Degradability studies of poly(l-lactide) after multi-reprocessing experiments in extruder

Poly(l-lactide), PLLA, belongs to the most widely used biodegradable polyesters. These polymers have attracted great attention in recent years due to their excellent material properties, which give a rise to potential applications in various commercial areas. Previously, PLLA reprocessing in a co-rotating twin-screw extruder was used as a model test for material recycling. It may be assumed basing on this study, that PLLA technological wastes may be suitable for reuse, for example, as an additive for neat PLLA. In order to verify such suitability of reprocessed PLLA its degradability under industrial composting conditions was studied at the static composting open-air pile. Incubation of this material in water at 70 °C (abiotic conditions) was also conducted. The progress of the degradation process was monitored by surface changes and measurements of sample weight loss and changes in the PLLA molecular weight. The molecular level structure of the water soluble degradation products of PLLA samples was determined by electrospray ionization multistage mass spectrometry (ESI-MSn). Lactic acid and its oligomers terminated by hydroxyl and carboxyl end groups were identified as hydrolytic degradation products. The obtained results indicated that in selected environments (industrial composting and incubation under abiotic conditions) hydrolytic degradation via random ester bond scission occurs preferentially. It was demonstrated, that multi-reprocessing of PLLA did not significantly affect the rate of degradation and only slightly affected the disintegration progress of the PLLA samples.