Single Step Depolymerization of Multiple Polyesters in Poly(Lactic Acid) Mixed Plastics: Process Optimization, Pure Monomers Extraction and Kinetics Evaluation

Nowadays, most of the chemical recycling approaches for plastic waste aim primarily for the depolymerization of single polymer plastics or the multiple steps depolymerization of a plastic mixture which in both cases is considered time, cost and energy consuming. Herein, we present an optimized, single step approach for non-catalyzed hydrolysis of multiple polyesters in Poly(lactic acid) (PLA) mixed plastics under mild conventional heating. The proposed depolymerization process simultaneously breaks down plastic polyesters (mainly PLA and polyethylene terephthalate (PET)) into their value-added monomers; lactic acid in the form of Ca lactate and terephthalic acid (TPA) which are further separated and purified in a reduced number of steps. Box-Behnken Design was employed to maximize the conversion of plastics and the yields of the produced monomers through optimization of the depolymerization and monomer extraction conditions, all while adhering to the green chemistry principles. Within 30 min, and at 85 ⁰C temperature, the proposed hydrolysis technique facilitated 83–100% conversion of various PLA products (PLA Polymaker fibers (containing PLA, PET and other additives), PLA pellets and postconsumer PLA cups) into 763.8-929.6 mg/g_plastic Ca lactate and 51.5 mg/g_plastic TPA if PET was present. The proposed non-catalyzed process followed first order reaction kinetics with a small activation energy of 78.92 kJ/mol, resulting in an acceptable total energy consumption of 176.0 kJ/g_plastic. The obtained monomers’ identity and purity were confirmed by FTIR analysis. Additionally, the waste reagents produced during the depolymerization and monomer extraction processes were regenerated for reuse in another cycle of depolymerization while maintaining good performance. The developed approach offers an economically attractive and ecologically sustainable solution for energy and cost-efficient recycling and upcycling of post-consumer plastic waste containing PLA combined with other polyesters.