Method for preparing refined lactide from recovered polylactic acid

摘要

A method for preparing lactide from recovered polylactic acid, the method including the following steps: A. pretreating the recovered polylactic acid; B. extruding the treated polylactic acid from a twin screw extruder to yield a polylactic acid melt, and introducing the polylactic acid melt to a pre-depolymerization kettle; C. carrying out a chain scission reaction to break molecular chains in the polylactic acid melt and to decrease the number-average molecular weight of the polylactic acid melt to below 5000; D. conducting depolymerization reaction to yield a crude lactide; and E. purifying the crude lactide and crystallizing the purified lactide.

主权项

1. A method for preparing a refined lactide from recovered polylactic acid, the method comprising:
a) crushing recovered polylactic acid by a crusher, washing and filtrating the polylactic acid to remove impurities, and desiccating the polylactic acid to remove water; b) transporting the polylactic acid obtained in a) to a hopper of a twin screw extruder, extruding the polylactic acid from the twin screw extruder to yield a polylactic acid melt, and introducing the polylactic acid melt to a pre-depolymerization kettle; c) stirring the polylactic acid melt in the presence of a catalyst at a temperature of between 180 and 250° C. for carrying out a chain scission reaction to break molecular chains of the polylactic acid melt and to decrease a number-average molecular weight of the polylactic acid melt to below 5000; d) transporting the polylactic acid melt after the chain scission reaction to a depolymerization system via a first delivery pump, and conducting depolymerization reaction at a temperature of between 150 and 250° C. and at a vacuum between 0.1 and 0.09 MPa to yield a crude lactide; and e) transporting the crude lactide to a purification system via a second delivery pump, and crystallizing the purified lactide, whereby yielding a refined lactide wherein the catalyst in c) is a mixture of zinc lactate, zinc dust, and propionic acid; an amount of the catalyst is between 1/10000 and 100/10000 of the polylactic acid melt by weight; and the crude lactide in d) comprises L-lactide and D-lactide, a concentration of the L-lactide exceeds 85 wt. % of the crude lactide; and a concentration of the D-lactide is less than 2 wt. % of the crude lactide.