Читать книгу Poly(lactic acid) - Группа авторов - Страница 77

Solvent‐assisted purification has been described in both the scientific and the patent literature. The most commonly used method for purifying lactide is by crystallization from ethyl acetate or from toluene and subsequent drying of the lactide under vacuum [95, 96]. A few industrial processes for manufacturing and purifying lactide from solvents have been described in the patent literature. In one reference, lactide is purified by washing with C_4–12 ethers [97]. In another approach, lactide is recrystallized in a suitable solvent followed by azeotropic distillation of the solvent [98]. In the third approach, lactide is dissolved in an organic solvent that is immiscible with water, the solution is extracted with water, and the lactide is isolated from the organic solvent [99]. One method utilizes the difference in the hydrolytic degradation rate of meso‐ and L‐lactide. The crude lactide is purified in water, whereby the meso‐lactide is hydrolyzed into lactoyllactic acid and solubilized, whereas the pure L‐lactide is crystallized and can be separated [100]. In a more recently described process, the crude lactide in vapor phase is cooled down, collected, and crystallized in a solvent (ethanol or propanol), and the crystals are filtrated. The crystallized lactide is dissolved and recrystallized in a subsequent process step [101].