A single step process for producing a cyclic ester of a hydroxy acid

Abstract

The present invention relates to a process of a producing cyclic ester of a hydroxy acid comprising the steps of: a) melting a polymer of the hydroxy acid, b) adding a catalyst, a co-catalyst and a recycle composition to the polymer melt obtained in step a) so as to prepare a reaction mixture, 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, so as to obtain a reaction mixture, c) partially reacting the reaction mixture obtained in step b) in a reactor to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 50 to 99% by mole so as to obtain a first crude product 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 and d) withdrawing the first crude product composition and the second composition from the reactor, wherein at least a portion of the second composition is recycled as recycle composition into the reaction mixture of step b).

Description

[0001] Sulzer Management AG S13802P

[0002] A single step process for producing a cyclic ester of a hydroxy acid

[0003] The present invention relates to a process of producing a cyclic ester of a hydroxy acid, in particular an co-hydroxy acid (omega-hydroxy acid) or an a-hydroxy acid (alpha-hydroxy acid), such as lactide, from polyhydroxy acid, in particular poly-co- hydroxy acid or poly-a-hydroxy acid, such as from polylactic acid, for example from waste polylactic acid, and thus for recycling the polyhydroxy 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. The cyclic esters may be produced by intramolecular esterification of an aliphatic hydroxy acid, such as in the case of a lactone, for instance caprolactone, or by condensation of two hydroxy acid molecules into a cyclic diester, such as the condensation of lactic acid to lactide or the condensation of glycolic acid to glycolide. Alternatively, the cyclic ester, such as in particular cyclic diester, may be produced by first oligomerizing a hydroxy acid and then subjecting the oligomer to a depolymerization reaction in order to obtain the cyclic diester. For instance, lactide is often prepared by the latter method, for instance by fermentation of carbohydrates from biomass, such as starch, sugar or com resulting in lactic acid, by then oligomerizing the lactic acid and by afterwards subjecting the oligomers to a depolymerization reaction in order to obtain lactide. After purification, the one or more cyclic esters of hydroxy acid as monomer(s) are then polymerized in the presence of a catalyst and optionally an initiator to form high molecular weight polymer. In view of the increasing amounts of articles made of polyhydroxy acid, the recycling of polyhydroxy acid waste becomes more and more important. In order to recycle polylactic acid waste, processes for converting polylactic acid to lactide have been already proposed. In these processes, the polylactic acid waste is first depolymerized to lactic acid oligomer, before the lactic acid oligomer is cyclodepolymerized to lactide. However, the known recycle processes have several drawbacks, such as that they require in both steps of depolymerizing polyhydroxy acid to lactic acid oligomer and of cyclodepolymerizing lactic acid oligomer to lactide a comparable long reaction time at an elevated temperature so that the respective processes are quite energy consuming. Moreover, these processes often lead to an undesired racemization of the lactide, which is chiral and exists in the form of enantiomeric L-lactide, D-lactide and meso-lactide. Analogously, lactic acid is chiral and exists in the form of enantiomeric L-lactic acid and D-lactic acid. Since the properties of the polylactic acid, which is produced from the lactide, such as the processing properties, crystallization properties and degradation behavior of polylactic acid, depend on the structure and composition of the polymer chains, in particular on the ratio of the L- to the D-stereoisomer of lactic acid, processes are desirable, in which lactide with a desired ratio of the three enantiomers is obtained.

[0006] In view of this, the object underlying the present invention is to provide a process of converting directly polyhydroxy acid, in particular co-hydroxy acid or a-hydroxy acid, such as polylactic acid, in particular waste polylactic acid, to a cyclic ester of the hydroxy acid, such as lactide, wherein the process is fast, may be performed within a comparable short reaction time, is energy efficient and allows to obtain the cyclic ester of the hydroxy acid in the desired stereochemical form.

[0007] In accordance with the present invention this object is satisfied by providing a process of producing cyclic ester of a hydroxy acid comprising the steps of: a) melting a polymer of the hydroxy acid, b) adding a catalyst, a co-catalyst and a recycle composition to the polymer melt obtained in step a) so as to prepare a reaction mixture, wherein the cocatalyst contains i) at least one alcohol and / or at least one primary amine and ii) water and / or the hydroxy acid, so as to obtain a reaction mixture, c) partially reacting the reaction mixture obtained in step b) in a reactor to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 30 to 99% by mole so as to obtain a first crude product 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 and d) withdrawing the first crude product composition and the second composition from the reactor, wherein at least a portion of the second composition is recycled as recycle composition into the reaction mixture of step b).

[0008] This solution bases on the surprising finding that by first melting a polymer of the hydroxy acid and then adding thereto a catalyst, a specific co-catalyst, which contains i) at least one alcohol and / or at least one primary amine and ii) water and / or the hydroxy acid, and a recycle composition from the reactor containing unconverted polymer of the hydroxy acid, unreacted catalyst and unreacted co-catalyst so as to obtain a reaction mixture, before the reaction mixture is reacted to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 30 to 99% by mole so as to obtain a first crude product 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 at least a portion of the second composition is recycled as recycle composition into the reaction mixture of step b), the conversion of the polymer of the hydroxy acid to the cyclic ester of the hydroxy acid can be performed in a single step so that the process is fast, may be performed within a comparable short reaction time and is energy efficient. Moreover, the process allows to obtain the cyclic ester of the hydroxy acid in the desired stereochemical form. On account of the intentional incomplete reaction in step c), i.e. by performing the reaction intentionally only with a conversion rate of 30 to 99% by mole, the unconverted material, such as polymer of the hydroxy acid with the residual catalyst and co-catalyst, is reused and mixed with the fresh feed composition containing polymer of the hydroxy acid, catalyst and co-catalyst, which increases the process economy due to the reduction of the consumption of catalyst and co-catalyst and the reduction of the reaction time. In particular, the use of the specific co-catalyst, which contains i) at least one alcohol and / or at least one primary amine and ii) water and / or the hydroxy acid, allows to drastically reduce the reaction time, the reaction temperature, the operational expenditures (OPEX) and the plant complexity. Furthermore, sidereactions and polymer degradation during the reaction in step c) are significantly reduced thereby, which among others allows to improve the quality of the produced cyclic ester of the hydroxy acid, because no low molecular weight oligomers of the hydroxy acid are obtained during the reaction in step c) so that no such oligomer mixes with and hence contaminate the cyclic ester of the hydroxy acid being produced during the reaction in step c). Moreover, the use of the alcohol and / or primary amine in the co-catalyst drastically reduces the viscosity of the intermediary formed oligomer of the hydroxy acid during the reaction, which in turn increases the production rate of the cyclic ester of the hydroxy acid and reduces the mass transfer limitations. All in all, the process in accordance with the present invention is fast and may thus be performed within a comparable short reaction time, is energy efficient and allows to obtain the cyclic ester of the hydroxy acid in the desired stereochemical form.

[0009] The term polymer as used herein means in accordance with the present invention any kind of polymer and thus covers homopolymers as well copolymers.

[0010] In accordance with the present invention, polymers are macromolecules being composed of at least 70 repeating subunits, whereas oligomers are macromolecules being composed of 2 to 69 repeating subunits. 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.

[0011] In accordance with the present invention, a polymer of the hydroxy acid, from which the cyclic ester shall be formed, is molten in step a). The present invention is not particularly limited concerning the kind of used polymer of the hydroxy acid and of produced cyclic ester of hydroxy acid, as long as the kind of hydroxy acid of the polymer and of the cyclic ester is the same. Preferably, the hydroxy acid is a co-hydroxy acid or a-hydroxy acid and hence the polymer is a poly-co-hydroxy acid or a poly-a-hydroxy acid. The present invention is particularly suitable for using a polymer of a co-hydroxy acid or a a-hydroxy acid being 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 and thus for producing a cyclic ester being selected from the group consisting of lactide, glycolide, caprolactone, valerolactone and arbitrary combinations of two or more of the aforementioned cyclic esters. Hence, preferred hydroxy acids are those being 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. Most preferably, the hydroxy acid is lactic acid, i.e. the process produces lactide from polylactic acid.

[0012] Concerning the device, in which the polymer of the hydroxy acid is molten in step a), as well as concerning the temperature, at which the polymer of the hydroxy acid is molten in step a), the present invention is not particularly restricted. How- ever, in view of possibly low operational costs it is preferred to melt the polymer of the hydroxy acid in step a) at a temperature between higher than the melting point of the polymer of the hydroxy acid and 190°C or less. As set out further below, the reaction in step c) is preferably performed at a temperature of 190°C or less, in order to avoid a racemization of the produced lactide, so that there is no need to heat the polymer melt upstream thereof, i.e. in step a) and / or in step b), to a temperature of more than 190°C. The melting of the polymer of the hydroxy acid may be performed in step a) in any suitable device, such as in a heated vessel or in an extruder. Good results are in particular obtained, when the polymer of the hydroxy acid is molten in step a) in an extruder to a polymer melt having a temperature of 190°C or less.

[0013] As set out above, in step b) a co-catalyst is added to the polymer melt obtained in step a), 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 so as to obtain a reaction mixture. The term alcohol generally denotes any compound comprising at least one hydroxy group. Hence, an alcohol may or may not comprise any further functional group in addition to at least one hydroxy group. More specifically, the alcohol may be any monoalcohol, dialcohol or polyalcohol comprising at least three hydroxy groups. Good results are in particular obtained, when in step b) an alcohol is added, which is selected from the group consisting of 2-ethyl hexanol, fatty alcohols, ethylene glycol, polyethylene glycol, butane diol, glycerol, phloroglucinol, pentaerythritol, sorbitol, dipentaerythritol, tripentaerythritol, 1 ,1 ,1 -tris (hydroxyme- thyl)ethane, 1 ,1 ,1-tris(hydroxymethyl)propane, di-(trimethylolpropane), trime- thylolpropanethoxylate, polyphenols comprising at least three hydroxy groups, saccharides comprising at least three hydroxy groups, star-shaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned alcohols. In accordance with a particular preferred embodiment of the present invention, in step b) a co-catalyst is added, which contains at least one polyalcohol comprising at least three hydroxy groups. The use of such a multivalent alcohol allows in combination with the recycling of the recycle composition into the reaction mixture in step d) and in particular also in combination with the use of water in the cocatalyst to reduce the total amount of co-catalyst needed in the process. On account of this reason it is preferred that in step b) a co-catalyst is added to the polymer melt obtained in step a), which contains at least one polyalcohol comprising at least three hydroxy groups and more preferably at least one polyalcohol 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, star-shaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned polyalcohols.

[0014] In addition to at least one alcohol and preferably at least one polyalcohol or instead of alcohol, in step b) a co-catalyst containing at least one primary amine may added. The term primary amine denotes any compound comprising at least one primary amino group. Hence, a primary amine may or may not comprise any further functional group in addition to at least one primary amino group. For instance, the primary amine comprising at least one amino group may comprise one or more hydroxyl groups, i.e. may be an aminoalcohol, such as one according to the formula OH-R-NH2, wherein R is a hydrocarbon group, such as an alkylene group, such as a Ci-5-alkylene group. Preferably, the primary amine comprising at least one amino group is an aminoalcohol or a primary amine comprising no other functional group in addition to the at least one amino group. The at least one primary amine may be a primary amine comprising at least one amino group and is preferably a primary amine comprising at least two amino groups. Suitable exam- pies for primary amines comprising at least one amino group are those being selected from the group consisting of ethylenediamine, hexamethylenediamine, tris(2-aminoethyl)amine, 1-aminopropan-2-ol, 5-amino-1 -pentanol, 3-amino-1- propanol, 3-amino-1 ,2-propanediol and arbitrary combinations of two or more of the aforementioned primary amines and suitable examples for primary amines comprising at least two amino groups are those being selected from the group consisting of hexamethylenediamine, phenylenediamine, butanediamine and arbitrary combinations of two or more of the aforementioned primary amines.

[0015] In accordance with a further particular preferred embodiment of the present invention, in step b) 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, in step b) 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.

[0016] Still more preferably, in step b) 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 in step b) 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.

[0017] 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 mixture.

[0018] In accordance with the present invention, a catalyst is added in step b). The catalyst allows to minimize possible side-reactions and to increase the yield in the reaction of step c). 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.

[0019] Particular suitable examples for catalysts are catalysts being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin ox- ide, dibutyl tin oxide, tributyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts.

[0020] Preferably, the catalyst is added in step b) in an amount of 1 to 3,000 ppm and more preferably 20 to 1 ,000 ppm, based on 100% by weight of the reaction mixture.

[0021] In accordance with the present invention, at least a portion of the second composition obtained in the reaction of step c) is recycled and added as recycle composition to the reaction mixture. Good results are in particular obtained, when the recycle stream is added in in step b) an amount of 20 to 60% by weight and more preferably of 40 to 50% by weight, based on 100% by weight of the reaction mixture.

[0022] As set out above, it is preferred that the polymer of the hydroxy acid is molten in step a) to a melt having a temperature of 190°C or less. In line therewith, it is preferred that the temperature of the reaction mixture is kept during step b) always at a temperature of 190°C or less.

[0023] While step b) may be performed in any suitable device, such as in an extruder, in a continuous reactor or in a static mixer, it is preferred that step b) is at least partially performed in an extruder and more preferably in an extruder and a static mixer being arranged downstream of the extruder. For instance, the polymer of the hydroxy acid is molten in an extruder and the co-catalyst, the catalyst and the recycle composition are fed into the extruder and are added therein in step b) into the polymer melt. The so obtained reaction mixture may then be led directly into the reactor, in which the reaction of step c) is performed, or may be first led through a static mixer, in order to homogenize the reaction mixture, before the homogenized reaction mixture is led into the reactor. Still alternatively, the polymer of the hydroxy acid is molten in an extruder and the co-catalyst is fed into the ex- truder and is added therein in step b) into the polymer melt, before the so obtained mixture is led through a line, into which the catalyst and the recycle composition are led and hence added to the mixture, before the so obtained reaction mixture is led through a static mixer, in order to homogenize the reaction mixture, before the homogenized reaction mixture is led into the reactor.

[0024] If a static mixer is used in the process of the present invention, it may be any kind of static mixer. Static mixer means any mixer, which does not comprise any moving part and does in particular not comprise any rotating part and any such mixer may be used in the present invention. Static mixers usually produce a mixing effect by generating a turbulent flow due to static, i.e. non-moving elements, such as plates, bars, crossbars, baffles, helically formed deflection means, grids and the like. Suitable examples for static mixers, are x-type static mixers, spiral / helical- type static mixers, quattro-type static mixers, baffle plate-type static mixers, turbulator strips-type static mixers and any combination of two or more of the above- mentioned mixer types. X-type static mixers comprise deflection means in the form of bars, crossbars, plates or the like having in a plan view and / or side view and / or cross-sectional view a x-like form. Such x-type static mixers are described for instance in WO 2010 / 066457 A1 , EP 1 206 962 A1 , EP 2 158 027 B1 and EP 0 655275 B1 and are commercially available from Sulzer Chemtech Ltd, Winterthur, Switzerland under the tradenames SMX, SMXL and SMX plus as well as from Fluitec, Neftenbach, Switzerland under the tradename CSE-X. Spiral / hel ical-type static mixers have a helically formed deflection means and are described for instance in US 3,743,250 A, whereas quattro-type static mixers comprise deflection means forming chamber-like mixing sections and are described for instance in EP 2 548 634 B1 and in EP 0 815 929 B1 . While baffle plate-type static mixers comprise usually longitudinal deflection means and are described for instance in EP 1 510247 B1 and in US 4,093,188 A, turbulator strip-type static mixers comprise in a tube a plurality of elongated strips, each of which being formed by a series of alternating deflection panels successively joined together by for example substan- tially triangular bridging portions with the strips being held together and anchored substantially on the axis of the tube by alternate ones of the bridging portions and the other bridging sections being disposed adjacent the inner wall of the tube and are described for instance in US 4,296,779 A. Other suitable static mixers are distributed from Sulzer Chemtech AG under the tradenames CompaX, SMI, KVM, SMV and GVM and from Stamixco AG, Wollerau, Switzerland under the trade- name GVM. In view of the above, it is preferred to use in step b) a static mixer being selected from the group consisting of x-type static mixers, spiral / helical-type static mixers, quattro-type static mixers, baffle plate-type static mixers, turbulator strips-type static mixers and any combination of two or more of the above mentioned mixer types.

[0025] Concerning the reaction conditions of the reaction of step c) the present invention is not particularly limited. Good results are in particular obtained, when the reaction of step c) is performed at a temperature of 150 to 290°C. More preferably, the cyclodepolymerization reaction of step b) is performed at a temperature of 170 to 210°C and most preferably 170 to less than 200°C. As set out above, a reaction temperature of 190°C or less is particularly preferred in particular for the conversion of polylactic acid to lactide, since at such low temperatures a racemization of the enantiomers is reliably avoided. On account of this reason it is particularly preferred that the reaction of step c) is performed at a temperature of 210°C or less, preferably at 150 to 200°C or less and most preferably at 170 to 190°C or less.

[0026] In addition, it is preferred that the reaction of step c) is performed at ambient pressure or at subatmospheric pressure. If the reaction of step c) is performed at sub- atmospheric pressure, the pressure is preferably 0.1 to 10 kPa and most preferably 0.1 to 1 kPa. In accordance with the present invention, the reaction mixture is intentionally only partially reacted in step c), namely the reaction of the polymer of the hydroxy acid to the cyclic ester of the hydroxy ester is performed so that the conversion rate in the reactor is 30 to 99% by mole. Good results are in particular obtained, when the reaction of step c) is performed so that the conversion rate in the reactor is 40 to 90% by mole and preferably 50 to 70% by mole. Conversion rate is in this connection defined as the ratio of the moles of hydroxy acid contained in the produced cyclic ester of the hydroxy acid divided by the moles of hydroxy acid contained in the polymer of the hydroxy acid being fed into the reactor, whereas the remaining moles of the hydroxy acid (i.e. those being not contained in the produced cyclic ester of the hydroxy acid) remain in the second composition obtained in step b).

[0027] The conversion rate can be for instance influenced by the residence time of the reaction mixture within the reactor, i.e. by the short reaction time, by the reaction temperature and the amounts and kinds of catalyst and co-catalyst. Advantageously, the reaction of step c) is performed for a short reaction time of 30 to 240 minutes and more preferably of 60 to 120 minutes.

[0028] The reaction of step c) may be performed in any kind of reactor, such as in a batch reactor, in a continuous reactor or in a reactive extrusion reactor, such as in an extruder or a plug-flow reactor. However, it is particularly preferred that the reaction of step c) is performed in a devolatilization reactor or in a distillation column, which allows to easily separate the cyclic ester of the hydroxy acid as vapor product composition from the liquid second composition. While the cyclic ester of the hydroxy acid as vapor product composition is withdrawn from the devolatilization reactor or from the distillation column as overhead stream, the liquid second composition is withdrawn from the devolatilization reactor or from the distillation column as bottom stream. Preferably, not all of the second composition is recycled to step b), but a small portion thereof is removed from the process as purge stream in order to avoid the accumulation of undesired materials in the recycle composition. Consequently, in accordance with a further preferred embodiment of the present invention, 70 to 99% by weight and preferably 80 to 90% by weight of the second composition being withdrawn from the reactor are recycled as recycle composition into the reaction mixture of step b), whereas the remainder of the second composition is withdrawn as purge stream from the process.

[0029] In order to increase the purity of the cyclic ester of the hydroxy acid obtained in step c) and being withdrawn from the reactor in step d), it is proposed in a further development of the idea of the present invention that the first crude product composition being withdrawn from the reactor is purified in a further step e). Preferably, the first crude product composition being withdrawn from the reactor is purified in step e) by a method being selected from the group consisting of distillation, static crystallization, falling film crystallization, suspension crystallization and arbitrary combinations of two or more thereof.

[0030] For instance, the purification of step e) comprises at least one static crystallization step, wherein the at least one static crystallization step preferably comprises one to four static crystallization stages and more preferably two static crystallization stages.

[0031] In accordance with an alternative variant, the purification of step e) comprises at least one dynamic crystallization step, wherein the at least one dynamic crystallization step preferably comprises one to four dynamic crystallization stages. More preferably, the dynamic crystallization is a falling film crystallization.

[0032] In accordance with an alternative variant, the purification of step e) comprises at least one distillation step each of which being performed in a distillation column. More preferably, the hydroxy acid is lactic acid and step c) comprises two distillation steps each of which being performed in a distillation column, wherein in the first distillation column the composition containing lactide as the cyclic ester of lactic acid obtained in step b) is separated into an overhead fraction, into a bottom fraction and into a side fraction, wherein the side fraction of the first distillation column is fed as side fraction into the second distillation column, in which a mesolactide enriched composition is produced as overhead fraction and a L-lactide enriched composition is produced as side fraction or as bottom fraction.

[0033] Subsequently, the present invention is described by means of an illustrative, but not limiting figure, in which:

[0034] Fig. 1 shows a scheme of a process of producing lactide in accordance with one embodiment of the present invention.

[0035] The process being schematically shown in figure 1 is performed in a plant comprising extruder 10, a static mixer 12 and a reactor 14. While the extruder 10 is connected with the static mixer 12 via a first connection line 16, the static mixer 12 and the reactor 14 are connected with each other via a second connection line 18. The extruder 10 comprises an inlet line 20 for polylactic acid as well as an inlet line 22 for catalyst and / or co-catalyst (preferably for co-catalyst), whereas an inlet line 24 for catalyst and / or co-catalyst (preferably for catalyst) and a recycle line 26 lead into the first connection line 16. The reactor 14 comprises an overhead outlet line 28 for crude lactide as well as a bottom outlet line 30 for heavy residue, which splits into a withdrawal line 32 and into a recycle line 26.

[0036] During the operation, waste polymer consisting mainly of polylactic acid is continuously fed via the inlet line 20 into the extruder 10 and is molten therein, before co-catalyst is added into the polymer melt via the inlet line 22. The mixture of polymer melt and co-catalyst is then continuously led through the first connection line 16, into which catalyst is fed via the inlet line 24 and recycle composition of heavy residue is fed via the recycle line 26. The so obtained reaction mixture is led through the static mixer 12, in which it is homogenized, then and through the second connection line 18 into the reactor 14, which is preferably a distillation column. The reaction mixture containing polylactic acid, co-catalyst, catalyst and recycle composition is only partially converted in the reactor 14 to lactide, namely with a conversion rate of 30 to 99% by mole. The produced lactide is withdrawn from the reactor 14 via the overhead outlet line 28, whereas the heavy residue containing unreacted polylactic acid together with unreacted catalyst and unreacted cocatalyst is withdrawn from the reactor 14 via the bottom outlet line 30. While a portion of the heavy residue is recycled as recycle composition through the recycle line 26 to the reaction mixture, the remaining portion of the heavy residue is withdrawn via the withdrawal line 32.

[0037] Subsequently, the present invention is described by means of an illustrative, but not limiting example.

[0038] Example 1

[0039] In a plant as shown in figure 1 , 1 kg / h of plastic waste containing 90 wt.% of polylactic acid PLA (0.9 kg / h) was fed via inlet line 20 into an extruder 12, in which it was molten and then mixed with 0.2 wt.% of co-catalyst (1 .8 g / h) being fed into the extruder 12 via the inlet line 22. The co-catalyst was a mixture of 75% by weight of butane diol, of 20% by weight lactic acid and of 5% by weight water. Furthermore, tin octanoate was added with a concentration of 100 ppm (0.09 g / h) and 0.7 kg / h of recycling stream from the reactor were added to the polymer melt via the lines 24, 26. The so obtained mixture was then fed as reaction mixture through the static mixer 12 and then into the reactor 14, in which the reaction took place at a temperature of 190°C and at a pressure of 0.7 kPa. The reactor 14 was operated with a ratio of lactide production / reaction mixture feed of 0.5 (0.85 kg / h of pro- duced lactide), i.e. at a conversion ratio of 50% by mole. While the crude lactide composition was withdrawn as vapor stream from the reactor 14 via the overhead outlet line 28, the heavy residue was withdrawn as liquid stream from the reactor 14, wherein the liquid stream was split to a 0.15 kg / h purge stream (0.1 kg / h of other plastic materials plus 0.05 kg / h unconverted polylactic acid) and the aforementioned 0.7 kg / h recycling stream.

[0040] The overall recovery yield of lactide from the polylactic acid waste stream was 94%.

[0041] Reference numerals 10 Extruder

[0042] 12 Static mixer

[0043] 14 Reactor

[0044] 16 First connection line

[0045] 18 Second connection line 20 Inlet line for polylactic acid

[0046] 22 Inlet line for catalyst and / or co-catalyst

[0047] 24 Inlet line for catalyst and / or co-catalyst

[0048] 26 Recycle line

[0049] 28 Overhead outlet line 30 Bottom outlet line

[0050] 32 Withdrawal line

Claims

Claims:1 . A process of producing a cyclic ester of a hydroxy acid comprising the steps of: a) melting a polymer of the hydroxy acid, b) adding a catalyst, a co-catalyst and a recycle composition to the polymer melt obtained in step a) so as to prepare a reaction mixture, 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, so as to obtain a reaction mixture, c) partially reacting the reaction mixture obtained in step b) in a reactor to cyclic ester of the hydroxy acid so that the conversion rate in the reactor is 30 to 99% by mole so as to obtain a first crude product 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 and d) withdrawing the first crude product composition and the second composition from the reactor, wherein at least a portion of the second composition is recycled as recycle composition into the reaction mixture of step b).

2. The process in accordance with claim 1 , wherein the hydroxy acid is a cohydroxy acid or a a-hydroxy acid, which is 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 polymer of the hydroxy acid is molten in step a) to a polymer melt having a temperature of 190°C or less, wherein preferably step a) is performed in an extruder.

4. The process in accordance with any of the preceding claims, wherein in step b) a co-catalyst containing at least one alcohol is added, wherein the at least one alcohol is a monoalcohol, a dialcohol or a polyalcohol comprising at least three hydroxy groups, wherein the alcohol is preferably selected from the group consisting of 2-ethyl hexanol, fatty alcohols, ethylene glycol, polyethylene glycol, butane diol, glycerol, phloroglucinol, pentaerythritol, sorbitol, dipentaerythritol, tripentaerythritol, 1 ,1 ,1 -tris (hydroxyme- thyl)ethane, 1 ,1 ,1 -tris(hydroxymethyl)propane, di-(trimethylolpropane), tri- methylolpropanethoxylate, polyphenols comprising at least three hydroxy groups, saccharides comprising at least three hydroxy groups, star-shaped oligomers comprising at least three hydroxy end groups, cyclitol and arbitrary combinations of two or more of the aforementioned alcohols.

5. The process in accordance with any of the preceding claims, wherein in step b) a co-catalyst containing at least one primary amine is added, wherein the at least one primary amine is preferably selected from the group consisting of ethylenediamine, hexamethylenediamine, tris(2-aminoethyl)amine, 1 -aminopropan-2-ol, 5-amino-1 -pentanol, 3-amino-1 -propanol, 3-amino-1 ,2- propanediol and arbitrary combinations of two or more of the aforementioned primary amines.

6. The process in accordance with any of the preceding claims, wherein in step b) 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 and / or a primary amine and of reminder to 100% by weight of ii) water and / or the hydroxy acid.

7. The process in accordance with any of the preceding claims, wherein in step b) 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.

8. The process in accordance with any of the preceding claims, wherein the co-catalyst is added in step b) in an amount of 0.01 to 5% by weight and preferably of 0.05 to 1 % by weight based on 100% by weight of the reaction mixture.

9. The process in accordance with any of the preceding claims, wherein in step b) a catalyst being selected from the group consisting of tin oxide, iron oxide, copper oxide, tin octanoate, butyl tin oxide, dibutyl tin oxide, tributyl tin oxide, calcium carbonate, potassium carbonate and arbitrary combinations of two or more of the aforementioned catalysts is added in an amount of 1 to 3,000 ppm and more preferably 20 to 1 ,000 ppm based on 100% by weight of the reaction mixture.

10. The process in accordance with any of the preceding claims, wherein in step b) the recycle stream is added in an amount of 20 to 60% by weight and preferably of 40 to 50% by weight of the reaction mixture.11 . The process in accordance with any of the preceding claims, wherein the temperature of the reaction mixture is kept during step b) always at a temperature of 190°C or less, wherein preferably at least the addition of the co- catalyst is performed in step b) in an extruder.

12. The process in accordance with any of the preceding claims, wherein the reaction of step c) is performed at a temperature of 150 to 290°C and the cyclodepolymerization reaction of step b) is performed at a temperature of 170 to 210°C and most preferably 170 to less than 200°C, wherein preferably the reaction of step c) is performed at a temperature of 210°C or less, more preferably at 150 to 200°C or less and most preferably at 170 to190°C or less at ambient pressure or at a pressure of 0.1 to 10 kPa and preferably of 0.1 to 1 kPa.

13. The process in accordance with any of the preceding claims, wherein the reaction of step c) is performed so that the conversion rate in the reactor is 40 to 90% by mole and preferably 50 to 70% by mole.

14. The process in accordance with any of the preceding claims, wherein the reaction of step c) is performed in a devolatilization reactor or in a distillation column, wherein the first crude product composition is withdrawn as vapor overhead stream and the second composition is withdrawn as liquid stream from the devolatilization reactor or distillation column.

15. The process in accordance with any of the preceding claims, wherein 70 to 99% by weight and preferably 80 to 90% by weight of the second composition being withdrawn from the reactor are recycled as recycle composition into the reaction mixture of step b), whereas the remainder of the second composition is withdrawn as purge stream from the process.

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