Production of meso-lactide, D-lactide and L-lactide by back biting of polylactide

Abstract

Process for increasingly producing D-Lactide and meso lactide by depolymerizing by back biting polylactide (PLA) said process which comprises: (i) Depolymerizing polylactide into its corresponding dimeric cyclic esters by heating the polylactide in the presence of a catalyst system comprising a catalyst and a co-catalyst in a reaction zone at temperature and pressure at which the polylactide is molten; (ii) Forming a vapor product stream from the reaction zone; (iii) Removing the vapor product stream and optionally condense it; (iv) Recovering, either together or separately meso-lactide, D-lactide and L-lactide.

Claims

1. Process for increasingly producing D-Lactide and meso lactide by depolymerizing by back biting polylactide (PLA) said process which comprises: (i) Depolymerizing polylactide into its corresponding dimeric cyclic esters by heating the polylactide in the presence of a catalyst system which comprises a catalyst and a co-catalyst, wherein the catalyst is Sn octanoate, MgO, ZnO or a mixture thereof; the co-catalyst is selected from the group comprising an organosilane aliphatic or cycloaliphatic selected from the group comprising alkylalkoxysilane or the cycloalkylalkoxysilane represented by the general formula QQSi(O-alkyl).sub.2, where the Q and Q are the same or different and are alkyl or cycloalkyl radical containing from 1 to 8 carbon atoms, in a reaction zone at temperature and pressure at which the polylactide is molten; (ii) Forming a vapor product stream from the reaction zone; (iii) Removing the vapor product stream and optionally condense it; (iv) Recovering, either together or separately meso-lactide, D-lactide and L-lactide.

2. Process according to claim 1 wherein the catalyst system is constituted of MgO as catalyst and a cycloalkylalkoxysilane represented by the general formula QQSi(O-alkyl).sub.2, where the Q and Q are different and are alkyl or cycloalkyl radical containing from 1 to 8 carbon atoms as cocatalyst.

3. Process according to claim 2 wherein the catalyst system is constituted of MgO as catalyst and cyclohexylmethyldimethoxysilane as cocatalyst.

4. Process according to anyone of claim 1 wherein the catalyst of the catalyst system is used in an amount comprised between 0.05 and 3% by weight of PLA and the co-catalyst is used in an amount comprised between 0.1 and 10.0% by weight of PLA.

5. Process according to claim 1 wherein the depolymerization is carried out at a temperature between 200 and 290 C. and at a pressure under lactide vapour pressure.

6. Process of mixing D-lactide and meso lactide obtained according to claim 1 with L-lactide to prepare a copolymer of P(L-D)LA by ring opening polymerization, the content of enantiomer D- of said copolymer not exceeding 35% by weight.

7. Process according to claim 5 wherein the depolymerization is carried out at a temperature between 210 and 260 C. and at a pressure under lactide vapour pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is representing a process to prepare PLA starting with lactic acid containing more than 98.5% by weight enantiomer L, said process starting with an aqueous solution of lactic acid from which water is evaporated to obtain lactic acid which is then prepolymerized and subject to cyclization to form crude L-lactide which needs to be purified before being polymerized by ring opening of lactide, and finally the resulting PLA is purified by devolatilization.

DETAILED DESCRIPTION OF THE INVENTION

(2) According to the process of the present invention, it is now possible to introduce, at the level of the purification of the crude lactide, a stream either liquid or gaseous, of a mixture rich in meso-lactide and D-lactide resulting from the depolymerization by back biting; said stream rich in meso-lactide and D-lactide is mixed with the stream of L-lactide and both streams are subject together to the ring opening polymerization, in accordance with usual conditions generally applied for such reaction.

(3) So, a copolymer is now formed containing both enantiomers L and D-. These latter being in a proportion which may reach 30 to 35% by weight.

(4) It is understood that lower contents may also be reached, simply by controlling the feeding of the back biting stream in the main stream of crude L-lactide to be purified.

(5) Another advantage of the proces of the invention, based on the important proportion of D-lactide and meso-lactide produced during the depolymerization, even if the starting material is constituted substantially completely of enantiomer-L, is to envision an integrated process for the production of homopolymers or copolymers of PLA, i.e., PLLA or PLLA-PDLA, starting with an aqueous solution of lactic acid not containing more than 10% of D-lactic acid.

(6) Such an integrated process is generally consisting of the removal of water from the starting lactic acid aqueous solution, the oligomerization of lactic acid into oligomers of 400 to 5,000 Dalton, the cyclisation of said oligomers to produce crude lactide then subject the obtained crude lactide to purification, and finally the polymerization by ROP of the lactide into PLA.

(7) Owing to the process of the invention which can produce more D-lactide and meso-lactide by depolymerization of PLA waste and off spec of the integrated process, this stream may be introduced directly in the purification step of the lactide of the integrated process, according to which the amount of D-lactide and/or meso lactide introduced is controlled in order to achieve the desired composition of copolymers.

(8) Generally, the composition of the copolymer may contain up to 25 to 35% by weight of D-lactide and/or meso-lactide.

(9) It is agreed that the process of the invention is not limited to such an integrated process, particularly when the feed of the back biting step is comprising waste of PLA coming from other sources than the present process; PLA waste from such sources may of course contain different amount of D-enantiomer which may reach 20% by weight;

EXAMPLES

(10) The back biting of various PLA samples, all having an enantiomer L content of 99.4% by weight has been carried out.

(11) First the samples were ground and then deposited into a recipient (the reaction zone) which was introduced into the reactor. Then the catalyst and co-catalysts were added in the reaction zone. The Sn octanoate (Sn (oct).sub.2) used as catalyst was added in an amount of 1% by weight of the PLA. Regarding the co-catalyst, when fumaric acid was used (pKa.sub.1=3.03; pKa.sub.2=4.44), it was added in an amount of 5% by weight of the PLA. When triphenylphosphine was used as co-catalyst, it was added in an amount of 0.7% by weight of the PLA.

(12) The temperature was then raised up to 250 C. and maintained during 60 minutes.

(13) The pressure was adjusted to 10 millibar.

(14) During the period of time the reaction mixture was maintained at said temperature, a vapor product was formed and further extracted from the reaction zone while the vapor product was then subject to a condensation step.

(15) The condensed product was recovered and analysed by gas chromatography (GC) to determine its constituants and their respective contents in % by weight in L-lactide, D-lactide and meso-lactide. The lactide yield (%) represents the quantity of lactide recovered and condensed.

(16) The results are presented here below.

(17) TABLE-US-00001 lactide L- D- Meso- Impur- yield lactide lactide Lactide ities Ex. Catalyst Co-Cata (%) (%) (%) (%) (%) 1 Sn (oct).sub.2 None 90.7 83.3 1.7 14.0 1.0 2 Sn (oct).sub.2 Fumaric 65.0 68.0 6.0 25.0 1.0 Acid 3 Sn (oct).sub.2 TPP 83.8 68 8.5 22.3 0.2

(18) A comparative example was conducted under the same conditions as those described for the examples according to the invention excepted that the sulfamic acid having a pKa of 0.99 was used as co-catalyst at 5.3% by weight of the PLA.

(19) TABLE-US-00002 lactide L- D- Meso- Impur- Yield lactide lactide lactide ities Ex. Catalyst Co-Cata (%) (%) (%) (%) (%) 4 Sn (oct).sub.2 Sulfamic 1.1 95 None 0.9 4.1 Acid

(20) Other examples within the process of the invention were conducted under the same operating conditions as those described above excepted that another catalyst than Sn octanoate and cocatalyst were used. MgO was used as catalyst in an amount of 1% by weight of the PLA and lactic acid (pKa: 3.86) was used as cocatalyst, it was added in an amount of 5% by weight of the PLA (see example 5). In example 6, MgO was used as catalyst in an amount of 1% by weight of the PLA and cyclohexylmethyldimethoxysilane was used as cocatalyst in an amount of 5% by weight of the PLA. In example 7, the same example was conducted without any cocatalyst (comparative example).

(21) TABLE-US-00003 lactide L- D- Meso- Impur- Yield lactide lactide lactide ities Ex. Catalyst Co-Cata (%) (%) (%) (%) (%) 5 MgO Lactic Acid 81.0 70.3 11.0 18.2 0.5 6 MgO cyclohexyl- 90.0 37.4 30.7 28.5 4.4 methyl dimethoxy- silane 7 MgO None 60.9 92.3 1.4 6.1 0.2