Process for producing a polymeso-lactide

Abstract

The present invention relates to a process for producing a polymeso-lactide (PML), to a polymeso-lactide obtainable or obtained according to such process and to various uses thereof. The process in particular involves a step of polymerising a portion of a purified meso-lactide stream, that has been separated from a crude lactide mixture and subsequently purified, to form polymeso-lactide. In certain embodiments, the present process further involves the preparation of a lactide mixture by blending another portion of the purified meso- lactide with a lactide-rich stream, and also involves the polymerisation of such mixture to form a polylactide (PLA) that comprises polymerised meso-lactide. The present invention also relates to a polylactide comprising polymerised meso-lactide obtainable or obtained according to such process, and uses thereof.

Description

[0001] PROCESS FOR PRODUCING A POLYMESO-LACTIDE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of polylactide processing. In particular, the present invention relates to a process for producing polymeso-lactide. The present invention also relates to a polymeso-lactide, produced according to a process as provided herein, and to various uses thereof. The present invention also relates to a process for preparing and polymerising lactide mixtures comprising meso-lactide, and to the polylactide polymers thereby obtained.

[0004] BACKGROUND

[0005] Bio-plastics have gained tremendous attention, due to the increasing environmental pressure on global warming and plastic pollution. Among them, polylactide, which is also referred to as polylactic acid and abbreviated as PLA, is both bio-based and bio-degradable polymer, which has, due to its good processability and mechanical properties, been widely used in many applications, such as for making packaging products like food packaging, or for making single used items, e.g. for medical applications.

[0006] Polylactide can be made industrially by converting lactic acid to lactide (the cyclic dimer of lactic acid), which is then polymerized via Ring-Opening Polymerization (ROP) catalysed by organometal catalysts. The process for making polylactide generally involves forming a low molecular weight poly(lactic acid) first, 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 as may be present. This can be done by distillation or by other methods such as recrystallization, either from a solvent or from a melt.

[0007] Lactic acid is a molecule with one chiral center, and so it exists in two enantiomeric forms, the so-called R-(or D-) enantiomer and the S-(or L-) enantiomer. The optical purity of lactic acid strongly affects the characteristics of the resulting PLA. Therefore, the starting lactic acid is usually of very high optical purity. However, the starting material is subjected to elevated temperatures as it is converted to the low molecular weight poly(lactic acid) polymer and subsequently depolymerized. Some racemization, i.e. , conversion of one enantiomeric form to the other, occurs under those conditions. Because this racemization occurs, the lactide obtained from the process will be a mixture of L-lactide, D-lactide and meso-lactide.

[0008] It is often the case that the lactide mixture produced as described above contains more mesolactide than is wanted in the downstream polymerization step. For instance, if very high optical purity of the PI_A is needed, all or nearly all of the meso-lactide must be removed from the lactide stream that is taken to polymerization. Hence, in such cases, meso-lactide is an undesired by-product obtained during PLA preparation, and its occurrence must be minimised, or the meso-lactide must be removed from the lactide mixture.

[0009] In principle, meso-lactide that is removed during the PLA preparation process can be converted back into lactic acid by hydrolysis with water. Also, meso-lactide that is removed from the lactide mixture during the PLA preparation process can be stockpiled and added back into a predominately S,S-lactide (or predominantly R,R-) stream, if it is desired at some later time to produce a more amorphous polylactide grade that has a higher proportion of the R- enantiomer (or S-enantiomer as the case may be).

[0010] However, the current market of PLA uses PLA products with on global product average incorporating only minor amounts of meso-lactide, typically less than 10%. Hence, the demand for amorphous PLA grades, and hence the demand to add meso-lactide during a PLA preparation process is relatively low.

[0011] In addition, this procedure requires additional effort in terms of handling, storying, and transport of meso-lactide.

[0012] Moreover, in practice, the removed meso-lactide stream may be highly contaminated with impurities, and difficulties and costs of removing such impurities from the meso-lactide have the consequence that the meso-lactide is usually discarded, or used in other, lower-value applications, e.g. for the preparation of lactic acid esters for solvents and the preparation of alkali metal and alkaline earth metal salts of lactic acid for feedstuffs and preservatives.

[0013] Overall, for a PLA process, removal of meso-lactide streams, storing thereof, and / or other recycle thereof in the process, represent a loss of overall yield and an increase in the overall cost of the PLA preparation process.

[0014] It is therefore an object of the present invention to provide an improved process which overcomes at least some of the above-mentioned drawbacks, and in which meso-lactide streams, that are obtained during PLA processing, can be valorised, preferably as polymer products which have highest value.

[0015] It is an object of the present invention to provide a process wherein meso-lactide, formed during the PLA preparation process, can be convert and / or re-used to make technically usable polymer(s).

[0016] It is further an object of the present invention to raise the overall process yield of a PLA preparation process. SUMMARY OF THE INVENTION

[0017] It has now surprisingly been found that some or all of the above demands and objectives can be attained either individually or in any combination by a process as defined herein. The process of the invention optimises the use of meso-lactide streams, that are obtained during PLA processing.

[0018] To that end, the present invention provides a process for producing a polymeso-lactide (PML) comprising the steps of: a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said 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%, based on the total weight of the stream, of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt%, based on the total weight of the stream, of meso-lactide and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide- enriched stream to crystallisation; and e) Polymerising at least a portion of said purified meso-lactide stream to form polymesolactide, and f) Optionally blending at least a portion of the purified meso-lactide stream that is not polymerised 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 said lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight of the polylactide.

[0019] In preferred embodiments, the present invention provides a process for producing a polymesolactide (PML), comprising the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said crude lactide to purification, thereby separating an L-lactide-rich stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt%, based on the total weight of the stream, of mesolactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt%, based on the total weight of the stream, of meso-lactide and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide- enriched stream to crystallisation; and e) Polymerizing at least a portion of said purified meso-lactide stream to form polymesolactide.

[0020] In certain preferred embodiments of a process of the invention, the crude lactide formed in step b) comprises L-lactide and D-lactide, and the weight ratio of the L-lactide to D-lactide (L / D ratio) is different from 1.0, and preferably higher than 1.0.

[0021] In certain preferred embodiments of a process of the 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%, with wt% based on the total weight of the crude lactide.

[0022] In certain preferred embodiments of a process of the invention, the lactide-rich stream is an L-lactide-rich stream which comprises 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, with wt% based on the total weight of the lactide-rich stream.

[0023] In certain preferred embodiments of a process of the invention, the meso-lactide-enriched stream is purified to an acidity level of equal to or higher than 1 .0 meq / kg, preferably equal to or higher than 1.5 meq / kg.

[0024] In certain preferred embodiments of a 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 said purification is carried out by means of crystallisation such as solvent or melt crystallisation.

[0025] In certain preferred embodiments of a process of the invention, the purified meso-lactide stream comprises at least 95.0 wt% of meso-lactide, preferably at least 97.0 wt% of mesolactide, with wt% based on the total weight of the stream. In certain preferred embodiments of a process of the invention, the portion of purified mesolactide stream that is polymerised in step e) amounts to at least 30.0 wt% of the purified mesolactide 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.

[0026] In certain preferred embodiments of a process of the invention crystallisation as applied in step d) is solvent crystallisation or melt crystallisation, wherein the solvent or melt crystallisation is performed as a suspension crystallisation or layer crystallisation.

[0027] The lactic acid as applied in step a) as starting material in the present process may have different origins. According to certain embodiments of a process of the invention part of the lactic acid applied in step a):

[0028] - has an isomeric purity of the L-isomer of 80.0 % or more, such as 85.0 % or more, and / or

[0029] - has an isomeric purity of the D-isomer of 80.0 % or more, such as 85.0 % or more, and / or

[0030] - is a racemic lactic acid.

[0031] It is preferred that the lactic acid applied in step a) comprises at most 40.0 wt% racemic lactic acid, with wt% based on the total amount of lactic acid applied in step a).

[0032] Alternatively, or additionally thereto, at least part of the lactic acid applied in step a) may also be prepared by depolymerising a poly-L-lactic acid and / or a poly-D-lactic acid. For instance, in certain embodiments, of such process, the poly-L-lactic acid and / or poly-D-lactic acid is depolymerised by hydrolysing said poly-L-lactic acid and / or a poly-D-lactic acid in the presence of water and / or lactic acid as co-reactant(s), preferably for 30 minutes to 24 hour, under conditions ranging from atmospheric conditions up to 10 bar, and at a temperature ranging from 120 to 200°C.

[0033] Regarding step b) of the present process, and according to certain embodiments of the present process, depolymerisation is carried out in step b) at a temperature of between 175 and 220°C.

[0034] A process according to the present invention may further by fine-tuned and tailored according to certain needs. For instance, according to certain embodiments of the present process, it is possible to increase the amount of meso-lactide that is formed during the process. In certain embodiment of the present process, the amount of meso-lactide in the crude lactide is up- regulated (increased), by stimulating racemization during step a) and / or b) of the process.

[0035] Alternatively, or additionally thereto, in certain embodiments, said racemization during step a) and / or b), is stimulated by adding a racemizing agent, during these step a) and / or b), preferably in an amount ranging from 5-5000 ppm. Preferably said racemizing agent is selected from the group comprising sodium salts like sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen-containing ligands such as pyridines, 1 ,5,7- triazabicyclo[4.4.0]dec-5-ene and 1 ,8-diazabicyclo[5.4.0]undec-7-ene, or any combinations thereof.

[0036] In certain embodiment of a process of the invention, polymerisation of a portion of the purified meso-lactide stream into polymeso-lactide, during step e) of the process, is carried out by means of a ring-opening polymerisation at a temperature comprised between 125 and 225°C in the presence of a catalyst, preferably a metal catalyst, and optionally one of more initiators, preferably wherein said initiator is an alcohol. As meso-lactide is more volatile than L-lactide and polymeso-lactide shows lower viscosities than known PLA resins in the art at the same molecular weight, polymeso-lactide production may be optimized by down-tuning polymerization temperatures. This may for example be below 190°C and even below 175°C in the polymerization section. In the devolatilization section, stripping temperatures may be below 200°C to still afford effective removal of the residual meso-lactide.

[0037] In certain embodiments of a process of the invention, polymerisation of a lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight of the polylactide in accordance with the present process is also carried out by means of a ringopening polymerisation at a temperature comprised between 125 and 225°C, or between 125 and 190°C, or between 125 and 175°C, in the presence of a catalyst, preferably a metal catalyst, and optionally one of more initiators, preferably wherein said initiator is an alcohol.

[0038] Advantages of the present process involve one or more of the following:

[0039] The process of the invention allows to convert or re-use meso-lactide to prepare polymeso-lactide; The process of the invention also allows to re-use meso-lactide for making lactide mixtures comprising meso-lactide, and to prepare valuable PLA polymers from such mixtures;

[0040] The process of the invention further provides a versatile process, wherein the amount of meso-lactide, and polymers prepared therewith, may be adapted (e.g. increased) in function of needs. The present invention for instance allows to increase the amount of meso-lactide used during the PLA preparation process by using less stereochemically pure lactic acid as starting product, or by using lactic acid from chemical recycling of stereochemically diverse PLA waste, or by applying higher synthesis temperatures when making crude lactide, or by deliberately increasing racemization during the process, or by a combination of any of the foregoing. It follows that the present invention may start from starting materials of lower stereochemical purity and / or to apply less stringent polymerisation conditions in a costefficient way.

[0041] The preparation of meso-lactide may be tailored, such that more meso-lactide, including meso-lactide of lower purity, can be made and re-used. Re-use of mesolactide containing streams within the PLA process also excludes or reduces the need for stockpiling meso-lactide, and reduces the costs associated therewith.

[0042] The present invention thus provides a process wherein, during the same process, mesolactide is polymerised into polymeso-lactide, and optionally also lactide mixtures comprising (re-cycled) meso-lactide may be subjected to polymerisation for preparing (producing) PLA polymers with well-defined properties. The present process therefor provides for an optimal valorisation and use of meso-lactide formed during the PLA preparation process.

[0043] Overall, the present invention also provides a process for producing polylactide, including polymeso-lactide, in which the overall yield of the process is optimised and improved. The present invention allows to separate a highly purified L-lactide stream, which may be further polymerised into (crystallisable) PLA, as well as a highly purified meso-lactide stream, which may be further polymerised into (crystallizable) polymeso-lactide. The present invention thus also provides an efficient process for preparing a crystallizable polymeso-lactide.

[0044] In another aspect, the present invention also provides a polymeso-lactide obtainable or obtained according to a process as provided herein, and preferably wherein the polymesolactide has one or more of the following properties:

[0045] (i) a stereochemical purity of 40.0 to 60.0 % L-lactate; or a stereochemical purity of 40.0 to 60.0 % D-lactate;

[0046] (ii) a weight-average molecular weight (Mw) of between 50 to 500 kg / mol, or between 100 and 350 kg / mol;

[0047] (iii) a glass transition temperature between 30 and 50°C, or between 35 and 50°C.

[0048] In certain embodiments, the polymeso-lactide obtainable or obtained according to a process as provided herein is amorphous. In certain other embodiments, the polymeso-lactide obtainable or obtained according to a process as provided herein is semi-crystalline.

[0049] In yet another aspect, the present invention also relates to the use of a polymeso-lactide as defined herein, or as obtainable or obtained by carrying out a process according to the invention. In certain embodiments, present invention relates to the use of a polymeso-lactide as defined herein, or as obtainable or obtained by carrying out a process according to the invention for preparing an article or composition.

[0050] The present invention also relates to the use of a polymeso-lactide as defined herein, or as obtainable or obtained by carrying out a process according to the invention, for preparing a degradable article, preferably a solid degradable article. For instance, a polymeso-lactide as defined herein, or obtainable or obtained by carrying out a process according to the invention can be used for making articles that (slowly) degrade during use, such as articles capable of providing a slow release of lactic acid.

[0051] The present invention also relates to the use of a polymeso-lactide as defined herein, or obtainable or obtained by carrying out a process according to the invention, as a (temporary) diverting agent, for instance for use in down-hole applications or in fracking applications.

[0052] The present invention also relates to the use of a polymeso-lactide as defined herein, or obtainable or obtained by carrying out a process according to the invention, for preparing a polymer composition, wherein the polymer composition comprises (A) polylactide, and (B) said polymeso-lactide, and wherein the polymer composition comprises less than 10 wt%, based on the total weight of the polymer composition, of said polymeso-lactide.

[0053] The present invention also relates to the use of a polymeso-lactide as defined herein, or as obtainable or obtained by carrying out a process according to the invention as a resin, e.g. a coating resin or as an adhesive resin, e.g. for preparing an adhesive film or adhesive layer or a multilayer structure.

[0054] In another aspect, the present invention also relates to a polymer composition comprising (A) polylactide, and (B) a polymeso-lactide as defined herein, or as obtainable or obtained by carrying out a process according to the invention, wherein said polymer composition comprises less than 10 wt%, based on the total weight of the polymer composition, of said polymeso-lactide.

[0055] 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 or other dependent claims as appropriate.

[0056] The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. DETAILED DESCRIPTION OF THE FIGURE

[0057] Figure 1 is a schematic diagram illustrating an embodiment of the process of the invention.

[0058] DETAILED DESCRIPTION OF THE INVENTION

[0059] When describing the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

[0060] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0061] In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0062] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while certain embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

[0063] The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of", "consists" and "consists of".

[0064] 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. By way of example, "a step" means one step or more than one step.

[0065] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art. The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0066] The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / -10% or less, preferably + / -5% or less, more preferably + / -1% or less, of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" refers is itself also specifically, and preferably, disclosed.

[0067] The terms “wt%,” “vol%”, or “mol%” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, which includes the component.

[0068] The terms “isomeric purity” and “stereochemical purity” are used herein interchangeably, and are expressed as wt%, and refer to the amount of a stated stereoisomer expressed as percentage of the total amount of stereoisomers having a given chiral center.

[0069] When describing the present invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

[0070] Preferred statements (features) and embodiments and uses of this invention are set herein below. Each statement and embodiment of the invention so defined may be combined with any other statement and / or embodiment unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features or statements indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below numbered statements and embodiments, with any other aspect and / or embodiment.

[0071] 1 . Process for producing a polymeso-lactide (PML) comprising the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said 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%, based on the total weight of the stream, of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt%, based on the total weight of the stream, of meso-lactide and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide- enriched stream to crystallisation; and e) polymerizing at least a portion of said purified meso-lactide stream to form polymesolactide.

[0072] 2. Process for producing a polymeso-lactide (PML) comprising the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said crude lactide to purification, thereby separating an L-lactide-rich stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt%, based on the total weight of the stream, of mesolactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt%, based on the total weight of the stream, of meso-lactide and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide- enriched stream to crystallisation; and e) polymerising at least a portion of said purified meso-lactide stream to form polymesolactide.

[0073] 3. 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 the L-lactide to D-lactide, L / D ratio, is different from 1.0, and preferably is higher than 1.0.

[0074] 4. Process according to any one of the preceding statement, wherein said 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.

[0075] 5. Process according to any one of the preceding statements, wherein said meso-lactide- enriched stream is purified to an acidity level of equal to or higher than 1.0 meq / kg, preferably equal to or higher than 1 .5 meq / kg. 6. Process according to any one of the preceding statements, wherein said meso-lactide- enriched stream is purified to a purified meso-lactide stream comprising an acidity level of 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.

[0076] 7. Process according to any one of the preceding statements, wherein the portion of purified meso-lactide stream that is polymerised 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.

[0077] 8. Process according to any one of the preceding statements, further comprising the step f) of blending at least a portion of the purified meso-lactide stream that is not polymerised in step e) with at least a portion of the lactide-rich stream, preferably an L-lactide rich stream, obtained in step c) to provide a lactide mixture, and subsequently polymerizing said lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight of the polylactide.

[0078] 9. Process according to any one of the preceding statements, wherein the meso-lactide content in said crude lactide is between 4.0 and 30.0 wt%, based on the total weight of the crude lactide, or between 6.0 and 30.0 wt%, or between 7.0 and 25.0 wt%, or between 8.0 and 20.0 wt%.

[0079] 10. Process according to any one of the preceding statements, wherein the purification in step c) is done by means of distillation.

[0080] 11. Process according to any one of the preceding statements, wherein said crude lactide is purified by subjecting said crude lactide to a distillation, preferably a 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.

[0081] 12. Process according to any one of the preceding statements, wherein said crude lactide is purified by

[0082] - subjecting said crude lactide to a pre-treatment, preferably a crystallisation, thereby obtaining a mother liquor comprising meso-lactide, and L-lactide and / or D-lactide, and

[0083] - subjecting said mother liquor to a distillation, preferably a 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, which 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 / kg.

[0084] 13. Process according to any one of the preceding statements, wherein crystallisation is solvent crystallisation or melt crystallisation, wherein said either solvent or melt crystallisation is performed as a suspension crystallisation or layer crystallisation.

[0085] 14. Process according to any one of the preceding statements, wherein crystallisation is melt crystallisation, and preferably wherein crystallisation is performed using a static melt crystalliser.

[0086] 15. Process according to any one of the preceding statements, wherein the polymeso-lactide is further processed into an article, and preferably an article selected from the group comprising a molded article, film, sheet, fiber, filament, fabric, spunmelt nonwoven, and a layer of a multilayer article.

[0087] 16. Process according to any one 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.

[0088] 17. Process according to any one of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of the L-isomer lower than 95.0 % or lower than 90.0 %.

[0089] 18. Process according to any one of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of the D-isomer of 80.0 % or more, such as 85.0 % or more.

[0090] 19. Process according to any one of the preceding statements, wherein at least part of the lactic acid applied in step a) has an isomeric purity of the D-isomer lower than 95.0 % or lower than 90.0 %.

[0091] 20. Process according to any one of the preceding statements, wherein part of the lactic acid applied in step a) is racemic lactic acid, preferably wherein said part is at most 40.0 wt% of racemic lactic acid, or at most 30.0 wt% of racemic lactic acid, or at most 20.0 wt% of racemic lactic acid, or at most 10.0 wt% of racemic lactic acid, based on the total amount of lactic acid applied in step a).

[0092] 21. Process according to any one of the preceding statements, wherein at least part of the lactic acid applied in step a) is prepared by depolymerising a poly-L-lactic acid and / or a poly- D-lactic acid. 22. Process according to any one of the preceding statements, wherein at least part of lactic acid applied in step a) is prepared by depolymerising a 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.

[0093] 23. 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 depolymerised by hydrolysing said poly-L-lactic acid and / or a poly-D-lactic acid in the presence of water and / or lactic acid as co-reactant(s), 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.

[0094] 24. Process according to any one of the preceding statements, wherein lactic acid is polycondensed into lactic acid oligomers at a temperature of between 100 and 200°C, such as between 110 to 175°C, under conditions of 10 to 500 mBar, and preferably during 1 to 24 hours.

[0095] 25. 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.

[0096] 26. Process according to any one 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%, of meso-lactide, based on the total weight of the crude lactide.

[0097] 27. Process according to any one of the preceding statements, wherein the crude lactide comprises up to 40.0 wt%, or up to 35.0 wt%, or up to 30.0 wt%, of meso-lactide, based on the total weight of the crude lactide.

[0098] 28. Process according to any one of the preceding statements, wherein the crude lactide comprises at most 93.0 wt% of L-lactide, or at most 90.0 wt%, or at most 80.0 wt% or at most 60.0 wt%, of L-lactide, based on the total weight of the crude lactide.

[0099] 29. Process according to any one of the preceding statements, wherein depolymerisation is carried out in step b) at a temperature of between 175 and 220°C.

[0100] 30. Process according to any one of the preceding statements, wherein the depolymerisation, in step b), is carried in the presence of a metal catalyst, preferably a metal catalyst selected from tin oxide, tin(ll)2-ethyl hexanoate, titanium tetrabutoxide, and titanium isopropoxide.

[0101] 31 . Process according to any one of the preceding statements, wherein the amount of mesolactide in said crude lactide is up-regulated or increased, by stimulating racemization during step a) and / or b) 32. Process according to any one of the preceding statements, wherein racemization during step a) and / or b) is stimulated by increasing temperature during step a) and / or b).

[0102] 33. Process according to any one of the preceding statements, wherein racemization during step a) and / or b) is stimulated by adding a racemizing agent, during step a) and / or b), preferably in an amount ranging from 5-5000 ppm, preferably said racemizing agent is selected from the group comprising sodium salts like sodium lactate, sodium hydroxide, sodium phosphate, and nitrogen-containing ligands such as pyridines, 1 ,5,7- triazabicyclo[4.4.0]dec-5-ene, and 1 ,8-diazabicyclo[5.4.0]undec-7-ene, or any combinations thereof.

[0103] 34. Process according to any one 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, with wt% based on the total weight of the lactide- rich stream.

[0104] 35. Process according to any one of the preceding statements, wherein the purified mesolactide stream comprises at least 95.0 wt% of meso-lactide, preferably at least 97.0 wt% of meso-lactide, with wt% based on the total weight of the purified meso-lactide stream.

[0105] 36. Process according to any one of the preceding statements, wherein said purified mesolactide stream comprises less than 8.0 wt% such as less than 6.0 wt%, or less than 5.0 wt% of L-lactide, preferably less than 4.0 wt% of L-lactide, more preferably less than 2.5 wt% of L- lactide, with wt% based on the total weight of the purified meso-lactide stream.

[0106] 37. Process according to any one of the preceding statements, wherein said purified mesolactide stream comprises less than 8.0 wt% such as less than 6.0 wt%, or less than 5.0 wt% of D-lactide, preferably less than 4.0 wt% of D-lactide, more preferably less than 2.5 wt% of D-lactide, with wt% based on the total weight of the purified meso-lactide stream.

[0107] 38. Process according to any one of the preceding statements, wherein the enriched mesolactide stream obtained in step c) is purified by crystallisation, preferably by melt crystallisation, to an acidity level which is at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, such as between 0.001 meq / kg and 7.0 meq / kg.

[0108] 39. Process according to any one of the preceding statements, wherein the enriched mesolactide stream obtained in step c) is purified by crystallisation, preferably by melt crystallisation, to an acidity level which is at least 1.0 meq / kg, preferably at least 1.5 meq / kg. 40. Process according to any one of the preceding statements, wherein the enriched mesolactide stream obtained in step c) is purified by multiple stages of melt crystallization, particularly to an acidity level which is at most 15.0 meq / kg, preferably at most 10.0 meq / kg, preferably at most 7.0 meq / kg, such as between 0.001 meq / kg and 7.0 meq / kg.

[0109] 41. Process according to any one of the preceding statements, wherein the enriched mesolactide stream obtained in step c) is purified by multiple stages of melt crystallization, particularly to an acidity level which is at least 1.0 meq / kg, preferably at least 1.5 meq / kg.

[0110] 42. Process according to any one of the preceding statements, wherein the polymerisation of a portion of the purified meso-lactide stream into polymeso-lactide is carried out by means of a ring-opening polymerisation at a temperature comprised between 125 and 225°C in the presence of a catalyst, preferably a metal catalyst as defined in statement 30, and optionally one or more initiators, preferably wherein said initiator is an alcohol.

[0111] 43. Process according to any one of the preceding statements, wherein said ring-opening polymerisation is carried out continuously with a residence time between 30 min and 300 min.

[0112] 44. Process according to any one of the preceding statements, wherein said ring-opening polymerisation is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, phosphoric acid ester or polyacrylic acid (co) polymer.

[0113] 45. Process according to any one of the preceding statements, further comprising the step of demonomerising the polymeso-lactide by means of evaporation, preferably at a pressure below 50 mbar and a temperature above 175°C, for example in a flash drum, degassing extruder or wiped film evaporator.

[0114] 46. Process according to any one of the preceding statements, wherein polymerisation of a lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight of the polylactide in step f) of the present process is also carried out by means of a ring-opening polymerisation at a temperature comprised between 125 and 225°C in the presence of a catalyst, preferably a metal catalyst, and optionally one of more initiators, preferably wherein said initiator is an alcohol.

[0115] 47. Process according to statement 46, wherein said ring-opening polymerisation is carried out continuously with a residence time between 30 min and 300 min.

[0116] 48. Process according to any one of the preceding statements 46-47, wherein said ringopening polymerisation is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, phosphoric acid ester or polyacrylic acid (co) polymer. 49. Polymeso-lactide obtainable or obtained according to the process of any one of the preceding statements 1-48, and preferably wherein the polymeso-lactide has one or more of the following properties:

[0117] - a stereochemical purity of 40.0 to 60.0 % L-lactate, or a stereochemical purity of 40.0 to 60.0 % D-lactate,

[0118] - a weight-average molecular weight (Mw) of between 50 and 500 kg / mol, or between 100 and 350 kg / mol,

[0119] - a glass transition temperature between 30 and 50°C, or between 35 and 50°C.

[0120] 50. Use of a polymeso-lactide according to the preceding statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, for preparing an article, preferably an article selected from the group comprising molded articles, films, sheets, spunmelt nonwovens, fibers, filaments, fabrics, and a layer of a multilayer article.

[0121] 51 . Use of a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, as a resin for (paper) coating or as an adhesive resin, e.g. for preparing an adhesive film or an adhesive layer of a multilayer structure.

[0122] 52. Use of a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, for preparing a polymer composition, wherein the polymer composition comprises polylactide and polymeso-lactide and wherein the polymer composition comprises less than 10 wt%, based on the total weight of the polymer composition, of said polymeso-lactide.

[0123] 53. Use of a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, for preparing a degradable article or degradable composition, preferably a solid degradable article or degradable composition.

[0124] 54. Use of a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, for preparing a slow release article or composition, preferably wherein said article or composition is in a solid form, and capable of slowly releasing lactic acid.

[0125] 55. Use of a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, as a diverting agent, for instance for use in down-hole applications or in fracking applications.

[0126] 56. Polylactide obtainable or obtained by carrying out a process according to any one of the preceding statements 1-48, and preferably wherein the polylactide comprises polymerized meso-lactide in an amount of less than 20.0 % by weight of the polylactide, or less than 15.0 % by weight, or less than 10.0 % by weight of the polylactide.

[0127] 57. Polymer composition comprising (A) polylactide, and (B) a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, wherein said polymer composition comprises less than 10.0 wt%, based on the total weight of the polymer composition, of said polymeso-lactide.

[0128] 58. Article prepared with a polymeso-lactide according to statement 49, or obtainable or obtained according to the process of any one of the preceding statements 1-48, preferably wherein said article is selected from a molded article, film, film layer, tape, filaments, fibers, sheet, fabric, and spunmelt nonwoven.

[0129] 59. Article prepared with a polylactide according to statement 56, or obtainable or obtained according to the process of any one of the preceding statements 1-48, preferably wherein said article is selected from a molded article, film, film layer, tape, mono- or multicomponent filaments, fibers, sheet, fabric, coated paper, coated textile and spunmelt nonwoven.

[0130] 60. Article prepared with a polymer composition according to statement 57, preferably wherein said article is selected from a molded article, film, film layer, tape, mono- or multicomponent filaments, fibers, sheet, fabric, coated paper, coated textile and spunmelt nonwoven.

[0131] Polylactide resins are made industrially by converting lactic acid to lactide, which is then polymerized. Two molecules of lactic acid can condense, with the elimination of two molecules of water, to form a 3, 6-dimethyl-1 ,4-dioxane-2, 5-dione, which is commonly referred to as "lactide". A lactide molecule 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 perceived as a less valuable by-product.

[0132] The inventors have now found a way to valorise meso-lactide and product streams comprising meso-lactide. Thereto, the present invention provides a process for polymerizing meso-lactide into polymeso-lactide. In certain preferred embodiments of the present invention, also a process for preparing and polymerising lactide mixtures comprising meso-lactide is provided. Moreover, the present invention provides processes for polymerization of meso-lactide and / or for preparing and polymerising lactide mixtures comprising meso-lactide that can be integrated in a PLA production plant. By means of the processes as described in the present invention the total yield in a production process for preparing PLA can be improved, and the preparation of PLA products can be optimised and steered according to needs. In certain preferred embodiments of the present invention, a process for producing a polymeso-lactide (PML) is provided, wherein the process comprises the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said 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%, based on the total weight of the stream, of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt%, based on the total weight of the stream, of meso-lactide and having an acidity level of at most 20.0 meq / kg, and preferably to an acidity level at most 15.0 meq / kg, or at most 10.0 meq / kg, or at most 7.0 meq / kg, such as between 0.001 meq / kg and 7.0 meq / kg, by subjecting said meso-lactide-enriched stream to crystallisation; e) polymerising at least a portion of said 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 a polymeso-lactide, and f) optionally blending at least a portion of the purified meso-lactide stream that is not polymerised 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 said lactide mixture to form a polylactide with polymerized meso-lactide, preferably with a content of polymerized mesolactide which is below 20.0 % by weight, or below 15.0 % by weight, or below 10.0 % by weight, of the polylactide.

[0133] In certain preferred embodiments of the present invention, a process for producing a polymeso-lactide (PML) is provided, comprising the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said crude lactide to purification, thereby separating a L-lactide-rich stream and a meso-lactide-enriched stream; wherein the meso-lactide- enriched stream comprises at least 70.0 wt% of meso-lactide, with wt% based on the total weight of the meso-lactide-enriched stream, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt% of meso-lactide, with wt% based on the total weight of the purified meso-lactide stream, and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide-enriched stream to crystallisation; and e) polymerizing at least a portion of said purified meso-lactide stream to form polymesolactide.

[0134] The terms “stream”, “product stream”, and the like, may be used interchangeably herein. Preferably the acidity level of the streams obtained by carrying out the present process and steps thereof are determined using the method as explained in the example section.

[0135] The process of the present invention involves the polymerization of at least a portion of a purified meso-lactide into polymeso-lactide. To obtain a purified meso-lactide, the present process includes the step of subjecting a crude lactide, having a defined composition and meso-lactide content, to a purification in order 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 70wt% based on the separated stream.

[0136] In some embodiments, the present process involves the direct separation of a stream enriched in meso-lactide from the crude lactide. This is achieved by subjecting the crude lactide directly to a distillation, such as a fractional distillation, thereby separating a (L-)lactide-rich stream from a meso-lactide-enriched stream as defined herein.

[0137] In some other embodiments, the present process involves the indirect separation of a stream enriched in meso-lactide from the crude lactide. This may be done by first subjecting the crude lactide as defined herein, to a pre-treatment, e.g. a crystallization step, yielding lactide-rich crystals, preferably L-lactide-rich crystals, and a mother liquor comprising remaining lactide (L- and / or D- lactide) and meso-lactide. The content of meso-lactide in the such mother liquor is below 70 wt%, with wt% based on the total weight of the mother liquor, and therefore, the mother liquor may be subsequently subjecting to further purification, e.g. by means of a distillation, allowing to separate a (L-)lactide-rich stream and a meso-lactide-enriched stream, both as defined herein.

[0138] The meso-lactide-enriched stream is then further purified separately from the separated lactide-rich stream, preferably the L-lactide-rich stream, by means of crystallization(s), to yield a purified meso-lactide stream of high meso-lactide content and high purity (i.e. low acidity level), which is particularly suitable for being polymerized into polymeso-lactide.

[0139] Hereunder, particularities and properties of the process steps of the invention, the starting materials applied therein, and resulting polymers will be discussed in more detail.

[0140] A first step (step a) in a process for preparing a polymeso-lactide (PML) according to the invention involves the formation of lactic acid oligomers by polycondensing lactic acid. Lactic acid is a molecule with one chiral center, and so it exists in two enantiomeric forms, the so- called R- (or D-) enantiomer and the S- (or L-) enantiomer. A mixture of the two enantiomeric forms in equal amounts is called DL-lactic acid or racemic lactic acid.

[0141] In processes involving the conversion of lactic acid into lactide, followed by the polymerisation thereof to prepare PLA, racemization (i.e., conversion of one enantiomeric form to the other) occurs during the prepolymerization and conversion into crude lactide. Because this racemization occurs, the lactide obtained from the 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 the process, the smaller amount of racemization that may occur.

[0142] Contrary to what would be expected from the prior art, the present process is not limited to a particular enantiomeric form or purity of the lactic acid or to the use of a lactic acid enantiomeric form having a high isomeric (optical) purity as starting material. The present process may advantageously be applied starting from any enantiomeric form of lactic acid, or from mixtures thereof, including racemic mixtures thereof, and from lactic acid enantiomers having a relatively low isomeric purity. In other words, racemization that occurs during certain steps of the present process are not limiting or disadvantageous to the process of the invention.

[0143] According to certain embodiments of the present process, at least part of the lactic acid applied in step a) may for instance have an isomeric purity of the L-isomer of 80.0 % or more, such as 85.0 % or more. In certain embodiments of the present process, at least part of the lactic acid applied in step a) may have an isomeric purity of the L-isomer which is lower than 95.0 % or lower than 90.0 %.

[0144] Likewise, according to certain embodiments of the present process, at least part of the lactic acid applied in step a) may for instance have an isomeric purity of the D-isomer of 80.0 % or more, such as 85.0 % or more. In certain embodiments of the present process, at least part of the lactic acid applied in step a) has an isomeric purity of the D-isomer which is lower than 95.0 %, or lower than 90.0 %.

[0145] According to certain embodiments of the present process, part of the lactic acid applied in step a) may also be racemic lactic acid. In some embodiments, the present process provides that a part, preferably a minor part, of the lactic acid in step a) is racemic lactic acid (i.e., DL-lactic acid). Hence, the present process may allow to start from lactic acid of lower isomeric purity to increase the amount of mesolactide formed in downstream process steps, and hence to produce more polymeso-lactide.

[0146] In certain embodiments wherein DL-lactic acid (i.e., racemic lactic acid) is used in step a) of a process as described herein, it is preferred that the amount of DL-lactic acid, based on the total amount of lactic acid in step a), does not exceed an upper limit. Preferably, the part of the lactic acid applied in step a) that is racemic lactid acid is at most 40.0 wt% of racemic lactic acid, or at most 35.0 wt% of racemic lactic acid, or at most 30.0 wt% of racemic lactic acid, or at most 25.0 wt% of racemic lactic acid, or at most 20.0 wt% of racemic lactic acid, or at most 15.0 wt% of racemic lactic acid, or at most 10.0 wt% of racemic lactic acid, with wt% based on the total amount of lactic acid applied in step a). In an example, the amount of DL-lactic acid present in the lactic acid applied in step a) may be between 0.001 and 15.0 wt%. The process of the invention thus allows the use of certain amounts of racemic acid (i.e., DL-lactic acid), and hence starting materials having a lower isomeric purity.

[0147] In certain preferred embodiments, lactic acid as used herein may comprise a mixture of the two enantiomeric forms in non-equal amounts, e.g. but not limited to a mixture of 40.0 wt% of the D-enantiomer and 60.0 wt% of the L-enantiomer, or a mixture of 20.0 wt% of the D- enantiomer and 80.0 wt% of the L-enantiomer, or a mixture of 10.0 wt% of the D-enantiomer and 90.0 wt% of the L-enantiomer

[0148] The lactic acid used as starting material in the present process of the invention may also be obtained starting from polymers of lactic acid (i.e. poly(lactic acid) or PLA).

[0149] In certain embodiments of the present process, at least part of the lactic acid applied in step a) of the process can also be prepared by depolymerising a poly-L-lactic acid (PLLA) and / or a poly-D-lactic acid (PDLA). In certain preferred embodiments, a poly-L-lactic containing up to 50.0 wt%, such as up to 20.0 wt%, or up to 15.0 wt% of D-isomer, is used and depolymerisation to yield lactic acid for use in step a) of the present process. In certain other embodiments it is preferred that a poly-L-lactic acid containing less than 45 wt% of D isomer, or less than 30 wt%, or less than 25 wt%, or less than 15 wt%, or less than 10 wt%, is depolymerised. The process of the invention thus allows the use of PLLA or PDLA, starting materials containing relatively important amounts of respectively D and L isomers, and hence starting materials having a lower isomeric purity.

[0150] In addition thereto, poly-L-lactic acid and / or poly-D-lactic acid as defined herein above may be depolymerised under conditions that allow to obtain lactic acid of lower isomeric purity, than would be acceptable and wanted in conventional industrial processes. For instance, in certain embodiments of the present process, the poly-L-lactic acid and / or poly-D-lactic acid can be depolymerised by hydrolysing said poly-L-lactic acid and / or a poly-D-lactic acid in the presence of water and / or lactic acid as co-reactant(s), preferably for 30 minutes to 24 hour, under conditions ranging from atmospheric conditions up to 10 bar, and at a temperature ranging from 120 to 200°C, preferably from 130 to 200°C.

[0151] The present process advantageously allows to start from lactic acid of lower isomeric purity to increase the amount of meso-lactide formed in downstream process steps, and hence to produce more polymeso-lactide. In addition, the present process also allows to steer and control the amount of meso-lactide, and / or of mixtures comprising lactide and meso-lactide, formed in downstream process steps (more or less depending on needs), by adapting the isomeric purity of the starting lactic acid material and / or by starting from PLA materials of lower quality / purity and / or that can be hydrolysed under less stringent conditions (e.g. lower depolymerisation temperatures) as explained above. It is generally understood that a majority of PLA applications typically use PLA resins with a stereochemical purity of <98%, which implies that the recycled lactic acid is generally of lower purity than a high purity lactic acid as obtained from any fermentative process. The invention allows the use of such lower stereochemical purity lactic acid as starting material too, as it is able to cope with increased amounts of meso-lactide.

[0152] Step a) of the present process involves a polycondensation of lactic acid into lactic acid oligomers. Preferably the lactic acid is polycondensed into lactic acid oligomers at a temperature of between 100 and 200°C, such as between 110 to 175°C, under conditions of 10 to 500 mBar. Preferably polycondensation is carried out during 1 to 24 hours. The term “lactic oligomers” as used herein refers to low molecular weight polylactic acid.

[0153] Preferably, the lactic acid oligomers prepared in step a) have a degree of polymerization (DP) of 5 to 30. The "Degree of Polymerisation (DP)" refers to the number of lactic acid monomeric units in the oligomer according to the invention. The "degree of polymerization" (DP) as used herein is calculated by taking the inverse of the value of free acid in wt%. The free acid content may be measured as indicated in the methodology section given below.

[0154] The lactic acid oligomers are depolymerized by subjecting them to an elevated temperature and sub-atmospheric pressures, typically in the presence of a depolymerization catalyst, to form a crude lactide. 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 usually some other reaction by-products or those stemming from fermentation processes. In accordance with 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 said crude lactide. For instance, in certain 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%, of meso-lactide, based on the total weight of the crude lactide. In certain 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%, of meso-lactide based on the total weight of the crude lactide. In certain embodiments, the meso-lactide content in said crude lactide is between 4.0 and 30.0 wt%, based on the total weight of the crude lactide, or between 6.0 and 30.0 wt%, or between 7.0 and 25.0 wt%, or between 8.0 and 20.0 wt%.

[0155] In certain preferred embodiments of the present process, the crude lactide produced during step b) comprises different amounts (i.e., wt%) of L-lactide and D-lactide. In certain embodiments it is preferred that the weight ratio of L-lactide to D-lactide, herein expressed as “L / D ratio” or “L / D weight ratio”, in the crude lactide is different from 1.0.

[0156] In some preferred embodiments of the present process, the obtained crude lactide comprises L-lactide and D-lactide in a L / D ratio of more than 1.0, preferably more than 1.5, more preferably more than 2.0. For instance the L / D ratio may be more than 3.0, or more than 5.0, or more than 8.0, or more than 10.0, or more than 12.0. A relative higher content of L-lactide in the crude lactide provides the advantage that subsequent purification of the crude lactide in separate product streams, and crystallization of the separated product streams may be done more efficiently and provide a higher product yield, thereby resulting in a more cost-effective and improved process. Higher contents of L-lactide in crude lactide thus allows for efficient separations down-stream. For example, L-lactide contents in crude lactides of 90 wt% or more allow for efficient and high yield (downstream) crystallization of L-lactide. In addition, commercial PLA products typically require high L-lactide contents, e.g., 80 wt% or more.

[0157] In some preferred embodiments of the present process, the amount of L-lactide in the crude lactide is at least 50.0 wt%, such as at least 55.0 wt%, preferably at least 60.0 wt%, preferably at least 65.0 wt%, preferably at least 70.0 wt%, with wt% based on the total weight of the crude lactide. It is also preferred that the crude lactide comprises lower than 40.0 wt%, preferably lower than 30.0 wt%, preferably lower than 20.0 wt%, preferably lower than 10.0 wt% of D- lactide, with wt% based on the total weight of the crude lactide.

[0158] In a non-limiting exemplary embodiment of the present process, a crude lactide may comprise: between 50.0 and 97.5 wt% of L-lactide, such as between 65.0 and 97.0 wt%, with wt% based on the total weight of the crude lactide, and between 0.1 and 20.0 wt% of D-lactide; preferably between 0.5 and 10.0 wt% of D- lactide, with wt% based on the total weight of the crude lactide, and between 2.0 and 40.0 wt% of meso-lactide, such as between 5.0 and 30.0 wt% of meso-lactide, with wt% based on the total weight of the crude lactide.

[0159] It is preferred that the depolymerisation is carried out in step b) at a temperature of between 175 and 220°C. In some preferred embodiments of the present process, depolymerisation in step b), is carried in the presence of a metal catalyst, preferably a metal catalyst selected from tin oxide, tin(ll)2-ethyl hexanoate, titanium tetrabutoxide, and titanium isopropoxide.

[0160] In certain preferred embodiments of the present process, the amount of meso-lactide in the crude lactide can be up-regulated (increased), by stimulating racemization during step a) and / or b) of the process. Increasing racemisation during step a) and / or b), in order to obtain more meso-lactide in the crude lactide, can be stimulated by various means, e.g. by selecting starting materials of lower isomeric purity and / or by adapting polycondensation and / or depolymerisation conditions during these steps, as explained herein. For instance, in certain embodiments of the present process, racemisation during step a) is stimulated by increased reaction time at above 180°C and / or racemization during step b) is stimulated by increasing temperature during step a) and / or b) beyond 200°C.

[0161] Advantageously, the present process allows the depolymerisation conditions to be more lenient than what is usually applied in the present technical field, e.g. by admitting longer residence times and / or higher temperatures that what is usually applied in this technical field. In addition, in certain embodiments, the polycondensation depolymerisation conditions may even be steered in order to obtain a certain degree of racemisation, and consequently more or less meso-lactide is obtained and in line with certain needs.

[0162] Alternatively, or additionally thereto, in certain embodiments of the present process, racemization during step a) and / or b), is stimulated by adding a racemizing agent, during these step a) and / or b). Preferably, such racemizing agent is selected from the group comprising sodium salts like sodium lactate, sodium hydroxide, sodium phosphate, and nitrogencontaining ligands such as pyridines, 1 ,5,7-triazabicyclo[4.4.0]dec-5-ene and 1 ,8- diazabicyclo[5.4.0]undec-7-ene, and any combinations thereof. It is preferred that, when added, such racemizing agent is added in a range from 5-5000 ppm.

[0163] The crude lactide produced during step b) of a process of the 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. In accordance with step c) of the present process, this crude lactide is subjected to a purification whereby a lactide-rich stream, preferably an L-lactide-rich stream as defined herein, and a meso-lactide-enriched stream, as defined herein, is separated.

[0164] In accordance with the process of the present invention, the crude lactide is separated (purified) into isolated product streams (fractions), including a meso-lactide-enriched stream, before subjecting said isolated and meso-lactide-enriched stream 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 separately crystallized in a subsequent step with sufficient purity and yield. Advantageously, the relative amount of meso-lactide and acidity level (i.e., remaining free acid impurities) present in a meso-lactide-enriched stream as defined herein and obtained in step c) facilitates purification via crystallization in downstream step d).

[0165] Advantageously, the present process provides an efficient separation of a crude lactide comprising different lactide stereoisomers with (very) similar chemical properties into separate product streams that may be purified further with high efficiency. In particular, it has been found that the present process is effective for separation of a crude lactide into an L-lactide- rich stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt% based on the total weight of the stream of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg.

[0166] In the context of the present process, it will be understood that the crude lactide may be directly or indirectly separated into a lactide-rich stream, preferably an L-lactide rich stream as defined herein, and a meso-lactide-enriched stream, as defined herein.

[0167] In some embodiments, the present process involves the direct separation of a stream enriched in meso-lactide and a lactide-rich stream, preferably an L-lactide rich stream as defined herein, from the crude lactide. This may be achieved by subjecting the crude lactide directly to a distillation thereby separating a (L-)lactide-rich stream from a meso-lactide-enriched stream as defined herein. The terms ‘direct separation’ or ‘direct purification’ in this context are meant to refer to a direct treatment of the crude lactide to separate the fractions or product streams, i.e. without intermediate purification or treatment step after depolymerizing the lactic acid oligomers to form the crude lactide. Direct purification, e.g. by means of distillation, such as fractional distillation, applied directly on the crude lactide, results in a lactide-rich stream, preferably an L-lactide rich stream as defined herein, separately from a meso-lactide-enriched stream, as defined herein.

[0168] Hence, in certain embodiments, the present process involves the step of purifying said crude lactide by subjecting said crude lactide to a distillation, preferably a 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.

[0169] A preferred method to purify the crude lactide, is to fractionally distil the crude lactide stream in one or more steps. In certain embodiments, it is preferred to remove the 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 L-lactide-rich stream as defined herein, especially when the meso-lactide is removed by fractional distillation. One approach can be to distil off some or all of the 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 prior to or simultaneously with a fractional distillation step(s) in which meso-lactide-enriched stream is separated from the lactide rich stream.

[0170] The term “fractional distillation” as used herein refers to a process step used for separating components of a liquid mixture, e.g. the crude lactide produced in step b) of the present process, based on their different boiling points. This technique involves heating the mixture to create vapor and then cooling it to condense the vapor back into liquid form. Advantageously, the vaporization and condensation steps can be repeated multiple times in a fractionating column, which allows for the separation of components with similar boiling points. The result is the collection of distinct fractions, each enriched in a specific component of the original mixture.

[0171] In some other embodiments, the present process involves the indirect separation of a stream enriched in meso-lactide 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 herein interchangeably and refer to the treatment of the crude lactide in more than one step, e.g. with one or more intermediate purification or treatment steps of the crude lactide. Such additional purification or treatment steps are carried out after depolymerizing the lactic acid oligomers to form the crude lactide. For instance, the crude lactide obtained in step b) may be subjected to a crystallization yielding lactide-rich crystals, preferably L-lactide-rich crystals, and a mother liquor comprising remaining L and / or D lactide and meso-lactide. Such mother liquor contains less than 70wt% of meso-lactide. Subsequently, the obtained mother liquor is distilled to separate a (L-)lactide-rich stream from the meso-lactide-enriched stream (having a meso-lactide content of more than 70wt%) as defined herein.

[0172] In certain other preferred embodiments, the present process thus involves the step of purifying said crude lactide by (c1) subjecting said crude lactide to a pre-treatment, preferably a crystallisation step, thereby obtaining a mother liquor comprising meso-lactide, and L and / or D lactide, and

[0173] (c2) subjecting said mother liquor to a 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-enriched stream comprising at least 70.0 wt%, based on the total weight of the stream of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg.

[0174] In preferred embodiments, 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 instance contain at least 90.0 wt%, such as at least 95.0 wt% of L-lactide, with wt% based on the total amount of the crystals. The mother liquor obtained with a process as given herein, contains less than 70.0 wt% of meso-lactide. In certain embodiments, such mother liquor may even contain less than 50.0 wt% of meso-lactide. In preferred embodiments, the mother liquor may contain at least 5.0 wt% of meso-lactide, preferably at least 7.5 wt%, preferably at least 10.0 wt% of meso-lactide, with wt % based on the total amount to the mother liquor. In preferred embodiments, the mother liquid may contain at least 50.0 wt% of L-lactide, preferably at least 60.0 wt%, preferably at least 70.0 wt% of L-lactide with wt % based on the total amount to the mother liquor.

[0175] The present process step c) advantageously reduces or even avoids that impurities remain in the two separated streams, and could crystallize separately, or incorporate into the product crystals in downstream steps.

[0176] It is known to those skilled in the art of processing lactides under what conditions to distil lactide streams. Typically, distillation may occur over one or multiple distillation columns, each equipped with structured packings. Distillation is applied under reduced pressure, typically at 5-100 mbar and reflux ratios above 20.

[0177] In the process of the invention, the crude lactide stream is separated into a lactide-rich stream, such as an L-lactide-rich stream, and a meso-lactide-enriched stream. In certain embodiments, the lactide-rich stream that is obtained in step c) contains the bulk of the L- and / or D-lactide that were present in the crude lactide. The process may be operated such that the meso-lactide is not completely separated out from the crude lactide via the meso- lactide-enriched stream, in which case the lactide-rich stream will contain some meso-lactide.

[0178] In preferred embodiments of the present process, the lactide-rich stream separated in step c) comprise 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% of L- lactide, with wt% based on the total weight of the lactide-rich stream.

[0179] The term “L-lactide-rich stream” as used herein is used to describe the composition of a stream (i.e., product stream or fluid or fluid flow in a process) in terms of the stereochemistry of the lactide (molecules) present, and in the present context refers to ta stream that comprises at least 50.0 wt% L-lactide. In preferred embodiments the L-lactide-rich stream comprises at least 55.0 wt% L-lactide, preferably 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, with wt% based on the total weight of the lactide-rich stream.

[0180] In certain preferred embodiments of the present process, the L-lactide-rich stream, comprises no more than 10.0 wt% of D-lactide, preferably no more than 7.5 wt%, preferably no more than 5.0 wt%, preferably no more than 2.5 wt% of D-lactide, with wt% based on the total weight of the L-lactide-rich stream.

[0181] The meso-lactide-enriched stream contains mainly meso-lactide. When step c) is performed in a distillation column, the meso-lactide-enriched steam is taken from the column as the more volatile fraction compared to the L-lactide rich streams. In accordance with the process of the invention, the meso-lactide-enriched stream that is 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%, or 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 quantities of L- or D-lactide, but these together generally constitute no more than about 15.0 % and more preferably no more than 10.0 % by weight of the meso- lactide-enriched stream.

[0182] Further in accordance with the process of the invention, the meso-lactide-enriched stream that is 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) may be determined as indicated in the example section below.

[0183] In certain 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 preparing lactide mixtures and / or for preparing polylactide.

[0184] The meso-lactide-enriched stream obtained in step c) is subjected to a further purification in order to yield a purified meso-lactide stream. In some embodiments of the present process said purification is obtained by subjecting said meso-lactide-enriched stream to crystallisation, such as to multiple stages of crystallisation. In certain embodiments of the present process, crystallisation is solvent crystallisation or melt crystallisation. As used herein the terms solvent crystallisation and solution crystallisation are used as synonyms, and refers to a crystallization that is carried out using a solvent. In melt crystallization, no solvent is added. Crystals are generated by cooling of the melt and are formed when sufficiently cooled below the solid / liquid equilibrium. The meso-lactide is then collected in its melt form. For instance melt crystallisation may be performed in the present process as a suspension crystallisation or layer crystallisation. In preferred embodiments of the present process, the meso-lactide-enriched stream as defined herein is subjected to solvent or melt crystallisation, for instance wherein crystallisation is performed using static melt crystalliser(s). In certain embodiments of the present invention, the meso-lactide-enriched stream as defined herein is subjected to multiple stages (cycles) of solvent or melt crystallisation, for instance wherein crystallisation is performed using static melt crystalliser(s).

[0185] In some embodiments, the purified meso-lactide stream comprises: at least 94.0 wt%, based on the total weight of the stream, of meso-lactide, such as at least 95.0 wt% of meso-lactide, preferably at least 96.0 wt% or at least 97.0 wt% of meso-lactide, based on the total weight of the stream, and is therefore further enriched in meso-lactide as compared to the meso-lactide-enriched stream; and 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, such as 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,.

[0186] In some other embodiments, the purified meso-lactide stream comprises: at least 94.0 wt%, based on the total weight of the stream, of meso-lactide, such as at least 95.0 wt% of meso-lactide, preferably at least 96.0 wt% or at least 97.0 wt% of meso-lactide, based on the total weight of the stream, and is therefore further enriched in meso-lactide as compared to the meso-lactide-enriched stream; and 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 an acidity level of at least 1.0 meq / kg, preferably at least 1 .5 meq / kg.

[0187] In some preferred embodiments, a purified meso-lactide stream is provided which has at least 94.0 wt%, based on the total weight of the stream, of meso-lactide, or least 95.0 wt%, or at least 96.0 wt%, or at least 97.0 wt% of meso-lactide, with wt% based on the total weight of the purified meso-lactide stream, and an acidity level of 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.

[0188] The acidity level as referred to in the present process can be measured as defined in the example section below.

[0189] Advantageously, the present process may provide a purified meso-lactide stream with an excellent purity level, making the obtained meso-lactide stream particularly useful for preparing polymeso-lactide of suitable quality for use in various downstream applications and / or for use as a component in mixtures or blends.

[0190] The purified meso-lactide stream may contain amounts of one or more other acids such as e.g. acetic acid, succinic acid, pyruvic acid, levulinic acid, etc., which may be present as their free acids, as esters and as anhydrides, that are reduced by at least 50.0 %, at least 60.0 % or at least 70.0 % as compared to amounts in the meso-lactide-enriched stream. Reduction of free acid content (and hence purification of the stream) is preferably effected in the crystallization step.

[0191] In some embodiments of the invention, the purified meso-lactide stream comprises less than 8.0 wt% of L-lactide, preferably less than 6.0 wt% of L-lactide, more preferably less than 4.0 wt% of L-lactide, with wt% bas ed on the total weight of the stream. In some embodiments of the invention, the purified mesolactide stream comprises less than 8.0 wt% of D-lactide, preferably less than 6.0 wt% of D- lactide, more preferably less than 4.0 wt% of D-lactide with wt% based on the total weight of the stream.

[0192] A next step of the present process (i.e. step e)) involves the polymerisation of at least a portion of the purified meso-lactide stream to form polymeso-lactide. The portion of the purified mesolactide stream that is polymerised is separately polymerised. The term “separate”’ or “separately” in that context, refers to the polymerisation of a portion of the purified meso-lactide stream without the presence of other streams that may be obtained in the process of the invention, and in particular separately from (i.e. not in the presence of) the lactide-rich stream or a portion thereof as obtained in step c) of the present process.

[0193] In addition, it is preferred in accordance with the present process that the portion of purified meso-lactide stream that is separately polymerised 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%, 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 valorised, preferably as a valuable polymer product. Hence, the present invention raises the overall process yield for producing a polymeso-lactide.

[0194] For the polymerization of the meso-lactide, use can be made of catalysts and reaction conditions customary for lactide polymerization. This is an important finding as generally in the art it is assumed that the meso-lactide fraction contains most of the color forming impurities, which the current invention has been shown to be able to remove and overcome. For instance, the meso-lactide stream is fed directly into a polymerization system, where it is polymerized at elevated temperature in the presence of a catalyst, preferably a metal catalyst, and optionally one of more initiators, preferably wherein said initiator is an alcohol. The same catalysts as used in the polymerisation of L-lactide or D-lactide may be used, such as for instance from tin oxide, tin(ll)2-ethyl hexanoate, titanium tetrabutoxide, and titanium isopropoxide. However, also stereoselective catalyst may be used in a process of the invention for the polymerisation of meso-lactide, as for instance published in Ovitt et al, J. Am. Chem. Soc. 1999, 121, 4072-4073. Using such stereoselective catalysts during polymerisation may yield syndiotactic or heterotactic polymeso-lactides. Such stereoregular polymeso-lactides may show possibility to crystallize. Alcohol initiators for use in lactide polymerisation reactions are well-known to the person skilled.

[0195] Polymerization can be conducted batch-wise, semi-continuously or continuously. Continuous stirred tank reactors (CSTRs), static mixer-based and tube or pipe reactors are suitable types of polymerization vessels. Suitable polymerization temperatures preferably are from about 125°C to about 225°C. A more preferred temperature range is from 130°C to about 220°C and especially from about 135°C to about 200°C. Residence times at polymerization temperatures are selected to produce a meso-lactide polymer of a desired molecular weight, low yellowness index and / or desired conversion of monomers. The polymerizations are preferably carried out in an inert gas (dry N2) atmosphere under high pressures (generally 4-300 bar). The reaction times are preferably between 0.5 and 8 hours.

[0196] In an example, polymerisation of a portion of the purified meso-lactide stream into polymesolactide can be carried out by means of a ring-opening polymerisation, preferably at a temperature comprised between 125 and 225°C, in the presence of a catalyst, preferably a metal catalyst as defined herein. Optionally ring-polymerisation is carried out in the presence of one of more initiators, preferably said initiator is an alcohol. Also preferably, said ringopening polymerisation is carried out continuously with a residence time between 30 min and 300 min. In some embodiments of the present invention ring-opening polymerisation is terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, phosphoric acid ester or polyacrylic acid (co) polymer.

[0197] Ring-opening polymerization allows to control the polymerization process and thereby the structure of the produced polymeso-lactide (PML). This method can be used to manufacture PML of high molecular weight. The molecular weights of the polymer fabricated by the ring opening polymerization can be controlled by residence time, catalyst and initiator concentration, and temperature.

[0198] In certain embodiments of the present invention, it may be preferred that the purified mesolactide 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 before polymerisation of a portion thereof. In such embodiments, the purified meso-lactide stream may be purified by means of the various methods as disclosed herein, such as by means of crystallisation. The high purity of the purified meso-lactide allows mild polymerization conditions in turn producing polymeso-lactide with low yellowness indices generally <40 as measured by a method as described herein. Surprisingly, the polymesolactide products produced show lower viscosities than PLA resins in the art which allows lower reactor pressures and minimization of reaction temperatures.

[0199] In some preferred embodiments of the present invention, the process further comprises the step of de-monomerising the polymeso-lactide by means of evaporation, preferably at a pressure below 50 mbar and a temperature above 175°C, for example in a flash drum, degassing extruder or wiped film evaporator. After polymerization and de-monomerising, the PML can be extruded or granulated by means of a granulator, preferably an underwater granulator. Advantageously, according to the present invention, lower temperatures are needed for stripping off residual meso-lactide remaining in the PML as compared to similar processes for stripping off residual L-lactide from PLA.

[0200] The present process may further comprise the step of processing the polymeso-lactide into an article. Examples of articles that can be made from or using PML include for instance, but are not limited to a molded article, film, tape, filaments, fibers, sheet, fabric, spunmelt nonwoven. The PML as obtained in the present process may also be processed into a layer of a multilayer article, e.g. a layer of a multilayer film. The PML may also be used as a (temporary) blocking agent (diverting agent) for use in in down-hole and fracking applications. The PML may also be used as slow lactic acid release agent.

[0201] In certain embodiments of the present invention, a PML as obtained herein may be blended with other polymers, in particular with polylactides of different composition (e.g. PLLA and / or PLDA). Preferably, a resulting polymer blend may comprise PLA and PML, wherein the polymer blend comprises at most 10.0 wt%, or at most 8.0 wt% or at most 5.0 wt% of polymeso-lactide, with wt% based on the total weight of the blend.

[0202] In certain embodiments of the present invention, the process further comprises the step f) of blending a portion of the purified meso-lactide stream that is not polymerised with a portion of the lactide-rich stream in the present process to provide a lactide mixture, and subsequently polymerizing said lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight, or below 10.0 % by weight, of the polylactide. In accordance with this step f), at least a portion of the purified meso-lactide is recombined with the lactide-rich stream and the recombined streams (also denoted by “lactide mixture” herein) are polymerized together. To that end, the purified meso-lactide stream can be combined with the lactide-rich stream and the resulting lactide mixture is fed into a polymerization unit.

[0203] In certain embodiments of the present process, the portion of the purified meso-lactide stream that is recombined with the lactide-rich stream and polymerised therewith may amount to at most 50.0 wt%, or at most 40.0 wt%, or at most 30.0 wt%, or at most 25.0 w%, based on the total weight of the purified meso-lactide stream. Preferably the portion of the purified mesolactide and the portion of the lactide-rich stream that are recombined to form a lactide mixture, may be selected in order to produce a lactide mixture having a certain desired lactic acid enantiomer ratio. For example, the purified meso-lactide stream may 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.

[0204] According to preferred embodiments of the present process, polymerisation of the lactide mixture yields a polylactide (PLA) with a content of polymerised meso-lactide below 20.0 % by weight of the polylactide, and preferably below 15.0 wt% or below 10.0 wt% by weight of the polylactide.

[0205] For the polymerization of the lactide mixture, use can be made of catalysts and reaction conditions customary for lactide polymerization. Preferably polymerization of the lactide mixture is carried out by means of a ring-opening polymerisation at a temperature comprised between 125 and 225°C in the presence of a catalyst, preferably a metal catalyst, and optionally one of more initiators, preferably wherein said initiator is an alcohol.

[0206] For instance, the lactide mixture is fed directly into a polymerization system, where it is polymerized at elevated temperature in the presence of a metal catalyst, and optionally an alcohol as initiator. The same catalysts as used in the polymerisation of L-lactide or D-lactide may be used to polymerise the lactide mixture defined herein, such as for instance a metalcontaining catalyst. Examples of these catalysts include various tin compounds such as SnCI2, SnBr2, SnCI4, SnBr4, SnO, tin (II) bis(2-ethyl hexanoate), butyltin tris(2-ethyl hexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate, tetraphenyltin and the like, and preferably selected from tin oxide, tin(ll)2-ethyl hexanoate. Also, group 4 metals complexes such as zirconium, titanium and hafnium-based oxides and trisphenolates may be used. Polymerization can be conducted batch-wise, semi-continuously or continuously. Continuous stirred tank reactors (CSTRs) and tube or pipe reactors are suitable types of polymerization vessels. Suitable polymerization temperatures preferably are from about 125°C to about 225°C, or from about 160 to about 200°C. Residence times at polymerization temperatures are selected to produce a PLA polymer of a desired molecular weight, low yellowness and / or desired conversion of monomers. In an example, a lactide mixture as defined herein is polymerised via ring-opening polymerisation, which is carried out continuously with a residence time between 30 min and 300 min. In certain embodiments, the ring-opening polymerisation can be terminated by adding a catalyst inhibitor, preferably a (hydro)peroxide, phosphoric acid ester or polyacrylic acid (co) polymer.

[0207] In certain embodiments, the present invention also relates to a polylactide obtainable or obtained by carrying out a process according to the present invention and in particular by polymerization a lactide mixture as described herein. Preferably, such polylactide comprises polymerized meso-lactide in an amount of less than 20.0 % by weight of the polylactide, such as less than 15.0 % by weight or less than 10.0 % by weight of the polylactide.

[0208] In certain preferred embodiments, said polylactide is a poly-L-lactide (PLLA) obtainable or obtained by polymerization of the lactide rich stream, preferably L-lactide-rich stream, as described herein. Preferably, said PLLA has a stereochemical purity of at least 85.0% L- lactate, or at least 90.0%, or at least 95.0%, or at least 98.0%, preferably equal to 99.0% or higher.

[0209] The present process may further comprise the step of processing a polylactide as provided herein, or as obtained with a method as provided herein, into an article, such as e.g. casting into a sheet, optionally as heat seal layer in a multilayer sheet.

[0210] In another example a polylactide as provided herein, or obtained with a method as provided herein may be used as coating layer onto paper.

[0211] In another example a polylactide as provided herein, or obtained with a method as provided herein may be used as fiber component, e.g. as the sheath component of a bicomponent fiber.

[0212] Figure 1 is a schematic diagram illustrating an embodiment of the process of the invention. The embodiment illustrated in Figure 1 illustrates various preferred or optional features. Figure 1 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, analytical, control devices and the like are not shown, but of course can be used as necessary or desirable.

[0213] In Figure 1 , a lactic acid stream 7 is fed into prepolymer reactor 1. The lactic acid concentration in feed stream such as stream 7 preferably is at least 60.0% by weight, and may be as high as 95.0% by weight, or as high as 99.0% by weight, or the feed stream consists of lactic acid. Lactic acid may be obtained from a fermentation broth, which is preferably concentrated to within the aforementioned ranges in an upstream process step which is not shown in Figure 1 . Lactic acid may also be obtained by depolymerisation of a PLLA or a PDLA polymer as described herein.

[0214] The starting material is heated in prepolymer reactor 1 to cause the lactic acid to condense to form a low molecular weight poly(lactic acid) as described before; as the degree of polymerization increases, typically pressure is decreased and temperature increased. Most of the free water and water produced by polycondensation is removed from prepolymer reactor 1 as stream 16. Stream 16 can be discarded, or all or any portion of it 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 may take place in a series of reactors rather than a single reactor, whereby the degree of polymerization is increased over the reactors and different vacuum and temperature settings may be used.

[0215] As with all other process steps in which the lactic acid and its derivatives are exposed to elevated temperatures, racemization can occur in 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 is reduced until for instance in the extreme a racemic lactic acid mixture is obtained. In accordance with the present process, some racemization can be tolerated and even encouraged in this process, e.g. by applying conditions that favour racemization in the prepolymer reactor, as well as in every other process step that involves exposure of the lactic acid and downstream reaction products to elevated temperatures.

[0216] A stream of low molecular weight poly(lactic acid) (i.e. lactic acid oligomers) stream 8 is removed from prepolymer reactor 1 and transferred to 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, agitated thin-film and disk evaporators. The lactide reactor 2 is preferably operated at a pressure of from about 1 to about 100 mbar, more preferably from about 2 to about 60 mbar. A temperature, such as from about 175 to 210°C is used. The depolymerization reaction that occurs in lactide reactor 2 is usually catalysed. In the embodiment shown, catalyst is introduced directly into lactide reactor 2 through catalyst stream 17.

[0217] Crude lactide and a bottoms mixture are formed in lactide reactor 2. The bottoms mainly include (linear and cyclic) oligomers of lactic acid, and high boiling materials. The bottoms are withdrawn as bottoms stream 18. These can be discarded or recycled, with or without treatment, to an earlier step in the process. The crude lactide produced in lactide reactor 2 contains mainly L-lactide, D-lactide, meso-lactide, water, lactic acid, and some lactic acid oligomers.

[0218] Crude lactide formed in lactide reactor 2 is withdrawn as stream 9 and, in the embodiment shown, is transferred to a distillation column 3. In the embodiment shown, crude lactide stream 9 is introduced into the distillation column 3, where it is separated into a lactide-rich stream 11 and a meso-lactide-enriched stream 10. This meso-lactide-enriched stream comprises at least 70.0 wt%, based on the total weight of the stream, of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg. The lactide-rich stream 11 contains mainly L-lactide and D-lactide, and minor amounts of meso-lactide, such as below 10 wt% or between 2 and 8 wt%. A bottoms stream (not illustrated) may also be withdrawn from the distillation column. It may be understood that a series of distillation columns may be used, for example to further achieve separation of lactic acid monomer from the meso-lactide stream.

[0219] In accordance with the present invention, the meso-lactide-rich stream 10 that is removed from the system is used as a monomer source for producing a polymeso-lactide. In the embodiment shown in Figure 1 , the meso-lactide-rich stream is taken from the distillation column 3 and first fed to a crystallisation unit 4, wherein the meso-lactide-enriched stream 10 is purified. Purification may be carried out via melt crystallisation or via crystallization from a solvent. The crystallisation unit may include for example, static crystallizers, falling film crystallizers, suspension crystallizers, suspension mixed product removal crystallizers, and the like. Preferably, static crystallisers are applied. Impurities are largely excluded from the meso-lactide crystals that form in the crystallizer 4, and a purified meso-lactide stream 12 is produced. The purified meso-lactide stream may comprise at least 94.0 wt% based on the total weight of the stream of meso-lactide and has an acidity level of 20.0 meq / kg or lower. In certain embodiments, the purified meso-lactide stream may comprise an acidity level as low as 7.0 meq / kg or even lower. In certain embodiments, such low acidity level may be obtained by subjecting the meso-lactide-rich stream to multiple crystallization stages.

[0220] At least a portion of the purified meso-lactide stream 12, is separated (see stream 14A) and sent to a polymerization unit 5 to produce a polymeso-lactide 13. The portion 14A of purified meso-lactide stream that is separated and separately polymerised into polymeso-lactide 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%, with wt% based of the total weight of the purified meso-lactide stream 12.

[0221] The lactide-rich stream 11 that is removed from the system in most cases can be used as a monomer source for producing a polylactide. In certain optional embodiments of the present invention, it may be a further option to separate at least a portion 14B of the purified mesolactide and to recombine this portion 14A with the lactide stream 11 so as to form a recombined stream 19. The recombined stream 19 can then be sent to a polymerization unit 6 to produce a polylactide (PLA) that comprises a certain level of polymerised meso-lactide 15. In certain embodiments, the present process allows to produce a polylactide with a content of polymerised meso-lactide which is below 20.0 % by weight of the polylactide. In certain embodiments, polymerization units 5 and 6 may be the same polymerization unit, thereby producing polymeso-lactide and polylactide in alternating campaigns.

[0222] The present process allows using the obtained polymeso-lactide 13, and the obtained polylactide 15 in various applications, e.g. for making articles as described herein.

[0223] The present invention also relates to a polymeso-lactide obtainable or obtained by carrying out a process according to the present invention. Preferably the polymeso-lactide as described herein has one or more of the following properties:

[0224] (i) a stereochemical purity of 40.0 to 60.0 % L-lactate, or a stereochemical purity of 40.0 to 60.0 % D-lactate;

[0225] (ii) a weight-average molecular weight (Mw) of between 50 and 500 kg / mol, or between 100 and 350 kg / mol;

[0226] (iii) a glass transition temperature between 30 and 50°C, or between 35 and 50°C.

[0227] Each of these features may be determined using methods known in the art, and as further described in the example section.

[0228] Preferably, a polymeso-lactide obtainable or obtained by carrying out a process according to the present invention has one or more of the following properties:

[0229] (i) a stereochemical purity of 40.0 to 60.0 % L-lactate; or a stereochemical purity of 40.0 to 60.0 % D-lactate as determined by chiral gas chromatography after full methylation (as explained in the example section);

[0230] (ii) a weight-average molecular weight (Mw) of between 50 and 500 kg / mol, or between 100 and 350 kg / mol; as determined by gel permeation chromatography in chloroform versus polystyrene standards (as explained in the example section); (iii) a glass transition temperature between 30 and 50°C, or between 35 and 50°C, as determined by DSC analysis at 1-20 K / min scanning rate (as explained in the example section).

[0231] In certain embodiments, a polymeso-lactide as described herein is amorphous. In certain other embodiments, a polymeso-lactide as described herein is semicrystalline.

[0232] Polymeso-lactide polymers may be studied by means of homonuclear decoupled1H-NMR analysis. This type of analysis permits for example to distinguish a polymer made of 100% meso-lactide from a polymer made of a 50 / 50 mix of L- and D-lactide.

[0233] Examples of articles that can be made from or using PML as defined herein include for instance, but are not limited to a molded article, film, tape, filaments, fibers, sheet, fabric, spunmelt nonwoven. The PML as obtained in the present process may also be processed into a layer of a multilayer article, e.g. a layer of a multilayer film.

[0234] Other examples of articles that can be made from or using PML as defined herein include for instance a degradable article or degradable composition, preferably wherein said article or composition is in a solid form. For example, a degradable article or degradable composition comprising or consisting of a polymeso-lactide as defined herein, or as obtained according to a process as described herein, may include a solid component that is capable of slowly releasing acid, such as lactic acid.

[0235] Examples of compositions comprising a polymeso-lactide as defined herein, or as obtained according to a process as described herein, may for instance include slow-release compositions capable of slowly releasing a component such as lactic acid. Such compositions may be provided in a solid form. As used herein the terms “slow-release” or “slowly releasing’ is understood to be defined as a controlled-release wherein the release of the active ingredient, in the present case an acid and preferably lactic acid, is delayed for a period of time or gradually released over an extended period of time.

[0236] In certain embodiments of the present invention a polymeso-lactide according as provided herein, or obtainable or obtained according to a process of the invention may be used as a resin for (paper) coating or as an adhesive resin, e.g. for preparing an adhesive film or an adhesive layer of a multilayer structure.

[0237] In certain embodiments of the present invention a polymeso-lactide as provided herein, or obtainable or obtained according to a process of the invention may be used as a diverting agent, for instance in down-hole applications or in fracking applications. A “diverting agent” and “blocking agent” are used herein as synonyms herein. “Fracking” is the process of injecting liquid at high pressure into subterranean rocks, boreholes, etc. so as to force open existing fissures and extract oil or gas. Diverting agents are known the art to function by creating a temporary blocking effect in such type of applications,

[0238] In certain embodiments of the present invention, a polymeso-lactide as provided herein, or obtainable or obtained according to a process of the invention may also be used for preparing a polymer composition, wherein the polymer composition comprises (A) polylactide and (B) polymeso-lactide. Preferably, said “polymer composition” (herein also “polymer blend”) comprises one or more polylactides, for instance a mixture of different PLA grades, and one or more polymeso-lactides, as defined or obtained herein, wherein the total content of polymeso-lactide(s) in said polymer composition is less than 10.0 wt%, and for instance 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 a polymeso-lactide as defined herein, or as obtained according to a process as described herein, for preparing an article or a composition.

[0239] In certain 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) coating.

[0240] In certain 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 preparing an adhesive film or adhesive layer in a multilayer film or structure.

[0241] In certain 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 preparing a degradable article or composition, preferably a solid degradable article or composition.

[0242] In certain 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 preparing a slow release article or composition, preferably wherein said article or composition is in a solid form, and capable of slowly releasing lactic acid.

[0243] In certain 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 temporary blocking agents in down-hole applications or in fracking applications.

[0244] In certain embodiments of the present invention, a PML as obtained herein may be blended with other polymers, in particular with polylactides of different composition (e.g. PLLA and / or PLDA), including PLA polymers as provided herein. Preferably resulting polymer blends may comprise polylactide and a polymeso-lactide, wherein the polymer blend comprises at most 10.0 wt%, or at most 8.0 wt% or at most 5.0 wt% of polymeso-lactide, with wt% based on the total weight of the blend. In an example, such polymer blend may be used to make articles, such as fibers, and for instance bicomponent fibers, wherein the PLA is the sheath and the PML is the core material. The following examples serve to merely illustrate the invention and should not be construed as limiting its scope in any way. While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the scope of the invention.

[0245] EXAMPLES

[0246] METHODOLOGY

[0247] Free Acid content (acidity level)

[0248] The free acid content (free acidity) of a stream, obtained in a process of the invention, particularly in a lactide containing stream, such as the meso-lactide-enriched stream, the purified meso-lactide stream, or the lactide-rich stream, and expressed as amount of free carboxylic acid groups in milli-equivalents per kg (meq / kg), may be measured by potentiometric titration of a sample of the stream, using sodium methylate or potassium methylate in water-free methanol. A suitable titration device may be a Mettler Toledo T5 titrator.

[0249] In an example, a sample from an L-lactide rich stream was titrated by this method. A titration vial was filled with 2.7844 g of the sample and subsequently dissolved at room temperature in a solution consisting of 40 ml of 30% v / v dichloromethane in dried methanol. Subsequently, the solution was titrated by a 0.0108 M solution of potassium methylate in dried methanol until reaching the equivalent point at pH 8.4. The was reached at a consumption of 1.8976 ml of the titrant. A blank sample was also titrated using 0.1720mL titrant. Free acidity was then calculated by the following formula:

[0250] V. T. 1000 where

[0251] V = titrant volume in ml of sample - titrant volume in ml of blank

[0252] T = titer of titrant in mol / L m = sample weight (g)

[0253] The free acid content for this example is 6.7 meq / kg.

[0254] Stereochemical purity (L / D content)

[0255] The stereochemical purity of polymeso-lactide (PML) can be assessed after destructive methylation. To that end 0.1 g of PML is brought into a crimp cap vial, subsequently 2.0 g of dichloromethane (pure, Acros Organics) and 5.0 g of methanol (J.T. Baker) are added and the sample is allowed to dissolve for 2 hours at 70°C. After cooling down to room temperature, 3.0 g of Amberlyst 15 (ion exchange resin, dry, Acros Organics) is added and the reaction is allowed to proceed for 22 hours at 80°C. After cooling down to room temperature, the sample is subjected to chiral gas chromatography separation on a Thermo Focus GC equipped with a CP-Chirasil-dex CB separation column. This achieves separation of the R- (or D) and S- (or L-)methyl lactates, the ratio of which finally determines the stereochemical purity of the sample.

[0256] Relative molecular weight parameters Mn, Mwand polydispersity index (PDI) can be determined using Gel Permeation Chromatography (GPC) with an ELSD detector. In particular, an Agilent HPLC Infinity II system can be used with chloroform (HPLC grade, stabilized with 1 % ethanol) and 5% methanol (HPLC grade) as solvent at a flow rate of 1.0 mL / min. The size exclusion columns used can be a PLgel 5pm Guard column (50x7.5mm) and two PLgel 5pm MIXED-C columns (300x7.5mm) connected in series at 35°C column oven temperature. 1-15 mg of a sample is weighed in a 20 ml crimp cap vial and 15 mL chloroform is added thereto. The suspension obtained is shaken for 2h at 50°C, filtered over a 0.45 pm filter, transferred to a 2 ml vial and injected to the GPC system for analysis. A calibration curve of narrow dispersity polystyrene standards is prepared versus which the molecular weights are reported.

[0257] The glass transition describes the temperature region where the mechanical properties of materials change from hard and brittle to more soft, deformable or rubbery. Glass transition temperature (Tg) can be determined by Differential Scanning Calorimetry (DSC) analysis at 1-20K / min scanning rate with sample weights of 4-8 mg. Values for Tg as reported herein are recorded as the midpoint of the glass transition region.

[0258] Residual lactides

[0259] The amount of residual lactides (i.e. the amount of L-, D-, meso-lactide) in a polymeso-lactide sample was determined by a precipitative method to separate the monomeric lactides from the polymeric meso-lactide. A polymeso-lactide sample (comprising polymeso-lactide and residual lactide monomers) was dissolved in a known amount of dicholoromethane (including an internal standard). The polymeso-lactide fraction of the sample was then removed by precipitation by introducing the dichloromethane solution into an excess amount of 5 / 95 acetone / hexane solvent mixture. After half an hour of precipitation, the polymeric fraction was removed by filtration over a 0.45 pm filter. The remaining solution was then analyzed using Gas Liquid Chromatography, to determine the amount of lactide monomers in the sample. The final amount of residual lactides is calculated by taking the sum of L-, D- and meso-lactide.

[0260] Yellowness index

[0261] The Yellowness Index may be directly measured on the polymer pellets, particularly PML pellets, as described in ASTM D 1925, using a Konica Minolta CR-410 Chromameter.

[0262] EXAMPLES

[0263] Example 1 : Prepolymer synthesis based on L-lactic acid

[0264] 1765g of high purity lactic acid (HS100, obtained from Corbion) was brought into a 2L, fournecked, round-bottom flask with distillation setup, temperature probe and overhead stirrer. After heating the contents of the flask to 190°C by a heating mantle while stirring, vacuum was lowered to 30mbar over the course of 30min and oligomerization was allowed to proceed for 1 h. Water and other volatiles were collected in a measuring cylinder and after breaking vacuum, the product was collected from the 2L flask. The product, Prepolymer 1 , of this synthesis is a lactic acid prepolymer with DP of 10.4 and a stereochemical purity of 1.59%D. This example shows that racemization readily occurs already at the prepolymerization stage and also here different conditions may be chosen to influence the final meso-lactide production rates.

[0265] Example 2: Lactide synthesis under various conditions providing flexibility in mesolactide production

[0266] Example 2 illustrates the synthesis of lactide under varying conditions and effects thereof on racemization.

[0267] In a first experiment, 349g of Prepolymer 1 (see example 1) was brought into a 500mL, fournecked, round-bottom flask with heating mantle, temperature probe, distillation setup and overhead stirrer. Next, 600ppm 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 vacuum was gradually lowered to 5 mbar over the course of 14 minutes. Crude lactide product was obtained and was condensed by water of 96°C, and finally collected in a measuring cylinder. The reaction was stopped after 100min. 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.

[0268] In a second experiment, the procedure of the first experiment was repeated, but the crude lactide synthesis temperature was 220°C, i.e. the mixture was heated to 220°C. Reaction residue and crude lactide were weighed and analyzed and the weighted stereochemical purity was 3.10%D indicating that 1.51 % racemization had occurred.

[0269] In a third experiment, the procedure of the first experiment was again repeated, but 1.82g sodium lactate (Purasal S100, obtained from Corbion), corresponding to 1000 ppm sodium, was added to the reaction mixture at start of the procedure. Reaction residue and crude lactide were weighed and analyzed and the weighted stereochemical purity was 17.5%D indicating that 15.9% racemization had occurred.

[0270] An overview of the experiments and resulting product analyses is provided in Table 1. The amount of D-lactate is determined using the method indicated in the methodology section above.

[0271] Table 1 : lactide synthesis under various conditions

[0272] This example illustrates that racemization and the proportion of meso-lactide in the crude lactide may be steered, and in particular that the amount of meso-lactide in the crude lactide can be up-regulated by stimulating racemization during lactide synthesis.

[0273] Example 3: Prepolymerization and lactide synthesis starting from stereochemically impure lactic acid

[0274] The same procedure for prepolymerization as mentioned in example 1 was repeated, yet with a mixture of 461g L-lactic acid (HS100, Corbion) and 56g D-lactic acid (D-LA93, Corbion). The resulting prepolymer product had a DP of 10.5 and a stereochemical purity of 12.2% D.

[0275] 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 , apart from the fact that the synthesis reaction was stopped at 50% yield.

[0276] The crude lactide that was obtained contained 15 wt% meso-lactide showing that increased meso-lactide production may be achieved by use of less stereochemically pure lactic acid mixtures as starting material. Example 4. Distillation of a crude lactide to obtain an L-lactide rich stream and a meso- lactide-enriched stream

[0277] The following example illustrates the purification by distillation of a mixture of lactides, lactic acid oligomers and other organic compounds, which is representative for a crude lactide.

[0278] The mixture was fed to two continuously operated distillation towers in order to produce a meso-lactide enriched process stream and an L-lactide-rich process stream. The composition of the feed mixture is summarized in Table 2.

[0279] Table 2: Composition of mixture

[0280] The reflux ratio and distillate flow of the first column were optimized to maximize the removal of light organic acids and small lactic acid oligomers without compromising too much of mesolactide yield. The bottom flow and bottom temperature of the first column were optimized to maximize the removal of heavy oligomers and non-volatile impurities. The side of the first column served as feed to the second column. The second distillation column produces a meso-lactide-enriched distillate stream and a polymer-grade L-lactide rich stream as side product. The reflux ratio and distillate flow of the first column were optimized to maximize purity of the meso-lactide enriched stream as well as the quality of the polymer-grade side product rich in L-lactide.

[0281] Both distillation columns were operated under vacuum at a pressure of 35 mbar, and reflux ratios of 30 and 40 respectively for column 1 and column 2. Resulting compositions of the meso-lactide enrich stream and the L-lactide-rich stream are given in Table 3.

[0282] Table 3: Distillation stream compositions As can be seen in Table 3 the distillation process described can effectively separate streams rich in meso-lactide and L-lactide, and enables variable meso-lactide contents (in this example of about 19 wt%, see Table 2) in crude lactide to be separated into a stream for production of poly(meso-lactide).

[0283] Example 5: Purification of a meso-lactide stream by melt crystallization

[0284] The present example illustrates the purification of a meso-lactide stream by melt crystallization, and in particular by suspension crystallization (no solvent used). A crude mesolactide with a meso-lactide content of 97 wt% with a free acid content of 16.0 meq / kg was submitted to a suspension crystallization. To that end, the crude meso-lactide was molten overnight in an oven at 70°C and 1.75 kg was transferred to a 2L Buchner jacketed glass filter funnel which allowed in situ filtration. The mixture was cooled at 20°C / h to 51 °C, seed crystals were added and subsequently the content was further cooled at 2°C / h. Crystallization was allowed to proceed at 51 °C for 30 minutes which resulted in first crystals being formed. After 30 minutes the suspension was further cooled at a rate of 2°C / h to 46°C at which point a slurry with about 50 wt% solids content was obtained. The suspension was filtered in situ by applying a slight vacuum on the bottom of the Buchner flask. The mother liquor was collected in the Buchner flask, while purified meso-lactide crystals were collected on the glass filter. The resulting meso-lactide had a free acid of 9.8 meq / kg.

[0285] The resulting purified meso-lactide is an example of a purified meso-lactide which may be recombined with a lactide mixture as obtained in a process of the invention, and polymerized to form a polylactide comprising polymerized meso-lactide.

[0286] Example 6: Purification of a meso-lactide stream by solvent crystallization

[0287] The present example illustrates the purification of a meso-lactide stream by solvent crystallization in a static crystallizer. A 5L drum filled with a meso-lactide-enriched stream obtained by distillation (meso-lactide content 95 %, free acid 52 meq / kg) was molten in an oven at 70°C overnight and 6.8 kg was transferred to a 10L scale static crystallizer with an interplate distance of 42 mm, which was pre-heated to 55°C. Next, 1 .7 kg of acetone (technical grade, Thermo Scientific) was added and the mixture was homogenized by manual agitation. By external water bath, the temperature of the crystallizer was regulated. The jacket temperature was thus cooled at 20°C / h from 55°C to 26°C and kept at that temperature for 15 min at which point crystallization on the plates started to occur. Then cooling was continued from 26°C to 15°C at 2°C / h followed by an isothermal period until next morning (about 12 hours isothermal). A significant crystal layer was observed on the plates and the mother liquor, about 4kg, was drained from the crystallizer. Subsequently, the crystallizer was heated to 44°C for 15 min while collecting 288g of a partial melt. Finally, the meso-lactide crystals were molten by heating the crystallizer to 60°C and maintaining that temperature for 4.5 hours. Three different fractions of crystals were collected separately, and the last two fractions were combined which yielded 2.1 kg of meso-lactide product (31 % yield) at a free acid level of 3.7 meq / kg.

[0288] Example 7: Polymeso-lactide production

[0289] This example illustrates the polymerisation of a purified meso-lactide stream as defined herein to form polymeso-lactide in accordance with step e) of the present process.

[0290] A purified meso-lactide stream with a free acid content below 4.0 meq / kg was collected in a feeding tank for use in a pilot-scale PLA polymerization unit. The feed rate of meso-lactide was 65 kg / h, and 40-50 ppm tin octoate was applied to catalyze the ring-opening polymerization in the presence of an alcohol initiator to control the molecular weight. The polymerization residence time was about 3 hours and polymerization temperatures were between 130 and 200°C. After deactivation of the catalyst, residual meso-lactide was stripped off in a devolatilization unit. The polymeso-lactide product was subsequently underwater pelletized, quench cooled with 20°C water, separated from the water by cyclone and dried at 30°C dehumidified air conditions. In Table 4 product properties of the polymeso-lactide obtained in three different runs are listed. The listed properties were determined using the methodology described in the above section.

[0291] Table 4. Properties of polymeso-lactide products

[0292] From the above data, it may be concluded that polymeso-lactide polymer products with high molecular weights, acceptable yellowness and low residual monomer contents can be produced. The products could be dried to <500 ppm water levels.

[0293] Example 8. Disintegration behavior of an extruded sheet

[0294] Polymeso-lactide with a weight-average molecular weight of 219 kg / mol as determined with the method given above and a residual lactide amount of 0.13 wt% (see Run 2 in Table 2) was converted into a 100 micron thick sheet through melt extrusion and by passing through a sheet die. Samples of the sheet were subjected to industrial composting tests according to EN 13432 at 58 ± 2°C. After one week of testing, the sheet was fully disintegrated.

[0295] Similar sheets made of primarily L-lactide based PLA do not disintegrate this quickly. Typically such sheets would only fully disintegrate under the same conditions after more than two weeks. Therefore, this example illustrates a faster hydrolysis of articles made of polymesolactide.

[0296] Example 9. Polymerization of 80% L-lactide and 20% purified meso-lactide

[0297] A 2L batch polymerization reactor was subjected to three vacuum / nitrogen cycles to remove oxygen and moisture. Subsequently, the reactor was filled with 739g L-lactide (Lumilact L, TotalEnergies Corbion, free acid content of 2 meq / kg) and 82g meso-lactide (free acid content of 10 meq / kg). After the lactides were transferred to the reactor, the reactor was again subjected to three vacuum / nitrogen cycles.

[0298] The lactide mixture was heated to 130°C after which 20 meq 1 -decanol (2-Ethyl-1 -Hexanol) (Alfa Aesar) and 150ppm tin octoate (Sigma, as a 10wt% solution in dry toluene) were added to start the 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 deactivation of the catalyst, the reactor contents were discharged and quenched.

[0299] The resulting product was a copolymer of meso-lactide and L-lactide, with a stereochemical purity of 5.2%D, Mw of 106 kg / mol with high thermal stability.

Claims

CLAIMS1 . Process for producing a polymeso-lactide (PML) comprising the steps of a) Forming lactic acid oligomers by polycondensing lactic acid; b) Depolymerising said lactic acid oligomers to form a crude lactide, wherein said crude lactide comprises L- 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 said crude lactide to purification thereby separating an L-lactide-rich stream and a meso-lactide-enriched stream; wherein the meso-lactide-enriched stream comprises at least 70.0 wt% based on the total weight of the stream of meso-lactide, and has an acidity level of 30.0 to 1000.0 meq / kg; d) Purifying said meso-lactide-enriched stream to a purified meso-lactide stream comprising at least 94.0 wt% based on the total weight of the stream of meso-lactide and having an acidity level of at most 20.0 meq / kg, by subjecting said meso-lactide- enriched stream to crystallisation; and e) Polymerising at least a portion of said purified meso-lactide stream to form polymesolactide.

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

3. 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%, with wt% based on the total weight of the crude lactide.

4. 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, with wt% based on the total weight of the lactide-rich stream.

5. Process according to any one of the preceding claims, wherein said meso-lactide-enriched stream is purified to an acidity level of equal to or higher than 1 .0 meq / kg, preferably equal to or higher than 1.5 meq / kg.

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

8. Process according to any one of the preceding claims, wherein the portion of purified meso-lactide stream that is polymerised 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. Process according to any one of the preceding claims, further comprising the step f) of blending at least a portion of the purified meso-lactide stream that is not polymerised 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 said lactide mixture to form a polylactide with polymerized meso-lactide content below 20.0 % by weight of the polylactide.

10. Process according to any one of the preceding claims, wherein crystallisation is solvent crystallisation or melt crystallisation, wherein said solvent or melt crystallisation is performed as a suspension crystallisation or layer crystallisation.11 . Process according to any one of the preceding claims, 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 and / or has an isomeric purity of the D-isomer of 80.0 % or more, such as 85.0 % or more.

12. 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 said part is at most 40.0 wt% of racemic lactic acid, with wt% based on the total amount of lactic acid applied in step a).

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

14. Process according to claim 13, wherein the poly-L-lactic acid is depolymerised by hydrolysing said poly-L-lactic acid in the presence of water and / or lactic acid as co- reactant(s), preferably for 30 minutes to 24 hour, under conditions ranging from atmospheric conditions up to 10 bar, and at a temperature ranging from 120 to 200°C.

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

16. Process according to any one of the preceding claims, wherein the amount of meso-lactide in said crude lactide is up-regulated, by stimulating racemization during step a) and / or b).

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

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

19. Polymeso-lactide obtainable or obtained 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:- a stereochemical purity of 40.0 to 60.0 % L-lactate, or a stereochemical purity of 40.0 to 60.0 % D-lactate,- a weight-average molecular weight of between 50 and 500 kg / mol,- a glass transition temperature between 30 and 50°C.

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

21. Use of a polymeso-lactide according to the claim 19, or obtainable or obtained according to the process of 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%, based on the total weight of the polymer composition, of said polymesolactide, and / or as a resin for coating or as an adhesive resin, e.g. for preparing an adhesive film or adhesive layer or a multilayer structure.

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