Valorisation of d-lactic acid stream in the production process of l-polylactic acid

A method of valorizing a flux containing undesired D-lactic acid and/or one or more undesired D-lactic acid esters in the production process of L-polylactic acid, said process comprising the steps of:

• i. oligomerization of a substantially pure L-lactic acid feed;
• ii. cyclisation of the lactic acid oligomers to obtain lactides and a first residual stream, wherein the first residual stream comprises unreacted oligomers, impurities and catalytic residues transferred, at least partially, to a transesterification and a hydrolysis step;
• iii. purification of the lactides to obtain purified lactides, a second, and a third residual stream, wherein the purified lactides comprise L-lactide and meso-lactide and wherein the second residual stream comprises unreacted lactic acid and oligomers transferred, at least partially, to the oligomerization step, and wherein the third residual stream comprises unreacted lactic acid, oligomers, impurities and residual L-lactide, D-lactide and meso-lactide transferred, at least partially, to a transesterification and a hydrolysis step;
• iv. polymerization by ring opening of the purified lactides to obtain polylactic acid;
• v. purification of the polylactic acid to obtain purified polylactic acid comprising substantially L-polylactic acid and unreacted L-lactide, unreacted meso-lactide, and impurities transferred, at least partially, to the lactide purification step;
• vi. transesterification and hydrolysis of the at least partially transferred first and third residual streams to obtain a fourth and a fifth residual stream, wherein the fourth residual stream comprises undesired D-lactic acid and wherein the fifth residual stream comprises one or more undesired D-lactic acid esters;
• wherein the lactic acid of the fourth residual stream and the one or more lactic acid esters of the fifth residual stream are used, at least partially, as a base for the synthesis of molecules insensitive to the optical isometry D or L of lactic acid and/or lactic acid ester(s).