Читать книгу Poly(lactic acid) - Группа авторов - Страница 45

Metal cations such as Sn, Zn, Fe, Al, and Ti not only accelerate polymerization, but can also affect hydrolysis, oxidation, racemization, or other degradation mechanisms of PLA and lactides [4, 6]. Consequently, the lactic acid used for lactide preparation should be very low (ppm) in metal cations in order to avoid considerable racemization during lactide synthesis.

O'Brien has shown that the formation of dark color of lactide was a direct function of the iron content of the material in which the lactide was in contact [86]. Other examples in the patent (Examples 7 and 8) demonstrate the desirability of having low alkali (e.g., sodium) content and minimizing the depolymerization temperature.

Cationic impurities such as sodium ions have no direct effect on lactide production rate, but the sodium content has a direct correlation with the meso‐lactide content in the crude lactide [67, 87].