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

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.