Deep-eutectic-solvent-assisted hydrothermal carbonization of sewage sludge for green wood bioadhesive production

This study reports the use of deep-eutectic-solvent- (DES-) assisted hydrothermal carbonization (HTC) to disrupt the floc structure of sewage sludge (SS) for deep carbonization, with the resulting hydrochars employed in the preparation of formaldehyde-free plywood bioadhesives. Sewage-sludge-based bioadhesive exhibits an excellent wet shear strength, complying with the requirements of Chinese national standard GB/T 9846–2015 (≥0.7 MPa). The molecular weight of proteins and the formation of covalent bonds via dehydration have a notable role in improving adhesive performance (wet shear strength). The Maillard reaction is a key reaction during HTC to destroy the secondary structure of proteins, resulting in the release of more OH and NH₂. The main reaction during hot-press treatment is dehydration. High ash content in bioadhesives improves flame resistance potential, particularly on addition of DES. A plausible mechanism is proposed for this. This work provides a new method for the valorization of SS-derived hydrochars and contributes to the development of greener formaldehyde-free wood bioadhesives.