Heterogeneous Catalyst for Prepolycondensation of Lactic Acid

Polyactic Acid (PLA) is a biodegenerable, thermoplastic, aliphatic polyester that can be used in a variety of applications, e.g. as packaging goods or biomedical devices. In recent years the market for bioplastics was steadily increasing and it is expected to continue growth until 2020 with a rate of 20 25%. PLA is produced by direct condensation of lactic acid or ring-opening polymerization (ROP) of lactide. However, the reaction is very slowly even under high temperatures. Thereby, the most time consuming step is the precondensation of lactic acid into a prepolymer that can be processed further on. Furthermore leads the addition of organic or inorganic acids that shall fasten the reaction process to the formation of undesired side reactions, thus contaminating the product.

The development of catalysts that fasten the polymerization without any undesired side effects is of great interest. We are now able to present oxo-anion modified metal oxides as catalysts that fasten the precondensation of lactic acid, especially zirconium sulfate tetrahydrate (ZrS2O8) that was calcinated at 450 550°C. A second positive effect is the insolubility of the catalyst within the reaction mass making it easily separable. First experiments produced nearly 100mol% L-lactic acid units even at high conversion and catalyst concentration.