Читать книгу Poly(lactic acid) - Группа авторов - Страница 47

In conclusion, the most important quality specifications for lactide monomers are those of free acidity, water, metal ion content, and stereochemical purity.

 Free acidity, for example, lactic acid or lactoyl lactic acid, slows down the rate of polymerization and limits the achievable degree of polymerization. According to the patent literature, free acidity of polymer grade lactide should be <10 meq/kg, and preferably no more than 5 meq/kg.

 Water causes hydrolysis of lactide and also limits the attainable degree of polymerization of PLA.

 Metal ions need to be specified in low quantities, because Sn, Zn, Fe, and Al cations accelerate polymerization, but may also affect hydrolysis, oxidation, or other degradation mechanisms. Sodium in particular causes racemization even in ppm amounts.

 Stereochemical purity expresses the sum of meso‐lactide and D‐lactide in L‐lactide and vice versa. The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity, and mechanical strength.