EP4127037A1 - Methods for depolymerisation of polylactic acid - Google Patents

Abstract

The present invention relates to a method of polylactic acid (PLA) depolymerisation. PLA is partially depolymerised with volatile solvent and acid. This step may be followed by enzymatic hydrolysis.

Description

METHODS FOR DEPOLYMERISATION OF POLYLACTIC ACID

The present application relates to methods of recycling plastic, especially to a method of polylactic acid (PLA) depolymerisation.

Polylactic acid (PLA) has found use in various applications such as in cups, food packaging, fibers, 3-D printing etc., and its production volume is increasing. Polylactic acid (PLA) is a polymer derived from lactic acid. Since lactic acid is produced biotechnologically, PLA is classified as a biopolymer. However, PLA degrades very slowly in the nature and cannot be regarded as biodegradable. Hence, methods for recycling PLA are needed. L-lactic acid is more reactive than a racemate and thus optical purity is an advantage in many applications. Thus, it is desired to avoid racemisation while hydrolysing PLA to lower oligomers and to monomers to be used as a raw material for production new PLA based products.

There is a need for energy efficient methods for depolymerisation of polylactic acid, preferably without racemisation.

An aspect of the invention is a method of depolymerisation of polylactic acid (PLA). Characteristic steps of said method are depicted in claim 1. Further embodiments are disclosed in the dependent claims and the description. The features recited in dependent claims and in the embodiments are mutually freely combinable unless otherwise explicitly stated.

Brief description of the Figures

Figure 1a. shows results of SEC analysis in chloroform of hydrolysed PLA by acid treatment at room temperature.

Figure 1b. shows results of SEC analysis in chloroform of hydrolysed PLA by acid treatment at 50 °C.

Figure 1c. shows results of SEC analysis in chloroform of hydrolysed recycled PLA pellets by acid treatment at room temperature.

Figure 1d. shows results of SEC analysis in chloroform of hydrolysed recycled PLA pellets by acid treatment at 50 °C.

Figure 1e. shows results of SEC analysis in chloroform of hydrolysed recycled PLA films by acid treatment at 50 °C.

Figure 2a. shows molar mass distributions of PLA (Ingeo 3251 D, NatureWorks) sample before and after its acid hydrolysis at room temperature. Figure 2b. shows molar mass distributions of recycled PLA pellets (IngeoTM 8052D, NatureWorks) sample before and after its acid hydrolysis at 50 °C.

Figure 3 shows a chiral GC analysis of hydrolysed PLA samples

Figure 4 shows a chromatogram of PLA hydrolysates produced by 67.5 hours. treatment.

Detailed description

The invention relates to chemoenzymatic depolymerisation and recycling of polylactic acid. Polylactic acid is mixed in a volatile solvent and depolymerised with an acid such as trifluoroacetic acid, formic acid or acetic acid or any combination of those, to oligomers. The inventors have surprisingly found that it is possible to perform a chemical depolymerisation at moderate temperatures (55 °C or lower). No energy-consuming grinding is needed. Also racemisation can be avoided. Racemisation generates a mixture of lactic acid enantiomers which are less reactive. L-lactic acid is more reactive than a racemate and thus optical purity is an advantage in many applications. The solvent and acid are evaporated. Optionally, the oligomers are hydrolysed further in acid conditions with an enzymatic hydrolysis into shorter oligomers of lactic acids, especially into lactic acid monomers. Polylactic acid depolymerisation has previously been carried out using neutral or alkaline esterases wherein a lot of alkali needed for neutralising the lactic acid released and the benefit in comparison to chemical alkaline hydrolysis is negligible. In the current invention only low amounts of alkali are used since an enzyme that is functional at low pH is utilized for hydrolysis to monomers.

The present invention relates to a method of depolymerisation of polylactic acid (PLA) comprising the steps of (a) mixing PLA pellets in a volatile solvent to form a homogeneous or clear solution; and

(b) adding trifluoroacetic acid (TFA), formic acid or acetic acid or any combination of those to the solution of item (a); and

(c) incubating at temperature between 20 to 60 °C; and (d) evaporating the solvent and the acid; and

(e) recovering the partially depolymerised PLA.

Solid polylactic acid (PLA) to be depolymerised may have different forms; it may be for example foil, granulated or any solid material. There is no need to crush the material which reduces the cost. Mixing a solid sample into a solvent to form a homogeneous (i.e. transparent or clear solution) results a full dissolution of the sample in the solvent with formation of a homogeneous system. Any means of mixing can be used. Partially depolymerised PLA may be collected as a solid residue. Volatile solvent may be chloroform or e.g. hexafluoroisopropanol. Hexafluoro- isopropanol is expensive and thus not commercially attractive. In one embodiment the volatile solvent is chloroform.

An acid hydrolysis decreases the molecular weight without altering the molecular weight distribution of PLA hydrolysates. Molecular weight distribution/polydispersity (PDI) reflects the differences between the length of generated oligomers. For further repolymerisation oligomers with narrow PDI or molecular weight in a defined range are desired. This is due to the difference in the reactivity and polymerisation process.

Trifluoroacetic acid (acidity characterized by pKa of 0.23) is the preferred acid and highly reactive towards PLA and volatile thereby allowing an easy separation from the partially depolymerised PLA. It is toxic to aquatic life, thus recycling it in the process is an advantage over non-volatile inorganic acids. Also possibility to reuse the acid is an advantage. TFA may be added to lactic acid into molar ratio (lactic acid : TFA) of 1 :1 to 1 : 2 or even to 1 : 8, such as 1 : 5 lactic acid : TFA. In performed experiments the TFA : chloroform ratio was varied between 1.38 and 3.46 mol/l. The ratio can be optimized based on desired oligomers to be produced.

Incubation time is dependent on the reaction temperature due to the differences in the reaction rates at selected temperatures. Also desired depolymerisation level defines the incubation time. The current examples describe acid hydrolyses of PLA at mild temperature conditions (meaning less than 55 °C) when reaction times were selected based on desired molecular weight of oligomers for further enzymatic depolymerisation. The incubation time of item (c) may be for example from 1 hour to 2 weeks at 25 °C or 1 hour to 5 days at 50 °C. The incubation time may be from 30 min to two weeks or 1 hour to one week, for example 4 hours to 3 days, or 6 hours to 2 days.

The incubation temperature of item (c) may vary between room temperature or 25 °C to 55 °C, for example 45 to 55 °C, such as about 50 °C or 48 to 52 °C. Higher temperatures, such as above 55 °C expedite the depolymerisation rate but also have a tendency to change the properties of monomers. When PLA hydrolysis is performed at higher temperatures, racemisation of D- and L- lactic acid enantiomers takes place. It is desired to avoid racemisation due to the difference in the reactivity of lactic acid enantiomers. Mild temperature conditions do not change the D/L ratio of the lactic acid isomers and thereby allows maintaining optical purity of non- amorphous starting material. In addition to the temperature, time and desired depolymerisation level (target molecular weight) the quality of PLA may have an impact to suitable conditions. Depolymerisation of a recycled PLA may be faster when compared to industrial PLA.

In this connection term industrial PLA means a commercial PLA which has not been thermally processed before the depolymerisation experiments described here. In one embodiment PLA is incubated at 48 to 52 °C for one to four days, such as 40 to 60 or 45 to 55 hours or 60 to 70 hours.

In one embodiment PLA is incubated at 48 to 52 °C for one to five days, such as 45 to 125 hours, or 45 to 55 hours.

In one embodiment PLA is incubated at 48 to 52 °C for one hour to 5.5 days; such as 70 to 125 hours. Without binding to the theory, it seems that recycled PLA may be easier to depolymerise.

Term “recycled PLA” as used here means PLA which has been thermally processed prior recycling. The number of cycles for chemical recycling is limited due to the partial degradation during every thermal processing and losing the strength of the polymer after every cycle.

In one embodiment PLA is incubated at 23 to 28 °C for one to five days, such as 45 to 110 hours, or 40 to 60 hours.

In one embodiment PLA is recycled PLA. In one embodiment PLA industrial PLA.

The volatile solvent and TFA may be evaporated, preferably using vacuo, after which the partially depolymerised PLA may be recovered as a solid fraction. The volatile solvent and TFA may be recovered by evaporation using a rotavapor.

Evaporation to dryness for the proper characterisation of PLA hydrolysates as well as for their further applications such as repolymerisation may be done in vacuo. By evaporation using a rotavapor the solvent and TFA are separated from the reaction mixture as a liquid phase. Their further separation and purification may be done by distillation. At this stage PLA hydrolysation is not complete but so called “partially depolymerised PLA” is obtained. This means that the average molecular weight of the PLA has been reduced due to depolymerisation of PLA comprising a mixture of lactic acid oligomers and lower molecular weight compounds. In this connection partially depolymerised PLA compounds may have molecular weight of varying e.g. from 500 or 1 000 to 100000 g/mol such as less than 55000 g/mol or less than 10000 g/mol depending on the conditions used. In one embodiment the molecular weight after acid hydrolysis is less than 10000 g/mol, less than 8000 g/mol, less than 6000 g/mol, less than 4000 g/mol or even less than 2000 g/mol. After recovery e.g. by evaporation the solvent and/or TFA may be reused.

In one embodiment the method comprises a further step (f), wherein water and an enzyme having hydrolytic activity towards lactic acid oligomers are added to the partially depolymerised PLA recovered in item (e) and the mixture formed is incubated. In this connection expression “an enzyme having hydrolytic activity” means any enzyme having activity towards ester bonds between lactic acid molecules. Examples on such enzymes are esterases belonging to EC-class 3.1., such as and lipases and cutinases.

The enzyme should be active at low pH in order to gain benefit in comparison to hydrolysis with alkali. Use of enzymes being active in low pH is preferred as it reduced the need of adjusting pH by an alkali after evaporation of the acid. In one embodiment the pH in step (f) is adjusted to below 5, preferably below 3.86. The enzyme having hydrolytic activity is active below the pKa of lactic acid, i.e. pH 3.86.

The enzyme may be active on temperatures ranging between 10 °C to 60 °C. The higher the temperature, the faster the reaction. The temperature of the enzymatic treatment is dependent on the properties of the enzyme used.

Enzymatic depolymerisation does not change the properties of lactic acid units. Depending on the properties of the enzyme and the incubation conditions and time the hydrolysis may be continued to complete monomerisation of PLA. Monomers are preferred as they are easier to use in a synthesis. In addition, desired isomers may be selectively produced by enzymes. A person skilled in the art is able to select the enzyme and optimize the conditions.

It is to be understood that the terminology employed herein is for description and should not be regarded as limiting. It must be understood that the embodiments given in the description above are for illustrative purposes only, and that various changes and modifications are possible within the scope of the disclosure.

The features of the invention described here as separate embodiments may also be provided in combination in a single embodiment. Also various features of the described here in the context of the method are usable in connection with the compositions and uses, and vice versa.

In the following the present invention will be illustrated in more detail by means of examples. The purpose of the examples is not to restrict the scope of the claims but illustrate some embodiments.

Examples

Example 1. Chemical treatment of PLA by acid hydrolysis

Commercial PLA (Ingeo 3251 D, NatureWorks) pellets were mixed in chloroform at 5 wt% concentration to form a homogeneous solution due to the full dissolution of polymer in the solvent. Trifluoroacetic acid (TFA) was added to the PLA solution in a ratio LA:TFA = 1 :5 (moles). The reaction mixture was stirred either at room temperature or at 50°C for a defined reaction time. Samples were taken at 1 , 2, 3, 4, 6, 20, 22, 24, 48.5, 67.5 hours, 5 days, 6 days, 8 days and 11 days. Isolated samples were evaporated to dryness and drying in vacuo and divided for characterizations as explained under Examples 4 to 7.

Example 2. Chemical treatment of recycled PLA pellets by acid hydrolysis

Acid hydrolysis of processed PLA (IngeoTM 8052D, NatureWorks) pellets was performed as described in Example 1. The recycled PLA sample was processed once through twin-screw extruder and pelletized. Samples for analysis were isolated from the reaction mixture at 1 , 2, 3, 4, 6, 24, 48.5, 72.5, 96.5 and 120.5 hours. Dried samples were divided for characterization as explained under Examples 4 to 7.

Example 3. Chemical treatment of recycled PLA films by acid hydrolysis

Acid hydrolysis of recycled PLA (LX175, Corbion) films (2x2 cm) was performed as described in Example 1. Samples for analysis were isolated from the reaction mixture at 1, 2, 3, 4, 6, 24, 72.5, 97 and 121 h. Dried samples were divided for characterization as explained under Examples 4 to 7. Example 4. Evaluation of molar mass and molar mass distributions by SEC

Molar masses and molar mass distributions were determined by size exclusion chromatography (SEC) at 40 °C. The system was equipped with Waters Styragel columns and Waters 2410 refractive index detector. The eluent used was chloroform and was delivered at a rate of 0.5 ml/min. The results were calibrated against the poly(methyl methacrylate) standards.

Dried samples for SEC analysis obtained as explained in Examples 1 to 3 were dissolved in chloroform at concentration of 3 mg/ml.

Results are shown is Tables 1a to 1e, Figures 1a to 1e, and Figures 2a and 2b. Table 1a. SEC analysis in chloroform of hydrolysed PLA by acid treatment at room temperature, see also Fig. 1a. (FIA - acid hydrolysis) Table 1b. SEC analysis in chloroform of hydrolysed PLA by acid treatment at 50 °C, see also Fig. 1b.

Temperature accelerates the acid hydrolysis of PLA. Table 1c. SEC analysis in chloroform of hydrolysed recycled PLA pellets by acid treatment at room temperature, see also Fig. 1c.

Table 1d. SEC analysis in chloroform of hydrolysed recycled PLA pellets by acid treatment at 50 °C, see also Fig. 1d. Table 1e. SEC analysis in chloroform of hydrolysed recycled PLA films by acid treatment at 50 °C, see also Fig. 1e.

Example 5. Evaluation of molar mass distributions of PLA samples after acid hydrolysis at room temperature or 50 °C by SEC

Acidic hydrolysis decreases the molecular weight of PLA. When the starting PLA polymers have molar mass, Mw, below 200000 g/ mol, the molar mass distribution remains unimodal. In case of starting PLA polymers with molar mass, Mw, below 300 000 g/mol, the molar mass distributions are slightly altered for reaction times longer than 24h.

The results of SEC analysis are shown as Figure 2a. Molar mass distributions of PLA (Ingeo 3251 D, NatureWorks) sample before and after its acid hydrolysis at room temperature and Figure 2b. Molar mass distributions of recycled PLA pellets (IngeoTM 8052D, NatureWorks) sample before and after its acid hydrolysis at 50 °C.

Example 6. Evaluation of D and L-lactic acid enantiomers by chiral gas chromatography

The evaluation of the relative amounts of D- and L-lactic acid enantiomers in PLA samples was performed by using a chiral gas chromatography method.

The established method (Nature Works LLC: analytical method) involves hydrolysis of samples (0.2 g) in 1N methanolic potassium hydroxide solution (4 ml), followed by acidification with concentrated sulfuric acid (400 pi) or 3N methanolic hydrochloric acid (4 ml) to catalyze esterification. To the acidified solution is then added methylene chloride (10 ml) and deionized water (5 ml) to bring about the partitioning of methyl lactate enantiomers into the organic layer. The bottom organic layer is collected and analysed by gas chromatography (GC) using a flame ionization detector (FID). Separation of methyl lactate enantiomers is achieved using Agilent CycloSil-B chiral capillary column. Table 2. Chiral GC analysis of PLA samples and the corresponding hydrolysates produced by acid hydrolysis at 50 °C.

Figure 3 shows a chiral GC analysis of hydrolysed PLA samples. The results from Table 2 indicates the same D/L ratio of lactic acid enantiomers as in PLA starting polymer. It is desired to avoid racemisation due to the different reactivity of D- and L-lactic acid enantiomers. Racemisation did not occur due to the mild conditions of acid treatment. Example 7. Evaluation of monomeric and oligomeric PLA hydrolysates by GC/MS analysis

The hydrolysates were analysed by gas chromatography mass spectrometry, after derivatisation by trimethylsilylation. The dry hydrolysates (c. 4 mg samples) were treated in pyridine (0.2 ml_) with a 0.2 ml_ mixture (3:1) of N,0- bis(trimethylsilyl)trifluoroacetamide and trimethylchlorosilane. The GC/MS runs were performed with an Agilent 6890 series GC system, equipped with an Agilent 5973 mass selective detector and a DB-5 MS capillary column (30 m ^c 0.25 mm, film thickness 0.25 pm). The temperature program applied was 1 min at 70 °C, 10 °C min^-1 to 300 °C, and 11 min at 300 °C. The injection split ratio was 50:1. The relative shares of different lactic acid isomers were calculated directly from the peak areas of their trimethylsilyl derivatives.

Table 3. Composition of PLA hydrolysates obtained by acid hydrolysis at room temperature or 50 °C.

A chromatogram of PLA hydrolysates produced by 67.5 h. treatment is shown as Figure 4. Higher oligomers may be present but cannot be detected by GC/MS.

Example 8. Enzymatic hydrolysis of PLA oligomers The oligomers were suspended at 10 g/L in 15 mL of distilled water and the suspension was placed in a glass vessel under the control of a Metier Toledo DL53 titrator at 40 °C. The pH of the enzymatic reaction was maintained at pH 3.5 by the addition of 5 M NaOH. Novozymes 51032 lipase was added at a dose of 2% (protein mass/mass of oligos) at the onset of the reaction. Same dose of enzyme was added after 9 d. The amount of lactic acid released was determined using the Megazymes D-/L-Lactic Acid (D-/L-Lactate) (Rapid) Assay Kit. The reaction was continued for 14 d. The initial concentration of monomeric lactic acid was determined as 11%. Of the oligomers, 72% were hydrolysed to lactic acid in 14 d as determined using the lactic acid assay kit. As a control the commercial PLA (Ingeo 3251 D, NatureWorks) pellets at 10 g/L were placed in Mclllvaine buffer at pH 3.5 and incubated in the presence of the Novozymes 51032 lipase as described above. Hydrolysis, determined as loss of PLA mass, was below 10%. Example 9.

Test procedure explained in Example 1 was performed using varying polymer and/or TFA concentrations in chloroform and varying reaction times shown in table 4 below.

Table 4. Reaction conditions of acid treatment at 50 °C for hydrolysis of PLA (Ingeo 3251 D, NatureWorks).

Molar masses and molar mass distributions were determined by size exclusion chromatography (SEC) as described in Example 4 above. Results are shown in table 5 below. Table 5. SEC analysis in chloroform of hydrolysed PLA by acid treatment at 50 °C.

Claims

Claims

1. Method of depolymerisation of polylactic acid (PLA) comprising the steps of

(a) mixing PLA pellets in a volatile solvent to form a homogeneous or clear solution; and

(b) adding trifluoroacetic acid (TFA), formic acid or acetic acid or any combination of those to the solution of item (a); and

(c) incubating at temperature between 20 to 60 °C; ad

(d) evaporating the solvent and the acid; and (e) recovering the partially depolymerised PLA.

2. Method of claim 1 , wherein trifluoroacetic acid (TFA) is added to the solution of item (a).

3. The method of claim 1 or 2, wherein the volatile solvent is chloroform.

4. The method of claim 2 or 3, wherein TFA is added into a molar ratio of 1 : 1 to 1 : 2 or to 1 : 8 lactic acid : TFA, such as 1 : 5 (lactic acid : TFA).

5. The method of any of the preceding claims, wherein the incubation time of item

(c) is from 1 hour to 2 weeks.

6. The method of any of the preceding claims, wherein the incubation temperature of item (c) is 25 to 55 °C, preferably 45 to 55 °C, such as about 50 °C.

7. The method of any of the preceding claims, wherein the incubation time of item

(c) is 40 to 60 hours.

8. The method of any of the preceding claims, wherein the volatile solvent and TFA, formic acid or acetic acid or any combination of those, are evaporated, preferably using vacuo, and partially depolymerised PLA is recovered as a solid fraction.

9. The method of any of the preceding claims, wherein the volatile solvent and TFA, formic acid or acetic acid or any combination of those, preferably the volatile solvent and TFA, are recovered by evaporation using a rotavapor.

10. The method of claim9, wherein said solvent and/or TFA, formic acid or acetic acid or any combination of those, preferably the volatile solvent and TFA, are reused after a purification and a separation from each other.

11. The method of any of the preceding claims, comprising a further step (f), wherein water and an enzyme having hydrolytic activity towards lactic acid oligomers are added to the partially depolymerised PLA recovered in item (e) and the mixture formed is incubated.

12. The method of claim 11, wherein the pH in step (f) is adjusted to below 5, preferably below 3.86.

13. The method of claim 11 or 12, wherein the enzyme has activity towards ester bonds between lactic acid molecules.