(636c) Towards Closed-Loop Polymer Recycling: Enhancing Poly(lactide) Depolymerization into Methyl Lactate

Almost 350 million tonnes of plastic waste is generated every year. This waste is managed by several processes such as landfilling, incineration and recycling. Among these methods, recycling is the key process for achieving a sustainable future and contribute to circular economy. Poly(lactide) (PLA), which is a biodegradable polymer, is gaining attention as it replaces conventional plastics. While PLA production increases, the biodegradation requires an environment with high temperature and humidity, and it takes a few months for complete degradation. Hence, many studies focus on the recycling and depolymerization of PLA. One of the prevalent recycling processes for PLA is methanolysis with the advantage of methyl lactate production for further use. Various catalysts such as 4-dimethylaminopyridine, ionic liquids, metal amides and metal complexes are used to convert PLA into methyl lactate. However, challenges related to reaction time and reaction temperature remain. In this work, we investigated PLA methanolysis and its reaction mechanism systematically. Depolymerization reaction parameters were reaction time (up to 1 hour), temperature (50°C to 90°C) and stirring speed (200 – 800 rpm). Experiments were conducted with 200 mg PLA and 1,8-Diazabicyclo(5.4.0)undec-7-ene (DBU, 5% based on monomeric unit) was used as the catalyst. PLA and the depolymerization products were analyzed by H-NMR, GPC and GC-MS. The efficiency of the depolymerization process was calculated using two different methods. First, PLA conversion was calculated by the ratio of initial and final PLA amount. Secondly, methyl lactate conversion was the ratio of H-NMR peak areas of methyl lactate and PLA. Although all reactions resulted in a complete depolymerization into methyl lactate with longer reaction time, the methyl lactate conversion reached 96.1±2.4 % in 25 minutes of reaction at optimum conditions, resulting in almost 50% w/w methanol-methyl lactate in the product. We approached PLA recycling holistically by adding a membrane process, which is an energy efficient process, to separate depolymerization products. Five different commercial nanofiltration and reverse osmosis membranes are examined for methanol/methyl lactate separation. We managed to increase the methyl lactate content to 84%w/w in the permeate by using NF90 membrane in a dead-end filtration set up at 4 bars.