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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.

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First Claim