Cellulose Acetates in Hydrothermal Carbonization: A Green Pathway to Valorize Residual Bioplastics

Abstract

Bioplastics possess the potential to foster a sustainable circular plastic economy, but their end‐of‐life is still challenging. To sustainably overcome this problem, this work proposes the hydrothermal carbonization (HTC) of residual bioplastics as an alternative green path. The focus is on cellulose acetate – a bioplastic used for eyewear, cigarette filters and other applications – showing the proof of concept and the chemistry behind the conversion, including a reaction kinetics model. HTC of pure and commercial cellulose acetates was assessed under various operating conditions (180‐250 °C and 0‐6 h), with analyses on the solid and liquid products. Results show the peculiar behavior of these substrates under HTC. At 190‐210 °C, the materials almost completely dissolve into the liquid phase forming 5‑hydroxymethylfurfural and organic acids. Above 220 °C, intermediates repolymerize into carbon‐rich microspheres (secondary char), achieving solid yields up to 23 %, while itaconic and citric acid form. A comparison with pure substrates and additives demonstrates that the amounts of acetyl groups and derivatives of the plasticizers are crucial in catalyzing HTC reactions, creating a unique environment capable of leading to a total rearrangement of cellulose acetates. HTC can thus represent a cornerstone in establishing a biorefinery for residual cellulose acetate.