Chemical degradation of end-of-life poly(lactic acid) into methyl lactate by a Zn(II) complex

The catalyzed methanolysis of end-of-life poly(lactic acid) (PLA) products by an ethylendiamine Zn(II) complex to form biodegradable methyl lactate was studied experimentally at 70 °C, 90 °C and 110 °C. The PLA samples consisted of typical consumer waste materials including a cup, a toy and 3D printing material. High selectivities and yields (>94%) were possible depending on temperature and reaction time. Additionally, and in order to develop a predictive kinetic model, kinetic parameters (pre-exponential factor and activation energies) of the PLA transesterification reaction were first obtained from virgin PLA. These parameters were subsequently used to estimate the conversion of PLA, selectivity and yield of methyl lactate after 1 h and 4 h of reaction, and the results were compared with the experimental values of the end-of-life PLA. Despite the presence of unknown additives in the PLA waste material and uncontrolled particle size, the model was able to predict the overall conversion, selectivity and yield to an average deviation of 5, 7 and 12% respectively. Greater agreement between model and experimental values are observed for the higher temperatures and the longer reaction time. Larger deviations were observed for the PLA toy which we attribute to the presence of additives, since despite its lower molecular weight, it possessed a higher structural strength.