Hydrochar-based facile and greenwood bio-adhesives production from distilled spent grain

The use of urea-, melamine-, and phenol–formaldehyde resin adhesives is endangering human health and damaging the environment. Developing formaldehyde-free plywood bio-adhesive is becoming a hot spot, and this study, for the first time, reports the preparation of facile and green wood bio-adhesives using hydrochar derived from distilled spent grain (DSG) by hydrothermal carbonization (HTC), which is promising for treating and valorizing high-moisture bio-waste such as DSG. HTC contributed to the dewatering, screening, and activation of active components in DSG, benefiting their interactions with cross-linkers and wood panels. The results show that the HTC temperature can significantly affect the reaction activity of hydrochar, thus influencing bonding strength. BD-180 bio-adhesive (mixture of cross-linkers glycerol triglycidyl ether/polyvinyl alcohol and hydrochar from HTC of DSG at 180 °C) had an excellent wet shear strength (1.24 MP), meeting the Chinese national standard GB/T 9846–2015 (≥0.7 MPa). Maillard reaction was a key reaction during HTC to destroy the secondary structure of proteins, resulting in the release of abundant –OH and –NH2. These functional groups in hydrochar were the major active sites available for the formation of covalent bonds (e.g., C–N and C–O–C) between bio-adhesive and wood panels, promoting the improvement in wet shear strength of bio-adhesives. This work provides a new method for the valorization of DSG and contributes to the development of green wood bio-adhesives without formaldehyde problems.