Process Development for Chemical Recycling of Bio-based Polymers

Though synthetic polymers have become indispensable in our daily life, the current global reserves of fossil fuels are only sufficient for the next 30 years at the current rate of consumption. Thus, new eco-friendly solutions synchronous with the concept of sustainability are being developed. One such attempt is the production of bio-based plastics. Amongst these plastics, poly-(lactic) acid (PLA) is the most promising polyester. With increased global production and its several utilities, it has become a high volume commodity material. Though PLA is a sustainable alternative to petrochemical plastics, the process of production of PLA is both, raw material and energy intensive. The bio-degradation of PLA is relatively slow while its degradation in an industrial compost leads to the formation of low-value products. Currently, there is no infrastructure in place to recycle PLA. Its presence contaminates other plastic waste streams, thereby, disturbing the existing recycling strategies. Thus, large scale disposal of PLA waste can lead to an acute source of plastic pollution. In order to address this issue, several recycling methods are proposed. Though mechanical recycling is applicable to PLA, this strategy is not infinite. Thus, chemical recycling of PLA, which leads to the production of valuable chemicals, is a promising strategy, thereby, completely utilizing the valuable polyhydroxy acid. Possible routes for chemical recycling of PLA include thermal degradation, hydrolytic and/or alcoholytic depolymerization and enzymatic processes. While the use of high temperatures (200 °C −400 °C) in hydrolytic and thermal degradation leads to chemical and optical degradation of the product, the enzymatic processes have relatively slower conversion rates and thus, suffer from scalability issues. This work deals with the transformation of PLA into corresponding lactate esters by alcoholytic depolymerization with the help of an organic catalyst in an eco-friendly solvent. Though severely limited by the insolubility of PLA in the alcohol, which necessitates the use of a solvent, the process has several advantages, such as a high yield of lactate ester and retention of stereo chemistry. Further, it adds value to the supply chain, as the market price of the lactate ester is higher as compared to PLA. In addition to choosing an appropriate system for selective dissolution of PLA, the current work estimates the dissolution kinetics and the subsequent effect of several operating parameters, namely, stirrer speed, molecular weight of PLA and particle size, on it. It also analyzes the effect of the chosen solvent on other polymeric impurities that might be present in the waste stream. Further, in order to devise an optimum reaction system for the alcoholysis of PLA, several catalysts are screened and an optimal reaction scheme is isolated for the proposed strategy. In addition to this, the effect of various operating parameters, namely, concentration of the alcohol and the molecular weight of PLA, on the reaction is also analyzed.