Highlights

Abstract

Keywords

Nomenclature

۱٫ Introduction

۲٫ Choice of the recycling strategy

۳٫ Materials and methods

۴٫ Results and discussions

۵٫ Conclusions

Declaration of Competing Interest

Acknowledgements

References

Abstract

Though synthetic plastics are indispensable in our everyday life, the uncertainty surrounding the sustainability of fossil reserves has led to the development of a novel class of plastics, referred to as bio-based plastics. Poly(lactic) acid (PLA) is the most frequently used member of this family. However, due to the lack of a holistic recycling strategy, its large scale utilization can turn out to be an acute source of plastic pollution in the future. Unlike other attempts directed towards chemical recycling of PLA which violate the basic principles of green chemistry, the following research establishes an eco-friendly recycling concept aimed at the production of a valuable lactate ester through solvent assisted transesterification of PLA waste. The scope of this research is not only limited to the selection of an appropriate system (solvent, nucleophile and catalyst) but also extends to analysing the selectivity of the solvent towards the PLA fraction in a commingled stream and the effect of the concentration of nucleophile and different PLA substrates on the yield of the lactate ester. It was observed that, irrespective of the source of PLA, a high yield of ethyl lactate (approx. 80%) with complete retention of stereochemistry was obtained for a molar ratio of nucleophile per mole repeat unit of PLA (nnuc:nrpu) equivalent to 3. Thus, this work represents an attempt towards instituting circular bio-economy by overcoming the engineering and environmental challenges associated with PLA-waste management and production of ethyl lactate; while strictly adhering to the principles of green chemistry and sustainable chemical engineering.

۱٫ Introduction

Although the mass production of synthetic plastics only dates back to 1950, they have become the most abundant anthropogenic materials and serve as a geological indicator of the Anthropocene era [1]. The annual plastic production is expected to reach 1124 million tons by 2050; thereby, consuming 20% of the crude oil produced globally as opposed to 6% in 2014 [2], [3]. Though these synthetic polymers are designed for their durable performance, their rapid growth as “materials of everyday use”, indiscriminate disposal and resistance to biological degradation presents an extensive threat to the environment [4]. In lieu of rising awareness about sustainability coupled with the pressure from global climate change over the past two decades, bio-based plastics have gained impetus as novel materials synchronous to the concept of sustainable production and utilization [4], [5]. Though this class accounts for less than 1% of the global annual plastic production, its global production capacity is estimated to increase from 2.11 million tons in 2020 to 2.87 million tons in 2025 at a compounded annual growth rate of 6.3% [6], [7].

Amongst other bio-based polymers, PLA is the most promising polyester [3], [5], [8]. It is regarded as a sustainable alternative to synthetic, petrochemical plastics such as PET and PS on account of its similar mechanical properties [5]. Accounting for 62.5% of its total annual production, NatureWorks® (۱۵۰,۰۰۰ tons; USA) and Total Corbion PLA (75,000 tons; Thailand) are the major producers of PLA in the global market [9]. PLA has expanded itself in several markets, ranging from disposable cutlery and degradable sutures to rigid packaging and extrusion coatings [5], [10], [11], [12], [13]. This is evident from the fact that, the relative share of PLA in the total global production of bio-plastics has increased from 13.9% in 2019 to 18.7% in 2020 [7]. However, the food packaging sector continues to be the most dominant market for PLA [11], [12], [13].