Method for the Manufacture of a Polyhydroxy-Carboxylic Acid

Abstract

Disclosed is a method to prepare a polylactic acid comprising the steps of: performing a ring opening polymerization using a catalyst and either a catalyst killer compound or an endcapping additive to obtain a raw polylactic acid of MW greater than 10,000 g/mol, purifying the raw polylactic acid by removing and separating low boiling compounds comprising lactide and impurities from the raw polylactic acid by devolatization of the low boiling compounds as a gas phase stream, purifying the lactide from the devolatization and removing the impurities from the gas phase stream of evaporated low boiling compounds by means of condensing the evaporated gas phase stream to give a condensed stream and a subsequent melt crystallization of the condensed stream, wherein the lactide is purified and the removed impurities include a catalyst residue and a compound containing at least one hydroxyl group such that the purified lactide is polymerized by feeding it back into the ring opening polymerization. The invention further relates to an apparatus for carrying out the method comprising: a polymerization reactor for performing a ring opening polymerization to obtain a raw polylactic acid, a devolatization apparatus for removing and separating low boiling compounds comprising lactide and impurities from a raw polylactic acid, and a crystallization apparatus for purifying a lactide and removing impurities from a condensed stream, wherein a condenser for condensing a gas phase stream to give a condensed stream is arranged between the devolatization apparatus and the crystallization apparatus.

1 . A method to prepare a polylactic acid comprising the steps of:
 (i) performing a ring opening polymerization using a catalyst and either a catalyst killer compound or an endcapping additive to obtain a raw polylactic acid of MW greater than 10,000 g/mol,   (ii) purifying the raw polylactic acid by removing and separating low boiling compounds comprising lactide and impurities from the raw polylactic acid by devolatization of the low boiling compounds as a gas phase stream,   (iii) purifying the lactide from the devolatization and removing the impurities from the gas phase stream of evaporated low boiling compounds by means of condensing the evaporated gas phase stream to give a condensed stream and a subsequent melt crystallization of the condensed stream, wherein the lactide is purified and the removed impurities include a catalyst residue and a compound containing at least one hydroxyl group such that the purified lactide is polymerized by feeding it back into the ring opening polymerization.   
     
     
         2 . The method of  claim 1 , wherein the melt crystallization is performed by means of a layer or a suspension crystallization. 
     
     
         3 . The method of  claim 1 , wherein the evaporated gas phase stream from the devolatization contains at least 30% of lactide, preferably at least 60%, most preferred at least 90%. 
     
     
         4 . The method of  claim 1 , wherein a crystal resulting from the melt crystallization of the condensed stream is crystallized in a further crystallization stage. 
     
     
         5 . The method of  claim 1 , wherein the layer crystallization comprises a sweating step followed by a melting step of a solidified fraction present in a crystalline form on a crystallization surface. 
     
     
         6 . The method of  claim 1 , wherein the removed impurities include either an organometallic compound or a carboxylic acid. 
     
     
         7 . The method of  claim 1 , wherein an apparatus is used for the melt crystallization which does not have an inert gas stream. 
     
     
         8 . The method of  claim 1 , wherein at least a portion of a purge stream from the crystallization is recycled to a raw lactide purification step, a pre-polymerization and dimerization step, or a solvent removal step in the production of a purified lactide. 
     
     
         9 . The method of  claim 1 , wherein a mother liquor from the crystallization and/or a liquid from the sweating step is collected and recrystallized in order to recover the lactide. 
     
     
         10 .- 13 . (canceled)