solvent is used.

11. The method of claim 1, wherein the solid meso-lactide recovered from the final step of solvent crystallization and subsequent product meso-lactide recovery has a free acid content of 15 meq / kg or less.

12. 10. The method of claim 1, wherein at least one of the solvent crystallization steps is carried out at a reduced temperature of the solvent and feedstock mixture below 40°C.

13. 10. The method of claim 1, wherein at least one of the solvent crystallization steps is carried out under evaporation of the solvent.

14. 10. The method of claim 1, wherein at least one of the steps of solvent crystallization is carried out in a static crystallizer, a scraped-wall crystallizer, or a stirred vessel.

15. 15. The method of claim 14, wherein at least one of the solvent crystallization steps is carried out in a stirred vessel, wherein the solid lactide is recovered through filtration.

16. The method of claim 1, wherein the solid meso-lactide recovered from the solvent crystallization and subsequent product recovery steps is subjected to a washing step.

17. 1. A process for producing a purified meso-lactide and purified L-lactide and / or D-lactide stream, comprising: a) reacting lactic acid to form a low molecular weight polylactic acid; b) depolymerizing the low molecular weight poly(lactic acid) to form a crude lactide comprising meso-lactide and L-lactide and / or D-lactide, wherein either L-lactide or D-lactide is the non-predominant lactide; c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which contains at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, wherein the purification process is a method according to any one of claims 1 to 16; and recovering the purified meso-lactide and purified L-lactide and / or D-lactide streams. The method, comprising the steps of:

18. 10. The method of claim 1, wherein the purified meso-lactide is converted to high-purity lactic acid by hydrolysis with water.

19. The method of claim 17, wherein the purified L-lactide and / or D-lactide stream is converted to high purity lactic acid by hydrolysis with water.

20. 1. A method for producing polylactide, comprising: a) reacting lactic acid to form a low molecular weight polylactic acid; b) depolymerizing the low molecular weight poly(lactic acid) to form a crude lactide comprising meso-lactide and L-lactide and / or D-lactide, wherein either L-lactide or D-lactide is the non-predominant lactide; c) separating meso-lactide from the crude lactide in one or more steps, resulting in the formation of a meso-lactide stream comprising at least 75% by weight meso-lactide having a free acid content of at least 30 meq / kg, and a purified L-lactide and / or D-lactide stream; d) subjecting the meso-lactide stream, which comprises at least 75% by weight of meso-lactide having a free acid content of at least 30 meq / kg, to a purification process, wherein the purification process is a process according to any one of claims 1 to 16; and e) subjecting at least a portion of the purified L-lactide and / or D-lactide stream, at least a portion of the purified meso-lactide, or a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide to a polymerization process to form polylactide. The method, comprising the steps of:

21. 21. The method of claim 20, wherein a combination of at least a portion of the purified L-lactide and / or D-lactide stream and at least a portion of the purified meso-lactide is provided to a polymerization process to form polylactide.

22. 21. The method of claim 20, wherein at least a portion of the purified meso-lactide formed in step d) is provided to step a) or step b).

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