Enhancing the Hydrolytic Stability of Poly(lactic acid) Using Novel Stabilizer Combinations

Abstract

Commercially available poly(lactic acid) exhibits poor hydrolytic stability, which makes it impossible for use in durable applications. Therefore, a novel hydrolysis inhibitor based on an aziridine derivative as well as a novel stabilizer composition, containing an aziridine derivative and an acid scavenger, were investigated to improve the hydrolytic stability. To evaluate the stabilizing effect, the melt volume rate (MVR) and molecular weight were monitored during an accelerated hydrolytic aging in water at elevated temperatures. Temperatures were selected according to the glass transition temperature (~60 °C) of PLA. It was shown that the novel hydrolysis inhibitor as well as the novel stabilizer composition exhibited excellent performance during hydrolytic aging, exceeding commercially available alternatives, e.g., polymeric carbodiimides. A molecular weight analysis resulted in a molecular weight decrease of only 10% during approximately 850 h and up to 20% after 1200 h of hydrolytic aging, whereas poly(lactic acid) stabilized with a commercial polycarbodiimide revealed comparable molecular weight reductions after only 300 h. Furthermore, the stabilization mechanism of the aziridine derivative alone, as well as in the synergistic combination with the acid scavenger (calcium hydrotalcite), was investigated using nuclear magnetic resonance (NMR) spectroscopy. In addition to an improved hydrolytic stability, the thermal properties were also enhanced compared to polymeric carbodiimides.

Long-Term Thermal Stabilization of Poly(Lactic Acid)

“…The additives used were a synergistic hydrolysis stabilizer combination consisting of an aziridine-based inhibitor (PolyU) and an acid scavenger (calcium hydrotalcite). The high stabilization efficiency of this combination against hydrolysis was demonstrated in a previous study [ 40 ]. On the other hand, commercially available antioxidants were also added to stabilize against possible oxidative degradation.…”

“…By combining both stabilization mechanisms in a synergistic manner, particularly extended stabilization is achieved. The specific stabilization mechanisms have already been described in more detail in a previous publication [ 40 ].…”