An integrated process for efficiently producing polyhydroxy acid, such as polylactic acid

Abstract

The present invention relates to a process for producing polyhydroxy acid by ring-opening polymerization of a cyclic ester of the hydroxy acid comprising the steps of: a) preparing a cyclic ester of hydroxy acid containing composition and adding thereto the recycle composition being obtained in step f), so as to obtain a crude composition, b) purifying the crude composition so as to obtain a purified composition containing the cyclic ester of the hydroxy acid, c) subjecting the purified composition to a ring-opening polymerization so as to obtain a polyhydroxy acid containing composition, d) withdrawing at least one purge stream containing oligomer and / or polymer of the hydroxy acid from one or more of steps a), b) and c), e) subjecting at least one purge stream obtained in step d) to a cyclodepolymerization reaction so as to obtain a recycle composition containing cyclic ester of the hydroxy acid and f) adding at least a portion of the recycle composition to the crude composition in step a).

Description

[0001] Sulzer Management AG S13812PEP - Pl / Fa

[0002] An integrated process for efficiently producing polyhydroxy acid, such as polylactic acid

[0003] The present invention relates to an integrated process for producing polyhydroxy acid by ring-opening polymerization of a cyclic ester of the hydroxy acid.

[0004] Polyhydroxy acid homo- or copolymers are of particular interest, because they can be obtained from renewable resources and are mostly compostable and / or biodegradable. Moreover, the technological properties of these polymers come quite close to the properties of those polymers derived from fossil-based resources, which explains why these polymers are regarded as highly promising substitutes for the latter. One example for a commercially important polyhydroxy acid homopolymer is polylactic acid, which is based on an a-hydroxy acid, namely on lactic acid, and which has a wide range of applications. For instance, polylactic acid is used in the biomedical field in chirurgical implants, in films, such as e.g. in packaging, in fibers, such as e.g. for garments, in hygienic articles, in carpets and in disposable plastic products, such as e.g. disposable cutlery or containers. Moreover, polylactic acid has found wide application in composite materials, such as in fiber- reinforced plastics. Another important example for a respective polyhydroxy acid homopolymer is polycaprolactone, which is a poly-co-hydroxy acid and is derived from a cyclic ester, caprolactone, originated from the intramolecular esterification of 6-hydroxyhexanoic acid. This polymer finds widespread applications in the production of speciality polyurethanes and is characterized by a good resistance to water, to oil and to solvent. Other examples are polyglycolic acid, polyvalerolactone and the like. Polyhydroxy acid copolymers are interesting alternatives, since by an appropriate selection of the comonomers and their relative amounts to each other and by adjusting an appropriate molecular weight, certain properties of the copolymers may be tailored to the intended use.

[0005] Generally, two alternative principal methods are known for synthesizing polyhydroxy acid homo- or copolymers. The first principal method is the direct polycondensation of one or more aliphatic hydroxy acid(s) to the respective homopolymer or copolymer, respectively, such as the direct polycondensation of lactic acid to polylactic acid. However, this principal method only leads to low molecular weight (co)polymers and is thus limited to specific (co)polymers. The second principal method is the ring-opening-polymerization of one or more cyclic esters of hydroxy acid(s), such as the ring-opening-polymerization of lactide (which is the cyclic diester of lactic acid), glycolide (which is the cyclic diester of glycolic acid), lactones (which are cyclic monoesters) or the like. This is the preferred method nowadays for the industrial production of polylactic acid and other polyhydroxy acid homo- or copolymers, because the ring-opening-polymerization of a cyclic ester of a hydroxy acid is, compared to the direct polycondensation reaction, a more controlled polymerization technique that can be easily tuned to produce polyhydroxy acids with different molecular weights with less reaction time at a lower temperature, thus leading to less yellowing and degradation during the process. However, a drawback of the synthesis of polyhydroxy acid via a ring-opening-polymerization of a cyclic ester of the hydroxy acid is that it is a two-step process requiring in a first step the synthesis of cyclic ester of the hydroxy acid from the hydroxy acid and in a second step the polymerization of the cyclic ester to polyhydroxy acid by ring-opening polymerization. Typically, during the single process steps, i.e. during the synthesis of the cyclic ester of the hydroxy acid from the hydroxy acid, during the purification of the obtained cyclic ester containing composition as well as during and after the ring-opening polymerization step purge streams containing hydroxy acid oligomer and / or hydroxy acid polymer are withdrawn from the process, in order to avoid the accumulation of catalyst and long oligomers in the process streams, in order to remove unconverted cyclic ester of the hydroxy acid remaining after the ring-opening polymerization step and in order to remove polyhydroxy acid being out of the specification from the process streams. In order to improve the efficiency of the process, it has been already proposed to re-use the purge streams, namely to hydrolyze the hydroxy acid oligomers and / or hydroxy acid polymers contained therein to the hydroxy acid, which is then recycled into the starting mixture containing the hydroxy acid, which is reacted in the first process step to the cyclic ester of the hydroxy acid. However, the efficiency of this process is still not optimal.

[0006] In view of this, the object underlying the present invention is to provide an efficient process for producing in high yield polyhydroxy acid, in particular polylactic acid, by ring-opening polymerization of a cyclic ester of the hydroxy acid, in particular lactide, with minimal operational costs.

[0007] In accordance with the present invention this object is satisfied by providing a process for producing polyhydroxy acid by ring-opening polymerization of a cyclic ester of the hydroxy acid comprising the steps of: a) preparing a cyclic ester of hydroxy acid containing composition and adding thereto the recycle composition being obtained in step f), so as to obtain a crude composition, b) purifying the crude composition so as to obtain a purified composition containing the cyclic ester of the hydroxy acid, c) subjecting the purified composition to a ring-opening polymerization so as to obtain a polyhydroxy acid containing composition, d) withdrawing at least one purge stream containing oligomer and / or polymer of the hydroxy acid from one or more of steps a), b) and c), e) subjecting at least one purge stream obtained in step d) to a cyclodepolymerization reaction so as to obtain a recycle composition containing cyclic ester of the hydroxy acid and f) adding at least a portion of the recycle composition to the crude composition in step a).

[0008] This solution bases on the surprising finding that by converting the hydroxy acid oligomers and / or hydroxy acid polymers being contained in the purge stream(s), such as lactic acid oligomer and / or lactic acid polymer, to cyclic diester of the hydroxy acid, such as to lactide, and by recycling the cyclic diester of the hydroxy acid into the crude composition before the ring-opening polymerization of the crude composition instead of hydrolyzing the hydroxy acid oligomers and / or hydroxy acid polymers being contained in the purge stream(s) to lactic acid, as in the prior art, the efficiency of the process can be further improved and the operational costs can be reduced. Without being wished to be bound to any theory it is considered that this is due to an operational cost reduction of higher than 13%. Thanks to the more efficient process and atom economy it is also possible to reduce the overall investment costs by at least 7%.

[0009] In accordance with the present invention, the term “oligomer” as used herein is defined broadly as any macromolecule being composed of 2 to 99 repeating units, whereas polymers are macromolecules being composed of at least 100 repeating units.

[0010] In turn, cyclic ester means in accordance with the present invention any cyclic molecule containing an ester group in the cyclic structure. In other words, a cyclic molecule containing an ester group only as attached side group, but not in the cyclic structure, is not considered in the present invention as cyclic ester. The cyclic ester may be a cyclic monoester, such as in the case of lactones, for instance s-caprolactone or y-valerolactone, or a cyclic diester, such as in the case of lactide and glycolide. In accordance with the present invention, in step a) a cyclic ester of hydroxy acid containing composition is prepared, which is then converted in a different process step to polyhydroxy acid. The present invention is not particularly limited concerning the kind of used hydroxy acid, of produced cyclic ester of hydroxy acid and of produced polyhydroxy acid, as long as the kind of the hydroxy acid, from which the cyclic ester and the polyhydroxy acid are formed, are the same. Preferably, the hydroxy acid is a co-hydroxy acid or a a-hydroxy acid, which is more preferably selected from the group consisting of lactic acid, glycolic acid, 6-hydroxyhexanoic acid, 5-hydroxypentanoic acid and arbitrary combinations of two or more of the aforementioned acids. Consequently, preferably the method of the present invention leads in a first step to a cyclic ester, which is selected from the group consisting of lactide, glycolide, caprolactone, valerolactone and arbitrary combinations of two or more of the aforementioned cyclic esters, which is then reacted in the ringopening polymerization of step c) to polylactic acid, poly glycolic acid, polycaprolactone and polyvalerolactone, respectively. Most preferably, the hydroxy acid is lactic acid, i.e. the method produces first lactide and then therefrom in the ringopening polymerization of step c) polylactic acid.

[0011] The present invention is not particularly limited concerning the kind, how in step a) the cyclic ester of hydroxy acid containing composition is obtained. For instance, the cyclic ester may be prepared i) by oligomerizing the hydroxy acid to hydroxy acid oligomer, before the hydroxy acid oligomer is cyclized to the cyclic ester, or ii) by depolymerizing polyhydroxy acid to hydroxy acid oligomer, before the hydroxy acid oligomer is cyclized to the cyclic ester.

[0012] In accordance with a first preferred embodiment of the present invention, the composition containing cyclic ester of the hydroxy acid is prepared in step a) by oligomerizing a starting mixture containing the hydroxy acid to hydroxy acid oligomer and by then subjecting the hydroxy acid oligomer to a cyclization reaction so as to obtain the cyclic ester of hydroxy acid containing composition. The oligomerization of the hydroxy acid may be performed in the presence or in the absence of catalyst. If the oligomerization of the hydroxy acid is performed in the presence of a catalyst, it is preferred that the starting mixture contains at least one catalyst being selected from the group consisting of metal oxides, metal alkoxides, metal carbonates, metal bicarbonates, organometallic compounds and arbitrary mixtures of two or more thereof. The catalyst may be a homogenous catalyst or a heterogeneous catalyst. While homogeneous catalyst means a catalyst that is present in the same phase as the reaction mixture of the oligomerization reaction, a heterogeneous catalysts is in a different phase. Suitable examples for a metal alkoxide catalyst are tin alkoxides, zinc alkoxides, aluminum alkoxides and titanium alkoxides, whereas suitable examples for a metal carboxylate catalyst are tin carboxylates, zinc carboxylates, aluminum carboxylates and titanium carboxylates. Preferably, the metal carboxylate catalyst comprises tin carboxylate comprising one or more C2-i4-carboxylate groups, more preferably one or more Ce-14- carboxylate groups and most preferably tin octoate. In turn, suitable examples for a metal oxide catalyst are transition metal oxides, such as tin oxide, iron oxide and copper oxide, whereas suitable examples for organometallic compounds are those comprising a metal selected from the group consisting of magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth and any combination of two or more of the aforementioned metals and an organic residue being selected from the group consisting of alkyl groups, aryl groups, halides, oxides, alkanoates, alkoxides and any combination of two or more of the aforementioned groups. In addition, suitable examples for metal bicarbonates are alkali metal bicarbonates and alkaline earth metal bicarbonates. Most preferably, the starting mixture contains at least one catalyst being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate, tin alkoxides, zinc alkoxides, aluminum alkoxides, titanium alkoxides, tin carboxylates, zinc carboxylates, aluminum carboxylates, titanium carboxylates and arbitrary mixtures of two or more of these compounds. Good results are in particular obtained, when the starting mixture contains, based on 100% by mole of the starting mixture, 0 to 50,000 ppm and more preferably 500 to 5,000 ppm of catalyst.

[0013] In a further development of the idea of the present invention, it is proposed that the starting mixture further comprises at least one alcohol. Good results are in particular obtained, when the alcohol is at least one polyalcohol comprising at least two hydroxyl groups per molecule. Suitable examples of polyalcohols are those being selected from the group consisting of alkane polyols, polyether polyols, polyester polyols, polyamide polyols, polyphenols, saccharides and arbitrary combinations thereof. Good results are in particular obtained, when the at least one polyalcohol being contained in the starting mixture is selected from the group consisting of butanediol, pentanetriol, pentaerythritol, dipentaerythritol, tripentaerythritol, diglycerol, triglycerol, multi-armed oligo(ethylene glycol), multi-armed oligo(lactic acid), monosaccharides, disaccharides, cyclitols and arbitrary combinations thereof.

[0014] Most preferably, the at least one polyalcohol being contained in the starting mixture is tripentaerythritol, triglycerol or trehalose. Good results are in particular obtained, when the molar ratio of the hydroxy acid carboxylic groups divided by the polyalcohol hydroxy groups in the starting mixture is at least 2, and when during the oligomerization reaction at least a portion of the reaction mixture, which is formed during the oligomerization reaction, is at least temporarily distilled so as to remove water from the reaction mixture.

[0015] For instance, the starting mixture being subjected to the oligomerization reaction is prepared i) by mixing an aqueous solution of hydroxy acid with at least one polyalcohol and optionally with at least one catalyst and by dewatering the so obtained blend, wherein the blend is preferably dewatered so that 90 to 98% by weight of the water being contained in the aqueous solution of hydroxy acid before having mixed it with at least one polyalcohol and optionally with at least one catalyst is removed during the dewatering from the blend, or ii) by dewatering an aqueous solution of hydroxy acid in water, wherein during the dewatering preferably 90 to 98% by weight of the water are removed from the aqueous solution, and by then mixing the dewatered aqueous solution with at least one polyalcohol and optionally with at least one catalyst.

[0016] Preferably, the oligomerization reaction is performed at a temperature of 80 to 250°C and at a pressure of 0.1 to 90 kPa and more preferably at a temperature of 120 to 200°C and at a pressure of 1 to 35 kPa. The reaction time may be 1 minute to 48 hours and more preferably 1 to 8 hours.

[0017] In turn, the cyclization reaction is preferably performed at a temperature of 180 to 230°C and at a pressure of 100 Pa to 5 kPa and more preferably at a temperature of 190 to 210°C and at a pressure of 100 to 800 Pa. Preferably, the reaction time is 1 minute to 24 hours and more preferably 30 minutes to 4 hours.

[0018] Also the cyclization reaction may be performed with or without catalyst. Preferably, the cyclization reaction is performed with catalyst, i.e. at least one catalyst is already present in the oligomer composition because it has been added to the starting mixture, and / or at least one catalyst is added to the oligomer composition. If at least one catalyst is added to the starting mixture and at least one catalyst is added to the oligomer composition, the catalyst may be any of the catalysts mentioned above as suitable for performing the oligomerization. Moreover, the at least one catalyst used in the cyclization reaction) may be the same or a different one as that used for the oligomerization.

[0019] In accordance with a second preferred embodiment of the present invention, the composition containing cyclic ester of the hydroxy acid is prepared in step a) by subjecting a polymer composition containing polymer of the hydroxy acid to a depolymerization reaction so as to obtain a depolymerized composition comprising oligomer of the hydroxy acid and then subjecting the depolymerized composition comprising oligomer of the hydroxy acid to a cyclization reaction so as to obtain the cyclic ester of hydroxy acid containing composition.

[0020] Preferably, the polymer composition of this embodiment further comprises as depolymerizing agent at least one alcohol, which is preferably at least one polyalcohol comprising at least two hydroxyl groups per molecule, and / or a primary amine comprising at least one amino group. The polyalcohol and / or primary amine is / are not consumed during the depolymerization reaction of the polymer of the hydroxy acid to the oligomer of the hydroxy acid and is / are thus still contained in the depolymerized composition and consequently in the crude composition, which is then subjected to the cyclization reaction. Good results are in particular obtained, when in step a) a trivalent and / or tetravalent polyalcohol and in particular a polyalcohol is used, which is selected from the group consisting of glycerol, phloroglucinol, pentaerythritol, sorbitol, dipentaerythritol, tripentaerythritol, 1 ,1 ,1- tris (hydroxymethyl)ethane, 1 ,1 ,1-tris(hydroxymethyl)propane, di- (trimethylolpropane), trimethylolpropanethoxylate, polyphenols comprising at least three hydroxy groups, saccharides comprising at least three hydroxy groups, starshaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned polyalcohols. Also, good results are in particular obtained, when in step a) a primary amine comprising at least one amino group and preferably a primary amine comprising at least two amino groups is used. Suitable examples for primary amines comprising at least one amino group are those being selected from the group consisting of ethylenediamine, hexamethylenediamine, tris(2-aminoethyl)amine and arbitrary combinations of two or more of the aforementioned primary amines.

[0021] Preferably, the depolymerization reaction is performed with a polymer composition comprising, based on 100% by mol of the reaction composition, 50 to 100% by mol and preferably 90 to 99.9% by mol of the polymer of the hydroxy acid and 0.1 to 10% by mol of the polyalcohol and / or the primary amine.

[0022] In a further development of the idea of the present invention, it is proposed that the reaction composition further comprises a catalyst. Thereby, possible sidereactions may be minimized and the yield may be increased. Moreover, the presence of catalyst assists in the shortening of the required reaction time and of the required reaction temperature. Examples for suitable catalysts are metal oxides, metal carbonates, metal bicarbonates and organometallic compounds. Suitable examples for metal oxides are transition metal oxides, and suitable examples for organometallic compounds are those comprising a metal selected from the group consisting of magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth and any combination of two or more of the aforementioned metals and an organic residue being selected from the group consisting of alkyl groups, aryl groups, halides, oxides, alkanoates, alkoxides and any combination of two or more of the aforementioned groups. Suitable examples for metal carbonates and metal bicarbonates are alkali metal carbonates, alkaline earth metal carbonates, alkali metal bicarbonates and alkaline earth metal bicarbonates. Particular suitable examples for catalysts are catalysts being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts.

[0023] Preferably, the polymer composition contains, based on 100% by mol of the polymer composition, 0 to 5,000 ppm and more preferably 100 to 1 ,000 ppm of catalyst.

[0024] The depolymerization reaction is preferably performed at a temperature of 120 to 220°C, more preferably at 160 to 200°C and more preferably at 170 to less than 190°C. In accordance with a further preferred embodiment of the present invention, in step a) a purge stream is withdrawn, which comprises hydroxy acid oligomers having a weight average molecular weight of 1 ,000 to 10,000 g / mol and preferably of 4,000 to 5,000 g / mol. In accordance with the present invention, the number- and weight-average molecular weight (Mnand Mw) of polymers is determined by gel permeation chromatography using a poly(methyl methacrylate) standard and a sample concentration of 1 to 5 mg / ml in 1ml HFIP depending on the sample’s molecular weight, wherein the column temperature is 40°C, the temperature of the Rl-Detector (refractive index) is 40°C and the flow rate 1 ml / min. As instrument, (GPC Viscotek TDA max from Malvern Panalytical, UK equipped with a Viscotek VE 2001 solvent / sample module, a precolumn HFIP guard (50 mm length and 8 mm internal diameter), two columns (Viscotek HFIP6000M and HFIP3000, Viscotek, Switzerland; 300 mm length and 8 mm internal diameter), and a triple detector Viscotek TDA 305 (Rl, UV and viscosimeter) may be used. The calibration curve may be constructed using poly(methylmethacrylate) (PMMA) standard (Mn,max = 50,352 g / mol and D of 1 .023).

[0025] The present invention is not particularly limited concerning the purification performed in step b). Good results are in particular obtained, when step b) of purifying the crude composition comprises at least one distillation step and / or at least one crystallization step. For instance, the process step b) comprises one to ten subsequent distillation steps, preferably one to five subsequent distillation steps, more preferably one to three subsequent distillation steps, still more preferably one or two subsequent distillation steps and most preferably two distillation steps. Alternatively, the process step b) comprises a static crystallization step, a dynamic crystallization step, a combination of a static crystallization step followed by a dynamic crystallization step or a dynamic crystallization step followed by a static crystallization step, wherein the at least one crystallization step may comprise one to ten crystallization stages, preferably one to five crystallization stages, more preferably one to three crystallization stages and most preferably two or three crystallization stages.

[0026] In a further development of the idea of the present invention, it is proposed that in step b) a purge stream is withdrawn, which comprises hydroxy acid oligomers having a weight average molecular weight of 50 to 10,000 g / mol and preferably of 100 to 3,000 g / mol.

[0027] Preferably, the purified composition comprises at least one catalyst or at least one catalyst is added to the purified composition before starting the ring-opening- polymerization step c) or at least one catalyst is added to the purified composition during the ring-opening-polymerization step c), wherein the at least one catalyst is preferably selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts.

[0028] It is further preferred that the purified composition comprises at least one initiator or at least one initiator is added to the purified composition before starting the ring- opening-polymerization step or at least one initiator is added to the purified composition during the ring-opening-polymerization step. Good results are in particular obtained, when the molar ratio of the cyclic ester of the hydroxy acid being contained in the purified composition to initiator is more than 100 to 10,000, and when the at least one initiator is selected from the group consisting of monohydroxy compounds, dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds and arbitrary combinations of two or more of the aforementioned initiators.

[0029] Concerning the kind of reactor, in which the ring-opening-polymerization step c) is performed, the present invention is not particularly restricted. For instance, the ring-opening-polymerization step c) may be performed in a reactor system, which comprises at least one continuous stirred-tank reactor, at least one loop reactor or a, in series, a combination of at least one loop reactor and at least one plug flow reactor, wherein preferably at least one of these reactors of the reactor system comprises at least one mixer and / or at least one heat transfer element.

[0030] Good results are in particular obtained, when the ring-opening-polymerization step c) is performed at a temperature of 160 to 240°C.

[0031] Furthermore, it is preferred that the polyhydroxy acid contained in the polyhydroxy acid containing composition obtained in step c) has a number average molecular weight of more than 10,000 g / mol as well as a polydispersity of at most 1 .8, preferably of 1 .6 or less and more preferably of 1 .4 or less.

[0032] In order to stop the polymerization reaction, it is further preferred that at least one catalyst inhibitor is added after step c) to the polyhydroxy acid containing composition. Good results are in particular obtained, when the at least one catalyst inhibitor is a phosphate ester or an alkyl phosphite having a total carbon number per molecule of 8 to 18. Preferably, 0.01 to 1% by weight and more preferably 0.5 to less than 0.1 % by weight of catalyst inhibitor are added to 100% by weight of polyhydroxy acid containing composition, wherein the so obtained mixture may be mixed using one or more mixers.

[0033] As usual, the polyhydroxy acid containing composition may be devolatilized, for example at a temperature of at least 180°C, preferably of 180 to 250°C and more preferably of 210 to 230°C, and at a pressure of 1 ,000 Pa or less, preferably of 100 to 500 Pa and more preferably of less than 200 Pa in at least one devolatilization stage so as to produce a purified polyhydroxy acid containing composition and a gaseous composition containing unreacted cyclic ester of the hydroxy acid. The obtained gaseous composition containing unreacted cyclic ester of the hydroxy acid may then be condensed to a liquid composition containing unreacted cyclic ester of the hydroxy acid, which may be at least partially recycled to the crude composition obtained in step a).

[0034] The process may further comprise a step of cooling and pelletizing the polyhydroxy acid containing composition to polyhydroxy acid pellets and drying them in vacuum, in inert atmosphere or hot air.

[0035] In accordance with a further preferred embodiment of the present invention, the polyhydroxy acid containing composition contains, based on 100% by weight of the polyhydroxy acid containing composition, 99.7% by weight or more, preferably 99.8% by weight or more, more preferably 99.9% by weight or more and most preferably at least 99.95% by weight of polyhydroxy acid.

[0036] In a further development of the idea of the present invention, it is suggested that in step c) at least one purge stream is withdrawn, wherein one purge stream comprises unconverted cyclic ester of the hydroxy acid and / or one purge stream comprises polyhydroxy acid being out of the specification.

[0037] In accordance with the present invention, in step e) the purge stream obtained in step d), if only one purge stream is withdrawn from any of steps a), b) and c), is subjected to a cyclodepolymerization reaction or, if two or more purge streams are withdrawn from steps a), b) and c), at least one of the purge streams and preferably all of the purge streams obtained in step d) are - preferably combined to one combined purge stream - subjected to a cyclodepolymerization reaction so as to obtain a recycle composition containing cyclic ester of the hydroxy acid. In accordance with a particular preferred embodiment of the present invention, step e) comprises that to the purge stream, if only one purge stream is withdrawn from any of steps a), b) and c), or to at least one of the purge streams and preferably to all of the purge streams - preferably combined to one combined purge stream -, if two or more purge streams are withdrawn from steps a), b) and c), i) an alcohol or a polyalcohol comprising at least two hydroxy groups and / or ii) a primary amine comprising at least one amino group is added so as to obtain a reaction composition, wherein the reaction composition is subjected to a depolymerization reaction so as to obtain a depolymerized composition comprising oligomer of the hydroxy acid, wherein the depolymerized composition is subjected to a cyclization reaction so as to obtain the recycle composition containing cyclic ester of the hydroxy acid.

[0038] Good results are in particular obtained, when in step e) a polyalcohol comprising at least three hydroxy groups is used, which is selected from the group consisting of glycerol, phloroglucinol, pentaerythritol, sorbitol, dipentaerythritol, tripentaeryth- ritol, 1 ,1 ,1 -tris (hydroxymethyl)ethane, 1 ,1 ,1-tris(hydroxymethyl)propane, di- (trimethylolpropane), trimethylolpropanethoxylate, polyphenols comprising at least three hydroxy groups, saccharides comprising at least three hydroxy groups, starshaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned polyalcohols.

[0039] Instead of the polyalcohol or in addition to the polyalcohol a primary amine comprising at least one amino group may be used in step e). Suitable examples for respective primary amines comprising at least one amino group are those being selected from the group consisting of ethylenediamine, hexamethylenediamine, tris(2-aminoethyl)amine and arbitrary combinations of two or more of the aforementioned primary amines.

[0040] Good results are in particular obtained, when the reaction composition comprises, based on 100% by mol of the reaction composition, 50 to 100% by mol and preferably 90 to 99.9% by mol of the at least one purge stream and 0.1 to 10% by mol of the i) polyalcohol comprising at least three hydroxy groups and / or ii) the primary amine comprising at least one amino group. Even more preferably, the reaction composition further comprises water and / or the hydroxy acid, preferably, based on 100% by mol of the reaction composition, 0 to 20% by mol and more preferably 0.1 to 10% by mol of water and / or 0 to 10% by mol and more preferably 0.1 to 10% by mol of the hydroxy acid.

[0041] The depolymerization reaction is preferably performed at a temperature of 120 to 220°C, more preferably at 160 to 200°C and more preferably at 170 to less than 190°C.

[0042] Preferably, the oligomer of the hydroxy acid contained in the depolymerized composition obtained in the depolymerization reaction of step a) has a weight average molecular weight of 150 to 5,000 g / mol and preferably of 200 to 1 ,000 g / mol.

[0043] In a further development of the idea of the present invention, it is proposed that the cyclization reaction is performed at a temperature of 150 to 290°C, more preferably at 170 to 210°C and most preferably at 170 to less than 200°C, wherein the cyclization reaction is preferably performed at ambient pressure or at a pressure of 0.1 to 10 kPa and more preferably of 0.1 to 1 kPa.

[0044] In accordance with an alternative particularly preferred embodiment of the present invention, step e) comprises that to the purge stream, if only one purge stream is withdrawn from any of steps a), b) and c), or to at least one of the purge streams and preferably to all of the purge streams, if two or more purge streams are withdrawn from steps a), b) and c), a catalyst, a co-catalyst and optionally a re-used composition are added so as to prepare a reaction composition. Thereafter, the reaction composition is partially reacted in a reactor to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 30 to 90% by mole so as to obtain the recycle composition containing cyclic ester of the hydroxy acid and a second composition containing unconverted polymer and / or oligomer of the hydroxy acid, residual catalyst and co-catalyst, before the recycle composition and the second composition are withdrawn from the reactor, wherein at least a portion of the second composition is recycled as re-used composition into the reaction composition. The co-catalyst preferably contains i) at least one alcohol and / or at least one primary amine and ii) water and / or the hydroxy acid. Preferably, the alcohol at least one polyalcohol comprising at least three hydroxy groups. Suitable examples therefore are polyalcohols being selected from the group consisting of glycerol, phloroglucinol, pentaerythritol, sorbitol, dipentaerythritol, tripentaerythritol, 1 ,1 ,1- tris (hydroxymethyl)ethane, 1 ,1 ,1-tris(hydroxymethyl)propane, di- (trimethylolpropane), trimethylolpropanethoxylate, polyphenols comprising at least three hydroxy groups, saccharides comprising at least three hydroxy groups, starshaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned polyalcohols. Suitable examples for primary amines are those being selected from the group consisting of ethylenediamine, hexamethylenediamine, tris(2-aminoethyl)amine and arbitrary combinations of two or more thereof.

[0045] Particular good results are obtained, when a co-catalyst is added, which contains, based on 100% by weight of the co-catalyst, 50 to 95% by weight and preferably of 65 to 85% by weight of i) an alcohol being preferably a polyalcohol comprising at least three hydroxy groups and / or a primary amine being preferably a primary amine comprising at least two amino groups and 5 to 50% by weight and preferably 15 to 35% by weight of ii) water and / or the hydroxy acid. Even more preferably, a co-catalyst is added, which consists, based on 100% by weight of the co- catalyst, of 50 to 95% by weight and preferably of 65 to 85% by weight of i) an alcohol being preferably a polyalcohol comprising at least three hydroxy groups and / or a primary amine being preferably a primary amine comprising at least two amino groups and of reminder to 100% by weight of ii) water and / or the hydroxy acid. Most preferably, the aforementioned co-catalysts contain as component i) only alcohol being preferably a polyalcohol comprising at least three hydroxy groups, but no amine. Still more preferably, a co-catalyst is added, which contains i) an alcohol being preferably a polyalcohol comprising at least three hydroxy groups and / or a primary amine being preferably a primary amine comprising at least two amino groups and ii) water as well as the hydroxy acid. The presence of water and the hydroxy acid in the co-catalyst allows to reduce the amount of alcohol and / or primary amine needed and hence the total amount of co-catalyst. More preferably, the aforementioned co-catalysts contain as component i) only alcohol being preferably a polyalcohol comprising at least three hydroxy groups, but no amine. Good results are in particular obtained, when a co-catalyst is added, which consists, based on 100% by weight of the co-catalyst, of 50 to 95% by weight and preferably of 65 to 85% by weight of an alcohol, of 1 to 20% by weight and preferably of 2 to 8% by weight of water and of 10 to 40% by weight and preferably of 15 to 35% by weight of the hydroxy acid.

[0046] The total amount of co-catalyst being added in step b) is preferably 0.01 to 5% by weight and more preferably 0.05 to 1% by weight, based on 100% by weight of the reaction composition.

[0047] The catalyst being added in this embodiment allows to minimize possible sidereactions and to increase the yield in the reaction of step e). Moreover, the presence of catalyst assists in the shortening of the required reaction time and in the reduction of the required reaction temperature. Examples for suitable catalysts are metal oxides, metal carbonates, metal bicarbonates and organometallic compounds. Suitable examples for metal oxides are transition metal oxides, and suitable examples for organometallic compounds are those comprising a metal selected from the group consisting of magnesium, titanium, zinc, aluminum, indium, yttrium, tin, lead, antimony, bismuth and any combination of two or more of the aforementioned metals and an organic residue being selected from the group consisting of alkyl groups, aryl groups, halides, oxides, alkanoates, alkoxides and any combination of two or more of the aforementioned groups. Suitable examples for metal carbonates and metal bicarbonates are alkali metal carbonates, alkaline earth metal carbonates, alkali metal bicarbonates and alkaline earth metal bicarbonates.

[0048] Particular suitable examples for catalysts are catalysts being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts.

[0049] Preferably, the catalyst is added in step b) in an amount of 1 to 2,000 ppm and more preferably 20 to 300 ppm, based on 100% by weight of the reaction mixture.

[0050] In accordance with the present invention, at least a portion of the recycle composition is added to the crude composition in step. Good results are in particular achieved, when at least 50% by weight, more preferably at least 80% by weight, yet more preferably at least 90% by weight, still more preferably at least 95% by weight and most preferably 100% by weight of the recycle composition are added to the crude composition in step a).

[0051] Subsequently, the present invention is described by means of an illustrative, but not limiting figure, in which:

[0052] Fig. 1 shows a scheme of a process of producing polyhydroxy acid in accordance with one embodiment of the present invention.

[0053] The process being schematically shown in figure 1 is described with reference to polylactic acid as polyhydroxy acid. More specifically, the process for producing polylactic acid being schematically shown in figure 1 is performed in a plant comprising a reactor 10 for preparing lactide as cyclic ester of lactic acid, a lactide purification unit 12, such as a distillation column, a polymerization reactor 14 as well as a lactide recovery unit 16. The reactor 10 comprises an inlet line 18 for starting mixture, an outlet line 20 for crude lactide composition and an outlet line 22 for a purge stream, which leads into the lactide recovery unit 16. In turn, the lactide purification unit 12 comprises an inlet line being connected with the outlet line 20 of the reactor 10, an outlet line 24 for purified lactide composition as well as an outlet line 26 for a purge stream. Moreover, a recycle line 28 leads from the polymerization reactor 14 to the line 20. Furthermore, the polymerization reactor 14 comprises an inlet being connected with the outlet line 24 for purified lactide composition of the lactide purification unit 12, an outlet line 30 for a purge stream, an outlet line 32 for purge stream containing polylactic acid being within the specification as well as an outlet line 34 for polylactic acid being out of the specification. All of the outlet lines 22, 26, 30, 34 for purge stream combine to the combined inlet line 36, which leads into the lactide recovery unit 16. In turn, the lactide recovery unit 16 comprises an outlet line 38, which leads into the line 20 for crude lactide composition leading into the lactide purification unit 12.

[0054] During the operation of the plant, an aqueous lactic acid composition is fed as starting mixture through the inlet line 18 into the reactor 10, in which the lactic acid is converted to lactide. The lactide is led through line 20, in which it is mixed with the recycle composition containing lactide being led through line 38 into line 20 and with the re-used mixture containing lactide being led through line 28 into line 20, wherein the so obtained mixture is led as crude lactide composition into the lactide purification unit 12, in which the crude lactide composition is purified to a purified lactide composition. The purified lactide composition is then led through the line 24 into the polymerization reactor 14, in which the lactide is polymerized via ring-opening polymerization to polylactic acid. Unconverted lactide is recycled via line 28 into the line 20, whereas polylactic acid being within the specification is withdrawn via line 32. The purge streams are lead via lines 22, 26, 30, 34, which combine to the combined inlet line 36, through which the purge streams are led as combined purge stream into the lactide recovery unit. In the lactide recovery unit 16, hydroxy acid oligomers as well as hydroxy acid polymers are depolymerized and cyclized to lactide. The so obtained lactide is led through line 38 into line 20 and is mixed there with the crude lactide composition and the re-used stream led through line 28 into line 20, wherein the so obtained crude lactide composition is, as described above, led into the lactide purification unit 12.

[0055] In the following two sets (A and B) of examples and comparative examples are provided to illustrate but not to limit the scope of the invention.

[0056] Set of examples A

[0057] Example 1

[0058] (Lactide production from alcohol-capped lactic acid oligomers)

[0059] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 68.6 g (0.657 mol lactic acid) of aqueous lactic acid and 15.3 g (41.06 mmol) of tripentaerythritol were mixed. Molar mass aim was 1525.4 g / mol, 144.12 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 90 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.2 wt% and a chiral purity of 98.5 wt% L-lactide.

[0060] Example 2 (Lactide production from alcohol-capped lactic acid oligomers with early catalyst addition, 1525.4 g / mol, 144.12 g / mol per hydroxyl group)

[0061] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 73.3 g (0.700 mol lactic acid) of aqueous lactic acid, 15.7 g (42.16 mmol) of tripentaerythritol and 92 mg of stannous octoate were mixed at 130°C. Molar mass aim was 1525.4 g / mol, 144.12 g / mol per oligomer arm for this composition. A pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.9 wt% and a chiral purity of 97.9 wt% L-lactide, demonstrating the catalyst can be added at earlier time points in the process.

[0062] Example 3

[0063] (Recycling of cyclization residues for application as a polyalcohol and subsequent lactide production)

[0064] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 33.84 g (0.324 mol lactic acid) of aqueous lactic acid and 44.3 g of cyclization residue from example 2 (estimated 42 mmol of oligomers and 92 mg of stannous octoate) were mixed at 130°C. Molar mass aim was 1610.7 g / mol, 154.8 g / mol per oligomer arm for this composition. A pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.5 wt% and a chiral purity of 93 wt% L-lactide, demonstrating that residues from the process may be re-used, although at a finite cost of chiral purity.

[0065] Example 4

[0066] (Lactide production from alcohol-capped lactic acid oligomers using an alternative alcohol)

[0067] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 70 g (0.670 mol lactic acid) of aqueous lactic acid and 16.10 g (67.0 mmol) of triglycerol were mixed. Molar mass aim was 1393.2 g / mol, 144.12 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was increased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 60 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.3 wt% and a chiral purity of 98.0 wt% L-lactide.

[0068] Example 5

[0069] (Lactide production from intermediate length, alcohol-capped oligomers) In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 62.48 g (0.590 mol lactic acid) of aqueous lactic acid and 4.57 g (12.30 mmol) of tripentaerythritol were mixed. Molar mass aim was 3831 .4 g / mol, 432.37 g / mol per oligomer arm for this composition. Under stirring, the mixture was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 180°C was set and pressure was decreased to 40 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 53 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 0.4 wt% and a chiral purity of 98.1 wt% L-lactide.

[0070] Set of comparative Examples A

[0071] Comparative Example 1 (Comparative experiment without addition of alcohol, short oligomers with free acid ends)

[0072] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 74.22 g (0.700 mol lactic acid) of aqueous lactic acid was added. Under stirring, the lactic acid was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which a temperature of 160°C was set and pressure was decreased to 310 mbara. The reaction mixture was left to stir for 2 hours under these conditions, whilst removing water. Final molar mass aim under these conditions was 260.4 g / mol. Thereafter, the vigreux column and water- cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 112 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 4.5 wt% and a chiral purity of 98.7 wt% L-lactide.

[0073] Comparative Example 2

[0074] (Comparative experiment without addition of alcohol, long oligomers with free acid ends, 647.0 g / mol)

[0075] In a 2-necked round-bottom flask equipped with a vigreux column, a water-cooled condenser and a collection flask, 70.24 g (0.663 mol lactic acid) of aqueous lactic acid was added. Under stirring, the lactic acid was heated to 130°C and a pressure of 82 mbara was applied to distill out water. The reaction mixture was left under these conditions for 2 hours, after which temperature and pressure was adjusted to 160 °C and 310 mbara respectively, and kept for 2 hours. Temperature and pressure were subsequently adjusted to 180 °C and 80 mbara respectively, which was kept for 2 hours. Lastly, temperature and pressure were adjusted at 190 °C and 20 mbara respectively, and kept for another 2 hours. Water was removed from the reaction flask during all previous stages. Final molar mass aim under these conditions was 647.0 g / mol. Thereafter, the vigreux column and water-cooled condenser were exchanged to a heated condenser with collection flask, which was set to 103°C. The reaction flask was heated to 195°C and 100.4 mg of stannous octoate was added into the reaction mixture, after which a pressure of 6 mbara was applied. Crude lactide was collected under these conditions over 2 hours, with a water content of 1 .0 wt% and a chiral purity of 96 wt% L-lactide.

[0076] The foregoing Examples demonstrate that the present invention provides a method for preparing a cyclic ester of hydroxy acid that achieves both low water content and high chiral purity, thereby combining the advantages of short- and long- oligomer routes while avoiding their respective drawbacks. It should be noted that the comparatively higher apparent molar masses observed in Examples 1-5, relative to Comparative Examples 1—2, arise from the fact that multiple lactic-acid oligomers become bound to the employed polyalcohols. Importantly, each individual oligomeric arm is not necessarily long, as reflected by the measured molar masses of the oligomer arms themselves. This structural characteristic explains the difference in overall molar mass without indicating the formation of excessively long oligomers.

[0077] The Examples further show that both water content and chiral composition were explicitly measured. All Examples 1-5 yield lactide with water contents below 1 wt%, markedly lower than the value obtained in the mild, short-oligomer Comparative Example 1 (4.5 wt%). Even when compared with the intentionally optimized long-oligomer route (Comparative Example 2), designed to suppress water, the water levels achieved in Examples 1-5 remain consistently and significantly lower.

[0078] With respect to chiral purity, Examples 1 , 2, 4, and 5 exhibit L-lactide purities of about 98%. These values are comparable to those observed in the mild shortoligomer process (Comparative Example 1 ) and are superior to the chiral purities obtained via the long-oligomer process (Comparative Example 2). Thus, the inventive process achieves the desired chiral stability without relying on harsh processing conditions or excessive oligomer growth.

[0079] Taken together, these findings confirm that the invention delivers the best attributes of both conventional approaches. A low water content, surpassing both short- and long-oligomer comparative processes, and high chiral purity, matching or exceeding the values seen in established mild short-oligomer methods. Accordingly, the inventive process provides a robust and advantageous route for producing high-quality lactide from alcohol-capped lactic acid oligomers. Set of examples B

[0080] Example 1

[0081] A chain scission reaction of polylactic acid waste has been performed in the presence of 2% by weight of trimethylolpropane ethoxylate having a number average molecular weight of 170 g / mol under stirring and constantly heating at 180°C for 40 minutes. The obtained oligomer has a wight average molecular weight of 14,000 g / mol.

[0082] The addition of 0.4% by weight of Sn(Oct)2 for further depolymerization yielded 7.8 kg / mol oligomers.

[0083] Thereafter, the oligomer mixture has been heated to 195°C at a vacuum of 5 to 6 mbar for 70 minutes, which yielded 95% of crude lactide having a purity of 97%.

[0084] Compared with the comparative example 1 , example 1 was characterized by a higher reactivity, i.e. lower reaction time, by a higher purity of the collected lactide and by less racemization.

[0085] Example 2:

[0086] A chain scission reaction of polylactic acid waste has been performed in the presence of 1 % by weight of dipentaerythritol under stirring and constantly heating at 195°C for 60 minutes. The obtained oligomer has a wight average molecular weight of 30,000 g / mol.

[0087] Thereafter, the oligomer mixture has been heated to 190°C at a vacuum of 5 to 6 mbar for 60 minutes, which yielded 92% of crude lactide having a purity of 98%. Example 3:

[0088] A chain scission reaction of polylactic acid waste has been performed in the presence of 5% by weight of tripentaerythritol under stirring and constantly heating at 200°C for 60 minutes. The obtained oligomer has a wight average molecular weight of 15,000 g / mol.

[0089] Thereafter, 0.2% by weight of Sn(Oct)2 were added as catalyst and the so obtained oligomer mixture was heated to 190°C at a vacuum of 5 to 6 mbar for 60 minutes, which yielded 96% of crude lactide having a purity of 93%.

[0090] Example 4:

[0091] A chain scission reaction of a mixture of 50 g polylactic acid waste and 50 g polycprolactone waste has been performed in the presence of 12% by weight of trimethylolpropane ethoxylate having a number average molecular weight of 170 g / mol under stirring and constantly heating at 160°C for 60 minutes. The obtained oligomer has a wight average molecular weight of 20,000 g / mol.

[0092] Thereafter, 0.3% by weight of Sn(Oct)2 were added as catalyst and the so obtained oligomer mixture was heated to 195°C at a vacuum of 5 to 6 mbar for 90 minutes, which yielded 95% of crude lactide having a purity of 90%.

[0093] Example 5:

[0094] A chain scission reaction of polylactic acid waste has been performed in the presence of 3% by weight of Hexamethylenediamine having a number average molecular weight of 116 g / mol under stirring and constantly heating at 180°C for 40 minutes. The obtained oligomer has a wight average molecular weight of 24,000 g / mol. The addition of 0.1 % by weight of Sn(Oct)2 for further depolymerization yielded 5.2 kg / mol oligomers.

[0095] Thereafter, the oligomer mixture has been heated to 195°C at a vacuum of 5 to 6 mbar for 70 minutes, which yielded 96% of crude lactide having a purity of 97%.

[0096] Comparative example B

[0097] A chain scission reaction of polylactic acid waste in the presence of ethylene alcohol was performed by adding polylactic acid waste and 2% by weight of ethylene glycol into a reactor and heating, stirring and reacting the mixture in the reactor at a temperature of 200°C for a reaction time of 120 minutes so as to prepare polylactic acid oligomer having a weight average molecular weight of 2,592 g / mol.

[0098] Thereafter, 0.3% by weight of a cracking catalyst, namely Sn(Oct)2, have been added to the prepared polylactic acid oligomer and mixed, before the so obtained mixture was subjected to a reduced pressure distillation operation at a vacuum of 0.5 kPa and to a continuously cracking of the lactic acid oligomer at a temperature of 195°C, before the so obtained lactide was collected after 180 minutes.

[0099] The collected crude lactide has a purity of 93%.

[0100] Compared with the comparative example 1 , example 1 was characterized by a higher reactivity, i.e. lower reaction time, by a higher purity of the collected lactide and by less racemization. Reference numerals

[0101] Reactor for preparing lactide

[0102] Lactide purification unit

[0103] Polymerization reactor

[0104] Lactide recovery unit

[0105] Inlet line of the reactor for starting mixture

[0106] Outlet line of the reactor

[0107] Outlet line for purge stream

[0108] Outlet line of the lactide purification unit

[0109] Outlet line for purge stream

[0110] Recycle line

[0111] Outlet line for purge stream

[0112] Outlet line of the polymerization reactor for polyhydroxy acid being within the specification

[0113] Outlet line purge stream containing polyhydroxy acid being out of the specification

[0114] Combined inlet line of the lactide recovery unit

[0115] Outlet line of the lactide recovery unit

Claims

Sulzer Management AG S13812PEP - Pl / FaClaims:1 . A process for producing polyhydroxy acid by ring-opening polymerization of a cyclic ester of the hydroxy acid comprising the steps of: a) preparing a cyclic ester of hydroxy acid containing composition and adding thereto the recycle composition being obtained in step f), so as to obtain a crude composition, b) purifying the crude composition so as to obtain a purified composition containing the cyclic ester of the hydroxy acid, c) subjecting the purified composition to a ring-opening polymerization so as to obtain a polyhydroxy acid containing composition, d) withdrawing at least one purge stream containing oligomer and / or polymer of the hydroxy acid from one or more of steps a), b) and c), e) subjecting at least one purge stream obtained in step d) to a cyclodepolymerization reaction so as to obtain a recycle composition containing cyclic ester of the hydroxy acid and that to at least one and preferably to all purge streams i) a polyalcohol comprising at least three hydroxy groups and / or ii) a primary amine comprising at least one amino group is added so as to obtain a reaction composition, wherein the reaction composition is subjected to a depolymerization reaction so as to obtain a depolymerized composition comprising oligomer of the hydroxy acid, and wherein the depolymerized composition is subjected to a cyclization reaction so as to obtain the recycle composition containing cyclic ester of the hydroxy acid andf) adding at least a portion of the recycle composition to the crude composition in step a).

2. The process in accordance with claim 1 , wherein the hydroxy acid is a cohydroxy acid or a a-hydroxy acid and preferably selected from the group consisting of lactic acid, glycolic acid, 6-hydroxyhexanoic acid, 5- hydroxypentanoic acid and arbitrary combinations of two or more of the aforementioned acids, wherein the hydroxy acid is most preferably lactic acid.

3. The process in accordance with claim 1 or 2, wherein the composition containing cyclic ester of the hydroxy acid is prepared in step a) by oligomerizing a starting mixture containing the hydroxy acid to hydroxy acid oligomer and by then subjecting the hydroxy acid oligomer to a cyclization reaction so as to obtain the cyclic ester of hydroxy acid containing composition.

4. The process in accordance with claim 3, wherein the starting mixture further comprises at least one catalyst and at least one alcohol and preferably at least one polyalcohol comprising at least four hydroxyl groups per molecule.

5. The process in accordance with claim 1 or 2, wherein the composition containing cyclic ester of the hydroxy acid is prepared in step a) by subjecting a polymer composition containing polymer of the hydroxy acid to a depolymerization reaction so as to obtain a depolymerized composition comprising oligomer of the hydroxy acid and then subjecting the depolymerized composition comprising oligomer of the hydroxy acid to a cyclization reaction so as to obtain the cyclic ester of hydroxy acid containing composition, wherein preferably the polymer composition further comprises at least one catalyst and at least one alcohol and preferably at least one polyalcohol comprisingat least three hydroxyl groups per molecule and / or a primary amine comprising at least one amino group.

6. The process in accordance with any of the preceding claims, wherein in step a) a purge stream is withdrawn, which comprises hydroxy acid oligomers having a weight average molecular weight of 2,000 to 10,000 g / mol and preferably of 4,000 to 5,000 g / mol.

7. The process in accordance with any of the preceding claims, wherein the step b) of purifying the crude composition comprises at least one distillation step and / or at least one crystallization step, wherein preferably in step b) a purge stream is withdrawn, which comprises hydroxy acid oligomers having a weight average molecular weight of 50 to 10,000 g / mol and preferably of 100 to 3,000 g / mol.

8. The process in accordance with any of the preceding claims, wherein the purified composition comprises at least one catalyst or at least one catalyst is added to the purified composition before starting the ring-opening- polymerization step c) or at least one catalyst is added to the purified composition during the ring-opening-polymerization step c), wherein the at least one catalyst is preferably selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts, and wherein the ring-opening-polymerization step c) is performed at a temperature of 160 to 240°C.

9. The process in accordance with claim 8, wherein the purified composition comprises at least one initiator or at least one initiator is added to the purified composition before starting the ring-opening-polymerization step or at least one initiator is added to the purified composition during the ring-opening-polymerization step, wherein the molar ratio of the cyclic ester of the hydroxy acid being contained in the purified composition to initiator is more than 100 to 10,000, and wherein the at least one initiator is preferably selected from the group consisting of monohydroxy compounds, dihydroxy compounds, trihydroxy compounds, tetrahydroxy compounds and arbitrary combinations of two or more of the aforementioned initiators.

10. The process in accordance with any of the preceding claims, wherein the polyhydroxy acid contained in the polyhydroxy acid containing composition obtained in step c) has a number average molecular weight of more than 10,000 g / mol as well as a polydispersity of at most 1 .8, preferably of 1 .6 or less and more preferably of 1 .4 or less.11 . The process in accordance with any of the preceding claims, wherein in step c) at least one purge stream is withdrawn, wherein one purge stream comprises unconverted cyclic ester of the hydroxy acid and / or one purge stream comprises polyhydroxy acid being out of the specification.

12. The process in accordance with any of the preceding claims, wherein the reaction composition comprises, based on 100% by mol of the reaction composition, 50 to 100% by mol and preferably 90 to 99.9% by mol of the at least one purge stream and 0.1 to 10% by mol of the i) alcohol or polyalcohol comprising at least two hydroxy groups and / or ii) the primary amine comprising at least one amino group, wherein preferably the reaction composition further comprises water and / or the hydroxy acid, preferably, based on 100% by mol of the reaction composition, 0 to 20% by mol and more preferably 0.1 to 10% by mol of water and / or 0 to 10% by mol and more preferably 0.1 to 10% by mol of the hydroxy acid.

13. The process in accordance with any of the preceding claims, wherein the depolymerization reaction is performed at a temperature of 120 to 220°C, preferably at 160 to 200°C and more preferably at 170 to less than 190°C, wherein the cyclization reaction is performed at a temperature of 150 to 290°C, preferably at 170 to 210°C and more preferably at 170 to less than 200°C, and wherein the cyclization reaction is performed at ambient pressure or at a pressure of 0.1 to 10 kPa and preferably of 0.1 to 1 kPa.

14. The process in accordance with any of claims 1 to 11 , wherein step e) comprises that to at least one and preferably to all purge streams a catalyst, a co-catalyst and optionally a re-used composition are added so as to prepare a reaction composition, wherein the co-catalyst contains i) at least one alcohol and / or at least one primary amine and ii) water and / or the hydroxy acid, before the reaction composition is partially reacted in a reactor to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 30 to 90% by mole so as to obtain the recycle composition containing cyclic ester of the hydroxy acid and a second composition containing unconverted polymer and / or oligomer of the hydroxy acid, residual catalyst and co-catalyst, wherein the recycle composition and the second composition are withdrawn from the reactor, wherein at least a portion of the second composition is recycled as re-used composition into the reaction composition.

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