Improved method for producing polylactic acid with a low yellow index

Abstract

The present invention relates to a continuous method for producing polylactic acid with a low yellow index, comprising a step of purifying a crude lactide stream, the crude lactide stream being formed upstream by the cyclization of lactic acid oligomers, the oligomers being obtained by oligomerization of a lactic acid stream, and the purification of the crude lactides stream comprising a step of crystallization without prior distillation.

Description

Improved process for the production of low yellow index polylactic acid Object of the invention

[0001] The present invention relates to a competitive continuous process for the production of polylactic acid (PLA), and in particular to an improved process which makes it possible to eliminate coloring molecules, and to generate by polymerization at variable ratios a whole range of PLA, with a low or high content of D isomer with high stability, a very high overall yield and a very low yellowing rate. State of the art

[0002] As environmental concerns grow, biodegradable and bio-based polymers are becoming increasingly attractive commercially as substitutes for conventional synthetic polymers such as polyolefins and polyurethanes. Consequently, the use of biodegradable polymers in packaging and textiles continues to expand. As a result, considerable research is being conducted on the synthesis of these new materials. Among the various biodegradable polymers, polylactic acid (PLA) is one of the most commonly used and studied.

[0003] We know that among the usual processes for producing PLA there is a first type which consists of a direct polycondensation of lactic acid, as described in patent JP733861; but this type of process is limited by the use of a solvent and therefore by the difficulties in eliminating water from the reaction medium.

[0004] Another common method for producing PLA involves starting with lactic acid using a multi-step process. This process includes oligomerizing the lactic acid, cyclizing the resulting oligomers into lactide, purifying the crude lactide, and polymerizing the purified lactide into PLA. The resulting PLA is then devolatilized to extract the unconverted lactide before the final step, which involves granulating, crystallizing, and drying the PLA granules. This is currently the preferred method for industrial PLA production.

[0005] Lactic acid has two optically active enantiomers, D-lactic acid (or R-lactic acid) and L-lactic acid (or S-lactic acid). Therefore, three stereoisomers of lactide can be produced: L-lactide, D-lactide, and meso-lactide. These three stereoisomers of lactide can give rise to three stereochemical forms of polylactic acid: D-PLA, L-PLA and a racemic mixture called D,L-PLA. The three stereochemical forms of PLA possess different properties.

[0006] It is known that the physical properties of PLA can be controlled by adjusting the proportion of D-enantiomers and L-enantiomers. Therefore, amorphous PLA can be obtained by adding more than approximately 8 mol% of the D-enantiomer. When the amount of D-enantiomer is less than 8 mol%, the resulting PLA is considered semi-crystalline. The degree of crystallization of PLA affects its melting point (Tm), glass transition temperature (Tg), and mechanical properties, including its modulus of elasticity and tensile strength. In particular, as the amount of D-enantiomers in L-PLA increases, both the Tm and Tg decrease.

[0007] To synthesize high-quality PLA in a controlled manner, it is therefore crucial to have access to highly purified lactide. This is why the lactide must be extensively purified before ring-opening polymerization can be performed.

[0008] During the various stages of PLA production, some racemization can occur, and the resulting lactide will therefore be a mixture (hereafter referred to as "lactides") of L-lactide, mesolactide, and, to a much lesser extent, D-lactide. Currently, most PLA applications require a low D-enantiomer content (between 0.3 and 4%). In this context, the majority of production processes include steps to purify and separate the L-lactide from the mesolactide. The various PLA compositions are obtained by simply mixing these two purified fractions.

[0009] During these crude lactide purification steps, the predominant L-lactide form is generally extracted and purified first, generating a stream composed of mesolactide, residual L-lactide, and also certain impurities such as water, lactic acid, lactic acid dimers and trimers, and other impurities whose exact identity and structure are not fully known. These impurities have the particularity of affecting both the chemistry of the polymerization reaction and its color (generally expressed as the "yellowing" of the polymer).

[0010] With conventional separation methods, such as distillation and / or melt crystallization, impurities concentrate in the mesolactide stream, primarily in that obtained by distillation, as described in patents EP2994462, EP2406245, and EP2406246. Although it is common to blend this mesolactide stream, which always contains some of the Impurities, along with the pure L-lactide stream in a proportion not exceeding approximately 20% by weight, lead to the production of a polylactic acid that is potentially less crystalline but, more importantly, characterized by a more pronounced yellowing. It is therefore not uncommon to find that a large part or all of the mesolactide recovered in such processes is simply discarded, which significantly reduces the overall yield of the PLA production process.

[0011] Furthermore, in order to remain competitive with conventional polymers, it is important to guarantee a high yield in the PLA production process. This requires significant internal recycling of intermediate fractions, as described in application W02023025820. Unfortunately, these practices can lead to undesirable consequences, such as an increase in mesolactide content during the synthesis of crude lactide or the pre-purified stream. Indeed, this content can increase from 4-5% (generally obtained with little or no recycling) to levels of up to 10% or even 15% mesolactide (with significant recycling), thus increasing the yellowness index of the resulting polymer, as described above.

[0012] The processes described in patents EP3406605 and EP4269465 both necessarily require purifying the crude lactide stream, obtained after the cyclization step of lactic acid oligomers, through a first distillation step. This step yields a fraction enriched in mesolactide and a fraction enriched in L-lactide. However, this separation is complex, carried out at high temperatures, and requires a large number of theoretical stages as well as high reflux ratios. This increases residence times in the unit in the presence of reactive molecules (water, lactic acid, etc.), which promotes the degradation of lactide, a heat-sensitive molecule, and therefore yield losses. Furthermore, this distillation step is very energy-intensive. The mesolactide-enriched stream from this distillation step is then purified by crystallization.This distillation step is actually necessary because it allows the removal of highly reactive volatile molecules which would otherwise disrupt crystallization, reducing its yield and thus requiring more purification stages.

[0013] Document WO2023 / 036948 A1 (Purac) discloses a lactic acid obtained by fermentation and converted into a prepolymer by condensation polymerization. The prepolymer is then converted into lactide. The lactide then undergoes a first distillation step to remove water and lactic acid monomers. Next, the lactide undergoes a second distillation step during which the meso- Lactide is separated, resulting in one lactide stream enriched in meso-lactide and another lactide stream depleted in meso-lactide and therefore enriched in L- and / or D-lactide.

[0014] Document US2004 / 014991 A1 (Van Gansberhe) discloses a lactide purification process, it does not disclose a "continuous" PLA manufacturing process.

[0015] Document EP4335887 discloses a PLA that is produced using an unspecified manufacturing process and exhibits yellow indices of a value of 4 (example 1), a value of 6 (example 2) and a value of 8 (example 3).

[0016] Document WO2014 / 115899A1 discloses PLAs with a yellow index of 2 to 5 respectively (examples at 5) and a D-lactide content of 10%.

[0017] There is therefore a need for a low yellow index PLA production process with a very high overall yield, without impacting the flexibility of the process (i.e. the possibility of producing a range of PLA with variable levels of D isomer with the same process). Objectives of the invention

[0018] The present invention aims to overcome the drawbacks of the prior art. It aims to disclose a process for obtaining PLA with low yellowing with a very high overall yield, and without impacting the flexibility of the process (i.e. the possibility of producing a range of PLA with variable levels of D isomer with the same process).

[0019] The process of the present invention differs from the prior art in particular by treating the crude lactide stream with a crystallization purification technology rather than distillation. This is much less thermally aggressive, which drastically reduces degradation reactions and yield loss. The meso-lactide-enriched stream is first treated by distillation, representing only a small fraction of the initial stream, to extract a pre-purified meso-lactide-enriched stream, which is then purified by crystallization. The purpose of this purification is to ensure chemical purification of the stream (removing water and residual acidity in accordance with "polymer grade" specifications) but not to obtain a pure meso-lactide stream (a mixture of meso-lactide and L-lactide) in order to avoid additional and unnecessary lactide yield losses. Summary of the invention

[0020] The present invention discloses a continuous process for the production of low yellow index polylactic acid comprising a step of purification of a crude lactide stream, this crude lactide stream being previously formed by cyclization of lactic acid oligomers, said oligomers being obtained by oligomerization of a lactic acid stream, characterized in that the purification of the crude lactide stream is carried out by crystallization without prior distillation.

[0021] Preferred embodiments of the invention include at least one, or any suitable combination of the following features: the crude lactide stream contains more than 85% by weight of lactides, preferably more than 90% by weight of lactides and more preferably more than 95% by weight of lactides, comprising 70% to 95% by weight of L-lactide, 10% to 20% by weight of meso-lactide, 1% to 8% by weight of lactic acid and 1% to 10% by weight of impurities; - the crude lactide stream contains more than 95% by weight of lactides, said lactide stream comprising between 70 and 95% by weight of L-lactide and less than 10% by weight of meso-lactide, 1 to 8% by weight of lactic acid and 1 to 10% by weight of impurities; - the purification of the crude lactide stream by crystallization includes solvent crystallization, molten wall crystallization and molten suspension crystallization; - The purification by crystallization of the crude lactide stream is a molten suspension crystallization coupled with a centrifugal separation and / or a washing column; a pre-purification step of the stream partially enriched in meso-lactide from the purification step of the crude lactide stream; the stream partially enriched in meso-lactide from the purification step of the crude lactide stream is pre-purified by distillation in order to obtain a pre-purified stream enriched in meso-lactide and a pre-purified stream enriched in L-lactide; the pre-purified stream enriched in meso-lactide has a meso-lactide content greater than 60% by weight, preferably greater than 70% by weight and more preferably greater than 80% by weight and whose residual acidity is less than 5%, preferably less than 1% and even more preferably less than 0.5%; the pre-purified stream enriched in L-lactide from the pre-purification step of the partially meso-lactide-enriched stream has a higher L-lactide content. at 80% by weight in L-lactide, preferably greater than 90% by weight and more preferably greater than 95% by weight; a purification step of the pre-purified meso-lactide-enriched stream to obtain a purified meso-lactide-enriched stream with a meso-lactide content greater than 70% by weight, preferably greater than 80% by weight, preferably greater than 90% by weight, an L-lactide content less than 30% by weight, preferably less than 20% by weight, preferably less than 10% by weight, an acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content less than 100 ppm, or even less than 50 ppm; the purification of the pre-purified meso-lactide-enriched stream includes a crystallization step;The purification by crystallization of the purification step of the pre-purified meso-lactide-enriched stream includes solvent crystallization, molten wall crystallization and molten suspension crystallization; the purification by crystallization of the purification step of the pre-purified meso-lactide-enriched stream is molten suspension crystallization coupled with centrifugal separation and / or a washing column; the process includes an additional step of recycling the pre-purified L-lactide-enriched stream from the pre-purification of the partially meso-lactide-enriched stream to the lactic acid oligomer cyclization step to form a crude lactide stream or to the crude lactide stream purification step or upstream of the process;The process further comprises a step of polymerizing the purified L-lactide stream alone, or polymerizing the purified stream enriched with meso-lactide alone, or polymerizing a composition of these two streams to obtain a polylactic acid grade containing from 0.5 to 45% D; the process further comprises a step of devolatilizing the polylactic acid to separate the residual lactide; the process comprises an additional step of recycling the residual lactide from the devolatilization step with the crude lactide stream from the oligomer cyclization step and / or the partially meso-lactide-enriched stream from the purification step of the crude lactide stream.

[0022] The present invention also discloses a purified lactide comprising an L-lactide content greater than 99.5% by weight, an acidity lower than at 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content of less than 100 ppm, preferably less than 50 ppm.

[0023] The present invention also discloses a polylactic acid obtained by the process as claimed which contains from 0.5 to 45% of the D isomer and which has a yellow index of less than 20, preferably less than 15, preferably less than 10, preferably less than 5, more preferably less than 2 and ideally less than 1.

[0024] Finally, the present invention discloses a polylactic acid having a D isomer content between 0.5% and 45% by weight, preferably between 0.5% and 20% by weight and a yellow index of less than 4, the yellow index having been measured using a BYK 6836 Guide spectrograph. Brief description of the figures

[0025] Figure 1 represents the PLA production scheme according to the present invention, which does not, however, constitute a limitation of its scope. List of reference symbols 10: lactic acid 20: pre-oligomerization unit 24: water 30: oligomerization unit 32: Lactic acid release (mostly) 34: Water outlet 40: cyclization reactor 42: Purge flow 42a: Recycled purge flow at the inlet of the cyclization reactor 42b: Recycled purge flow at the inlet of the oligomerization unit 50: crystallization unit 52: first distillation column 53: footflow (residue) 54: Second distillation column 55: heavy fractions 55a: Heavy fractions recycled at the inlet of the first column reboiler; 55b: Heavy fractions recycled at the inlet of the crystallization unit; 55c: Heavy fractions recycled at the inlet of the cyclization reactor 56: Prepurified stream enriched with L-lactide 58: crystallization unit 59: Purified stream enriched with "polymer grade" meso-lactide 60: polymerization unit 70: devolatilization unit 72: vapor stream containing unreacted lactide 72a: Recycled steam stream at the inlet of the crystallization unit 72b: Recycled steam stream at the inlet of the first distillation column 72c: Recycled steam stream at the inlet of the second distillation column 75: Granulation / Crystallization / Drying Unit 100: Commercial PLA grade 105: L-lactide flux of "polymer grade" valued on the market 110: Purified stream enriched with "polymer grade" meso-lactide, valued on the market Detailed description of the invention

[0026] The present invention discloses a continuous process which makes it possible to reduce the disadvantages mentioned above, this process comprises the following steps: a) oligomerization of a lactic acid stream; b) cyclization of the lactic acid oligomers to form a crude lactide stream containing more than 85% lactides, preferably more than 90% lactides and more preferably more than 95% lactides, comprising 70 to 95% L-lactide, 10 to 20% meso-lactide, 1 to 8% lactic acid and 1 to 10% heavy compounds (the whole being brought to 100%); c) purification of the crude lactide stream by crystallization in order to obtain a "polymer grade" L-lactide stream, i.e. an L-lactide content greater than 99% by weight, preferably greater than 99%.5% by weight; acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm and a partially meso-lactide-enriched stream still containing residual L-lactide but also certain impurities such as water, lactic acid, dimers and trimers, or even tetramers of lactic acid and other impurities; d) pre-purification of the partially meso-lactide-enriched stream from step c) by distillation in order to obtain a pre-purified meso-lactide-enriched stream with a meso-lactide content greater than 60% by weight, preferably greater than 70% by weight and more preferably greater than 80% by weight and a pre-purified L-lactide-enriched stream; e) purification of the pre-purified stream enriched with meso-lactide from step d) by crystallization in order to obtain a purified stream enriched with meso-lactide of "polymer grade" quality, i.e. a meso-lactide content greater than 70% by weight, preferably greater than 80% by weight, preferably greater than 90% by weight, L-lactide content less than 30% by weight, preferably less than 20% by weight, preferably less than 10% by weight, acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm; f) optionally recycling of the pre-purified stream enriched with L-lactide from step d) to step b) or c);g) polymerization of the polymer grade L-lactide stream from step c) or polymerization of the purified meso-lactide-enriched polymer grade stream from step e) or polymerization of a composition of these two streams to obtain a PLA grade containing 0.5 to 45% D, preferably 0.5 to 20% D; h) devolatilization of the PLA grade from step g) to separate the residual lactide from the PLA grade; i) optionally recycling the residual lactide from step h) with the crude lactide stream from step b) and / or the partially meso-lactide-enriched stream from step c);

[0027] Advantageously, when recycling steps f) and i) are carried out, this improves the yield of the process of the invention (for example, step i) recovers about 5% of residual lactide which would otherwise be lost).

[0028] Thanks to the process steps mentioned above, it is possible to generate by polymerization at varying ratios a whole range of PLA (low or high D content) with high stability, high yield (thanks to recycling) and a very low yellowing rate.

[0029] According to the present invention, step a) consists of the production of oligomers by polycondensation reaction of lactic acid according to the formula: n LH → Ln H + (n-1) H2O The objective is to have an n > 10, or even > 20, or even > 30 to obtain good functioning of the downstream cycling.

[0030] This endothermic reaction is in equilibrium and generates water as a byproduct. Given the extent of the reaction, the applied conditions, and the fact that the process is governed by equilibrium reactions, lactide is also produced. The applied conditions aim to promote the reaction while limiting thermal degradation and capacity loss through the elimination of acid or lactide with water.

[0031] The starting lactic acid must contain at least 90%, preferably at least 95%, more preferably at least 98%, and even more preferably at least 99.5% of the L(+) enantiomer, and no more than 10%, preferably no more than 5%, more preferably no more than 2%, and even more preferably no more than 0.5% of the D(-) enantiomer. It must also preferably have a concentration close to 100% and is continuously fed into one or more pre-polymerization reactors. The resulting pre-oligomer and catalyst are then continuously fed into one or more oligomerization reactors.

[0032] The oligomers obtained in step a) are then depolymerized in step b). The purpose of this step is the production of lactide by backbiting of oligomers according to the formula: LnH → LL + Ln-2H This balanced reaction is endothermic and generates, in addition to the targeted lactide, unreacted oligomers as well as degradation by-products.

[0033] The conditions applied are generally adjusted to minimize the residence time, in order to limit racemization, and to promote the reaction by directly extracting the generated lactide, which nevertheless remains crude (presence of water, lactic acid, counter-ion of the catalyst and oligomers).

[0034] This crude lactide stream contains more than 85% lactides, preferably more than 90% lactides, and more preferably more than 95% lactides. It comprises 70 to 95% by weight of L-lactide, 10 to 20% by weight of mesolactide, 1 to 8% by weight of lactic acid, and 1 to 10% by weight of heavy compounds, the percentages of the ingredients influencing each other, the whole being brought to 100%. The expression "crude lactide stream" should be interpreted as the lactide stream originating exclusively from the cyclization of lactic acid oligomers. This stream therefore does not include secondary fractions resulting from any recycling and reinjected into the crude lactide stream.

[0035] Obtaining a raw lactide stream with such a high lactide content reduces its content of unstable and reactive molecules (such as lactic acid or oligomers); therefore, according to the present invention, it becomes It is conceivable to avoid the usual pre-purification step by distillation, a step which proves to be more thermally aggressive and very energy-intensive.

[0036] In this way, advantageously, the crude lactide stream is sent to step c) of purification by crystallization, without prior distillation, allowing to obtain a "polymer grade" quality L-lactide stream (L-lactide content greater than 99%, or even greater than 99.5% by weight; acidity less than 20 mEq / kg, preferably less than 10 mEg / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm) and a partially enriched meso-lactide stream still containing residual L-lactide but also some impurities such as water, lactic acid, dimers and trimers, or even tetramers of lactic acid and other impurities.

[0037] Regarding the type of crystallization used, the present invention is not particularly limited. Among the preferred crystallization purification technologies, solvent crystallization, molten wall crystallization, and molten suspension crystallization are selected. In a preferred embodiment of the invention, molten suspension crystallization coupled with centrifugal separation and / or a TNO / GEA NIRO type wash column is chosen.

[0038] Indeed, in this type of technology the energies of the Solid / Liquid phase change are much lower than those of the Liquid / Vapor phase and the processing temperature is much lower, which is very important when working with thermosensitive compounds.

[0039] However, crystallization in suspension, more specifically, offers a larger surface area for crystal growth (5000 vs 50m²). 2 / m 3 ) but also greater heat transfer efficiency compared to molten wall crystallization, which allows not only the size of the equipment but also that of the exchangers, thus making the process more economical.

[0040] Furthermore, this is a continuous process, which eliminates the need for repeated heating and cooling of equipment with the product, unlike what is done in molten crystallization on walls. There is also no need for storage tanks for intermediate fractions, further reducing process costs and minimizing chemical degradation of the products (no need to maintain them in a molten state for extended periods). Furthermore, two crystallization stages may suffice to achieve the required purity, whereas three or even four are generally necessary for crystallization on walls in the state melted (without taking into account the efficiency stages), which greatly reduces energy consumption.

[0041] The purified L-lactide stream can be sent to step g) to a polymerization reactor to synthesize an L-PLA using a very small amount of catalyst, its content being a function of the amount of impurities present, which makes it possible to produce polylactic acids with high thermal stability and low colour.

[0042] The partially meso-lactide-enriched stream from step c) is sent to step d) to a first distillation column which includes three outlets, one of which is lateral, through which the lactides, composed of a condensed mixture of L-lactide and meso-lactide, are drawn off and then sent to a second distillation column, controlled to extract: In the upper lateral, a prepurified fraction enriched in meso-lactide and more specifically whose meso-lactide content is greater than that of the L-lactide / meso-lactide eutectic, i.e. greater than 60% by weight in meso-lactide of the prepurified fraction enriched in meso-lactide, preferably greater than 70% by weight in meso-lactide and more preferably greater than 80% by weight and whose residual acidity is less than 5%, preferably less than 1%, and even more preferably less than 0.5%. In the lower lateral, a pre-purified fraction enriched in L-lactide and more specifically whose content is greater than 80% by weight in L-lactide and preferably greater than 90% by weight and more preferably greater than 95% by weight and which can be recycled all or part upstream of the process such as for example at one of the stages of the crystallization process in order to maximize the yield. - The head and foot fractions include respectively the light fractions such as water, lactic acid or light degradation products which can be sent to a yield maximization step, and the heavy compounds such as lactic acid oligomers or heavy degradation products which can be sent all or part upstream of the process such as for example to the oligomerization step.

[0043] The initial separation is preferably performed using a single column, but for technical or operational reasons, two columns may be required in series. In this case, the extracted fractions will be adjusted while maintaining the same principle as described above.

[0044] The second separation is also preferably achieved with a single column but may require, for the same reasons, the use of two columns in series. The extracted fractions will in this case be adapted while keeping the same principle as described above.

[0045] In order to achieve the purity criteria required to obtain high D PLA grades with a very low yellowing rate, the pre-purified meso-lactide-enriched lactide stream obtained in step d) will be subjected to one (or more) additional step(s) e) of purification by crystallization. The purified meso-lactide-enriched stream obtained at the end of this step is of "polymer grade" quality (meso-lactide content greater than 70% by weight, or even greater than 80% by weight, or even greater than 90% by weight, or even greater than 95% by weight; L-lactide content less than 30% by weight, or even less than 20% by weight, or even less than 10% by weight, or even less than 5% by weight; acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm).The pre-purified stream enriched with L-lactide obtained in step d) is recycled in step c) or further upstream in the process.

[0046] Regarding the type of crystallization used in step e), the present invention is not particularly limited. Among the preferred crystallization purification technologies, solvent crystallization, molten wall crystallization, and molten suspension crystallization are considered. In a preferred embodiment of the invention, and as in step c), molten suspension crystallization coupled with centrifugal separation and / or a TNO / GEA NIRO type wash column is chosen for the same reasons as explained above.

[0047] The purified meso-lactide-enriched stream from step e) is sent alone or at least in part to step g) in combination with the purified L-lactide stream from step c) in order to obtain a grade of PLA containing various levels of D ranging from 0.5 to 45% in D, preferably from 0.5 to 20% in D. These two purified lactide streams having been treated at least by a crystallization step allowing the removal of impurities and coloring molecules, this makes it possible to generate by polymerization at varying ratios of these two streams a whole range of PLA (low or high content of D) with high stability and a very low rate of yellowing.

[0048] The degree of yellowing (or yellow index) is measured using a BYK 6836 spectroguide or equivalent, according to the L*a*b model (where the brightness L* is derived from the surface luminance; the two parameters a* and b* express the deviation of the color from that of a gray surface of the same brightness). The color measurement conditions are the illuminant D65 (normalized daylight) and an angle of 10. The PLA sample is placed under a light source; the light reflected back from the sample is captured and analyzed by the spectroguide. The yellow index is calculated from this light measurement according to ASTM E313-98.

[0049] The PLA grade from step g) is sent after the addition of a catalyst deactivation agent (cata killer) to a devolatilization step h) in order to separate the residual lactide from the PLA grade.

[0050] The residual lactide from step h) is recycled with the crude lactide stream from step b) and / or the partially meso-lactide-enriched stream from step c) in order to improve the process yield.

[0051] The PLA grade obtained by the process described in the invention contains from 0.5 to 45% D and has a yellow index of less than 20, preferably less than 15, more preferably less than 10, and ideally less than 5 (depending on the amount of D enantiomer present in the PLA grade). For comparison, commercially available grades containing 10 to 12% D enantiomer have a yellow index greater than 30, or even greater than 35.

[0052] The process of the invention as described above offers several advantages over state-of-the-art processes: • A technology requiring significantly less energy compared to those where distillation on the main stream is required. • Crude lactide, which contains more impurities, is processed under gentler conditions and with shorter residence times, which can only be beneficial in reducing degradation reactions and therefore yield loss. • The more aggressive distillation step now only involves a portion of the stream (stream partially enriched in meso-lactide), which reduces constraints on the product, as well as reducing investments and operating costs and generating fewer coloring molecules. • The residual lactide from devolatilization step h) can be advantageously recycled, to maximize yield, with the crude lactide stream from step b) and / or the partially meso-lactide-enriched stream from step c). Preferably, it will be recycled to the inlet of the first distillation column or the inlet of the second distillation column. This allows for double purification of the residual lactide stream and thus results in a higher quality purified, meso-lactide-enriched stream for use in the polymerization step.

[0053] Figure 1 is a schematic diagram of an embodiment of the process according to the invention, which does not, however, constitute a limitation of its scope.

[0054] The embodiment shown in Figure 1 illustrates various preferred or optional features. The figure is not intended to show specific features or technical details, including the design of the various components illustrated. Furthermore, auxiliary equipment such as various valves, pumps, heating and cooling equipment, analysis, control and similar devices are not shown, but can of course be used if necessary or desirable.

[0055] During the operation of the installation, lactic acid (10) is continuously introduced into the pre-oligomerization unit (20), in which the lactic acid is pre-oligomerized by releasing water (24). The pre-oligomer of lactic acid is brought into the oligomerization unit (30), in which it is oligomerized with the aid of a catalyst so as to produce a lactic acid oligomer, the oligomerization unit includes a water outlet (34) and an outlet comprising mainly lactic acid (32) which is recycled to the inlet of the pre-oligomerization unit.

[0056] The lactic acid oligomer is led into the cyclization reactor (40), in which the lactic acid oligomer is depolymerized in the presence of a catalyst into a crude lactide mixture, which generally includes mesolactide, L-lactide and to a much lesser extent D-lactide. While the raw lactide stream is brought from the cyclization reactor to the suspension crystallization unit (50), the purge stream (42) can be recycled all or part of it either to the inlet (42a) of the cyclization reactor, or to the inlet (42b) of the oligomerization unit or to a yield maximization step (not shown in Figure 1).

[0057] The crude lactide stream is purified in the suspension crystallization unit (50) into a polymer grade L-lactide stream (105) (L-lactide content greater than 90% by weight, preferably greater than 99.5% by weight; acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm) and into a partially enriched meso-lactide stream. While the partially enriched meso-lactide stream is fed into a first distillation column (52), the polymer-grade L-lactide stream is sent to the polymerization unit (60) or directly sold on the market (105).

[0058] In the first distillation column (52), the light products are separated from the lactides as the overhead stream and recycled to a yield-maximizing stage (not shown in Figure 1), while the residue is sent as a foot stream (53) to the oligomerization unit. The resulting meso-lactide enriched stream is drawn off laterally and sent to the second distillation column (54).

[0059] In the second distillation column (54), the light impurities that may still be generated by the degradation reactions are separated as the top stream and recycled to a yield maximization stage (not shown in Figure 1). At the bottom, the last heavy fractions (55) are recycled to the reboiler of the first column (55a) to allow the recycling of any remaining residual L-lactide and / or to the inlet of the crystallization unit (55b) and / or to the inlet of the cyclization reactor (55c) and / or even further upstream in the process.The pre-purified meso-lactide enriched stream, whose meso-lactide content is greater than that of the L-lactide / meso-lactide eutectic, i.e. greater than 60% by weight of meso-lactide in the pre-purified meso-lactide enriched fraction and preferably greater than 70% by weight and more preferably greater than 80% by weight and whose residual acidity is less than 5%, preferably less than 1% and even more preferably less than 0.5%, is sent to the crystallization unit (58) while the pre-purified L-lactide enriched stream (more specifically greater than 80% by weight of L-lactide and preferably greater than 90% by weight and more preferably greater than 95% by weight) is drawn off at the bottom lateral (56) and recycled to the inlet of the crystallization unit in suspension (50) and / or further upstream in the process.

[0060] The pre-purified meso-lactide-enriched stream is separated in the suspension crystallization unit (58) into a purified meso-lactide-enriched stream of "polymer grade" quality (59) (meso-lactide content greater than 70% by weight, or even greater than 80% by weight, or even greater than 90% by weight, or even greater than 95% by weight; L-lactide content less than 30% by weight, or even less than 20% by weight, or even less than 10% by weight, or even less than 5% by weight; residual acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg, residual water less than 100 ppm, or even less than 50 ppm) and into a secondary stream which may be wholly or partly recycled either to a yield maximization step (not shown in Figure 1). or be valued outside the process.

[0061] The purified stream enriched with "polymer grade" meso-lactide (59) is either fed at least in part with the "polymer grade" L-lactide stream in the polymerization unit (60) or is valued directly on the market (110). It will be fed into the polymerization unit in a proportion determined by the desired PLA grade, the PLA composition being able to vary from 0.5% to 45% in D-.

[0062] The raw PLA stream is introduced into the devolatilization unit (70), in which it is separated into a PLA stream and a vapor stream comprising unreacted lactide (72). While the purified PLA is sent to the granulation / crystallization / drying unit (75) to obtain a commercial grade of PLA (100) with a yellow index of less than 20, preferably less than 15, more preferably less than 10, even more preferably less than 5, more preferably less than 2 and ideally less than 1 (depending on the amount of D enantiomer present in the grade of PLA), the vapor fraction (72) is all or part recycled to the inlet (72a) of the suspension crystallization unit (50) and / or to the inlet (72b) of the first distillation column (52) and / or to the inlet (72c) of the second distillation column (54). The process of the present invention is also described and illustrated with the help of the examples below, but these do not constitute a limitation thereof. Example 1:

[0063] A crude lactide stream from the cyclization unit (40), the composition of which is shown in Table 1, was sent to a molten suspension crystallization unit (50). Table 1

[0064] The output yielded a purified L-lactide (105) comprising 99.5% L-lactide, 0.5% meso-lactide and a residual acidity of 1.6 mEq / kg. The partially meso-lactide-enriched stream (drain) recovered in this test is composed of 58% L-lactide and 33% meso-lactide, with the remainder consisting of lactic acid, oligomers and other impurities.

[0065] This drain was sent to two distillation columns (52 and 54). At the top of the columns, a distillate composed of light impurities is extracted, which will be sent back to the yield maximization section.

[0066] At the outlet of the first column (52), a stream composed of 68.08% L-lactide and 31.11% mesolactide is drawn off laterally, the remainder being composed of lactic acid, oligomers, and other coloring impurities. At the bottom of the column, the heavier particles (consisting mainly of lactic acid oligomers) are drawn off, and this stream (53) is returned to the oligomerization section (30).

[0067] At the outlet of the second column (54), the pre-purified stream enriched in gaseous meso-lactide, drawn from the upper lateral side and composed of 23.3% L-lactide, 75.2% meso-lactide, and 0.1% lactic acid, is collected as condensate and sent directly to a fractional crystallization unit on molten walls. The crystallizers operate in batch, sequential, and multi-stage operation, the typical sequence of which is: crystallization - residue removal - sweating - melting and recovery of purified crystals. (58).

[0068] After several stages of purification, the composition of the purified meso-lactide-enriched stream obtained is 91.7% meso-lactide and 6.7% L-lactide and a residual acidity of 4.7 mEq / kg.

[0069] This purified meso-lactide-enriched stream can be injected, at varying rates, into the purified L-lactide stream during the polymerization step (60). By proceeding in this way, the D content of the synthesized PLA can be modulated (even to D contents well above 4%) while maintaining high stability and a very low yellowing rate.

[0070] In this example, a PLA characterized by an MFI at 190°C (2.16 kg) of 4g / 10 min and a D content of 4% by weight was synthesized from a mixture of the purified L-lactide stream of "polymer grade" quality and the purified meso-lactide enriched stream generated above. The yellow index was measured using a BYK 6836 spectro guide and is equal to 0.2, whereas market references for an identical grade are usually between 7 and 12.

[0071] In this example, optional recycling was not carried out. This will not affect the quality of the PLA produced; however, when it is performed, the much smaller the lost fractions, the process yield is improved by approximately 5 to 10%. Example 2:

[0072] Example 1 was reproduced identically up to the polymerization step, where the purified meso-lactide-enriched stream was injected into the purified polymer-grade L-lactide stream at a rate sufficient to obtain PLA. characterized by a MFI at 190°C (2.6 kg) of 4.4 and a D content of 12% by weight. The yellow index was measured using a BYK 6836 spectroguide and is equal to 0.4. Example 3:

[0073] A crude lactide stream from the cyclization unit (40), the composition of which is shown in Table 2, was sent to a molten wall crystallization unit (50). Table 2

[0074] The output yielded a purified L-lactide (105) comprising 99.5% L-lactide, 0.5% meso-lactide and a residual acidity of 1.7 mEq / kg.

[0075] The partially meso-lactide enriched stream (drain) was sent to two distillation columns (52 and 54).

[0076] At the outlet of the second column (54), the pre-purified flow enriched in gaseous meso-lactide drawn off laterally at the top, the composition of which is 23.3% in L-lactide, 75.2% in meso-lactide and 0.1% in lactic acid, is recovered as condensate and is sent directly to a fractional crystallization unit on walls in a molten medium (58).

[0077] After several stages of purification, the composition of the purified meso-lactide-enriched stream obtained is 81% meso-lactide and 18.9% L-lactide and a residual acidity of 3.1 mEq / kg.

[0078] This purified meso-lactide-enriched stream is injected into the purified L-lactide stream, at the polymerization step (60) in order to obtain a PLA characterized by an MPI at 190°C (2.16 kg) of 4g / 10 min and a D content of 10% by weight. The yellow index was measured using a BYK 6836 spectroguide and is equal to 0.4. Comparative example

[0079] The crude lactide stream in Example 1 was purified by distillation, unlike in Example 2 where this stream was directly purified by crystallization. The crude lactide stream is therefore sent to a distillation column at vacuum-operated packing (<30mbara) comprising three outlets, one of which is lateral. Through the side outlet, a stream of pre-purified lactides (L-lactide / meso-lactide mixture) is drawn off in vapor form and then condensed at a temperature of approximately 90-100°C.

[0080] This stream of pre-purified lactides, composed of 89.1% L-lactide and 10.2% mesolactide, with the remainder consisting of lactic acid, oligomers, and other impurities, is then purified by fractional crystallization on walls in a molten medium. The crystallizers operate in batch, sequential, and multi-stage processes, with the typical sequence being: crystallization - residue removal - sweating - melting and recovery of the purified crystals.

[0081] At the output, after several crystallization stages, we obtained a purified L-lactide comprising 99.2% L-lactide, 0.7% mesolactide, and a residual acidity of 2.1 mEq / kg The meso-lactide enriched drain is composed of 60.3% L-lactide and 37% meso-lactide, the balance being composed of lactic acid, oligomers and other impurities.

[0082] This drain undergoes a distillation step to obtain a purified stream enriched in mesolactide, with a composition of 64.8% L-lactide, 35.2% mesolactide, and a residual acidity of 7.2 mEq / kg. Unlike Example 1, it does not undergo purification by crystallization.

[0083] As in Example 1, the purified L-lactide stream and the purified meso-lactide-enriched stream were injected into the polymerization step to obtain a PLA characterized by a MFI at 190°C (2.6 kg) of 4 g / 10 min and a D content of 4 wt%. The yellow index was measured using a BYK 6836 guide spectrometer and was equal to 10.1.

[0084] By proceeding in this way, in addition to lower quality, we observed in this example an energy consumption per kg of purified lactide, 75% higher compared to example 1.

[0085] Clause The present invention discloses a continuous process for manufacturing low-yellow polylactic acid comprising the following steps: a) oligomerization of a lactic acid stream to obtain lactic acid oligomers; b) cyclization of the lactic acid oligomers to form the crude lactide stream containing more than 85% lactides, preferably more than 90% lactides and more preferably more than 95% lactides, comprising 70% to 95% L-lactide, 10% to 20% meso-lactide, 1% to 8% lactic acid and 1% to 10% impurities; c) purification of the crude lactide stream to obtain an L-lactide stream with an L-lactide content greater than 99% by weight, preferably greater than 99.5% by weight, an acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, preferably less than 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content less than 100 ppm, preferably less than 50 ppm, and a partially enriched meso-lactide stream still containing residual L-lactide, but also certain impurities such as water, lactic acid, dimers and trimers, or even tetramers of lactic acid and other impurities; d) pre-purification of the partially enriched meso-lactide stream from step c);e) purification of the pre-purified meso-lactide-enriched stream from step d) in order to obtain a purified meso-lactide-enriched stream with a meso-lactide content greater than 70% by weight, preferably greater than 80% by weight, preferably greater than 90% by weight, an L-lactide content less than 30% by weight, preferably less than 20% by weight, preferably less than 10% by weight, an acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content less than 100 ppm, or even less than 50 ppm; f) polymerization of the L-lactide stream from step c) alone or in combination with at least part of the purified meso-lactide-enriched stream from step e) to obtain a grade of polylactic acid that may contain from 0.5 to 45% in D;(g) devolatilization of the polylactic acid from step (f) in order to separate the residual lactide; wherein step (c) of purification of the crude lactide stream includes a crystallization step without prior distillation; the partially meso-lactide-enriched stream from this step (c) is then pre-purified in step (d) by distillation in order to obtain a pre-purified meso-lactide-enriched stream and a pre-purified L-lactide-enriched stream; step (e) of purification of the pre-purified meso-lactide-enriched stream from step (d) includes a crystallization step.

Claims

DEMANDS 1. Continuous process for the production of low yellow index polylactic acid comprising a step of purification of a crude lactide stream, this crude lactide stream being previously formed by cyclization of lactic acid oligomers, said oligomers being obtained by oligomerization of a lactic acid stream characterized in that the purification of the crude lactide stream is carried out by crystallization without prior distillation.

2. A process according to claim 1 wherein the crude lactide stream contains more than 85% by weight of lactides, preferably more than 90% by weight of lactides and more preferably more than 95% by weight of lactides, comprising 70% to 95% by weight of L-lactide, 10% to 20% by weight of meso-lactide, 1% to 8% by weight of lactic acid and 1% to 10% by weight of impurities.

3. A process according to claim 1 or 2 wherein the crude lactide stream contains more than 95% by weight of lactides, said lactide stream comprising between 70 and 95% by weight of L-lactide and less than 10% by weight of meso-lactide, 1 to 8% by weight of lactic acid and 1 to 10% by weight of impurities.

4. A process according to any one of the preceding claims, characterized in that the purification of the crude lactide stream by crystallization comprises solvent crystallization, molten wall crystallization and molten suspension crystallization.

5. A process according to any one of the preceding claims characterized in that the purification by crystallization of the crude lactide stream is a molten suspension crystallization coupled with a centrifugal separation and / or a washing column.

6. A process according to any one of the preceding claims comprising a pre-purification step of the partially meso-lactide-enriched stream from the crude lactide stream purification step.

7. Process according to claim 6 wherein the partially meso-lactide-enriched stream from the crude lactide stream purification step is pre-purified by distillation to obtain a pre-purified meso-lactide-enriched stream and a pre-purified L-lactide-enriched stream.

8. A process according to any one of claims 6 or 7, characterized in that the pre-purified mesolactide-enriched stream has a mesolactide content greater than 60% by weight, preferably greater than 70% by weight and more preferably greater than 80% by weight, and whose residual acidity is less than 5%, preferably less than 1% and even more preferably less than 0.5%.

9. A process according to any one of claims 6 to 8, characterized in that the pre-purified stream enriched in L-lactide from the pre-purification step of the partially enriched meso-lactide stream has an L-lactide content greater than 80% by weight in L-lactide, preferably greater than 90% by weight and more preferably greater than 95% by weight.

10. A process according to any one of claims 6 to 9 comprising a step of purifying the pre-purified meso-lactide-enriched stream to obtain a purified meso-lactide-enriched stream with a meso-lactide content greater than 70% by weight, preferably greater than 80% by weight, preferably greater than 90% by weight, an L-lactide content less than 30% by weight, preferably less than 20% by weight, preferably less than 10% by weight, an acidity less than 20 mEq / kg, preferably less than 10 mEq / kg, or even less than 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content less than 100 ppm, or even less than 50 ppm.

11. A process according to any one of claims 6 to 10 wherein the purification of the prepurified meso-lactide-enriched stream comprises a crystallization step.

12. A process according to any one of claims 6 to 11 characterized in that the purification by crystallization of the purification step of the prepurified meso-lactide-enriched stream comprises solvent crystallization, molten wall crystallization and molten suspension crystallization.

13. Process according to claim 10, characterized in that the purification by crystallization step of the purification step of the prepurified meso-lactide enriched stream is a molten suspension crystallization coupled to a centrifugal separation and / or a washing column.

14. A process according to any one of the preceding claims comprising an additional step of recycling the pre-purified L-lactide-enriched stream from the pre-purification of the partially meso-lactide-enriched stream to the cyclization step of lactic acid oligomers to form a crude lactide stream or to the purification step of the crude lactide stream or upstream of the process.

15. A process according to any one of the preceding claims, further comprising a step of polymerizing the L-lactide stream purified alone or polymerization of the purified stream enriched in meso-lactide alone or polymerization of a composition of these two streams to obtain a grade of polylactic acid that may contain from 0.5 to 45% in D.

16. A process according to any one of the preceding claims further comprising a step of devolatilizing polylactic acid in order to separate the residual lactide.

17. A process according to any one of the preceding claims comprising an additional step of recycling the residual lactide from the devolatilization step with the crude lactide stream from the oligomer cyclization step and / or the partially meso-lactide-enriched stream from the crude lactide stream purification step.

18. Purified L-lactide comprising an L-lactide content greater than 99.5% by weight, an acidity of less than 5 mEq / kg, preferably less than 1 mEq / kg and a residual water content of less than 100 ppm, preferably less than 50 ppm.

19. Polylactic acid obtained by the process as claimed in any one of claims 1 to 17, characterized in that it contains at least 12 wt% of D isomer and having a yellow index of less than 20, preferably less than 15, preferably less than 10, preferably less than 5, more preferably less than 2 and ideally less than 1, the yellow index having been measured using a BYK 6836 Guide spectrograph.

20. Polylactic acid having a D isomer content between 0.5% and 45% by weight, preferably between 0.5% and 20% by weight and a yellow index less than 1, the yellow index having been measured using a BYK 6836 Guide spectrograph.

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