Process for producing poly-meso-lactide

Abstract

The present invention relates to a process for the production of poly meso-lactide (PML), poly meso-lactide obtainable according to or according to such a process, and various uses thereof. The process particularly involves the step of polymerizing a portion of the purified meso-lactide stream that has been separated from the crude lactide mixture and subsequently purified to form poly meso-lactide. In some embodiments, the process further relates to the preparation of a lactide mixture by blending another portion of the purified meso-lactide with a lactide-rich stream, and also to the polymerization of such mixture to form a polylactide (PLA) comprising a polymerized meso-lactide. The invention also relates to polylactide comprising polymerized meso-lactide, obtainable according to or according to such a process, and to the use thereof.

Description

Technical Field

[0001] The present invention relates to the field of polylactide processing. Specifically, the present invention relates to a process for producing polymeso-lactide. The present invention also relates to polymeso-lactide produced according to the process as provided herein and various uses thereof. The present invention also relates to a process for preparing and polymerizing lactide mixtures including meso-lactide, and to the polylactide polymers obtained thereby. Background Art

[0002] Bioplastics have attracted significant attention due to increasing environmental pressures from global warming and plastic pollution. Among them, polylactide, also known as polylactic acid and abbreviated as PLA, is a bio-based and biodegradable polymer that has been widely used in many applications due to its good processability and mechanical properties, such as for the manufacture of packaging products such as food packaging, or for the manufacture of disposable items, such as for medical applications.

[0003] Polylactide can be manufactured industrially by converting lactic acid into lactide (a cyclic dimer of lactic acid), which is then polymerized via ring-opening polymerization (ROP) catalyzed by an organometallic catalyst. The process for making polylactide generally involves first forming a low molecular weight poly (lactic acid) and then depolymerizing the low molecular weight poly (lactic acid). The depolymerization step produces lactide. The lactide is then purified to separate it from water, residual lactic acid, linear lactic acid oligomers, and other impurities that may be present. This can be done from a solvent or from a melt by distillation or by other methods such as recrystallization.

[0004] Lactic acid is a molecule with a chiral center, and therefore it exists in two enantiomeric forms (the so-called R-(or D-) enantiomer and S-(or L-) enantiomer). The optical purity of lactic acid strongly affects the properties of the resulting PLA. Therefore, the starting lactic acid usually has a very high optical purity. However, the starting material is subjected to elevated temperatures when it is converted into a low molecular weight poly(lactic acid) polymer and subsequently depolymerized. Under those conditions, some racemization occurs, i.e., the conversion of one enantiomeric form to another. Because this racemization occurs, the lactide obtained from the process will be a mixture of L-lactide, D-lactide, and meso-lactide.

[0005] It is often the case that the lactide mixture produced as described above contains more meso-lactide than is desired in the downstream polymerization steps. For example, if very high optical purity of PLA is desired, all or nearly all of the meso-lactide must be removed from the lactide stream used for polymerization. Therefore, in such cases, meso-lactide is an undesirable by-product obtained during PLA production, and its occurrence must be minimized, or the meso-lactide must be removed from the lactide mixture.

[0006] In principle, meso-lactide removed during the PLA production process can be converted back into lactic acid by hydrolysis with water. Furthermore, if it is desired to produce a more amorphous polylactide grade with a higher proportion of the R-enantiomer (or S-enantiomer, as the case may be) at a later time, the meso-lactide removed from the lactide mixture during the PLA production process can be stored and added back to the predominantly S,S-lactide (or predominantly R,R-) stream.

[0007] However, the current PLA market uses PLA products that incorporate only small amounts of meso-lactide, typically less than 10% on a global product average. Therefore, the demand for amorphous PLA grades, and therefore the need to add meso-lactide during the PLA manufacturing process, is relatively low.

[0008] Furthermore, this procedure requires additional efforts in the handling, storage and transportation of meso-lactide.

[0009] Furthermore, in practice, the removed meso-lactide stream can be highly contaminated with impurities, and the difficulty and cost of removing such impurities from the meso-lactide has the result that the meso-lactide is often discarded or used in other, lower-value applications, such as in the preparation of lactic acid esters for solvents, and in the preparation of alkali and alkaline earth metal salts of lactic acid for use in feed and preservatives.

[0010] In general, for a PLA process, the removal of the meso-lactide stream, its storage and / or its other recycling in the process represents a loss in overall yield and an increase in overall costs of the PLA production process.

[0011] It was therefore an object of the present invention to provide an improved process which overcomes at least some of the above-mentioned disadvantages and in which the meso-lactide stream obtained during PLA processing can be valorized, preferably as polymer product with the highest value.

[0012] It is an object of the present invention to provide a process in which meso-lactide formed during the PLA production process can be converted and / or reused for the production of technically usable polymers.

[0013] Another object of the present invention is to increase the overall process yield of the PLA preparation process. Summary of the Invention

[0014] It has now surprisingly been found that some or all of the above requirements and aims can be achieved by the process defined herein, alone or in any combination.The process of the invention optimizes the use of the meso-lactide stream obtained during PLA processing.

[0015] To this end, the present invention provides a process for producing polymeso-lactide (PML), comprising the following steps:

[0016] a) forming lactic acid oligomers by polycondensing lactic acid;

[0017] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0018] c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0019] d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg; and

[0020] e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide, and

[0021] f) optionally blending at least a portion of the purified meso-lactide stream not polymerized in step e) with at least a portion of the lactide-rich stream obtained in step c) to provide a lactide mixture, and subsequently polymerizing the lactide mixture to form polylactide, wherein the polymerized meso-lactide content is less than 20.0 wt. % of the polylactide.

[0022] In a preferred embodiment, the present invention provides a process for producing polymeso-lactide (PML), comprising the steps of:

[0023] a) forming lactic acid oligomers by polycondensing lactic acid;

[0024] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0025] c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0026] d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg; and

[0027] e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide.

[0028] In some preferred embodiments of the process of the present invention, the crude lactide formed in step b) comprises L-lactide and D-lactide, and the weight ratio of L-lactide to D-lactide (L / D ratio) is different from 1.0, and is preferably higher than 1.0.

[0029] In some preferred embodiments of the process of the present invention, the amount of L-lactide in the crude lactide is at least 50.0 wt %, preferably at least 55.0 wt %, preferably at least 60.0 wt %, preferably at least 65.0 wt %, preferably at least 70.0 wt %, wherein the wt % is based on the total weight of the crude lactide.

[0030] In some preferred embodiments of the processes of the present invention, the lactide-rich stream is an L-lactide-rich stream comprising at least 50.0 wt% L-lactide, at least 60.0 wt% L-lactide, preferably at least 65.0 wt% L-lactide, preferably at least 70.0 wt% L-lactide, preferably at least 75.0 wt% L-lactide, preferably at least 80.0 wt% L-lactide, preferably at least 85.0 wt% L-lactide, preferably at least 90.0 wt% L-lactide, preferably at least 95.0 wt% L-lactide, wherein the weight % are based on the total weight of the lactide-rich stream.

[0031] In some preferred embodiments of the process of the present invention, the meso-lactide enriched stream is purified to an acidity level equal to or higher than 1.0 meq / kg, preferably equal to or higher than 1.5 meq / kg.

[0032] In some preferred embodiments of the process of the invention, the meso-lactide-enriched stream is purified to an acidity level of at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, and preferably, the purification is carried out by means of crystallization, such as solvent crystallization or melt crystallization.

[0033] In some preferred embodiments of the inventive process, the purified meso-lactide stream comprises at least 95.0 wt% meso-lactide, preferably at least 97.0 wt% meso-lactide, wherein the wt% are based on the total weight of the stream.

[0034] In some preferred embodiments of the process of the present invention, the portion of the purified meso-lactide stream that is polymerized in step e) amounts to (is equivalent to) at least 30.0 wt.-% of the purified meso-lactide stream, or at least 40.0 wt.-%, or at least 50.0 wt.-%, or at least 60.0 wt.-%, or at least 70.0 wt.-%, or at least 80.0 wt.-%, or at least 90.0 wt.-% of the purified meso-lactide stream.

[0035] In some preferred embodiments of the process of the present invention, the crystallization applied in step d) is solvent crystallization or melt crystallization, wherein the solvent or melt crystallization is performed as suspension crystallization or layer crystallization.

[0036] The lactic acid applied as starting material in the process in step a) may have different origins. According to some embodiments of the process of the present invention, part of the lactic acid applied in step a) is:

[0037] - having an isomeric purity of 80.0% or more, such as 85.0% or more, of the L-isomer, and / or

[0038] - having an isomeric purity of 80.0% or more, such as 85.0% or more, of the D-isomer, and / or

[0039] - is racemic lactic acid.

[0040] Preferably, the lactic acid applied in step a) comprises at most 40.0 wt.-% racemic lactic acid, wherein the wt.-% is based on the total amount of lactic acid applied in step a).

[0041] Alternatively or in addition, at least part of the lactic acid applied in step a) can also be prepared by depolymerizing poly-L-lactic acid and / or poly-D-lactic acid. For example, in some embodiments of such a process, poly-L-lactic acid and / or poly-D-lactic acid is depolymerized by hydrolyzing the poly-L-lactic acid and / or poly-D-lactic acid in the presence of water and / or lactic acid as a co-reactant, preferably for 30 minutes to 24 hours, under conditions ranging from atmospheric conditions up to 10 bar and at a temperature ranging from 120 to 200°C.

[0042] With regard to step b) of the present process, and according to some embodiments of the present process, the depolymerization in step b) is carried out at a temperature between 175 and 220°C.

[0043] The process according to the present invention can be further fine-tuned and customized according to specific needs. For example, according to some embodiments of the present process, the amount of meso-lactide formed during the process can be increased. In some embodiments of the present process, the amount of meso-lactide in the crude lactide is increased by stimulating racemization during steps a) and / or b) of the process.

[0044] Alternatively or in addition, in some embodiments, the racemization during steps a) and / or b) is stimulated by adding a racemizer during these steps a) and / or b), preferably in an amount ranging from 5 to 5000 ppm. Preferably, the racemizer is selected from the group consisting of sodium salts such as sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen-containing ligands such as pyridine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene and 1,8-diazabicyclo[5.4.0]undec-7-ene, or any combination thereof.

[0045] In some embodiments of the process of the present invention, during step e) of the process, a portion of the purified meso-lactide stream is polymerized into polymeso-lactide by ring-opening polymerization in the presence of a catalyst, preferably a metal catalyst, and optionally one or more initiators at a temperature between 125 and 225°C, preferably wherein the initiator is an alcohol. Since meso-lactide is more volatile than L-lactide and exhibits a lower viscosity than PLA resins known in the art at the same molecular weight, polymeso-lactide production can be optimized by lowering the polymerization temperature. In the polymerization section, this can be, for example, below 190°C and even below 175°C. In the devolatilization section, the stripping temperature can be below 200°C to still provide effective removal of residual meso-lactide.

[0046] In some embodiments of the process of the present invention, polymerizing the lactide mixture to form polylactide in which the polymerized meso-lactide content is below 20.0 wt. % of the polylactide according to the present process is also carried out by means of ring-opening polymerization in the presence of a catalyst, preferably a metal catalyst, and optionally one or more initiators, at a temperature comprised between 125 and 225° C., or between 125 and 190° C., or between 125 and 175° C., preferably wherein the initiator is an alcohol.

[0047] The advantages of this process relate to one or more of the following:

[0048] - The process of the present invention allows the conversion or reuse of meso-lactide to prepare polymeso-lactide; the process of the present invention also allows the reuse of meso-lactide to produce lactide mixtures including meso-lactide and the preparation of valuable PLA polymers from such mixtures;

[0049] The process of the present invention further provides a versatile process in which the amount of meso-lactide and polymers produced therefrom can be adjusted (e.g., increased) as desired. For example, the present invention allows for increasing the amount of meso-lactide used during the PLA production process by using less stereochemically pure lactic acid as the starting product, by using lactic acid chemically recycled from stereochemically diverse PLA waste, by applying higher synthesis temperatures when making crude lactide, by intentionally increasing racemization during the process, or by any combination of the foregoing.

[0050] - Thus, the present invention makes it possible to start from starting materials of lower stereochemical purity and / or to apply less stringent polymerization conditions in a cost-effective manner.

[0051] The production of meso-lactide can be customized, allowing for the production and reuse of larger quantities of meso-lactide, including lower purity meso-lactide. Reusing meso-lactide-containing streams within the PLA process also eliminates or reduces the need for storage of meso-lactide and its associated costs.

[0052] The present invention thus provides a process in which meso-lactide is polymerized to polymeso-lactide during the same process, and optionally a lactide mixture including (recycled) meso-lactide can also be subjected to polymerization to produce (produce) PLA polymer having well-defined properties. The present process thus provides for optimal valorization and use of the meso-lactide formed during the PLA production process.

[0053] In general, the present invention also provides a process for producing polylactide (including polymeso-lactide) in which the overall yield of the process is optimized and improved. The present invention allows for the separation of a highly purified L-lactide stream, which can be further polymerized into (crystallizable) PLA, and a highly purified meso-lactide stream, which can be further polymerized into (crystallizable) polymeso-lactide. Thus, the present invention also provides an efficient process for preparing crystallizable polymeso-lactide.

[0054] In another aspect, the present invention also provides a polymeso-lactide obtainable according to or according to a process as provided herein, and preferably wherein the polymeso-lactide has one or more of the following properties:

[0055] (i) an L-lactate having a stereochemical purity of 40.0 to 60.0%; or a D-lactate having a stereochemical purity of 40.0 to 60.0%;

[0056] (ii) a weight average molecular weight (Mw) between 50 and 500 kg / mol, or between 100 and 350 kg / mol;

[0057] (iii) a glass transition temperature between 30°C and 50°C, or between 35°C and 50°C.

[0058] In some embodiments, the polymeso-lactide obtainable according to or according to the processes as provided herein is amorphous. In some other embodiments, the polymeso-lactide obtainable according to or according to the processes as provided herein is semi-crystalline.

[0059] In a further aspect, the present invention also relates to the use of polymeso-lactide as defined herein or as obtainable by or by performing the process according to the invention.

[0060] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtainable by or by performing a process according to the present invention for the preparation of an article or composition.

[0061] The present invention also relates to the use of polymeso-lactide as defined herein or as obtainable by or by performing the process according to the invention for the preparation of a degradable article, preferably a solid degradable article. For example, polymeso-lactide as defined herein or as obtainable by or by performing the process according to the invention can be used to make an article that (slowly) degrades during use, for example an article capable of providing a slow release of lactic acid.

[0062] The present invention also relates to the use of polymeso-lactide as defined herein or as obtainable by or by performing the process according to the invention as a (temporary) diverting agent, e.g. in downhole applications or in fracturing applications.

[0063] The present invention also relates to the use of a polymeso-lactide as defined herein or as obtainable by or by carrying out a process according to the invention for preparing a polymer composition, wherein the polymer composition comprises (A) polylactide, and (B) the polymeso-lactide, and wherein the polymer composition comprises less than 10 wt.-% of the polymeso-lactide, based on the total weight of the polymer composition.

[0064] The present invention also relates to the use of polymeso-lactide as defined herein or as obtainable by or by carrying out the process according to the invention as a resin, for example a coating resin, or as an adhesive resin, for example for the preparation of an adhesive film or adhesive layer or a multilayer structure.

[0065] In another aspect, the present invention also relates to a polymer composition comprising (A) polylactide, and (B) polymeso-lactide as defined herein or as obtainable by or by performing the process according to the invention, wherein the polymer composition comprises less than 10 wt.-% of said polymeso-lactide, based on the total weight of the polymer composition.

[0066] The independent and dependent claims set out particular and preferred features of the invention. Features from the dependent claims may be combined with features of the independent claim or other dependent claims as appropriate.

[0067] The present invention will now be further described. In the following paragraphs, different aspects of the present invention are defined in more detail. Each aspect so defined can be combined with any other one or more aspects, unless clearly indicated otherwise. In particular, any feature that is indicated as preferred or advantageous can be combined with any other one or more features that are indicated as preferred or advantageous. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 is a schematic diagram illustrating one embodiment of the process of the present invention. DETAILED DESCRIPTION

[0069] When describing the present invention, the terms used should be interpreted according to the following definitions, unless the context requires otherwise.

[0070] Unless otherwise defined, all terms used to disclose the present invention, including technical and scientific terms, have the meaning commonly understood by those skilled in the art to which the present invention belongs. By way of further guidance, term definitions are included to better understand the teachings of the present invention.

[0071] In the following paragraphs, different aspects of the present invention are defined in more detail. Each aspect so defined can be combined with any other one or more aspects, unless explicitly indicated otherwise. In particular, any feature indicated as preferred or advantageous can be combined with any other one or more features indicated as preferred or advantageous.

[0072] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Thus, the phrases "in one embodiment" or "in an embodiment" appearing in various places throughout this specification do not necessarily all refer to the same embodiment, but may refer to the same embodiment. Furthermore, in one or more embodiments, the particular features, structures, or characteristics may be combined in any suitable manner, as will be apparent to those skilled in the art from this disclosure. Furthermore, while some embodiments described herein include some features included in other embodiments and not other features, combinations of features from different embodiments are intended to be within the scope of the invention and to form different embodiments, as will be appreciated by those skilled in the art.

[0073] As used herein, the terms "comprising," "comprises," and "comprised of" are synonymous with "including," "includes," or "containing," and are inclusive or open-ended and do not exclude additional, unrecited members, elements, or method steps. It will be understood that the term "comprising," as used herein, includes the terms "consisting of," "consists of," and "consists of."

[0074] As used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. For example, "a step" means one step or more than one step.

[0075] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art.

[0076] Numerical ranges expressed by endpoints include all integers and, where appropriate, fractions contained within the range (e.g., 1 to 5 when referring to, for example, the number of elements, may include 1, 2, 3, and 4, and when referring to, for example, measurements, may also include 1.5, 2, 2.75, and 3.80). The recitation of endpoints also includes the endpoint values ​​themselves (e.g., 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all subranges contained therein.

[0077] As used herein, the term "about" when referring to a measurable value, such as a parameter, amount, duration, etc., is intended to encompass variations of the specified value, including + / - 10% or less, preferably + / - 5% or less, and more preferably + / - 1% or less, from the specified value, provided such variations are suitable for use in the disclosed invention. It is understood that the value to which the modifier "about" refers is itself also specifically and preferably disclosed.

[0078] The terms "wt%", "vol%" or "mol%" refer to the weight percent, volume percent or mole percent of a component, respectively, based on the total weight, total volume or total moles of the material including the component.

[0079] The terms "isomer purity" and "stereochemical purity" are used interchangeably herein and are expressed as weight % and refer to the amount of the recited stereoisomer expressed as a percentage of the total amount of stereoisomers having a given chiral center.

[0080] When describing the present invention, the terms used should be interpreted according to the following definitions, unless the context requires otherwise.

[0081] Preferred statements (features) and embodiments and uses of the present invention are set forth herein below. Each statement and embodiment of the present invention so defined may be combined with any other statement and / or embodiment, unless expressly indicated to the contrary. In particular, any feature indicated as preferred or advantageous may be combined with any other feature or features or statements indicated as preferred or advantageous. To this end, the present invention is particularly captured by any one or one or more of the following numbered statements and embodiments in any combination with any other statement and / or embodiment.

[0082] 1. A process for producing polymeso-lactide (PML), comprising the following steps:

[0083] a) forming lactic acid oligomers by polycondensing lactic acid;

[0084] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0085] c) subjecting the crude lactide to purification, thereby separating a lactide-rich stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0086] d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg; and

[0087] e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide.

[0088] 2. A process for producing polymeso-lactide (PML), comprising the following steps:

[0089] a) forming lactic acid oligomers by polycondensing lactic acid;

[0090] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0091] c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0092] d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg; and

[0093] e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide.

[0094] 3. A process according to statement 1 or 2, wherein the crude lactide formed in step b) comprises L-lactide and D-lactide, and wherein the weight ratio of L-lactide to D-lactide, L / D ratio, is different from 1.0, and is preferably higher than 1.0.

[0095] 4. A process according to any preceding statement, wherein the meso-lactide enriched stream is purified to an acidity level of at most 15.0 meq / kg, such as at most 10.0 meq / kg, or at most 7.0 meq / kg, such as between 0.001 and 7.0 meq / kg.

[0096] 5. The process according to any of the preceding statements, wherein the meso-lactide enriched stream is purified to an acidity level equal to or higher than 1.0 meq / kg, preferably equal to or higher than 1.5 meq / kg.

[0097] 6. A process according to any of the preceding statements, wherein the meso-lactide-enriched stream is purified to a purified meso-lactide stream comprising an acidity level between 1.0 and 20.0 meq / kg, or between 1.5 and 17.5 meq / kg, or between 2.0 and 15.0 meq / kg.

[0098] 7. The process according to any of the preceding statements, wherein the amount of the portion of the purified meso-lactide stream that is polymerized in step e) amounts to at least 30.0 wt.-%, or at least 40.0 wt.-%, or at least 50.0 wt.-%, or at least 60.0 wt.-%, or at least 70.0 wt.-%, or at least 80.0 wt.-%, or at least 90.0 wt.-% of the purified meso-lactide stream.

[0099] 8. The process according to any of the preceding statements, further comprising the step f) of blending at least a portion of the purified meso-lactide stream not polymerized in step e) with at least a portion of the lactide-rich stream, preferably the L-lactide-rich stream, obtained in step c) to provide a lactide mixture, and subsequently polymerizing the lactide mixture to form polylactide, wherein the polymerized meso-lactide content is less than 20.0 wt. % of the polylactide.

[0100] 9. The process according to any of the preceding statements, wherein the meso-lactide content in the crude lactide is between 4.0 and 30.0 wt.-%, or between 6.0 and 30.0 wt.-%, or between 7.0 and 25.0 wt.-%, or between 8.0 and 20.0 wt.-%, based on the total weight of the crude lactide.

[0101] 10. The process according to any of the preceding statements, wherein the purification in step c) is carried out by means of distillation.

[0102] 11. A process according to any of the preceding statements, wherein the crude lactide is purified by subjecting the crude lactide to distillation, preferably fractional distillation, thereby separating a lactide-rich stream, preferably an L-lactide-rich stream, preferably an L-lactide-rich stream as defined herein, and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.-% meso-lactide, based on the total weight of the meso-lactide-enriched stream, and has an acidity level of 30.0 to 1000.0 meq.

[0103] 12. The process according to any one of the preceding statements, wherein the crude lactide is purified by:

[0104] - subjecting the crude lactide to a pretreatment, preferably crystallization, thereby obtaining a mother liquor comprising meso-lactide, and L-lactide and / or D-lactide, and

[0105] - subjecting the mother liquor to distillation, preferably fractional distillation, thereby separating a lactide-rich stream, preferably an L-lactide-rich stream, preferably an L-lactide-rich stream as defined herein; from the meso-lactide-enriched stream comprising at least 70.0 wt.-% meso-lactide, based on the total weight of the meso-lactide-enriched stream, and having an acidity level of 30.0 to 1000.0 meq / kg.

[0106] 13. The process according to any of the preceding statements, wherein the crystallization is solvent crystallization or melt crystallization, wherein the solvent crystallization or melt crystallization is performed as suspension crystallization or layer crystallization.

[0107] 14. A process according to any one of the preceding statements, wherein the crystallization is melt crystallization, and preferably wherein the crystallization is performed using a static melt crystallizer.

[0108] 15. The process according to any of the preceding statements, wherein the polymeso-lactide is further processed into an article, and preferably an article selected from the group consisting of: a molded article, a film, a sheet, a fiber, a filament, a fabric, a spunmelt nonwoven, and a layer of a multilayer article.

[0109] 16. Process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of the L-isomer of 80.0% or more, such as 85.0% or more.

[0110] 17. Process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of less than 95.0% or less than 90.0% of the L-isomer.

[0111] 18. Process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of 80.0% or more, such as 85.0% or more, of the D-isomer.

[0112] 19. Process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of less than 95.0% or less than 90.0% of the D-isomer.

[0113] 20. The process according to any of the preceding statements, wherein part of the lactic acid applied in step a) is racemic lactic acid, preferably wherein the part is at most 40.0 wt.-% racemic lactic acid, or at most 30.0 wt.-% racemic lactic acid, or at most 20.0 wt.-% racemic lactic acid, or at most 10.0 wt.-% racemic lactic acid, based on the total amount of lactic acid applied in step a).

[0114] 21. Process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) is prepared by depolymerizing poly-L-lactic acid and / or poly-D-lactic acid.

[0115] 22. A process according to any of the preceding statements, wherein at least part of the lactic acid applied in step a) is prepared by depolymerizing poly-L-lactic acid containing less than 45 wt%, or less than 30 wt%, or less than 20 wt%, or less than 15 wt%, or less than 10 wt% of D-isomer.

[0116] 23. The process according to any one of the preceding statements 21 or 22, wherein the poly-L-lactic acid and / or poly-D-lactic acid is depolymerized by hydrolyzing the poly-L-lactic acid and / or poly-D-lactic acid in the presence of water and / or lactic acid as co-reactants, preferably for 30 minutes to 24 hours, under conditions ranging from atmospheric conditions up to 10 bar and at a temperature ranging from 120 to 200°C.

[0117] 24. Process according to any of the preceding statements, wherein lactic acid is polycondensed to lactic acid oligomers at a temperature between 100 and 200° C., for example between 110 and 175° C., under conditions of 10 to 500 mbar and preferably during 1 to 24 hours.

[0118] 25. The process according to any one of the preceding statements, wherein the lactic acid oligomers prepared in step a) have a degree of polymerization (DP) of 5 to 30.

[0119] 26. The process according to any of the preceding statements, wherein the crude lactide comprises at least 4.0 wt%, or at least 6.0 wt%, or at least 7.0 wt%, or at least 8.0 wt%, or at least 12.0 wt%, or at least 15.0 wt%, or at least 20.0 wt%, or at least 30.0 wt%, or at least 35.0 wt% meso-lactide, based on the total weight of the crude lactide.

[0120] 27. A process according to any preceding statement, wherein the crude lactide comprises up to 40.0 wt%, or up to 35.0 wt%, or up to 30.0 wt% meso-lactide, based on the total weight of the crude lactide.

[0121] 28. A process according to any preceding statement, wherein the crude lactide comprises at most 93.0 wt% L-lactide, or at most 90.0 wt%, or at most 80.0 wt%, or at most 60.0 wt% L-lactide, based on the total weight of the crude lactide.

[0122] 29. The process according to any of the preceding statements, wherein the depolymerization in step b) is carried out at a temperature between 175 and 220°C.

[0123] 30. Process according to any of the preceding statements, wherein in step b) the depolymerization is carried out in the presence of a metal catalyst, preferably a metal catalyst selected from tin oxide, tin(II) 2-ethylhexanoate, titanium tetrabutoxide and titanium isopropoxide.

[0124] 31. A process according to any of the preceding statements, wherein the amount of meso-lactide in the crude lactide is upregulated by stimulating racemization during steps a) and / or b).

[0125] 32. The process according to any of the preceding statements, wherein racemization during steps a) and / or b) is stimulated by increasing the temperature during steps a) and / or b).

[0126] 33. Process according to any of the preceding statements, wherein racemization during step a) and / or b) is stimulated by adding a racemizer during step a) and / or b), preferably in an amount in the range of 5-5000 ppm, preferably said racemizer is selected from the group comprising sodium salts such as sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen containing ligands such as pyridine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene and 1,8-diazabicyclo[5.4.0]undec-7-ene, or any combination thereof.

[0127] 34. The process according to any of the preceding statements, wherein the lactide-rich stream is an L-lactide-rich stream, preferably wherein the L-lactide-rich stream comprises at least 50.0 wt.% L-lactide, preferably at least 60.0 wt.% L-lactide, preferably at least 65.0 wt.% L-lactide, preferably at least 70.0 wt.% L-lactide, preferably at least 75.0 wt.% L-lactide, preferably at least 80.0 wt.% L-lactide, preferably at least 85.0 wt.% L-lactide, preferably at least 90.0 wt.% L-lactide, preferably at least 95.0 wt.% L-lactide, wherein the wt.% are based on the total weight of the lactide-rich stream.

[0128] 35. The process according to any of the preceding statements, wherein the purified meso-lactide stream comprises at least 95.0 wt% meso-lactide, preferably at least 97.0 wt% meso-lactide, wherein the wt% are based on the total weight of the purified meso-lactide stream.

[0129] 36. A process according to any of the preceding statements, wherein the purified meso-lactide stream comprises less than 8.0 wt% L-lactide, e.g., less than 6.0 wt%, or less than 5.0 wt% L-lactide, preferably less than 4.0 wt% L-lactide, more preferably less than 2.5 wt% L-lactide, wherein the wt% are based on the total weight of the purified meso-lactide stream.

[0130] 37. A process according to any of the preceding statements, wherein the purified meso-lactide stream comprises less than 8.0 wt. % D-lactide, e.g., less than 6.0 wt. %, or less than 5.0 wt. % D-lactide, preferably less than 4.0 wt. % D-lactide, more preferably less than 2.5 wt. % D-lactide, wherein the wt. % are based on the total weight of the purified meso-lactide stream.

[0131] 38. Process according to any of the preceding statements, wherein the enriched meso-lactide stream obtained in step c) is purified by crystallization, preferably by melt crystallization, to an acidity level of at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, e.g. between 0.001 meq / kg and 7.0 meq / kg.

[0132] 39. Process according to any of the preceding statements, wherein the enriched meso-lactide stream obtained in step c) is purified by crystallization, preferably by melt crystallization, to an acidity level of at least 1.0 meq / kg, preferably at least 1.5 meq / kg.

[0133] 40. Process according to any of the preceding statements, wherein the enriched meso-lactide obtained in step c) is purified by passing through a multi-stage melt crystallization, in particular to an acidity level of at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, e.g. between 0.001 meq / kg and 7.0 meq / kg.

[0134] 41. Process according to any of the preceding statements, wherein the enriched meso-lactide obtained in step c) is purified by passing through a multi-stage melt crystallization, in particular to an acidity level of at least 1.0 meq / kg, preferably at least 1.5 meq / kg.

[0135] 42. A process according to any of the preceding statements, wherein the polymerization of a portion of the purified meso-lactide stream to polymeso-lactide is carried out by means of ring-opening polymerization in the presence of a catalyst, preferably a metal catalyst as defined in statement 30, and optionally one or more initiators at a temperature comprised between 125 and 225°C, preferably wherein the initiator is an alcohol.

[0136] 43. The process according to any of the preceding statements, wherein the ring-opening polymerization is carried out continuously with a residence time of between 30 minutes and 300 minutes.

[0137] 44. Process according to any of the preceding statements, wherein the ring-opening polymerization is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, a phosphate ester or a polyacrylic acid (co)polymer.

[0138] 45. Process according to any of the preceding statements, further comprising the step of demonomerizing the polymeso-lactide by means of evaporation, preferably at a pressure below 50 mbar and at a temperature above 175°C, for example in a flash tank, a degassing extruder or a wiped film evaporator.

[0139] 46. ​​Process according to any of the preceding statements, wherein in step f) of the process the lactide mixture is polymerized to form polylactide, wherein the polymerized meso-lactide content is below 20.0 wt. % of the polylactide, also by means of ring-opening polymerization in the presence of a catalyst, preferably a metal catalyst and optionally one or more initiators at a temperature comprised between 125 and 225° C., preferably wherein the initiator is an alcohol.

[0140] 47. A process according to statement 46, wherein the ring-opening polymerization is carried out continuously with a residence time between 30 minutes and 300 minutes.

[0141] 48. A process according to any of the preceding statements 46-47, wherein the ring-opening polymerization is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, a phosphate ester or a polyacrylic acid (co)polymer.

[0142] 49. Polymeso-lactide obtainable according to or according to a process according to any one of the preceding statements 1 to 48, and preferably wherein the polymeso-lactide has one or more of the following properties:

[0143] - 40.0 to 60.0% stereochemical purity of the L-lactate, or 40.0 to 60.0% stereochemical purity of the D-lactate,

[0144] - a weight average molecular weight (Mw) between 50 and 500 kg / mol, or between 100 and 350 kg / mol,

[0145] - a glass transition temperature between 30°C and 50°C, or between 35°C and 50°C.

[0146] 50. Use of the polymeso-lactide according to statement 49 above or obtainable according to or by the process according to any one of statements 1 to 48 above for the preparation of an article, preferably an article selected from the group consisting of: a molded article, a film, a sheet, a spunmelt nonwoven, a fiber, a filament, a fabric, and a layer of a multilayer article.

[0147] 51. Use of a polymeso-lactide according to statement 49 or obtainable according to or by a process according to any one of the preceding statements 1 to 48 as a resin for (paper) coatings or as an adhesive resin, for example for the production of adhesive films or adhesive layers of multilayer structures.

[0148] 52. Use of polymeso-lactide according to statement 49, or obtainable according to or according to the process of any one of the preceding statements 1 to 48, for the preparation of a polymer composition, wherein the polymer composition comprises polylactide and polymeso-lactide, and wherein the polymer composition comprises less than 10% by weight of the polymeso-lactide, based on the total weight of the polymer composition.

[0149] 53. Use of polymeso-lactide according to statement 49 or obtainable according to the process according to any one of the preceding statements 1 to 48 for the preparation of a degradable article or a degradable composition, preferably a solid degradable article or a degradable composition.

[0150] 54. Use of polymeso-lactide according to statement 49, or obtainable according to or by a process according to any one of the preceding statements 1 to 48, for the preparation of a slow-release preparation or composition, preferably wherein the preparation or composition is in solid form and is capable of slowly releasing lactic acid.

[0151] 55. Use of polymeso-lactide according to statement 49 or obtainable according to or by a process according to any one of the preceding statements 1 to 48 as a diverting agent, for example in downhole applications or in fracturing applications.

[0152] 56. A polylactide obtainable by or by carrying out a process according to any one of the preceding statements 1 to 48, and preferably wherein the polylactide comprises polymerized meso-lactide in an amount less than 20.0 wt. % of the polylactide, or less than 15.0 wt. % of the polylactide, or less than 10.0 wt. % of the polylactide.

[0153] 57. A polymer composition comprising (A) polylactide, and (B) polymeso-lactide according to statement 49, or obtainable according to or according to any one of the preceding statements 1 to 48, wherein the polymer composition comprises less than 10.0 wt. % of the polymeso-lactide, based on the total weight of the polymer composition.

[0154] 58. An article prepared with polymeso-lactide according to statement 49 or obtainable according to or according to a process according to any one of the preceding statements 1 to 48, preferably wherein the article is selected from the group consisting of a molded article, a film, a film layer, a tape, a filament, a fiber, a sheet, a fabric, and a spunmelt nonwoven.

[0155] 59. An article prepared with a polylactide according to statement 56 or obtainable according to or according to a process according to any one of the preceding statements 1 to 48, preferably wherein the article is selected from the group consisting of a molded article, a film, a film layer, a tape, a monocomponent or multicomponent filament, a fiber, a sheet, a fabric, a coated paper, a coated textile and a spunmelt nonwoven.

[0156] 60. An article prepared with a polymer composition according to statement 57, preferably wherein the article is selected from the group consisting of a molded article, a film, a film layer, a tape, a monocomponent or multicomponent filament, a fiber, a sheet, a fabric, a coated paper, a coated textile, and a spunmelt nonwoven.

[0157] Polylactide resins are manufactured industrially by converting lactic acid into lactide, which is then polymerized. Two molecules of lactic acid can condense, eliminating two molecules of water to form 3,6-dimethyl-1,4-dioxane-2,5-dione, commonly referred to as "lactide." Lactide molecules can take one of three forms: 3S,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (S,S-lactide or L-lactide), 3R,6R-3,6-dimethyl-1,4-dioxane-2,5-dione (R,R-lactide or D-lactide), and 3R,6S-dimethyl-1,4-dioxane-2,5-dione (R,S-lactide or meso-lactide). L-lactide and D-lactide are a pair of enantiomers, while meso-lactide is a stereoisomer. Meso-lactide is often considered a less valuable by-product.

[0158] The present inventors have discovered a method for increasing the value of meso-lactide and product streams comprising meso-lactide. Furthermore, the present invention provides a process for polymerizing meso-lactide to form polymeso-lactide. In preferred embodiments of the present invention, processes for preparing and polymerizing lactide mixtures comprising meso-lactide are also provided. Furthermore, the present invention provides processes for polymerizing meso-lactide and / or for preparing and polymerizing lactide mixtures comprising meso-lactide, which can be integrated into a PLA production plant. The processes described herein can improve the overall yield of a production process for producing PLA, and the production of PLA products can be optimized and steered as desired.

[0159] In some preferred embodiments of the present invention, a process for producing polymeso-lactide (PML) is provided, wherein the process comprises the following steps:

[0160] a) forming lactic acid oligomers by polycondensing lactic acid;

[0161] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0162] c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0163] d) purifying the meso-lactide-enriched stream by subjecting it to crystallization to form a purified meso-lactide stream, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg, and preferably at most 15.0 meq / kg, or at most 10.0 meq / kg, or at most 7.0 meq / kg, e.g., between 0.001 meq / kg and 7.0 meq / kg;

[0164] e) polymerizing at least a portion of the purified meso-lactide stream, and preferably at least 30.0 wt.%, or at least 40.0 wt.%, or at least 50.0 wt.%, or at least 60.0 wt.%, or at least 70.0 wt.%, or at least 80.0 wt.%, or at least 90.0 wt.% of the purified meso-lactide stream, to form polymeso-lactide, and

[0165] f) blending at least a portion of the purified meso-lactide stream not polymerized in step e) with at least a portion of the lactide-rich stream obtained in step c) to provide a lactide mixture, and subsequently polymerizing the lactide mixture to form a polylactide having polymerized meso-lactide, preferably having a content of polymerized meso-lactide of less than 20.0 wt. %, or less than 15.0 wt. %, or less than 10.0 wt. % of the polylactide.

[0166] In some preferred embodiments of the present invention, there is provided a process for producing polymeso-lactide (PML), comprising the steps of:

[0167] a) forming lactic acid oligomers by polycondensing lactic acid;

[0168] b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide;

[0169] c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt% meso-lactide, wherein the wt% is based on the total weight of the meso-lactide-enriched stream, and has an acidity level of 30.0 to 1000.0 meq / kg;

[0170] d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, wherein the wt% is based on the total weight of the purified meso-lactide stream and having an acidity level of at most 20.0 meq / kg; and

[0171] e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide.

[0172] The terms "stream", "product stream" and the like are used interchangeably herein. Preferably, the acidity level of the stream obtained by carrying out the present process and its steps is determined using the method as described in the Examples section.

[0173] The process of the present invention involves polymerizing at least a portion of the purified meso-lactide to form polymeso-lactide. To obtain purified meso-lactide, the process comprises the steps of subjecting crude lactide having a defined composition and meso-lactide content to purification to separate a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, from a meso-lactide-enriched stream, the latter having a relatively high meso-lactide content of at least 70% by weight, based on the separated stream.

[0174] In some embodiments, the process involves separating a meso-lactide-rich stream directly from crude lactide. This is achieved by subjecting the crude lactide directly to distillation, such as fractional distillation, to separate a (L-)-lactide-rich stream from a meso-lactide-rich stream as defined herein.

[0175] In some other embodiments, the process involves indirectly separating a meso-lactide-enriched stream from crude lactide. This can be accomplished by first subjecting the crude lactide, as defined herein, to a pretreatment, such as a crystallization step, to produce lactide-enriched crystals, preferably L-lactide-enriched crystals, and a mother liquor comprising the remaining lactide (L-lactide and / or D-lactide) and meso-lactide. The meso-lactide content of such a mother liquor is below 70% by weight, where the % by weight is based on the total weight of the mother liquor, and thus, the mother liquor can subsequently be subjected to further purification, such as by distillation, thereby allowing separation of a (L-)-lactide-enriched stream and a meso-lactide-enriched stream, both as defined herein.

[0176] The meso-lactide-enriched stream is then further purified separately from a separated lactide-rich stream, preferably an L-lactide-enriched stream, by means of crystallization to produce a purified meso-lactide stream of high meso-lactide content and high purity (i.e., low acidity level) that is particularly suitable for polymerization into polymeso-lactide.

[0177] In the following, the process steps of the present invention, the starting materials used therein and the particularities and properties of the resulting polymers will be discussed in more detail.

[0178] The first step (step a) in the process for preparing polymeso-lactide (PML) according to the present invention involves forming lactic acid oligomers by polycondensing lactic acid. Lactic acid is a molecule with one chiral center and therefore exists in two enantiomeric forms, the so-called R- (or D-) enantiomer and the S- (or L-) enantiomer. A mixture of equal amounts of the two enantiomeric forms is called DL-lactic acid or racemic lactic acid.

[0179] In processes involving the conversion of lactic acid to lactide, which is then polymerized to produce PLA, racemization (i.e., the conversion of one enantiomeric form to the other) occurs during prepolymerization and conversion to crude lactide. Because of this racemization, the lactide obtained from this process will be a mixture of L-lactide, D-lactide, and meso-lactide. It is known that the more purified the starting material (lactic acid) used in this process, the less racemization may occur.

[0180] Contrary to what would be expected from the prior art, the present process is not limited to a specific enantiomeric form or purity of lactic acid, nor is it limited to the use of enantiomeric forms of lactic acid with high isomeric (optical) purity as starting materials. The present process can advantageously be applied starting from any enantiomeric form of lactic acid, or from mixtures thereof (including racemic mixtures thereof), and from lactic acid enantiomers with relatively low isomeric purity. In other words, racemization occurring during some steps of the present process is not limiting or detrimental to the process of the present invention.

[0181] According to some embodiments of the present process, at least a portion of the lactic acid applied in step a) can, for example, have an isomeric purity of the L-isomer of 80.0% or more, such as 85.0% or more. In some embodiments of the present process, at least a portion of the lactic acid applied in step a) can have an isomeric purity of the L-isomer of less than 95.0% or less than 90.0%.

[0182] Likewise, according to some embodiments of the present process, at least a portion of the lactic acid applied in step a) can, for example, have an isomeric purity of the D-isomer of 80.0% or more, such as 85.0% or more. In some embodiments of the present process, at least a portion of the lactic acid applied in step a) has an isomeric purity of the D-isomer of less than 95.0% or less than 90.0%.

[0183] According to some embodiments of the present process, part of the lactic acid applied in step a) may also be racemic lactic acid.

[0184] In some embodiments, the present process provides a portion, preferably a minor portion, of the lactic acid provided in step a) as racemic lactic acid (i.e., DL-lactic acid). Thus, the present process can allow starting with lactic acid of lower isomer purity to increase the amount of meso-lactide formed in downstream process steps and, thus, produce more polymeso-lactide.

[0185] In some embodiments in which DL-lactic acid (i.e., racemic lactic acid) is used in step a) of the processes described herein, it is preferred that the amount of DL-lactic acid does not exceed the upper limit, based on the total amount of lactic acid in step a). Preferably, the portion of the lactic acid applied in step a) that is racemic lactic acid is at most 40.0% by weight of racemic lactic acid, or at most 35.0% by weight of racemic lactic acid, or at most 30.0% by weight of racemic lactic acid, or at most 25.0% by weight of racemic lactic acid, or at most 20.0% by weight of racemic lactic acid, or at most 15.0% by weight of racemic lactic acid, or at most 10.0% by weight of racemic lactic acid, wherein the wt% is based on the total amount of lactic acid applied in step a). In one example, the amount of DL-lactic acid present in the lactic acid applied in step a) may be between 0.001 and 15.0% by weight. Therefore, the process of the present invention allows the use of a certain amount of racemic acid (ie DL-lactic acid) and therefore the starting material has a lower isomeric purity.

[0186] In some preferred embodiments, lactic acid as used herein may include a mixture of unequal amounts of two enantiomeric forms, such as, but not limited to, a mixture of 40.0 wt% D-enantiomer and 60.0 wt% L-enantiomer, or a mixture of 20.0 wt% D-enantiomer and 80.0 wt% L-enantiomer, or a mixture of 10.0 wt% D-enantiomer and 90.0 wt% L-enantiomer.

[0187] The lactic acid used as starting material in the process of the invention can also be obtained starting from polymers of lactic acid, ie poly(lactic acid) or PLA.

[0188] In some embodiments of the process of the present invention, at least a portion of the lactic acid applied in step a) of the process can also be prepared by depolymerizing poly-L-lactic acid (PLLA) and / or poly-D-lactic acid (PDLA). In some preferred embodiments, poly-L-lactic acid containing up to 50.0 wt %, such as up to 20.0 wt % or up to 15.0 wt % of the D-isomer, and depolymerization is used to produce the lactic acid used in step a) of the process. In some other embodiments, it is preferred that poly-L-lactic acid containing less than 45 wt %, or less than 30 wt %, or less than 25 wt %, or less than 15 wt %, or less than 10 wt % of the D-isomer is depolymerized. Thus, the process of the present invention allows the use of starting materials of PLLA or PDLA that contain relatively large amounts of D and L isomers, respectively, and therefore the starting materials have a lower isomeric purity.

[0189] In addition, the poly-L-lactic acid and / or poly-D-lactic acid as defined above herein can be depolymerized under conditions that allow obtaining lactic acid of a lower isomeric purity than would be acceptable and desirable in conventional industrial processes. For example, in some embodiments of the present process, the poly-L-lactic acid and / or poly-D-lactic acid can be depolymerized by hydrolyzing the poly-L-lactic acid and / or poly-D-lactic acid in the presence of water and / or lactic acid as a co-reactant, preferably for 30 minutes to 24 hours, under conditions ranging from atmospheric conditions up to 10 bar and at a temperature ranging from 120 to 200° C., preferably 130 to 200° C.

[0190] The present process advantageously allows starting with lactic acid of lower isomeric purity to increase the amount of meso-lactide formed in downstream process steps, thereby producing more polymeso-lactide. Furthermore, the present process also allows for the manipulation and control of the amount of meso-lactide and / or mixtures comprising lactide and meso-lactide formed in downstream process steps (to a greater or lesser extent as desired) by adjusting the isomeric purity of the starting lactic acid material and / or by starting with lower quality / purity PLA material and / or PLA material that can be hydrolyzed under less stringent conditions (e.g., lower depolymerization temperatures), as described above. It is generally understood that most PLA applications typically use PLA resins with a stereochemical purity of <98%, meaning that recycled lactic acid generally has a lower purity than high-purity lactic acid obtained from any fermentation process. The present invention also allows for the use of such lower stereochemically pure lactic acid as a starting material, as it can handle increased amounts of meso-lactide.

[0191] Step a) of the present process involves polycondensing lactic acid into lactic acid oligomers. Preferably, the lactic acid is polycondensed into lactic acid oligomers at a temperature between 100 and 200°C, for example, between 110 and 175°C, under conditions of 10 to 500 mbar. Preferably, the polycondensation is carried out for 1 to 24 hours. As used herein, the term "lactic acid oligomer" refers to low molecular weight polylactic acid.

[0192] Preferably, the lactic acid oligomer prepared in step a) has a degree of polymerization (DP) of 5 to 30. "Degree of polymerization (DP)" refers to the number of lactic acid monomer units in the oligomer according to the present invention. As used herein, "degree of polymerization" (DP) is calculated by taking the reciprocal of the value of free acid in wt%. The free acid content can be measured as shown in the method section given below.

[0193] Crude lactide is formed by depolymerizing lactic acid oligomers by subjecting them to elevated temperatures and sub-atmospheric pressures, typically in the presence of a depolymerization catalyst. The crude lactide formed in the depolymerization step contains a mixture of L-lactide, meso-lactide, and D-lactide. It often also contains various types of impurities, such as residual water, some lactic acid, some linear oligomers of lactic acid, and often other reaction byproducts or those derived from the fermentation process.

[0194] According to the present process, the meso-lactide content is between 2.0 and 40.0 wt %, based on the total weight of the crude lactide in the crude lactide. For example, in some embodiments of the present process, the crude lactide comprises at least 4.0 wt %, or at least 6.0 wt %, or at least 7.0 wt %, or at least 8.0 wt %, or at least 12.0 wt %, or at least 15.0 wt %, or at least 20.0 wt %, or at least 30.0 wt %, or at least 35.0 wt %, based on the total weight of the crude lactide. In some embodiments of the present process, the crude lactide comprises up to 40.0 wt %, or up to 35.0 wt %, or up to 30.0 wt %, based on the total weight of the crude lactide. In some embodiments, the meso-lactide content in the crude lactide is between 4.0 and 30.0 weight percent, or between 6.0 and 30.0 weight percent, or between 7.0 and 25.0 weight percent, or between 8.0 and 20.0 weight percent, based on the total weight of the crude lactide.

[0195] In some preferred embodiments of the present process, the crude lactide produced during step b) includes different amounts (i.e., weight %) of L-lactide and D-lactide. In some embodiments, it is preferred that the weight ratio of L-lactide to D-lactide in the crude lactide (expressed herein as "L / D ratio" or "L / D weight ratio") is different from 1.0.

[0196] In some preferred embodiments of this process, the crude lactide obtained comprises L-lactide and D-lactide at an L / D ratio greater than 1.0, preferably greater than 1.5, and more preferably greater than 2.0. For example, the L / D ratio may be greater than 3.0, or greater than 5.0, or greater than 8.0, or greater than 10.0, or greater than 12.0. A relatively high content of L-lactide in the crude lactide offers the following advantages: subsequent purification of the crude lactide in a separate product stream and crystallization of the separated product stream can be performed more efficiently, providing higher product yields, thereby leading to a more cost-effective and improved process. Therefore, a higher content of L-lactide in the crude lactide allows for efficient downstream separation. For example, an L-lactide content of 90% by weight or greater in the crude lactide allows for efficient and high-yield (downstream) crystallization of L-lactide. Furthermore, commercial PLA production typically requires a high L-lactide content, for example, 80% by weight or greater.

[0197] In some preferred embodiments of the present process, the amount of L-lactide in the crude lactide is at least 50.0 wt %, for example, at least 55.0 wt %, preferably at least 60.0 wt %, preferably at least 65.0 wt %, or preferably at least 70.0 wt %, where the wt % is based on the total weight of the crude lactide. It is also preferred that the crude lactide contains less than 40.0 wt %, preferably less than 30.0 wt %, preferably less than 20.0 wt %, or preferably less than 10.0 wt % of D-lactide, where the wt % is based on the total weight of the crude lactide.

[0198] In one non-limiting exemplary embodiment of the present process, crude lactide may include:

[0199] - between 50.0 and 97.5 wt. % L-lactide, for example between 65.0 and 97.0 wt. %, wherein the wt. % are based on the total weight of the crude lactide, and

[0200] - between 0.1 and 20.0 wt. % D-lactide; preferably between 0.5 and 10.0 wt. % D-lactide, wherein the wt. % are based on the total weight of the crude lactide, and

[0201] - between 2.0 and 40.0 wt% meso-lactide, for example between 5.0 and 30.0 wt% meso-lactide, wherein the wt% are based on the total weight of the crude lactide.

[0202] Preferably, the depolymerization in step b) is carried out at a temperature between 175° C. and 220° C. In some preferred embodiments of the process, the depolymerization in step b) is carried out in the presence of a metal catalyst, preferably a metal catalyst selected from tin oxide, tin(II) 2-ethylhexanoate, titanium tetrabutoxide and titanium isopropoxide.

[0203] In some preferred embodiments of the present process, the amount of meso-lactide in the crude lactide can be increased by stimulating racemization during steps a) and / or b) of the process. To obtain more meso-lactide in the crude lactide, increasing racemization during steps a) and / or b) can be stimulated by various means, such as by selecting starting materials of lower isomer purity and / or by adjusting the polycondensation and / or depolymerization conditions during these steps, as described herein. For example, in some embodiments of the present process, racemization during step a) is stimulated by increasing the reaction time above 180°C, and / or racemization during step b) is stimulated by increasing the temperature during steps a) and / or b) to above 200°C.

[0204] Advantageously, the present process allows for more relaxed depolymerization conditions than those typically employed in the art, for example by allowing for longer residence times and / or higher temperatures than typically employed in the art. Furthermore, in some embodiments, polycondensation depolymerization conditions can even be manipulated to achieve a certain degree of racemization, thereby yielding more or less meso-lactide as desired.

[0205] Alternatively or additionally, in some embodiments of the present process, racemization during steps a) and / or b) is stimulated by adding a racemizing agent during these steps a) and / or b). Preferably, such a racemizing agent is selected from the group consisting of sodium salts such as sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen-containing ligands such as pyridine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene and 1,8-diazabicyclo[5.4.0]undec-7-ene, and any combination thereof. Preferably, when added, such a racemizing agent is added in a range of 5-5000 ppm.

[0206] The crude lactide produced during step b) of the process of the present invention contains mainly L- and D-lactide, meso-lactide, but it often also contains various types of impurities, such as residual water, some lactic acid, some linear oligomers of lactic acid and usually some other reaction by-products.

[0207] According to step c) of the present process, the crude lactide is subjected to purification, thereby separating a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, and a meso-lactide-rich stream as defined herein.

[0208] According to the process of the present invention, crude lactide is separated (purified) into separate product streams (fractions), including a meso-lactide-enriched stream, and the separated and meso-lactide-enriched stream is then subjected to crystallization. Step c) of the present process has the advantageous effect of providing a sufficiently concentrated (enriched) meso-lactide stream, which advantageously allows meso-lactide to be crystallized separately in sufficient purity and yield in a subsequent step. Advantageously, the relative amount and acidity level of meso-lactide (i.e., the remaining free acid impurity) present in the meso-lactide-enriched stream, as defined herein and obtained in step c), facilitates purification via crystallization in the downstream step d).

[0209] Advantageously, the present process provides for the efficient separation of crude lactide comprising different lactide stereoisomers having (very) similar chemical properties into separate product streams that can be further efficiently purified. In particular, the present process has been found to be effective for separating crude lactide into an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg.

[0210] In the context of the present process, it will be understood that the crude lactide can be separated directly or indirectly into a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, and a meso-lactide-enriched stream as defined herein.

[0211] In some embodiments, the process involves directly separating a meso-lactide-rich stream and a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, from the crude lactide. This can be achieved by subjecting the crude lactide directly to distillation, thereby separating the (L-)lactide-rich stream from the meso-lactide-rich stream as defined herein. In this context, the term "direct separation" or "direct purification" means directly treating the crude lactide to separate fractions or product streams, i.e., without intermediate purification or processing steps after depolymerizing the lactic acid oligomers to form the crude lactide. Direct purification, for example by distillation, such as fractionation, applied directly to the crude lactide results in a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, separated from the meso-lactide-rich stream as defined herein.

[0212] Thus, in some embodiments, the process involves a step of purifying the crude lactide by subjecting it to distillation, preferably fractional distillation, thereby separating a lactide-rich stream, preferably an L-lactide-rich stream, preferably an L-lactide-rich stream as defined herein, and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt% meso-lactide and has an acidity level of 30.0 to 1000.0 meq.

[0213] A preferred method for purifying crude lactide is to fractionate the crude lactide stream in one or more steps. In some embodiments, it is preferred to remove more volatile impurities, such as water and lactic acid, from the crude lactide stream before separating the meso-lactide-enriched stream from the lactide-rich stream, preferably the L-lactide-enriched stream as defined herein, particularly when meso-lactide is removed by fractional distillation. One approach can be to distill off some or all impurities that are significantly more volatile than meso-lactide, such as water, residual lactic acid, and other small organic compounds. Such a distillation step can be performed before or simultaneously with the fractionation step in which the meso-lactide-enriched stream is separated from the lactide-rich stream.

[0214] As used herein, the term "fractionation" refers to a process step used to separate the components of a liquid mixture (e.g., the crude lactide produced in step b) of this process) based on their different boiling points. This technique involves heating the mixture to generate vapor, and then cooling it to condense the vapor back into a liquid form. Advantageously, the evaporation and condensation steps can be repeated multiple times in a fractionating column, allowing for the separation of components with similar boiling points. The result is the collection of different fractions, each enriched in a specific component of the original mixture.

[0215] In some other embodiments, the process involves indirectly separating a meso-lactide-rich stream and a lactide-rich stream, preferably an L-lactide-rich stream, as defined herein, from the crude lactide. The terms "indirect separation" and "indirect purification" are used interchangeably herein and refer to processing the crude lactide in more than one step, for example, with one or more intermediate purification or treatment steps of the crude lactide. Such additional purification or treatment steps are performed after depolymerizing the lactic acid oligomers to form the crude lactide. For example, the crude lactide obtained in step b) can be subjected to crystallization to produce lactide-rich crystals, preferably L-lactide-rich crystals, and a mother liquor comprising residual L-lactide and / or D-lactide, and meso-lactide. Such a mother liquor contains less than 70% by weight of meso-lactide. Subsequently, the mother liquor obtained is distilled to separate a stream rich in (L-)lactide from a stream rich in meso-lactide as defined herein (having a meso-lactide content of greater than 70 wt.-%.).

[0216] In some other preferred embodiments, the present process therefore involves the step of purifying the crude lactide by:

[0217] (c1) subjecting the crude lactide to a pretreatment, preferably a crystallization step, thereby obtaining a mother liquor comprising meso-lactide, and L-lactide and / or D-lactide, and

[0218] (c2) subjecting the mother liquor to distillation, preferably fractional distillation; thereby separating a lactide-rich stream, preferably an L-lactide-rich stream, preferably an L-lactide-rich stream as defined herein; from a meso-lactide-rich stream, the meso-lactide-rich stream comprising at least 70.0 wt. % meso-lactide, based on the total weight of the stream, and having an acidity level of 30.0 to 1000.0 meq / kg.

[0219] In a preferred embodiment, during step c1), crystals may be separated from the mother liquor. Preferably, such crystals are rich in lactide, preferably rich in L-lactide, and may, for example, contain at least 90.0% by weight, for example at least 95.0% by weight, of L-lactide, wherein the % by weight is based on the total amount of the crystals. The mother liquor obtained using the process described herein contains less than 70.0% by weight of meso-lactide. In some embodiments, such a mother liquor may even contain less than 50.0% by weight of meso-lactide. In a preferred embodiment, the mother liquor may contain at least 5.0% by weight of meso-lactide, preferably at least 7.5% by weight, preferably at least 10.0% by weight of meso-lactide, wherein the % by weight is based on the total amount of the mother liquor. In a preferred embodiment, the mother liquor may contain at least 50.0% by weight of L-lactide, preferably at least 60.0% by weight, preferably at least 70.0% by weight of L-lactide, wherein the % by weight is based on the total amount of the mother liquor.

[0220] Step c) of the present process advantageously reduces or even avoids impurities remaining in the two separate streams and can be crystallized separately or incorporated into the product crystals in a downstream step.

[0221] Those skilled in the art of lactide processing know the conditions under which lactide streams are distilled. Typically, distillation occurs in one or more distillation columns, each equipped with structured packing. Distillation is performed under reduced pressure, typically at 5-100 mbar and a reflux ratio of 20 or greater.

[0222] In the process of the present invention, a crude lactide stream is separated into a lactide-rich stream, such as an L-lactide-rich stream, and a meso-lactide-enriched stream. In some embodiments, the lactide-rich stream obtained in step c) contains the majority of the L-lactide and / or D-lactide present in the crude lactide. The process can be operated so that meso-lactide is not completely separated from the crude lactide via the meso-lactide-enriched stream, in which case the lactide-rich stream will contain some meso-lactide.

[0223] In a preferred embodiment of the present process, the lactide-rich stream separated in step c) comprises at least 50.0 wt.% L-lactide, preferably at least 60.0 wt.%, preferably at least 65.0 wt.%, preferably at least 70.0 wt.%, preferably at least 75.0 wt.%, preferably at least 80.0 wt.%, preferably at least 85.0 wt.%, preferably at least 90.0 wt.%, preferably at least 95.0 wt.% L-lactide, wherein the wt.% are based on the total weight of the lactide-rich stream.

[0224] As used herein, the term "L-lactide-rich stream" is used to describe the composition of a stream (i.e., a product stream or a fluid or fluid stream in a process) in terms of the stereochemistry of the lactide (molecules) present, and in this context refers to a stream that comprises at least 50.0 wt.% L-lactide. In a preferred embodiment, the L-lactide-rich stream comprises at least 55.0 wt.% L-lactide, preferably at least 60.0 wt.% L-lactide, preferably at least 70.0 wt.% L-lactide, preferably at least 75.0 wt.% L-lactide, preferably at least 80.0 wt.% L-lactide, preferably at least 85.0 wt.% L-lactide, preferably at least 90.0 wt.% L-lactide, preferably at least 95.0 wt.% L-lactide, wherein the wt.% are based on the total weight of the lactide-rich stream.

[0225] In some preferred embodiments of the present process, the L-lactide-rich stream comprises no more than 10.0 wt% D-lactide, preferably no more than 7.5 wt%, preferably no more than 5.0 wt%, preferably no more than 2.5 wt% D-lactide, wherein the wt% are based on the total weight of the L-lactide-rich stream.

[0226] The meso-lactide-enriched stream primarily contains meso-lactide. When step c) is carried out in a distillation column, the meso-lactide-enriched stream is withdrawn from the column as a more volatile fraction compared to the L-lactide-enriched stream. According to the process of the present invention, the meso-lactide-enriched stream obtained comprises at least 70.0 wt. % meso-lactide, based on the total weight of the stream, and, for example, at least 80.0 wt. %, at least 85.0 wt. %, or at least 90.0 wt. % meso-lactide, based on the total weight of the stream. It may contain small amounts of L-lactide or D-lactide, but these together generally constitute no more than about 15.0%, and more preferably no more than 10.0 wt. % of the meso-lactide-enriched stream.

[0227] According to the process of the present invention, the meso-lactide-enriched stream obtained in step c) has an acidity level of 30.0 to 1000.0 meq / kg, or between 35.0 and 900.0 meq / kg, or between 50.0 and 800.0 meq / kg, or between 55.0 and 750.0 meq / kg, or between 60.0 and 300.0 meq / kg. The acidity level (free acid level) can be determined as shown below in the Examples section.

[0228] In some embodiments, the lactide-rich stream, preferably the L-lactide-rich stream as defined herein, obtained in step c) may be further used, optionally after further purification, for the preparation of a lactide mixture and / or for the preparation of polylactide.

[0229] The meso-lactide-enriched stream obtained in step c) is subjected to further purification to produce a purified meso-lactide stream. In some embodiments of the present process, the purification is obtained by subjecting the meso-lactide-enriched stream to crystallization, for example, by subjecting it to multiple crystallization stages. In some embodiments of the present process, the crystallization is solvent crystallization or melt crystallization. As used herein, the terms solvent crystallization and solution crystallization are used as synonyms and refer to crystallization performed using a solvent. In melt crystallization, no solvent is added. Crystals are produced by cooling the melt and form when sufficiently cooled to below solid / liquid equilibrium. The meso-lactide is then collected in its molten form. For example, melt crystallization can be carried out in the present process as suspension crystallization or layer crystallization. In a preferred embodiment of the present process, the meso-lactide-enriched stream as defined herein is subjected to solvent or melt crystallization, for example, wherein crystallization is performed using a static melt crystallizer. In some embodiments of the invention, the meso-lactide-enriched stream as defined herein is subjected to multi-stage (cyclic) solvent crystallization or melt crystallization, for example wherein the crystallization is performed using a static melt crystallizer.

[0230] In some embodiments, the purified meso-lactide stream comprises:

[0231] - at least 94.0 wt.% meso-lactide, based on the total weight of the stream, such as at least 95.0 wt.% meso-lactide, preferably at least 96.0 wt.% or at least 97.0 wt.% meso-lactide, based on the total weight of the stream, and thus further enriched in meso-lactide than the meso-lactide-enriched stream; and

[0232] - an acidity level of at most 20.0 meq / kg, and preferably at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, preferably at most 6.0 meq / kg, preferably at most 5.0 meq / kg, preferably at most 4.0 meq / kg, for example between 0.001 and 7.0 meq / kg, or between 0.001 and 6.0 meq / kg, or between 0.001 and 5.0 meq / kg, or between 0.001 and 4.0 meq / kg.

[0233] In some other embodiments, the purified meso-lactide stream comprises:

[0234] - at least 94.0 wt.% meso-lactide, based on the total weight of the stream, such as at least 95.0 wt.% meso-lactide, preferably at least 96.0 wt.% or at least 97.0 wt.% meso-lactide, based on the total weight of the stream, and thus further enriched in meso-lactide than the meso-lactide-enriched stream; and

[0235] - an acidity level of at most 20.0 meq / kg, and preferably at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, preferably at most 6.0 meq / kg, preferably at most 5.0 meq / kg, preferably at most 4.0 meq / kg, and

[0236] - an acidity level of at least 1.0 meq / kg, preferably at least 1.5 meq / kg.

[0237] In some preferred embodiments, a purified meso-lactide stream is provided having

[0238] - at least 94.0 wt% meso-lactide, or at least 95.0 wt%, or at least 96.0 wt%, or at least 97.0 wt% meso-lactide, based on the total weight of the stream, wherein the wt% are based on the total weight of the purified meso-lactide stream, and

[0239] - Acidity levels between 1.0 and 20.0 meq / kg, or between 1.5 and 15.0 meq / kg, or between 2.0 and 12.0 meq / kg, or between 3.0 and 10.0 meq / kg.

[0240] The acidity levels referred to in this process can be measured as defined below in the Examples section.

[0241] Advantageously, the present process can provide a purified meso-lactide stream having excellent purity levels, making the obtained meso-lactide stream particularly useful for preparing polymeso-lactide of suitable quality for use in various downstream applications and / or as a component in mixtures or blends.

[0242] The purified meso-lactide stream can contain one or more other acids, such as acetic acid, succinic acid, pyruvic acid, levulinic acid, and the like, which may be present as their free acids, as esters, and as anhydrides, in an amount reduced by at least 50.0%, at least 60.0%, or at least 70.0% compared to the amount in the meso-lactide-enriched stream. Reduction of the free acid content (and thus purification of the stream) is preferably achieved in a crystallization step.

[0243] In some embodiments of the invention, the purified meso-lactide stream comprises less than 8.0 wt% L-lactide, preferably less than 6.0 wt% L-lactide, more preferably less than 4.0 wt% L-lactide, wherein the wt% are based on the total weight of the stream. In some embodiments of the invention, the purified meso-lactide stream comprises less than 8.0 wt% D-lactide, preferably less than 6.0 wt% D-lactide, more preferably less than 4.0 wt% D-lactide, wherein the wt% are based on the total weight of the stream.

[0244] The next step of the process (i.e., step e)) involves polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide. The portion of the polymerized purified meso-lactide stream is polymerized separately. In this context, the terms "separate" or "separately" refer to polymerizing a portion of the purified meso-lactide stream in the absence of other streams obtainable in the process of the present invention, and in particular separately from (i.e., without the presence of) the lactide-rich stream or a portion thereof obtained in step c) of the process.

[0245] Furthermore, according to the present process, it is preferred that the portion of the separately polymerized purified meso-lactide stream accounts for at least 30.0% by weight, or at least 40.0% by weight, or at least 50.0% by weight, or at least 60.0% by weight, or at least 70.0% by weight, or at least 80.0% by weight, or at least 90.0% by weight, based on the total weight of the purified meso-lactide stream. This has the advantage that the purified meso-lactide stream obtained in the present process can be value-added, preferably as a valuable polymer product. Thus, the present invention improves the overall process yield for producing polymeso-lactide.

[0246] For the polymerization of meso-lactide, the catalyst and reaction conditions commonly used in the polymerization of lactide can be used. This is an important discovery, because generally in the art, it is assumed that the meso-lactide fraction contains most of the impurities forming color, and the present invention has been shown to be able to remove and overcome these impurities. For example, meso-lactide stream is directly fed into the polymerization system, wherein the meso-lactide stream is polymerized at an elevated temperature in the presence of a catalyst, preferably a metal catalyst and optionally one or more initiators, preferably wherein the initiator is an alcohol. The same catalyst used in the polymerization of L-lactide or D-lactide can be used, such as tin oxide, 2-ethylhexanoic acid tin (II), titanium tetrabutoxide and titanium isopropoxide. However, stereoselective catalysts can also be used in the process for polymerization of meso-lactide of the present invention, as for example at the people such as Ovitt, J.Am.Chem.Soc.1999, 121, announced in 4072-4073. The use of such stereoselective catalysts during polymerization can produce syndiotactic or heterotactic polymeso-lactide. Such stereoregular polymeso-lactide may exhibit the potential for crystallization. Alcohol initiators used in lactide polymerization reactions are well known to those skilled in the art.

[0247] The polymerization can be carried out batchwise, semi-continuously or continuously. Continuously stirred tank reactors (CSTRs), reactors based on static mixers, and tube or pipe reactors are suitable types of polymerization vessels. Suitable polymerization temperatures are preferably from about 125°C to about 225°C. A more preferred temperature range is from 130°C to about 220°C and particularly from about 135°C to about 200°C. The residence time at the polymerization temperature is selected to produce a meso-lactide polymer having a desired molecular weight, low yellowness index, and / or desired monomer conversion. The polymerization is preferably carried out in an inert gas (dry N2) atmosphere at high pressure (generally 4-300 bar). The reaction time is preferably between 0.5 and 8 hours.

[0248] In one example, a portion of the purified meso-lactide stream is polymerized to form polymeso-lactide by ring-opening polymerization, preferably at a temperature comprised between 125 and 225° C. in the presence of a catalyst, preferably a metal catalyst as defined herein. Optionally, the ring polymerization is carried out in the presence of one or more initiators, preferably an alcohol. Also preferably, the ring-opening polymerization is carried out continuously with a residence time between 30 and 300 minutes. In some embodiments of the invention, the ring-opening polymerization is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, a phosphate ester, or a polyacrylic acid (co)polymer.

[0249] Ring-opening polymerization allows control of the polymerization process, thereby controlling the structure of the polymeso-lactide (PML) produced. This method can be used to produce high molecular weight PML. The molecular weight of the polymer produced by ring-opening polymerization can be controlled by residence time, catalyst and initiator concentrations, and temperature.

[0250] In some embodiments of the present invention, it may be preferred that, prior to polymerization of a portion thereof, the purified meso-lactide stream is purified to an acidity level of at most 10.0 meq / kg, preferably to an acidity level of at most 7.0 meq / kg, or to an acidity level of at most 5.5 meq / kg, or to an acidity level of at most 4.0 meq / kg. In such embodiments, the purified meso-lactide stream can be purified by various methods as disclosed herein, for example by crystallization. The high purity of the purified meso-lactide allows for mild polymerization conditions, thereby producing polymeso-lactide having a low yellowness index of generally <40 as measured by the method described herein. Surprisingly, the polymeso-lactide product produced exhibits a lower viscosity than PLA resins in the art, which allows for lower reactor pressures and minimized reaction temperatures.

[0251] In some preferred embodiments of the present invention, the process further comprises a step of demonomerizing the polymeso-lactide by evaporation, preferably at a pressure below 50 mbar and a temperature above 175°C, for example, in a flash tank, degassing extruder, or wiped-film evaporator. Following polymerization and demonomerization, the PML can be extruded or pelletized using a pelletizer, preferably an underwater pelletizer. Advantageously, according to the present invention, lower temperatures are required to strip residual meso-lactide remaining in the PML compared to similar processes for stripping residual L-lactide from PLA.

[0252] This process may further include the step of processing the polymeso-lactide into an article. Examples of articles that can be made from or using PML include, for example, but not limited to, molded articles, films, tapes, filaments, fibers, sheets, fabrics, and spunmelt nonwovens. The PML obtained in this process may also be processed into layers of multilayer articles, such as layers of multilayer films. PML may also be used as a (temporary) blocking agent (diverter) for use in downhole and fracturing applications. PML may also be used as a slow lactic acid releaser.

[0253] In some embodiments of the present invention, the PML as obtained herein can be blended with other polymers, in particular with polylactides of different compositions (e.g., PLLA and / or PLDA). Preferably, the resulting polymer blend can include PLA and PML, wherein the polymer blend includes up to 10.0 wt%, or up to 8.0 wt%, or up to 5.0 wt% of polymeso-lactide, wherein the wt% is based on the total weight of the blend.

[0254] In some embodiments of the present invention, the process further comprises step f) of blending at least a portion of the purified meso-lactide stream that was not polymerized in step e) with a portion of the lactide-rich stream from the process to provide a lactide mixture, and subsequently polymerizing the lactide mixture to form polylactide, wherein the polymerized meso-lactide content is less than 20.0% or less than 10.0% by weight of the polylactide. According to step f), at least a portion of the purified meso-lactide is recombined with the lactide-rich stream, and the recombined streams (also referred to herein as the "lactide mixture") are polymerized together. To this end, the purified meso-lactide stream can be combined with the lactide-rich stream, and the resulting lactide mixture can be fed to a polymerization unit.

[0255] In some embodiments of the present process, the portion of the purified meso-lactide stream that is recombined and polymerized with the lactide-rich stream may amount to up to 50.0 wt.%, or up to 40.0 wt.%, or up to 30.0 wt.%, or up to 25.0 wt.%, based on the total weight of the purified meso-lactide stream. Preferably, the portion of the purified meso-lactide and the portion of the lactide-rich stream that are recombined to form the lactide mixture are selected so as to produce a lactide mixture having a desired ratio of lactic acid enantiomers. For example, the purified meso-lactide stream can constitute at least 0.1%, at least 0.25%, at least 0.5%, at least 1.0%, at least 2.0%, at least 3.0%, or at least 4.0% up to 25.0%, up to 20.0%, up to 10.0%, up to 8.0%, or up to 6.0% of the combined weight of the purified meso-lactide stream and the lactide-rich stream.

[0256] According to a preferred embodiment of the present process, the polymerization of the lactide mixture produces polylactide (PLA) in which the content of polymerized meso-lactide is less than 20.0 wt % of the polylactide, and preferably less than 15.0 wt % or less than 10.0 wt % of the polylactide.

[0257] For the polymerization of the lactide mixture, catalysts and reaction conditions customary for lactide polymerization can be used. Preferably, the polymerization of the lactide mixture is carried out by ring-opening polymerization at a temperature between 125 and 225° C. in the presence of a catalyst, preferably a metal catalyst, and optionally one or more initiators, preferably wherein the initiator is an alcohol.

[0258] For example, the lactide mixture is directly fed into the polymerization system, wherein the lactide mixture is polymerized at an elevated temperature in the presence of a metal catalyst and an optional alcohol as an initiator. The same catalyst as the catalyst used in the polymerization of L-lactide or D-lactide can be used for polymerizing the lactide mixture defined herein, such as a metal-containing catalyst. Examples of these catalysts include various tin compounds such as SnCl , SnBr , SnCl , SnBr , SnO, bis(2-ethylhexanoate) tin(II), tris(2-ethylhexanoate) butyltin, hydrated monobutyltin oxide, dibutyltin dilaurate, tetraphenyltin, etc., and are preferably selected from tin oxide, 2-ethylhexanoate tin(II). In addition, Group 4 metal complexes can be used, such as oxides and trisphenolates based on zirconium, titanium and hafnium. Polymerization can be carried out in batches, semi-continuously or continuously. Continuously stirred tank reactors (CSTRs) and pipes or tubular reactors are suitable types of polymerization vessels. Suitable polymerization temperatures are preferably from about 125° C. to about 225° C., or from about 160° C. to about 200° C. The residence time at the polymerization temperature is selected to produce a PLA polymer having a desired molecular weight, low yellowness, and / or desired monomer conversion. In one example, a lactide mixture as defined herein is polymerized via ring-opening polymerization, which is carried out continuously with a residence time between 30 minutes and 300 minutes. In some embodiments, the ring-opening polymerization can be terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, a phosphate ester, or a polyacrylic acid (co)polymer.

[0259] In some embodiments, the present invention also relates to a polylactide obtainable by or by performing a process according to the present invention, and in particular by polymerizing a lactide mixture as described herein. Preferably, such a polylactide comprises polymerized meso-lactide in an amount less than 20.0% by weight of the polylactide, e.g., less than 15.0% or less than 10.0% by weight of the polylactide.

[0260] In some preferred embodiments, the polylactide is poly-L-lactide (PLLA) obtainable by or by polymerization of a lactide-rich stream, preferably an L-lactide-rich stream, as described herein. Preferably, the PLLA has a stereochemical purity of at least 85.0%, or at least 90.0%, or at least 95.0%, or at least 98.0%, preferably equal to 99.0% or more L-lactate.

[0261] The process may further comprise the step of processing the polylactide as provided herein or as obtained with the method as provided herein into an article, such as casting into a sheet, optionally as a heat seal layer in a multilayer sheet.

[0262] In another example, the polylactide as provided herein or obtained using the methods provided herein can be used as a coating layer on paper.

[0263] In another example, the polylactide as provided herein or obtained with the methods as provided herein can be used as a fiber component, for example, as the sheath component of a bicomponent fiber.

[0264] Figure 1 is a schematic diagram illustrating one embodiment of the process of the present invention. Figure 1 The embodiments shown in FIG. 5 illustrate various preferred or optional features. Figure 1 It is not intended to show specific engineering features or details, including the design of the various components shown. In addition, auxiliary equipment such as various valves, pumps, heating and cooling equipment, analysis, control equipment, etc. are not shown, but can of course be used as needed or desired.

[0265] exist Figure 1 In the embodiment of the present invention, a lactic acid stream 7 is fed to the prepolymer reactor 1. The lactic acid concentration in the feed stream, e.g., stream 7, is preferably at least 60.0 wt. %, and may be as high as 95.0 wt. %, or as high as 99.0 wt. %, or the feed stream consists of lactic acid. Lactic acid may be obtained from a fermentation broth, which is preferably obtained in Figure 1 Lactic acid can also be obtained by depolymerization of PLLA or PDLA polymers as described herein.

[0266] The starting materials are heated in the prepolymer reactor 1 to condense the lactic acid to form a low molecular weight poly(lactic acid) as previously described; as the degree of polymerization increases, the pressure typically decreases and the temperature increases. Most of the free water and the water produced by the polycondensation are removed from the prepolymer reactor 1 as stream 16. Stream 16 can be discarded, or all or any portion thereof can be recycled to an earlier stage in the process. Any recycled portion of stream 16 can be purified before being recycled. It is understood in the art that prepolymerization can be carried out in a series of reactors rather than a single reactor, whereby the degree of polymerization increases across the reactors, and different vacuum and temperature settings can be used.

[0267] As with all other process steps in which lactic acid and its derivatives are exposed to elevated temperatures, racemization can occur in the prepolymer reactor 1. Racemization is random, as L-lactic acid can racemize to R-lactic acid and vice versa. However, because one enantiomer is predominant, the net effect of racemization is that the concentration of the non-predominant enantiomer increases at the expense of the predominant enantiomer, and stereochemical purity decreases, until, for example, a racemic lactic acid mixture is obtained in extreme cases. According to the present process, some racemization can be tolerated or even promoted in the process, for example by applying conditions that favor racemization in the prepolymer reactor and in each other process step involving exposure of lactic acid and downstream reaction products to elevated temperatures.

[0268] A stream of low molecular weight poly (lactic acid) (i.e., lactic acid oligomers) stream 8 is removed from the prepolymer reactor 1 and transferred to the lactide reactor 2, where it is depolymerized to form (crude) lactide. Examples of suitable lactide reactors include, for example, stirred tank reactors, forced circulation, short path or short tube, long tube vertical, long tube horizontal, falling film, stirred thin film, and disk evaporators. The lactide reactor 2 is preferably operated at a pressure of about 1 to about 100 mbar, more preferably about 2 to about 60 mbar. A temperature of, for example, about 175 to 210° C. is used. The depolymerization reaction occurring in the lactide reactor 2 is typically catalyzed. In the embodiment shown, the catalyst is introduced directly into the lactide reactor 2 via catalyst stream 17.

[0269] In lactide reactor 2, crude lactide and a bottoms mixture are formed. The bottoms primarily consist of lactic acid oligomers (linear and cyclic) and high-boiling materials. The bottoms are withdrawn as bottoms stream 18. This can be discarded with or without treatment or recycled to an earlier step in the process. The crude lactide produced in lactide reactor 2 primarily contains L-lactide, D-lactide, meso-lactide, water, lactic acid, and some lactic acid oligomers.

[0270] The crude lactide formed in lactide reactor 2 is withdrawn as stream 9 and, in the illustrated embodiment, transferred to distillation column 3. In the illustrated embodiment, crude lactide stream 9 is introduced into distillation column 3, where it is separated into a lactide-rich stream 11 and a meso-lactide-enriched stream 10. The meso-lactide-enriched stream comprises at least 70.0 wt. % meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg. Lactide-rich stream 11 primarily contains L-lactide and D-lactide, with a small amount of meso-lactide, for example, less than 10 wt. % or between 2 and 8 wt. A bottoms stream (not shown) may also be withdrawn from the distillation column. It will be appreciated that a series of distillation columns may be used, for example, to further separate the lactic acid monomer from the meso-lactide stream.

[0271] According to the present invention, the meso-lactide-rich stream 10 removed from the system is used as a monomer source for the production of polymeso-lactide. Figure 1 In the illustrated embodiment, a meso-lactide-rich stream is withdrawn from distillation column 3 and first fed to a crystallization unit 4, where a meso-lactide-rich stream 10 is purified. Purification can be performed via melt crystallization or via crystallization from a solvent. The crystallization unit may comprise, for example, a static crystallizer, a falling film crystallizer, a suspension crystallizer, a suspension mixed product removal crystallizer, or the like. Preferably, a static crystallizer is employed. Impurities are largely excluded from the meso-lactide crystals formed in crystallizer 4, resulting in a purified meso-lactide stream 12. The purified meso-lactide stream may comprise at least 94.0 wt. % meso-lactide, based on the total weight of the stream, and have an acidity level of 20.0 meq / kg or less. In some embodiments, the purified meso-lactide stream may comprise an acidity level as low as 7.0 meq / kg or even lower. In some embodiments, such a low acidity level may be achieved by subjecting the meso-lactide-rich stream to multiple crystallization stages.

[0272] At least a portion of purified meso-lactide stream 12 is separated (see stream 14A) and sent to polymerization unit 5 to produce polymeso-lactide 13. The portion 14A of the purified meso-lactide stream that is separated and separately polymerized into polymeso-lactide is in an amount of at least 30.0 wt.%, or at least 40.0 wt.%, or at least 50.0 wt.%, or at least 60.0 wt.%, or at least 70.0 wt.%, or at least 80.0 wt.%, or at least 90.0 wt.%, where the wt.% are based on the total weight of purified meso-lactide stream 12.

[0273] In most cases, the lactide-rich stream 11 removed from the system can be used as a monomer source for producing polylactide. In some optional embodiments of the present invention, separating at least a portion 14B of the purified meso-lactide and recombining this portion 14A with the lactide stream 11 to form a recombinant stream 19 can be a further option. The recombinant stream 19 can then be sent to the polymerization unit 6 to produce polylactide (PLA) including a certain level of polymerized meso-lactide 15. In some embodiments, the present process allows the production of polylactide in which the content of polymerized meso-lactide is less than 20.0% by weight of the polylactide. In some embodiments, polymerization units 5 and 6 can be the same polymerization unit, thereby producing polymeso-lactide and polylactide in alternating activity.

[0274] The present process allows the use of the obtained polymeso-lactide 13 and the obtained polylactide 15 in various applications, for example for the manufacture of articles as described herein.

[0275] The present invention also relates to polymeso-lactide obtainable by or by carrying out the process according to the present invention. Preferably, the polymeso-lactide as described herein has one or more of the following properties:

[0276] (i) 40.0 to 60.0% stereochemical purity of the L-lactate or 40.0 to 60.0% stereochemical purity of the D-lactate;

[0277] (ii) a weight average molecular weight (Mw) between 50 and 500 kg / mol or between 100 and 350 kg / mol;

[0278] (iii) a glass transition temperature between 30°C and 50°C or between 35°C and 50°C.

[0279] Each of these characteristics can be determined using methods known in the art and as further described in the Examples section.

[0280] Preferably, the polymeso-lactide obtainable by or by carrying out the process according to the invention has one or more of the following properties:

[0281] (i) 40.0 to 60.0% stereochemical purity of the L-lactate; or 40.0 to 60.0% stereochemical purity of the D-lactate, as determined by chiral gas chromatography after complete methylation (as described in the Examples section);

[0282] (ii) a weight average molecular weight (Mw) between 50 and 500 kg / mol or between 100 and 350 kg / mol; as determined by gel permeation chromatography in chloroform relative to polystyrene standards (as described in the Examples section);

[0283] (iii) a glass transition temperature between 30°C and 50°C or between 35°C and 50°C, as determined by DSC analysis at a scan rate of 1-20 K / min (as explained in the Examples section).

[0284] In some embodiments, the polymeso-lactide as described herein is amorphous. In some other embodiments, the polymeso-lactide as described herein is semi-crystalline.

[0285] Polymeso-lactide polymers can be homonuclear decoupled 1 This type of analysis allows, for example, to distinguish between a polymer made from 100% meso-lactide and a polymer made from a 50 / 50 mixture of L-lactide and D-lactide.

[0286] The example of the goods that can be made by or use the PML as defined herein includes, for example, but not limited to, molded articles, films, bands, long filaments, fibers, sheets, fabrics, spunmelt nonwovens.The PML obtaining in this process also can be processed into the layer of multilayer articles, for example the layer of multilayer films.

[0287] Other examples of articles that can be made from or using PML as defined herein include, for example, degradable articles or degradable compositions, preferably wherein the article or composition is in solid form. For example, a degradable article or degradable composition comprising or consisting of polymeso-lactide as defined herein or as obtained according to a process as described herein may include a solid component capable of slowly releasing an acid, such as lactic acid.

[0288] Examples of compositions comprising polymeso-lactide as defined herein or as obtained according to a process as described herein may, for example, include sustained-release compositions capable of slowly releasing components such as lactic acid. Such compositions can be provided in solid form. As used herein, the terms "slow-release" or "slowly releasing" are understood to be defined as controlled release, wherein the release of the active ingredient (in the case of the present invention, acid and preferably lactic acid) is delayed for a period of time or is gradually released over an extended period of time.

[0289] In some embodiments of the present invention, the polymeso-lactide according to the process as provided herein or obtainable according to or in accordance with the present invention can be used as resin for (paper) coatings or as adhesive resin, e.g. for the preparation of adhesive films or adhesive layers of multilayer structures.

[0290] In some embodiments of the present invention, poly-meso-lactide as provided herein or obtainable according to or in accordance with the process of the present invention can be used as a diverting agent, for example, in downhole applications or in fracturing applications. "Diverting agent" and "sealing agent" are used as synonyms herein. "Fracturing" is the process of injecting a liquid into underground rock, a borehole, etc. under high pressure in order to force open existing cracks and extract oil or gas. Diverting agents known in the art work by creating a temporary sealing effect in such types of applications.

[0291] In some embodiments of the present invention, polymeso-lactide as provided herein or obtainable according to or in accordance with the process of the present invention can also be used to prepare a polymer composition, wherein the polymer composition comprises (A) polylactide and (B) polymeso-lactide. Preferably, the "polymer composition" (also referred to herein as a "polymer blend") comprises one or more polylactides as defined or obtained herein, for example a mixture of different PLA grades, and one or more polymeso-lactides, wherein the total content of polymeso-lactide in the polymer composition is less than 10.0 wt%, and for example less than 7.5 wt%, or less than 5.0 wt%, based on the total weight of the polymer composition. The present invention also relates to the use of polymeso-lactide as defined herein or as obtained according to the process as described herein for preparing an article or composition.

[0292] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtained according to a process as described herein as resin for (paper) coatings.

[0293] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtained according to a process as described herein as an adhesive resin, e.g. for the preparation of an adhesive film or adhesive layer in a multilayer film or structure.

[0294] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtained according to a process as described herein for the preparation of a degradable article or composition, preferably a solid degradable article or composition.

[0295] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtained according to a process as described herein for the preparation of a slow release preparation or composition, preferably wherein the preparation or composition is in solid form and is capable of slowly releasing lactic acid.

[0296] In some embodiments, the present invention relates to the use of a polymeso-lactide as defined herein or as obtained according to a process as described herein as a temporary sealant in downhole applications or in fracturing applications.

[0297] In some embodiments of the present invention, the PML as obtained herein can be blended with other polymers, in particular with polylactides (e.g., PLLA and / or PLDA) of different compositions, including PLA polymers as provided herein. Preferably, the resulting polymer blend can include polylactide and polymeso-lactide, wherein the polymer blend includes up to 10.0 wt%, or up to 8.0 wt%, or up to 5.0 wt% of polymeso-lactide, wherein the wt% is based on the total weight of the blend.

[0298] In an example, such polymer blends can be used for preparing goods, for example fiber, and for example bicomponent fiber, wherein PLA is sheath and PML is core material.The following examples are only used to illustrate the present invention, and should not be construed as limiting its scope in any way.Although only some forms of the present invention are shown, it should be clear to those skilled in the art that the present invention is not limited thereto, but is easy to carry out various changes and modifications without departing from the scope of the present invention.

[0299] Example

[0300] Methodology

[0301] Free acid content (acidity level)

[0302] The free acid content (free acidity) of a stream obtained in the process of the present invention, in particular a lactide-containing stream, such as a meso-lactide-enriched stream, a purified meso-lactide stream, or a lactide-enriched stream, and expressed as the amount of free carboxylic acid groups in milliequivalents per kilogram (meq / kg), can be measured by potentiometric titration of a sample of the stream with sodium methoxide or potassium methoxide in anhydrous methanol. A suitable titration apparatus may be a Mettler Toledo T5 titrator.

[0303] In one embodiment, a sample from a stream rich in L-lactide was titrated using this method. A titration vial was filled with 2.7844 g of sample and subsequently dissolved in 40 mL of a 30% v / v solution of dichloromethane in dry methanol at room temperature. The solution was then titrated with 0.0108 M potassium methoxide in dry methanol until the pH 8.4 equivalence point was reached. This was achieved when 1.8976 ml of titrant was consumed. A blank sample was also titrated using 0.1720 mL of titrant. The free acidity was then calculated using the following formula:

[0304]

[0305] in

[0306] V = titrant volume of sample in ml - titrant volume of blank in ml

[0307] T = titrant titration in mol / L

[0308] m = sample weight (g)

[0309] The free acid content of this example was 6.7 meq / kg.

[0310] Stereochemical purity (L / D content)

[0311] The stereochemical purity of poly-meso-lactide (PML) can be assessed after destructive methylation. For this reason, 0.1 g of PML is put into a crimp cap vial, followed by addition of 2.0 g of dichloromethane (pure, Acros Organics) and 5.0 g of methanol (JT Baker), and the sample is allowed to dissolve at 70 ° C for 2 hours. After being cooled to room temperature, 3.0 g of Amberlyst 15 (ion exchange resin, drying, Acros Organics) are added, and the reaction is allowed to proceed at 80 ° C for 22 hours. After being cooled to room temperature, the sample is subjected to chiral gas chromatography separation on a ThermoFocus GC equipped with a CP-Chirasil-dex CB separation column. This achieves the separation of R- (or D) and S- (or L-) methyl lactate, and its ratio ultimately determines the stereochemical purity of the sample.

[0312] Relative molecular weight

[0313] Relative molecular weight parameter M n 、M wThe polydispersity index (PDI) and polydispersity index (PDI) can be determined using gel permeation chromatography (GPC) with an ELSD detector. In particular, the Agilent HPLC Infinity II system can use chloroform (HPLC grade, stabilized with 1% ethanol) and 5% methanol (HPLC grade) as solvents at a flow rate of 1.0 mL / min. The size exclusion column used can be a PLgel 5μm guard column (50x7.5mm) and two PLgel 5μm MIXED-C columns (300x7.5mm), which are connected in series at a column oven temperature of 35°C. 1-15 mg of sample is weighed in a 20 mL crimp cap vial, and 15 mL of chloroform is added thereto. The obtained suspension is shaken at 50°C for 2h, filtered on a 0.45 μm filter, transferred to a 2 mL vial and injected into the GPC system for analysis. A calibration curve of narrow dispersity polystyrene standards is prepared and the molecular weight relative to it is reported.

[0314] Glass transition temperature (Tg)

[0315] The glass transition describes the temperature region where a material's mechanical properties change from hard and brittle to softer, deformable, or rubbery. The glass transition temperature (Tg) can be determined by differential scanning calorimetry (DSC) analysis at a scan rate of 1-20 K / min with a sample weight of 4-8 mg. The Tg values ​​reported herein are reported as the midpoint of the glass transition region.

[0316] Residual lactide

[0317] The amount of residual lactide (i.e., L-lactide, D-lactide, meso-lactide) in the polymeso-lactide sample is measured by precipitation method to separate monomer lactide from polymerized meso-lactide. The polymeso-lactide sample (including polymeso-lactide and residual lactide monomer) is dissolved in a known amount of dichloromethane (including internal standard). The polymeso-lactide fraction of the sample is then removed by precipitation by being introduced into an excessive 5 / 95 acetone / hexane solvent mixture with a dichloromethane solution. After precipitation for half an hour, the polymer fraction is removed by filtering through a 0.45 μm filter. The remaining solution is then analyzed using gas-liquid chromatography to determine the amount of lactide monomer in the sample. The final amount of residual lactide is calculated by taking the sum of L-lactide, D-lactide and meso-lactide.

[0318] Yellowness Index

[0319] Yellowness index can be measured directly on polymer pellets, particularly PML pellets, using a Konica Minolta CR-410 colorimeter as described in ASTM D 1925.

[0320] Example

[0321] Example 1: Synthesis of prepolymer based on L-lactic acid

[0322] 1765 g of high-purity lactic acid (HS100, available from Corbion) was placed in a 2L four-necked round-bottom flask with a distillation apparatus, a temperature probe, and an overhead stirrer. After the contents of the flask were heated to 190°C by a heating mantle while stirring, the vacuum was reduced to 30 mbar over a period of 30 minutes, and oligomerization was allowed to proceed for 1 hour. Water and other volatiles were collected in a graduated cylinder, and after breaking the vacuum, the product was collected from the 2L flask. The resulting product, prepolymer 1, is a lactic acid prepolymer with a DP of 10.4 and a stereochemical purity of 1.59%D. This example shows that racemization readily occurs during the prepolymerization stage, and different conditions can also be selected to influence the final meso-lactide production rate.

[0323] Example 2: Lactide Synthesis under Various Conditions Providing Flexibility in Meso-Lactide Production

[0324] Example 2 illustrates lactide synthesis under different conditions and their effect on racemization.

[0325] In the first experiment, 349 g of prepolymer 1 (see Example 1) was placed in a 500 mL four-neck round-bottom flask with a heating mantle, temperature probe, distillation apparatus, and overhead stirrer. Next, 600 ppm of tin octoate was added by injecting a 10 wt % toluene solution of the catalyst using a syringe. The mixture was heated to 200 ° C, and the vacuum was gradually reduced to 5 mbar over a 14-minute period. A crude lactide product was obtained and condensed through 96 ° C water and finally collected in a graduated cylinder. The reaction was stopped after 100 minutes. The reaction residue and crude lactide were weighed and analyzed, and the weighted stereochemical purity was 1.95%D, indicating that 0.36% racemization had occurred.

[0326] In the second experiment, the procedure of the first experiment was repeated, except that the crude lactide synthesis temperature was 220° C., i.e., the mixture was heated to 220° C. The reaction residue and the crude lactide were weighed and analyzed, and the weighted stereochemical purity was 3.10% D, indicating that 1.51% racemization had occurred.

[0327] In the third experiment, the procedure of the first experiment was repeated, but 1.82 g of sodium lactate (Purasal S100, available from Corbion) was added to the reaction mixture at the beginning of the procedure, corresponding to 1000 ppm of sodium. The reaction residue and the crude lactide were weighed and analyzed, and the weighted stereochemical purity was 17.5% D, indicating that 15.9% racemization had occurred.

[0328] An overview of the experiments and analysis of the resulting products is provided in Table 1. The amount of D-lactate was determined using the method shown above in the Methods section.

[0329] Table 1: Lactide synthesis under various conditions

[0330]

[0331] This example demonstrates that the ratio of racemized and meso-lactide in crude lactide can be manipulated, and in particular, the amount of meso-lactide in crude lactide can be upregulated by stimulating racemization during lactide synthesis.

[0332] Example 3: Prepolymerization and lactide synthesis starting from stereochemically impure lactic acid

[0333] The same procedure for prepolymerization as mentioned in Example 1 was repeated, but using a mixture of 461 g of L-lactic acid (HS100, Corbion) and 56 g of D-lactic acid (D-LA93, Corbion). The resulting prepolymer product had a DP of 10.5 and a stereochemical purity of 12.2% D.

[0334] This stereochemically impure prepolymer was subsequently used for crude lactide synthesis at 200°C according to the same procedure as in Example 2, Experiment 1, except for the fact that the synthesis reaction was stopped at 50% yield.

[0335] The crude lactide obtained contained 15 wt% meso-lactide, showing that increased meso-lactide production can be achieved by using a less stereochemically pure lactic acid mixture as the starting material.

[0336] Example 4. Distillation of crude lactide to obtain an L-lactide-rich stream and a meso-lactide-enriched stream

[0337] The following examples illustrate the purification by distillation of a mixture of lactide, lactic acid oligomers, and other organic compounds, which represents crude lactide.

[0338] The mixture was fed to two continuously operated distillation columns to produce a meso-lactide-rich process stream and an L-lactide-rich process stream. The composition of the feed mixture is summarized in Table 2.

[0339] Table 2: Composition of the mixture

[0340]

[0341] The reflux ratio and distillate flow rate of the first tower are optimized to maximize the removal of light organic acids and small lactic acid oligomers without compromising excessive meso-lactide yield. The bottom flow and bottom temperature of the first tower are optimized to maximize the removal of heavy oligomers and non-volatile impurities. The side line of the first tower is used as the feed for the second tower. The second distillation tower produces a distillate stream enriched in meso-lactide and a stream enriched in polymer-grade L-lactide as a by-product. The reflux ratio and distillate flow rate of the first tower are optimized to maximize the purity of the stream enriched in meso-lactide and the quality of the polymer-grade by-product enriched in L-lactide.

[0342] Both distillation columns were operated under vacuum at a pressure of 35 mbar, and the reflux ratios for columns 1 and 2 were 30 and 40, respectively. The resulting compositions of the meso-lactide-rich and L-lactide-rich streams are given in Table 3.

[0343] Table 3: Distillation Stream Composition

[0344]

[0345] As can be seen from Table 3, the described distillation process can effectively separate streams rich in meso-lactide and L-lactide, and is capable of separating the variable meso-lactide content in the crude lactide (about 19 wt % in this example, see Table 2) into a stream for producing poly(meso-lactide).

[0346] Example 5: Purification of a meso-lactide stream by melt crystallization

[0347] This example illustrates the purification of a meso-lactide stream by melt crystallization and, in particular, by suspension crystallization (without a solvent). A crude meso-lactide having a meso-lactide content of 97% by weight and a free acid content of 16.0 meq / kg was subjected to suspension crystallization. To this end, the crude meso-lactide was melted overnight at 70°C in an oven, and 1.75 kg was transferred to a 2L Büchner jacketed glass filter funnel, which allowed in-situ filtration. The mixture was cooled to 51°C at 20°C / h, seed crystals were added, and the contents were subsequently further cooled at 2°C / h. Crystallization was allowed to proceed at 51°C for 30 minutes, which resulted in the formation of the first crystal. After 30 minutes, the suspension was further cooled to 46°C at a rate of 2°C / h, now obtaining a slurry with a solids content of approximately 50% by weight. The suspension was filtered in situ by applying a slight vacuum at the bottom of the Büchner flask. The mother liquor was collected in a Buchner flask while the purified meso-lactide crystals were collected on a glass filter. The resulting meso-lactide had a free acid content of 9.8 meq / kg.

[0348] The resulting purified meso-lactide is an example of purified meso-lactide that can be recombined with a lactide mixture as obtained in the process of the present invention and polymerized to form a polylactide comprising polymerized meso-lactide.

[0349] Example 6: Purification of a meso-lactide stream by solvent crystallization

[0350] This embodiment illustrates the purification of meso-lactide stream by solvent crystallization in a static crystallizer. A 5 L drum (bucket, drum) filled with a stream (meso-lactide content 95%, free acid 52 meq / kg) enriched in meso-lactide obtained by distillation was melted overnight in an oven at 70 ° C, and 6.8 kg was transferred to a 10 L scale static crystallizer with a plate spacing of 42 mm, which was preheated to 55 ° C. Next, 1.7 kg of acetone (technical grade, ThermoScientific) was added and the mixture was homogenized by manual stirring. By an external water bath, the temperature of the crystallizer was regulated. Therefore, the jacket temperature was cooled to 26 ° C from 55 ° C with 20 ° C / h, and kept at this temperature for 15 minutes, when crystallization began to occur on the plate. Then, cooling continued to 15 ° C from 26 ° C with 2 ° C / h, followed by an isothermal period, until the next morning (about 12 hours isothermal). A noticeable layer of crystals was observed on the plates, and approximately 4 kg of mother liquor was discharged from the crystallizer.

[0351] The crystallizer was then heated to 44°C for 15 minutes while collecting 288 g of a partial melt. Finally, the meso-lactide crystals were melted by heating the crystallizer to 60°C and maintaining this temperature for 4.5 hours. Three different fractions of the crystals were collected separately, and the last two fractions were combined to yield 2.1 kg of meso-lactide product with a free acid level of 3.7 meq / kg (31% yield).

[0352] Example 7: Polymeso-lactide Production

[0353] This example illustrates the polymerisation of a purified meso-lactide stream as defined herein to form polymeso-lactide according to step e) of the process.

[0354] A purified meso-lactide stream with a free acid content below 4.0 meq / kg was collected in a feed tank for use in a pilot-scale PLA polymerization unit. The meso-lactide feed rate was 65 kg / h, and 40-50 ppm of tin octoate was applied to catalyze ring-opening polymerization in the presence of an alcohol initiator to control molecular weight. The polymerization residence time was approximately 3 hours, and the polymerization temperature was between 130 and 200°C. After catalyst deactivation, residual meso-lactide was stripped in a devolatilization unit. The polymeso-lactide product was then pelletized underwater, quenched with 20°C water, separated from the water by a cyclone, and dried under dehumidified air at 30°C. Table 4 lists the product properties of the polymeso-lactide obtained in three different runs. The properties listed were determined using the methods described in the previous section.

[0355] Table 4. Properties of poly-meso-lactide products

[0356]

[0357] From the above data, it can be concluded that a polymeso-lactide polymer product can be produced having a high molecular weight, acceptable yellowness and low residual monomer content. The product can be dried to a water level of <500 ppm.

[0358] Example 8. Disintegration behavior of extruded sheets

[0359] Polymeso-lactide (see Run 2 in Table 2) having a weight-average molecular weight of 219 kg / mol and a residual lactide content of 0.13% by weight, as determined using the method given above, was converted into a 100-micrometer-thick sheet by melt extrusion and passing through a sheet die. Samples of the sheet were subjected to an industrial composting test at 58 ± 2°C according to EN 13432. After one week of testing, the sheet completely disintegrated.

[0360] Similar sheets made from PLA, which is primarily based on L-lactide, do not disintegrate as quickly. Typically, such sheets completely disintegrate only after more than two weeks under the same conditions. Thus, this example demonstrates the faster hydrolysis of articles made from poly-meso-lactide.

[0361] Example 9. Polymerization of 80% L-lactide and 20% purified meso-lactide

[0362] A 2 L batch polymerization reactor was subjected to three vacuum / nitrogen cycles to remove oxygen and moisture. Subsequently, the reactor was filled with 739 g of L-lactide (Lumilact L, TotalEnergies Corbion, 2 meq / kg free acid content) and 82 g of meso-lactide (10 meq / kg free acid content). After the lactide was transferred to the reactor, the reactor was again subjected to three vacuum / nitrogen cycles.

[0363] The lactide mixture was heated to 130°C, after which 20 meq of 1-decanol (2-ethyl-1-hexanol) (AlfaAesar) and 150 ppm of tin octoate (Sigma, as a 10 wt% solution in dry toluene) were added to initiate polymerization. The temperature of the melt was increased to 180°C, and polymerization was allowed to proceed for 120 minutes. After 120 minutes, the reactor was sampled, and after catalyst deactivation, the reactor contents were drained and quenched.

[0364] The obtained product is a copolymer of meso-lactide and L-lactide with a stereochemical purity of 5.2% D, an Mw of 106 kg / mol, and high thermal stability.

Claims

1. A process for producing polymeso-lactide (PML), comprising the following steps: a) forming lactic acid oligomers by polycondensing lactic acid; b) depolymerizing the lactic acid oligomers to form crude lactide, wherein the crude lactide comprises L-lactide and / or D-lactide, and meso-lactide, and wherein the meso-lactide content is between 2.0 and 40.0 wt % based on the total weight of the crude lactide; c) subjecting the crude lactide to purification to separate an L-lactide-enriched stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt.% meso-lactide, based on the total weight of the stream, and has an acidity level of 30.0 to 1000.0 meq / kg; d) purifying the meso-lactide-enriched stream into a purified meso-lactide stream by subjecting the meso-lactide-enriched stream to crystallization, the purified meso-lactide stream comprising at least 94.0 wt% meso-lactide, based on the total weight of the stream, and having an acidity level of at most 20.0 meq / kg; and e) polymerizing at least a portion of the purified meso-lactide stream to form polymeso-lactide.

2. The process of claim 1 , wherein the crude lactide formed in step b) comprises L-lactide and D-lactide, and wherein the weight ratio of L-lactide to D-lactide (L / D ratio) is different from 1.0, and preferably higher than 1.

0.

3. The process according to any one of the preceding claims, wherein the amount of L-lactide in the crude lactide is at least 50.0 wt. %, preferably at least 55.0 wt. %, preferably at least 60.0 wt. %, preferably at least 65.0 wt. %, preferably at least 70.0 wt. %, wherein the wt. % are based on the total weight of the crude lactide.

4. The process according to any one of the preceding claims, wherein the L-lactide-rich stream comprises at least 50.0 wt.% L-lactide, preferably at least 60.0 wt.% L-lactide, preferably at least 65.0 wt.% L-lactide, preferably at least 70.0 wt.% L-lactide, preferably at least 75.0 wt.% L-lactide, preferably at least 80.0 wt.% L-lactide, preferably at least 85.0 wt.% L-lactide, preferably at least 90.0 wt.% L-lactide, preferably at least 95.0 wt.% L-lactide, wherein the wt.% are based on the total weight of the lactide-rich stream.

5. The process according to any one of the preceding claims, wherein the meso-lactide enriched stream is purified to an acidity level equal to or higher than 1.0 meq / kg, preferably equal to or higher than 1.5 meq / kg.

6. The process according to any one of the preceding claims, wherein the meso-lactide enriched stream is purified to an acidity level of at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg.

7. The process according to any one of the preceding claims, wherein the purified meso-lactide stream comprises at least 95.0 wt% meso-lactide, preferably at least 97.0 wt% meso-lactide, based on the total weight of the stream.

8. The process according to any one of the preceding claims, wherein the portion of the purified meso-lactide stream that is polymerized in step e) amounts to at least 30.0 wt.%, or at least 40.0 wt.%, or at least 50.0 wt.% of the purified meso-lactide stream.

9. The process according to claim 1 , further comprising the step f) of blending at least a portion of the purified meso-lactide stream not polymerized in step e) with at least a portion of the lactide-rich stream obtained in step c) to provide a lactide mixture, and subsequently polymerizing the lactide mixture to form polylactide, wherein the polymerized meso-lactide content is less than 20.0 wt. % of the polylactide.

10. The process according to any one of the preceding claims, wherein the crystallization is solvent crystallization or melt crystallization, wherein the solvent or melt crystallization is performed as suspension crystallization or layer crystallization.

11. The process according to any of the preceding claims, wherein at least part of the lactic acid applied in step a) has an isomeric purity of 80.0% or more, such as 85.0% or more, for the L-isomer, and / or has an isomeric purity of 80.0% or more, such as 85.0% or more, for the D-isomer.

12. The process according to any one of the preceding claims, wherein part of the lactic acid applied in step a) is racemic lactic acid, preferably wherein the part is at most 40.0 wt.-% racemic lactic acid, wherein the wt.-% is based on the total amount of lactic acid applied in step a).

13. The process according to any one of the preceding claims, wherein at least part of the lactic acid applied in step a) is prepared by depolymerizing poly-L-lactic acid.

14. The process according to claim 13, wherein poly-L-lactic acid is depolymerized by hydrolyzing the poly-L-lactic acid in the presence of water and / or lactic acid as co-reactants, preferably for 30 minutes to 24 hours, under conditions ranging from atmospheric conditions up to 10 bar and at a temperature ranging from 120 to 200°C.

15. The process according to any one of the preceding claims, wherein the depolymerization is carried out in step b) at a temperature between 175 and 220°C.

16. The process according to any one of the preceding claims, wherein the amount of meso-lactide in the crude lactide is upregulated by stimulating racemization during steps a) and / or b).

17. The process according to claim 16, wherein racemization is stimulated by adding a racemizer during step a) and / or b), preferably in an amount ranging from 5 to 5000 ppm, preferably the racemizer is selected from the group comprising sodium salts such as sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen containing ligands such as pyridine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene and 1,8-diazabicyclo[5.4.0]undec-7-ene, or any combination thereof.

18. The process according to any one of the preceding claims, wherein the polymerization of a portion of the purified meso-lactide stream to polymeso-lactide is carried out by means of ring-opening polymerization in the presence of a catalyst, preferably a metal catalyst, and optionally one or more initiators, at a temperature comprised between 125 and 225°C, preferably wherein the initiator is an alcohol.

19. Polymeso-lactide obtainable according to or according to the process of any one of the preceding claims 1 to 18, and preferably wherein the polymeso-lactide has one or more of the following properties: - 40.0 to 60.0% stereochemical purity of the L-lactate, or 40.0 to 60.0% stereochemical purity of the D-lactate, - a weight-average molecular weight between 50 and 500 kg / mol, - Glass transition temperature between 30°C and 50°C.

20. A polymer composition comprising (A) polylactide, and (B) polymeso-lactide according to claim 19 or obtainable according to or according to the process of any one of the preceding claims 1 to 18, wherein the polymer composition comprises less than 10.0 wt.% of the polymeso-lactide, based on the total weight of the polymer composition.

21. Use of polymeso-lactide according to claim 19 or obtainable according to or by a process according to any one of the preceding claims 1 to 18, for preparing an article or composition, and / or for preparing a polymer composition, wherein the polymer composition comprises polylactide and polymeso-lactide, and wherein the polymer composition comprises less than 10 wt % of the polymeso-lactide, based on the total weight of the polymer composition, and / or As a resin for coatings or as an adhesive resin, for example, for producing adhesive films or adhesive layers or multilayer structures.

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